Transition with cyanocarbons

King, R.B. and Diefenbach, S.P., Transition-metal cyanocarbon derivatives. 5. Reactions of (l-chloro-2,2-dicyanovinyl)manganese derivatives with trialkyl phosphites. A novel variant of the Michaelis-Arbuzov reaction leading to [2,2-dicyanovinylphosphonato]metal complexes, Inorg. Chem., 18, 63, 1979. 

For the malononitriles and l,4-dicyano-2-butene, very low primary isotope effects are observed (Table 3). For 2-methyl-3-phenylpropionitrile reacting with methoxide ion in pure methanol, fcH/fcD = 1.15 [119]. Isotope effects [121] on proton transfer from ketones [89] and nitro-paraffins [5] are much higher than these values. It will now been shown that the low primary isotope effects observed for cyanocarbons support [113] the conclusion [64] reached from the high Bronsted exponents that proton transfer occurs through a transition state in which the proton is almost fully transferred. The equilibrium isotope effect (KH /KD = Kr /l) on the ionization of malononitriles (80)... 

The importance of the low isotope effects which have been observed for proton transfer from cyanocarbons has been emphasized [125]. Until recently it has been assumed that a rate-determining proton transfer would give rise to an appreciable isotope effect. The results for malononitriles show that this is not always the case and small isotope effects can be expected when the transition state in a proton transfer reaction is strongly reactant-like or product-like and this has been confirmed by theoretical calculation [126]. Inverse primary isotope effects (kH/kD < 1) may even be possible with a product-like transition state when the equilibrium isotope effect is in this direction [126]. The observation of an appreciable kinetic isotope effect for a reaction indicates that a hydrogen transfer is involved, but a small isotope effect does not prove the contrary and this may have led, in the past, to errors in the interpretation of... 

A value of kH/kD = 1.4 was obtained [114] for the rate of proton transfer compared with deuteron transfer from chloroform to hydroxide ion and this result is similar to the values determined earlier for several haloforms [164, 166]. In the most recent work [171(b)] a value kH /kD = 1.11 0.05 was determined for chloroform. These values are close to those observed for reaction of cyanocarbon acids (though a different base catalyst is involved) and in Sect. 4.3 it was argued that isotope effects as low as these are expected for a transition state in which proton transfer is almost complete. The isotope effect for proton transfer from chloroform was measured using a new and useful method [114]. It can be shown that the ratio of initial rates of uptake of tritium for the first ten per cent of reaction from tritiated water into CHC13 and CDC13 is identical to the primary isotope effect for proton loss (feH /fcD). The procedure can be used for measuring isotope effects on proton transfer from carbon acids to hydroxide ion or buffer catalysts and is more convenient than other methods. Other methods which have been used, for example, involve the comparison of rates of detritiation and dedeuteration or the comparison of rates of bromination for isotopically different acids (RCH and RCD) [113]. 

The small isotope effects observed in proton transfer from cyanocarbon acids to various bases shown in Table 3 (for example feH/feD = 1.46 for proton transfer from malononitrile to water) are compatible with an extremely product-like transition state in which the proton is almost fully transferred [113] (Sect. 4.3). Similar conclusions may be reached from the small isotope effects observed for chloroform (feH/feD = 1.41 0.01 [114] and 1.11 0.05 [171]) and phenylacetylene (kH/kD = 0.95 0.09 [143]) for reaction with hydroxide ion, and for reaction of disulphones with water (feH/feD = 2.2 0.1 [65]). In all these cases the magnitude of the Bronsted exponent is close to the limiting value of unity as expected for a product-like transition state. 

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