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

• Published in: Journal of the American Chemical Society Vol. 121; no. 42; pp. 9769 - 9780
• Main Authors: Reich, Hans J, Holladay, Johnathan E, Walker, Tamara G, Thompson, Jennifer L
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 27.10.1999; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; Physical Sciences; Science & Technology; CARBANION MECHANISMS; LITHIUM COMPOUNDS; CARBONYL-COMPOUNDS; LOW-TEMPERATURE C-13-NMR; ORGANOLITHIUM REAGENTS; ION-PAIR SEPARATION; STEREOSELECTIVE SYNTHESIS; STEREOCONTROLLED SYNTHESIS; COUPLING-CONSTANTS; CONFIGURATIONAL STABILITY
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja991719d

Analysis of 13C 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 (ΔG ⧧ - 150) below 4 kcal/mol. Several of the organolithium reagents studied had diastereotopic carbon signals (SiMe2, CMe2, or CPh2 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.