Comparison of an internally coordinated 2-pentenyllithium with its 4-sila analog. Structure and dynamic behavior: unexpected 13C7Li spin coupling.
Four allylic lithium compounds have been prepared with a tethered ligand, (CH(3)OCH(2)CH(2))(2)NCH(2)C(CH(3))(2)-L, attached to a terminal allyl carbon. They are equimolar equilibrium mixtures of 3-endo-L-allyllithium with 3-exo-L-allyllithium, 9en and 9ex, respectively, and (separately) 1-exo-TMS-3-endo-L-allyllithium with 1-exo-TMS-3-exo-L-allyllithium, 13en and 13ex, respectively. Carbon-13 NMR analysis shows that all four compounds are monomeric and internally coordinated, the allyl moiety in each one is partially localized, and (7)Li is spin coupled to both (31)C(1) and (13)C(2) of allyl. NMR line shape changes show that 9en, 9ex, 13en, and 13ex all undergo fast transfer of the lithium-coordinated ligand between faces of the allyl planes at low temperatures. Within each equilibrium system the compound with the exo-tethered ligand inverts faster than its endo analog. All four compounds invert faster in THF-d(10) compared to in diethyl ether-d(10) as solvent. Corresponding Eyring activation parameters are reported. Also, inversion in THF-d(8) for all four compounds is characterized by large negative DeltaS(double dagger) values, whereas in diethyl ether-d(10) DeltaS(double dagger) values for inversion are near neutral. NMR line shape changes due to the dynamics of bimolecular C,Li exchange and rotation around the C(2)-C(3) bonds are just detected via selective line broadening effects above 290 K. These processes are much slower than inversion.
Fraenkel
Gideon
G
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA. fraenkel@mps.ohio-state.edu
Cabral
José
J
Chen
Xiao
X
Chow
Albert
A