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ComputationalMethodsinOrganometallicCatalysis
FromElementaryReactionstoMechanisms
YuLan
Author
Prof.YuLan ZhengzhouUniversity GreenCatalysisCenter,andCollegeofChemistry
450001Zhengzhou
China
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Contents
Foreword xv Preface xvii
PartITheoreticalViewofOrganometallicCatalysis 1
1IntroductionofComputationalOrganometallicChemistry 3
1.1OverviewofOrganometallicChemistry 3
1.1.1GeneralViewofOrganometallicChemistry 3
1.1.2ABriefHistoryofOrganometallicChemistry 6
1.2UsingComputationalTooltoStudytheOrganometallicChemistry Mechanism 8
1.2.1MechanismofTransitionMetalCatalysis 8
1.2.2MechanisticStudyofTransitionMetalCatalysisbyTheoretical Methods 10 References 13
2ComputationalMethodsinOrganometallicChemistry 19
2.1IntroductionofComputationalMethods 19
2.1.1TheHistoryofQuantumChemistryComputationalMethods 19
2.1.2Post-HFMethods 21
2.2DensityFunctionalTheory(DFT)Methods 23
2.2.1OverviewofDensityFunctionalTheoryMethods 23
2.2.2Jacob’sLadderofDensityFunctionals 25
2.2.3TheSecondRungin“Jacob’sLadder”ofDensityFunctionals 25
2.2.4TheThirdRungin“Jacob’sLadder”ofDensityFunctionals 26
2.2.5TheFourthRungin“Jacob’sLadder”ofDensityFunctionals 26
2.2.6TheFifthRungin“Jacob’sLadder”ofDensityFunctionals 26
2.2.7CorrectionofDispersionInteractioninOrganicSystems 27
2.3BasisSetandItsApplicationinMechanismStudies 29
2.3.1GeneralViewofBasisSet 29
2.3.2Pople’sBasisSets 30
2.3.3PolarizationFunctions 31
2.3.4DiffuseFunctions 31
2.3.5Correlation-ConsistentBasisSets 31
2.3.6PseudoPotentialBasisSets 32
2.4SolventEffect 33
2.5HowtoChooseaMethodinComputationalOrganometallic Chemistry 34
2.5.1WhyDFTMethodIsChosen 34
2.5.2HowtoChooseaDensityFunctional 34
2.5.3HowtoChooseaBasisSet 36
2.6RevealingaMechanismforAnOrganometallicReactionbyTheoretical Calculations 37
2.7OverviewofPopularComputationalPrograms 37
2.8TheLimitationofCurrentComputationalMethods 40
2.8.1TheAccuracyofDFTMethods 40
2.8.2ExactSolvationEffect 41
2.8.3EvaluationofEntropyEffect 41
2.8.4TheComputationofExcitedStateandHighSpinState 41
2.8.5SpeculationontheReactionMechanism 41 References 42
3ElementaryReactionsinOrganometallicChemistry 51
3.1GeneralViewofElementaryReactionsinOrganometallicChemistry 51
3.2CoordinationandDissociation 52
3.2.1CoordinationBondandCoordination 52
3.2.2Dissociation 55
3.2.3LigandExchange 57
3.3OxidativeAddition 59
3.3.1ConcertedOxidativeAddition 60
3.3.2Substitution-typeOxidativeAddition 62
3.3.3Radical-typeAddition 67
3.3.4OxidativeCyclization 68
3.4ReductiveElimination 70
3.4.1ConcertedReductiveElimination 71
3.4.2Substitution-typeReductiveElimination 73
3.4.3Radical-Substitution-typeReductiveElimination 74
3.4.4BimetallicReductiveElimination 75
3.4.5EliminativeReduction 78
3.5Insertion 78
3.5.11,2-Insertion 79
3.5.21,1-Insertion 80
3.5.3ConjugativeInsertion 83
3.5.4Outer-SphereInsertion 84
3.6Elimination 86
3.6.1 β-Elimination 86
3.6.2 α-Elimination 90
3.7Transmetallation 92
3.7.1ConcertedRing-TypeTransmetallation 92
3.7.2TransmetallationThroughElectrophilicSubstitution 98
3.7.3StepwiseTransmetallation 99
3.8Metathesis 100
3.8.1 σ-BondMetathesis 100
3.8.2OlefinMetathesis 102
3.8.3AlkyneMetathesis 106 References 106
PartIIOntheMechanismofTransition-metal-assisted Reactions 125
4TheoreticalStudyofNi-Catalysis 127
4.1Ni-MediatedC—HBondActivation 128
4.1.1Ni-MediatedAreneC—HActivation 128
4.1.2Ni-MediatedAldehydeC—HActivation 132
4.2Ni-MediatedC—HalogenBondCleavage 133
4.2.1ConcertedOxidativeAdditionofC—HalogenBond 133
4.2.2Radical-TypeSubstitutionofC—HalogenBond 135
4.2.3C—HalogenBondCleavageby β-HalideElimination 137
4.2.4NucleophilicSubstitutionofC—HalogenBond 139
4.3Ni-MediatedC—OBondActivation 140
4.3.1EtherC—OBondActivation 140
4.3.2EsterC—OBondActivation 142
4.4Ni-MediatedC—NBondCleavage 148
4.5Ni-MediatedC—CBondCleavage 151
4.5.1C—CSingleBondActivation 151
4.5.2C=CDoubleBondActivation 152
4.6Ni-MediatedUnsaturatedBondActivation 153
4.6.1OxidativeCyclizationwithUnsaturatedBonds 153
4.6.2ElectrophilicAdditionofUnsaturatedBonds 156
4.6.3UnsaturatedCompoundsInsertion 157
4.6.4NucleophilicAdditionofUnsaturatedBonds 159
4.7Ni-MediatedCyclization 160
4.7.1Ni-MediatedCycloadditions 161
4.7.2Ni-MediatedRingSubstitutions 163
4.7.3Ni-MediatedRingExtensions 166 References 168
5TheoreticalStudyofPd-Catalysis 181
5.1Pd-CatalyzedCross-couplingReactions 182
5.1.1Suzuki–MiyauraCoupling 183
5.1.2NegishiCoupling 186
5.1.3StilleCoupling 189
5.1.4HiyamaCoupling 190
5.1.5Heck–MizorokiReaction 192
5.2Pd-MediatedC—HeteroBondFormation 196
5.2.1C—BBondFormation 196
5.2.2C—SBondFormation 197
5.2.3C—IBondFormation 200
5.2.4C—SiBondFormation 201
5.3Pd-MediatedC—HActivationReactions 204
5.3.1Chelation-FreeC(sp3 )—HActivation 206
5.3.2Chelation-FreeC(sp2 )—HActivation 208
5.3.3CoordinativeChelation-Assisted ortho-C(aryl)—HActivation 210
5.3.4CovalentChelation-Assisted ortho-C(aryl)—HActivation 212
5.3.5Chelation-Assisted meta-C(aryl)—HActivation 214
