Nitroacetals cycloaddition reactions

In addition to conventional generation of carbenes from nitrodiazo compounds (22) (79), target intermediates C can be prepared by oxidation of functionalized AN CH2XNO2 with phenyliodonium diacetate. The reactions of Rhodium intermediates with certain olefins afford the corresponding cyclopropanes (23). The cycloaddition reaction was performed in the presence of a catalyst. (The successful synthesis of nitrocyclopropanes from trinitromethane derivatives and nitroacetic ester was also documented (81)). 

There are many reports for nitration of alkenes using various nitrating agents, which proceeds via an ionic or radical addition process.49 Nitration of cyclohexene with acetyl nitrate gives a mixture of 3-and y-nitrocyclohexenes, 1,2-nitroacetate, and 1,2-nitronitrate. This reaction is not a simple ionic or radical process instead, [2+2] cycloaddition of nitryl cation is proposed.50... 

Primary nitro ketones, ethyl nitroacetate, and (phenylsulfony l)nitromethane react with alkenes in the presence of Lewis acids to give nitrile oxide cycloaddition.61a Similarly, the reaction of a-nitro ketones with TeCl4 generates the corresponding nitrile oxides, as shown in Eq. 6.36.61b... 

A further specific behavior of bases in chloroform marks a difference between the two model nitro compounds the nitroketone, unlike the nitroacetate, with some bases undergoes dehydration prior to cycloaddition, at least in part. This is indicated by the production of the corresponding furoxan as an intermediate of consecutive transformation products (see Section 8.2.5). With other bases (e.g., imidazole and its derivatives) condensation is the sole observed reaction, even for the nitroketone. 

For intramolecular reactions, the authors have verified that the E and Z stereoisomers of 2-butenyl nitroacetate maintain their stereochemistry in the bicyclic condensation products, in agreement with a concerted cycloaddition step. However, it might be argued that cycloaddition of nitronic acid followed by elimination would give the same result, without intermediate nitrile oxide. 

The dipolar cycloaddition of nitrile oxides with species containing unsaturation is a very attractive means of forming crosslinks in polymer systems, since the reaction is rapid and yields thermally stable links. Latent nitrile oxide precursors can be prepared by reaction of an isocyanate and an alkyl nitroacetate. These precursors release alkanol and carbon dioxide when heated to liberate the highly reactive nitrile oxide species. An improved synthetic procedure is developed to afford novel crosslinking agents based on difunctional, trifunctional and aliphatic precursors. Application of these agents to polymer crosslinking is demonstrated. 9 refs. 

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