Radicals with isocyanides

Reaction (7.13) shows the simplest and most popular type of hex-5-en-l-yl radical cyclization mediated by (TMS)3SiH. Thus, a 50 mmol solution of 5-hexenyl bromide and silane in the reported conditions leads to a 24 1 ratio of cyclized vs uncyclized products [6]. Similar results were obtained by replacing the bromide with isocyanide [7]. 

Tris(trimethylsilyl)silane is found to be an efficient reducing agent for a variety of functional groups. In particular, the reduction of halides, chalcogen groups, thiono esters and isocyanides are the most common ones. The efficiency of these reactions is also supported by available kinetic data. The rate constants for the reaction of (TMS)3Si radicals with a variety of organic substrates are collected in Table 2. 

Imidoyl radicals are often prepared by the addition of carbon- or heteroatom-centred radicals to isocyanides, but can also be prepared by hydrogen atom abstraction from imines. This latter method has been adopted in a new annulation approach to quinoline synthesis. Thus, reaction of the imine 1 with phenylacetylene and di-isopropyl peroxydicarbonate in benzene at 60°C gave a mixture of the quinolines 2 and 3 in 65% yield (2 3 = 4.4). 

The catalytic substitution reactions of metal carbonyl clusters, including [M3(CO)i2] (M = Fe, Ru, or Os), [Ru4H4(CO)i2], [Rh6(CO)i6], and [Co3(CO)9(/it-CCl)], with isocyanides or Group V-donor ligands may be induced by either electrochemical or chemical (benzophenone ketyl) reduction. The most favorable conditions for efficient substitution include (1) the formation of a radical anion with a significant lifetime and (2) the use of a ligand which is not reduced by [Ph2CO], and which is less of a tt acid than CO (166). 

Miyazoe et al. introduced silyltellurides as partners for group-transfer three-component couplings with isocyanides (Scheme 60) [ 170]. The reaction probably started through initial addition of silyl radicals to a carbonyl derivative, yielding ketyl radical 185, which underwent addition to phenylisocyanide (a Cl radical synthon). Final phenyltelluride group transfer to intermediate... 

The reaction of the dimers (Fp>2, Fp 2 with isocyanides in the presence of a catalytic quantity of triethylborane to give a mono substituted product is proposed to go via a radical chain process.The photoelectron spectra of the n -acetylides CpFe(CO>2C=CR R = H, Bu and Ph have been analysed and show significant interaction of the ir-orbitals with the metal dx orbitals based on the splitting of metal-based ionization bands, shifts in Cp-based and acetylide based ionizations, changes in ionization cross sections (Hel/Hell) and the vibrational fine structure in the metal based ionizations. The 19e complex [CpFtffCsMee) has been used as an electron reservoir in the multiple electron reduction of Buckminster fullerene. ... 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

A small library of highly functionalized pyrrolines 95 was synthesized by reaction of allylic and propargylic isocyanides 94 with thiols followed by radical cyclization (Scheme 33). The radical reaction was carried out using a radical initiator (AIBN) under flash heating microwave irradiation [67]. 

Substitution at the SiH moiety has been carried out with alkylthio groups, such as MeS and i-PrS. Tn s(alkylthio)silanes, (RSlsSiH, are radical-based reducing agents which can effect the reduction of bromides, iodides, xanthates, phenylselenides, and isocyanides in toluene, using AIBN as the initiator at 85... 

It has been shown by Zanardi, Nanni and coworkers that, instead of isocyanides, isothiocyanates can be used in the radical process [90]. Treatment of the mixture of the diazonium tetrafluoroborates 3-217 and the isothiocyanates 3-218 with pyridine or a mixture of 18-crown-6 and potassium acetate in ethyl acetate, furnished the benzothienoquinoline derivatives 3-219. In addition, in some cases the rearranged products 3-220 were furnished in low to reasonable yields (Scheme 3.57). 

Polymerization of isocyanides to polyiminomethylenes is best achieved with cationic initiators, although anionic and radical polymerizations also occur [Millich, 1988]. 

Trifluoromethyl isocyanide traps the bis(trifluoromethyI)nitroxyl radical at the terminal carbon atom with the formation of an intermediate radical that decomposes spontaneously to give trifluoromethyl isocyanate as the end product (Table 14).238... 

---