Benzylic a-alkoxy

The intrinsic barrier for the addition of solvent to an a-alkoxy benzyl carbocation is several kcal mol-1 smaller than that for the corresponding reaction of ring-substituted 1-phenylethyl carbocations. This result is consistent with the conclusion that these nucleophile addition reactions become intrinsically easier as stabilizing resonance electron donation from an a-phenyl group to the cationic center is replaced by electron donation from an a-alkoxy group. 

V-Benzyl and A -Alkoxy Pyridinium Salts as Thermal and Photochemical Initiators for Cationic Polymerization... 

Ester derivatives of benzoin are known to display usually quite a low photoinitiation activity in the polymerization of vinyl monomers [85,104]. By contrast, ben2 in alkyl ethers are claimed to generate [17], by a photofragmentation mechanism, benzoyl and a-alkoxy benzyl radicals resulting in a much more active polymerization and crosslinking initiating species (Scheme 25). Thus, polymeric systems having the above moieties in the side chains have been prepared and their photoreactivity studied in more detail [105-107],... 

The photochemistry of benzoin ethers (a-alkoxy-a-phenyl-acetophenones) has been examined in considerable detail, recently, by quenching, sensitization (, CIDNP (, and radical scavenging studies. These investigations indicate that benzoin ethers undergo a facile, photocleavage (Norrish type I) to yield benzoyl and benzyl ether radicals, as shown in eqn 1. This abscission is not retarded by conventional triplet... 

From an extensive examination of the addition of allylindium reagents to a-oxygenated aldehydes 291 it has been established that the stereochemical outcome is dependent on both the a-alkoxy substituent and the solvent employed (Table 10-33) [197]. The silyl and benzyl ethers favor the formation of the anti homoallylic alcohol, whereas the MOM and the hydroxyl aldehydes favor the syn alcohol. The rate of the reaction is dependent upon the solvent (faster in a mixed THF/water solvent) and the pH (faster at lower pH). 

Deprotonation of ethers is another route to the a-alkoxy anions, but this pathway is often precluded by a kinetic barrier. Unless the a-carbon is benzylic [175], surmounting this barrier usually requires conditions that are not favorable to the survival of the anion [164]. Notable exceptions are the hindered aryl esters studied by Beak [176], Figure 3.13a, and the carbamates studied by Hoppe [177], shown in Figure 3.13b. In both cases, ec-butyllithium is required for deprotonation, and the carbonyls which direct the metalation by a complex-induced proximity effect [178] must be shielded from the base by large alkyl groups. Once formed, the organo-lithiums are chelated and stabilized by the heteroatom-induced dipole [179]. 

Depending on the nature of the substituent R, the radical 76 (Scheme 3.53) may be slow to add to double bonds and primary radical termination can be a severe complication (see 3.2,9). The problems associated with formation of a relatively stable radical are mitigated with certain a-alkoxy (77) and tx-alkanesulfonyl derivatives (79). In both cases the substituted benzyl radicals formed by a-scission (78 and 80 respectively) can themselves undergo a facile fragmentation to fonn a more reactive radical which is less likely to be involved in primary radical termination (Scheme 3.55, Scheme 3.56). 

Allylstannanes undergo similar reaction with a-alkoxy aldehydes under Lewis acid catalysis. The treatment of 464 with allyl tri- -butylstannane in the presence of either MgBr2 Et20 [150] or lithium perchlorate-diethyl ether [153] furnishes protected syn-dio 468 with a dia-stereoselectivity of at least 25 1. This intermediate has been carried on to TBS-protected L-( — )-rhodinose (302) in an overall yield of 46% starting from (9-benzyl ethyl lactate 271b [150] (Scheme 69). 

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