to produce unisolable allylchromium species which add efficiently to aldehydes or ketones with high degree of stereo- and chemoselectivity. Particularly, high threo selectivity is observed in the reaction of aldehydes and 1-bromo-2-butene and is ascribed to a chair-like six-membered transition state. Simple reduction of allylic and benzylic halides produces biallyls and bibenzyls, while gem-dibromocyclopropanes
Alkynyl halides are reduced by chromium(II) chloride in DMF to give unisolable alkynylchromium compounds which add selectively to an aldehyde moiety without affecting the coexisting ketone group of the substrate.
inclination of other 17-electron metal carbonyl complexes to undergo associative ligand displacement reactions, it seems ironic that the incorporation of a pentadienyl ligand, which would be expected to promote associative attack via $ 73 coordination changes, should lead instead to a series of compounds for which much lower rates of substitution are observed and for which the substitutions also take place