Radical allyloxy

Alkoxy (R0 ) radicals react at near diffusion controlled rates with trialkyl phosphites to give phosphoranyl radicals [ROP(OR )3] that typically undergo very fast -scission to generate alkyl radicals (R ) and phosphates [OP(OR )3]. In a mechanistic study, trimethyl phosphite, P(OMe)3, has been used as an efficient and selective trap in oxiranylcarbinyl radical systems formed from haloepoxides under thermal AIBN/n-Bu3SnH conditions at about 80 °C (Scheme 27) [64]. The formation of alkenes resulting from the capture of allyloxy radicals by P(OMe)3 fulfils a prior prediction that, under conditions close to kinetic control, products of C-0 cleavage (path a. Scheme 27), not just those of C-C cleavage (path b. Scheme 27) may result. 

The Arrhenius expression for the reaction of the o(allyloxy)phenyl radical (9) with (Me3Si)3SiH relative to this unimolecular rearrangement [Eq. (4)] has been measured, v/z., og(kclku) (M) = 2.6 - 1.6/0.36 When the competition study was performed, however, reliable absolute rate constants for the cyclization of radical 9 to radical 10 were not available, although a... 

Unsaturated carboxylic acids can be de-carboxylated to alkyl radicals that undergo an intramolecular addition. The S-exo-trig-cyclization of fi-allyloxy radicals, generated from an appropriate carboxylic acid, combined with a final heterocoupling has been applied to synthesize a precursor of prostaglandine PGF2q (Fig. 47) [246] and a branched carbohydrate (ratio of diastereo-mers 1.8 1) (Fig. 48) [247]. A radical tandem cyclization of a doubly unsaturated monocyclic carbocyclic acid provides a... 

The Arrhenius expression for the reaction of the o-(allyloxy)phenyl radical (15) with (Me3 Si)3 SiH relative to this unimolecular rearrangement (Reaction 3.8) has been established [10]. A rate constant kc = 8 x 10 s has been estimated for... 

Sequences (or cascades) of radical reactions involving the 5-endo-trig cyclization of silyl radical to an allyloxy-type substituent as the key step have been developed and applied to the synthesis of natural products [7 11]. The concept and the sequence of radical reactions is outlined in Scheme 6.7. A radical of... 

Typical procedure for group-transfer radical cyclization preparation of compound 3bP (Ar = p-CF3CgH4) To a solution of 2-(allyloxy)cyclohexyl 4-(trifluoromethyl)phenyl... 

The UV irradiation of the 2-allyloxy-4,5-diphenyloxazole 41 results in the predominant formation of 5-substituted-2(5/T)-oxazolones 44, in contrast to the thermal aza-Claisen rearrangement which readily affords the 2(3/T)-oxazolones 42. The photolysis proceeds via allyl-O bond scission with the generation of a radical pair that subsequently recombines to produce 5-substituted-2(5/T)-oxazolones. Similar results were obtained with 2-(benzyloxy)-4,5-diphenyloxazole (Fig. 5.75). °... 

Allyloxy)iodoarenes react with Sml2 to yield radical anions, which undergo thermal fragmentation. The resulting aryl radicals can cyclize to dihydrobenzofurans (Entries 8-10, Table 15.9). The radical obtained after cyclization can be reduced and treated with a proton source such as water or an alcohol to yield alkanes, or with carbonyl compounds to yield alcohols (Entry 10, Table 15.9). 

The hydroperoxide decomposes to form an allyloxy radical and a hydroxyl radical, each of which continues the chain reaction. 

The peroxy and allyl radicals may couple to generate dialkyl peroxides which decompose to form additional allyloxy radicals. 

The formation of cyclic peroxides has been proposed (14) in the oxidation of poly (2,3-dimethylbutadiene). The same reaction in cts-1,4-polybutadiene would also yield additional allyloxy radicals. 

The allyloxy radicals generated from hydroperoxide, dialkyl peroxide and cyclic peroxide intermediates, in accordance with Reactions 6, 9, and 10, respectively, are converted to allyl alcohol moieties, as shown in Reaction 7. The latter are particularly susceptible to reaction with hydrogen chloride. 

