Interception of a Uranium Alkyl-Carbene Complex Utilizing a Bulky Phosphaylide

We report the use of the bulky phosphaylide reagent H₂C=P(Ar^tt)₃ (Ar^tt = 3,5-^tBu₂C₆H₃) to kinetically trap the alkyl-carbene molecule Cp*₂U(Me)[=C(H)P(Ar^tt)₃] (Cp* = C₅Me₅) (1-mono). However, despite the steric bulk of the Ar^tt groups, a significant amount of highly encumbered bis(carbene) species Cp*₂U[=C(H)P(Ar^tt)₃]₂ (2-bis) is produced. To circumvent this issue, an alternative route was devised in which the iodide compound Cp*₂U(I)[=C(H)P(Ar^tt)₃] (4-I) was synthesized. The subsequent addition of MeLi afforded 1 in good yields. The identity of the methylating reagent plays a key role in the success of the reaction, as addition of the Grignard reagent MeMgBr generates a mixture of products including Cp*₂UMe₂ as a major species. Finally, we demonstrate that access to other mixed carbene complexes such as Cp*₂U(O^tBu)[=C(H)P(Ar^tt)₃] (6-O^tBu) is possible.