Lewis acids radical reactions

A potential method for the preparation of novel amino acids via the highly selective addition of radicals to the glyoxylic oxime derivative of Oppolzer s camphor sultam (88) has been reported.181 Both Lewis acid and non-Lewis acid-mediated reaction conditions for the addition of alkyl radicals generated from alkyl iodides and Et3B/Bu3SnH were examined. A new chiral auxiliary based upon (R,R)-2,5-diphenylpyiTolidine has been used in the addition of phenylthiyl radicals to unsaturated methacrylamides. The selectivity was found to be better than that reported for the structurally related 2,5-dimethylpyrrolidine derivative.182... 

Addition of halogens to fluoroolefins is a common reaction which usually proceeds by a radical mechanism. However, some halogenations are catalyzed by Lewis acids. This reaction has been reviewed [6,14,77]. Among recently reported reactions are ionic addition of Br2 to RC6H4CF=CFX that was found not to be stereoselective [78], and the ratio of syn- and anti-addition products was thought to be determined by a combination of steric and electronic factors. 

Although copper reagents, hahdes and triflates, are widely used in atom-transfer polymerization reactions (ATRP) [63], these processes do not fall under the category of Lewis acid-mediated reactions. Sherrington and co-workers have shown that a vinyl monomer coordinated to a chiral copper Lewis acid (122) undergoes stereoselective polymerization (Sch. 29) [64]. A chiral block-copolymer 124 was prepared under radical conditions. 

Another interest in the use of triplet oxygen lies in the oxidation of dienes with photochemical activation (Type 1, above) with formation of endoperoxides as products. The first example of this reaction was observed in the early 1970 s. Thus, reaction of ergosteryl acetate (107) in the presence of trityl tetrafiuoroborate and Lewis acids in the presence of light yielded the endoperoxide 108 (equation 28), With certain Lewis acids this reaction could be thermally, rather than photochemically, activated. Cation radicals were shown to be the intermediate active species, as was borne out by a comparative oxidation of the isomeric lumisteryl acetate which was inactive under these conditions but reacted easily with singlet oxygen . This reaction was later extended to other substrates. Thus, the intermediacy of cation radicals was also indirectly observed by the fact that the r-butyl substituted 1,3-cyclodiene 109 gave a dimeric product 110 (equation 29) via the cation radical intermediate in addition to the usual endoperoxide llOa ,... 

One of the more commonly used organotin reagents in both free-radical reactions and Lewis acid-mediated reactions is allyltributyltin [3, 7]. This reagent permits the construction of new carbon-carbon bonds from free radical precursors such as alkyl halides however, reactions with a-ketocydopropanes were poorly understood. Tin(IV) enolates generated from a-ketocyclopropane 44 and allyltributyltin undergo both radical allylation and electrophilic quench as shown in Scheme 11, forming O-stannyl ketyl 45 with allyltributyltin and subsequent scission of the cyclopropane... 

A 2,5-disubstituted C g tetrahydrofuran fatty ester [13] was obtained from methyl ricinoleate by addition of bromine to the isomerized substrate, followed by hydrogenation over palladium on charcoal (68). Free radical and Lewis acid-induced reactions involving flie double bonds of unsaturated fetty esters have been conducted by Metzger et al. (69-74) these have resulted in the production of a large number of functionalized, cyclic, and branched fetty ester derivatives (e.g., [14], [15]). The synthesis of methyl rac-2-dodecyl-cyclopentane carboxylate from methyl 2-iodo-18 1(6Z) is presented in Scheme 5. 

The chemistry described by Saegusa, Kobayashi, and others (Sehemes 1 to 4 and Table 1) is reminiscent of the earlier work by Zeeh (Scheme 5) [16-18]. These Lewis acid-catalyzed reactions of isocyanides with ketones are shown in equation 3. The A/-/crt-butyl group in 37 is removable by boron trifluoride etherate. A suggested pathway is illusttated in equation 3 and is initiated by BE3 activation of the aryl ketone (38). Some indoles prepared by Zeeh are 39-41 [17]. The elegant free-radical cyclization of aryl isocyanides developed by Fukuyama is presented in Chapter 49. 

