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Nucleophilic addition to C,Cdouble bonds. Part IX . Kinetics and mechanism of the base‐catalyzed intramolecular cyclization of olefinic alcohols
Author(s) -
Tombo Gerardo M. Ramos,
Ganter Camille
Publication year - 1985
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19850680817
Subject(s) - chemistry , protonation , carbanion , intramolecular force , nucleophile , steric effects , double bond , medicinal chemistry , alkoxide , stereochemistry , ring (chemistry) , base (topology) , catalysis , polymer chemistry , organic chemistry , ion , mathematical analysis , mathematics
The base‐catalyzed intramolecular cyclization of polycyclic olefinic alcohols of type a (10‐ endo ‐hydroxy‐ anti 9,10 ‐tricyclo [4.2.1.1 2,5 ]dec‐7‐en‐9‐ones (type h ), anti 9,10 ‐tricyclo[4.2.1.1 2,5 ] dec‐3‐en‐9‐ endo ‐ols (type j ), and anti 10,11 ‐tricyclo[4.3.1.1 2,5 ]undec‐3‐en‐10‐ endo ‐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,Cdouble 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 c , which is protonated stereoselectively on the exo ‐side by the hydroxylic solvent. (b) Compounds without a carbonyl group ( j and l ) cyclize 10 2 ‐10 4 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,Obond formation. Steric compression in the olefinic alcohols a influences the cyclization rate: (a) Alcohols with a smaller ring ( h , X = CH 2 CH 2 ) cyclize 70–200 times faster than the ones with a larger ring ( 1 , X = CH 2 CH 2 CH 2 ). (b) Replacement of the H‐atom at the carbinol C‐atom by a CH 3 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 = CHCH) cyclize 20–300 times faster than the corresponding monoolefinic ones ( h and j , X = CH 2 CH 2 ).