Rearrangement of Carbenes

Cydizations and Other Intramolecular Rearrangements of Carbene Complexes... 

Figure 1. Energy profile for the rearrangement of carbene 26, based on calculations at the B3LYP/6-311 G(d,p)//B3LYP/6-31 G(d) level of theory.1 ...

Direct dynamics calculations were carried out with quasiclassical normalmode sampling from a canonical ensemble at 923 K (the experimental reaction temperature). Simulations initiated at the vicinity of TS for rearrangement of carbene 13 to 14 via oxirene 12, and 300 trajectories were obtained at DFT methods. The preliminary results reported in the manuscript showed that preferred formation of 15a over 15b by the ratio of 1.8 7.6 depends on the method used. The results were qualitatively consistent with the value of 2.5 deduced from the experiment. The non-unity ratio likely arises from the situation that two methyl groups in 14 are dynamically unequal on the carbene formation process. 

Relative Migratory Aptitudes of Substituted Phenyl Groups (Ar) in Rearrangements of Carbenes and Carbonium Ions... 

The thermal decompositions of diazirines appear to be homogeneous, uni-molecular processes " . Cyclic diazirines decompose with a slightly smaller activation energy (Table 14) due to ring strain. Product distributions presumably result from intramolecular rearrangement of carbene intermediates, and the fact that the product ratios differ from those obtained on photolysis supports the hypothesis that the carbenes generated photolytically contain considerable excess vibrational energy. 

Photolysis of diazirine 63 included within the cavities of CyDs presumably formed carbene 64 CyD ICs, i.e., 64 CyD (Equation (7)). The lifetime (t) of carbene 64 was expected to be prolonged due to the preclusion of intermolecular reactions (Scheme 14), such as azine 70 formation and solvent insertion, i.e., 64->72. However, interfering innermolecular reactions21 between the host and guest were indicated (vide infra). Therefore, the latent intramolecular rearrangement of carbene 64 to cyclic allene 65, a rare transformation seen under the forbidding, low-temperature... 

The cleavage of silane into alkylfluorocarbene requires temperatures of at least 140°C. A competitive reaction is the rearrangement of carbene to fluoroalkene. 1-Fluoro-l-fluoromethyl-2,2-dimethylcyclopropane was prepared from trifluoro(l,l,2-trifluoroethyl)silane and 2-methyl-propene see Houben-Weyl, Vol. E19b, pp 1466-1467. Similarly, fluoro(methyl)carbene was generated and underwent addition to 2-methylpropene to give 2. ... 

Platz, M. S., Huang, H. Y., Ford, F., Toscano, J.,Photochemical Rearrangements of Diazirines and Thermal Rearrangements of Carbenes Pure Appl. Chem. 1997, 69, 803 807. 

A similar anti diastereoselectivity has been proposed for the rearrangement of carbene-derived ammonium ylides obtained from crotyl- and cinammylamines 14105. 

As is the case for the [2,3] Wittig rearrangement, the stereochemical consequences of the [2,3] ylide rearrangement are sensitive to perturbation by external steric and stereoelectronic factors, presenting a useful opportunity for both substrate- and reagent-based asymmetric induction. Rearrangements of carbene-derived ylides of allylic sulfide 1 provide a simple example of substrate-directed diastereosclcction, in which diastereoface selectivity results from attack on the exocyclic olefin via the less-hindered equatorial approach vector112. 

Benzyne (1) and cyclopentylidenecarbene (121) are isoelectronic. The formation of benzyne from 121 in the gas phase was first proposed in 1979, and the ensuing studies have recently been reviewed. The rearrangement of 121 -> 1 was suggested from pyrolysis results with the alkylidene Meldrum s acid 122. Besides high yields of the expected carbon dioxide, acetone and cyclopentadiene, substantial amounts of aromatic products such as biphenylene (27%) and triphenylene (12%) were obtained that were most easily accounted for as a consequence of rearrangement of carbene 121 to benzyne and subsequent di- or trimerization. 

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