Methylene insertion reaction

The synthetic method (c), is a base-catalysed ring opening of an oxirane.246 Since the oxirane may be formed by the epoxidation of an olefin (Section 8.1.3, p. 1132), or by a methylene insertion reaction into a carbonyl group (Section 8.1.2, p. 1131), this method is of some versatility. 

Little is as yet understood concerning the transition states involved in the insertion process. In fact, there has been considerable recent controversy on this subject. For methylene insertion reactions the transition state was visualized earlier as a three-membered cyclic complex. 

Methylene insertion reactions into the olefinic CH bond of 164, 168 and 171/172 by diazomethane have been reported several times 292-294.29s. 305. these cases... 

They suggested that dimethyl ether (or methanol) gave CH2 as the primary step, with C2H4 being formed via concerted methylene insertion reactions. However, the necessity for strong acidity in the catalyst implies cationic intermediates. Protonated methylene, the methyl carbonium ion CHa", is improb-able, especially in the presence of water. 

Figure 5. Capillary GC separation of the methylene insertion reaction products of methylcychdodecane. Column was 100 m X 0.25 mm DC-200, and run isothermally at 120° C with H2 as a carrier gas. (7)...

The intensities of the Si-H stretching band ( 2080 cm ) and the Si-CH2-Si deformation band ( 1050 cm ) of the IR spectrum increase with an increase in pyrolysis temperature from 250 to 400 °C. Methylene insertion reactions are also suggested by changes in the CRAMPS, C MAS, and Si MAS NMR spectra. The observation of more than one distinct methylene carbon/proton resonance is consistent with the complicated chemistry of the system, wherein different reactions generate chemically distinct methylene environments. 

The first methylene insertion reaction in ionic hquids involves the reaction of a trialkyl sulfonium salt with an aldehyde to give an epoxide [4]. Here a sulfide is... 

Evidence which was consistent with the postulate of methylene-insertion reactions in the formation of hexanes from pentane and xylene from toluene (both in the liquid phase) has already been mentioned. The formation of the three labeled hexanes from pentane in strict ratio relative to the number of available hydrogen positions on pentane strongly suggested that hot methylene was involved. The butanes product from propane by recoiling C (MacKs,y el cd., 1961) were also found in a ratio corresponding to what might he expected from the reaction of the species in the gas phase. 

The methylene insertion reactions have been found to proceed better in ionic liquids such as [bmin] [PFJ or [bmin] [BFJ on treatment with sulphur ylides. Sulphur ylides are obtained in situ by the reaction of alkyl halides with sulphides. The reaction works equally well with preformed sulphonium salts (Scheme 10). 

It is also possible to carry out asymmetric methylene insertion reaction in good enantiomeric excess by using a chiral sulphide such as 2R,5R-tetrahydrothiophene (Scheme 11). 

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