The reaction of biphenyl radical anion and dianion with alkyl fluorides. From ET to SN2 reaction pathways and synthetic applications

The reaction of dilithium biphenyl (Li2C12H10) with alkyl fluorides has been studied from the point of view of the distribution of products. Two main reaction pathways, the nucleophilic substitution (SN2) and the electron transfer (ET), can compete to yield the same alkylation products in what is known as the SN2-ET dichotomy. SN2 seems to be the main mechanism operating with primary alkyl fluorides (n-RF). Alkylation proceeds in good yields, and the resulting alkylated dihydrobiphenyl anion (n-RC12H10Li) can be trapped with a second conventional electrophile (E+) affording synthetically interesting dearomatized biphenyl derivatives (n-RC12H10E). The reaction gives a higher amount of ET products as we move to secondary (s-RF) and to tertiary alkyl fluorides (t-RF), in which case the mechanism seems to be dominated by ET. In this case, alkylation by radical coupling is still feasible, giving access to the synthesis of t-RC12H10E, although in lower yields. A rational interpretation of this SN2-ET dichotomy is given on the basis of the full distribution of products observed when 5-hexenyl fluoride and 1,1-dimethyl-5-hexenyl fluoride were are used as radical probes in their reaction with Li2C12H10 and LiC12H10.