Carbanions. 21. Reactions of 2- and 3-p-Biphenylylalkyl Chlorides with Alkali Metals. Preparation of Labile Spiro Anions

Article abstract of DOI:10.1021/jo00136a025

The present work was undertaken to see if (3-p-biphenylylpropyl)- and (2-p-biphenylethyl)cesium cyclize like (4-p-biphenylbutyl)cesium to stable spiro anions.Reaction of 1-p-biphenylyl-3-chloropropane (5) with Cs-K-Na alloy in THF at -75 deg C gave, under the optimum conditions found, 36percent of 7-phenylspiro<3.5>nona-6,5-dien-5-yl anion (8) besides 3-p-biphenylylpropyl (7), 1-p-biphenylylpropyl (9), and p-biphenylylmethyl (10) anions, according to the products of carbonation.The spiro anion 8 (Cs⁺ as the counterion) has a half-life of about 13 min at -75 deg C.Incontrast, 5 reacts with lithium to give predominantly (3-p-biphenylylpropyl)lithium.Reaction of 1-p-biphenylyl-2-chloroethane with Cs-K-Na alloy gave no appreciable spiro anion under conditions which were successful with 5.In the reaction of 1-p-biphenylyl-2-chloro-2-methylpropane (28) with Cs-K-Na alloy the α-gem-dimethyl group accelerates <1,2> migration of the p-biphenylyl group to give products similar to those from 2-p-biphenylyl-1-chloro-2-methylpropane (24); however, the expected intermediate spiro anion 26 was undetectable by the carbonation technique.With both α- and β-gem-dimethyl groups, 2-p-biphenylyl-3-chloro-2,3-dimethylbutane (41) reacts with Cs-K-Na alloy to give 1,1,2,2-tetramethyl-6-phenylspiro<2.5>octa-5,7-dien-4-yl anion (43) and 2-p-biphenylyl-1,1,2-trimethylpropyl anion (42) in about a 2:1 ratio according to the results of carbonation.The open anion 42 and the spiro anion 43 appear to be in mobile equilibrium (Cs⁺ as the counterion) with half-lives of about 22 min in THF at -75 deg C.With lithium as counterion, only the open product (2-p-biphenylyl-1,1,2-trimethylpropyl)lithium (50) was detectable by carbonation.