FLP chemistry update

21 March 2010 - Part V

In our continuing coverage of frustrated Lewis pairs or FLP's (see part I , part II , part III and part IV) a brief update. Miller et al. have successfully dehydrocoupled amine-boranes using a phosphine - borane system (DOI). Formation of a polyaminoborane proceeds through the R2N=BH2 monomer. Although technically it is a three-component reaction the phosphine has to be added first.

Schultz et al. did the reverse , taking a TMS protected amine R2NTMS which in the presence of B(C6F5)3 reverts to the amine R2NH and the silyl hydride TMS-H (DOI) More FLP mediated cleavage has been found by Dureen et al. of the Stephan lab with the disulfide PhSSPh (DOI) in reaction with tBu2P(C6F4)B(C6F5)2

In earlier work in 2009 Dureen and Stephan activated terminal alkynes with B(C6F5)3 and phosphines DOI ) and recently Geier et al. tried out phosphorus dimers , cyclic tetramers and pentamers to form novel zwitterions ( DOI) . Jiang et al. also cleaved the C-H bond of a terminal alkene (DOI) but this time with amines instead of phosphines. In the reaction of acetylene, B(C6F5)3 and TMP, the first intermediate is attacked by a second borane unit and one of the C6F5 units shifts a bond (something similar occurs in FLP carbon dioxide reduction).

We are already used to FLP mediated hydrogenations: Schwendemann et al. hydrogenated complex organometallic dienamines (DOI). A new frustrated pair was presented by Stepowska et al. who split hydrogen with a Ru(II) - carbanionic zwitterion DOI)

On the computational / mechanistic front Grimme et al. (DOI) responded to a reaction mechanism proposed by Imre Pápai in 2008 that has a linear P--H--H--B unit in the transition state. Not true says Grimme, the bottleneck in the reaction is the entrance of the dihydrogen molecule (not at all dissociated in the TS) into the FLP interior and once the H2 molecule is within the FLP, the dissociation is practically barrierless.