Nitrenes styrene

A more practical, atom-economic and environmentally benign aziridination protocol is the use of chloramine-T or bromamine-T as nitrene source, which leads to NaCl or NaBr as the sole reaction by-product. In 2001, Gross reported an iron corrole catalyzed aziridination of styrenes with chloramine-T [83]. With iron corrole as catalyst, the aziridination can be performed rmder air atmosphere conditions, affording aziridines in moderate product yields (48-60%). In 2004, Zhang described an aziridination with bromamine-T as nitrene source and [Fe(TTP)Cl] as catalyst [84]. This catalytic system is effective for a variety of alkenes, including aromatic, aliphatic, cyclic, and acyclic alkenes, as well as cx,p-unsaturated esters (Scheme 28). Moderate to low stereoselectivities for 1,2-disubstituted alkenes were observed indicating the involvement of radical intermediate. 

Diphenylphosphorylazide (DPPA) has also been shown to be an excellent nitrene source in aziridination reactions <06JOC6655>. The reaction of styrene and substituted styrenes with DPPA and tetraphenylporphyrin cobalt (CoTPP) provided the A-diphenylphosphinyl aziridines in moderate yields. 

Investigations into the mechanism of this reaction revealed several interesting facts (61). Compelling evidence was presented that a discreet Cu nitrenoid was involved in the catalytic cycle. Photolysis of a solution of tosyl azide and styrene in the presence of the catalyst afforded aziridine with the same enantioselectivity as obtained from the PhI=NTs reaction, Eq. 69. Since photolysis of tosyl azide is known to extrude dinitrogen and form the free nitrene, the authors argue that this is indicative of a common Cu-nitrenoid intermediate in this reaction. 

This indicates that the prebaking temperature higher than the melting point of the azide decomposes the azide (50%) and it totally decomposes upto 100 mJ/cm2 irradiation. It is possible that subsequent reactions of the nitrene, generated from the azide thermolysis and photolysis, with the styrene resin could be responsible for solubility modulation of this type resist (16). 

This indicates that the thermally or photochemically decomposed azide (Figure 4) inhibits the dissolution of the styrene resin into the alkaline developer. The inhibition may be due to the increase of the molecular weight of the styrene resin in the presence of the decomposed azide. Hydrogen abstraction from the polymer by nitrene of the decomposed azide and subsequent polymer radical recombination result in a increase in the molecular weight of the polymer (17). 

The first catalytic, asymmetric aziridination of an alkene in good yield and high enantioselectivity was recently reported56. Thus styrene (63) was treated with [N-(p-toluenesulphonyl)imino]phenyliodinane (64) and an asymmetric copper catalyst to yield (/ )-Ar-(p-toluenesulphonyl)-2-phenylaziridine [(/ )-65] in 97% yield with an ee of 61%, the catalyst being the complex formed in situ in chloroform from the chiral bis[(5 ) 4-ferf-butyloxazoline] [(S,S)-66] and copper triflate (CuOTf)56, the reaction proceeding by way of a nitrene transfer57. 

Catalytic methods are suitable for nitrene transfer," and many of those found to be effective for carbene transfer are also effective for these reactions. However, 5- to 10-times more catalyst is commonly required to take these reactions to completion, and catalysts that are sluggish in metal carbene reactions are unreactive in nitrene transfer reactions. An exception is the copper(ll) complex of a 1,4,7-triaza-cyclononane for which aziridination of styrene occurred in high yield, even with 0.5 mol% of catalyst. Both addition and insertion reactions have been developed. 

TaUI nitrenes have been prepared by reduction of Tav nitrenes under argon (equation 59). The lability of the PMe3 ligands allowed the production of [Ta(NPh)ClL3(alkene)] (alkene = ethylene or styrene). 

Copper complexes catalyze formally related aziridination of olefins with ]7V-(p-toluenesulfonyl)imino]phenyliodinane, a nitrene precursor (219b). As exemplified in Scheme 98, catalysts formed from Cu(I) tri-flate and optically active bis(oxazolines) effect enantioselective reaction of styrene (Scheme 98) (218b, 219a). 

Although it has been established that the HOMO (diazoalkane)-LUMO (alkene) controlled concerted cycloaddition occurs without intervention of any intermediate for the reactions of simple diazoalkanes with alkenes, Huisgen once proposed a mechanistic alternative 4 namely an initial hypothetical nitrene-type 1,1-cycloaddition reaction of phenyldiazomethane to styrene followed by a vinylcyclopropane-cy-clopentene-type 1,3-sigmatropic rearrangement Control experiments, however, excluded this hypothesis for the bimolecular 1,3-dipolar cycloaddition reaction of diazomethane (Scheme 60).204... 

Enantioselectivity of copper-catalyzed aziridination is dependent on the nitrene precursor used (Scheme 6B.32) [77]. Although the precursor of choice varies with the substrates, /j-Me0C6H4S02N=lPh orp-02NC6H4S02N=IPh is superior to TsN=IPh in many cases. For example, the aziridination of styrene in the presence of copper-bisoxazoline complex 29b gives the product with 78% ee using p-Me0C6H4S02N=IPh as the nitrene precursor, whereas the enantioselectivity is 52% ee when TsN=IPh is used as the precursor. 

A review has appeared on the synthesis of enantiomerically enriched aziridines by the addition of nitrenes to alkenes and of carbenes to imines.45 A study of the metal-catalysed aziridination of imines by ethyl diazoacetate found that mam group complexes, early and late transition metal complexes, and rare-earth metal complexes can catalyse the reaction.46 The proposed mechanism did not involve carbene intermediates, the role of the metal being as a Lewis acid to complex the imine lone pah. Ruthenium porphyrins were found to be efficient catalysts for the cyclopropana-tion of styrenes 47 High diastereoselectivities in favour of the //-product were seen but the use of chiral porphyrins gave only low ees. 

