property of carbon in pi bond that determine a reaction mechanism

Question

This is a mechanism provided in a problem set in my ochem course.

As far as I know, double bond is electron rich, which is nucleophilic and attacks electrophilic species. In this case, why does one double bond attack another carbon in double bond?

IMO, wouldn't a SN2 reaction make more sense?

Answer 1

Several points about the question itself. First, the enamine of n-butyraldehyde is unquestionably of the (E)-configuration and its preparation does not require an acid catalyst. Mannich^[1] prepared 1,1-dipiperidinobutane (1,1'-(butane-1,1-diyl)dipiperidine) from piperidine and n-butyraldehyde in the presence of potassium carbonate. Elimination to the enamine and piperidine occurred upon distillation. Secondly, the enamine employed in the allylation is that of propionaldehyde.

Whether or not $\ce{S_N2 or S_N2'}$ allylation occurs is not straightforward. N-Allylation may be kinetically favored over direct C-allylation. Stork, et al.^[2] have noted that Cope rearrangement of the N-allylated product leads to a C-allylation aldehyde which would, if C-allylation were kinetically favored, lead to allylation at the more hindered end of the allylic electrophile. The following is from the Stork paper:

1. C. Mannich and H. Davidsen, Chem. Ber., 1936, 69, 2106.
   
2. G. Stork, A. Brizzolara, H. Landesman, J. Szmuszkovicz, and R. Terrell, J. Am. Chem. Soc. 1963, 85, 207–222.