Allyl(allyloxy)dimethylsilane (18) was also proposed by Barton and coworkers as being the source of 10 in gas-phase pyrolysis24. It has been suggested that the generation of 10 by pyrolysis of 18 results from consecutive loss of two allyl radicals. 1,5-Hexadiene and cyclosiloxanes 19 (D3) and 20 (D4) were the final pyrolysis products of 18 (Scheme 6). 

Pyrolytic fragmentations of other suitable precursors also lead to dimethylsilanone (97). For example, 6-oxa-3-silabicyclo[3.1.0]hexane 96 is reported to be split into 97 and 1,3-butadiene upon pyrolysis104. Other possible routes to 97 consist in the pyrolysis of (allyloxy)dimethylsilane (99)105 or Diels-Alder adduct 98104. Matrix-isolated 97 gives its trimer hexamethylcyclotrisiloxane when the matrix is warmed up to 35-40 K. The SiO stretching vibration of 97 was found at 1210 cm-1. This frequency fits the calculated force constant and bond order and has to be considered as evidence for significant double bonding in 97104. Octamethylcyclotetrasiloxane, allyl(allyloxy)dimethylsilane and 2,2,6-trimethyl-2-silapyrane failed as precursors for 97 and only the SiO molecule (cf Section V.B) and CH3 radicals were found on the matrix holder105. 

Bicyclic ji-lactamsJ This radical cyclization has been used for cyclization of the 4-allyloxy-2-azetidinone (1) to the bicyclic / -lactam 2. In this case, only endo addition is observed. 

Kunishima, M. Hioki, K Kono, K Kato, A. Tani, S. Sml2-Induced 2,3-Wittig rearrangement regioselective generation of a-allyloxy carbanions via 1,5-hydrogen transfer of vinyl radicals./. Org. Chem. 1997, 62, 7542-7543. 

Aryl radical cyclization.10 Sml2 in HMPA/THF at 25° can effect cyclization of l-allyloxy-2-iodobenzene to a Sm(III) intermediate (a) that can be trapped by electrophiles, including aldehydes or ketones. The report suggests that a similar mechanism operates in the Barbier-type coupling generation of an alkyl radical followed by formation of RSml2, which adds to a carbonyl compound to form an adduct that is hydrolyzed to an alcohol. 

A reductive radical cyclization methodology, which is catalyzed by zirconocene complexes, furnished butyrolactols 91 (Fig. 27) [168, 169]. When 2-allyloxy-3-halotetrahydropyrans or bromoacetaldehyde allyl acetals 90 were treated with 20-30 mol% of zirconocene dichloride, 1 equiv. of BEt3 and an excess Red-Al as the stoichiometric reductant, tetrahydrofurotetrahydropyrans or butyrolactols 91... 

S-Allyloxy tellurides also underwent similar radical 5-exo cyclizations catalyzed by 7 mol% of Ni(acac)2 and 2 equiv. of Et2Zn [113]. The reaction proceeded with high r/.v-selectivity in 56% yield. In contrast to the reactions of 5-hexenyl iodides shown above, 5-exo cyclization reactions of 5-hexynyl iodides were proposed to proceed by a two-electron pathway consisting of alkyne coordination/oxidative addition/intramolecular carbonickelation and reductive elimination, resulting in alkylidenecyclopentanes [114]. 

In an application of a novel radical chain reaction based on O-alkyl tin dithiocarbonates, the disubstituted dihydrofuro[3,2-/i]quinoline (39) was prepared in good yield from an 8-allyloxy-7-iodoquinoline precursor (Equation (14)) <92JA7909>. 

The mechanism of the ligand coupling or reductive elimination step is not well understood and rather speculative. However, the possible intermediacy of radical species, either free or in-cage, has been considered unlikely, since the use of free-radical traps did not alter the outcome of the reactions. The use of an external trap (1,1-dipheny-lethylene)31,40 (Equation (13)) or the use of the classical internal intramolecular free-radical probe [(o-allyloxy)phenyl radical]41 or its diphenyl analog ]( -2,2-diphenylallyloxy)phenyl radical]42 (Equation (14)) did not interfere with the outcome of the arylation reaction, thus excluding the possibility of the intermediacy of free radicals. 

Chuard et al. introduced a two-step radical alternative to the oxy-Cope reaction [95]. It relied on the opening of a strained cz-oxetanyl radical to the allyloxy radical. Because of the favorable geometry, further fragmentation delivered enone radicals that could undergo a 6-endo-trig cyclization, which led to bicyclic ketones in 69% yield. 

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