D. Griller

Homolytic organometallic reactions. Part VII. An electron spin resonance study of the reaction of t-butoxyl radicals with trialkyl phosphites: kinetics of β-scission of the t-butoxytriethoxyphosphoranyl radical

The oxidation in solution of trialkyl phosphites by t-butoxyl radicals has been studied using e.s.r. spectroscopy, and intermediate tetra-alkoxyphosphoranyl radicals [a(31P)ca. 900 G] have been detected. Rates of reactions (i) and (ii) were determined using e.s.r. spectroscopy to monitor the radical concentrations. ButO·+ P(OEt)3→ But·+ OP(OEt)3(i), ButOP(OEt)3→ But·+ OP(OEt)3(ii)The rate constants are given by the expressions log (k1/l mol–1 s–1)= 9·83 – 2·24/θ and log (kii/s–1)= 12·95 – 10·34/θ where θ= 2·303RT kcal mol–1.Tetra-alkoxyphosphoranyl radicals react with oxygen to give tetra-alkoxyphosphoranylperoxyl radicals [a(31P)ca. 9 G] which are detected by e.s.r. at low temperatures.

Homolytic organometallic reactions. Part IX. An electron spin resonance study of the mechanism of β-scission of tetra-alkoxyphosphoranyl radicals. Formation of trialkoxy(methyl)phosphoranyl radicals

The rate of β-scission of the tetra-alkoxyphosphoranyl radicals (RO)4P·, where the alkyl groups may be the same or different, is determined by configurational, steric, electronic, and solvent effects. The rate of C–O cleavage increases along the series Rp < Rs < Rt < PhCH2. In the distorted trigonal bipyramidal radical (RO)4P·, alkyl groups in apical or equatorial positions are not lost at the same rate, probably those in the equatorial position under-going more rapid β-scission. As a result of this configurational selectivity, bulkyl alkyl groups are retained preferentially when (RO)4P· decomposes. The rate of scission of (EtO)3POBut increases somewhat with the solvent polarity, but there appears to be little steric acceleration when (RO)4P· contains bulky alkyl groups. Methyl radicals react with trialkyl phosphites to give the radicals (RO)3[graphic omitted]Me at a rate which is governed primarily by steric effects. PhCH2O[graphic omitted](OEt)2Me undergoes rapid β-scission to give a benzyl radical.

Homolytic organometallic reactions. Part III. An electron spin resonance study of homolytic t-butoxydealkylation of organometallic compounds. Rate constants for the reaction at boron

When di-t-butyl peroxide is photolysed in the presence of a variety of organometallic compounds (RM) in an e.s.r. cavity, the alkoxyl radicals which are formed induce an SH2 reaction at the metal centre, and the e.s.r. spectrum of the displaced alkyl radical R· can be observed [equation (i)]. The rates of these SH2 reactions for a variety of trialkyl- [graphic omitted] boranes and trialkylboroxines have been studied by causing the organoboranes to compete with cyclopentane for reaction with the t-butoxyl radicals [equation (ii)], and monitoring the relative concentrations of the alkyl and cyclopentyl radicals by e.s.r. The reactions are very fast (ki= 105–107 M–1 s–1) and are characterised by low activation energies (0–5 kcal mol–1), and A factors generally in the range 107–108.

Electron spin resonance studies of the structure and reactivity of some spirophosphoranyl radicals in solution

Spirophosphoranyl radicals have been generated by abstraction of hydrogen from 1,4,6,9-tetraoxa-5-phosphaspiro[4,4]nonane (IV; R = H) and related compounds. The e.s.r. spectra of these radicals have been observed and correlated with their structures. The radical (VII) from 2,2,7,7-tetramethyl-1,6-diaza-4,9-dioxa-5-phosphaspiro[4,4]nonane was also detected. These spirophosphoranyl radicals possess trigonal bipyramidal structures in which the unpaired electron is stereochemically active and occupies an equatorial ligand site. There is no evidence that any rapid exchange of apical and equatorial ligands (e.g. by pseudo-rotation) occurs on the e.s.r. time scale. A tertiary alkoxy-group occupies an equatorial position in preference to a primary alkoxy-group in these radicals The spirophosphoranyl radicals are remarkably stable towards unimolecular α- or β-scission, but they will add to terminal olefins and to 2-methyl-2-nitrosopropane.

The electron spin resonance spectra and structure of chlorophosphoranyl radicals in solution

Chlorophosphoranyl radicals of the types ButO(R)nṖCl3–n and ButO(RO)nṖCl3–n have been prepared by addition of photochemically generated t-butoxyl radicals to chlorophosphines in fluid solution. The e.s.r. spectra of these radicals are characterised by very large phosphorus-31 hyperfine splittings (ca. 1000 G) and readily resolvable 35Cl and 37Cl splittings (ca. 40 G). The chlorine atoms are probably held rigidly (on the e.s.r. time scale) at apical sites in these chlorophosphoranyl radicals, the apicophilicity of chlorine being greater than that of an alkoxy-group. The reactions of chlorophosphoranyl radicals are discussed.

The electron spin resonance spectra and decarboxylation of alkoxycarbonyl radicals

The e.s.r. spectrum of the t-butoxycarbonyl σ-radical, generated by u.v. irradiation of a liquid mixture of di-t-butyl peroxide and t-butyl formate, has been observed. Decarboxylation of this radical has been monitored by kinetic e.s.r. spectroscopy and the unimolecular rate constant for the reaction But·ĊO → But·+ CO2 is given by log (k/s–1)= 13·4 – 12·1/θ where θ= 2·303 RT/kcal mol–1.These kinetic parameters differ from those reported in an earlier communication because it was assumed that the rate constants for reaction of a t-butyl radical with a t-butyl radical or with a t-butoxycarbonyl radical had the same value. This assumption has been tested experimentally and shown to be incorrect.Abstraction of hydrogen from the alkoxy-groups of alkyl carbonates gives radicals which do not decompose to form alkoxycarbonyl radicals.

Homolytic substitution at boron by the triplet state of ketones

In the triplet state, ketones (R21CO) will react with trialkylboranes (R32B) by a bimolecular homolytic substitution to give the radicals R21COBR22 and R2.

Absolute rate constants for homolytic alkoxydealkylation at boron

Absolute rate constants for the SH2 process RO·+ BBu3→ ROBBu2+ Bu· have been obtained by two competition techniques involving the analysis of products by g.l.c., or of the radical intermediates by e.s.r.

The electron spin resonance spectrum and decarboxylation of the t-butoxycarbonyl-radical in solution

The t-butoxycarbonyl radical (ButOĊO), produced by photolysis of di-t-butyl peroxide in the presence of t-butyl formate, has been observed by e.s.r. spectroscopy and shown to be a σ-radical; the kinetics of the decarboxylation of ButOĊO have been measured and the reaction shown to have a low A-factor (1010·8s–1) for a unimolecular scission.