Boron compounds carbanions

When the boron compound coordinates with the carboxylic acid in this way, two things result. First it labilizes the potential carbanion. One can prove this by infrared observations where, in going from BH3 to BH4, such coordination weakens the BH linkage. At the same time, this should increase the acidity of the... 

Diboryl compounds do not yield aldehydes with alkaline hydrogen peroxide. Instead there is r d hydrolysis, presumably via a boron-stabilized carbanion (see Volume 1, Ch ter 2.6) which is protonated and then oxidized to the alcohol (equation 16). ... 

There are currently three general methods for the production of boron-stabilized carbanions, each of which has analogies in caibonyl chemistry. The cleavage of a 1,1-diboiylalkane by base readily yields the desired anion in a fashion similar to the base cleavage of a -dicaibonyl compound. Deprotonation a to a boron atom can be accomplished directly in special circumstances, as can the addition of an organo-metallic compound to a vinylborane, which is similar to conjugate addition to an a,p-unsaturated carbonyl compound. Each of these methods is treated in detail in Ae next three sections. 

Although cleavage of gem-diboryl compounds is a mild and general process for die production of boron-stabilized carbanions, it nevertheless suffers from two disadvantages. Firstly, although... 

As much recent work has concerned dimesitylboryl species, it is worth remarking that the related compounds (22)-(24) also readily yield carbanions. Unlike mesityl compounds, (22) cannot be deproton-ated at the 4 -methyl group, while compounds (23) have some special properties due to the great increase in steric hindrance around boron. Compounds (24) promise to be of utility due to their very ready solvolysis with water or alcohols in the presence of caudytic quantities of mineral acid. n>ey are even selectively hydrolyzed in preference to vinyl as well as alkyl groups (equation 23). ... 

In general, all types of boron-stabilized carbanions are readily alkylated by primary alkyl halides. The situation for the i ylation of dimesitylboryl compounds is sununarized in Scheme 2. These reactions represent highly efficient homologation processes. Oxidation of the tertiary all l organoboranes is slow, but use of 4-methoxy-2,6-dimethylphenyl groups instead of mesityl groups renders the products sensitive to solvolysis. ... 

Boron-stabilized carbanions. Ordinarily bases coordinate with the boron atom of organoboranes. However, Rathke and Kow report that a highly hindered lithium amide such as lithio-2,2,6,6-tetramethylpiperidine or lithio-t-butylneopentylamine2 can remove the a-proton from an organoborane to generate carbanions. Thus treatment of the boron compound B-methyl-9-borabicyclononane (1)J in benzene with the former base for 12 hr. at room temperature followed by quenching with deuterium oxide results in deuterium incorporation of 50% (equation I). 

Thiophilic addition to dithioesters followed by capture of the thioacetal anion with an electrophile provides a useful approach to the formation of carbonyl compounds such as (42) and (43), and has been elegantly employed in the total synthesis of the macrocycles ( )-maysine and ( )-4,5-deoxymaysine. Trimethyl borate has been used to convert bis(phenylthio)methyl-lithium into the keten thioacetal (45) via the boron-containing carbanion (44), as shown in Scheme 4. Bis(phenylthio)methyl-lithium and related compounds are also the key reagents in the preparation of 2-(phenylthio)-enones, 2-(phenylthio)-butadienes, a-(phenyl-... 

The reactions described so far can be considered as alkylation, alkenylation, or alkynylation reactions. In principle all polar reactions in syntheses, which produce monofunctional carbon compounds, proceed in the same way a carbanion reacts with an electropositive carbon atom, and the activating groups (e.g. metals, boron, phosphorus) of the carbanion are lost in the work-up procedures. We now turn to reactions, in which the hetero atoms of both the acceptor and donor synthons are kept in a difunctional reaction produa. 

In contrast to the transition metals, where there is often a change in oxidation level at the metal during the reaction, there is usually no change in oxidation level for boron, silicon, and tin compounds. The synthetically important reactions of these three groups of compounds involve transfer of a carbon substituent with one (radical equivalent) or two (carbanion equivalent) electrons to a reactive carbon center. Here we focus on the nonradical reactions and deal with radical reactions in Chapter 10. We have already introduced one important aspect of boron and tin chemistry in the transmetallation reactions involved in Pd-catalyzed cross-coupling reactions, discussed... 

Boron trichloride appears to form with methyl /3-D-ribofuranoside a complex which, after treatment with appropriate nucleophiles, such as sodium methoxide, or the carbanion derived from diethyl 2-(ethox-ycarbonylmethyl)malonate, gives the corresponding a-glycosyl compound.147... 

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