UNUSUAL STRUCTURES, ENERGIES, AND BONDING OF LITHIUM-SUBSTITUTED ALLENES, PROPYNES, AND CYCLOPROPENES

Propyne and aliene undergo sequential hydrogen-lithium exchange experimentally to give C3H3Li (two isomers), C3H2Li2, C3HLi3, and C3Li4. Mono- and dilithiated cyclopropenes are also known. The structures and bonding of hypothetical isolated monomers with these compositions, investigated by ab initio molecular orbital methods, reveal remarkable features. While acetylide bonding, e.g., in propynyllithium (1), is the most favorable energetically, bridging lithiums are preferred over conventional placements. Allenyllithium (11), the simplest bridged molecule of this type, has a bent carbon skeleton (ZCCC = 157.6°, STO-3G optimization) which better accommodates simultaneous Li bonding to C-1 and C-3. Even though Li is closest to C-2, bonding to that atom is indicated by the overlap populations to be negligible. The vinyl hydrogen of cyclopropene. with its enhanced acidity, exhibits a larger energy of replacement than that of ethylene. The methylene hydrogen behaves in the opposite way owing to the antiaromatic character of 3-cyclopropenyllithium (5). Examination of a number of structural possibilities indicated the most favorable forms, e.g., 17 for C3H2Li2, with an acetylide and a bridging lithium. Two bridging lithiums in CLiC planes roughly at right angles are found in the lowest energy structures of C3HLi3 (23) and of C3Li4 (36), which in addition have one and two acetylide-type C-Li bonds, respectively. The orthogonal tt systems, each involved in bonding a bridging lithium, are responsible for this arrangement. Numerous other forms of these isomers were considered. With few exceptions, classical structures based on the corresponding hydrocarbons were much less stable than alternative arrangements in which the multicenter bonding capabilities of lithium could be better utilized. Another example is 1, 2-dillthiocyclopropene, which prefers the doubly bridged (4a) over the classical (4) structure. Copyright © 1979, American Chemical Society. All rights reserved.