Functionalization of Carborane via Carboryne Intermediates



Author:
Da Zhao
Published in: Springer Science+Business
Release Year: 2012
ISBN: 978-981-10-1569-4
Pages: 1257
Edition: First Edition
File Size: 8 MB
File Type: pdf
Language: English



Description of Functionalization of Carborane via Carboryne Intermediates


The functionalization of Carborane via Carboryne Intermediates Boron compounds, such as borax, has been known and used by ancient cultures for thousands of years. However, element boron itself was not identified until 1808. In contrast to hydrocarbons, boranes avoid the formation of chain structures and clearly prefer the formation of polyhedral clusters. Their structures cannot be described in terms of common organic bond diagrams, in which a connection line between two atoms explicitly represents an electron pair. Instead, connecting lines in their structures only indicate the polyhedral geometry. typical molecular frameworks of boranes consist of close, Nido, arachnoid, and hypho structures, and the latter three are formally derived from the close frameworks by removal of one, two, or three missing vertices, respectively. 
As intuitively expressed in its formal nomenclature, carboranes are generally accepted as polyhedral boranes with one or more BH vertices being replaced by CH. The range of composition in carborane cages extends from boron-rich clusters such as CB11H12− and C2B10H12 to species having as many as six skeletal carbon atoms, but those with high boron content are still dominant. Different from classical organoboranes, the skeletal carbon atoms in carboranes typically have at least three neighbors including hydrogen or other attached substituents in the cluster.

Content of Functionalization of Carborane via Carboryne Intermediates


1 Introduction ......................................... 1
1.1 Skeletal Transformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.1 Thermal Cage Rearrangement . . . . . . . . . . . . . . . . . . . . . 2
1.1.2 Deboronation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1.3 Reductive Cage Opening . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Boron Substitution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 Electrophilic Substitution . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Nucleophilic Substitution . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.3 Direct B–H Bond Functionalization . . . . . . . . . . . . . . . . . 5
1.2.4 Carbene/Carbenoid Insertion of B–H Bond. . . . . . . . . . . . 5
1.3 Carbon Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1 Nucleophilic Substitution . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.2 Transmetalation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.3 Lithium–Halogen Exchange . . . . . . . . . . . . . . . . . . . . . . 7
1.4 Dehydrogenation to o-Carboryne . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4.1 Generation and Trapping of o-Carborynes . . . . . . . . . . . . 8
1.4.2 Reactivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.4.3 Transition Metal–Carboryne Complexes . . . . . . . . . . . . . . 19
1.4.4 Bonding and Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 20
1.5 Research Objectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 Regioselective Insertion of o-Carborynes into α-C−H Bond
of Tertiary Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
2.2 Reaction of o-Carborynes with Tertiary Amines . . . . . . . . . . . . . 30
2.3 Mechanistic Studies on α-Carboranylation of Amines. . . . . . . . . . 34
2.4 DFT Calculations on α-Carboranylation of Amines . . . . . . . . . . . 37
2.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3 Synthesis of Carborane-Functionalized Heterocycles:
Dearomative [2 + 2] Cycloaddition and sp2 C–H Insertion
Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.2 Reaction of o-Carboryne with N-TMS Indoles. . . . . . . . . . . . . . . 49
3.3 Effects of N-Substituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
3.4 Reaction of o-Carborynes with N-Methyl Indoles . . . . . . . . . . . . 53
3.5 Reaction of o-Carboryne with Other N-Heterocycles . . . . . . . . . . 53
3.6 Transformation of [2 + 2] Cycloadducts . . . . . . . . . . . . . . . . . . . 54
3.7 Mechanistic Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
3.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4 Reaction of o-Carboryne with Nitrones: A Formal [5 + 2]
Cycloaddition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4.2 Reaction of o-Carboryne with Nitrones . . . . . . . . . . . . . . . . . . . 74
4.3 Mechanistic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5 1,3-Dehydro-o-Carborane: Generation and Reaction
with Arenes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
5.2 Generation of 1,3-Dehydro-o-Carborane: Precursor Design . . . . . . 89
5.3 3-(N2+BF4)-o-C2B10H11: A More Efficient Precursor . . . . . . . . . 90
5.4 Reactivity of 1,3-Dehydro-o-Carborane Toward Arenes . . . . . . . . 91
5.5 Thermal Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
5.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
6 Ene Reaction of 1,3-Dehydro-o-Carborane . . . . . . . . . . . . . . . . . . . 103
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
6.2 Ene Reaction of 1,3-Dehydro-o-Carborane: Reactivity Toward
Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6.3 Ene Reaction of 1,3-Dehydro-o-Carborane: Reactivity Toward
Alkynes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6.4 Mechanistic Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
7 Cage Boron Arylation of o-Carborane via Metal-Free,
Visible-Light-Mediated Radical Coupling . . . . . . . . . . . . . . . . . . . . 117
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.2 B(3)-Arylation of o-Carborane: A Radical Approach . . . . . . . . . . 118
7.3 Mechanistic Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
Experimental Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
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