Electrophilic Addition to 1-Cyclopropylallenes A Highly Efficient and Stereoselective Method for

Electrophilic Addition to 1-Cyclopropylallenes: A Highly Efficient and Stereoselective Method for the Preparation of 6-Substituted-1,3-hexadienes

LETTER1331

Electrophilic Addition to 1-Cyclopropylallenes: A Highly Efficient and Stereoselective Method for the Preparation of 6-Substituted-1,3-hexadienes

Lei Yu,a Bo Meng,a Xian Huang*a,b

Department of Chemistry, Zhejiang University (Campus Xixi), Hangzhou 310028, P. R. of ChinaFax +86(571)88807077; E-mail: huangx@zju.edu.cnb

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai a

200032, P. R. of ChinaReceived 13 January 2008

Abstract: Electrophilic additions to 1-cyclopropylallenes were in-vestigated, providing an efficient and stereoselective method for thesynthesis of 6-substituted-1,3-hexadienes.

Key words: cyclopropyl, allene, 1,3-hexadiene, electrophilic addi-tion, stereoselective

Methylenecyclopropanes 1 (MCP), which are highlystrained but readily accessible molecules, are of currentinterest in synthetic organic chemistry. The relief of theirring strain provides a potent thermodynamic drivingforce, which facilitates the construction of complex andinteresting organic molecules under mild conditions. Re-cently, the reactions and applications of MCP have beenwidely investigated.1 Allenes 2 are also useful buildingblocks in organic synthesis and have attracted much atten-tion of chemists for a long period. Due to their higher re-activity than noncumulated C–C double bonds, a varietyof reactions could take place to produce a series of inter-esting products. During the last decade, allenes haveplayed very important roles in organic methodology stud-ies.2

Previous investigations on MCP have showed that whenan electrophile (E+) reacted with MCP, it first added to theC=C bond to produce an intermediate carbocation 4. Be-cause of the high strain of the cyclopropyl ring, C–C bondcleavage of 4 takes place and gives the intermediate 5(Scheme1, equation 1),3 which can be transformed to ho-moallylic compounds in the subsequent reactions. Simi-larly, when E+ reacts with allenes, the central carbon ofallenes is attacked first to give intermediate 6 (Scheme1,equation 2),2c,4 which could be attacked by nucleophiles toproduce the corresponding allyl derivatives.

1-Cyclopropylallenes 3, which contain both a cyclopropylgroup and an allene structural unit, could be convenientlyprepared by the classical reaction of cyclopropyl Grignardreagent and toluene-4-sulfonic acid prop-2-ynyl ester cat-alyzed by copper(I) bromide (Scheme2).5 Due to the spe-

SYNLETT 2008, No. 9, pp1331–1334Advanced online publication:07.05.2008

DOI: 10.1055/s-2008-1072631; Art ID:W00908ST© Georg Thieme Verlag Stuttgart · New York

Scheme 1

Scheme 2

cial structure of compound 3, we assumed that when anelectrophile E+ reacts with 3, the most probable interme-diate would be 7. The structure of 7 was very similar tothat of the intermediate 4. Therefore, if the same b-scis-sion of C–C bond in the cyclopropyl group takes place,the corresponding 1,3-hexadiene cation 8 might be gener-ated (Scheme1, equation 3). Subsequent reactions of 8could produce a compound that contains a conjugate dienestructural unit. Conjugate dienes are useful buildingblocks in organic synthesis.6 Hence, the investigations onthe reactions of 1-cyclopropylallenes and their applica-tions are of value.

Recently, we reported the reaction of 1-cyclopropyl-al-lenes with phenylselenyl bromide (Scheme3).7 In that re-action, the phenylselenyl cation first added to compound3 to produce the intermediate 9. Due to the large sterichindrance of phenylselenyl group, the cyclopropyl groupin 9 should be at the same side with the group R. Hence,10 was obtained as E-isomer selectively in the subsequentreaction. Herein, we wish to report additional findings onthe electrophilic additions to 1-cyclopropyl-allenes. As itturned out, the reaction selectivity could be reversed whenelectrophiles (E+) with less steric hindrance were em-ployed.

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