The Reactivity of Phosphagermaallene Mes*P=C=Ge(t-Bu)Tip toward Aldehydes and Ketones: an Experimental and Theoretical Study

Phosphagermaallene Mes*P=C=Ge(t-Bu)Tip 1 (Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl) has been obtained by an improved synthesis in relation to the previously reported preparation starting from t-BuGeCl3. Very short Ge=C (1.761(2) angstrom) and P=C (1.625(2) angstrom) double bond lengths, trigonal planar geometry around the Ge atom, and GeCP bond angle of 166.57 degrees are indicative of an heteroallenic structure for 1. Its reactions with crotonaldehyde and cinnamaldehyde lead to 1-oxa-2-germacyclobutanes by a [2 + 2] cycloaddition between the Ge=C and C=O double bonds; with methyl vinyl ketone, both a [2 + 4] cycloaddition between the Ge=C and the O=C-C=C moieties and an ene-reaction are observed leading to a 1-oxa-2-germacyclohex-5-ene and a germyl(butadienyl)ether, respectively. With acetophenone, an ene-reaction occurs to afford exclusively a germyl(vinyl)ether. DFT calculations have been performed to explain the regiochemistry observed.