Brenda Muggleton

Homolytic organometallic reactions : XII. Homolytic dealkylation of di-n-butyl-t-butyltin chloride: preferential displacement of the n-butyl radical

Abstract t-Butoxyl radicals react with di-n-butyl-t-butyltin chloride to show the ESR spectrum of the n-butyl rather than the t-butyl radical. Cumyloxyl radicals behave similarly, but trimethylsiloxyl and benzoyloxyl radicals react to give both n-butyl and t-butyl radicals. The unexpected preferential formation of the less stable n-alkyl radical is tentatively ascribed to steric constraints in a 5-coordinate intermediate.

An electron spin resonance study of 3-oxypropenoyl radicals derived from glycidols

Glycidols with blocked OH groups (A; M = alkyl or trialkylsilyl) react with t-butoxyl radicals to show the e.s.r. spectra of the corresponding 3-oxypropenoyl radicals (D), and 24 examples of these acyl radicals are reported. The [graphic ommitted] reaction is thought to proceed through the formation of the allyloxyl radicals (B), which, in part, are converted into the aldehyde (C) which is very reactive towards loss of hydrogen to give the acyl radical (D). Glycidyl pivalate (A; M = COCMe3) reacts cleanly in this way, but glycidyl acetate (E; R = Me) also undergoes intramolecular 1,5-transfer of the acyl group to show the spectrum of the enoxyl radical (F). Glycidyl propionate and butyrate do not undergo this acyl transfer, but show the spectra of the radicals OĊCHCHOCOCH2R′ and [graphic omitted]HCH2OCOĊHR′(R′= Me or Et). [graphic ommitted]

Direction of ring-opening of ring-substituted cyclopropyl(stannyloxy)methyl and cyclopropyl(hydroxy)methyl radicals: preferential formation of primary alkyl radicals

cis- and trans-2-Methylcyclopropyl-stannyloxymethyl and -hydroxymethyl radicals (A) have been generated by treating either the corresponding tin alkoxides or alcohols with t-butoxyl radicals, or the corresponding ketones with tributylstannyl radicals. The e.s.r. spectra show that, at low temperature (< –60°C), the cis-(A) radicals undergo ring-opening to give principally secondary alkyl radicals, but the trans-(A) radicals give only primary alkyl radicals.

Homolytic abstraction of enolic hydrogen

The e.s.r. spectrum of the homoallylic radical (C) is observed when a mixture of di-t-butyl peroxide and a cyclopropylcarbinol (A; R = H or alkyl) is photolysed at low temperature, but near room temperature the spectrum shows that the enolic hydrogen has been transferred to the alkyl radical to give the enoxyl radical (D). Studies of the thermolysis of di-t-butyl peroxide or of di-t-butyl hyponitrite in the presence of 1-cyclopropylethan[2H]ol confirmed that deuterium was located in the 5-position of the pentan-2-one which was formed. The same sequence [graphic omitted] of radicals (B)→(C)→(D)(R = Me) occurs when cyclopropyl methyl ketone is photolysed in the presence of 1-cyclopropylethanol. Aryl(cyclopropyl)methanols (A; R = Ar) yield only the radical (B), and no ring opening could be detected. 2-(Hydroxymethyl)oxiran shows the formation of the 3-hydroxyprop-2-enoxyl radical, consistent with the occurrence of ring opening and hydrogen transfer from enolic oxygen to oxygen, but no evidence could be found for transfer of hydrogen from lactim oxygen to carbon, or from enolic oxygen to nitrogen, in the reactions of 1- or 2-hydroxyalkylaziridines.

Direction of ring-opening of 2-methyl-substituted cyclopropyl(stannyloxy)methyl and cyclopropyl(hydroxy)methyl radicals: kinetic and thermodynamic control

trans-2-Methylcyclopropyl(stannyloxy or hydroxy)methyl radicals undergo ring-opening to give principally the thermodynamically less stable primary alkyl radicals; use has been made of this effect to establish the conditions under which the cyclopropylmethyl → homo-allyl rearrangement is reversible.