Mechanism of Heterocumulene Insertion Implication for Other Electrophiles

Mechanism of Heterocumulene Insertion Implication for Other Electrophiles 

Braunstein has suggested that precoordination is necessary for heterocumulene insertion [64]. If this suggestion is correct, THF must dissociate from 2a or 2b, or PMe3 from 2o, before insertion can take place. Such a mechanism is supported by the kinetics of the reaction (Eq. 27). The inverse dependence upon [THF] in the rate law (Eq. 28) reflects the reversible dissociation of THF from 2p in Eq. 27 [49]. (The dissociation is an unfavorable, but rapidly maintained, preequilibrium.) 

The reaction can be run in reverse at high [THF], with bis-(trimethylsilyl) carbodiimide replaced by THF. The rate is independent of [THF], consistent with rate-limiting extrusion of carbodiimide, i.e., with the operation of a similar dissociative mechanism in reverse. Similarly, addition of trimethylphos-phine to 35 results in extrusion of the carbodiimide to form the phosphine-stabilized zirconaaziridine [49]. In contrast, the isocyanate insertions are effectively irreversible. 

Isocyanate insertions presumably go through the same 16-electron intermediate, which explains why the Zr-N/Zr-C ratio from those reactions is inde- 

---