Trichloromethyl carbanion

The catalytic conditions (aqueous concentrated sodium hydroxide and tetraalkylammonium catalyst) are very useful in generating dihalo-carbenes from the corresponding haloforms. Dichlorocarbene thus generated reacts with alkenes to give high yields of dichlorocyclopropane derivatives,16 even in cases where other methods have failed,17 and with some hydrocarbons to yield dicholromethyl derivatives.18 Similar conditions are suited for the formation and reactions of dibromocar-benc,19 bromofluoro- and chlorofluorocarbene,20 and chlorothiophenoxy carbene,21 as well as the Michael addition of trichloromethyl carbanion to unsaturated nitriles, esters, and sulfones.22... 

When no electrophile is present, reduction of carbon tetrachloride leads to di-chlorocarbene by elimination of chloride ion from the trichloromethyl carbanion intermediate. Dichloromethane is the best solvent for this process [79], The car-bene is trapped by reaction with an alkene to form a dichlorocyclopropane (Table... 

It was found that alkylidenemalononitriles 41 undergo 1,1-dichlorocyclopropane (223) formation by reaction with trichloroacetic acid. This reaction is presumed not to involve a dichlorocarbene addition, but instead a trichloromethylated carbanionic intermediate (equation 64) Substitution of one of the cyano groups by another EWG such as an ester... 

The trichloromethyl carbanion, CC13-, which is formed by the deprotonation of trichloromethane, CHCI3, may, under favourable conditions, lose a chloride anion. Write down the balanced equation... 

Trichloromethyl carbanions can dissociate into dichlorocarbenes also into the interfacial region where they hydrolyze, giving chloride and formate anions. However, in the system with and without the catalyst this hydrolysis is a slow process because the chloride anions produced, being less hydrophilic than hydroxide anions, occupy preferentially the interfacial region, thus shifting equilibrium (95) to the left ... 

A very convincing piece of evidence for the interfacial generation and reaction of carbanions provides addition of trichloromethyl carbanions to A-alkylpyridinium salts when the latter in chloroform solution are stirred with concentrated aqueous NaOH. Treatment of a solution of A-alkylpyridinium salt in chloroform with concentrated aqueous NaOH results in fast addition of trichloromethyl anions in positions 2 and 4 of the pyridinium ring, giving the corresponding adduct in high yield. On the other hand, treatment of these salts in toluene or chlorobenzene with concentrated aqueous NaOH results in rapid decomposition, obviously via formation of unstable adducts of hydroxide anions. These results exclude formation of Q OH type ion pairs as a path for generation of trichloromethyl anions [108]. 

This reasoning is supported by the observation that the reaction of 2-nitrofuran with trichloromethyl carbanion proceeds in both 3- and 5-positions, because in this case the base-induced p-elimination of HCl from the intermediate o adducts is a fast process [54]. Also the VNS reactions of nitro derivatives of other 5-membered heterocycles, imidazoles [20, 56] and thiazoles [34], with a variety of a-halogeno carbanions have been shown to proceed efficiently. 

The VNS of hydrogen in l-benzyl-4-nitroimidazole by action of trichloromethyl carbanion results in the formation of 5-dichloromethyl derivative (Scheme 29). Hydrolysis and condensation of the resulting aldehyde with diethyl malonate afford the corresponding alkene that, upon reduction of the nitro group, undergoes cycli-... 

The precursor 66 of the DATE triflate reagent 69 can only be synthetized from 61 by bromination with NBS and subsequent hydrolysis, or from 62 by addition of the trichloromethyl carbanion. The attempted preparation of 66 from 63 and 64 via 65 does not succeed. Presently we are exploring the use of the DATE group as a protective group for other functions than the 5 -hydroxy group of nucleosides (see above). 

On the other hand, the VNS reaction of chloroform with nitroarenes carried out in the presence of t-BuOK that gives dichloromethyl nitroarenes [65] proceeds via addition of trichloromethyl carbanions to electrophihc aromatic rings of nitroarenes and formation of anionic intermediates [65]. One can therefore consider VNS as an umpolimg of the Friedel-Crafts reaction. It is also a process complementary to the Friedel-Crafts reaction, because it proceeds with nitroarenes that usually do not enter the Friedel-Crafts reaction (Scheme 11.41). 

Carbanion amidoalkylations provide a versatile means of generating a-amino acids. The ability to oxidize alkenes or hydrolyze trichloromethyl groups of amidoalkylation products (entries 5 and 6, Table 7) to acids provides access to these compounds. a-Halo-A -(t-butoxycarbonyl)glycine esters have been used as electrophilic glycinates for the preparation of broadly substituted amino acids (entry 7, Table 7). Several approaches have been examined as possible enantioselective routes to relatively inaccessible, synthetic amino acids. The reactipns of vinylmagnesium bromide with A -Cbz-t-Phe-a-chloro-Gly-OMe resulted in no asymmetric induction as a 1 1 mixture of diastereomeric 3,y-unsaturated... 

Esters of (trichloromethyl)phosphonic add form good starting points for the preparation, albdt through several stages (Scheme 10), of the (1-formylalkyl)-phosphonates (181). The formylation (using ethyl formate) of carbanions from (2-alkenyl)phosphonic esters is regiospecific at the a-position. The carbonyl-protected a-phosphinoylated propenals (182) (R = H or Ph) have been noted. 

It is therefore most remarkable that the conversion of the trichloromethyl compound [72] into [73] can be achieved in very good yield with ammonia in aqueous THF at room temperature. As shown below, this reaction presumably starts from the red carbanion formed by proton abstraction, which easily eliminates chloride ion from the a-position forming violet carbanion through quinodimethane. Under the same reaction conditions, perchlorotoluene remains unaltered (M. Ballester and C. Fernandez-Llamazares, unpublished). 

An alternative to the synthesis of thioethers in which sulfur is a nucleophile, is a synthesis in which sulfur bears a leaving group. The reaction of a carbanion with a sul-fenyl halide should, in principle, constitute a synthesis of thioethers. Such a reaction of sulfenyl halides has not yet been reported but the case in which sulfur is rendered electrophilic by attached cyano, has been [10]. Thus, benzylthiocyanate reacts with trichloromethide ion (generated from chloroform under phase transfer conditions) according to equation 13.6 to yield (80%) of benzyl trichloromethyl thioether [10]. 

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