5.3.6CoordinativeChelation-AssistedC(sp3 )—HActivation 216
5.3.7CovalentChelation-AssistedC(sp3 )—HActivation 216
5.3.8C—HBondActivationThroughElectrophilicDeprotonation 219
5.3.9C—HBondActivationThrough σ-Complex-AssistedMetathesis 221
5.3.10C—HBondActivationThroughOxidativeAddition 223
5.4Pd-MediatedActivationofUnsaturatedMolecules 224
5.4.1AlkeneActivation 225
5.4.2AlkyneActivation 225
5.4.3EnyneActivation 226
5.4.4ImineActivation 229
5.4.5COActivation 229
5.4.6IsocyanideActivation 231
5.4.7CarbeneActivation 231
5.5AllylicPdComplex 234
5.5.1FormationfromAllylicOxidativeAddition 235
5.5.2FormationfromAllylicNucleophilicSubstitution 236
5.5.3FormationfromtheNucleophilicAttackontoAllene 237
5.5.4FormationfromAllylicC—HActivation 237
5.5.5FormationfromAlleneInsertion 238
References 239
6TheoreticalStudyofPt-Catalysis 257
6.1MechanismofPt-CatalyzedC—HActivation 258
6.1.1OxidativeAdditionofC—HBond 259
6.1.2ElectrophilicDehydrogenation 259
6.1.3CarbeneInsertionintoC—HBonds 261
6.2MechanismofPt-CatalyzedAlkyneActivation 264
6.2.1NucleophilicAdditions 264
6.2.2Cyclopropanation 266
6.2.3OxidativeCycloaddition 268
6.3MechanismofPt-CatalyzedAlkeneActivation 270
6.3.1HydroaminationofAlkenes 270
6.3.2HydroformylationofAlkenes 272
6.3.3IsomerizationofCyclopropenes 273 References 274
7TheoreticalStudyofCo-Catalysis 289
7.1Co-MediatedC—HBondActivation 289
7.1.1HydroarylationofAlkenes 291
7.1.2HydroarylationofAllenes 293
7.1.3HydroarylationofAlkynes 294
7.1.4HydroarylationofNitrenoid 296
7.1.5OxidativeC—HAlkoxylation 297
7.2Co-MediatedCycloadditions 297
7.2.1Co-MediatedPauson–KhandReaction 298
7.2.2Co-Catalyzed[4+2]Cyclizations 299
7.2.3Co-Catalyzed[2+2+2]Cyclizations 299
7.2.4Co-Catalyzed[2+2]Cyclizations 300
7.3Co-CatalyzedHydrogenation 301
7.3.1HydrogenationofCarbonDioxide 301
7.3.2HydrogenationofAlkenes 304
7.3.3HydrogenationofAlkynes 306
7.4Co-CatalyzedHydroformylation 307
7.4.1DirectHydroformylationbyH2 andCO 308
7.4.2TransferHydroformylation 309
7.5Co-MediatedCarbeneActivation 310
7.5.1ArylationofCarbene 310
7.5.2CarboxylationofCarbene 312
7.6Co-MediatedNitreneActivation 313
7.6.1AziridinationofOlefins 314
7.6.2AminationofIsonitriles 314
7.6.3AminationofC—HBonds 315 References 317
8TheoreticalStudyofRh-Catalysis 329
8.1Rh-MediatedC—HActivationReactions 330
8.1.1Rh-CatalyzedArylationofC—HBond 330
8.1.2Rh-CatalyzedAlkylationofC—HBond 332
8.1.3Rh-CatalyzedAlkenylationofC—HBond 335
8.1.4Rh-CatalyzedAminationofC—HBond 338
8.1.5Rh-CatalyzedHalogenationofC—HBond 340
8.2Rh-CatalyzedC—CBondActivationsandTransformations 341
8.2.1Strain-drivenOxidativeAddition 341
8.2.2TheCarbon—CyanoBondActivation 342
8.2.3 β-CarbonElimination 343
8.3Rh-MediatedC—HeteroBondActivations 343
x Contents
8.3.1C—NBondActivation 344
8.3.2C—OBondActivation 345
8.4Rh-CatalyzedAlkeneFunctionalizations 345
8.4.1HydrogenationofAlkene 346
8.4.2DiborationofAlkene 347
8.5Rh-CatalyzedAlkyneFunctionalizations 349
8.5.1HydroacylationofAlkynes 349
8.5.2HydroaminationofAlkynes 349
8.5.3HydrothiolationofAlkynes 351
8.5.4HydroacetoxylationofAlkynes 351
8.6Rh-CatalyzedAdditionReactionsofCarbonylCompounds 352
8.6.1HydrogenationofKetones 352
8.6.2HydrogenationofCarbonDioxide 353
8.6.3HydroacylationofKetones 353
8.7Rh-CatalyzedCarbeneTransformations 354
8.7.1CarbeneInsertionintoC—HBonds 354
8.7.2ArylationofCarbenes 357
8.7.3CyclopropanationofCarbenes 358
8.7.4CyclopropenationofCarbenes 359
8.8Rh-CatalyzedNitreneTransformations 359
8.8.1NitreneInsertionintoC—HBonds 360
8.8.2AziridinationofNitrenes 361
8.9Rh-CatalyzedCycloadditions 362
8.9.1(3+2)Cycloadditions 365
8.9.2Pauson–Khand-type(2+2+1)Cycloadditions 367
8.9.3(5+2)Cycloadditions 367
8.9.4(5+2+1)Cycloadditions 369 References 369
9TheoreticalStudyofIr-Catalysis 387
9.1Ir-CatalyzedHydrogenations 387
9.1.1HydrogenationofAlkenes 388
9.1.2HydrogenationofCarbonylCompounds 391
9.1.3HydrogenationofImines 393
9.1.4HydrogenationofQuinolines 396
9.2Ir-CatalyzedHydrofunctionalizations 397
9.2.1Ir-CatalyzedHydroaminations 397
9.2.2Ir-CatalyzedHydroarylations 397
9.2.3Ir-CatalyzedHydrosilylations 399
9.3Ir-CatalyzedBorylations 401
9.3.1BorylationofAlkanes 402
9.3.2BorylationofArenes 403
9.4Ir-CatalyzedAminations 405
9.4.1AminationofAlcohols 405
9.4.2AminationofArenes 407
9.5Ir-CatalyzedC—CBondCouplingReactions 407 References 409
10TheoreticalStudyofFe-Catalysis 419
10.1Fe-MediatedOxidations 420
10.1.1AlkaneOxidations 420
10.1.2AreneOxidations 422
10.1.3AlkeneOxidations 423
10.1.4OxidativeCatecholRingCleavage 425
10.2Fe-MediatedHydrogenations 426
10.2.1HydrogenationofAlkenes 427
10.2.2HydrogenationofCarbonyls 429
10.2.3HydrogenationofImines 430
10.2.4HydrogenationofCarbonDioxide 430
10.3Fe-MediatedHydrofunctionalizations 431
10.3.1HydrosilylationofKetones 431
10.3.2HydroaminationofAllenes 432
10.4Fe-MediatedDehydrogenations 434
10.4.1DehydrogenationofAlcohols 434
10.4.2DehydrogenationofFormaldehyde 435
10.4.3DehydrogenationofFormicAcid 435
10.4.4DehydrogenationofAmmonia-Borane 437
