Aluminum reaction with alkynes

Reaction with Alkynes. Treatment of a dichloromethane solution of the sulfonimidoyl chloride (1) with a Lewis acid in the presence of an alk)nie affords 2,1-benzothiazines in good-to-excellent yields with high Markownikoff regioselectivity (eq 2) Finely powdered aluminum chloride is the Lewis acid of choice for this reaction, though titanium chloride works reasonably well. 

Flynn et al., also described the synthesis of the fused indoles [73]. The o-iodotrifluoroacetanilide 110 was coupled to aryl alkyne 111 under Sono-gashira conditions followed by subsequent reaction with aryl iodide, 107 with gaseous carbon dioxide produced the fused indole 158. Lewis acid dealkylation with aluminum trichloride produced the deprotected alcohol 159. 

The reaction of alkynes with AIX3 at —78 °C has been shown, by NMR spectroscopy, to generate a zwitterionic cr-cyclobutadiene aluminum species 225 (Scheme 58)211a. Transfer of the cyclobutadiene ligand from 225 to a variety of transition metals has been reported211. 

The conversion of an alkyne to a trans-alkene can be accomplished by heating with lithium aluminum hydride (LAH), by reaction with lithium in liquid ammonia (Li, NH3). Thus all of these reagents (H2/P-2 Ni, LAH, and Li, NH3) are reducing agents for alkynes and give alkenes as the reduced products. 

Several of the trialkylaluminum and alkylaluminum halides and hydrides mentioned above are commercially available. Alkynyl, alkenyl, cyclopentadienyl, and aryl derivatives are, in general, not commercially available and must be synthesized for laboratory use. Alkynyl derivatives can be prepared by salt metathesis, as in the reaction of Et2AlCl with NaC=CEt to give Et2AlC=CEt. The acidity of terminal alkynes is sufficient for preparation of alkynyl aluminum compounds by alkane or hydrogen elimination upon reaction with a trialkylaluminum or an aluminum hydride (equation 17), respectively. TriaUcynyl aluminum compounds are typically isolated as Lewis base adducts to stabilize them against otherwise facile polymerization. Alkenyl compounds of aluminnm have similarly been prepared. 

Hydroalumination. The treatment of alkynes with diisobutylaluminum hydride in hydrocarbon solvents results in a aT-addition of the Al-H bond to the triple bond to produce stereodefined alkenylalanes. The hydroalumination of alkynes is more limited in scope than the corresponding hydroboration reaction of alkynes (see Chapter 5) with regard to accommodation of functional groups and regioselectivity. Whereas hydroalumination of 1-alkynes is highly regioselective, placing the aluminum at the terminal position of the triple bond, unsymmetrically substituted alkynes produce mixtures of isomeric alkenylalanes. 

Reaction of Alkynes with Titanium-Aluminum-Alkyl Complexes. . . 270... 

In another interesting variant of this process, disilacyclopentenes (e.g., (57)) were prepared by the reaction of diisobutylaluminum hydride with alkynes such as (56) (Scheme 8) <70ZAAC(377)48>. In this case, cu-addition of A1—H to the alkyne triple bond results in an increased reactivity of the —CH2Br group to the extent that spontaneous ring closure takes place in solvents of weak Lewis basicity (ether or dioxane) to afford the corresponding disilacyclopentene after hydrolytic loss of aluminum. 

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