Grignard reaction centers

In contrast to the widely investigated stereochemistry of nucleophilic substitution at optically active tricoordinate sulfur, there have been few similar studies with optically active tetracoordinate sulfur systems. Sabol and Andersen (174) were the first to show that the reaction of p-tolylmagnesium bromide with (-)-menthyl phenyl-methane[ 0- 0]sulfonate 140 proceeds with inversion of configuration. Thus, the Grignard reaction at the sulfinyl and sulfonyl centers takes place with the same stereochemistry. 

This is consistent with DFT calculations on [CH3MgCl2], which reveal that the six-centered transition state for the enolisation reaction 66 is entropically favored over the four-centered transition state for the Grignard reaction 67 (Scheme 14). 

The first system to be studied involved the addition of methyltitanium tri-isopropoxide 6 (one part) to a 1 1 mixture of hexanal 74 and 2-ethylbutanal 75 21). The product ratio 76 77 turned out to 92 8 (95 % conversion), showing that reactions of organotitanium reagents are very sensitive to the steric environment around the reaction center. It is interesting to note that in Grignard reactions a reversed mode of addition is required, i.e., the aldehyde is added to the very basic organometallic reagent, otherwise rapid aldolization sets in. For competition experiments this is unsuitable 21). 

The variation of organo-substituents at the silicon centers performed by Grignard reactions or by hydrosilylation of H-silanes or H-siloxanes with alkenes and alkines, respectively, gives even today materials with new industrially exploitable properties. Thus, besides the classical routes of condensation and addition curing mechanisms to build silicon network structures, new opportunities of photocrosslinking have been put into practice. This counts as well for radical as for cationic systems. 

The initial strategy to fix both side chains containing the Diels-Alder reactants to a sp center was maintained. Compound 63 was synthesized by alkylation and esterification. The second chain with the diene unit was introduced by a Grignard reaction. Ketalization and transformation of the vinyl bromide with tert-... 

There has been some controversy over the nature of intermediates in the Grignard reaction (54-56). Available evidence indicates that organic radicals form through cleavage of the carbon-halogen linkage everyone agrees on this. Controversy centers on the question of whether the alkyl radical remains adsorbed on the metal surface ( A model ) or whether it diffuses into solution ( D model ). Evidence appears to be more consistent with the D model (56), but the question cannot be considered as settled. 

Allylic halides couple effectively with vinyltin reagents to provide the corresponding 1,4-dienes in high yields.2 Like nickel- and palladium-catalyzed Grignard reactions with allylic ethers, this reaction also proceeds with net inversion of configuration at the allylic center. Similarly, vinyl epoxides react... 

Butenoic esters bearing a chiral quaternary a-carbon center are prepared by Grignard reaction of a-substituted y-chlorocrotonic esters in the presence of (4R, 5i )-diphenyl-l,3-dimesitylimidazolylidene. ... 

Grignard reaction of 4-pentenyl magnesium bromide with aldehyde 27 afforded alcohols 28 and 29 in 89% yield in a 97 3 ratio. At this point the stereochemistry of the C-22 center of 28 and 29 was not clear. Compound 28 was easily transformed into lactone 30 by ozonolysis followed by an oxidation of the resulting hemiacetal. Triflic acid catalyzed solvolysis of 30 afforded lactone 26 in 97% yield. The stereochemistry it C-22 of 28 was... 

The simulation of sonochemical effects on liquid-solid reactions is accomplished by a two-center model that allows for mechanically activated areas on a solid with increased reactivity. The parameters have been fitted to experiments and can be used to find suitable reaction parameters as well as a prediction whether or not a given Grignard reaction will perform better under ultrasound activation or not. 

Caution new chiral centers are often created in Grignard reactions with aldehydes or ketones. When neither the aldehyde/ketone nor the Grignard reagent is chiral but the product has a new chiral center, a racemic mixture is formed. 

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