Diimine ligands aziridination

Experimental observations of the aziridination of styrene-type alkenes, catalyzed by CuPF6 in the presence of chiral diimine ligands (such as (lR,2R,A i4A i4)-A A -bis(2,6-dichlorobenzylidene)cyclohexane-l,2-diamine 425), have been taken as evidence of the intermediacy of a discrete, monomeric Cu(lll)-nitrene complex, (diimine)Cu=NTs 423. Variation of the steric properties of the aryl group in the oxidant TsN=IAr (Ar = Ph, 2-/-Bu, 5,6-Me3C6H) has no effect on the enantioselectivities in forming the aziridination products 424 (Scheme 108) <1995JA5889>. 

Scheme 6.14 Catalytic asymmetric aziridination using chiral diimine ligands ...

Katsuki described the utility of chiral bisoxazolines (chiral ligand A) and 1,2-diimine ligands (chiral ligand B) in enantioselective alkylative cyclization of a range of alkenes using metal-stabilized nitrenes originated from A-tosyliminophenyliodinane and CuOTf to produce chiral aziridines 166 and 168 (Scheme 40.32). 

In a study published concurrently with the Evans bis(oxazoline) results, Jacobsen and co-workers (82) demonstrated that diimine complexes of Cu(I) are effective catalysts for the asymmetric aziridination of cis alkenes, Eq. 66. These authors found that salen-Cu [salen = bis(salicylidene)ethylenediamine] complexes such as 88b Cu are ineffective in the aziridination reaction, in spite of the success of these ligands in oxo-transfer reactions. Alkylation of the aryloxides provided catalysts that exhibit good selectivities but no turnover. The optimal catalyst was found to involve ligands that were capable only of bidentate coordination to copper. 

Further, a comparison of the effects of ligand on enantioselectivity in the cyclo-propanation and aziridination reactions revealed a linear relationship. Jacobsen argues that this reinforces the mechanistic analogy between these group-transfer reactions and suggests that the transition states are subject to similar selectivity determining factors. Finally, Jacobsen observed ligand acceleration with the diimines in this reaction. 

The mesityl diimine 88d was as effective a ligand in the aziridination as the 2,6-dichlorophenyl diimine 88a ( 65% ee vs 66% ee) (61). The bound face of the styrene undergoes aziridination (in contrast with Fu s selective crystallization of the wrong face of styrene in his copper-catalyzed cyclopropanation reaction, cf. Section II.A.8). Unfortunately, the potential racemization of 118 (by the mechanism... 

Jacobsen and co-workers have reported that chiral diimine 33a serves as an effective chiral auxiliary for the copper-catalyzed aziridination of aryl-substituted Z-olefins (Scheme 6B.35) [80], For example, the aziridination of 6-cyanochromene proceeds with high enantioselectivity (>98% ee). Comparison of ligands 33a-33c has revealed that the o-substituents in the ligands sterically and electronically influence the enantioselectivity of the reaction, that is, the introduction of chlorines at o-positions not only prolongs catalyst lifetime but also enhances enantioselectivity. The reactions of other Z-substrates and cinnamate esters catalyzed by 33a show moderate-to-high enantioselectivity, whereas that of -stilbene gave low enantioselectivity (Table 6B.3). 

N-Tosylaziridines. Aziridination of alkenes in the presence of a (diimine)copper complex is accelerated by ligands, and evidence strongly implicates a redox mechanism. 

Li Z, Quan RW, Jacobsen EN. Mechanism of the (diimine) copper-catalyzed asymmetric aziridination of alkenes. Nitrene transfer via ligand-accelerated catalysis. 7. Am. Chem. Soc. 1995 117(21) 5889-5890. 

In more recent years, a number of discoveries involving the use of nitre-noid or carbenoid intermediates have been made [35-37]. In 1993, Evans [139] and Jacobsen [140] independently described the asymmetric aziridination of olefins with Phi = NTs mediated by Cu(I) catalysts [141]. The chiral ligands included bisoxazoline 173 (Equation 29) [139] and diimine 176 (Equation 30) [140]. The methods displayed complementary substrate... 

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