10.5Fe-CatalyzedCouplingReactions 438
10.5.1C—CCross-CouplingswithArylHalide 438
10.5.2C—NCross-CouplingswithArylHalide 438
10.5.3C—CCross-CouplingswithAlkylHalide 441
10.5.4Iron-MediatedOxidativeCoupling 441 References 441
11TheoreticalStudyofRu-Catalysis 451
11.1Ru-MediatedC—HBondActivation 452
11.1.1MechanismoftheRu-MediatedC—HBondCleavage 452
11.1.2Ru-CatalyzedC—HBondArylation 456
11.1.3Ru-Catalyzed ortho-AlkylationofArenes 460
11.1.4Ru-Catalyzed ortho-AlkenylationofArenes 461
11.2Ru-CatalyzedHydrogenations 464
11.2.1HydrogenationofAlkenes 464
11.2.2HydrogenationofCarbonyls 465
11.2.3HydrogenationofEsters 466
11.2.4HydrodefluorinationofFluoroarenes 467
11.3Ru-CatalyzedHydrofunctionalizations 468
11.3.1Hydroacylations 469
11.3.2Hydrocarboxylations 470
11.3.3Hydroborations 471
11.4Ru-MediatedDehydrogenations 472
11.4.1DehydrogenationofAlcohols 473
11.4.2DehydrogenationofFormaldehyde 473
11.4.3DehydrogenationofFormicAcid 474
11.5Ru-CatalyzedCycloadditions 475
11.5.1Ru-Mediated(2+2+2)Cycloadditions 475
11.5.2Ru-MediatedPauson–KhandType(2+2+1)Cycloadditions 478
11.5.3Ru-MediatedClickReactions 478
11.6Ru-MediatedMetathesis 480
11.6.1Ru-MediatedIntermolecularOlefinMetathesis 482
11.6.2Ru-MediatedIntramolecularDieneMetathesis 484
11.6.3Ru-MediatedAlkyneMetathesis 484 References 485
12TheoreticalStudyofMn-Catalysis 499
12.1Mn-MediatedOxidationofAlkanes 500
12.1.1C—HHydroxylations 500
12.1.2C—HHalogenations 501
12.1.3C—HAzidations 501
12.1.4C—HIsocyanations 501
12.2Mn-MediatedC—HActivations 502
12.2.1ElectrophilicDeprotonation 503
12.2.2 σ-Complex-AssistedMetathesis 504
12.2.3ConcertedMetalation–Deprotonation 505
12.3Mn-MediatedHydrogenations 507
12.3.1HydrogenationofCarbonDioxide 507
12.3.2HydrogenationofCarbonates 508
12.4Mn-MediatedDehydrogenations 510
12.4.1DehydrogenationofAlcohols 510
12.4.2DehydrogenativeCouplings 511 References 512
13TheoreticalStudyofCu-Catalysis 517
13.1Cu-MediatedUllmannCondensations 518
13.1.1C—NBondCouplings 520
13.1.2C—OBondCouplings 522
13.1.3C—FBondCouplings 522
13.2Cu-MediatedTrifluoromethylations 524
13.2.1TrifluoromethylationsThroughCross-Coupling 524
13.2.2TrifluoromethylationsThroughOxidativeCoupling 524
13.2.3Radical-TypeTrifluoromethylations 525
13.3Cu-MediatedC—HActivations 527
13.3.1C—HArylations 527
13.3.2C—HAminations 529
13.3.3C—HHydroxylation 531
13.3.4C—HEtherifications 532
13.4Cu-MediatedAlkyneActivations 533
13.4.1Azide–AlkyneCycloadditions 533
13.4.2NucleophilicAttackontoAlkynes 535
13.4.3AlkynylCuTransformations 536
13.5Cu-MediatedCarbeneTransformations 539
13.5.1[2+1]CycloadditionswithAlkenes 539
13.5.2CarbeneInsertions 541
13.5.3RearrangementofCarbenes 542
13.6Cu-MediatedNitreneTransformations 542
13.6.1[2+1]CycloadditionswithAlkenes 543
13.6.2AminationofNitrenes 543
13.6.3NitreneInsertions 544
13.7Cu-CatalyzedHydrofunctionalizations 545
13.7.1Hydroborylations 547
13.7.2Hydrosilylation 547
13.7.3Hydrocarboxylations 548
13.8Cu-CatalyzedBorylations 549
13.8.1BorylationofAlkenes 551
13.8.2BorylationofAlkynes 552
13.8.3BorylationofCarbonyls 553
References 554
14TheoreticalStudyofAg-Catalysis 567
14.1Ag-MediatedCarbeneComplexTransformations 568
14.1.1Silver–CarbeneFormation 568
14.1.2CarbeneInsertionintoC—ClBond 572
14.1.3CarbeneInsertionintoO—HBond 573
14.1.4NucleophilicAttackbyCarbonylGroups 573
14.1.5CarbeneInsertionintoC—HBond 574
14.2Ag-MediatedNitreneTransformations 576
14.2.1Silver–NitreneComplexFormation 577
14.2.2NucleophilicAttackbyUnsaturatedBonds 580
14.2.3NucleophilicAttackbyAmines 582
14.3Ag-MediatedSilyleneTransformations 583
14.4Ag-MediatedAlkyneActivations 585
14.4.1 π-ActivationofAlkynes 586
14.4.2C—HActivationofAlkynes 587
References 588
15TheoreticalStudyofAu-Catalysis 597
15.1Au-MediatedAlkyneActivations 598
15.1.1IsomerizationofAlkynes 599
15.1.2NucleophilicAttackbyOxygen-InvolvedNucleophiles 599
15.1.3NucleophilicAttackbyNitrogen-InvolvedNucleophiles 602
15.1.4NucleophilicAttackbyArenes 606
15.2Au-mediatedAlkeneActivations 607
15.2.1NucleophilicAdditionofAlkenes 607
15.2.2AllylicSubstitutions 608
15.3Au-mediatedAlleneActivations 609
15.3.1HydroaminationofAllenes 610
15.3.2HydroalkoxylationofAllenes 610
15.3.3CycloisomerizationofAllenylKetones 613
15.4Au-mediatedEnyneTransformations 613
15.4.11,5-EnyneCycloisomerizations 614
15.4.21,6-EnyneCycloisomerizations 615
15.4.3AllenyneCycloisomerizations 617
15.4.4ConjugativeEnyneCycloisomerizations 617
References 618
Index 629
Foreword
Computationalchemistrybeganinthe1940swiththeearliestelectroniccomputers anddrasticapproximationstotheSchrödingerequation,suchasHückelmolecular orbitaltheory.Sincethe1960s,thepossibilitiesofdoingquantummechanicscalculationsonlargesystemscontainingmetals,indeedtostudytheheartoforganometallicchemistry,beganwiththeMulliken–Wolfsberg–Helmholtzapproachandthen RoaldHoffmann’sExtendedHückelTheory(EHT)inthe1950sand1960s.Whilean amazinglyusefulmethod,EHTisreallyonlyofqualitativevalue.Butactuallywhat weneedisaconceptualframeworkthatisusefuleventoday.
