Free-radical rearrangements in telomerization

Direct electrochemical reduction of a bromo-propargyloxy ester at vitreous carbon cathodes in dimethylformamide

“…In previous research dealing with the electrochemical reductions of 6-bromo-and 6-iodo-1-phenyl-1-hexyne [11], 1-bromo-and 1-iododecane [27], and 1,10-dibromo-and 1,10-diiododecane [28] in DMF, gas chromatography-mass spectrometry indicated that mixtures of telomers (of relatively high molar mass) are formed via attack of solvent-derived radicals on olefinic electrolysis products, and a similar conclusion was reached inferentially concerning the reduction of 1-bromo-and 1-iodo-5-decyne [12,13]. As pointed out by Freidlina and TerentÕev [29], any olefinic compound can play the part of a monomer in telomerization. Therefore, telomers could be produced via reactions of 3 or 4 with solvent-derived radicals, although no supporting experimental evidence has yet been acquired.…”

Carbocations in the synthesis of prostaglandins by the cyclooxygenase of pgh synthase? a radical departure!

“…The process constitutes a sigmatropic shift allowed by orbital symmetry rules~Lowry & Richardson, 1981!. In pot chemistry, the H shifts characteristic of radicals are mostly 1,5-shifts because the steric requirements dominate the energetic requirements~Heusler & Kalvoda, 1963;Wilt, 1973;Freidlina & Terent'ev, 1977;March, 1992!. However, 1,4-shifts are found when 1,5-shifts are less easily accessible, for example, across a ring system~Fisch & Ourisson, 1965;!, and even 1,3-shifts~but no 1,2-shifts! have been reported~Freidlina & Terent'ev, 1977!.…”

A homopropargyl radical rearrangement. Kinetics of the rearrangement of the 2,2,5,5,-tetramethyl-3-hexyn-1-yl radical

“…kc&. would not be the k, of [3] but instead, kc* = k,h,/(k, + A_,). Detailed comparison of kc& for the present homopropargyl radical rearrangement with rate constants for homoallyl and homobenzyl radical rearrangements could then be misleading since the latter have no stereoelectronic constraint against direct ring opening of the cyclic radical intermediate in the thermodynamically favoured direction.…”

“…the intramolecular addition step in the overall rearrangement of 4 is expected to lead to radical 5a in preference to 5b,' eq. [3].…”

“…In other cases the the C z N~o n d (9) expected strain in the intermediate methylenecyc- ' 6 lopropane system (e.g., 5). It was not known, for 1 2 3 example, whether that strain would provide a barring reopens in another sense, yielding an acyclic rier to ring closure high enough to suppress cornrearranged radical 3 from its acyclic progenitor 1. A pletely the rearrangement of a h o m o~r o~a r g~l radinumber of systems have been studied to date, in-cal 'elative to competing Processes.…”