Halo compounds

All known commercial syntheses are based on this approach with improvements in preparations of the three components (23). Shordy after the first synthesis, similar methods were pubUshed employing other C -halo compounds, such as l,l,3-tribromo-2-propanone, 2,2,3-tribromopropanal (24), 2,2,3-trichloropropanal, and l,l,3-trichloro-2-propanone (23). 

In contrast to the trichloro compound, these mixed halo compounds, as well as trihromodiphenylantimony [62170-61 -0], (CgH3)2SbBr2, are monomeric ia the sohd state. 

Other synthetic routes to pyrazinones and quinoxalinones are from the halo compounds, by dealkylation of ethers (8lJCS(Pl)3111> or by diazotization of the corresponding amines, but since the halo derivatives are normally derived from the hydroxy compounds, and the amines from the halo derivatives, direct synthesis seems to represent the most practical approach. 

The most widely used method for the synthesis of thiazoles (see Chapter 4.19) is of this type and involves the reaction of a-halo compounds (Y = halogen in Scheme 2) with a reactive component containing an —C(=S)— structural entity. Reaction of the a-... 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

The preparation of e/n-difluoro compounds by the oxidative fluorodesul-furization ot 1,3-dithiolanes readily proceeds by treatment with a pyridinium polyhydrogen fluoride-Af-halo compound reagent the latter serves as a bromonium ion source [2], l,3-Dibromo-5,5-dimethylhydantoin is the most effective of several At-halo oxidants. It is believed that /V-halo compounds combine with hydrogen fluoride to generate in situ halogen fluorides, the oxidants. Formation of gem-difluorides from dithiolanes derived from ketones is efficient and rapid, even at -78 °C, whereas the reaction of dithiolanes derived from aldehydes requires higher temperature (0 °C) (equation 4). 

A degassed suspension of lhe halo compound 14 and an alkyne (1.2 equiv) in Et3N (0.1-0.2 mol) was stirred in the presence of dichlorobis(triphenylphosphane)palladium(II) (0.02 equiv) and coppcr(I) iodide (0.04 equiv) at 20 C for 20 h. EtOAc was added and the mixture was washed with H20 and brine the organic phase was dried (MgSOi) and evaporated to yield the product. 

The facile and reversible reaction of propagating species with transition metal halide complexes to form a polymeric halo-compound is one of the key steps in atom transfer radical polymerization (ATRP, see Section 9.4). 

The efficiency of the halide- radical transformation is reported to be near quantitative. The yield of block or graft is then limited by the efficiency of the halide synthesis. Whether AB or ABA blocks are formed depends on the termination mechanism. Similar halo-compounds have been used to initiate A TRP (Section 9.4). 

These terms were coined by Otsu and Yoshida24 based on the similar terminology introduced by Kennedy25 to cover analogous cationic systems. Except for the case of the dithiuram disulfides and related species (Section 9.3.2.1), these expressions have now fallen from favor and are no longer used as a generic terminology. In this chapter, wc use the term initiator to denote alkoxyamincs in NMP and halo-compounds in ATRP despite the confusion this can create, especially when the process also involves added conventional initiators. 

TABLE 6. Oxidation of sulphides to sulphoxides, R1R2SO, using N-halo compounds... 

A comprehensive set of rate coefficients for water-soluble halo compounds has been published indicating the same trends apparent in Table 26. Activation parameters for a selection of these compounds fall in a range E = 6.9 3.0 kcal.mole , AS = -28 8 eu ... 

Biologically important arylamines, various kinds of heterocycles, and macrocyclic compounds have been prepared by using resin-bound nitro halo compounds via SNAr reactions. Such a process is very important for combinatorial synthesis of biologically important compounds. Typical examples are presented in Eqs. 9.13—9.21,22-31... 

Aldehyde Halo compound Reaction conditions % yield... 

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