Tetrahedron Letters,Vo1.30,No.30,pp 3901-3904,1989 0040-4039/89 $3.00 + .OO Printed in Great Britain Maxwell Pergamon Macmillan plc PREPARATION AND FACILE CYCLIZATION OF 5-ALKYN-l-YLLITHIUMS William F. Bailey*, Timo V. Ovaska, and Thomas K. Leipert Department of Chemistry, university of Connecticut, Stows, Connecticut 06269-3060 Absiracf: Primary 5-hexyn-I-yllithiums, which may be generated in virtually quantitative yield by low-temperature lithium-iodine interchange, undergo regiospecific 5-exo-dig cyclization via stereoselectively syn-addition of CH,Li to the alkyne moiety to give vinyllithiums that may be trapped with any of a variety of electrophiles to deliver functionalized cyclopentylidene- containing products. Some twenty years ago, Ward reported’ that treatment of 6-bromo-l-phenyl- I-hexyne with 9-butyllithium (n-BuLi) in hexane-ether (5:l by vol) at room temperature, followed by hydrolysis with water, gave a mixture of products that included 60% of benzylidenecyclopentane. Ward’ and others2 have presented convincing arguments that Ph = /IrBr Ph n43uLi _ + Ph = -0~ + Ph = N + .. c-C6H14 I Et20 H these products resulted from radical intermediates generated in the reaction of 9 -BuLi with the bromide. This analysis is consistent with a body of recent evidence3-5 which suggests that the lithium-bromine interchange reaction between an alkyllithium and a primary alkyl bromide is an outer sphere process involving single-electron transfer (SET). Since the lithium-iodine interchange, by contrast, is an inner sphere reaction that does not involve radical intermediates when conducted under appropriate conditions,3z6 it occurred to us that the use of an acetylenic alkyl iodide rather than the bromide should allow for the preparation of the corresponding lithio derivative. Indeed, treatment of 6-iodo I-phenyl-1-hexyne (1) in n-pentane-diethyl ether (32 by vol) at -78°C with 2.2 equiv of f-BuLi in n-pentane served to cleanly generate the acetylenic alkyllithium, 2 Quench of the cold reaction mixture with deoxygenated methanol afforded the hydrocarbon quantitatively (96% isolated yield: d, content of 91% above natural abundance upon quench with MeOD). Ph = /\A, t-BuLi * Ph-Li MeOH Pha /~C5H12 / Et20 -78’C -78'C zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1 2 At temperatures above ca. -25”C, 2 undergoes fairly rapid isomerization to vinyllithium 3 via a 5-exo-dig cyclization. Quench of a reaction mixture that had been warmed briefly at room temperature gave benzylidenecyclopentane in 94% isolated yield. Ph T> -25’C Ph Ph MeOH t - o+ Li Li H 3901