Grignard reagents calculations

The 1-alkyl-diaziridines can easily be hydrolyzed to alkyl hydrazines. Hence alkyl hydrazines are easily available from Grignard reagents and thus from alkyl halides. The three last examples of Table XII show the yield of alkyl hydrazine calculated on the diazirine used. The reaction has preparative interest because the alkylation of hydrazine with alkyl halides only gives monoalkyl hydrazines in exceptional cases. ... 

Cyclohexylcarbinol (6, 22), In the preparation of cyclohexylcarbinol as described in Vol. 6, 22, a high-boiling by-product, the cyclohexylcarbinol acetal of formaldehyde, is sometimes obtained. The by-product becomes the main product if the steam distillation of the reaction mixture is omitted. The by-product can usually be avoided if twice the calculated amount to decompose the Grignard reagent of io per cent sulfuric acid is added to the reaction mixture before steam distillation is earned out. The acetal which may be present is thus hydrolyzed. 

In an investigation of the stereoselectivity of nucleophilic addition to larger ring systems, ethyl-, vinyl-, and ethynyl-lithium and -Grignard reagents have been added to 2-(3 -phenylpropyl)cycloheptanone (69). In all cases, the predominant product is the cw-alcohol, and calculations have been used to identify the steric and torsional effects in the transition state that favour this stereochemistry. 

Nucleophilic additions of ethyl-, vinyl-, and ethynyl-lithium and Grignard reagents to a 2-alkyl-substituted cycloheptanone have been found to yield the corresponding cw-cycloheptanol preferentially. The selectivity, which increases with size of the nucleophile, has been attributed to a combination of steric repulsions and torsional effects in the transition state. Application of the MM2 force field to analyse nucleophilic attack of hydride and ethynyl lithium on 2-methylcycloheptanone gave results comparable to those obtained experimentally and from ab initio calculations. 

Ab initio calculations also confirm that the use of an allyl magnesium alkoxide in place of the alcohol functionality will lead to high or complete stereoselectivity (138). When homoallylic alcohols are used, the Kanemasa protocol afforded the respective isoxazolines with poor stereoselectivity ( 55 45) in the case of terminal aUcenes, but with very high diastereoselectivity (up to 96 4) in the reaction of cis-1,2-disubstituted olefins (136). Extension of this concept to the reaction of a-silyl allyl alcohols also proved feasible and produced the syn (threo) adducts as nearly pure diastereomers (>94 6) (137). Thus, the normal stereoselectivity of the cycloaddition to the Morita-Baylis-Hillman adducts (anti > syn, see above) can be reversed by prior addition of a Grignard reagent (176,177). Both this reversal... 

TABLE 5. Enthalpies of reaction between ketones and Grignard reagents and calculated enthalpies of formation of alkoxymagnesium bromides (kJ mol )... 

Semiempirical [PM3(tm)] calculations indicated that Cu complex A adopts a distorted tetrahedral structure with the positioning of the Grignard reagent at the bottom face of the complex (Figure 8). 

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