From haloforms

The classic work of Hine and co-workers [42, 43] established that carbenes could be generated by base hydrolysis of haloforms and that the process could be divided into two steps (Eigure 6.33). 

The deprotonation step was deduced from H/D exchange studies and the second stage from a steady-state treatment of the overall rates of hydrolysis. Stabilisation of the intermediate carbanions by halogen follows the order I Br Cl F (Chapter 4, Section VII) and loss of halide ion in stage 2 is in the order of leaving group ability, I Br Cl F. Therefore, it was possible to conclude that the effect of fluorine on the stability of carbenes is in the order F Cl Br 1 [42]. 

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More useful for synthetic purposes, however, is the combination of the zinc-copper couple with methylene iodide to generate carbene-zinc iodide complex, which undergoes addition to double bonds exclusively to form cyclopropanes (7). The base-catalyzed generation of halocarbenes from haloforms (2) also provides a general route to 1,1-dihalocyclopropanes via carbene addition, as does the nonbasic generation of dihalocarbenes from phenyl(trihalomethyl)mercury compounds. Details of these reactions are given below. 

When carbenes are generated from haloforms with potassium hydroxide, exclusive insertion of carbene into a /3-C-H bond has been reported <1980JOM(198)29> (Scheme 57). 

The formation of dihalocarbene is one of the most useful phase-transfer processes developed in organic chemistry. The dihalocarbenes, once generated from haloform and base, can undergo addition or insertion reactions (3, 11, 12). If the double bond is complexed to a metal, addition by the carbene will not occur, but rather insertion into a saturated carbon-hydrogen bond will take place. 

Dihalocarbenes generated from haloforms and strong bases e.g. alkoxides) or from haloforms and alkali metal hydroxides under phase transfer conditions " or organomeicuiytrihalomethanes, add to azomethines to yield 2,2-dihaloaziridines (49 equation 30). The following carbenes have been used for such purposes dichlorocarbene, " dibromocarbene, " difluorocarbene, bromochlorocar-bene and bromofluorocarbene. The addition of dihalocarbenes to azoxybenzene or azobenzene affords dihaloaziridines e.g. 50 equation 31) among other products. 

A simple example of the hazards involved in even short extrapolations is provided by studies of the reaction of bases with bromomalononitrile (29) in the presence of tetramethylethylene. The product (equation 8) is the corresponding 1,1-dicyanocyclopropane, analogous to the dihalocyclopropanes formed from haloforms and olefins. Bromomalononitrile is isoelectronic with bromodichloromethane, and the... 

Halocarbenes are relatively easy to use and can be made from haloforms with base or by decarboxylation of trihaloacetate ions. 

PROBABLE FATE photolysis, could be important, only identifiable transformation process if released to air is reaction with hydroxyl radicals with an estimated half-life of 8.4 months oxidation, has a possibility of occurring, photooxidation half-life in air 42.7 days-1.2 yrs hydroiysis too slow to be important, first-order hydrolytic half-life 275 yrs voiatilization likely to be a significant transport process, if released to water or soil, volatilization will be the dominant environmental fate process, volatilization half-life from rivers and streams 43 min-16.6 days with a typical half-life being 46 hrs sorption adsorption onto activated carbon has been demonstrated bioiogicai processes moderate potential for bioaccumulation, biodegradation occurs in some organisms, in aquatic media where volatilization is not possible, anaerobic degradation may be the major removal process other reactions/interactions may be formed from haloform reaction after chlorination of water if sufficient bromide is present... 

In the 1950s and 1960s chemists learned how to generate carbenes from haloform, diazirine and diazo compound precursors as transient intermediates in solution. [1,5,6] The chemists of this era identified the products of carbene reactions and learned how to make these reactions synthetically useful. They postulated a framework for carbenes (Scheme 1) with ground or low lying triplet states which is still the starting point in all mechanistic discussions. 

It should be stressed that this methodology is of general use for generation of other dihalocarbenes via oc-elimination of hydrogen halide from haloforms, and many other carbenes which can be generated via base-induced a-eliminations ... 

The Ciamician—Dennstedt reaction involves the addition of intermediate dihalocarbene, generated from haloforms (CHX3, X = Cl, Br, or I) and a strong base, to a pyrrole (330) to form an unstable dihalogenocyclopropane, which rearranges to a 3-halogenopyridine such as 331. The reaction was expanded for indoles to provide 3-chloroquinolines. ... 

The reaction involves the generation of dihalocarbene from haloform and a base, e.g., KOH, which then inserts into the pyrrole ring. The subsequent ring enlargement affords the pyridine derivative, as illustrated by the reaction of chloroform and pyrrole. 

The ease of carbanion formation from haloforms (58) depends on the a halogen as I > Br > Cl > F. This same order is valid for haloethylenes (59) as evidenced by the rate of deuterium exchange. 

Among the oldest and more spectacular applications of PTC is the generation of carbenes (mostly dihalocarbenes) from haloforms, aqueous concentrated alkaline hydrox-... 

The generation of carbenes under sonication from haloforms and strong bases in the solid state appeared in the literature in 1982 (Eq. 38). 05 method can be performed in common cleaning baths and is thus accessible to many laboratories. Addition to olefins occurs in yields at least equivalent to or higher than conventional methods, with the advantage of shorter times (p. 344). 

The production of carbenes from haloforms (Eq. 10.54) is an interesting reaction. The reaction sequence displays second-order kinetics, first order in both base and haloform. This supports a mechanism involving an equilibrium deprotonation prior to rate-determining a-halogen departure. The loss of an equivalent of HCl from HCCI3 constitutes an elimination reaction, and is specifically called a 1,1-elimination or an a-elimination. 

Caron aldehyde ester was of great help particularly for the casual preparation of many chrysanthemic acid analogues for insecticidal research. Instead of triphenyl phosphine the trisdimethylamino phosphine may be apphed preferentially [247]. The production of caronaldehyde 755 by the route outUned above is still essential for the production of deltamethrin. Alternatively to the Wittig reaction, the dichlorome-thylene moiety may be introduced by reaction of carbenoids or carbanions from haloforms with the hemiketal 158 of free cis-caronaldehyde, (reaction scheme 100) [248, 249] and subsequently completed by dehalogenation processes. 

Halocarbenes can be generated from haloforms and base, methylene halide and lithium alkyls, and by the decarboxylation of sodium... 

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