Isomerization, Rearrangement, and Redistribution of Alkylmetal Compounds

Isomerization, Rearrangement, and Redistribution of Alkylmetal Compounds Many organometallic compounds undergo various transformations that may be accounted for by the involvement of higher coordinate carbon atoms. Most information is available for boron and aluminum derivatives. 

In a similar fashion, the redistribution of alkyl groups between organo-alanes and organoboranes proceeds via an alkyl-bridged boron-aluminum species  

Hydroboration, the addition of borane (or diborane) to n-donor substrates such as alkenes, acetylenes, carbonyl compounds, and so on, is a most useful synthetic reaction and was developed by the extensive work of Brown et al. 2 52 In the absence of more nucleophiUc n- and n-donor sites, borane will also attack a bonds in alkanes, silanes, and aromatic compounds to yield addition, cleavage, and rearrangement products. The interaction of borane with hydrocarbons involves 3c-2e bonded pentacoordinate carbons. 

Alkylboranes rearrange intramolecularly (on heating) to thermodynamically more stable alkylboranes, typically with the boron atom attached to the StericaUy least hindered position on the alkyl chain (the hydroboration rear-rangement). 53 529.533 wjnjams proposed that this equilibrium occurs by a stepwise intramolecular attack of the electron-deficient center on the neighboring C-H o bonds. The transition state of the intramolecular migration incorporates a pentacoordinate carbon atom [Eq. (6.142)]  

An alternative mechanism for the hydroboration rearrangement has been advanced by Brown et al. -529,534,535 considered the rearrangement to be 

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