Aluminum alkynides

The carbaalanes [8, 9] possess clusters formed by aluminum and carbon atoms. They represent a new class of compounds which, in some respects, may be compared to the important class of carbaboranes. Usually, they were obtained by the reaction of aluminum alkynides with aluminum hydrides (hydroalumination) and the release of trialkylaluminum derivatives (condensation). The first carbaalane, (AlMe)g(CCH2Ph)5H 3 [10], was synthesized by the treatment of dimethylalumi-num phenylethynide with neat dimethylaluminum hydride. The idealized stoichiometric ratio of the components is given in Eq. (2), which also shows a schematic drawing of the molecular structure. Compound 3 was isolated in the form of colorless crystals in 60% yield. While 3 is only slightly air-sensitive, the less sterically shielded propynide derivative 4, also shown in Eq. (2), is highly pyrophoric [11],... 

The low acidity of 1-alkynes means that strong bases must be used to form the alkynide ions and that water is not a suitable solvent aqueous solutions have a very low concentration of alkynide ions. Some transition metal alkynides can be prepared by precipitation from aqueous solution because their solubilities are very low. Suitable solvents for the preparation of alkynide ions must be less acidic than the alkyne, and preferably allow the alkyne and the alkynide ion to remain in solution. Liquid ammonia, te-trahydrofuran, ether and hydrocarbons have all been used, particularly the first, the alkynide anion being readily formed by metal amides. Alkynides of many types have been prepared from various metals. Besides Groups I and III, copper(I), silver, gold(I), zinc, mercury and, more recently, aluminum alkynides have been synthesized. The alkynides of Groups I and II have been principally used as nucleophiles in alkylation reactions, but there are now many examples of other metal alkynides in this role. Palladium-catalyzed reactions, as remarked above, have become increasingly important for the reactions of alkynides of metals other than Groups I and II, but these have not usually involved alkylation. 

One of the characteristic features of this approach is the successful fert-alkyl-al-kynyl coupling with dialkylaluminum alkynides which enables the introduction of a quaternary carbon in a position adjacent to an alkynyl group. Such transformation was previously achieved by the cross-coupling of ferf-alkyl chlorides with trialkynyl-aluminums as already described in this section [92]. The reaction of 98 with dimethyl-aluminum phenylacetylide (1.5 equiv.), readily prepared from lithium phenylacetylide and Me2AlCl, in toluene at -78 °C for 30 min resulted in formation of a cross-coupling product in 70 % yield. This result indicates the efficient and selective transfer of the alkynyl group from the aluminum center in dialkylaluminum alkynides as depicted in Sch. 65. 

Treatment of lithium alkynides with aluminum trichloride leads to tri(ethynyl)aluminum intermediates, which on treatment with haloalkanes give the corresponding disubstituted acetylenes. " The reaction is successful with tertiary haloalkanes and a variety of alkynes have been prepared (Scheme 9). An interesting variation of this method has been reported by Trost and Ghadri who reacted a diethylethynylalumin-um with an allyl sulfone in the the presence of the Lewis acid aluminum trichloride. When the allyl sulfone was part of a six-membered ring, then the alkyne was introduced exclusively in a pseudo axial orientation (Scheme 10). 

EpoxycycIopentene (4) undergoes nucelophilic addition with the ethylaluminium alkynide (5) in a mixture of THF, toluene and hexane to give predominantly the isomer (6) in which the alkyne has been introduced at carbon 3, adjacent to the double bond (Scheme 18). When the reaction was run in toluene at low temperature, the unexpected 4-hydroxy-4-(l-hexynyl)cyclopentene (7) was obtained, which the authors suggest may arise from an aluminum -catalyzed rearrangement of the epoxide to cyclopent-3-enone, which then undergoes nucleophilic addition. 

---