Nucleophilic addition to C,Cdouble bonds. Part IX. Kinetics and mechanism of the base‐catalyzed intramolecular cyclization of olefinic alcohols

Abstract

The base-catalyzed intramolecular cyclization of polycyclic olefinic alcohols of type a ( 10-endo-hydroxy-anti9~'0-tricyclo[4.2.1 .12*']dec-7-en-9-ones (type h), an~i9~'0-tricyclo[4.2.1 .12~']dec-3-en-Y-endo-ols (type j), and ~nti'~~"-tricyclo[4.3.1. I2~']undec-3-en-lO-~n~fo-ols (type 1)) to the ethers d and f, resp., has been studied. A mechanism for the nucleophilic addition of the corresponding alkoxide anion b to the isolated C,C-double bond is discussed. It is proposed that b is formed (fast acid/base equilibrium) in the first step. For the subsequent reaction sequence, there are two well distinguishable pathways: a) Compounds with an additional carbonyl group (h) cyclize via a homoenolate-like intermediate e, which is protonated stereoselectively on the em-side by the hydroxylic solvent. b) Compounds without a carbonyl group (i and 1) cyclize 102-104 times slower, and the reaction proceeds via a carbanion-like transition state e. The proton transfer from the hydroxylic solvent is clearly coupled with the C,O-bond formation. Steric compression in the olefinic alcohols a influences the cyclization rate: a) Alcohols with a smaller ring (h, X = CH'CH,) cyclize 70-200 times faster than the ones with a larger ring (1, X = CH,CH2CH2). b) Replacement of the H-atom at the carbinol C-atom by a CH, group enhances the rate of ether formation by a factor of 50-100. Due to through-bond interactions between the C,C-double bonds, olefinic alcohols with an additional endocyclic C,C-double bond (h and j, X = CH=CH) cyclize 20-300 times faster than the corresponding monoolefinic ones (h and j, X = CH,CH,).In previous communications'), we have shown that in polycyclic olefinic alcohols of the general type a, the structurally close proximity of the reacting centers allows basecatalyzed intramolecular nucleophilic addition of an alkoxide anion to a C,C-double bond bearing no electron-attracting substituents (+d and f, resp.). Continuing our studies, we present a mechanistic model (Sect. I ) for this uncommon reaction, based on kinetic data (Sect.2).