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An isocyanide (also called isonitrile or carbylamine) is an organic compound with the functional group –. It is the isomer of the related nitrile (–C≡N), hence the prefix is isocyano.IUPAC Goldbook isocyanides The organic fragment is connected to the isocyanide group through the nitrogen atom, not via the carbon. They are used as building blocks for the synthesis of other compounds.
Akin to carbon monoxide, isocyanides are described by two resonance structures, one with a triple bond between the nitrogen and the carbon and one with a double bond between. The π lone pair of the nitrogen stabilizes the structure and is responsible of the linearity of isocyanides, although the reactivity of isocyanides reflects some carbene character, at least in a formal sense. Thus, both resonance structures are useful representations. They are susceptible to polymerization.
The electronic symmetry about the isocyanide 14N nucleus results in a slow quadrupolar relaxation so that 13C-14N nuclear spin coupling can be observed, with coupling constants of ca. 5 Hz for the isocyanide 13C nucleus and 5–14 Hz for the 13C nucleus which the isocyanide group is attached to.
Ivar Karl Ugi states that "The development of the chemistry of isocyanides has probably suffered only little delay through the characteristic odor of volatile isonitriles, which has been described by Hofmann and Gautier as 'highly specific, almost overpowering', 'horrible', and 'extremely distressing'. It is true that many potential workers in this field have been turned away by the odour, but this is heavily outweighed by the fact that isonitriles can be detected even in traces, and that most of the routes leading to the formation of isonitriles were discovered through the odor of these compounds." Isocyanides have been investigated as potential non-lethal weapons.
Some isocyanides convey less offensive odours such as malt, natural rubber, creosote, cherry or old wood. Non-volatile derivatives such as tosylmethyl isocyanide do not have an odor.B. E. Hoogenboom, O. H. Oldenziel, and A. M. van Leusen "Toluenesulfonylmethyl isocyanide" Organic Syntheses, Coll. Vol. 6, p.987 (1988).
In some cases, a phosphonite ester-amide can desulfurize to isocyanides.
Isocyanides are stable to strong base (they are often made under strongly basic conditions), but they are sensitive to acid. In the presence of aqueous acid, isocyanides hydrolyse to the corresponding formamides:
Isocyanides participate in many multicomponent reactions of interest in organic synthesis, two of which are: the Ugi reaction and the Passerini reaction.
Isocyanides also participate in cycloaddition reactions, such as the 4+1 cycloaddition with tetrazines. Depending on the degree of substitution of the isocyanide, this reaction converts isocyanides into or gives stable cycloadducts. They also undergo insertion into the C–Cl bonds of acyl chlorides in the Nef isocyanide reaction, a process that is believed to be concerted and illustrates their carbene character.
Isocyanides have also been shown to be a useful reagent in palladium catalysed reactions with a wide variety of compounds being formed using this method.
Much like nitriles, isocyanides are electron-withdrawing and easily deprotonate at the α position. For example, benzyl isocyanide has a p Ka of 27.4 and benzyl cyanide has a p Ka of 21.9, but toluene has a p Ka in the 40s. In the gas phase, is 1.8 kcal/mol less acidic than .
Chlorination of isocyanides gives isocyanide dichlorides.
The sometimes used old term "carbylamine" conflicts with systematic nomenclature. An amine always has three single bonds, IUPAC Nomenclature of Organic Compounds (Recommendations 1993) whereas an isocyanide has only one single and one multiple bond.
The Isodiazomethane functional group consists of an amino group attached to an isocyano moiety. The isonitrile suffix or isocyano- prefix is used depending upon priority table.
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