Solution structure and stereochemistry of alkyl- and silyl-substituted allenyl-propargyllithium reagents
Solution structure and stereochemistry of alkyl- and silyl-substituted allenyl-propargyllithium reagents
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DOI：
10.1021/ja991719d
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发表时间：
1999-10-27
影响因子：
15
通讯作者：
Thompson, JL
中科院分区：
文献类型：
--
作者：
Reich, HJ;Holladay, JE;Thompson, JL
C-13化学位移和Li-C偶联分析表明，烯基锂（SA-Li）在四氢呋喃中是单体和二聚体的混合物，均具有烯基结构。同样，THF中2-丁炔（6A-Li）、4-甲基戊炔（BA-Li）和4,4-二甲基戊炔（21A-Li）的金属化产物，以及金属化或Li/Sn交换形成的几种1,3 -二烷基（32A-Li、33A-Li、34A-Li）丙炔烯锂，都是THF中的单体锂烯。化合物6A-Li的丙基锂异构体（6P-Li, 21P-Li）小于5%，而化合物21A-Li的丙基锂异构体（6P-Li, 21P-Li）小于3%。二环丙基乙炔金属化制备的试剂（8P-Li）具有丙炔结构，而相邻的两个环丁烷横跨3,3位的试剂（37A-Li和39A-Li）具有烯丙基结构。研究了几种三有机硅基取代试剂。在烯基位置（28A-Li, 31A-Li）有硅基的是烯基锂，在丙基位置（22-Li至26P-Li）有硅基的是介于烯基和丙基异构体之间的化学位移，在某些条件下，这种位移强烈依赖于温度。这些化合物可能是烯基锂和丙基锂的平衡混合物或不对称pi络合结构的平衡混合物，其相互转化的势垒(δ G（-150)（双dagger））低于4 kcal/mol。所研究的几种有机锂试剂具有非对映位碳信号（SiMe2， CMe2或CPh2基团），这可以确定手性烯基片段构型反转的障碍。从6.1 kcal/mol （26A-Li在二甲醚中）到14.5 kcal/mol （39A-Li在3:2 THF/醚中）的势垒测定。
Analysis of C-13 chemical shifts and Li-C couplings showed that allenyllithium (SA-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 (BA-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, bur 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 pi-complexed structures, with barriers to interconversion (Delta G(-150)(double dagger)) 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.