Hydrogen carbene transfer reactions

Triplet carbenes have a singly occupied p orbital, as is the case for radicals, and hence react like those radicals. Hydrogen atom transfer reactions are fundamental reaction pathways of triplet carbenes. The reaction of a triplet carbene with a hydrocarbon is quite analogous to the free radical hydrogen atom transfer process (Scheme 9.6). 

Analogous to Robinson s NHG-supported Ga" digallane complex 106, the NHG-supported E" complexes 109 (E = Al, Ga, In) were also reported by Jones and Gole (Scheme 5.20). The Ga and In complexes are prepared by disproportionation of Ga or In halides in the presence of the NHG ligand or by direct coordination of the carbene to Ga" or In". In contrast, dialane complex 110 was prepared via a hydrogen atom transfer reaction using a procedure developed by Jones and Stasch et al. ... 

Finally, some diradicals can be made in situ by an internal hydrogen-transfer reaction from a suitable hydrogen donor to a carbene or nitrene. In benzene derivatives, this is a well-tested route to o-quinoid compounds, which are not biradicals, although a biradical valence structure probably makes a significant contribution to their electronic structure. However, if the donor and the carbene or nitrene are... 

Some phosphonium ylids may be prepared by simply heating a phosphonium salt (6.391), or by heating a vinyl phosphonium salt with a metal alkyl (6.411). Dihalophosphoranes react with active methylene compounds under conditions in which hydrogen halide may be removed (6.412). Another reaction is that of carbene transfer (6.413) or the action of sodamide on a phosphonium salt at 0°C. 

Partially because of last minute cancellations, several important topics have not been included in the present work. Papers on fluorine atom chemical lasers, high-energy fluorine atom reactions, and classical trajectory simulations of hydrogen transfer reactions by atomic fluorine are conspicuously absent. Other more speciahzed topics not included were olefin addition reactions by fluorine-rich carbenes, H2/F2 explosions, unimolecular reaction d3mamics of fluorine-containing aliphatic radicals, and high-power fluorine atom lasers in energy research. 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

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