Nitrene chemistry oxides

If metal carbene chemistry can be said to be mature, metal nitrene chemistry is in its infancy. Although the first report of a catalytic process used benzenesulfonyl azide,high temperatures were required, and no one has yet provided a synthetically viable method to use azides as sources of nitrenes. Instead, iminophenyliod-inanes (44), formed from the corresponding sulfonamide by oxidation with... 

The ensuing decomposition reactions are light-catalysed and show a first-order dependence on the ruthenium(m) complex. Although back-bonding (dn pn) is important in metal-nitrene chemistry, the trend in rates observed with the complexes [Ru(bipy)aL(N8)] + (L = Ng", MeCN, or py) does not reflect the ability of the metal centre to donate ar-electron density but instead correlates with the oxidizing power of the complexed ruthenium(ni). No free azide radicals were detected in solution and it is suggested that the rate-determining step involves either metal-nitrene formation,... 

This efficient intermolecular C(sp )—H amination reaction has recently been used to address the issue of the unavoidable formation of iodobenzene as a side-product in iodine(III)-mediated oxidative aminations, as depicted in Scheme 1. In line with the recent reports on Phi-catalyzed reactions, the search for iodine-catalyzed amination involving a cooxidant has been investigated but this strategy has been unsuccessful in nitrene chemistry, until... 

In connection with the chemistry of the reactive transient species, nitrene, the chemistry of azepines is well documented u. Also, the chemistry of oxepins has been widely developed due to the recent interest in the valence isomerization between benzene oxide and oxepin and in the metabolism of aromatic hydrocarbons 2). In sharp contrast to these two heteropins, the chemistry of thiepins still remains an unexplored field because of the pronounced thermal instability of the thiepin ring due to ready sulfur extrusion. Although several thiepin derivatives annelated with aromatic ring(s) have been synthesized, the parent thiepin has never been characterized even as a transient species3). 

Preliminary efforts to examine the mechanism of C-H amination proved inconclusive with respect to the intermediacy of carbamoyl iminoiodinane 12. Control experiments in which carbamate 11 and PhI(OAc)2 were heated in CD2CI2 at 40°C with and without MgO gave no indication of a reaction between substrate and oxidant by NMR. In Hne with these observations, synthesis of a carbamate-derived iodinane has remained elusive. The inability to prepare iminoiodinane reagents from carbamate esters precluded their evaluation in catalytic nitrene transfer chemistry. By employing the PhI(OAc)2/MgO conditions, however, 1° carbamates can now serve as effective N-atom sources. The synthetic scope of metal-catalyzed C-H amination processes is thus expanded considerably as a result of this invention. Details of the reaction mechanism for this rhodium-mediated intramolecular oxidation are presented in Section 17.8. 

Imido (NH2-) and arylimido (NR2-) complexes have been prepared for MoVI as well as for lower oxidation states of Mo. The alternative formulation for the ligand is that of a nitrene wherein the ligand is neutral. This, of course, makes the formal oxidation state of the metal two units lower for each ligand compared to the corresponding arylimido formulation. We prefer the arylimido formulation as the resultant complexes all have analogs in the chemistry of oxo MoVI species and do not have analogs in lower oxidation state Mo chemistry. 

We have used the reaction extensively to prepare the indole moiety of several natural products. For example, the key step in the synthesis of the bacterial coenzyme methoxatin (36) is the formation of the indole (35) by intramolecular nitrene insertion from the azide (34), readily prepared from commercially available 4-aminosalicyclic acid. The third ring was annelated onto the indole (35) using conventional chemistry to give, after oxidation to the orrho-quinone, the natural product (36). 

Although silver-mediated oxidative decarboxylation was known for years, its application in synthetic chemistry was very limited (107-110). Systematic studies of this chemistry and other silver-mediated oxidation chemistry in homogeneous solution is rare. This result may be due to the inherent difficulties in working with silver catalysts, which include sensitivity to ligand environment and relative inertness toward oxidation. However, these drawbacks may be overcome with carefully tuned reaction conditions and/or supporting ligand systems. Some of the recent successes with silver nitrene and carbene-transfer reactions will be discussed in detail in Sections VI and Vll. 

