Organohalides, naturally

Organohalide(s), 332 biological uses of, 352 naturally occurring, 351-352 number of, 351... 

The Heck reaction, a palladium-catalyzed vinylic substitution, is conducted with olefins and organohalides or pseudohalides are frequently used as reactants [15, 16], One of the strengths of the method is that it enables the direct monofunctionalization of a vinylic carbon, which is difficult to achieve by other means. Numerous elegant transformations based on Heck chemistry have been developed in natural and non-natural product synthesis. Intermolecular reactions with cyclic and acyclic al-kenes, and intramolecular cyclization procedures, have led to the assembly of a variety of complex and sterically congested molecules. 

The sulfide derivative of furocoumarin based natural products can be synthesized by treatment of a thiadiazole 107 with KO-f-Bu and an organohalide in an one-pot operation. This process presumably involves the generation of a phenolate 108 and is followed by an intramolecular proton shift and a rearrangement to give an alkynethiolate 109, which undergoes a sequence of intramolecular proton shift, cyclization and alkylation to lead to the furocoumarin product. The thiadiazole, in turn, can be prepared by reaction of the corresponding acetyl precursor with carbethoxyhydrazine, followed by treatment with thionyl chloride . 

Bromodichloromethane is found in chlorinated drinking-water as a consequence of the reaction between chlorine, added during water treatment, and natural organic substances in the presence of bromide. The major route of human exposure is via drinking-water. Bromodichloromethane has also been detected in some untreated waters, but at much lower levels. It is a major component of the organohalides produced by marine algae (lARC, 1991). 

Chlorinated aliphatic compounds are globally distributed. Natural product organohalides such as chloromethane and chloramphenicol are probably produced in diverse soil environments. Many halogenated organic compounds are biosynthesized by marine organisms (Neidleman Geigert, 1986), so the oceans area source as well. 

The chemistry of organohalides in the atmosphere is dictated by both physical and chemical factors, but the nature of their substituents also serves to determine the type... 

The turnover-limiting step in this catalytic cycle depends on the steric and electronic properties of both the organohalide and the organometallic reagent as well as the nature of the main-group metal, and can also be affected by the structure of the metal catalyst. The order of halide reactivity in oxidative addition processes is I > Br = OTf > Cl, and as noted above, the relative rate of oxidative addition of various aromatic halides is roughly... 

A cursory analysis of the catalytic cycle, as shown in Fig. 4.3, reveals the simplicity of the overall process [2]. An oxidative addition of the organohalide (RrX) with a derivative of the palladium catalyst (L Pd ) affords an intermediate (L XPdRi) that then undergoes a transmetalation with an organometallic reagent (M-Rj). It is the nature of this species that gives rise to the named-reaction variation, as illustrated. Finally, the resultant... 

---