Stepwise Base-Promoted Elimination of Hydrochloric Acid via Hydrogen-Bonded Carbanions Studied by Reaction Branching and Extreme Deuterium Isotope Effects. Parallel Carbocationic and Carbanionic Reactions

Article abstract of DOI:10.1021/ja00311a045

Reaction of 1-(2-chloro-2-propyl)indene (h-1-Cl) with pyridine (P) in methanol at 30 deg C results in base-promoted 1,2 elimination competing with base-catalyzed 1,3-proton transfer to give 1-isopropylideneindene (h-3) and 3-(2-chloro-2-propyl)indene (h-2-Cl), repectively.The latter product solvolyzes rapidly, yielding the two isomeric methyl ethers 3-(2-methoxy-2-propyl)indene (h-2-OMe) and 1-isopropylidene-2-methoxyindan (h-4) and the elimination product 3-(2-propenyl)indene (h-6).Compound 1 also solvolyzes, slowly, yielding mainly 1-(2-methoxy-2-propyl)indene (h-1-OMe) and a small amount of 1-(2-propenyl)indene (h-5).Reaction of (1,3-(2)H2-1-(2-chloro-2-propyl)indene (d-1-Cl) with P yields the same spectrum of products as the reaction of h-1-Cl.However, some of the products are formed in strikingly different amounts.These results provide important mechanistic information.The reactions have been studied quantitatively by using an HPLC procedure.The base-catalyzed 1,3-proton transfer, i.e., the formation of 2-Cl, shows an unusually large kinetic deuterium isotope effect, 14.6+/-1.0, while the corresponding kinetic deuterium isotope effect on the reaction of 1-Cl with P is only 5.6+/-0.3.The extreme deuterium isotope effect is proposed to originate from reaction branching, i.e., that the base-promoted elimination and the base-catalyzed rearrangement of 1-Cl make use of at least one common hydrogen-bonded carbanion intermediate.Thus, it is included that the potent leaving group Cl^- may also eliminate by an ElcB mechanism.