Subject with Grignard reagents

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

Matsuda and Sugishita found that cyclooctatetraene monoepoxide (233 equation 99) gave only skeletally rearranged products, e.g. (234), when treated with Grignard reagents. In an effort to isolate the presumed intermediate cycloheptatrienecarbaldehyde, (233) was subjected to a catalytic amount of MgBra, but this resulted in the formation of phenylacetaldehyde. 

Allylic chlorides are generally more useful than allylic alcohols for further synthetic operations since they are subject to SN2 displacements and, particularly, to coupling with Grignard reagents. Thus (2) on coupling with methallyl chloride in THF-HMPT gives the 1,5-diene (3) in high yield. Use of HMPT as solvent is essential for satisfactory yields. 

On the subject of Grignard reagents, a review of their mechanism of reaction with ketones has been made by Ashby. 

In -keto-aldehydes the aldehyde group is easily and preferentially converted into a stable monoacetal, in which the free ketone group can be reduced by metal hydrides [229, 230], or by sodium and ethanol [231], made to react with lithium acetylide [232] or with Grignard reagents [233, 234], converted into a cyanohydrin [235] or subjected to a glycidic ester condensation [236] all in a normal manner and in high yields. The use of the acetals of acetoacetaldehyde as synthetic intermediates has been reviewed [237]. 

Determination of the structures of Grignard reagents continues to be of interest, and reviews on this subject have appeared.126,127 Most of the structure authentications are done on crystalline materials, although solution studies performed with extended X-ray absorption fine structure (EXAFS) spectroscopy are also available. The Grignard compounds MeMgBr and EtMgBr in BunzO were studied at room temperature and —85 °C with EXAFS. At both temperatures, dimers are observed (Mg-(/r-Br) = 2.5 A Mg-O = 2.0 A).128... 

O - P h e n y 1 - /V - e r y t h r o s y 1 nitrone (336), as a Ci,C i-bis-electrophile, when subjected to the double addition of Grignard reagents (in a domino style), leads to acyclic hydroxylamine (338) via the formation of open-chain nitrone (337 ). The reaction proceeds at 0°C with variable diastereoselectivity ranging from medium to good, depending on the organometalic reagent used (Scheme 2.140) (564). 

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