Halogenated products, organohalogen

Table 7.4 Typical mass fractions, w, of the halogenated products found in the combustion of organohalogen compounds [42,64]...

Equations such as (36)-(40) suggest that radical-radical recombination could result in undesirable organohalogen products. However, such reactions are likely only when the initial halomethane concentrations are very high. Under normal circumstances radical-radical recombination would be unlikely, simply because the radicals are at low concentration. Further, once produced they would decompose by radiolysis under continued irradiation. Halogenated products, other than stoichiometric chloride and bromide ion, have not been detected [13], Even at intermediate doses, dehalogenation to inorganic halogen anions was stoichiometric. 

The Organohalogen Component. Just as for Mg and the solvent, the organic halide must be dry (less than 0.02 wt % of H Ol and free of CK The relative reactivity of the halogens is reflected in the rate of disappearance of Mg. which follows ihc general ordei I > Br > Cl > F. Unfortunately, the rate of disappearance Mg of does not always correlate with the formation of active Grignard. Typically, the more reactive the RX is. ihe higher ihc probability of forming a homocoupled product. Therefore. 

The first survey of natural organohalogens documented 570 halogenated terpenes (1). The present update describes many additional new members of this important class of marine and terrestrial natural products. 

The biosynthesis of organohalogens has seen enormous interest since the first survey, and several examples are mentioned earlier in the present review. Space does not allow for full coverage of this topic, but some additional examples are presented here. The reader is also directed to general reviews on the biosynthesis of marine natural products, many of which contain halogen (2398-2401), terrestrial fungal (basidiomycetes) metabolites (2402), and halogenated alkaloids (2403). 

Jordan A (2003) Volcanic Formation of Halogenated Organic Compounds. In Gribble GW (ed) Natural Production of Organohalogen Compounds, The Handbook of Environmental Chemistry, vol 3, part P. Springer, Berlin, p 121... 

Although naturally occurring organohalogens represent a relatively new category of natural products, the discovery of such compounds has increased rapidly in recent years, and halogenated pyrroles and indoles are a major subset of this class. 

Of the more than 4500 known naturally occurring organohalogen compounds, a large fraction are alkaloids [1,3]. Most of these halogenated pyrroles, indoles, carbazoles, carbolines, tyrosines, and others have a marine origin. The present chapter surveys the occurrence, structure, and biosynthesis of these fascinating natural products. However, given their sheer number, this review focuses mainly on recent examples. 

The greater nucleophilic reactivity of silyl phosphites towards organohalogen compounds results in a greater complexity in product composition thus, dialkyl trimethylsilyl phosphites and CCI4 afford the dialkyl (trichloromethyl)phosphonates in yields of 50-60%, together with various halogenated silicon-containing products and a dialkyl phosphorochloridate (in up to 30% yield)". ... 

Reaction of a metallated carbon nucleophile R —M with an organohalogen compound R-X leads to an ate complex 12 [71, 72], the relevance of which in lithium-halogen exchange reactions was postulated as early as 1958 by Wittig [71]. Experimental evidence for this intermediate was subsequently obtained by Reich [73, 74], and in 1986 Famham reported the first isolation of such an (iodine-based) 10-1-2 [75] ate complex [76]. In 1998, Hoffmann reported a related ate complex as an observable intermediate in an iodine-magnesium exchange reaction [77]. In the further course of the reaction, these intermediates are converted to the end products R -X and R-M [78]. 

In this chapter, the applications of stereoselective halogenations (C—Cl, C—Br, and C—I) in the total synthesis of natural or bioactive organohalogens will be shown. The main focus of this chapter will be on the enantioselective synthesis of natural products and bioactive molecules... 

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