Initiators halo-compounds

Haloall lation. Haloalkyl groups can be introduced directiy by processes similar to Friedel-Crafts alkylation into aromatic and, to some extent, ahphatic compounds. Because halo alkylations involve bi- or polyfunctional alkylating agents, they must be performed under conditions that promote the initial halo alkylation but not, to any substantial degree, subsequent further alkylations with the initially formed haloalkylated products. 

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. 

Construction of carbocycles. Formation of cyclopentane rings can be initiated by treatment of unsaturated thiocarbamates, selenides, and halo compounds with BujSnH. " When the radical adds to a triple bond, an alkylidenecyclopentane unit is formed. 

While the range of available initiators for ATRP is much larger than for other CRP methods, the use of various halo-compounds as initiators described above leads to halogen end groups of the polymers formed. They can be replaced, however, by many synthetic methods to provide more useful fimctionalities and halogen-free products. Furthermore, as shown later, the use of multifunctional activated halides enables simultaneous growth of chains in several directions, leading to macromolecular stars, combs, and brushes. 

Epoxides bearing electron-withdrawing groups have been most commonly synthesized by the Darzens reaction. The Darzens reaction involves the initial addition of an ct-halo enolate 40 to the carbonyl compound 41, followed by ring-closure of the alkoxide 42 (Scheme 1.17). Several approaches for inducing asymmetry into this reaction - the use of chiral auxiliaries, reagents, or catalysts - have emerged. 

Similar additions have been successfully carried out with carboxylic acids, anhydrides, acyl halides, carboxylic esters, nitriles, and other types of compounds. These reactions are not successful when the alkene contains electron-withdrawing groups such as halo or carbonyl groups. A free-radical initiator is required, usually peroxides or UV light. The mechanism is illustrated for aldehydes but is similar for the other compounds ... 

There are three possible products when NOCl is added to alkenes. The initial product is always the [3-halo nitroso compound, but these are stable only if the carbon bearing the nitrogen has no hydrogen. If it has, the nitroso compound tautomerizes to the oxime ... 

With some alkenes, the initial p-halo nitroso compound is oxidized by the NOCl to a P-halo nitro compound. Many functional groups can be present without interference (e.g., COOH, COOR, CN, OR). The mechanism in most cases is probably simple electrophilic addition, and the addition is usually anti, though syn addition has been reported in some cases. Markovnikov s rule is followed, the positive NO going to the carbon that has more hydrogens. 

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar... 

Darzens reaction, the reaction between a carbonyl compound and an a-halo ester in the presence of a base, consists of an initial aldol-type addition and a subsequent intramolecular Sn2 reaction, forming an epoxide as its final product. Its high stereoselectivity thus relies on the stereoselectivity of the nucleophilic addition of an a-halo ester onto the carbonyl substrate, which can be either an aldehyde or a ketone. 

Oshima [30] reported a radical alkenylation of a-halo carbonyl compounds under mild conditions by utilizing alkenylindium reagents. Using 0.5 equivalent of triethylborane as a radical initiator at ambient temperature, we demonstrated that this process affords the alkenylation products in high yield (Scheme 10, Eq. 10a). Styrylation reaction showed retention of the stereochemistry from starting alkenylindium (Eq. 10b). 

Methyl ketones are degraded to the next lower carboxylic acid by reaction with hypochlorite or hypobromite ions. The initial step in these reactions involves base-catalyzed halogenation. The halo ketones are more reactive than their precursors, and rapid halogenation to the trihalo compound results. Trihalomethyl ketones are susceptible to alkaline cleavage because of the inductive stabilization provided by the halogen atoms. 

Several significant pyrrole syntheses involve the formal tricomponent cyclization of type III ace (equation 126). The Hantzsch pyrrole synthesis involves a dicarbonyl compound, an a -halo ketone and ammonia or an amine. The mechanistic pattern is similar to that involved in the Knorr synthesis (Section 3.06.3.4.1). In addition to a-halo ketones and a-haloal-dehydes, compounds such as 1,2-dichloroethyl acetate, 1,2-dibromoethyl acetate and 1,2-dichloroethyl ethyl ether can serve as a -haloaldehyde equivalents (equation 127) (70CJC1689, 70JCS(C)285>. It is believed that the initial step in these reactions is the formation of a stabilized enamine from the amine and the /3 -dicarbonyl compound. A structural ambiguity... 

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