Aziridines, coordinated

Oxidative cleavage of P-aminoacyl complexes can yield P-amino acid derivatives (320,321). The rhodium(I)-catalyzed carbonylation of substituted aziridines leads to P-lactams, presumably also via a P-aminoacyl—metal acycHc compound as intermediate. The substituent in the aziridine must have 7T or electrons for coordination with the rhodium (322,323). 

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. 

When the reaction is conducted in the presence of added fumarate, the yield of pyrrolidine (130) increases at the expense of the aziridine. Jacobsen suggests that the aziridines and pyrrolidines arise from a common intermediate, azo-methine ylide (132), Scheme 6, which may also be partly responsible for the poor levels of asymmetric induction in this reaction. Electrocyclic ring closure of the azo-methine while still within the coordination sphere of the metal (131) may provide aziridine with some induction, while decomplexation (132) will lead to the formation of racemic aziridine and pyrrolidine. 

The formation of the A-vinylaziridine 70 in the photoreaction of 68 deserves additional comment. Depending on the multiplicity, the intermediate 72 formed by path b could be a triplet 1,3-biradical. However, if intersystem crossing occurs along the reaction coordinate, the singlet biradical must be considered as a dipolar azomethine ylide. According to literature precedents, both intermediates, the 1,3-biradical and the ylide, will cyclize to form the observed aziridine. This is the first case in a DPM process where a zwitterion can be postulated as a possible intermediate. 

Lastly, Antilla has disclosed a novel asymmetric desymmetrization of a wide range of aliphatic, aromatic, and heterocyclic meso-aziridines with TMS-N3 promoted by 11 and related 12 (Scheme 5.31) [56]. Uniquely, this is one of only several reports of electrophilic activation of nonimine substrates by a chiral phosphoric acid. Mechanistic studies suggest that silylation of 11 or 12 by displacement of azide generates the active catalytic species A. Consequently, the aziridine is activated through coordination of it carbonyl with chiral silane A to produce intermediate B. Nucleophilic ring opening by azide furnishes the desymmetrized product and regenerates 11 or 12. 

In typical organic crystals, molecular pairs are easily sorted out and ab initio methods that work for gas-phase dimers can be applied to the analysis of molecular dimers in the crystal coordination sphere. The entire lattice energy can then be approximated as a sum of pairwise molecule-molecule interactions examples are crystals of benzene [40], alloxan [41], and of more complex aziridine molecules [42]. This obviously neglects cooperative and, in general, many-body effects, which seem less important in hard closed-shell systems. The positive side of this approach is that molecular coordination spheres in crystals can be dissected and bonding factors can be better analyzed, as examples in the next few sections will show. 

An intermediate product was identified and characterized in the reaction outlined in Scheme 27. X-Ray analysis has shown that this intermediate product contains a hexadentate monoanionic ligand formed through the reaction of a phenol group with an aziridine group once coordinated to nickel(II). 4,2335... 

In this chapter, the recent development of catalytic asymmetric epoxidation and aziridination of simple olefins bearing no pre-coordinating substituent is discussed. 

The aziridination of imines catalyzed by the CpFe(CO)2+ fragment by carbene transfer is a most remarkable reaction [26], The imine is consumed first to form both the cis and trans products. The catalyst then coordinates predominantly to the trans product and leads to its decomposition, leaving the cis product untouched [27]. 

In the absence of organic solvents, Sc(OTf)3 catalysed the cycloaddition of aziridines to nitriles to produce substituted imidazolines in good to excellent yields at room temperature and in an air atmosphere. The reaction is believed to progress through a highly reactive cationic intermediate in which the aziridine nitrogen is coordinated to Sc(OTf)3.65 The phosphine-catalysed enantioselective 3 + 2-cycloaddition of buta-2,3-dienoates with arylimines yielded 2-aryl-3-pyrrolidines with 64% ee.66... 

The nature of the /V-subsiiiuen1 of the aziridine moiety has been found to play an important role in the deprotonation reaction of oxazolinylaziridines.16 An electron-donating group appeared to be the /V-subsiilucnt of choice when the oxazoline moiety has a cis relationship with respect to the proton to be removed. The high stability of the resulting aziridinyllithium may be due to the coordinative effect of the oxazoline ring. 

Compound 46 and benzaldehyde are considered to be most likely formed by the ring-opening reaction of aziridine 44, giving an azomethine ylide, followed by hydrolysis. Thermolysis of 32 also affords 44-46, and benzaldehyde in 79%, 100%, 15%, and 16% NMR yield, respectively. This is the first example of aziridine formation from heterocyclobutanes with high coordinate main-group elements. 

Tridentate Schiff base chromium(III) complexes were identified as the optimal catalysts for the enantioselective ring opening of meso-aziridines by TMSN3.51 Indeed, preliminary studies have shown that, although the (salen)chromium complexes catalyzed the reaction to some extent, they consistently led to low enantioselectivities (<14% ee). It was rationalized that the diminished reactivity and selectivity of the salen complexes with aziridines compared to epoxides was a result of the steric hindrance created by the /V-substituent of the coordinated aziridine. As expected, improved results were observed using tridentate ligands on the chromium center because they offer a less-hindered coordination environment (Figure 17.7).51... 

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