Additions of lower order and higher order mixed and homostannylcuprates, as well as the Cu(I) catalyzed additions of Bu3SnBBN(OMe)'Li +, Bu3SnAIEt 2, and Bu3SnMgMe to conjugated 1,3-enynes were investigated. Regioselective additions (> 90% of one isomer) of these bimetallic reagents to the alkyne moiety of 2-methyl-1-buten-3yne, 2-methyl-1-hexen-3-yne and 2-penten-4-yn-I-ol were obtained. The direction of the addition of the mixed HO cuprates could be affected by changing the size of the ligands bound to tin and copper. The regioselectivity of addition of Sn-AI and Sn-Mg reagents was influenced by solvent and reaction conditions.
Reactions of 1,3-enynes with organometallics offer efficient routes to 1,3-dienes and allenes (Scheme 1). Organolithium (RLi), calcium, strontium or barium reagents react with conjugated enynes with internal triple bonds (la) by 1,4-addition initiated by nucleophilic attack of the olefin 1 at C-1 to produce allenes (2a). When the triple bond is terminal (lb) this reaction with alkyllithiums proceeds by 1,2-addition to the olefin to give only alkynes (3b). 2 This is presumably due to the initial formation of an alkynyl anion which inhibits the 1,4-addition. Organomagnesium reagents add to internal conjugated enynes by both 1,2and 1 A-addition also initiated by attack on the olefin.
Attenuation of the reactivity of benzyl magnesium halides by addition of NiCI 2 promotes nucleophilic attack on C-1 of the alkyne of conjugated 1,3-enynes to produce 1-substituted-1,3-dienes (5a). 3 Similarly, homocuprates (R2CuMgBr), heterocuprates [(RCuBr)MgX] and di-t-butylzinc react with enynes possessing ~-hydroxy, alkoxy and amino groups (lc,ld) to produce 1,3-dienes (5ยข,5d). 4 Stoichiometric zirconium chemistry 5 also provides efficient routes to l-phenylseleno-, 1-phosphino and 1-bora-l,3-dienes. 6,7
With allylmagnesium halides additions proceed by attack of the nucleophile at the internal carbon of the alkyne (ld) to produce 2-substituted-1,3-dienes (4d). 8 Additions of allylic organozincs to enynes with terminal triple bonds (lb) also give 2-substituted-1,3-dienes (4b). 1,2,9 When excess organozinc reagent is used, alkenes (6b) resulting from double addition are produced. As well, silanes, 1ยฐ germanes,lO,11 stannanes,12 (R3MH) and alanes (R2MH)I react to give 2-substituted-1,3-dienes.
Recently, methods have been developed for metallometalation of 1-alkynes. 13 Bimetallic reagents reported to date that add to 1-alkynes include Sn-B 14, Sn-AI15,Sn-Cu 16, Sn-7~ 15, Sn-Sn 17, Si-A118, Si-Sn 19, Si-Si 20 and Si-Cu 21 reagents. Metallometallations of 1-alkynes involving these organometallics produce 1,2-dimetalloalkenes in which the metal accompanying Sn or Si yields a C-M bond more reactive than the simultaneously formed Si-carbon or Sn-carbon bond. The reactivities of the bimetallic adducts produced in these reactions usually require electrophilic consumption of the more reactive organometallic centre prior to isolation. Methods are available to react the C-M bonds of these adducts with a wide variety of electrophiles. 13,22 Since several methods have been developed for replacement of the Si-C bond by electrophilic reagents 23 and for the transmetalation of the Sn-carbon bond with organopalladium 24 or organocopper reagents 25 the synthetic opportunities emanating from metallometallation of alkynes are apDreciable.