Tantallacarborane Mediated Consecutive C–C and C–N Coupling Reactions of Alkyl Isonitriles: A Facile Route to N‑Heterocycles

Reactions of tantallacarborane methyl complexes ([η1:η5-(Me2NCH2CH2)­C2B9H10]­TaMe3 (1) and [η1:η5-(MeOCH2CH2)­C2B9H10]­TaMe3 (8)) with alkyl isonitriles have been studied. Complex 1 reacted with 1 equiv of RNC (R = TMSCH2, Cy, and iPr) to afford double migratory insertion products [η1:η5-(Me2NCH2CH2)­C2B9H10]­Ta­[η2-C,N-C­(Me2)­NCH2TMS]­Me (2) and [σ:η1:η5-{MeN­(CH2)­CH2CH2}­C2B9H10]­Ta­[N­(iPr)­R]­Me (R = Cy (3), iPr (4)). However, treatment of 1 or 8 with 4 equiv of alkyl isonitriles gave two fused six-membered N-heterocycles 5–7 and 9 via consecutive C–C/C-N bond-forming reactions. All new complexes were characterized by 1H, 13C, and 11B NMR spectra as well as elemental analyses. Their structures were further confirmed by single-crystal X-ray analyses. The results show that aryl and alkyl isonitriles exhibit significantly different reactivity patterns. This work also offers a very efficient method for the synthesis of N-heterocycles.