Alkyl Carbagermatranes Enable Practical Palladium-Catalyzed sp~2-sp~3 Cross-Coupling

摘要

Pd-catalyzed cross-coupling reactions have achieved tremendous accomplishments in the past decades. However, C(sp(3))-hybridized nucleophiles generally remain as challenging coupling partners due to their sluggish transmetalation compared to the C(sp(2))-hybridized counterparts. While a single-electron-transfer-based strategy using C(sp(3))-hybridized nucleophiles had made significant progress recently, fewer breakthroughs have been made concerning the traditional two-electron mechanism involving C(sp(3))-hybridized nucleophiles. In this report, we present a series of unique alkyl carbagermatranes that were proven to be highly reactive in cross-coupling reactions with our newly developed electron-deficient phosphine ligands. Generally, secondary alkyl carbagermatranes show slightly lower, yet comparable activity to its Sn analogue. Meanwhile, primary alkyl carbagermatranes exhibit high activity, and they were also proved stable enough to be compatible with various reactions. Chiral secondary benzyl carbagermatrane gave the coupling product under base/additive-free conditions with its configuration fully inversed, suggesting that transmetalation was carried out in an "S(E)2(open) Inv" pathway, which is consistent with Hiyama's previous observation. Notably, the cross-coupling of primary alkyl carbagermatranes could be performed under base/additive-free conditions with excellent functional group tolerance and therefore may have potentially important applications such as stapled peptide synthesis.