Trichloromethyl carbanion, formation

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... 

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... 

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]. 

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-... 

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). 

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. 

---