1,3 acyclic control in free radical reactions: Threo—selective additions

1,3-Stereoinduction in Radical Reactions

“…Porter et al reported on the allyltributyltin-mediated addition of iodoalkanes to oxazolidinonacrylamides, which afforded 1,3-disubstituted products with good stereoselectivity. [6] Remarkable effects in chelation-controlled reactions of g-alkoxy-a-ester radicals were observed by Nagano et al [7] For the first time now, we have examined the chelation-controlled 1,3-stereoinduction of the trapping reaction of g-alkyl-a-ester radicals 2 (Scheme 1), which are also of interest as models for radical polymerizations of acrylic esters. As trapping reaction we chose hydrogen transfer, and assumed that the stereochemical effects observed in this process should be transferable to the radical addition reaction in the chain propagation step of polymerizations.…”

1,3-Stereoinduktion bei Radikalreaktionen

“…Allerdings verlaufen radikalische Polymerisationen im Allgemeinen weitgehend stereounselektiv, [1a] wobei sterisch sehr anspruchsvolle Methacrylate als Ausnahmen bekannt sind. [6] Bemerkenswerte Effekte beschrieben Nagano et al bei chelatkontrollierten Reaktionen von g-Alkoxy-aesterradikalen. [4] Bisher wurde ganz überwiegend die Stereokontrolle durch 1,2-und 1,4-Induktion untersucht, während die 1,3-Induktion nur an wenigen Beispielen, [5] die zudem als Modelle für die radikalische Polymerisation weniger interessant sind, untersucht ist.…”

Radical Allylations with Trimethyl[2- [(tributylstannyl)methyl]-2-propenyl]silane or Trimethyl[2-[(triphenylstannyl)- methyl]-2-propenyl]silane

“…Radical allylation of the type illustrated in eq 1 (X = halogen) is a well-established and synthetically important reaction. − Various radical sources, besides alkyl halides, are suitable, 1a,b,d,e,h,k,l, and the reaction can be carried out both by thermal or photochemical procedures. A number of related processes have also been reported in which radical leaving groups other than R 3 Sn· are involved, 5a,b, and much work has been done to identify what substituents can be tolerated on the allyl unit.…”

“…A number of related processes have also been reported in which radical leaving groups other than R 3 Sn· are involved, 5a,b, and much work has been done to identify what substituents can be tolerated on the allyl unit. In the case of allylstannanes (general formula 1 ), alkyl substitution at C(1) or C(3) does not appear to be synthetically usefulat least as a general rulebecause of facile 1,3-tin migration 8 and the fact that crotylstannanes ( 2 ) are good hydrogen donors, so that, instead of being allylated, the initial radical is reduced (eq 2). 1a,j, Oxygen substituents at C(1) have also been examined; intramolecular processes with AcO at C(1) are successful, but intermolcular reactions when the C(1) substituent is MeOCH 2 O do not work 8a. It would seem that, besides the parent compound ( 1 11 ), only C(2)-substituted allylstannanes are of general synthetic utility for radical allylation.…”

“…In these compounds, of course, the 1,3-stannane migration is degenerate. Electron-withdrawing groups at C(2), 1c,h,5c,e,6b,d,8a, especially CO 2 R, provide useful reagents ( 3 , X = CO 2 R, R’ = Bu), as do alkyl ( 3 , X = Me, R‘ = Bu) 1c,g,h,4b,d,e,6b,9c,14 or silyl groups 1h,l,5c,15 ( 3 , X = SiMe 3 , R’ = Bu, Ph). , In the case of silane 3 (X = SiMe 3 , R‘ = Bu, Ph), allylation is facilitated when the attacking radical is nucleophilic 15 and this can be interpreted 15b in terms of a polar transition state that is favored by the ability of a trimethylsilyl group to stabilize an α negative charge. Reactions involving 3 (X = SiMe 3 ) serve as a link between radical and vinyl silane chemistry (as expressed by eq 3, E + = electrophile).…”