Carbenes generation case studies

Phenylene-linked bis(diradicals), where the diradical is a carbene or a ni-trene, are relatively simple and experimentally accessible and it is possible to vary the carbene site by chemical substitution. Thus, such compounds are reasonable models for the spin-spin interactions across conjugated systems and, in particular, across aromatic rings. This chapter is devoted in the recent efforts to generate and study such systems in order to progress our understanding of more complex cases. 

The great majority of matrix isolation studies of carbenes and nitrenes have employed their formal adducts with molecular nitrogen, that is, diazo compounds or diazirines in the case of carbenes, azides in the case of nitrenes, as precursors for their in situ generation. Usually, these compounds will readily release N2 on irradiation with a low-pressure mercury lamp (254 nm), and this fragment has the advantage that it will usually not react with or perturb the targeted reactive intermediate (see Scheme 17.2). 

Adlhart and Chen [126] reported a QM/MM study of the olefin-metathesis reaction catalyzed by [L2(Cl)2Ru=CHPh] complexes L are phosphane groups in the case of the first-generation catalyst, whereas for the second-generation catalysts an N-heterocyclic carbene ligand replaces one of the phosphanes. A study of first- and second-generation metathesis catalysts... 

The irradiation of the methyl substituted cyclic allene (163) has been studied to evaluate the influence of substitution on the reactions encountered. This work follows on an earlier study of the photoreactivity of the unsubstituted allene. Direct irradiation using light of wavelength > 220nm in pentane brought about the formation of the products shown in scheme 7 in the yields shown below the appropriate structure. The products obtained from this reaction were compared with those from a thermally generated carbene intermediate and the conclusion reached is that carbene intermediates are involved in the photochemical reaction. This is different from the unsubstituted case 80... 

Another comparative study of the CM reaction catalyzed by a second generation Grubbs carbene complex (without imidazolium tag) has been recently presented by Tang and coworkers [264]. They compared the catalytic transformation of styrene in [BMIM][BF4] and [BMIM PFs] with the same reaction in CH2Q2 (3 h, 45 °C) and obtained comparable yields of the CM product in all solvents in the first cycle. In the case of the two ionic liquids the product was extracted with diethyl ether and the remaining ionic catalyst solution could be recycled four times with only a small drop in catalytic activity (product yield 75% in the fourth cycle compared to 85% in the first cycle in [BMIMjfPFs]). The scope of the study also included the successful... 

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