Nitrenes transition metal complexes

Copper catalyzes the decomposition of sulphonyl azides in benzene very slowly. When methanesulphonyl azide was boiled under reflux in benzene solution in the presence of an excess of freshly reduced copper powder, some decomposition occurred to give methanesulphonamide and azide was recovered 78>. Transition metal complexes have been found to exert a marked effect upon the yields of products and isomer ratios formed in the thermal decomposition of methanesulphonyl azide in methyl benzoate and in benzotrifluoride 36>. These results will be discussed in detail in the section on the properties of sulphonyl nitrenes and singlet and triplet behaviour. A sulphonyl nitrene-iron complex has recently been isolated 37> and more on this species will be reported soon. 

Transition metal complex-catalyzed carbon-nitrogen bond formations have been developed as fundamentally important reactions. This chapter highlights the allylic amination and its asymmetric version as well as all other possible aminations such as crosscoupling reactions, oxidative addition-/3-elimination, and hydroamination, except for nitrene reactions. This chapter has been organized according to the different types of reactions and references to literature from 1993 to 2004 have been used. 

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. 

A substantial 267-article review covers the theoretical treatment of nitrenes.72 Further reviews address the kinetics, spectroscopy, and computational chemistry of arylnitr-enes73 and the generation of nitrene species catalysed by transition metal complexes.74... 

Attempts to achieve asymmetric nitrene insertion reactions catalyzed by chiral transition metal complexes have also been performed [41,42]. The reaction of the nosyl-imine derivative as the nitrene donor with indane 61 catalyzed by the chiral rhodium complex 63 gave the optically active allyl amine 62 in good yield and moderate ee (Eq. (15)) [41],... 

Such complexes have been referred to as nitrene complexes but, strictly, nitrenes are RN species and while zero valent NR compounds have been reported, none has been isolated. Imido compounds are commonly found in transition metal complexes with oxidation states 3 and above. The high capacity for electron donation by the imido group does of course, like that of O2-, act to stabilize high oxidation states prime examples of this are Os(NBu )4 vs. 0s04 and (Bu N)3MnCl vs. 03MnCl. 

The persistent radical effect must always play a role when transient and persistent radicals are formed with equal or nearly equal rates. It leads to the formation of the mutual reaction products in high yields and to the virtual absence of the self-termination reactions. In the few examples given earlier, the persistent species were radicals and transition metal complexes, but other reaction partners such as molecular ions and even normal molecules may take their place. Furthermore, the phenomenon can also work with other transient species, such as carbenes, nitrenes, and molecules in electronically excited states. A literature search would probably reveal a large variety of diverse reactions that exhibit the effect to some degree, although this went unnoticed, so far. Here, we restrict the survey to evident cases. A few of the reactions have even been designed to exploit the persistent radical effect in synthesis. 

A related transformation to the previous carbene transfer reaction involves a nitrene ligand bonded to the metal center, in a metallonitrene intermediate in situ generated upon the appropriate selection of the catalyst and the nitrene precursor. As shown in Scheme 17, some transition metal complexes react with such a precursor to generate an unsaturated intermediate, generally electrophilic in nature, which might react with olefins or C—H bonds affording aziridines or amines in a catalytic manner. The most employed nitrene sources are hypervalent I(III) compounds such as PhI=NTs, chloramine-T or organic azides. 

Like carbenes, nitrenes also insert into C-H bonds and add to double bonds. In some of the most useful nitrene addition reactions, however, free nitrenes are not involved rather a carrier such as a transition-metal complex or an iodine or bromine reagent is... 

For Type lb sonochemical effects one may draw a parallel between the ability of sonication to create coordinatively unsatured transition metal complexes and the photochemical activation of resistant C-H bonds in the presence of selected transition metal complexes.22 The only problem in these cases is that one is dealing here with stoichiometric activation in most cases. In photochemistry, most of the corresponding reactions would be characterized by quantum yields smaller than 1. Examples of such reactions can be found in Ch. 2. Therefore, this type of sonochemical activation which creates active species (radicals, nitrenes, transition metal compounds with vacancies) unable to trigger a chain or a catalytic reaction should consume far more energy than Type la. 

Breslow R, GeUman SH. Intramolecular nitrene C-H insertion mediated by transition-metal complexes as nitrogen analogues of cytochrome P-450 reactions. J Am Chem Soc. 

Despite the volume of work concerned with metal-catalyzed decomposition of diazo compounds and carbenoid reactions 28>, relatively little work has been reported on the metal-catalyzed decomposition of sulphonyl azides. Some metal-aryl nitrene complexes have recently been isolated 29 31>. Nitro compounds have also been reduced to nitrene metal complexes with transition metal oxalates 32K... 

Among other convenient nitrene precursors are chloramine-T (A-chloro-A-sodio-p-toluenesulfonamide), bromamine-T, sulfonamides in the presence of (diacetoxyiodo)benzene and various transition metal catalysts, and sulfonyl azides in the presence of ruthenium complexes . 

Most transition metal imido (nitrene) complexes are nucleophilic at nitrogen, but certain species exhibit electrophilic character. Procedures for the catalytic oxyamination and aziridination of alkenes have been developed where imidoosmium and manganese species, respectively, are generated in situ. 

Organoimido complexes are currently of interest in studies of ligands multiply bonded to transition metal centers. It is now possible to prepare a variety of monoimido complexes for a diversity of the early transition metals,but bis(organoimido) complexes are less well known, inasmuch as routes for their synthesis have been less well developed. The bis(organoimido) moiety is isoelectronic with the cis-dioxo group that is commonly found for higher oxidation states, and it shows potential utility as a stable source of nitrenes in organic syntheses. ... 

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