Dehydrogenation of Secondary Amines to Imines Catalyzed by an Iridium PCP Pincer Complex

62 Pages·2006·0.9 MB·English

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Dehydrogenation ofSecondary Amines to Imines
Catalyzed by an Iridium PCP Pincer Complex
A TIffiSIS SUBMITTED TO TIffi GRADUATE DIVISION OF
THE UNNERSITYOF HAWAI'I IN PARTIAL FULFILLMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTEROF SCIENCE
IN
CHEMISTRY
DECEMBER2002
By
Wei Cheng
Thesis Committee:
Craig M. Jensen, Chairperson
JohnD. Head
Roger E. Cramer
TABLE OF CONTENTS
Acknowledgements 11
Abstract
111
List ofSchemes tv
Chapter 1: Introduction 1
1.1 Condensation ofAldehydes and Ketones with Amines 2
1.2 Catalytic Imination ofKetones 3
1.3 Oxidative Dehydrogenation ofAmines 4
Chapter 2: Catalytic Reactions 10
2.1 Introduction 10
2.2 Experimental 11
2.2.1 CatalyticReactions GeneralProcedure 12
2.2.2 Synthesis ofAuthentic Samples ofImines 12
2.2.3 Preparative scale synthesis ofN-butylidenebenzy1amine 15
2.3 Results and Discussion 16
2.3.1 CatalyticReactions 16
2.3.2 Preparative scale catalytic reaction 20
Chapter 3: Mechanistic Studies 22
3.1 Introduction 22
3.2 Experimental 25
3.2.1 The preparationof2,2,2',2'-tetramethy1dibuty1aimine 25
3.3 Results and Discussion 28
3.3.1 SpectraStudy 28
3.3.2 Results 29
Chapter4: Conclusions 32
Reference 34
Appendix 36
ACKNOWLEDGEMENTS
I would like to give my sincere thanks to my advisor, Professor Craig M. Jensen,
forhis guidance andfor allowing me to contribute to this project.
I would also like to thank the members of the Jensen research group, past and
present, for theirhelp and companionship.
Many thanks go to Wesley Yoshiba for his help in obtaining NMR and mass
spectra.
ii
ABSTRACT
The PCPpincercomplex, IrH2{C6H3-2,6-(pBu/2)2}, catalyzes the transfer
dehydrogenation ofsecondary amines. Dehydrogenation occurs across C-N bonds rather
thanC-C bonds to give imines that are obtainedin good to excellent yieldswhen the
reactions are carried out in toluene solution. Theregioselectivityofthe dehydrogenation
ofaliphatic amines is stringentlycontrolledbystene factors while dehydrogenation of
aromatic amines leads to imineproducts favored thermodynamicallybyconjugated1t
bonds inthe aromatic system. The dehydrogenationreaction has beensuccessfully
carriedoutin large scale (separable) withN-butylbenzylamine with acceptable separation
yield. The dehydrogenation of2,2,2',2'-tetramethyldibutylamine leads exclusively to
productionofthe correspondingimine indicating thatthe catalytic reactionpathway
involves the initialintermolecularoxidativeaddition ofaN-Hbondratherthan a C-H
bond.
iii
LIST OF SCHEMES
Scheme Page
I. Amine Dehydrogenation with Ru-catalyst and PhIO 5
II. Catalytic dehydrogenation ofbenzyl amine 8
III. Possible Mechanisms
for the Dehydrogenation ofAmines 23
IV. Synthesis of2,2,2',2'-tetramethyldibutylaimine 26
V. Fragmentation ofImines 29
VI. Results ofdehydrogenation
of2,2,2',2'-tetramethyldibutylaimine 31
iv
LIST OF TABLES
Table Page
I. Some typical bond energies 11
II. Dehydrogenation ofamines using PCP pincer catalyst 19
v
TABLE OF CONTENTS
Acknowledgements 11
Abstract III
List ofSchemes IV
Chapter 1: Introduction 1
1.1 Condensation ofAldehydes and Ketones with Amines 2
1.2 Catalytic Imination ofKetones 3
1.3 Oxidative Dehydrogenation ofAmines 4
Chapter 2: Catalytic Reactions 10
2.1 Introduction 10
2.2 Experimental 11
2.2.1 Catalytic Reactions GeneralProcedure 12
2.2.2 Synthesis ofAuthentic Samples ofImines 12
2.2.3 Preparative scale synthesis ofN-butylidenebenzylamine 15
2.3 Results and Discussion 16
2.3.1 CatalyticReactions 16
2.3.2 Preparative scale catalytic reaction 20
Chapter 3: Mechanistic Studies 22
3.1 Introduction 22
3.2 Experimental 25
3.2.1 Thepreparationof2,2,2',2'-tetramethyldibutylaimine 25
3.3 Results and Discussion 28
3.3.1 SpectraStudy 28
3.3.2Results 29
Chapter4: Conclusions 32
Reference 34
Acknowledgements
11
Abstract
The PCPpincercomplex, IrHz{C H -2,6-(PBu'z)z}, catalyzes the transfer
6 3
dehydrogenation ofsecondary amines. Dehydrogenationoccurs across C-Nbonds rather
than C-C bonds to give imines that are obtainedin good to excellent yields whenthe
reactions are carriedout intoluene solution. The regioseleetivity ofthe dehydrogenation
ofaliphatic amines is stringentlycontrolledbysteric factors while dehydrogenation of
aromatic amines leads to imineproducts favored thermodynamicallyby conjugated1t
bonds inthe aromatic system. The dehydrogenation reactionhasbeen successfully
carriedout in large scale (separable) withN-butylbenzylaminewith acceptable separation
yield. The dehydrogenation of2,2,2',2'-tetramethyldibutylamine leads exclusively to
productionofthe corresponding imine indicating thatthe catalytic reactionpathway
involves the initial intermolecularoxidativeaddition ofaN-H bond ratherthan a C-H
bond.
iii
LIST OF SCHEMES
Scheme Page
I. Amine Dehydrogenation with Ru-catalyst and PhIO 5
II. Catalytic dehydrogenation ofbenzyl amine 8
III. Possible Mechanisms
for the Dehydrogenation ofAmines 23
IV. Synthesis of2,2,2',2'-tetramethyldibutylaimine 26
V. Fragmentation oflmines 29
VI. Results ofdehydrogenation
of2,2,2',2'-tetramethyldibutylaimine 31
IV