Some reactions of Pd(PPh3)4 are given in Fig. 5.18. The platinum derivative behaves similarly. Sulphur dioxide acts as a Lewis acid. The reactions with alkyl halides usually follow the Sj 2 mechanism, although free radical pathways can also participate. The reactions of aryl halides, which are normally rather unreactive towards nucleophiles are of interest. The palladium complexes catalyse the arylation of alkenes (the Heck reaction) by the following cycle ... 

While the Bronsted acid/base terms specifically refer to proton donors and acceptors, respectively, the Lewis approach (named after G. N. Lewis, who introduced the idea in 1923) greatly broadens the definitions of what is an acid and what is a base. Recall that a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor. All common organic reactions that do not involve radicals or concerted pericyclic processes can in some manner be discussed as Lewis acid-base reactions. Similarly, all these reactions can be considered to be occurring between electrophiles and nucleophiles. Recall that an electrophile is any species seeking electrons and a nucleophile is any species seeking a nucleus (or positive charge) toward which it can donate its electrons. In this context, a Lewis base is synonymous with a nucleophile, and a Lewis acid is synonymous with an electrophile it just de-... 

Allyltributylstannanes are common reagents in both radical and Lewis acid-mediated reactions, although they are unknown to react with enones. However, there has been the first report of a reaction of an electrophilic allylstannane with an enone (Scheme 1). While the parent allylstannane does not react with enones, those substituted with an ester substitute smoothly at the y3-position of enones. The authors claim this to be the first non-basic and non-nucleophilic alternative to the Michael reaction. 

Sibi et al reported the first example in acyclic systems where both highly diastereo-and enantioselective tandem radical reactions occurred with carbon-carbon bond formation to achieve the vicinal functionalization of cinnamates with chiral Mg(II) catalysts (Scheme 4.5) [3]. The reactions began with the addition of nucleophilic alkylradical to cinnamate (7) and trapping of the intermediate radical with allyl-stannane using a chiral bis(oxazoline)s-Mgl2 as a Lewis acid. The reaction with cinnamate (7) (1 equiv), t-Bul (10 equiv), allyltributyltin (5 equiv), and triethylborane... 

There are a lot of tools accumulated in the initial toolkit of Part 1, a dozen in all. Some of them are just slightly different versions of others, just as a mallet is a type of hammer. Thus, combustion is an oxidation, reduction is a component of redox reactions, and electrolysis, corrosion, and the generation of electricity are all aspects of redox reactions. Complex substitution is another type of Lewis acid-base reaction. Catalysis makes use of all kinds of the basic tools, and is more like a lubricant than a tool. When you stand back, it might seem that there are just five basic types of reaction precipitation, proton transfer, electron transfer, Lewis acid-base, and radical recombination. You used them all in the work of construction that followed their assembly. 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

Addition Chlorination. Chlorination of olefins such as ethylene, by the addition of chlorine, is a commercially important process and can be carried out either as a catalytic vapor- or Hquid-phase process (16). The reaction is influenced by light, the walls of the reactor vessel, and inhibitors such as oxygen, and proceeds by a radical-chain mechanism. Ionic addition mechanisms can be maximized and accelerated by the use of a Lewis acid such as ferric chloride, aluminum chloride, antimony pentachloride, or cupric chloride. A typical commercial process for the preparation of 1,2-dichloroethane is the chlorination of ethylene at 40—50°C in the presence of ferric chloride (17). The introduction of 5% air to the chlorine feed prevents unwanted substitution chlorination of the 1,2-dichloroethane to generate by-product l,l,2-trichloroethane. The addition of chlorine to tetrachloroethylene using photochemical conditions has been investigated (18). This chlorination, which is strongly inhibited by oxygen, probably proceeds by a radical-chain mechanism as shown in equations 9—13. 

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