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

Chemical Science View Article Online Open Access Article. Published on 22 September 2015. Downloaded on 14/05/2018 14:16:40. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE Cite this: Chem. Sci., 2015, 6, 7249 View Journal | View Issue Two-coordinate group 14 element(II) hydrides as reagents for the facile, and sometimes reversible, hydrogermylation/hydrostannylation of unactivated alkenes and alkynes† Terrance J. Hadlington,a Markus Hermann,b Gernot Frenking*b and Cameron Jones*a Reactions of the solution stable, two-coordinate hydrido-tetrylenes, :E(H)(L†) (E ¼ Ge or Sn; L† ¼ –N(Ar†)(SiPri3); Ar† ¼ 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 ¼ Ge or Sn, R ¼ H, Ph or But), [L†E{CH(CH2)3(CH2)n}] (E ¼ Ge, n ¼ 1, 2 or 3; E ¼ Sn, n ¼ 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 b-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 b-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 Received 8th September 2015 Accepted 22nd September 2015 chiral germanium(IV) complexes, [L†Ge(C5H9)(H)Cl] and [L†Ge(Et)(H)R] (R ¼ NH2 or OEt). In contrast, related reactions between [L†SnEt] and ButOH or TEMPOH (TEMP ¼ 2,2,6,6-tetramethylpiperidinyl) proceed via ethane elimination, affording the tin(II) products, [L†SnR] (R ¼ OBut or OTEMP). In addition, DOI: 10.1039/c5sc03376d the oxidation of [L†Ge(C6H11)] and [L†Sn(C2H4But)] with O2 yields the oxo-bridged metal(IV) dimers, www.rsc.org/chemicalscience [{L†(C6H11)Ge(m-O)}2] and [{L†(ButC2H4)Sn(m-O)}2], respectively. Introduction The 1,2-addition of element-hydrogen bonds across the carbon– carbon unsaturations of alkenes and alkynes is of immense importance to organic synthesis. In this respect, and since Brown's seminal work on the hydroboration of alkenes in the 1950's,1 boranes have become the reagent of choice for the reduction of olens and alkynes.2 One of the primary reasons for the efficacy of such hydroborations, is that electron decient, three-coordinate boranes (R2BH) possess an empty p-orbital which is thought to allow the formation of a loose a School of Chemistry, Monash University, PO Box 23, VIC, 3800, Australia. E-mail: ; Web: http://www.monash.edu/science/research-groups/ chemistry/jonesgroup b Fachbereich Chemie, Philipps-Universität Marburg, 35032, Marburg, Germany. E-mail: † Electronic supplementary information (ESI) available: Experimental procedures and characterisation data for all new compounds, full details of the computational studies. Crystal data, details of data collections and renements. CCDC 1422725–1422742. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc03376d This journal is © The Royal Society of Chemistry 2015 p-complex with the unsaturated substrate, prior to its insertion into the polar d+B–Hd linkage.3 This mechanism has also been used to explain the typically observed cis-/anti-Markovnikov addition of boranes to unsaturated hydrocarbons. While much less studied than boranes, a variety of electron decient, polar hydride complexes of aluminium, the heavier group 13 metals,2 and the s-4 and early d-block metals,5 have additionally been shown to be effective for the hydrometallation of alkenes and alkynes. Considering that neutral group 14 element(IV) hydrides (e.g. R3EH, E ¼ Si, Ge or Sn) do not possess any vacant valence orbitals, it is not surprising that they are poorly effective for the hydroelementation of alkenes and alkynes, at least in their own right. However, reactions of this type (particularly hydrosilylations) are of considerable synthetic importance, and can proceed, for example, in the presence of transition metal catalysts or radical initiators; and/or when subjected to UV irradiation or elevated temperatures.6,7 It would be a signicant advantage if the addition of group 14 element-hydrogen bonds to unsaturated hydrocarbons could be effected in the absence of catalysts or initiators, and in a Chem. Sci., 2015, 6, 7249–7257 | 7249 View Article Online Open Access Article. Published on 22 September 2015. Downloaded on 14/05/2018 14:16:40. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. Chemical Science facile manner under ambient conditions. The rst hints that this might be possible came with the kinetic stabilisation of group 14 element(II) hydride complexes, a small number of which (e.g. I–V, Scheme 1)8–11 have been reported since the turn of the millennium.12 Of these, the three-coordinate silicon(II) hydride, I, has been shown to hydrosilylate cyclopentene and a series of terminal olens at elevated temperatures (70–120  C) and in the presence of large excesses of the alkene substrate.8 The latter reactions give rise to mixtures of regioisomers, in which the anti-Markovnikov product predominates. In one case, i.e. the reaction with trimethylsilylethylene, the reaction proceeds via an isolated [2 + 1] cycloadduct, viz. the silirane [I {h2-H2C]C(H)(SiMe3)}], which exists in equilibrium with I and free H2C]C(H)(SiMe3) at ambient temperature. With respect to hydrogermylation and hydrostannylation reactions, the threecoordinate species, II and III, have been shown to cleanly hydrometallate activated (ester substituted) terminal and internal alkynes at ambient temperature.13 Furthermore, the dimeric, three-coordinate metal(II) hydride complexes, IV and V, react with tert-butylethylene at ambient temperature over 48 hours to give the alkyl/aryl substituted ditetrelenes [{Ar0 E(CH2CH2But)}2] (Ar0 ¼ C6H3(C6H3Pri2-2,6)2-2,6; E ¼ Ge or Sn). Contrastingly, aer 48 hours, the reaction of IV with excess cyclopentene at ambient temperature yielded only a monohydrogermylation product, viz. the hydrido-digermene, [Ar0 (H) Ge]Ge(Cp)Ar0 ] (Cp ¼ cyclopentyl).14 This suggests that the dissociation of IV to the two-coordinate hydrido-germylene, Ge(H)Ar0 , in solution is minimal. Recently, we have utilised extremely bulky amide ligands, developed in our group,15 to kinetically stabilise amido/hydridodigermenes, e.g. [L†(H)Ge]Ge(H)L†] 1 (L† ¼ –N(A (...truncated)