Isocyanide alkylation

The related mono-A/-alkylated carboranes, 7-(NH2R)-7-CB2qH22, can be prepared by treatment of decaborane(14) with alkyl isocyanides (125). 

Van Leusen and Possel described the use of mono-substituted tosylmethyl isocyanides (TosCHRN=C R = alkyl, benzyl, allyl) in the synthesis of 4,5-substituted oxazoles. For example, 4-ethyl-5-phenyloxazole (8) was prepared in 82% yield by refluxing a-tosylpropyl isocyanide (7) and benzaldehyde for 1 hr with 1.5 equivalent of K2CO3 in MeOH. 

Ji-Toldaldehyde, 2-bromo-, 46,13 />-ToJuenesult onamide, alkylation with o-xylylene dibromide, 47, 111 -Toluenesulfonvlbromide, 45, 88 -Toluenesulfonyl chloride m prepara tion of methvl isocyanide, 46,... 

ISONITRILES. This designation includes isocyanides (see Vol 3, C582-L to C583-L and Vol 7, I135-R for Cyanicic Acid and-Derivatives) and Carbylamines, R.NiC. The first isonitriles were prepd in 1886 by Gautier, by the action of Ag cyanide on alkyl iodides. Later, Hofmann prepd them by the action of chlf ale KOH on primary amines ... 

An attractive synthesis of cyclobutanone (253) has been recently described using 1-isocyano-l-tosylcyclobutane(252)(equation 151)144. l-Isocyano-l-tosylcyclobutanes252 can be prepared from (tosylmethyl)isocyanide and alkyl-substituted 1,3-dibromobutanes. This method appears to be superior to previously reported methods for the preparation of cyclobutanone because of high purity and high yields. 

When the reagent is the thiocyanate ion, S-alkylation is an important side reaction (10-43), but the cyanate ion practically always gives exclusive N-alkylation. ° Primary alkyl halides have been converted to isocyanates by treatment with sodium nitrocyanamide (NaNCNN02) and m-chloroperoxybenzoic acid, followed by heating of the initially produced RN(N02)CN. ° When alkyl halides are treated with NCO in the presence of ethanol, carbamates can be prepared directly (see 16-7). ° Acyl halides give the corresponding acyl isocyanates and isothiocyanates. For the formation of isocyanides, see 10-111. 

A). The alkyl complexes react rapidly with CO and isocyanides, but pure products could not be isolated. Fast reactions of CO2 and acetone with Sc(OEP)Me gave the well-characterized acetate and /-butoxide products. Sc(OEP)OAc and Sc(0EP)0-r-Bu, respectively, formed by insertion into the Sc—C bond. ... 

The valence-bond picture does not adequately express a difference between various isocyanides. There are interesting differences between alkyl and aryl isocyanides, discussed below. 

One additional point should be discussed here, concerning the substantial emphasis that has been placed on the differences between alkyl and aryl isocyanides. It has been suggested, primarily on the basis of infrared evidence, that aryl isocyanides are better 7r-acceptors than alkyl isocyanides (90). Qualitatively this difference is easily rationalized. One can see that delocalization of charge into 7r -orbitals on an aryl ring in aryl isocyanide-metal complexes should be possible, whereas no such possibility exists for alkyl isQcyanide-metal complexes this means that aryl isocyanides should be better ir-acceptors. Of course, the simple qualitative model gives one no measure of the relative importance of this effect. 

More important than this, however, is the fact that until recently there have been no substantial differences observed in the chemistry of metal complexes of alkyl and aryl isocyanides. In general, the choice of which isocyanide to use seems to be made largely at random, dictated perhaps by convenience as much as any other factor. Lacking any substantial chemical differences between the two groups of complexes, one might wish to minimize, rather than emphasize, this comparison. However, several observations have recently been made which seem to substantiate the earlier conclusion. 

Still, the ease of oxidation is obviously a property of considerable chemical importance. These differences require that the choice of aryl vs. alkyl isocyanides not be taken lightly. 

Substantially more work has been done on reactions of square-planar nickel, palladium, and platinum alkyl and aryl complexes with isocyanides. A communication by Otsuka et al. (108) described the initial work in this area. These workers carried out oxidative addition reactions with Ni(CNBu )4 and with [Pd(CNBu )2] (. In a reaction of the latter compound with methyl iodide the complex, Pd(CNBu )2(CH3)I, stable as a solid but unstable in solution, was obtained. This complex when dissolved in toluene proceeds through an intermediate believed to be dimeric, which then reacts with an additional ligand L (CNBu or PPh3) to give PdL(CNBu )- C(CH3)=NBu I [Eq. (7)]. 

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