Insertion of nitrenes

The deoxygenation of o-nitrobiphenyls followed by the insertion of nitrene into the o -carbon-hydrogen bond is a useful and versatile method for the synthesis of... 

Formation of the product of formal insertion of nitrene 16e into the aryl and benzylic CH-bonds of toluene was observed upon photolysis of 15e in toluene ... 

CHi generated by photolysis of CH2N2 in the liquid phase is indiscriminate—totally non-selective—in its reactivity (p. 199). CH, generated in other ways and other carbenes are less reactive and insert in the order tertiary > secondary > primary.232 Halocarbenes insert much less readily, though a number of instances have been reported.233 Nevertheless, even for less reactive carbenes, the insertion reaction has seldom been used for synthetic purposes.234 The carbenes can be generated in any of the ways mentioned in Chapter 5 (p. 198). For the similar insertion of nitrenes, see 2-12. 

Vinylsilanes also serve as a starting material for various SMA derivatives, mostly silylaziridines or their precursors silylpyrazolines via insertion of nitrenes, diazomethanes, azides, nitrile inline and nitrogen oxide, N203. 

Insertion of nitrenes occurs readily and may lead to ring expansion and a variety of rearrangements [195, 196] (Figure 9.82). 

When conventional rearrangements are not possible numerous reactions attributable to insertion of nitrenes or of abstraction of hydrogen from the environment to the same species have been reported. An early example was the attack on the solvent to give an azepine derivative (reaction 69) found when phenyl azide was... 

Scheme 6.29 Insertion of nitrene into a saturated hydrocarbon.

Scheme 6.40 (a) DMSO as a nucleophilic trap for nitrenes (b)insertion of nitrene into... 

Although the mechanism of Hemetsberger indole synthesis is not quite clear, the reaction is postulated to proceed via a highly electrophilic singlet nitrene species, which then inserts into an aromatic ring to form the indole derivatives. In addition, 2H-azirines are also proposed as the intermediates for this reaction, arising from the insertion of nitrene to the adjacent double bond. A tentative mechanism is given below. 

The insertion of nitrenes, generated via the photolysis or thermolysis of azides, into C-H bonds is well known, although the yields are generally poor. However, when ethyl azidoformate is thermolyzed in the presence of 1-chlorocyclohexene, the allylic insertion product is formed in 49% 3deld (eq 10). In this case it is not clear if the product is formed through direct insertion or by the ring opening of an aziridine intermediate. 

Like their carbene cousins, the insertion of nitrenes into C-H bonds is a useful method for functionalizing molecules (Scheme 8.148). " The nitrenes are typically generated by the oxidation of an amine derivative in the presence of the metal catalyst, most often, but not exclusively, a rhodium complex. Widely used oxidants are iodosobenzene diacetate and iodosobenzene. These react with the amine derivative to form an... 

Apart from hydrodesulphurization, reactions causing transformation or destruction of the thiophen ring are rather rare. Three interesting examples have been reported. In the first, photolysis of 3-cyanothiophen (22) in furan affords the adduct (23) as the major product. 2-Cyanothiophen behaves in a similar way, and an analysis of permutation patterns has indicated that there is a mechanism involving first the 2,5-bridged compound (24) followed by a walk of the sulphur atom. Anils (25) are converted into pyrroles (26) by triethyl phosphite a similar ring-opening by intramolecular insertion of nitrene has also been shown to occur with o-nitrophenyl-di-(2-thienyl)methanes. ... 

A copper-catalyzed enantioselective aziridination of aryl cinnamyl ethers and intramolecular arylation of the in situ generated tethered aziridine was developed by Hajra and Sinha. This method provides an easy access of N-sulfonyl-protected fra s-3-amino-4-arylchromans with high regio-, diastereo- (dr > 99 1), and enantioselectivity (ee up to 95%) (Scheme 8.113). Three C(sp )-H bonds were activated in this transformation under this Cu(0Tf)2/Cu(C10 )2-6H20 catalyst system. The major side reaction is the C-H insertion of nitrene at the O-CH2 unit followed by oxidative cleavage [188]. 

---