Reactivity of 1-aza-4-oxa-1,3-butadienes (.alpha.-imino ketones) toward triorganoaluminum reagents. 1. X-ray crystal structures of [(AlMe3)2{.sigma.,.sigma.-N,O-(MeN:C(Ph)C(Ph):O)}] with a N,O-bridge-bonded .alpha.-imino ketone, and of cyclic Me2Al(tert-Bu)N:CHC(Me)2OAlMe3, a carbonyl alkylated product which is O-coordinated to AlMe3