The1960ssawremarkableadvancesinmethods,approximations,andthe beginningsofthefloweringofcomputersforchemicalcalculations.Thedawnof themodernhybriddensityfunctionaltheoryinthemid-1990s,borrowingexact exchangefromwavefunctiontheory,madeitpossibletobeginthequantitativecalculationsofstructure,mechanics,andmechanisms,includingtheincrediblyuseful organometallicreactions.BothRuandMocatalystsforolefinmetathesis,andPd catalysisforcross-couplingreactions,haveledtoNobelprizesfortheirdiscoverers.
YuLanhasnowwrittenanintroduction,aguide,amasterworkaboutquantum mechanicalstudiesoforganometallicreactionmechanisms.Heisideallyequipped towritesuchabook,alreadydoingoutstandingworkinthefieldasastudentand postdoc,andbecomingaleaderinthefieldinhisindependentcareer.Hehasalso trainedmanyyoungexpertsinthefield,andhisinfluencewillspreadfurtherfrom theirachievementsandthroughthisbook.
Thebookincludesahistoricalintroductiontoorganometallicchemistry,asurvey ofmechanisms,andanextensiveintroductiontoquantummechanicalcomputationalmethods,especiallydensityfunctionaltheory,aswellasprogramsforquantumchemicalcalculations.
Thedescriptionoforganometallicstructuresandmechanismsispepperedwith numerouscalculationsfromtheLangroupwithrelativelyaccuratedensityfunctionals.Part2,thebulkofthisbook,isorganizedwithachapterforeachofthemost importantmetalsusedinorganometallicchemistry:Ni,Pd,Pt,Co,Rh,Ir,Fe,Ru,Mn, Cu,Ag,andAu.Thecomputationalstudiesofthereactionsofcomplexesofeachof thesemetalsarereviewedwithgreatinsightsintomechanismsusingcomputations.
Foreword
Thebookwillbeaboontoorganometallicchemistsandcomputationalchemists involvedinthestudyoforganometallicreactions.Whileanumberofbookson organometallicchemistrymechanismsareavailable,thisisTHEbookdescribing themethodsforcomputationandanalysisoforganometallicreactionsusing modernquantummechanicalmethods.
29August2020
KendallN.Houk
Preface
Alongtimeago,whenIfirstcameintocontactwithscience,Iwasfascinatedbythe uniquecharmoforganicchemistry.Thetetravalentcarbonatomsandtheirtetrahedralstructuresimpressedmewithelegantsimplicity.Throughthebrokenand formationofcovalentbonds,variousnewmoleculesthatpossessuniqueproperties couldbegenerated.Bymanipulatingthereactionconditions,catalysts,andligands inorganicreactions,chemistscaneffectivelysynthesizeplentyofcomplexnaturalproductmoleculesandpharmaceuticalmoleculesinhighregioselectivityand enantioselectivity.WhenIwasateenager,Itookeveryorganicchemicalreaction asapuzzle,andthereactionmechanismisliketheanswertothepuzzle.Inthis way,Ifoundgreatpleasureinthinkingaboutthemechanismoforganicreactions. Thedevelopmentoforganicchemistryalsorequiresacomprehensivemechanisticunderstanding.Generally,asignificantamountofinformationaboutreaction mechanismscouldbeobtainedbyexperimentaltechniques.However,theexperimentalmechanisticstudymainlyfocusesonthemacroscopicallyobservedexperimentalphenomena.Therefore,inmanycases,pureexperimentalobservationis notsufficientforrevealingthecompletereactionpathwayandclarifyingtheoriginofselectivity.DuringmydoctoralstudyatPekingUniversity,Iwasfortunate toworkwithmysupervisorProfessorYun-DongWu,fromwhomIlearnedhowto usecomputationalchemistrytoinvestigatetheorganicreactionmechanisms.Theoreticalcalculationsbasedonquantummechanics,especiallythedensityfunctional theorycalculation,haveconstitutedthemostpowerfultoolformechanisticstudy duetothedevelopmentofsupercomputer,computationaltheory,andcorresponding software.
Inthepastseveraldecades,oneofthemostimportantadvancesinorganicchemistryhasbeentheintroductionoftransitionmetalcatalyststoorganicsynthesis. Transitionmetalspeciescanreactwithorganiccompoundstogenerateintermediatesthatcontaincarbon–metalbonds.Subsequentconversionoftheseorganometallicintermediatescouldenrichthesyntheticapproachtowardnewmolecules.Differentfromorganocatalysis,theorganometalliccatalysisusuallygoesthroughmultiple stepsandcomplicatedcatalyticcycles,whichoriginatedfromthecomplexbonding patternoforganometalliccatalystsandthevariationofvalancestateofthecentral metalspecies.Thus,theutilizationoftheoreticalcalculationforunderstandingthe reactionmechanismisimperativeforthedevelopmentoforganicchemistry.My
post-doctoralresearchwithProfessorK.N.Houkwasworkingwithexperimental chemiststoexplorethemechanismoforganometallicreactionsthroughthecollaborationoftheoreticalcalculationwithexperimentalstudy.Thepromotionoftheoreticalstudyonexperimentaldevelopmentcouldbesummarizedinto“3D,”i.e. description,design,anddirection.Basedonthedataobtainedfromexperimental study,detaileddescriptionsoftheorganometallicreactionmechanismscouldbe fulfilledusingtheoreticalcalculation.Themechanisticstudythenprovidesfurther theoreticalguidancefortherationaldesignofnewreactions,whichpointsoutthe directionofexperimentaldevelopment.