In 2005, Cho and Bolm (152) used the previously discussed [Ag2(f-Bu3tpy)2] system to catalyze the imination of sulfoxides. Good to excellent yields could be achieved with various sulfoximines under mild conditions. If a chiral sulfoxide is employed, the corresponding sulfoximine can be prepared after oxidation and deprotection with retention of ee. This reaction further highlights the versatility of the disilver(I) system in catalytic nitrene-transfer chemistry (111,112). The silver-bathophenanthroline system was not employed in this chemistry, however, it may give interesting results as well (Fig. 38) (120). 

Indirect data suggest that ring enlargement sometimes occurs by a mechanism not involving an A -nitrene or by a masked modification, as shown in Eq. (86). Thus, a paradox in A-aminobenzimidazole chemistry is that whereas 1,2-diaminobenzimidazole and its Bz-substituted derivatives give, on oxidation, 3-aminobenzo-l, 2,4-triazines (309, R = H) in high yield (77JOC542), l-amino-2-alkylaminobenzimidazoles under the same condi-... 

The first breakthroughs in this chemistry appeared with the independent works of Breslow [5] and Mansuy [6] who reported the use of Fe(III) and Mn(ni) porphyrins complexes with tosylimidoiodobenzene (Phi = NTs) [7] as an N centered electro philic oxidant (Scheme 12.3). Treatment of an alkene with a high valent Mn nitrene favored the C H insertion product over the competing aziridination reaction. However, yields in these intermolecular reactions remained low. A mixture of N tosylated allylic products was obtained with the product distribution best ratio nalized by a radical C H abstraction/rebound process [6]. 

Upon heating, sulfonyl azide compounds containing a hindered phenolic antioxidant moiety graft to the polymer matrix. This technique is applicable to saturated and unsaturated hydrocarbon polymers, copolymers, and terpoly-mers such as NBR, NR, EPDM, polyethylene, and polypropylene. Even after extraction with an organic solvent, the oxidative stability of the polymers is much greater than that of the polymers containing conventional antioxidants. The preparation of the sulfonyl azide antioxidants and the chemistry and application of the nitrene insertion mechanism are described. 

The most prevalent complexes of this oxidation state are those which contain the [ReO] ", [Re02] and [RejOj] moieties, a reflection of the increasing stability of the rhenium-oxygen bond as the oxidation state increases. The tendency of Re to form multiple bonds (as in [Re=0] and [0=Re=0] ) is also seen in its behavior toward nitrogen, viz. the stabilization of the [Re=N] (nitrido) and [Re=NR] + (nitrene) moieties. With few exceptions, complexes of Re have been found to be essentially diamagnetic, i.e. to contain a spin-paired configuration. Because of the dominance of the [ReO] +, [Re02] and [RejOj] units in the chemistry of Re, such complexes will be considered first in the chemistry of this oxidation state. 

There are many other kinds of reactive intermediates, which do not fit into the previous classifications. Some are simply compounds that are unstable for various possible reasons, such as structural strain or an unusual oxidation state, and are discussed in Chapter 7. This book is concerned with the chemistry of carbocations, carbanions, radicals, carbenes, nitrenes (the nitrogen analogs of carbenes), and miscellaneous intermediates such as arynes, ortho-quinone methides, zwitterions and dipoles, anti-aromatic systems, and tetrahedral intermediates. This is not the place to describe in detail the experimental basis on which the involvement of reactive intermediates in specific reactions has been estabhshed but it is appropriate to mention briefly the sort of evidence that has been found useful in this respect. Transition states have no real hfetime, and there are no physical techniques by which they can be directly characterized. Probably one of the most direct ways in which reactive intermediates can be inferred in a particular reaction is by a kinetic study. Trapping the intermediate with an appropriate reagent can also be very valuable, particularly if it can be shown that the same products are produced in the same ratios when the same postulated intermediate is formed from different precursors. 

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