Pergamon Tetrahedron 55 (1999)7923-7934 TETRAHEDRON Friedel-Crafts Acylation of 2-Trimethylsilylnorbornene. Effect of Acyl Group on the Position of Attack GopalpurNagendrappa* Engier-Bunte Institut, BereichPetrochemie,UniversitatKarisruhe. 76128Karisruhe, Germany. Noor Shahina Begum Department of Chemistry, Bangalore University, Bangalore 560001, India. Received 5 March 1999; revised 13 April 1999; accepted 29 April 1999 Abstract: The acylation of 2-4rimethylsilylnorbornene 1 in the pcesence of aluminium chloride gives minor quantities of the expected 2-norbornenylketones 4. The formation of 3 and 5 as major products indicatesthat either or- or IS-attacktakes place wedominantly depending on the nature of the acyl group, and the l~-silicon effect is not a decisive factor. The I~--silylcation intermediate 2 mainlyleads to nortricyclyl ketones3 through 1.3-depmtonation.and the ct-silyl cation 8 undergoes rearrangement to give novelI~rid~-headsilylatedproductsS. © 1999Elsevier Science Ltd. All rights reserved. Introduction Vinylsilanes generally undergo facile regio- and stereo-specific displacement of silyl moiety by a variety of electrophiles due to the directive influence of silicon through stabilization of the intermediate 13 cation, called the [I-silicon effect.~ ~-Silylated substrates experience maximum rate enhancement and furnish products of high stereo~lectivity in their nucleophilic substitution 2 and ~-elimination ~ reactions if silicon and the leaving group can achieve a perfect antiperiplanar geometry in the transition state. The stereoelectronic effect of 13-silicon in the antiperiplanar position has been shown to be operative even in the ground state of l~-silyl esters. 4 The stereochemical outcome of electrophilic substitution reactions of the vinylsilanes is likewise governed by the B-silicon effect.~b The hyperconjugative overlap of C-Si o bond with the empty p orbital on the 15-carbenium ion formed by the addition of an electrophile can be achieved by a simple C~-Cp bond rotation of 60° when severe restrictions are not encountered) ~'5 In the case of the rigid 2-trimethylsilylnorbornene (1), the addition of an electrophile, e.g., R-C+=O, generates the carbenium ion 2 (Scheme 1), that would enjoy only a partial stabilization by silicon as the C2-Si o bond and C3-p empty orbital will be out of plane by a dihedral angle of about 300. 6 However, if maximum overlap is achieved through the needed bond twist in 2 the Friedel-Crafls reaction of 1 would become an easy access to 2-norbomenyl kctones, e.g., 4a-d, for which no simple preparative procedure exists, 7 though some of them find application in the synthesis ofbicyclie alkaloids and terpenes. 8 The acylation of norbornene itself is seldom attempted, as the results do not seem to be useful. 9 The Friedel-Crafls acylation of even simple oletins is not an efficacious synthetic method because of the complexity * ~ for correstmndence: ~ t of Chemistxy,BangaloreUniversity(Central CollegeCampus), Bangalore560001, India. 0040-4020/99/5 - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0040-4020(99)00402-0