A truly green synthesis of α-aminonitriles via Strecker reaction

Strecker reaction [1], the oldest known synthesis of α-aminonitriles, is one of the most general methods potentially useful for syntheses of amino acids and other bioactive compounds including natural products. In addition, the Strecker reaction represents one of the simplest and most economical methods for the preparation of α-amino acids for both laboratory and industrial scales [2]. Since 1850, a number of publications have appeared on this reaction. Still this reaction is under active investigation. Recently, synthesis of hepatitis C virus NS3 serine protease inhibitors [3], (±)-phthalascidin 622 [4] and novel boron-containing retinoids [5] have been reported following this strategy. A number of new catalysts have also been reported for this reaction which includes mesoporous aluminosilicate (Al-MCM-41) [6], lanthanum(III)-binaphthyl disulfonate [7], nanocrystalline magnesium oxide [8], BINOL-phosphoric acid [9, 10], Fe(Cp)₂PF₆ [11], Jacobsen's thiourea catalyst [12], N-heterocyclic carbene (NHC)-amidate palladium(II) complex [13], Yb(OTf)₃-pybox [14], K₂PdCl₄ [15], gallium (III) triflate [16], bisformamides [17], IBX/TBAB [18], Lewis base e. g. N,N-dimethylcyclohexylamine [19], superparamagnetic iron oxide [20], and ionic liquid [21]. To prepare α-aminonitriles (precursor to α-amino acids) generally an imine is reacted with a cyanide source. Notable among them are HCN [22], KCN [23], (EtO)₂P(O)CN [24, 25], Et₂AlCN [26, 27], Bu₃SnCN [28, 29], and TMSCN [3, 4, 6–20]. Among these cyanide sources, trimethylsilyl cyanide (TMSCN) is relatively easy to handle and highly soluble in organic solvents. In contrast, many of these reported methods involve the use of expensive reagents, hazardous solvents, longer reaction times and tedious workup procedure. Therefore, it is desirable to develop an efficient and practical method for the Strecker reaction under eco-friendly conditions.

A series of α-aminonitriles were synthesized by using diverse amines, aldehydes and TMSCN in the presence of indium metal (10 mol%) as catalyst in water. As shown in Table 4, the reaction proceeded equally well irrespective of the nature of the carbonyl compounds (aliphatic, aromatic, heteroaromatic) or amines (aliphatic, heterocyclic, and aromatic) to afford the corresponding products in excellent yield (79-98%). The catalytic system worked well with acid sensitive heteroaromatic aldehyde (entries 4, 6, 7), α, β unsaturated aldehyde (entry 3), aliphatic aldehyde (entry 5) and ketone (entry 10). Aromatic primary amine (aniline), benzyl amine (entry 6), heterocyclic amines (entries 7, 8 and 9) could effectively undergo Strecker reaction with aldehydes and TMSCN to give the corresponding products in excellent yields (94-97%). For aliphatic amines such as benzyl amine, piperidine and morpholine relatively slower reaction rate was observed.

A plausible mechanism may follow a two-step pathway. In the first step, indium acts as an Lewis acid to facilitate formation of the corresponding imine from the condensation of the amine and aldehyde. In the subsequent step, the imine is further activated due to the presence of indium, to form a more electrophilic C = N intermediate. As a result, an attack of TMSCN to the imine carbon can take place and thus the corresponding α-aminonitriles is formed via hydrolysis in water.

There is growing interest in the one-pot Strecker synthesis of α-aminonitriles from carbonyl compounds, amines and TMSCN, because of the significant importance of α-aminonitriles in preparing a wide variety of amino acids, amides, diamines, and nitrogen containing heterocycles. In summary, we have developed a rapid, convenient and efficient one-pot, three-component environmentally benign Strecker reaction using indium as catalyst at room temperature. A series of α-aminonitriles were obtained in excellent yields. This reaction will be applicable to the synthesis of various organic compounds of medicinal interests.