Overrecentdecades,massiveexperimentalandtheoreticalinvestigationson organometalliccatalysishavebeenreported.Inthoseworks,theoreticalstudies havebeenprovedtobeanindispensabletechniqueformodernorganicchemistry. Consequently,thisbookiswrittentosummarizeandgeneralizethetheoretical advancesinthemechanisticstudyoforganometalliccatalysis.Thisbookcomprises twoparts,whicharethegeneraloverviewoforganometalliccatalysisandthe computationalstudiesofreactionmechanismsclassifiedbytransitionmetals.I hopethisbookcouldinspirethemechanisticstudiesofcomplexreactionsfor theoreticalchemists,andenableabetterunderstandingofreactionmechanismsfor experimentalchemists.
29July2020
YuLan
Zhengzhou,P.R.China
TheoreticalViewofOrganometallicCatalysis
Itistimetowriteabookoncomputationalorganometallicchemistry.
Thefirstpartofthisbookcanbeconsideredastheintroductiontocomputational organometallicchemistry.Itisalonghistorysinceorganometalliccatalysishasbeen appliedinorganicsynthesis;however,themechanismofthosereactionsistoocomplicatedtounderstand.Indeed,computationalchemistryprovidedapowerfultool torevealthemechanismoforganometallicreactions.Duringrecenttwodecades,the combinationofcomputationalchemistryandorganometallicchemistryhasmadea seriesofprogressinmechanisticstudies,whichhasledtoanewdiscipline,computationalorganometallicchemistry.
Thefirstpartwouldbecomposedofthreechapters.InChapter1,abriefhistoryoforganometallicsisgiventorevealthesignificanceofthischemistry.Computationalchemistry,especiallycomputationalmethods,isdiscussedinChapter2, whichwouldbeusedinmechanismstudyoforganometalliccatalysis.Detailedprocessesforthefamiliarelementaryreactionsinorganometalliccatalysisdiscovered bytheoreticalcalculationsaresummarizedinChapter3. ComputationalMethodsinOrganometallicCatalysis:FromElementaryReactionstoMechanisms, FirstEdition.YuLan. ©2021WILEY-VCHGmbH.Published2021byWILEY-VCHGmbH.
IntroductionofComputationalOrganometallicChemistry
Thischapterprovidesabriefintroductionofcomputationalorganometallic chemistry,whichusuallyfocusesonthereactionmechanismofhomogeneous organometalliccatalysis.
1.1OverviewofOrganometallicChemistry
Inthissection,thehistoricalfootprintoforganometallicchemistryisconcisely given,whichwouldhelpthereadersbetterunderstandtheroleofcomputationin themechanisticstudyoforganometallicchemistry.
1.1.1GeneralViewofOrganometallicChemistry
Creatingnewmaterialisalwaysentrustedwiththeimportantresponsibilityforthe developmentofhumancivilization[1–3].Inparticular,syntheticchemistrybecomes apowerfultoolforchemists,asitexhibitsgreatvaluefortheselectiveconstructionof newcompounds[4–8].Varioususefulmoleculescouldbepreparedbythestrategies ofsyntheticchemistry,whichprovidesmaterialfoundation,technologicalsupport, anddriveforceforscience[9–20].Syntheticchemistryisalsothemotivatingforce fortheprogressofmaterialscience,pharmaceuticalscience,energyengineering, agriculture,andelectronicsindustry[21–41].Inthisarea,organicsynthesisreveals broadinterestsfromaseriesofresearchfields,whichcouldtargetsupplytomultifariousfunctionalmolecules.
Thesyntheticorganicchemistryusuallyfocuseson“carbon”towidenrelated research,whichcouldaffordvariousstrategiesforthebuildingofmolecular framework,functionalgrouptransformations,andcontrollingstereochemistry inmoresophisticatedmolecules[9,22,42–50].Therefore,selectiveformationof newcovalentbondbetweencarbonatomandsomeotheratominvolvingnitrogen, oxygen,sulfur,halogen,boron,andphosphorusbecomesoneofthemostimportant aimsforsyntheticorganicchemistry.Inparticular,nucleophilesandelectrophiles areimportantfortheconstructionofnewcovalentbonds.
ComputationalMethodsinOrganometallicCatalysis:FromElementaryReactionstoMechanisms, FirstEdition.YuLan. ©2021WILEY-VCHGmbH.Published2021byWILEY-VCHGmbH.
1IntroductionofComputationalOrganometallicChemistry
Anucleophile,whichisamoleculewithformallone-pairelectrophiles,candonate twoelectronstoitsreactionpartnerfortheformationofnewcovalentbond.Alternatively,anelectrophile,whichisamoleculewithformalunoccupiedorbitals,can accepttwoelectronsfromitspartnerfortheformationofnewcovalentbond.Thereinto,couplingreactionscouldbecategorizedasredox-neutralcross-couplingwith anelectrophileandanucleophile,oxidativecouplingwithtwonucleophiles,and reductivecouplingwithtwoelectrophiles(Scheme1.1).
Cross-coupling:
Oxidative cross-coupling:
Reductive cross-coupling: +2e
E + EE
Scheme1.1 Cross-couplingreactionswithnucleophilesandelectrophiles.
Inorganicchemistry,thenucleophileisanelectron-richmoleculethatcontainsa lonepairofelectronsorapolarizedbond,theheterolysisofwhichalsocouldyield alonepairofelectrons(Scheme1.2).Accordingtothisconcept,organometallic compounds,alcohols,halides,amines,andphosphineswithalonepairofelectrons arenucleophiles.Somenonpolar π bonds,includingolefinsandacetylenes,which coulddonatethe π-bondingelectrons,areoftenconsideredtobenucleophiles. Moreover,theC—Hbondsofhydrocarbonscanbeconsideredtobenucleophiles becausetheelectronegativityofcarbonishigherthanthatofhydrogen,which coulddeliveraprotontoformaformalcarbonanion.Correspondingly,theelectrophileisanelectron-deficientmoleculethatcontainsunoccupiedorbitalsor
Lone-pair of electrons:
Unsaturated Bonds: ��
Organometallic complexes:
C H bonds:
Scheme1.2 Someselectedexamplesofnucleophiles.
