Carbanions reagents

Posner and coworkers have published a series of papers in which they described a successful application of the Michael reaction between a variety of carbanionic reagents and chiral cycloalkenone sulphoxides 557 to the synthesis of chiral organic compounds (for reviews see References 257, 649, 650). In several cases products of very high optical purity can be obtained. Subsequent removal of the sulphinyl group, serving as a chiral adjuvant, leads to optically active 3-substituted cycloalkenones 558 (equation 356 Table 27). 

Reaction of a chlorophosphine with a stabilized carbanion reagent... 

Some carbyne complexes, in particular cationic ones with good Ji-accepting ligands, can react with nucleophiles to give carbene complexes [187,521]. Several reductions of carbyne complexes to carbene complexes by treatment with metal hydrides have been reported. Similarly, organolithium or other carbanionic reagents can react with electrophilic carbyne complexes to yield carbene complexes. Illustrative examples of both reactions are sketched in Figure 3.23. 

We here further demonstrate the merits of Fe -exchanged montmorillonite over the conventional homogeneous acid catalysts, applying it to other types of liquid-phase carbon-carbon bond-forming reactions between carbonyl compounds and useful nucleophilic carbanion reagents such as silyl ketene acetal (an ester... 

Nucleophilic Addition to Carbonyl Compounds and -Unsaturated Carbonyl Compounds.—Although the reactions of 17-oxo-steroids with various carbanion reagents are reported to have varying success/ the use of alcohols or DMF as solvents appears to facilitate the Wittig-Horner reaction/ For example, the ketone (97) was converted with (Et0)2P0CHC02Et into the E-ethyl ester (98) in... 

Carbocationic vs. Carbanionic Reagents. Some Novel Options for C-C Bond-forming Reactions... 

Dithiolium ions, particularly those having a hydrogen atom in position 3 or 5, have oxidative properties that appear, for example, in dehydrogenation processes which follow condensation of 3-aryl-1,2-dithiolium ions with carbanionic reagents. No precise information is available on the nature of the reduction products arising from 1,2-dithiolium ions in these dehydrogenations. 

Compared to the long history of synthetic applications of organomagnesium compounds, the investigation of their structure and bonding covers a much shorter period. In many textbooks they are still treated as simple carbanionic reagents, without a more detailed discussion of their nature. For the delayed development of this aspect of organomagnesium chemistry, three major factors are responsible. 

The addition of active methylene compounds of C-5 of 6-methyl-3-phe-nyl- 1,2,4-triazine demonstrates a wider applicability of the reactions with carbanionic reagents for the synthesis of 5-substituted triazines (Scheme 13) (87CPB1378). 

Using this method, precursors of thiophene containing tricyclic prostaglandin endoperoxide analogs are available by addition of thienyl organomercurials to norbornadiene in the presence of palladium(II) chloride/lithium chloride and subsequent conversion of the nortricyclenyl palladium product 32 with carbanion reagents. ... 

Several air-stable, storable, and non-carbanionic reagents including allyhc silanes and stannanes have been used for C-C bond formation with carbonyl compounds, usually under nonbasic conditions. Unfortunately, however, transferable groups are hmited to aUyl, alkyne, and enol groups. TetraaUcyUead compounds, on the other hand, reacted smoothly with aldehydes in the presence of TiCU to produce the corresponding alcohols in moderate to good yield, with exceUent diastereoselectivity (Scheme 13.26) [49]. 

Unquestionably, the most attractive procedure for the preparation of dialkyl l-(trimethylsilyl)meth-ylphosphonates is the carbanionic route. The obvious transmetallation difficulties occurring when carbanionic reagents are employed in conjunction with activated methylene groups have led to the development of procedures based on the trapping of a-metallated phosphonates with chlorosilanes in the presence of lithium diisopropylamide (LDA) in excess. For example, the addition at -70°C of chlorotrimethylsilane (1 eq) to a solution of dialkyl 1-lithioalkylphosphonates prepared from dialkyl alkylphosphonates (1 eq) and LDA (2 eq) produces, after workup, dialkyl l-(trimethylsilyl)alkylphos-phonates in 75-90% yields of isolated product, via the quantitative and clean generation of stable dialkyl l-lithio-l-(trimethylsilyl)alkylphosphonates (Scheme Varying the phosphorus reac-... 

The development of radical additions to C=N bonds is a field with great promise for applications in the synthesis of multifunctional compounds the functional group compatibility of radical chemistry complements that of carbanion reagents to expand... 

Carbanionic reagents, such as enolates and cyanide can also be used in place of the organometallic component. 

Nucleophilic substitution in electrophilic azine positions with nonstabi-lized carbanionic reagents is difficult. It has been found, however, that sulfoxides derived from an electrophilic azine position can react with a Grignard reagent with replacement of the sulfinyl group and carbosubstitu-tion [90AHC(48)1]. 

---