Alkenyl‐Alkynylgermanes Functionalised by Lewis Acids: Intramolecular Aluminium– and Gallium–Alkyne Interactions and Potential Ge–C Bond Activation

Abstract

Treatment of various diethynylgermanes, (R1)2Ge(C≡C–R2)2 (R1 = C6H5, CH3; R1–R1 = C6H4–C6H4; R2 = CH3, CMe3, nBu, C6H5), with equimolar quantities of di(tert‐butyl)aluminium or ‐gallium hydride, R2E–H (E = Al, Ga), afforded mixed alkenyl‐alkynylgermanes, (R1)2Ge(C≡C–R2)[C{E(CMe3)2}=C(H)–R2], by reduction of one of their C≡C triple bonds. The alkenyl groups have a cis arrangement of H and Al or Ga atoms across the C=C double bonds, which reflects the kinetically favoured situation. The cis/trans isomerisation is probably prevented from yielding thermodynamically favoured trans isomers; this is a result of an intramolecular bonding interaction between the α‐carbon atoms of the remaining ethynyl groups and the coordinatively unsaturated metal atoms. The higher Lewis acidity of Al compared to Ga results in relatively short Al···C(ethynyl) contacts (>231.2 pm) and a concomitant lengthening of the Ge–C bonds.

Cooperative GeN Bond Activation in Aluminium‐Functionalised Aminogermanes and Spontaneous Imine Elimination via an Intermediate Germyl Cation

“…[14] A higher ionic contribution causes an increasing approach to a coplanar arrangement of the remaining four atoms (Si and three substituents). In a perfectly ionic compound ([R 3 Si] + and F À ) the sum of the angles at Si is 3608. For the Ge compound 7, this consideration suggests an intramolecular activation of the GeÀC(alkyne) bond by the coordination of the a-C atom to Al, which is possibly supported by hyperconjugative interaction to the amino group.…”

“…Mixed alkenyl-alkynylsilanes (1, Scheme 1) and -germanes have been obtained by partial hydrometallation of dialkynylelement compounds. [2][3][4][5][6][7][8][9][10] They exhibit an interaction of the coordinatively unsaturated Group 13 metal atoms with the a-C atoms of the remaining alkynyl groups. The concomitant weakening of the SiÀC or GeÀC bonds favours the thermally induced rearrangement, which, by 1,1-carbometallation and insertion of the alkynyl C atom into an AlÀC(vinyl) bond, results in the formation of sila-or germacyclobutenes at elevated temperatures.…”

Germacyclobutenes: Generation by 1,1‐Carbalumination or 1,1‐Carbagallation and Their Photophysical Properties

“…The starting alkenylalkynylgermanes were obtained according to literature procedures. [6,7] Synthesis of the germacyclobutenes 9 to 14, general procedure: The mixed alkenylalkynylgermanium derivatives (3 to 8; ca. 1 mmol) were heated without a solvent in vacuum (1 10 À3 mbar; 9, 13: 160 8C, 30 min; 10: 170 8C, 3.5 h; 11: 155 8C, 1.5 h; 12: 170 8C, 4 h; 14: 170 8C, 1.5 h) in a closed vessel.…”

“…These gas phase data are in good agreement with the X-ray data published recently. [7] Data of the absorption spectrum were evaluated by TD-DFT calculations: The HOMO!LUMO excitation gave a transition at 296 nm. Additional weak transitions (HOMOÀ1!LUMO; HOMO!LUMO + 1) at 289 and 287 nm, respectively (see Table 3), and further lines at 275 and 256 nm are calculated, reflecting the different p-systems of this highly unsaturated molecule.…”

“…Hydroalumination [5] of the silane (H 5 C 6 ) 2 Si(CCÀC 6 H 5 ) 2 with equimolar quantities of HÀAl(CMe 3 ) 2 afforded a mixed alkenylalkynylsilane in which the coordinatively unsaturated Al atom approached the a-C atom of the unaffected CC triple bond. [3,[6][7][8][9] This interaction resulted in an activation of the SiÀC bond, and in refluxing toluene a slow rearrangement was observed to yield the corresponding silacyclobutene with conjugated endo-and exocyclic C=C double bonds. [3] A comparable Ge compound was obtained by dual hydroalumination of the tetraalkynylgermane Ge(CCÀC 6 H 5 ) 4 with the sterically highly shielded hydride HÀAl[CH(SiMe 3 ) 2 ] 2 (Scheme 1).…”