Alkynes reactions with ketyls

A ketone or aldehyde is reduced to an alcohol by reaction with sodium or lithium metal in liquid ammonia, in the presence of ethanol. This is called a dissolving metal reduction and it proceeds by an alkoxy-radical known as a ketyl 21, 22, 46, 49. Alkynes are reduced to E-alkenes under dissolving metal conditions. Benzene is reduced to a cyclohexadiene under the same conditions 23, 28, 33, 37, 41, 48. 

Kariv-Miller and coworkers have developed indirect electroreductive cyclizations with the dimethyl-pyrrolidinium ion (DMP") as a mediator. Preparative electrolysis of 6-hepten-2-one (9) at a graphite cathode afforded cu-dimethylcyclopentanol (10) in 90% yield (equation 5). The reduction is believed to occur via the ketyl radical anion, which cyclizes onto the alkenic bond. In the absence of DMP simple reduction to 6-hepten-2-ol takes place.Very recently it was shown that instead of DMP several aromatic hydrocarbons can be used as mediators to initiate the cyclization reaction. The carbonyl group can also be cyclized onto an alkynic bond and even an aromatic ring. - ... 

One of the most frequently eneountered reactions is that with proton sources, as observed with arenes (Birch reduction) [189], aldehydes [ 190], alkynes [2d], fullerenes [191], ketones [192] (even enantioselective protonation of ketyl radical anions [193]), nitriles [194], nitro [195] and nitroso compounds [196], and olefins [197]. Protons are often replaced as electrophiles by trialkylsilyl chloride [198],... 

Several examples have appeared of diversion of the normal photochemistry of ortho-alkyl aromatic ketones due to apparent interception of the intermediate biradical by an unsaturated group present elsewhere in the molecule.Thus the vinyl aryl ketones (376) are converted to (377) in reasonable yields upon irradiation with ultra-violet light,while irradiation of the ortho-alkynyl acetophenone (378) in methanol yields diastereomers of (379)The latter reaction is thought to proceed by coupling of the initially formed ketyl radical onto the alkyne to give a carbene (380). The photochemistry of the jS-diketone (381) gives a mixture of the benzocyclobutenol (382) and the tetralone (383), and the proportions depend on the identity of the R-substituent in (381). The formation of (383) can be rationalised in terms of interception of the ketyl radical in the initially formed 1,4-biradical by the remote carbonyl group. 

By far the most useful and highly utilized radical reactions promoted by Sml2 are couplings between ketyls and alkenes or alkynes. The selectivity that can be achieved in these transformations, combined with the versatility of the method for the construction of diverse targets, makes it an extremely attractive means to form carbon-carbon bonds. 

Toward this end, Caryn Kenny set out to investigate the scope and limitations of the process. Indeed, the reaction turned out to be quite versatile (Eqs. 4 and 5). Both p-keto esters and p-keto amides could be used as substrates. Unactivated alkenes, electron deficient alkenes, and alkynes all served as suitable ketyl acceptors under Sm(II)-promoted conditions. As anticipated, high diastereoselectivity over three stereocenters could be achieved. However, although five-membered rings could be formed with exceptional ease, in these systems virtually all attempts at forming six-membered rings met with disappointment. 

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