Carbenoid Chain Reactions: Substitutions by Organolithium Compounds at Unactivated 1-Chloro-1-alkenes

Abstract

The deceptively simple "cross-coupling" reactions Alk(2)C=CA-Cl + RLi --> Alk(2)C=CA-R + LiCl (A = H, D, or Cl) occur via an alkylidenecarbenoid chain mechanism in three steps without a transition metal catalyst. In the initiating step 1, the sterically shielded 2-(chloromethylidene)-1,1,3,3-tetramethylindans 2a-c (Alk(2)C=CA-Cl) generate a Cl,Li-alkylidenecarbenoid (Alk(2)C=CLi-Cl, 6) through the transfer of atom A to RLi (methyllithium, n-butyllithium, or aryllithium). The chain cycle consists of the following two steps: (i) A fast vinylic substitution reaction of these RLi at carbenoid 6 (step 2) with formation of the chain carrier Alk(2)C=CLi-R (8), and (ii) a rate-limiting transfer of atom A (step 3) from reagent 2 to the chain carrier 8 with formation of the product Alk(2)C=CA-R (4) and with regeneration of carbenoid 6. This chain propagation step 3 was sufficiently slow to allow steady-state concentrations of Alk(2)C=CLi-Aryl to be observed (by NMR) with RLi = C6H5Li (in Et2O) and with 4-(Me3Si)C6H4Li (in t-BuOMe), whereas these chain processes were much faster in THF solution. PhC[triple bond]CLi cannot perform step 1, but its carbenoid chain processes with reagents 2a and 2c may be started with MeLi, whereafter LiC[triple bond]CPh reacts faster than MeLi in the product-determining step 2 to generate the chain carrier Alk(2)C=CLi-C[triple bond]CPh (8g), which completes its chain cycle through the slower step 3. The sterically congested products were formed with surprising ease even with RLi as bulky as 2,6-dimethylphenyllithium and 2,4,6-tri-tert-butylphenyllithium.