Carbenoid chain reactions: Substitutions by organolithium compounds at unactivated 1-chloro-1-alkenes

摘要

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 = CLi cannot perform step 1, but its carbenoid chain processes with reagents 2a and 2c may be started with MeLi, whereafter LiC = CPh reacts faster than MeLi in the product-determining step 2 to generate the chain carrier Alk(2)C = CLi - C = 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.