Dehydrogenation of Amine-Boranes Using p-Block Compounds

Abstract

Amine-boranes have gained a lot of attention due to their potential as hydrogen storage materials and their capacity to act as precursors for transfer hydrogenation. Therefore, a lot of effort has gone into the development of suitable transition- and main-group metal catalysts for the dehydrogenation of amine-boranes. During the past decade, new systems started to emerge solely based on p-block elements that promote the dehydrogenation of amine-boranes through hydrogen-transfer reactions, polymerization initiation, and main-group catalysis. In this review, we highlight the development of these p-block based systems for stoichiometric and catalytic amine-borane dehydrogenation and discuss the underlying mechanisms.

Keywords: amine-boranes; ammonia-borane; dehydrogenation; hydrogen transfer; main-group catalysis.

Figure 1

B−N‐containing products.

Scheme 1

Reaction of iPr₂N=BH₂ with NH₃⋅BH₃.

Scheme 2

Equilibrium formation during the reaction of iPr₂N=BH₂ with Me₂NH⋅BH₃.

Scheme 3

Dihydrogen abstraction from Me₂NH⋅BH₃ with 2 (Fxyl = 3,5‐(F₃C)₂C₆H₃).

Scheme 4

H₂ abstraction by a phosphinoborane.

Scheme 5

Computed mechanism for dihydrogen transfer from AB to iminoboranes.

Scheme 6

Dihydrogen transfer from AB to imines.

Scheme 7

Hydroboration of ketones and aldehydes.

Scheme 8

Proposed mechanism by Zhou and Fan for alkyl borate formation.

Scheme 9

Chen's proposed mechanism for hydrogen transfer of AB to aldehydes.

Scheme 10

Reduction of C=C double bonds through hydrogen transfer of ammonia–borane.

Scheme 11

Transition‐metal‐free conversion of CO₂ to methanol.

Scheme 12

Formation and reactivity of aminodiborane 17.

Scheme 13

Proposed mechanism for the formation of aminodiborane from NH₃⋅BH₃ and THF⋅BH₃.

Scheme 14

Reactivity of a gallium(III)‐based complex with NH₃⋅BH₃.

Scheme 15

The reaction of an N‐heterocyclic carbene and germylene with AB.

Scheme 16

Dehydrogenation of RNH₂⋅BH₃ by an NHC.

Scheme 17

2:1 reaction of an NHC with MeNH₂⋅BH₃.

Scheme 18

Dehydrogenation of amine–boranes utilizing PtBu₃/B(C₆F₅)₃.

Scheme 19

Dehydrogenation of amine–boranes mediated by Me₃SiOTf/TMP.

Scheme 20

Initiation step of AB dehydrogenation by Brønsted and Lewis acids.

Scheme 21

Calculated mechanism for ammonia–borane dehydrogenation using triflic acid in diglyme.

Scheme 22

Anionic polymerization of AB dehydrogenation initiated by a Brønsted base.

Scheme 23

Formation and isolation of intermediates in the base‐promoted polymerization of AB.

Scheme 24

Imine reduction catalyzed by 43 with ammonia–borane as hydrogen source.

Scheme 25

Mechanism for transfer hydrogenation by 43 (shown schematically here).

Scheme 26

β‐hydride transfer to form 49.

Scheme 27

Mechanism for iPr₂NH⋅BH₃ dehydrogenation by 50.

Scheme 28

Catalytic dehydrogenation of NH₃⋅BH₃ by 52.

Scheme 29

Mechanism of AB dehydrogenation by 53.

Scheme 30

Proposed catalytic cycle for azobenzene hydrogenation.

Scheme 31

Calculated catalytic cycle for azobenzene transfer hydrogenation.

Scheme 32

Catalytic transfer hydrogenation by a pincer‐type phosphorus compound.

Scheme 33

Proposed catalytic cycle for azobenzene hydrogenation catalyzed by 62.

Scheme 34

Amine–borane dehydrogenation by a P/E (E=Al, Ga) FLP.

Scheme 35

FLP 69 as catalyst for imine hydrogenation.

Scheme 36

Phosphinoborane‐catalyzed transfer hydrogenation of imines.

Scheme 37

Stoichiometric reactions of 74 with amine–boranes.

Scheme 38

Stoichiometric and catalytic reactions of 74 with methylamine–boranes.

Scheme 39

Catalytic dehydrogenation of cyclic amine–boranes and diamine–boranes with 80.

Scheme 40

Stepwise and catalytic generation of phosphonium–borate 81.

Scheme 41

FLP‐catalyzed asymmetric transfer hydrogenation of imines.

Scheme 42

FLP‐catalyzed reduction reactions of 2,3‐disubstituted quinoxalines.

Scheme 43

FLP‐catalyzed transfer hydrogenation of pyridines.

Scheme 44

Reactivity of N‐heterocyclic iminoboranes towards different amine–boranes.

Scheme 45

Calculated mechanism for ammonia–borane dehydrogenation.

Scheme 46

Energy barriers for 103 for ammonia–borane dehydrogenation.

Scheme 47

Oligomerization of aminoborane monomers assisted by 103.