Mechanism Of Gattermann Synthesis Essay

Zinc Cyanide1

[557-21-1]  · C2N2Zn  · Zinc Cyanide  · (MW 117.43)

(mild Lewis acid used as an alternative to HCN in Gattermann formylation reactions, in the synthesis of -cyanoenamines, and in the cyanosilylation of carbonyl compounds)

Physical Data: mp 800 °C (dec).

Solubility: practically insol water, alcohols; sol aq solutions of alkali cyanides.

Form Supplied in: white crystalline powder, with alkali metal chlorides as possible impurities.

Gattermann Reactions.

In the classical Gattermann aldehyde synthesis, hydrogen cyanide and Hydrogen Chloride are used together with a Lewis acid such as Aluminum Chloride or Zinc Chloride. In the Adams modification,3 zinc cyanide is used together with hydrogen chloride as a more convenient reagent system, effectively generating hydrogen cyanide and zinc chloride. Any phenol that can be formylated using the classical Gattermann reaction conditions can be converted equally well into a hydroxy aldehyde by the Adams method. Pure zinc cyanide is said to be ineffective but may be activated by the addition of potassium chloride.4

Although the mechanism of the Gattermann reaction has not been investigated in detail, it is known that an initial nitrogen-containing product is hydrolyzed to the aldehyde (eq 1). It has been suggested that the electrophile may be a protonated dimer of HCN.5 In some cases the hydrocyanic acidium ion may be involved; in others, a Lewis acid complex could be formed between hydrogen cyanide and, for example, zinc chloride or aluminum chloride. In this connection, a very high acidity is required for reaction to occur with benzene; evidence has been presented in favor of a diprotonated HCN in that case.6 In reactions involving the system Zn(CN)2-HCl-AlCl3, a relative rate for the formylation of toluene versus benzene (KT/KB) of 128 was obtained.7

Early examples showed that both - and -naphthol could be formylated in good yields, in the absence of a Lewis acid other than the zinc chloride, as indicated (eq 2).3a Anisole (eq 3) and resorcinol dimethyl ether are converted in good yields into the expected aldehydes in the presence of added aluminum chloride.3b Another improvement in the experimental procedures involves the use of tetrachloroethane as the solvent; this method has been used in the formylation of hindered systems such as 1,3,5-triisopropylbenzene.8 A detailed and checked method for the preparation of mesitaldehyde is available.9 The first stage in the synthesis of a number of fungal metabolites involves the formylation of orcinol at the 4-position.10 Another recent example is shown in eq 4,11 and others


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