Monomere versus dimere Dialkylaluminiumalkinide R₂Al‐C≡C‐R′ – dreifach koordinierte Aluminiumatome gebunden an C≡C‐Dreifachbindungen

Abstract

Monomeric versus Dimeric Dialkylaluminum Alkynides R2Al‐C≡C‐R′ – Tricoordinated Aluminum Atoms Attached to C≡C Triple Bonds Addition of lithium ethynides, Li‐C≡C‐R (R = C6H5, CMe3), to cooled solutions of the chloroalane Cl‐Al[CH(SiMe3)2]2 (1) afforded the corresponding dialkylaluminum alkynides. Caused by the effective steric shielding the products (2 and 3) are monomeric in solution and in the solid state. They possess coordinatively unsaturated, tricoordinated aluminum atoms attached to C≡C triple bonds and relatively short bonds between aluminum and the alkinyl carbon atoms (<191.4 pm). Owing to quantum‐chemical calculations an interaction between the empty p‐orbital at aluminum with the π‐system of the triple bond can clearly be excluded. The sterically less shielded di(tert‐butyl)aluminum chloride yielded the dimeric aluminumalkynide 4 upon treatment with lithium tert‐butylalkynide. Lithium‐trimethylsilylethynide, Li‐C≡C‐SiMe3, and 1 gave always the alanate Li[(Me3Si‐C≡C)2Al{CH(SiMe3)2}2] (5) independently of the stoichiometric ratio of the starting compounds.

3H‐Phosphaallenes Revisited: Facile Synthesis by Hydroalumination of Alkynylphosphines and β‐Elimination, Stability and Trapping of Transient Species by Coordination to Transition Metal Atoms

“…Sterically better shielded Trip‐P(‐C≡C‐ t Bu) 2 9 reacted with H‐Al[CH(SiMe 3 ) 2 ] 2 to yield an addition product similar to 2 in the first step [ δ ( 31 P)=−55.2 ppm; δ ( 1 H)=0.12 ppm (AlCH), 6.46 ppm (C=CH), 3 J PH =37.5 Hz]. It is unstable in solution and decomposes by elimination of [(Me 3 Si) 2 CH] 2 Al−C≡C−CMe 3 and formation of Trip‐P=C=C(H)‐ t Bu ( 24 ). In an optimized procedure, 24 was obtained from the dialkynylphosphine and commercially available H‐Al i Bu 2 .…”

“…Hydroalumination of the dialkynylphosphines 8 to 16 with different dialkylaluminium hydrides and elimination of dialkylaluminium alkynides afforded the phosphaallenes 23 to 31 (Scheme ). The released alkynides were identified by NMR spectroscopy . The sterically less shielded mesityl compound 8 required the sterically encumbered hydride H‐Al[CH(SiMe 3 ) 2 ] 2 , otherwise the intermediately formed phosphaallene 23 reacted with the hydroalumination product to afford a five‐membered P 2 C 2 Al‐heterocycle analogous to 3 .…”

“…Details are given together with the procedures for the generation of the secondary products. The NMR spectra showed the resonances of the intermediate hydroalumination product, the phosphaallene 24 and [(Me 3 Si) 2 HC] 2 Al‐C≡C‐CMe 3 . Data of the phosphaallene 24 : 1 H NMR (C 6 D 6 , 300 K): δ =7.10 (s, 2 H, m ‐C H ), 5.81 (d, 3 J PH =25.7 Hz, 1 H, P=C=C H ), 4.01 (sept, 3 J HH =6.8 Hz, 4 J PH =4.8 Hz, 2 H, o ‐C H Me 2 ), 2.74 (sept, 3 J HH =6.9 Hz, 1 H, p ‐C H Me 2 ), 1.33 (d, 3 J HH =6.8 Hz, 6 H, o ‐CH Me 2 ), 1.31 (d, 3 J HH =6.7 Hz, 6 H, o ‐CH Me 2 ), 1.18 (d, 3 J HH =6.9 Hz, 6 H, p ‐CH Me 2 ), 0.98 ppm (s, 9 H, CMe 3 ); 13 C{ 1 H} NMR (C 6 D 6 , 300 K): δ =241.0 (P= C = C H), 151.8 (d, 2 J PC =7.2 Hz, o ‐C), 150.0 ( p ‐C), 131.8 ( ipso ‐C), 122.1 (d, 2 J PC =13.0 Hz, P=C=CH), 121.8 ( m ‐C), 34.7 (overlap, p‐C HMe 2 ), 33.8 (d, 3 J PC =2.1 Hz, o‐C HMe 2 ), 33.0 ( C Me 3 ), 29.9 (d, 4 J PC =17.3 Hz, C Me 3 ), 24.1 and 24.4 (overlap, o ‐CH Me 2 ), 24.0 ppm ( p ‐CH Me 2 ); 31 P{ 1 H} NMR (C 6 D 6 , 300 K): δ =56.0 ppm.…”

“…Bulky CH(SiMe 3 ) 2 groups bound to aluminium ( 2 ; Scheme ) hinder the contact between the Lewis centers and help to stabilize such Al/P compounds which represent nice examples of functionalized Al/P‐based frustrated Lewis pairs . The eliminated Al alkynides R 2 Al−C≡C−R′ (R=CH(SiMe 3 ) 2 ; R′=CMe 3 , Ph) are monomeric in these cases, and the missing dimerization energy contributes to the slow degradation of the addition products. The obtained phenyl or mesityl substituted phosphaallenes were unstable and started to decompose before the elimination reaction was finished, but were identified by NMR spectroscopy.…”

“…Bulky CH(SiMe 3 ) 2 groups bound to aluminium (2;S cheme 1) hinder the contact between the Lewis centers and help to stabilize such Al/P compounds [21] which represent nice examples of functionalized Al/P-based frustrated Lewis pairs. [23] The eliminated Al alkynides R 2 AlÀCCÀR' (R = CH(SiMe 3 ) 2 ;R ' = CMe 3 ,P h) are monomeric in these cases, [24] and the missing dimerization energy contributes to the slow degradation of the addition products.T he obtained phenylo rm esityl substituted phosphaallenes were unstable and startedt od ecompose before the elimination reactionw as finished, but were identified by NMR spectroscopy.I nother cases [20] the 3H-phosphaallenes could not be identified as intermediates.A ss hown by DFT calculations,t hey were immediatelyt rapped by the hydroalumination products which coordinated the P=C=Cgroups in atypical FLP fashion to afford five-membered AlC 2 P 2 heterocycles (3;S cheme1). [20,21] Previously the groups of Appel,M ärkl and Yoshifujii solated persistent3 H-phosphaallenes,w hich were kinetically protected by bulky tri(tert-butyl)phenyl groups.…”