Radicals, alkoxy allylic

Previous discussions (20, 39) on the propagation rate, kp, point out the effect caused by the resonance energy of the radical formed. Our results support this view and enable us to complete the arrangement by families according to the groups adjacent to the attacked function—alkyl, benzyl, alkoxy, allyl, hydroxyl. The steric effect does not reveal itself in any important way—e.g., a-methylbenzylic ether has a kp which is close to that of benzylic ether, and the tertiary carbons in the former product are generally attacked at rates comparable with that of a less-encumbered carbon. 

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

The polymer alkoxy allyl radicals (3.58) produced in reaction 3.245 may initiate some new degradation chains. 

The strength of the O—H bond in ROO—H is comparable to that of allylic and benzylic C—H bonds and is ca 15-20 kcalmor weaker than the corresponding RO—H bond. Therefore, alkoxy radicals like t-BuO are suitable initiators in reactions of the type described in Scheme 37. The hazardous di-icri-butyl peroxalate (DBPO) (ti/2 ca 12 h at 20 °C in solution) (equation 9) and the safer di-fert-butyl hyponitrite (DTBN) (ti/2 ca 12 h at 40 °C) (equation 10) ° have been widely used for the generation of tert-BuO radicals under very mild conditions . 

Primary or secondary allylic radical (27) a-Alkoxy-alkyl-radical (13, 14)... 

As illustrated in Scheme 3.26, the role of the Pd(II) porphyrin complex is the photochemically sensitized generation of singlet oxygen Oj), which forms the allylic hydroperoxide 14. Subsequent Fe3+ porphyrin-catalyzed hydrogen abstraction from 14 yields the peroxy radical 16, while the catalytic Fe3+ species is recycled by homolytic 0—0 bond cleavage of 14 to give the alkoxy radical 17. Both radicals 16 and 17 are involved in the dark reaction where the remaining cyclohexene is consumed. 

Stereoselective radical reduction or allylation.2 Radical reduction of (1-alkoxy-a-halo esters such as 1 shows marked onri-stereoselectivity with Bu3SnH (AIBN) at 50°, which is markedly improved when conducted under photochemical irradiation at —78°. Similar but even higher stereoselectivity obtains in reduction of the tetrahy-drofuran derivative 2. 

Allylation of these P-alkoxy-a-halo esters with allyltributyltin initiated with AIBN at 60° also shows good to impressive stereoselectivity, which can be improved by use of triethylborane as initiator at —78°. However, these reactions are generally slower than radical reductions. 

Oxidation of C—bonds by copper ion catalyzed reaction with an organic peroxy ester (the Kha-rasch-Sosnovsky reaction) was at one time very popular for allylic oxidation and has been thoroughly reviewed. The reaction is usually carried out by dropwise addition of peroxy ester (conunonly r-butyl peracetate or r-butyl perbenzoate) to a stirred mixture of substrate and copper salt (0.1 mol % commonly copper(I) chloride or bromide) in an inert solvent at mildly elevated temperature (60-120 C). The mechanism involves three steps (i) generation of an alkoxy radical (ii) hyttogen atom abstractitm and (iii) radical oxidation and reaction with carboxylate anion (Scheme 11). 

The photochemical reactions of arenecarboxylic acid esters with alkenes has received recent attention by Cantrell. - For example, irradiation of 2,3-dimethyl-2-butene and methyl benzoate gave a mixture of alkoxyoxetane (56), carbonyl-alkene metathesis product (57) and ketone (58), resulting from alkoxy radical allylic hydrogen abstraction and radical recombination. Such alkoxyoxetane photoproducts are... 

Satisfactory to good yields of adducts have been found for styrenes [Eq. (21a), Y = phenyl], conjugated dienes (Y = vinyl), enamines (Y = NR2), and enol ethers (Y = alkoxy), particularly if they are unsubstituted at the 6-carbon atom to Y. Nonactivated alkenes react less satisfactorily. In the oxidation of anionized 1,3-dicarbonyl compounds (Table 11, numbers 1-8) at potentials between 0.6 and 1.4 V (SCE) and in the presence of butadiene, only the additive dimer LXII is obtained in the presence of ethyl vinyl ether only the disubstituted monomers LXVI or LXVII arise, but with styrene both types of products LXII and LXVI are formed. This result indicates that the primary adduct LXIII is oxidized rapidly between 0.6 to 1.4 V to the carbenium ion in the case of an ethoxymethyl radical (Y = OEt), and slowly in the case of an allyl radical (Y = vinyl). 

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