Free radicals allylations, radical reactions

Enholm [12] has carried out free radical allyl transfer reactions using a non-cross-linked polystyrene, soluble polymer 77. Model reactions of bro-mo ester 78 with allyltin reagents gave products 79 and 80 in excellent yield (Scheme 17). 

Alkenes react with N bromosuccimmide (NBS) to give allylic bromides NBS serves as a source of Br2 and substitution occurs by a free radical mechanism The reaction is used for synthetic purposes only when the two resonance forms of the allylic radical are equivalent Otherwise a mixture of isomeric allylic bromides is produced... 

Allyltin reagents supported on polymer underwent free radical allylic transfer with a marked preference for electron-poor carbon radicals598. The fluorous method developed by Curran and coworkers has recently been successfully extended to four-component radical reactions using fluorinated allyltin reagents599. 

C-Glycosides. Thiophenyl glycosides undergo ready free-radical allylation with allyl- or methallyltri- -butyltin. Thus the L-lyxose derivative 1 reacts with allyltri-/ -butyltin under photochemical initiation to give 2 and 3 in the ratio 90 10. The reaction... 

Hanessian, S., Yang, H., and Schaum, R., Hydrogen-bonding as a stereocontrolling element in free-radical C-allylation reactions Vicinal, proximal, and remote a.symmetric induction in the amino acid series, J. Am, Chem. Soc. 118, 2507-2508 (1996). 

Enholm et al. [172] also examined free radical allylations of carbohydrate linked bromo esters. The D-xylose-derived ester 267 was reacted with allytributyltin and ZnCl2 as a Lewis acid to give the ester 268 (Scheme 10.87). The diastereoselectivity and yield of the allylation reaction were high. 

It has been observed that addition of Lewis acids to the free radical allylation improved the chemical yield [101]. When substrates with a chiral auxiliary were subjected to free radical allylation in the presence of a Lewis acid, the desired allylated products were obtained with high stereoselectivity [94 d]. In these reactions the Lewis acid plays a pivotal role in fixing the conformation of radical intermediates. Recently Sibi indicated that an elevated reaction temperature accelerated inversion of the stereochemistry of the radical-centered carbon giving rise to greater diastereoselectivity (Scheme 12.39) [102]. When enantiomerically pure Lewis acids were employed as chiral auxiliaries enantioselective free radical allylation of sulfones [103] and oxazolidinones [104] were realized. In the latter reaction two contiguous chiral centers were generated successfully in a single operation with excellent stereoselectivity via tandem C-C bond formation both enantiomers can be se-... 

The first synthesis of the tetracyclic substructure ABCE was published by Hart et al. As illustrated in Scheme 26, the route includes (a) a new approach to substituted perhydroisoquinolines that features a stereoselective free radical allylation, (b) a Mitsunobu reaction using a new iV-acylsulfonamide (this step was completed with net retention of configuration), and (c) final preparation of a A. -azocine via an intramolecular iV-alkylation in high yield (the system has limited degrees of freedom due to the presence of the Z-alkene) [88]. 

Bromination appears to involve substitution rather than addition. Baldwin and Kuntz (I960) report that bromination of butyl rubber with elemental bromine is accompanied by the formation of large amounts of HBr. Using ozonolysis techniques it was found that unsaturation did not greatly change with bromination. It has thus been inferred (Makowski, 1%9) that the reaction proceeds by a free radical allylic substitution rather than by an ionic mechanism, e.g. 

Stereoselective Allylation Reaction. The enantioselective allylation of aldehydes with allyltrimethylsilane is now possible with good control. One example is the use of a chiral acyloxy borane (CAB) catalyst. Besides that, chiral Ti(OiPr)2X2-BINOL and TiFa-BINOL catalysts are effective for the enantioselective allylation of glyoxylates and aldehydes, respectively (eq 35). Chiral homoallylamines can be prepared from the reaction of imines and allyltrimethylsilane using chiral tt-allylpalladium complexes with TBAF (eq 36) or an (5)-Tol-BINAP-CuPFe system. Free radical allyl transfers from allyltrimethylsilane provides another method for this enantioselective C-C bond formation. Promoted by chiral Lewis acids, both... 

Atom abstraction reactions also occur with // -cyclopentadienyl substituted metal carbonyl complexes. Laser photolysis (460-490 nm) of [ / -cpM(CO)3]2 in the presence of organic halides (RX) leads to the formation of the halo compounds / -cpM(CO)3X (M = Mo, W). The reactivity trends RI > RBr > RCl, and benzyl > allyl > 3 > 2 > 1 > CH3, follow those expected for a free radical pathway. The reaction involves formation of the 17-electron intermediate / -cpM(CO)3, which then abstracts a halogen atom from the substrate (Scheme 6.9). The cpM(CO)3 radical is trapped by oxygen at a rate that is close to diffusion controlled. 

Hydrogen bromide continually forms in the reaction. As its concentration increases, might it not add to the double bond in competition with the free-radical allylic bromination The answer is, it could if it was to accumulate as a reaction product. However, this possibility can be bypassed completely. We can use a reagent called N-bromosuccinimide (NBS) to carry out allylic bromination. NBS generates a low concentration of bromine and also prevents the continued production of HBr that would add to the double bond in an electrophihc addition reaction. 

As argued elsewhere in detail [12], the foregoing observations support a mechanism for antoacceleration that involves the reaction of HCI with polyenes to generate free radicals. These radicals, or radicals formed from them, then abstract methylene hydrogen from the ordinary monomer units of PVC, as shown in equation 3 (wherein the nature of X is not yet specified), and the resultant carbon-centered radicals eiqierience P-scission in order to give free chlorine atoms and allyl chloride defect structures (equation 4). These structures initiate the growth of new polyene sequences in the usual (non-... 

Reaction Mechanism. High temperature vapor-phase chlorination of propylene [115-07-17 is a free-radical mechanism in which substitution of an allyhc hydrogen is favored over addition of chlorine to the double bond. Abstraction of allyhc hydrogen is especially favored since the allyl radical intermediate is stabilized by resonance between two symmetrical stmctures, both of which lead to allyl chloride. 

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