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

We report the use of the bulky phosphaylide reagent H2C=P(tBuAr)3 (tBuAr = 3,5-tBu2C6H3) to kinetically trap the alkyl–carbene molecule Cp*2U(Me)[=C(H)P(tBuAr)3] (1-mono). However, despite the steric bulk of the tBuAr groups, a significant amount of the highly encumbered bis(carbene) species Cp*2U[=C(H)P(tBuAr)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(tBuAr)3] (4-I) was synthesized. The subsequent addition of MeLi affords 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 demon-strate that access to other mixed carbene complexes such as Cp*2U(OtBu)[=C(H)P(tBuAr)3] (6-OtBu) is possible.