1.1OverviewofOrganometallicChemistry 5
low-energyantibondingmolecularorbital,whichcouldaccepttheelectronsfrom nucleophiles.Inthischemistry,cationiccarbons,whichusuallycomefromthe heterolysisofcarbon—halogenbonds,areelectrophile.Polar π bonds,including carbonylcompoundsandimines,alsocouldbeconsideredtobeelectrophile, whichinvolvealow-energy π antibond.Interestingly,Fisher-typesingletcarbene hasanelectronpairfillingonesp2 hybridorbitalandanunoccupiedporbital, whichcouldbeconsideredtobeeithernucleophileorelectrophileincoupling reactions.
Superficially,atleast,thereactionbetweennucleophileandelectrophilecould constructacovalentbondundoubtedly.However,thefamiliarnucleophilesand electrophiles,usedincross-couplingreactions,areusuallyinactive,whichcould notreactwitheachotherrapidly.Moreover,whenmoreactivenucleophilesand electrophilesareusedincouplingreactions,itwouldbecomeoutofcontrol,which wouldnotselectivelyaffordtargetproducts.Ineffect,introducingtransitionmetal catalysiscanperfectlysolvethisproblem.Theappropriatetransitionmetalcanbe employedtoselectivelyactivatethenucleophilesandelectrophilesandstabilize someothers,whichledtoaspeciallyappointedcross-couplingreaction.
High-valencetransitionmetalcanobtainelectronsfromnucleophile,whichled tothetransformationofnucleophileintoelectrophile.Thenewlygeneratedelectrophilecancouplewithothernucleophilestoformcovalentbond,whichisnamed oxidativecouplingreaction[51–53].Meanwhile,thereducedtransitionmetalcan beoxidizedbyexogenousoxidantforregeneration.Correspondingly,low-valence transitionmetalcandonateelectronstoelectrophileleadingtothetransformation ofelectrophileintonucleophile,whichcanreactwithanotherelectrophiletoform covalentbond.Accordingly,itisnamedreductivecouplingreaction.Theoxidized transitionmetalalsocanbereducedbyexogenousreductant.
Thedorbitalofsometransitionmetalscouldbefilledbyunpairedelectrons,which ledtoauniquecatalyticactivityinradical-involvedreactions.Thehomolyticcleavageoftransitionmetal–carbon(orsomeotheratoms)bondisanefficientwayforthe generationofaradicalspecies,whichcanpromotefurthertransformations.Onthe otherhand,freeradicalcanreactwithsometransitionmetalleadingtothestabilizationofradical,whichcancausefurtherradicaltransformations[54–57].Moreover, nucleophilesandelectrophiles,activatedbytransitionmetals,alsocanreactwith radicaltoformnewcovalentbonds.
Althoughthereisnoelectronbarrierduetotheappropriatesymmetryoffrontiermolecularorbitals,agreatdealofuncatalyzedpericyclicreactionswouldoccur underharshreactionconditions,whichcouldbeoftenattributedtothelow-energy levelofhighestoccupiedmolecularorbitals(HOMOs)andhigh-energyleveloflowestunoccupiedmolecularorbitals(LUMOs)inreactingpartners.Transitionmetals canplayasaLewisacid,whichcouldsignificantlyreducetheLUMOofcoordinatedorganicmoiety.Therefore,ithasbeenwidelyadoptedtocatalyzepericyclic reactions,whichleadstomoderatereactionconditionsandadjustableselectivity [58–62].Moreover,thenodeofdorbitalcanchangethesymmetryofaconjugative compound,whichinvolvesatransitionmetal.Therefore,transitionmetalitselfalso couldparticipateinapericyclicreactiontorevealuniquecatalyticactivity.
1IntroductionofComputationalOrganometallicChemistry
Asanoverviewoforganometallicchemistry,thecoreistheformationofa metal–carbonbondanditsfurthertransformation.Differentfromorganocatalysis, organometalliccatalysisprocessusuallygoesthroughmultiplestepsaswellas complicatedcatalyticcycles,whichoriginatedfromthecomplexbondingpattern ofmetalliccatalystandthevariationofvalencestateforthecentralmetalelement. Consequently,improvingthereactionefficiencyandyieldfororganometallic catalysisencounteredmoredifficultythanconventionalorganocatalysis.Moreover, thedesignofcatalysisandligandfortransitionmetal-catalyzedreactionisstill facingbothopportunitiesandchallenges.Tosolvetheabove-mentionedissues, theunderstandingofreactionmechanismisimperative,whichcouldgivemore informationforthedetailedreactionprocess,andhelptoimprovethereaction efficiencyandyield.
1.1.2ABriefHistoryofOrganometallicChemistry
Asaninterdisciplineoforganicandinorganicchemistry,organometallicchemistry hasahistoryofalmost200yearssincethefirstcomplexK[(C2 H4 )PtCl3 ]⋅H2 O wasreportedbyZeisewhenheheatedethanolsolutionofPtCl4 /KCl[62].The historyoforganometallicchemistrycanberoughlydividedintofourstages.The chemistsmajorlyfocusedonmaingrouporganometalliccompoundsinthenineteenthcentury.Laterinthefirsthalfofthetwentiethcentury,chemistspaidmore attentiontounderstandingthestructuresoforganometalliccompoundsinvolving transitionmetals.Theninthelatterhalfofthetwentiethcentury,varioustransition metal-catalyzedreactionshadbeenwidelyreported.Sincethiscentury,chemists havebeenkeenonusingtransitionmetalcatalysistoselectivelyconstructmore complexorganiccompounds.Theoutlinedhistoryoforganometallicchemistry couldbeconcludedinScheme1.3[63].
Structural theory of organometallic complexes (1913)
Zeise salt
ZnMe2 (1827)(1849)
Al2Et3I3 (1859)
Ni(CO)4 (1890)
Grignard reagent (1901)
Main group organometallic chemistry
Cr(C6H6)2 (1919)
Ziegler–Nata catalyst (1953) -allylmetal (1959)
Fe(C5H5)2 (1951)
Structures of organometallic compounds
Asymmetric synthesis (2010 NP) Crosscoupling π
Metal–carbene complexes (1964)
β-hydride elimination (1970) Heck reaction (1971) Metathesis (2005 NP) (2001 NP)
Organometallic catalysis
Precise synthesis through organometallic catalysis
Scheme1.3 Abriefhistoryoforganometallicchemistry.Source:BasedonDidier[63].
