Radical deuteration

Renaud and co-workers reported an unusual effect of solvent on diastereoselectivity in the radical deuteration of the sulfinylated benzyl radical with BuaSnD-AIBN, and exceptional inversion of stereoselectivity was obtained with MAD (Sch. 139) [181]. 

Determined on radical with S—CH3 instead of S—CD,. ) Determined on isotopically enriched radical. Determined on isotopically enriched radical, deuterated in positions 2, 3, 5 and 6. This structure is to be preferred over the published one on the basis of the 3-value and the a-values, the large a-value of the S—H proton corresponds to an even larger one of a pyryl radical with S—H in position 4, see [78Dor1]. ... 

Only one proton (consequence of severe twisting at C(l)-C(2) bond). ) Assignment based on corresponding radicals deuterated at C(2) and C(3) (flD = 0.175 mT). Assignment by [71Rusl]. By reduction of the ketone with Na K alloy in DMOE a similar spectrum without resolution of t-butyl splittings was obtained. Carbon atoms of t-butyl groups at C(3). ... 

Assignment based on radical deuterated at this position. 

INDO calculation of spin densities for various geometries. Similar a- and g-values obtained from radicals generated by reaction of CHjMgBr and CHjCHjMgBr with acetyl acetonates of Cr, Mn, Co, Ni, Cu, Be. Assignment based on radical deuterated at this position. ... 

Disappears in the spectrum of the radical deuterated in this position u-values of this radical very similar to those of the undeuterated species. The same radical is obtained by photolysis of DTBP containing pentamethyldisilane and 3,5-di-t-butylphenyltrimethylsilane. ... 

Assignment based on comparison with data from 7,7-di-t-butylbenzyl. ) Assignment based on comparison with 7,7-di-t-butylbenzyl. °) One or two ortho carbon atoms. Assignment based on data from corresponding radical deuterated in both m-positions. ) Carbon atoms of ( 03)3 group. ) Carbon atoms of f-butyl groups. ... 

Only one CH3 group ) Assignment based on radicals deuterated at para or both meta positions. ... 

Assignment based on radical deuterated at position 2 (as 100% D) and 4(as50% D). 

Renaud and coworkers reported that in the radical deuteration of a-selenyl sulfoxide with BuaSnD the use of MAD afforded inversion of diastereoselectivity (Scheme 6.167) [197]. In the absence of any Lewis acids, the treatment of a-selenyl sulfoxide with A IB N/BusSnD system in CH2CI2 resulted in moderate syn selectivity. In contrast, the reaction in the presence of MAD proceeded with significantly high anti selectivity. 

Decomposition. Acetaldehyde decomposes at temperatures above 400°C, forming principally methane and carbon monoxide [630-08-0]. The activation energy of the pyrolysis reaction is 97.7 kj/mol (408.8 kcal/mol) (27). There have been many investigations of the photolytic and radical-induced decomposition of acetaldehyde and deuterated acetaldehyde (28—30). 

Further evidence for a bromine-bridged radical comes from radical substitution of optically active 2-bromobutane. Most of the 2,3-dibromobutane which is formed is racemic, indicating that the stereogenic center is involved in the reaction. A bridged intermediate that can react at either carbon can explain the racemization. When the 3-deuterated reagent is used, it can be shown that the hydrogen (or deuterium) that is abstracted is replaced by bromine with retention of stereochemistry These results are also consistent with a bridged bromine radical. 

A distinction between these four possibilities can be made on the basis of the kinetic isotope effect. There is no isotope effect in the arylation of deuterated or tritiated benzenoid compounds with dibenzoyl peroxide, thereby ruling out mechanisms in which a C5— bond is broken in the rate-determining step of the substitution. Paths (ii) and (iii,b) are therefore eliminated. In path (i) the first reaction, Eq. (6), is almost certain to be rate-determining, for the union of tw o radicals, Eq. (7), is a process of very low activation energy, while the abstraction in which a C—H bond is broken would require activation. More significant evidence against this path is that dimers, Arz, should result from it, yet they are never isolated. For instance, no 4,4 -dinitrobiphenyl is formed during the phenylation of... 

This last result bears also on the mode of conversion of the adduct to the final substitution product. As written in Eq. (10), a hydrogen atom is eliminated from the adduct, but it is more likely that it is abstracted from the adduct by a second radical. In dilute solutions of the radical-producing species, this second radical may be the adduct itself, as in Eq. (12) but when more concentrated solutions of dibenzoyl peroxide are employed, the hydrogen atom is removed by a benzoyloxy radical, for in the arylation of deuterated aromatic compounds the deuterium lost from the aromatic nucleus appears as deuterated benzoic acid, Eq. (13).The over-all reaction for the phenylation of benzene by dibenzoyl peroxide may therefore be written as in Eq, (14). 

The use of selective deuteration is a powerful tool in electron spin resonance (ESR) experiments, in order to establish unequivocal assignments of experimental spectra of radicals. The reason for this is, as is well known, the difference in magnetic properties between the deuteron and the proton, which can be exploited to distinguish chemically inequivalent hydrogens in the molecule. 

A particularly interesting case is when a set of hydrogens which are chemically equivalent in the unionized molecule become inequivalent in the positive ion. Obvious examples are Jahn-Teller active molecules, but the same phenomenon may be found also in Jahn-Teller inactive systems. Since deuteration fcr practical reasons must be done before ionization, it may happen that a single deuterated molecule may produce several inequivalent isomers of the radical cation, e.g., upon irradiation. This will obviously influence the recorded ESR spectrum. 

The inequality of the C—H bonds in the radical cation implies that all C—H bonds do not have the same force constants. In a simplistic approximation, the zero-point vibrational energy (ZPVE) of a C—H stretching vibration will be proportional to (k/mn), where k is the force constant of the C—H bond and j// is the mass of the hydrogen nucleus. The effect on the ZPVE of replacing one proton by a deuteron will hence depend on the deuteration site, such that the ZPVE will be lowered more if the deuteron occupies a site with a larger fcrce constant, i.e. a shorter bond. This, in general, means a site with low unpaired spin density. 

Table 3 Vibrational Frequency Components in the High- and Low-Frequency regions (cm ) and Total ZPVE (kJ mol ) for the Non- and Mono-Deuterated Isomers of the Benzene Radical Cation.

Benzyl radical, C6H5CH2, and its deuterated analogues (C6H5CD2, CfiDsCHz) were obtained by vacuum thermolysis of dibenzyl derivatives or by pyrolysis of the corresponding benzyl bromides (17) (Baskir, 1989). 

However, deuteration at carbons 9, 10, 12 and 13 did change the spectrum to one consisting of only a nitrogen triplet, indicating that the radical trapped had a deuterium attached to the carbon centre. This would imply that the carbon-centred radical lies at one of these four positions. 

The disappearance of the spectra of the biradicals was attributed to ring closure to form the corresponding bicyclobutanes. H-abstraction from the surrounding matrices was excluded because (a) rates of decay were much too fast below 65 K compared with known radical H-abstraction reactions, (b) no radical signals were observed to grow in the EPR spectra, and (c) no rate differences were observed in deuterated compared with protio matrices. NMR and GC analyses of the EPR samples showed... 

Khaled, M., A. Hadjipetrou et al. (1990). Electrochemical and electron paramagnetic resonance studies of carotenoid cation radicals and dications Effect of deuteration. J. Phys. Chem. 94 5164—5169. 

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