Carbanion reactivity moderation

Tri(cyclopentyl)borane Synthesis via lithium borates Moderation of carbanion reactivity... 

The problem can be solved by the transformation of the lithium carbanions into the more reactive trichlorotitanium intermediates via the stannanes. Finally, the (- )-sparteine complex of (5)-( )-l-methyl-2-butenyl diisopropylcarbamate105 (Section 1.3.3.3.1.2.) is apparently transmetalated by tetraisopropoxytitanium with inversion of configuration, leading to homoaldol products with moderate diastereomeric excess103. 

P-Keto esters have been prepared in moderate to high yields by treatment of aldehydes with diethyl diazoacetate in the presence of a catalytic amount of a Lewis acid such as SnCL, BF3, or GeCL. The reaction was successful for both aliphatic and aromatic aldehydes, but the former react more rapidly than the latter, and the difference is great enough to allow selective reactivity. In a similar process, aldehydes react with certain carbanions stabilized by boron, in the presence of (F3CC0)20 or NCS, to give ketones. 

Baizer and Chruma reported electrolytic reductive coupling, in a broad study in which reduction of organic halides was conducted in the presence of electrophiles as seen in equation 1034. Controlled potential electroreduction of reactive halides at a mercury cathode in the presence of olefinic substrates with electron-withdrawing groups (Michael receptors) gave moderate yields (50-75% in many cases) of carbanion addition products35. Current yields in excess of 100% in the case where chloroform was used as a cosolvent with carbon tetrachloride indicated the intervention of electrocatalytic reactions (equations 10 and 11). 

The transition metal-free addition of silylphosphines to styrenes, pyridines, and acrylates was promoted by a fluoride source (Scheme 4.17) [59]. The addition reaction occurred under extremely mild conditions and afforded moderate to excellent yields of the anti-Markovnikov addition prodnct. The reaction was proposed to proceed through initial formation of a phosphide anion that reacted with the alkenes through a phospha-Michael-type addition. The proton source for the generation of the final adduct was proposed to be adventitious moisture in the commercially available TBAF solution. While activated alkenes were quite amenable to this approach, unactivated alkenes were unresponsive. 2-Vmylpyridine was particularly reactive and was cleanly converted into the alkylphosphine (95%). While most of the substrates were terminal Michael acceptors, a number of internal alkenes as well as a tetrasubstituted alkene were converted into the alkylphosphines in moderate yields (Schane 4.18 and Example 4.17). The chemistry could be extended to terminal and internal alkynes, although a mixture of E- and Z-isomers were obtained with the EIZ> 1. Following this work, the authors were able take advantage of the need for another electrophile to complete the reaction and added an aldehyde to trap the intermediate carbanion [60]. This approach worked well and enabled the construction of gamma-hydroxyphosphonates in excellent yield (up to 88%). 

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