Two-coordinate group 14 element(ii) hydrides as reagents for the facile, and sometimes reversible, hydrogermylation/hydrostannylation of unactivated alkenes and alkynes

Abstract

Reactions of the solution stable, two-coordinate hydrido-tetrylenes, :E(H)(L^†) (E = Ge or Sn; L^† = –N(Ar^†)(SiPrⁱ₃); Ar^† = C₆H₂{C(H)Ph₂}₂Prⁱ-2,6,4), with a variety of unactivated cyclic and acyclic alkenes, and one internal alkyne, lead to the rapid and regiospecific hydrometallation of the unsaturated substrate at ambient temperature. The products of the reactions, [L^†E(C₂H₄R)] (E = Ge or Sn, R = H, Ph or Bu^t), [L^†E{CH(CH₂)₃(CH₂)_n}] (E = Ge, n = 1, 2 or 3; E = Sn, n = 1) and [L^†E{C(Ph)C(H)(Me)}], include the first structurally characterised examples of two-coordinate amido/alkyl germylenes and stannylenes. The cycloalkene hydrometallation reactions are cleanly reversible under ambient conditions, a process which computational and experimental van't Hoff analyses suggest proceeds via β-hydride elimination from the metal coordinated cycloalkyl ligand. Similarly, the reactions of :Ge(H)(L^†) with 1,5-cyclooctadiene and 2-methyl-2-butene, both likely proceed via β-hydride elimination processes, leading to the clean isomerisation of the alkene involved, and its subsequent hydrogermylation, to give [L^†Ge(2-cyclooctenyl)] and [L^†Ge{C₂H₄C(H)Me₂}], respectively. Reactions of [L^†GeEt] and [L^†Ge(C₅H₉)] with the protic reagents, HCl, NH₃ and EtOH, lead to oxidative addition to the germanium(II) centre, and formation of the stable chiral germanium(IV) complexes, [L^†Ge(C₅H₉)(H)Cl] and [L^†Ge(Et)(H)R] (R = NH₂ or OEt). In contrast, related reactions between [L^†SnEt] and Bu^tOH or TEMPOH (TEMP = 2,2,6,6-tetramethylpiperidinyl) proceed via ethane elimination, affording the tin(II) products, [L^†SnR] (R = OBu^t or OTEMP). In addition, the oxidation of [L^†Ge(C₆H₁₁)] and [L^†Sn(C₂H₄Bu^t)] with O₂ yields the oxo-bridged metal(IV) dimers, [{L^†(C₆H₁₁)Ge(μ-O)}₂] and [{L^†(Bu^tC₂H₄)Sn(μ-O)}₂], respectively.