The first reports on iron-catalyzed aziridinations date back to 1984, when Mansuy et al. reported that iron and manganese porphyrin catalysts were able to transfer a nitrene moiety on to alkenes [90]. They used iminoiodinanes PhIN=R (R = tosyl) as the nitrene source. However, yields remained low (up to 55% for styrene aziridination). It was suggested that the active intermediate formed during the reaction was an Fev=NTs complex and that this complex would transfer the NTs moiety to the alkene [91-93]. However, the catalytic performance was hampered by the rapid iron-catalyzed decomposition of PhI=NTs into iodobenzene and sulfonamide. Other reports on aziridination reactions with iron porphyrins or corroles and nitrene sources such as bromamine-T or chloramine-T have been published [94], An asymmetric variant was presented by Marchon and coworkers [95]. Biomimetic systems such as those mentioned above will be dealt with elsewhere. 

The nitrene transfer from PhI=NTos to alkenes catalyzed by the CpFe(CO)2+ fragment gave better results (85% for styrene) [25], but the characteristics of the chemistry of the cationic intermediates as postulated by the reaction mechanism are closely connected to the alternative formation of aziridines by a carbene transfer... 

It would appear that a similar mechanism probably applies to the many aziiidinations of alkenes with lead tetraacetate and a wide variety of other A -aminoheterocycles. In any case, such reactions are now thought unlikely to occur via a nitrene in competition eiqieriments, A -aminqphthalimide/lead tetraacetate reacts with styrene in preference to methyl aciylate (1.5 1), whereas with the supposed genuine nitrene (27) (whose nature is also in doubt " ) prepared by pyrolysis (Scheme 32), the ratio is reversed (1 3). Other sources of supposed (27) by pyrolysis include (28) and (29). Very recently, strong... 

Relative rates of the aziridination of styrene 397 versus a series of ra-substituted styrenes 398 furnishing the respective aziridines 401 and 402 have been determined using Tp Cu(C2H4) (Tp =hydridotris(3,5-dimethyl-l-pyr-azolyl)borate) 400 as the copper precatalyst in combination with PhI=NTs 399 as the nitrene source. The experimental data of the aziridination reaction can be fit with a two-term equation of the type log(>fx/ H) = -t-p cr" a are Jackson s radical substituent constants) leading to the values =—0.28 0.06 (polar contribution) and p = 0.34... 

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>. 

A one-pot procedure designed for the aziridination of a series of styrene derivatives employs commercially available iodobenzene diacetate [PhI(OAc)2] and sulfonamides (427, RSO2NH2) to generate the nitrene precursors [iV-(arene/methanesulfonyl)imino]phenyliodanes (RS02N=IPh) in situ. The reaction is carried out in the presence of the chiral catalyst CuIMeCNIaClOa-L (436 L = 2,2-bis[2-[(4A)-/-butyl-l,3-oxazolinyl]]propane) to give aziridine 437 (Scheme 113) <2004TL3965>. 

The efficient light-initiated decomposition of azides has been the basis for commercially important photoresist formulations for the semiconductor industry. A common approach is to mix a diazide, such as diazadibenzylidenecyclohexanone (I), with an unsaturated hydrocarbon polymer. Excitation of the difunction-al sensitizer produces highly reactive nitrenes which crosslink the polymer by a variety of paths including insertion into both carbon-carbon double bonds and carbon-hydrogen bonds, and by generation of radicals. The polymer component in the most widely used resists is polyisoprene which has been partially eye Iized by reaction with p-toluenesulfonic acid G). Other polymers used include polycyclopentadiene and the copolymer of cyclopentadiene and a-methyI styrene ( ). 

Styrene cyclopropanation continues to attract much interest. Cationic complex CpFe(CO)2(THF) BF4" mediates carbene transfer from ethyl diazoacetate with high cis selectivity cis trans = 85 15) [38]. On the other hand, Tp Cu(C2H4), where Tp is hydrotris(3,5-dimethyl-l-pyrazolyl)borate, is one of the rare catalysts to promote carbene transfer from ethyl diazoacetate to alkenes and also to alkynes. While cyclopropanes are formed in high yield, cyclopropenes are obtained only in moderate yield [39]. The same complex also catalyzes nitrene transfer from PhI=NTs to alkenes to produce aziridines in high yields. 

Compounds of type 303 were isolated by Atkinson and Kelly on cautious oxidation of 3-amino-2-ethylquinazolone-4 [Eq. (87)] (87CC1362). N-Acetoxyamino derivative 305 is stable only at temperatures below 0 C, and at -40°C it reacts with styrene to give aziridine 307, supposedly via intermediate 306. These data were interpreted as a proof of the possibility of forming aziridines without N-nitrene participation. Notice, however, that in principal there is no great difference between the nitrene mechanism and mechanisms presented in Eqs. (86) and (87), since if elimination of X from 304 and of MeCOj from 305 is a little ahead of the following reaction, then, in fact, the masked nitrene mechanism is described. 

Tp CuL complexes catalyze both reactions shown in Scheme 17. The aziridination reaction with such catalysts was discovered using Tp CulCjH ) and Phi=NTs as the nitrene source (Scheme 18). The influence of the hapticity of the Tp ligand and the oxidation state of the copper center were later studied demonstrating that tricoordination of the ligand and +1 as the copper oxidation state were the best choices. The use of the fluorinated version of the above catalyst, that is Tp< u(C2H ) also proved effective. Moreover, the already mentioned Tp Cu(NCMe) complex induced the aziridination reaction not only with the frequently employed olefins (styrene, 1-hexene, cyclooctene) but also with aaylates and using a stoichiometric mixture of olefin and PhI=NTs. ... 

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