Insertion of Isonitriles into the M–C Bonds of Group 4 Dialkyl Complexes

The 2,3-dimethyl­butadiene complexes
of Group 4 metals with
constrained geometry (cg) ligands have been prepared and found to
adopt a supine orientation with σ2,π bonding.
Treatment of cgTi­(2,3-dimethyl­butadiene) (1-Ti) with tBuNC leads to the formation of
a titana-aziridine (3) with a coordinated cyclo­penten­imine
that arises from the formal [4+1] addition of the diene to the isonitrile.
In contrast, the reactions of cgZr­(2,3-dimethyl­butadiene)
(1-Zr) or cgHf­(2,3-dimethyl­butadiene) (1-Hf) with 2 equiv of tBuNC or
XyNC proceeded in a more sophisticated manner to yield unsymmetrical
2,5-diazametallacyclopentane derivatives (4, 6-Zr, and 6-Hf) or symmetrical 2,5-diazametallacyclopentene
complexes (7-Zr and 7-Hf). The unsymmetrical
products contain coordinated cyclopropanes; the strength of the interaction
is measured by the reduction in the 1JCC of the C–C bond that is coordinated. A detailed
mechanistic analysis has been possible with the related cgM(Me)2 (M = Ti and Hf) complexes. The first insertion is too fast
to monitor, but allows complete conversion to an alkyl iminoacyl intermediate.
The second isonitrile (RNC) may react with that intermediate by either of
two different mechanisms, reductive elimination and coordination/insertion.
In the first mechanism (Ti), rate-determining C–C coupling
gives a titana-aziridine, followed by fast coordination of the isonitrile.
In the second mechanism (Hf), coordination is the slow step; insertion
to form a bis­(iminoacyl) Hf complex is rapid.