Solution Structure and Stereochemistry of Alkyl- and Silyl-Substituted Allenyl-Propargyllithium Reagents

Analysis of 13 C chemical shifts and Li−C couplings showed that allenyllithium ( 5A-Li ) was a mixture of monomer and dimer in THF, both with an allenyl structure. Similarly, the metalation products of 2-butyne ( 6A-Li ), 4-methylpentyne ( 9A-Li ), and 4,4-dimethylpentyne ( 21A-Li ) in THF, as well as several 1, 3-dialkyl ( 32A-Li, 33A-Li, 34A-Li ) propargyl-allenyllithiums formed by metalation or Li/Sn exchange were all monomeric lithioallenes in THF. Compound 6A-Li was shown to have less than 5% and 21A-Li less than 3% of the propargyllithium isomers ( 6P - Li, 21P - Li ) present from analysis of residual broadening of the propargyl carbon by Li−C coupling. The reagent prepared by metalation of dicyclopropylacetylene ( 8P - Li ) has a propargyl structure, but two related reagents with a cyclobutane spanning the 3,3-positions ( 37A-Li and 39A-Li ) had allenyl structures. Several triorganosilyl-substituted reagents were also investigated. Those with silyl groups at the allenyl position ( 28A-Li, 31A-Li ) are allenyllithiums, those with silyl groups at the propargyl position ( 22-Li to 26P - Li ) showed chemical shifts intermediate between those of allenyl and propargyl isomers, and the shifts were strongly temperature-dependent under some conditions. These compounds are probably equilibrating mixtures of allenyl and propargyllithiums or equilibrating mixtures of unsymmetrically π-complexed structures, with barriers to interconversion (Δ G ⧧ - 150 ) below 4 kcal/mol. Several of the organolithium reagents studied had diastereotopic carbon signals (SiMe 2, CMe 2, or CPh 2 groups), which allowed determination of barriers to configurational inversion of the chiral allenyl fragment. Barriers from 6.1 kcal/mol ( 26A-Li in dimethyl ether) to 14.5 kcal/mol ( 39A-Li in 3:2 THF/ether) were measured.