Amide formation nitrile hydrolysis

There are two pathways for the degradation of nitriles (a) direct formation of carboxylic acids by the activity of a nitrilase, for example, in Bacillus sp. strain OxB-1 and P. syringae B728a (b) hydration to amides followed by hydrolysis, for example, in P. chlororaphis (Oinuma et al. 2003). The monomer acrylonitrile occurs in wastewater from the production of polyacrylonitrile (PAN), and is hydrolyzed by bacteria to acrylate by the combined activity of a nitrilase (hydratase) and an amidase. Acrylate is then degraded by hydration to either lactate or P-hydroxypropionate. The nitrilase or amidase is also capable of hydrolyzing the nitrile group in a number of other nitriles (Robertson et al. 2004) including PAN (Tauber et al. 2000). 

Formally related reactions are observed when anthracene [210] or arylole-fines [211-213] are reduced in the presence of carboxylic acid derivatives such as anhydrides, esters, amides, or nitriles. Under these conditions, mono- or diacylated compounds are obtained. It is interesting to note that the yield of acylated products largely depends on the counterion of the reduced hydrocarbon species. It is especially high when lithium is used, which is supposed to prevent hydrodimerization of the carboxylic acid by ion-pair formation. In contrast to alkylation, acylation is assumed to prefer an Sn2 mechanism. However, it is not clear if the radical anion or the dianion are the reactive species. The addition of nitriles is usually followed by hydrolysis of the resulting ketimines [211-213]. 

To aqcuire more insight in amide formation we undertook to study the hydrolysis of enantiomerically pure cyanohydrins. Nitrile le racemises too readily but the stereochemical integrity of (R)- and (S)-la could be maintained with careful adjustment of the reaction conditions (pH 6, 0°C). Thus, enantiomerically pure (R)- and (S)-la were subjected to hydrolysis in the presence of PfNLase (see Figure 16.9) [5]. It became clear that only 11% of amide was formed from the (R)-enan-tiomer, whereas the (S)-enantiomer was hydrolyzed into 55% amide and 45% acid under otherwise identical conditions (see Figure 16.9). Stereochemical integrity was fully maintained under the reaction conditions and 3a and 4a were formed with complete retention of configuration, as would be expected. 

The replacement of a carboxylic acid group by nitrile functionality can also be used for the preparation of labeled compounds, and conditions for alkaline hydrolysis which did not lead to conjugation in skipped dienes like linoleic acid were developed by the Barton group. In this case, the free-radical chain sequence is straightforward, with the methanesulfonyl (or p-toluenesulfonyl) radical acting as the chain carrier [26], This methodology also represents an interesting way for the preparation of nitriles without the necessity for amide formation followed by dehydration. 

For organisms which express both pathways for nitrile hydrolysis, the stereochemical pathways can be very complex. The latter is illustrated by the microbial resolution of cx-aryl-substituted propionitriles using a Rhodococcus butanica strain (Scheme 2.109) [697]. Formation of the natural L-acid and the o-amide indicates the presence of an L-specific amidase and a nonspecific nitrile hydratase. However, the occurrence of the (5)-nitrile in case of Ibuprofen (R = i-Bu, e.e. 73%) proves the enantioselectivity of the nitrile hydratase [694]. In a related approach, Brevibacter-ium imperiale was used for the resolution of structurally related a-aryloxypropionic nitriles [698]. 

Mukherjee, C., Zhu, D.M., Biehl, E.R., Parmar, R.R., and Hua, L. (2006). Enzymatic nitrile hydrolysis catalyzed by nitrilase ZmNIT2 from maize. An unprecedented p-hydroxy functionality enhanced amide formation. Tetrahedron, 62,6150-6154. 

Hydrolysis of Nitriles. The chemical hydrolysis of nitriles to acids takes place only under strong acidic or basic conditions and may be accompanied by formation of unwanted and sometimes toxic by-products. Enzymatic hydrolysis of nitriles by nitrile hydratases, nittilases, and amidases is often advantageous since amides or acids can be produced under very mild conditions and in a stereo- or regioselective manner (114,115). 

Solvents influence the hydrogenation of oximes in much the same way as they do hydrogenation of nitriles. Acidic solvents prevent the formation of secondary amines through salt formation with the initially formed primary amine. A variety of acids have been used for this purpose (66 ), but acids cannot always be used interchangeably (43). Primary amines can be trapped also as amides by use of an anhydride solvent (2,/5,57). Ammonia prevents secondary amine formation through competition of ammonia with the primary amine in reaction with the intermediate imine. Unless the ammonia is anhydrous hydrolysis reactions may also occur. 

A very efficient and universal method has been developed for the production of optically pue L- and D-amino adds. The prindple is based on the enantioselective hydrolysis of D,L-amino add amides. The stable D,L-amino add amides are effidently prepared under mild reaction conditions starting from simple raw materials (Figure A8.2). Thus reaction of an aldehyde with hydrogen cyanide in ammonia (Strecker reaction) gives rise to the formation of the amino nitrile. The aminonitrile is converted in a high yield to the D,L-amino add amide under alkaline conditions in the presence of a catalytic amount of acetone. The resolution step is accomplished with permeabilised whole cells of Pseudomonas putida ATCC 12633. A nearly 100% stereoselectivity in hydrolysing only the L-amino add amide is combined with a very broad substrate spedfidty. 

A crude mixture of enzymes isolated from Rhodococcus sp. is used for selective hydrolysis of aromatic and aliphatic nitriles and dinitriles (117). Nitrilase accepts a wide range of substrates (Table 8). Even though many of them have low solubility in water, such as (88), the yields are in the range of 90%. Carboxylic esters are not susceptible to the hydrolysis by the enzyme so that only the cyano group of (89) is hydrolyzed. This mode of selectivity is opposite to that observed upon the chemical hydrolysis at alkaline pH, esters are more labile than nitriles. Dinitriles (90,91) can be hydrolyzed regioselectively resulting in cyanoacids in 71—91% yield. Hydrolysis of (92) proceeds via the formation of racemic amide which is then hydrolyzed to the acid in 95% ee (118). Prochiral 3-substituted glutaronitriles (93) are hydrolyzed by Phodococcus butanica in up to 71% yield with excellent selectivity (119). 

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