Two Different Structural Motifs Observed for Dimeric Dialkylaluminum and Dialkylgallium Alkynides [R₂E‐C≡C‐C₆H₅]₂

Abstract

The dialkylaluminum and dialkylgallium alkynides [R2E‐C≡C‐R′]2 (R = Me, CMe3; E = Al, Ga; R′ = Ph) containing C≡C triple bonds attached to their central aluminum or gallium atoms are easily obtained by the reactions of dialkylelement chlorides with lithium alkynides or by treatment of the corresponding alkyne R‐C≡C‐H with dialkylaluminum or dialkylgallium hydrides. The first reaction is favored by the precipitation of LiCl, the second one by the formation of elemental hydrogen. All products form dimers in which the carbanionic carbon atoms of the alkynido groups adopt bridging positions, but, interestingly, different types of molecular structures were observed depending on the steric demand of the substituents terminally attached to the aluminum or gallium atoms. The small methyl substituents gave structures in which the aluminum or gallium atoms seemed to be side‐on coordinated by the C≡C triple bonds of almost linear E‐C≡C groups. In contrast, the more bulky tert‐butyl groups forced an arrangement in which the C≡C triple bonds were perpendicular to the E‐E axis of the molecules. Different bonding modes result, which were analyzed by quantum‐chemical calculations.

Hydroalumination of diazo and azido compounds: An Al-H addition to end-on nitrogen of substrates

“…In the 1 H NMR spectra of 2-4, the NH proton resonates at high field for NH(1-Ad) Reactions of aluminum hydrides with terminal alkynes have been known to occur through deprotonation due to the acidic acetylenic hydrogen [1, 2,31,32] or alternatively through the AlH addition to the CC triple bond [1,33]. The initial deprotonation followed by multiple Al-H addition was also observed to form the carbaalane clusters.…”

Treatment of Terminal Alkynes R–C≡C–H with Dialkylaluminum Hydrides: Hydroalumination versus Deprotonation

“…In contrast, gas evolution was observed when phenylethyne, H-CϵC-C 6 H 5 , was treated with less shielded di(tert-butyl)-and dimethylaluminum hydrides. [9] By hydrogen elimination dialkyl-alkynylaluminum compounds were formed in very selective reactions without any detectable by-product. In contrast, gas evolution did not occur when the less acidic tert-butylethyne was allowed to react with various dialkylaluminum hydrides [products 1 to 5, R = Me to tBu; Equation (1)].…”

“…Dialkylgallium hydrides gave similar alkynyl compounds also with other, less acidic alkynes. [2,9] In contrast, diisobutylaluminum hydride was reported many years ago to give hydroalumination upon treatment with different aliphatic alkynes. [10] However, the characterization of the products was rather limited.…”

“…The sterically highly shielded hydride H-Al[CH(SiMe 3 ) 2 ] 2 gave exclusively hydroalumination (Scheme 2), [7] while hydrogen release with the formation of dimeric alkynylaluminum compounds occurred upon treatment of the relatively acidic phenylethyne with smaller dialkylaluminum hydrides (Scheme 2). [9] These alkynyl derivatives adopted dimeric structures in the solid state with two different structural motifs. One had linear Al-CϵC groups, which showed a side-on interaction with a second aluminum atom via a π-orbital localized at the α-carbon atom.…”

Ferrocenyl(dimethyl)aluminum dimer [(η5-C5H5)Fe(η5-C5H4)AlMe2]2

“…The present data for 3a extends the range of bridging Al-C(ferrocenyl) bond lengths from 202 pm [9a] to 227 pm (3a). For a bridging C"C-Ph group, where the aluminum can be considered as partly side-on coordinated to the C"C bond, the Al-C bond lengths also differ greatly (199.4(2) pm and 222.4(2) pm) [15].…”