Isocyanide-cyanide rearrangement

Preparation of cyanides.1 An attractive route to cyanides involves as the first step dehydration of alkylformamides with phosgene-tricthylamine (1,857) or phosphoryl chloride-diisopropylamine (13,249) to form isocyanidcs followed by isocyanidc-cyanide rearrangement. This rearrangement traditionally was conducted in the gas phase, but proceeds in almost quantitative yield when carried out by flash pyrolysis at 600°. This route to cyanides is attractive because allyl isocyanides rearrange without allylic rearrangement. Moreover, optically active carboxylic acids can be obtained from optically active amines without raccmization. 

On the basis of theoretical studies, it has been suggested that alkyl diazonium ions should undergo a rearrangement similar to the cyanide-isocyanide interconversion this reaction, which could only be detected by shift of labelled nitrogen, has not been observed. Reaction of a diazonium ion proceeds usually by one of four routes. 

Let us consider the rearrangement of hydrogen cyanide to hydrogen isocyanide ... 

Imidates, rearrangement of, 14, 1 Imines, additions of allyl, allenyl, propargyl stannanes, 64, 1 additions of cyanide, 70, 1 as dienophiles, 65, 2 synthesis, 70, 1 Iminium ions, 39, 2 65, 2 Imino Diels-Alder reactions, 65, 2 Indoles, by Nenitzescu reaction, 20, 3 by reaction with TosMIC, 57, 3 Ionic hydrogenation, 71, 1 Isocyanides, in the Passerini reaction, 65, 1... 

Both cyclic and acyclic ketoximes may be used in this transformation and the reaction is usually performed in an alcohol solution containing equimolar quantities of alkoxide. For a successful reaction, the starting material usually contains at least an a-methylene group but the presence of only one a-hydrogen may suffice. When treated with base the 0-acylated aldoximes do not react via the Neber rearrangement and instead they undergo an E2 elimination to cyanides or isocyanides. 

Neutral and cationic homoleptic (see Homoleptic Compound) tnngsten-isocyanide complexes W(CNR)6 and [W(CNR)y] + (R = alkyl, aryl) are known, and W(CO)e (CNR) (n=l-3) complexes may be prepared from W(CO)6 and excess isocyanide in the presence of catalytic amounts of C0CI2 or PdO. Iso-cyanides are isoelecfronic with CO and also insert into the W-R bonds of alkyl complexes (see Alkyl Complexes). For instance, the alkyl-nitrosyl complexes Cp W(NO)(X)(CH2Bfr) (X = NHBfr, OBfr) react with CNBu to afford -iminoacyl complexes, and the isocyanide complexes, Cp W(CO)2(Me)(CNR) (10, R = alkyl), rearrange to afford either -iminoacyl Cp W(CO)2( -MeCNR)(ll) or ij -l-azaallyl Cp W(CO)2( -CH2CHNR) (12) derivatives, depending upon the reaction conditions (equation 4). ... 

Potential surfaces for the isocyanide-cyanide rearrangement have been constructed from extended Hiickel calculations and theory predicts a small amount of charge separation in the transition state . The data on methyl and ethyl iso-... 

There are no restrictions with regard to the stmcture of the isocyanide (5), as long as it is a C-iso-cyanide. " An acid HX is only suitable as the acid component of the 4CC if it is not irreversibly converted into an a-aminoalkylation product, as happens with the cyanide and the thiolate ions." Furthermore, the acid component HX must yield an a-adduct (41) that is capable of rearranging into a stable product (42). 

Chapters on the rearrangements of cyanides, diazo and diazonium compounds, and acetylenes have appeared in books in this series in 1970 and 1978. Chapters on arynes(1983) and isocyanides (1983) have also appeared. This chapter aims to update these earlier works to the end of 1992. 

The isocyanide-cyanide (isonitrile-nitrile) rearrangement was discovered in 1873 by Weith and is best described as a cationotropic 1,2-shift. 

A high-yield access to cyanides from isocyanides is given by the isocyanide-cyanide rearrangement [1215, 1216], Isocyanides are converted to cyanides by flash thermolysis in excellent yields (often near 100%) under strict retention of conflgu-ration. The employed isocyanides were prepared by standard procedures from the corresponding formamides by the use of phosphoryl chloride. A 1-homocubyl isocyanide was synthesized in 51% yield [1216],... 

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