Regiospecificity of the Reaction

Since ketoximes exist predominantly in Z-configuration relative to the least bulky substituent [276,277], it becomes clear that, similar to the Beckman rearrangement, the reaction proceeds with participation of the radical, which is in -position towards 

SCHEME 1.101 Competition between the methyl and methylene gronps in the reaction of methyl alkyl ketoximes with acetylenes. 

The following questions are important for an understanding of the reaction mechanism and for better preparative use of the reaction Which of the two groups of the ketoxime, the methyl (when R =H) or methylene group, primarily participates in the construction of the pyrrole ring Are there differences in the reactivity of the two a-methylene moieties (R R, R =Alk) related to different alkyl groups with normal structures Do the reaction conditions affect the ratios of the structural isomers The answers to these questions have been found during a study of the interaction between a number of unsymmetrical ketoximes with acetylene in the presence of the KOH/DMSO system under conditions that lead to N-vinylpyrroles [188]. 

At 120°C, all methyl alkyl ketoximes react with acetylene exclusively at the methylene group of the alkyl radical irrespective of a structure of its other part Q.so- or normal, the exception makes methyl-iso-propyl ketoxime that reacts exclusively at the methyl group, forming the only one isomer). With temperature increase, the regioselectivity of reaction is broken at 140°C, pyrroles are already formed involving the methyl group in amount of 20%-50% from the mass of the mixture, though the total yield drops. 

by selection of the conditions, one can change the direction of the reaction and synthesize not only 2,3-diaIkyl-substituted but also 3-unsubstituted pyrroles 

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Information concerning the dependence of the regiospecificity of the reaction on the configuration of the starting material was provided by the reaction of methyl l,3 4,6-di-0-benzylidene-/3-D-allopyrano-side 277 with butyllithium277 the major product (contaminated by only a trace of the unsaturated derivative 279, the fi anomer of compound 259) was the 3-deoxyhexos-2-ulose 278 (fi anomer, the regio-... 

The regiospecificity of the reaction depends on the relative migratory ability of the substituents attached to the carbonyl. Substituents which are able to stabilize a positive charge migrate more readily, so that the order of preference is tert. alkyl > cyclohexyl > sec. alkyl > phenyl > prim, alkyl > CH3. In some cases, stereoelectronic or ring strain factors also affect the regiochemical outcome. 

Steric factors probably play a key role in determining the regiospecificity of the reactions leading to the phosphine oxides 4a and 4b. An intermediate involved in both cases is probably of type 5, in which ring closure preferentially occurs para rather than ortho to the bulky CF3 group. The preferential formation of 5b from the corresponding phosphoramidous dichloride is undoubtedly associated with the fact that... 

Regiospecificity of the reaction is normally high but with some alkenes, such as styrene, considerable amount of the /3-isomer (38.5%) is produced exceptionally in addition to the a-isomer (58.7%). Interestingly, alkenes having an inner double bond undergo hydrosilation with isomerization to give exclusively terminal products, as shown in equation (24). 

As with thiosilanes, so the selenosilanes open oxygen substituted rings, but better yields result if selenium is rendered hard through using 18-crown-6 and KF. Acid catalysis also influences the regiospecificity of the reactions (equation 63)58. 

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