Strikingly Different Reactivity Patterns of Fischer Alkoxycarbene and Thiocarbene Complexes in Experimental and Theoretical Studies

Abstract

Striking differences in the reactions of alkoxycarbene and thiocarbene complexes of chromium and tungsten are observed. Thus, (β-imino)ethoxycarbene complexes 10a − e, generated in situ from [(OC) 5 W C(OEt)CH 2 R] ( 7a − c; R = n -Pr, Me, c -C 7 H 7 ) and imidoyl chlorides R 1 ClC NCHR 2 R 3 ( 9a − f; R 1 = t -Bu, Ph, 2-furyl; R 2 = H, Me; R 3 = Me, Et, Ph), undergo a metalla(di-π-methane) rearrangement to ( N -enamino)ethoxycarbene complexes 12a − e, while the corresponding (β-imino)thiocarbene complexes 11a − l, derived from [(OC) 5 M C(SEt)CH 2 R] ( 8a − e; M = W, Cr; R = n -Pr, Me, c -C 7 H 7, c -C 6 H 7 Fe(CO) 3 ) and imidoyl chlorides under similar conditions, form pyrroles 16a − h and 17k, l by α-cyclization. On the basis of the calculated DFT/BP86 potential energy surfaces of the particular reaction channels it is shown that (β-imino)alkoxycarbene compounds 10 prefer a metalla(di-π-methane) rearrangement due to the kinetic stability of the ( N -enamino)ethoxycarbene products, while formation of pyrroles is not favored due to the presence of high energetic stationary structures in the α-cyclization pathway. For (β-imino)thiocarbene compounds 11, on the other hand, rearranged products are kinetically unstable, and α-cyclization reactions are strongly favored on thermodynamic grounds.