Methanol alkyne elimination

Disubstituted alkynes and terminal alkynes form E-dibromoalkenes [4] when the tribromide is formed in situ in an essentially basic medium, an addition reaction followed by elimination of hydrogen bromide results in the conversion of terminal alkynes into the 1-bromoalkynes [5]. When the addition reaction is conducted in methanol, l,l-dibromo-2,2-dimethoxyalkanes are produced, in addition to the 1,2-dibromoalkenes [6], The dimethoxy compounds probably result from the initial intermediate formation of bromomethoxyalkenes. Under similar conditions, alkenes yield methoxy bromo compounds [7]. 

In 1991, we reported that a nucleophilic vinylic substitution of (E)-fi-alkylvinyl-AModanes with halides (BuN4X, X = C1, Br, I) in dichloromethane, methanol, or acetonitrile at room temperature proceeds with exclusive inversion of configuration [Eq. (100)] [176,177]. This is the first clear example of a vinylic Sn2 reaction. This reaction competes with an alkyne-forming reductive syn /3-elimination. 

The cyclic oxo esters are very much a feature of osmium(VI) chemistry but analogous rings are rare with other elements they have been detected for ruthenium(VI) and manganese(V), but only for chromium(V) has any substantial oxo ester chemistry been uncovered. The osmium species are formed by two main routes (a) by reaction of 0s04 with alkenes, dienes or alkynes, in which case two electrons are transferred from the multiple bond to osmium(VIII) which is thus reduced to osmium(VI), or (b) by reaction of m-glycols with fra/w-[0s02(02 R)2] (R = H, Me) with elimination of water or methanol (see p. 582). 

Concurrent with this disclosure, Rossi reports a similar protocol wherein 2-acyl-l-(phenylsulfonyl)-3-iodoindoles 189 undergo room temperature palladium-catalyzed coupling with terminal alkynes to afford the corresponding 3-alkynylindoles 190 <01S2477>. Subsequent treatment with ammonia in methanol leads to formation of the p-carbolines 191 through sequential addition/elimination/cycloamination reactions accompanied by loss of the A -phenylsulfonyl group. 

The work of Garranti and Zecchi provides additional examples in which cyclization was shown to occur with alkynes and alkenes. They have, however, identified further limitations regarding chain length and hence ring size of final products. Pocar and co-workers have illustrated the intermolecular version of this reaction by use of nitrile oxides and tran5-1,3-diaminopropenes. A base is necessary for reaction to occur, and of several tried, triethylamine proved superior. The exo-methylene isoxazolines (363) were obtained after elimination of one of the amino-functions. When sodium methoxide in methanol was used, however, the reverse elimination occurred to give the isoxazole (362) (Scheme 52). 

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