Mechanistic aspects of the reactions of bis(tricarbonylchromium) coordinated biphenyl dianions with electrophiles and their subsequent oxidation

Article abstract of DOI:10.1021/om00037a051

Reaction of the (η⁶:η⁶-biphenyl)bis(tricarbonylchromium) dianion (2) with a wide variety of primary electrophiles results exclusively in an ipso addition. NMR data point out the formation of the endo isomer upon protonation. While a definitive answer is not possible for primary alkyl electrophiles, indirect evidence suggests either a carbanionic type S_N2 or a metal-assisted endo addition to the ipso position. Reaction with cyclopropylcarbinyl bromide as well as 6-bromo-1-hexene suggests an inner-sphere polar S_N2 mechanism for most of the primary electrophiles. However, easily reduced halides such as allylic and benzylic or those halides which have steric hindrance toward S_N2 processes may well proceed via an electron-transfer process or a combination of S_N2 and electron-transfer processes. 1,2-Aryl migration has been found to be the predominant pathway in the oxidative cleavage of the alkylated (η⁶:η ⁶-4,4′-dimethylbiphenyl)bis(tri-carbonylchromium) dianion (6), thereby indicating a cationic intermediate in the mechanism of this step. Alkylation followed by iodine oxidation of 4,4′-disubstituted biphenyls provides a short convenient approach to 2-alkyl-5,4′-disubstituted biphenyls.