Nitriles, coordinated Hydrolysis

The other way of reducing nitriles to aldehydes involves using a metal hydride reducing agent to add 1 mol of hydrogen and hydrolysis, in situ, of the resulting imine (which is undoubtedly coordinated to the metal). This has been carried out with LiAlH4, LiAlH(OEt)3, LiAlH(NR2)3, and DIBAL-H. The metal hydride method is useful for aliphatic and aromatic nitriles. 

Amides offer both O and N-donors for Co coordination. N-bound amides are accessible through hydrolysis of their corresponding coordinated nitrile or by linkage isomerization of the O-bound form. The preparation of an extensive series of pentaamminecobalt(III) complexes of (monoden-tate-coordinated) amides of the form RCONH2 (R = H, Me, CF3, CH2C1, CH2F, CH=CH2, Ph,... 

In 17, X may be POj (but COCH3, SO3 and other groups have also been examined by this means). In the type of structure shown in 18 we have already encountered the 2-nitrile hydrolyses. With X = POf in 18, divalent metal ions show a pronounced catalysis of the hydrolysis of the dianionic species. The metal is strongly chelated to the phenanthroline but in the product it is unlikely that the 0 is coordinated since a four-membered ring would result (see Sec. 6.8). The monoanionic form (X = POjH ) is the reactive species (Prob. 3). Reaction of the dianion in the absence of metal ion cannot be observed and with Cu +, for example, accelerating effects of >10 are estimated. ... 

Reactions between 5-cyanotetrazole and transition metals, when performed in boiling acetone, lead to hydrolysis of the cyano group and formation of 5-carbamyl tetrazolate complexes (68). Complexes containing 1- or 5-substituted tetrazolate anions can also be obtained by 1,3-dipolar cycloaddition of organic isonitriles (RNC) (15) or nitriles (RCN) (61), respectively, to coordinated azide ligands [Eqs. (3) and (4)]. 

In recent years there has been considerable interest in the reactions of nitriles in the coordination sphere of metal ions. Breslow et al.312 first reported that the hydrolysis of 2-cyano-l,10-phenanthro-line to the corresponding carboxamide is strongly promoted by metal ions such as copper(II), nickel(II) and zinc(II). Base hydrolysis of the 1 1 nickel complex is 107 times faster than that of the uncomplexed substrate. The entire rate acceleration arises from a more positive value of AS. Somewhat similar effects have been observed for base hydrolysis of 2-cyanopyridine to the corresponding carboxamide. In this case rate accelerations of 109 occurred with the nickel(II) complex.313... 

A number of studies have also been made of the hydrolysis of nitriles in the coordination sphere of cobalt(III). Pinnell et al.3 4 found that benzonitrile and 3- and 4-cyanophenol coordinated to pentaamminecobalt(III) are hydrolyzed in basic solution to the corresponding N-bonded carboxamide (equation 22). The reaction is first order in hydroxide ion and first order in the complex with koH= 18.8M 1s 1 at 25.6 °C for the benzonitrile derivative. As fc0H for the base hydrolysis of benzonitrile is 8.2 x 10-6 M-1 s at 25.6 °C, the rate acceleration is ca. 2.3 x 106-fold. The product of hydrolysis is converted to [(NH3)5CoNH2COPh]3+ in acidic solution and the pJC of the protonated complex is 1.65 at 25 °C. Similar effects have been observed with aliphatic nitriles.315 Thus, base hydrolysis of acetonitrile to acetamide is promoted by a factor of 2 x 106 on coordination to [Co(NH3)5]3+. 

Previous work has shown that the base hydrolysis of organonitriles is increased by a factor of ca. 108 on coordination to pentaamineruthenium(III) and by about 10s on coordination to Co111 and Rh111. A base-independent pathway is also observed with Rum, due to water attack on the coordinated nitrile.330 The relative nucleophilicities of hydroxide ion and water for attack on N-coordinated nitriles are ca. 109 (Table 22). 

The rate-determining step is likely to be the attack of an outer-sphere OH on the coordinated olefin (Equation 8) or the nitrile group (Equation 9). Reductive elimination regenerating PtL2 or hydrolysis giving the water adduct PtH(OH)L2 completes the catalytic cycle. 

Thus, many metal ions catalyze the hydrolysis of esters [7,8], amides [9], and nitriles [10] via electrophilic activation of the C=0 or C=N group. This type of catalysis is characteristic of coordination complexes and is very common in metalloenzyme-mediated processes. Zinc(II), for example, is a key structural component of more than 300 enzymes, in which its primary function is to act as a Lewis acid (see Chapter 4). The mechanism of action of zinc proteases, e.g., thermolysin, involves electrophilic activation of an amide carbonyl group by coordination to zinc(II) in the active site (Figure 4). 

The reactivity of coordinated nitriles towards nucleophiles, and electrophiles when deprotonated, has been of major interest. Addition of OH- to the nitrile carbon occurs readily in alkaline solution, kobs = oh[OH-]) and the rate is increased some 106—107 times by coordination (equation 24).176177 k0H has values of 1-50mol-1 dm3 s 1 for R = alkyl177 [e.g. 3.4 (Me) 35 (CH=CH,)j and varies over a wider range for substituted benzonitriles following a Hammett relationship.For 2-cyanoben-zonitrile addition of one equivalent of OH is followed by intramolecular amidine formation on treatment with further alkali (29), or by rearrangement in acid and hydrolysis to the diamide (31), or cyclization to the alternative amidine isomer (32 Scheme 17).179 The alkaline hydrolysis of the... 

Methylene groups adjacent to coordinated nitrile carbon are acidic (e.g. for R = CH2CN, P= 5.7) and rapidly undergo base-catalyzed proton exchange (for R = Me this is faster than hydrolysis), and addition of electrophiles other than H+ to the resulting carbanion (35 Scheme 19). 

Ruthenium(III), with a t t configuration, is, in contrast to Ru11, a very good TT-acceptor. This is dramatically demonstrated by the rates of hydrolysis of free and coordinated nitriles 23... 

In recent years there has been considerable interest in the reactions of nitriles in the coordination sphere of metal ions. Breslow et first reported that the hydrolysis of 2-cyano-l,10-phenanthro-... 

Neighbouring-group participation in the hydrolysis of coordinated nitriles has also been investi-gated. The hydrolysis of (93) has been studied for R = H (fcoH = 1050 s , LH =... 

---