Thenineteenthcenturycouldbeconsideredastheenlightenmenteraof organometallicchemistry.Franklandfirstsystemicallyinvestigatedorganometallic chemistryandpreparedaseriesofalkylmetalcompoundsin1850s.Inthelate nineteenthcentury,ZnMe2 (in1849byE.Frankland),Sn(C2 H5 )4 (in1859byE. Frankland),PbEt4 (in1853byC.Löwig),Al2 Et3 I3 (in1859byW.Hallwachsand
A.Schafarik),andRMgX(in1900byV.Grignard)hadbeenprepared,andthe chemicalpropertyofthosecompoundsalsohadbeenstudied[64–68].
In1890,Ni(CO)4 wasfoundasthefirstmetalcarbonylcomplexbyL.Mondetal. inthestudyofthecorrosionofstainlesssteelvalvesbyCO[69].Nextyear,Fe(CO)5 wasalsofoundbythesamegroup[70].Itcouldbeconsideredasthebeginningof thestructuralstudyoforganometalliccomplexes.Twoyearslater,Wernerproposed structuraltheoryoforganometalliccomplexesinvolvingthetetrahedral,octahedral, squareplanar,etc.whichwonhimtheNobelprizeinchemistryin1913[71].In1919, Cr(C6 H6 )2 waspreparedbyHeinusingMgPhBrtoreactwithCrCl3 [72].However, thesandwich-likestructureofthiscomplexwasprovedbyFischer36yearslater. In1951,Fe(C5 H5 )2 hadbeensynthesizedbyKealyandPausonindividually[73]. Thesandwich-likestructureofthatcomplexwasconfirmedbyG.Wilkinsonthefollowingyear,whicharousedchemists’enthusiasmforthestudyoftransitionmetal organiccompounds.In1964,tungstencarbenecomplexwasreportedbyFischer, whoshared1973NobelprizeinchemistrywithG.Wilkinson[74].Bythe1950s, withtheappearanceofrepresentationalmethods,involvingX-raycrystallography, infraredspectrum,andnuclearmagneticresonancespectrum,meansofcharacterizingtransitionmetalcompoundswerebecomingmoreandmoremature.Therefore, organometallicchemistrybecameanindependentdiscipline.
Fromthemiddleofthetwentiethcentury,organometalliccompoundswere graduallyconsideredasacatalystinorganicreactions.In1953,ZieglerandNatta foundthatTiCl4 /AlEt3 couldpromoteatmosphericpolymerizationofolefins, whichhelpedthemshare1963Nobelprizeinchemistry[75,76].In1959,allylic palladiumwaspreparedbySmidtandHafner,whichwasthebeginningof π-allyl metalchemistry[77].Thesameyear,ShawandRuddickreportedanelementary reactionofoxidativeaddition[78].In1974,Wilkinsonreportedanotherelementary reactionof β-hydrideelimination[79].Thoseworksledtoaseriesoffollowing mechanisticstudiesfororganometallicreactions.In1972,HeckandNolley reportedapalladium-catalyzedcouplingreactionbetweenarylhalidesandolefins, whichwasnamedHeckreaction[80].Meanwhile,aseriesofpalladium-catalyzed cross-couplingreaction,includingKumadacouplingwithGrignardreagent[81], Suzukicouplingwitharylborane[82],Negishicouplingwithorganozinc[83],Stille couplingwitharyltin[84],andSonogashiracouplingwithalkynylcopper[85], werereported.Thosereactionsmadetransitionmetal-catalyzedcross-coupling reactionsoneofthemostimportantwaystoconstructnewC—Ccovalentbondsin syntheticchemistry.Therefore,R.F.Heck,E.Negishi,andA.Suzukiwonthe2010 Nobelprizeinchemistry.Alsoin1971,W.S.Knowlesappliedchiralbisphosphine ligandsasligandinrhodium-catalyzedhydrogenationreactions,whichhadopened upawholenewfieldofasymmetriccatalysiswithtransitionmetals[86].W.S. Knowlesshared2001NobelprizeinchemistrywithK.B.SharplesandR.Noyori, whopromotedtheresearchupsurgeofasymmetriccatalysis.Moreover,Chauvin, Grubbs,andSchrockwonthe2005Nobelprizeinrecognitionoftheiroutstanding contributionsintransitionmetal-mediatedmetathesisofolefins.
Basedontheadvancesofmethodologystudyandliganddesign,transition metalcatalysishasbecomeoneoftheimportantmeansforsyntheticchemiststo
1IntroductionofComputationalOrganometallicChemistry constructmorecomplexnewsubstancesinthiscentury.Thecurrentpursuitis toselectivelyconstructmultiplecovalentbondsinonereactionsynchronously bytransitionmetalcatalysis.Toachievethisgoal,transitionmetalcatalysthas beenemployedtoselectivelyactivatesomeinertcovalentbonds.Themostfamous example–transitionmetal-mediatedC—Hbondactivation–becamethefocusof chemists.Thisprocesscouldaffordacarbon–metalbonddirectly,whichcouldbe usedasapowerfulnucleophileinfurthertransformations.Inmodernorganometallicchemistry,multistepelementaryreactionsinserieshavebeenextensively studied,whichcouldaffordabatteryofnewcovalentbondsthroughonecatalytic cycle.Syntheticefficiencyinorganometallicchemistryhasbecomethefocusof attention.Hereon,transitionmetalcatalysiswithhigherturnovernumberswas pursuedtofurtherimprovetheeconomyandenvironmentalprotection.Current researchontransitionmetalcatalysisisalsodevotedtoimprovingtheaccuracyof synthesis,aimingatachievingspecificfunctionalgrouptransformationintheexact location.Toachievethesegoals,thedesignoftransitionmetalcatalysisbecomes morecomplex,andtherequirementsforsuitableligandsarehigher.Itisnecessary todesignthecorrespondingligandsmanuallyaccordingtoaspectsofstructure, electronicproperties,stericeffect,andcoordinationability.Theseauxiliarydesigns alsomakethecatalyticcyclewithtransitionmetallengthier;meanwhile,the possibilityofsidereactionsincreases.Therefore,mechanisticstudiesfortransition metalcatalysisbecamemoreandmoreimportant,whichwerehelpfulfordesign ofnewcatalysis,enhancedefficiency,increasedselectivity,improvedturnover number,andaccuratesynthesis.
1.2UsingComputationalTooltoStudy theOrganometallicChemistryMechanism
Transitionmetalcatalysisisoneofthemostpowerfultoolsfortheconstruction ofneworganicmaterials,whosedevelopmenttrendismoreefficientaswellas morecomplex.Therefore,studyingthemechanismoforganometalliccatalysishas becomeevenmoreessential,andhasprovedtobethebasisforthedesignofnew ligands,catalysts,andreactions.
