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The rotational barriers about the C-N bonds in aminoalkyl radicals, H2NCHR, were found to be significantly greater when R is an acceptor group than when R is an alkyl group; this is good evidence for capto-dative stabilisation of the ground state radicals.The proposal1 that radicals would be stabilised by the combined interaction of donor and acceptor substituents at the same centre has been vigorously promoted by Viehe and co-workers in recent years.24 This effect, termed 'captodative' (C-D) stabilisation by Viehe had previously been suggested, though under different names, by several other groups of workers-7 Despite the wealth of applications, quantitative evidence in support of the C-D effect has been noticeably sparse. Some theoretical treatments provided supporting evidence ,8 while others indicated negligible C-D stabilisation.9~10 Kinetic studies of the addition of alkyl radicals to C-D substituted alkenes yielded ambiguous results. 11 Thermochemical studies on the decomposition of 2,3-dimethoxy-2,3-diphenylsuccinonitrile by Ruchardt and coworkers failed to find any C-D effect.12 The rates of dimerisation of C-D substituted methyl radicals were shown to be diffusion controlled; thus indicating the absence of kinetic stabilisation. 13 This does not, of course, preclude the possibility of thermodynamic stabilisation. 14 Recently, Sustmann and co-workers determined the barrier to internal rotation in the 1-cyano-1-methoxyallyl radical (l).15 In the transition state (2), i.e. on 90" rotation about the C(l)-C(2) bond, the unpaired electron is confined to the C-D substituted moiety. Thus, any stabilising C-D effect should cause a lowering in the transition state energy and hence give rise to lower rotational barriers in comparison with unstabilised ally1 radicals. Sustmann et al. found the rotation barrier of (1) to be ca. 3 kcal mol-IT lower than that of analogous radicals without C-D substitution.15 Aminoalkyl radicals (3) containing an acceptor substituent (R) should also be subject to C-D stabilisation. In the transition state for rotation about the C-N bond (4) the unpaired electron cannot be delocalised onto nitrogen and any C-D stabilisation is lost. It follows that if C-D stabilisation is significant the barriers to rotation about the C-N bonds in C-D substituted aminoalkyl radicals should be greater than those of analogous radicals without C-D substitution. We have developed an e.s.r. exchange broadening technique for determining rotational barriers in radicals with amino substituentsl6~17 and we have now applied this to several substituted aminoalkyl radicals, some of which could exhibit C-D stabilisation.The aminoalkyl radicals (3, R = Me, Pri, But, and C02But) were generated by photolysis of solutions of the appropriate amine in di-t-butyl peroxide, equations (1) and (2), in the

Hydroxyalkylcyclopropanes are oxidised by nicotinamide-dependent horse liver alcohol dehydrogenase without cleavage of the 3-membered ring, implying that radical intermediates are improbable in these reactions.

The syntheses of a number of 3-substituted prop-2-en-l -ols and -1 -als, required for studying the latent inhibition of horse liver alcohol dehydrogenase (E.C. 1.1.1 .I ), are described. Substituents were chosen to cover a range of alkoxide, phenolate, thiolate, and halide leaving groups. Of the compounds studied, only 3-ethylthioprop-2-en-l-01 proved to be a latent inhibitor through oxidation to the corresponding aldehyde, catalysed by the enzyme. The persistence of inhibition caused by this inhibitor is shown, by product studies and kinetic measurements, to be due to ethanethiol, formed by an enzyme-catalysed hydrolysis of the aldehyde. TABLE 1 Inhibition properties of the compounds studied a Krn IM Kl I M Inhibition propcrties Competitive, b slow inactivation 1.54 x 10-4 1.25 x lea 6.4 x 10-4 5.0 x los6 1 . 3 x 10-6 5.2 x 1 0 -4 Perkin I Competitive; no time-dependent inactiva-Neither substrate nor inhibitor No time-dependent inactivation tion, forms malonic dialdehyde No time-dependent inactivation Competitive, no time-dependent inactiva-Rapid covalent inhibition at 1 0 -s ~, 4 = 2 tion, forms malonic dialdehyde min Rapid covalent inhibition at 1 0 -6 ~, ti = 30s No time-dependent inactivation Competitive, reversible No time-dcycndciit inactivation 6.2 x lo-' Competitive, reversible Compounds were assayed at pH 9 in the presence of NAL) (1.5 x l P 3 ~) , using 3.76 x l0-'-1.5 x 1 W 6 ~ lIL.4DH. Q ' Conipetitive refers to competition with ethanol.[0/1182 Received, 28th JuZy, 108OJ