Hydridic Organometallic Reagents

Grignard reagents react sluggishly with oxabicydic compounds in the absence of a transition metal catalyst. In the presence of excess MgBr2, the products of 

The product from reductive ring opening was isolated along with the product from the nucleophilic addition in the reaction of f-Bu2CuCNLi2 with oxabicyclic substrate 253 b, Eq. 158, vide infra [202]. Reduction by of one of the -hydrogens of the ferf-butyl group of the cuprate must be responsible for this product. 

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The most common reactions of carboxylic acid derivatives are substitution by water (hydrolysis) to yield an acid, by an alcohol (alcoholysis) to yield an ester, by an amine (aminolysis) to yield an amide, by hydride ion to yield an alcohol (reduction), and by an organometallic reagent to yield an alcohol (Grignard reaction). 

Reduction by hydride transfer from the organometallic reagent can become predominant if bulky organometallics are employed. 

Treatment of 19b with phenylmagnesium bromide gives diphenylacetylene (66) and the salt of benzenesulfmic acid Lithium aluminium hydride reacts with 19b similarly. These ring-opening reactions are similar to the reactions of organometallic reagents with the analogous thiirane dioxides (equation 17 above). 

The stereochemistry of addition of organometallic reagents to chiral carbonyl compounds parallels the behavior of the hydride reducing agents, as discussed in Section 5.3.2. Organometallic compounds were included in the early studies that established the preference for addition according to Cram s rule.118... 

In the presence of organometallic reagents (R1 MgX, R11u, diisobutylaluminium hydride (DIBAL-H)), the Weinreb amides 186 lead selectively to the corresponding carbonyl derivatives 187 as a mixture of Z/E isomers which spontaneously undergo partial cyclization to dihydropyrrolizines 188 (Scheme 42). This cyclization was completed by refluxing the cmde mixture in chloroform in the presence of silica gel <2002S2450>. 

Other reactions employed in grafting to methodologies rely on the prior formation of carbanionic polymers. For example, strong bases or organometallic reagents such as sodium hydride or butyl lithium, can afford the polymeric anion which then reacts with the tube [45]. 

The stereoselectivity of reduction of carbonyl groups is effected by the same combination of steric and stereoelectronic factors which control the addition of other nucleophiles, such as enolates and organometallic reagents to carbonyl groups. A general discussion of these factors on addition of hydride is given in Section 3.10 of Part A. 

The stereochemistry of addition of organometallic reagents to acyclic carbonyl compounds parallels the behavior of the hydride reducing agents, as discussed in Section... 

Another method for the preparation of bicyclic lactams consists of formation of a succinimide 5, followed by reaction with a hydride or an organometallic reagent to give a carbonyl equivalent that is readily cyclized upon workup10 15. 

Acid chlorides are easily converted to 1° alcohols and aldehydes (see Section 5.7.21) and 3° alcohols and ketones through the choice of appropriate metal hydride and organometallic reagents (see Section 5.5.5). Acid chloride reacts with benzene in the presence of Lewis acid (AICI3) in Friedel-Crafts acylation (see Section 5.5.6). 

Primary and tertiary alcohols are obtained conveniently from esters by the reduction of LiAlH4 and two molar equivalents of organometallic reagents (R MgX or R Li), respectively (see Sections 5.7.22 and 5.5.5). A less powerful reducing agent, diisobutylaluminium hydride (DIBAH), reduce an ester to an aldehyde (see Section 5.7.22). 

The ff-adducts are obtained with the hydride ion under conditions similar to those derived from organometallic reagents and may therefore involve strong association or covalent bonding with the metal ions in much the same way (see initial statement regarding structural formulas in Section II, D,2,a). 

Triazasilatranes 179 and 180 react with various nucleophiles such as organometallic reagents (equation 176), metal alkoxides (equation 177) and amides (equations 178 and 179) to give the substitution products 172, 181-184 as well as hydride transfer products 169, 170. The relative ratios of these products depend on stereoelectronic factors, the nature of the nucleophilic reagents and the reaction conditions312. Thus, the reaction of triazasilatrane 180 with /i-butyllithiurn affords 181a, which is the product of substitution, while only 1-hydrotriazasilatrane (170) is formed from 180 and /e/t-butyllithiurn in a hydride transfer process. 

Lithium triethylborohydride (Super-Hydride) is a much more powerful reducing agent than lithium aluminium hydride. It is useful for the reductive dehalogenation of alkyl halides, but unlike lithium aluminium hydride does not affect aryl halides. It is available as solution in tetrahydrofuran in sealed containers under nitrogen. The solutions are flammable and moisture sensitive and should be handled with the same precautions as are taken with other organometallic reagents (see Section 4.2.47, p. 442). 

Addition of hydride sources or organometallic reagents to (3-ketosilanes... 

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