1.2.1MechanismofTransitionMetalCatalysis
Generally,reactionmechanismcouldbeconsideredtobeallelementaryreactions usedtodescribeachemicalchangepassinginareaction.Itistodecomposeacomplexreactionintoseveralelementaryreactionsandthencombinethemaccording tocertainrules,soastoexpoundtheinternalrelationsofcomplexreactionsand theinternalrelationsbetweentotalreactionsandelementaryreactions.Therate ofchemicalreactioniscloselyrelatedtothespecificpathwaysthroughwhichthe reactiontakesplace.
Tostudythelawofchemicalreactionrateandfindouttheintrinsiccauses ofvariouschemicalreactionrates,syntheticchemistsmustexplorethereaction
1.2UsingComputationalTooltoStudytheOrganometallicChemistryMechanism 9 mechanismandfindoutthekeytodeterminethereactionrate,soastocontrol thechemicalreactionratemoreeffectively.AsshowninScheme1.4,traditional researchmethodsforreactionmechanisminclude:(i)determiningtheimportant intermediateordecisivestepofareactionbyisotopetracing,(ii)determiningthe effectofdifferentfactors(e.g.reactiontemperature,solvent,substituenteffect,etc.) onreactionrateandselectivitybycompetitivetest,(iii)studyingtherelationship betweenthereactionrateandtheconcentrationofreactantsandcatalystsobtaining bykineticexperiments,and(iv)characterizingandtrackingintermediatesby instrumentalanalysis.However,thesemethodsareoftenmacroscopicobservation oftheaveragestateofmanymolecules,whichcannotwatchaprocessofthe transformationforonemoleculefromamicro-perspective.Fortunately,theoretical calculationsbasedonfirstprincipleshavebecomeoneoftheimportantmeans tostudythereactionmechanismwiththedevelopmentofsoftwareandthe improvementofhardwarecomputingcapabilityinrecentseveraldecades.Through theoreticalcalculationandsimulation,thetransformationofonemoleculein reactionprocesscanbe“watched”moreclearlyfromthemicroscopicpointofview. Actually,theoreticalcalculationcanbeconsideredtobeaspecialkindofmicroscope,whichcanseethegeometricalstructure,electronicstructure,spectrum,and dynamicprocessatatomiclevel,andishelpfulforchemiststounderstandthereal reactionmechanism.
Control reactions
Isotope tracing
Tracking intermediates
Kinetic experiments
Reaction mechanism of organometallic catalysis
Theoretical calculations
Scheme1.4 Revealingthereactionmechanismoforganometalliccatalysis.
Thecombinationoftheoreticalandexperimentaltechniquescouldnotonly greatlyimprovetheefficiencyofreactionandyieldofproduct,butalsouncover thefactorsthatcontroltheselectivityofproductmoreclearly.Thepromotionof theoreticalstudytoexperimentalinvestigationcouldbesummarizedinto“3D,”i.e. description,design,anddirection.Basedonthedataobtainedfromexperimental technique,detaileddescriptionforthemechanismoforganometalliccatalysis couldbefulfilledusingtheoreticalcalculations.Basedontheresultsofcomputations,themechanismscouldbeverifiedbythedesignedexperiment.Toputin anutshell,theoreticalcalculationscouldplayacriticalroleinthedirectionof transition-metal-organicsynthesis.
1.2.2MechanisticStudyofTransitionMetalCatalysisbyTheoretical Methods
Quantumchemicalcomputationbasedonfirstprincipleprovidesapowerfultool forthemechanisticstudyoftransitionmetalcatalysis.Sincethewholecontentof thisbookistodiscussthetheoreticalcalculation-basedstudyforthemechanismof transitionmetalcatalysis,wewillgiveonlyafewexamplestoshowhowtostudythe reactionmechanismbytheoreticalcalculations.
Generally,mechanismresearchoftransitionmetalcatalysisinitiallyfacesaseries ofstudiesinvolvingthemolecularstructureandelectronicstates.Asanexample, (Xantphos)Pd(CH2 NBn2 )+ isanimportantprecursorforaminomethylationreactions,thegeometricstructureofwhichhasbeenconfirmedbyX-rayanalysis[87]. However,whythiscomplexcouldbeformedandtheelectronicpropertiesofthis complexstillremainedunclear.AsshowninScheme1.5,inresonancestructure 1-1,thePd—Cbondisanormalsinglebond,andPd—Nisacoordinationbond. Theformalpositivechargeislocalizedonpalladium,andtheformaloxidationstate ofpalladiumis +2.Alternatively,theiminiummoietyactsasamonodentateligand coordinatedwithPd(0)inresonancestructure 1-2,andtheformalpositivechargeis mainlylocalizedontheiminiummoiety.Therealstructureofthiscomplexwouldbe amixtureofresonancestructures 1-1 and 1-2.Ontheotherhand,thebondordersof Pd—C,Pd—N,andC—Naredeterminedtobe0.322,0.135,and0.965,respectively, whichindicatethatthePd—CandPd—Nbondsareveryweak.Moreimportantly, thesedatasupportthattheC—Nisadoublebondand 1-2 ismostlikelytobethe mainstructureofthiscomplex.Furtherfrontiermolecularorbitalstudiesalsosupportedthispoint.
Tosummarize,computationalorganometallicchemistryfocusesonsomeofthe stationarypointsonpotentialenergysurfaceforthecorrespondingreactions,which couldbeusedtocomparethepossibleelementaryreactions.Thelowestenergy reactionpathwaycouldbefoundbytheoreticalcalculations,whichishelpfulfor chemiststounderstandthereactionmechanismanddesignnewreactions.
Asanexample,computationalmethodwasusedtostudythemechanismof rhodium-catalyzedcouplingreactionofquinoline N -oxideandacetylenes.As showninScheme1.6,fourpossiblepathwaysweretakenintoaccount(i.e.paths A–D)[88].Allthesefourpathwaysbeginwiththecoordinationofquinoline N -oxide toRh(III)precursor 1-6,whichisfollowedbyan N -oxide-directedelectrophilic deprotonationbyacetateandcoordinationofacetylenesubstratetogiverhodacycle 1-7.ThesubsequentinsertionoftheacetylenesubstrateintotheRh—Cbond ofintermediate 1-7 givesintermediate 1-8,whichisacommonintermediatefor
Scheme1.5 Theresonancestructuresof(Xantphos)Pd(CH2 NBn2 )+ .
