Nitrones diethylzinc

The reaction of nitrones with terminal alkynes proceeds in excellent yields and high purity, in the presence of stoichiometric quantities of diethylzinc and zinc triflate (219, 661-663). To optimize the process of diastereoselective addition of terminal alkynes to chiral nitrones, ZnCl2 and NEt3 in toluene were used. This reaction protocol is facile to perform, cost-effective and environmental friendly (664). 

Many other groups further studied the utility of bis(oxazolines) including Ikeda s use of biaryl bis(oxazolines) such as 250 (Fig. 9.75) in the zinc-catalyzed asymmetric alkylation of benzaldehyde with diethylzinc. " This reaction proceeded in yields up to 92% with ee up to 88%. Kodama and co-workers used a biaryl bis(oxazoline) ligand, namely, [l,l -]binaphthalenyl-2,2 -diol (BlNOL)-box 251, in the lanthanide-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrones to alkenes. ... 

On the other hand, approaches to the use of catalytic amounts of chiral ligands have been developed. Thus, the use of a sub-stoichiometric amount (50 mol%) of DBNE (1) affords A,A-diphenylphosphinylamine with 85% in 69% yield121a. Similarly, 25 mol% of chiral aziridinyl alcohol 56 (R = - ) affords (V,(V-diphenylphosphinylamine with 65% in 60% yield123. In the enantioselective addition reaction of diethylzinc to a nitrone, 20 mol% of the metal alkoxide of diisopropyl tartrate 62 catalyzed the formation of a... 

High enantioselectivity has been achieved on addition of diethylzinc to benzaldehyde catalysed by a chiral diamine, (,S )-2-(A,A -disubstitutcd aminomethyl)pyrrolidine,116 and by chiral helical titanate complexes of tetradentate ligands.117 Enantioselective additions of dialkylzinc reagents to A,-(diphcnylphosphiiioyl)imines, promoted by aziridino alcohols,118 and to the carbon-nitrogen double bond of the nitrone 3,4-dihydroisoquinoline A-oxide, promoted by dicyclopentyl(R,R)-tartrate,119 have also been reported. 

Catalytic enantioselective versions of the inverse electron demand cycloaddition have also been reported. For example, the C-(diisopropylamino)carbonyl A -phenyl nitrone reacted with 7-substituted allylic alcohols in the presence of diethylzinc, iodine, pyridine A -oxide, and a catalytic amount of enantiopure DIPT to afford 3,5-m-disubstituted isoxazolidines with high enantioselectivity (Scheme 124) <2002CL302>. Enantioselective cycloaddition of nitrones to enol ethers was achieved in the presence of chiral binaphthol-aluminium and chiral copper-bis(oxazoline) catalysts <1999JA3845, 1999CC811, 1999JOC2353, 2000JOC9080>. 

Chavant and Vallee have demonstrated dialkylzinc-promoted addition of pinacolyl alkenylboronates to nitrones (Equation 15) [85]. The reaction is tolerant of chloro and pivalate ester functionality on the alkenylboronate 85. Nitrones 86 derived from aromatic aldehydes with either N-benzyl or N-alkyl functionality were mainly used, although two examples employing nitrones derived from enolizable aliphatic aldehydes were also reported. The addition protocol is thought to rely on a transmetaUation of the pinacolyl alkenylboronates with dimethylzinc to give alkenylzinc species, which then undergo addition to give products such as 87. The use of diethylzinc resulted in some competitive direct ethylation of the nitrones. and NMR studies... 

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