Grignard-like derivatives

Organoboranes do not normally react with carbonyl compounds in Grignard-like fashion with the exception of allylboranes. Aromatic aldehydes reacted with dialkylboron chloride derivatives in the presence of base to generate arylalkylmethanols in good yields (Equation (132)). On the other hand, reactions of aromatic aldehydes with dialkylboron chlorides in the presence of oxygen resulted in chlorination (Equation (133)).595... 

As the number of nitrogen heteroatoms increases, the stability of lithium azoles decreases the 5-lithio derivatives of 1,2,3-triazoles 778 ring-open spontaneously. l-Methyltetrazol-5-yllithium decomposes to nitrogen and lithium methyl-cyanamide above 50 G, although it gives the expected Grignard-like reactions with bromocyanogen, esters, ketones, and sulfur at lower temperatures. 

Allylic Alcohols. The allylic oxidation of alkenes mediated by selenium dioxide produces reduced selenium by-products which can be a disadvantage in the practical separation of the allyl alcohol products. A new procedure uses only catalytic amounts of selenium dioxide in the presence of t-butyl hydroperoxide to reoxidize and thus recycle the reduced selenium species, and leads to clean allylic oxidation with yields comparable to the stoicheiometric method. B-Alkenyl derivatives (27) of 9-BBN (3) react in a Grignard-like fashion with aldehydes to give allyl alcohols (Scheme 7) in which the alkene stereochemistry is retained from the vinyl-borane. 

Derivatives with Ln(IIl)-C bonds have been prepared from metals and organic halides (30) and have been briefly studied as Grignard like reagents. As previously discussed, ytterbium and samarium metals do not result in pure trivalent organolanthanides, because of formation of divalent species. 

Unique to E-vinyl derivatives of 9-BBN is their Grignard-like addition to aldehydes [6] to afford stereodefined alcohols. trans-E-Vinyl-9-BBN synthesis via dehydroborylation [7] process is extended to the synthesis of trans-2-tnmeth-ylsilyl vinylborane [8]. trans-2-Trimethylsilylvinyl-9-BBN adds cleanly to aldehydes (Eq. 6.17) to provide pure frans-3-trimethylsilyl(TMS) allylic alcohols in excellent yields (Table 6.18) [8]. ... 

The detailed investigation of reactions of (Me5C5)2Ln(Et20) (Ln = Sm, Eu, Yb) with alkyl- and arylhalides RX, fulfilled by Finke, Watson and coworkers [59 - 61], has shown the complicated character of these processes. The proposed scheme of the reactions of ytterbium and samarium derivatives includes the interspheric electron transfer, oxidation of metal to Ln(III), free radicals R generation and Grignard-like reactions ... 

However, saturated organoborane derivatives appeared to be unreactive toward carbonyl compounds. Early studies involving alkylborane additions to aldehydes resulted in the reduction of the aldehyde via 3-hydrogen elimination. Grignard-like reactions were successful only after modification of the carbonyl group, the trialkylborane, or by adding transition metal catalysts. ... 

Upon addition of (—)methylacrylate to the hydride, two diastereomers were obtained, as evidenced by NMR, and separated. Racemic tetraorganotin derivatives can be obtained following analogous routes using alternatively Grignard reactions and bromodemethylations133,134. Bromodemethylations must necessarily be carried out in a polar solvent like methanol. 

Metals more electronegative than magnesium, like beryllium, zinc, cadmium and mercury, form useful reagents for specific purposes, but the metals themselves are not sufficiently active to form organic derivatives under normal laboratory conditions and are unwanted in the environment since they are toxic. Aluminum compounds are useful for industrial purposes, but their use in the laboratory is insignificant in comparison with Grignard reagents. 

High yields and diastereoselectivities have also been observed for the addition of Grignard reagents to imines like 221 derived from phenylglycinol (222), which are existing in equilibria with 1,3-oxazolidines. Also, the imine derived from methoxyacetone affords amino-ethers with excellent diastereoselectivities. The addition of a Lewis acid (MgBr2) has a strong effect on both the yield and the selectivity (equation 151) . ... 

The alcohol 10 looks like it might be formed by the addition of a Grignard reagent to an aldehyde. In fact, Patrick Steel of the University of Durham prepared 10 (Tetrahedron Lett. 44 9135,2003) by Diels-Alder addition of the transient silene derived from 7 to the diene 8. More highly substituted dienes lead to more complex arrays of stereogenic centers. The intermediate silacyclohexenes, exemplified by 9, should also engage in the other reactions of allyl silanes. 

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