Alkylation reactions photochemically induced

Autoxidation of secondary acetonitriles under phase-transfer catalytic conditions [2] avoids the use of hazardous and/or expensive materials required for the classical conversion of the nitriles into ketones. In the course of C-alkylation of secondary acetonitriles (see Chapter 6), it had been noted that oxidative cleavage of the nitrile group frequently occurred (Scheme 10.7) [3]. In both cases, oxidation of the anionic intermediate presumably proceeds via the peroxy derivative with the extrusion of the cyanate ion [2], Advantage of the direct oxidation reaction has been made in the synthesis of aryl ketones [3], particularly of benzoylheteroarenes. The cyanomethylheteroarenes, obtained by a photochemically induced reaction of halo-heteroarenes with phenylacetonitrile, are oxidized by air under the basic conditions. Oxidative coupling of bromoacetonitriles under basic catalytic conditions has been also observed (see Chapter 6). 

This very rich chemistry has been the subject of several comprehensive reviews, including recent ones sp carbon centres, general (Kornblum, 1975, 1982 Russell, 1970, 1987), photochemically induced reactions (Bowman, 1988a), substituted aliphatic nitro-compounds (Bowman, 1988b), alkyl mercurials (Russell, 1989) sp carbon centres, general (Norris, 1983 Rossi and... 

A hydroxymethyl group can be introduced (ArH —> ArCH2OH) by several variations of this method.345 Alkylation of these substrates can also be accomplished by generating the alkyl radicals in other ways from hydroperoxides and FeS04,346 from alkyl iodides and H Oi-FefH),347 from carboxylic acids and lead tetraacetate, or from the photochemically induced decarboxylation of carboxylic acids by iodosobenzene diacetate.348 The reaction has also been applied to acetophenone and ferrocene.349... 

The photosensitized addition of 1,3-dioxolane and 1,3,5-trioxane to, alkenes was developed a number of years ago as a route to a-alkylated ethers (68JOC805). It has now been shown that 2-methyl-l,3-dioxolane will undergo a photochemically induced conjugate addition reaction to cyclohexenone to afford an adduct (327) in 54% yield which can be hydrolyzed to the diketone (328) (77CJC3986). Functionalized dioxolane (330) was also... 

The formation of 3-pyrrolylcarbinols (280) from the photochemically induced reaction of pyrrole, or its 1-alkyl derivatives, with aliphatic aldehydes and ketones is thought to proceed via an oxetane intermediate (279) (79JOC2949). In contrast, the analogous reaction of 1 -phenylpyrrole with benzophenone leads to the formation of the diphenyl(2-pyrrolyl)car-binol, whilst the oxetane (281) has been isolated from the photoaddition of 1-benzoylpyrrole and benzophenone (76JHC1037, B-77MI30500). 2-Benzoyl-1-methylpyrrole undergoes a normal Paterno-Buchi photocyclization with 2,3-dimethylbut-2-ene, via the n -> v triplet... 

An example of the photodecomposition of a dithiin to give a stable dithiet, with elimination of ethylene, has been reported.206 A different reaction is preferred in the dithiins (261), which on irradiation, are converted to benzene derivatives (262) via the 1,4-dithiocins (263).207 The cyclic trimer of thio-acetophenone on irradiation in cyclohexane yields thioacetophenone.208 An initial carbon-sulfur bond homolysis is also responsible for the conversion, on irradiation in methanol, of a 3-cephem to two thiazoles,209 whereas a 1,2-alkyl migration followed by a further photochemically induced carbon-sulfur bond homolysis has been proposed to account for the photorearrangement of the sulfonium ylides (264) to the pyrimido-l,4-benzo[h]thiazepines (265).210 The novel photorearrangemenl of a 1,3-thiazine to a cyclopro-pathiazolidine has also been rationalized in terms of an initial carbon-sulfur... 

The alkyls CpMo(CO)3R (R = Me, Et, and CHaPh) readily insert SOa (57) these reactions were the subject of a kinetic study (71, 76). When R = CHaPh, the 0-sulfinato intermediate was detected spectroscopically (72, 73). Contrasting with the above behavior is the apparent lack of reactivity of CpMo(CO)3CFa (57) and CpMo(CO)3CHaCN (124) toward neat SOa- However, the dicarbonyl CpMo(CO)a(PPh3)Me does afford the corresponding >S -sulfinate, the reaction proceeding much more rapidly than that of its parent tricarbonyl, CpMo(CO)3Me (57). The photochemically induced desulfination of CpMo(CO)3S(0)a-CHaPh furnishes CpMo(CO)3CHaPh (35% yield) in contrast, irradiation of CpMo(CO)3S(0)aMe yields [CpMo(CO)3]a (57). 

Coixjugate Addition Reactions. a,3-Unsaturated N-acyloxazolidinones have been implemented as Michael acceptors, inducing chirality in the same sense as in enolate alkylation reactions. Chiral a,3-unsaturated imides undergo 1,4-addition when treated with diethylaluminum chloride (eq 55). Photochemical initiation is required for the analogous reaction with Dimethylaluminum Chloride. ... 

In contrast to the most synthetic strategies involving ionic elongation reactions at C-l of aldoses, a synthesis of KDO based on the photochemically induced radical cross-coupling of a-ethoxyacrylnitrile with an alkyl cobaltoxime derivative at C-6 of D-mannose has been reported by Branchaud et al. [106], Thus, treatment of 6-iodo-D-... 

Cyclizations of 5-alkyl-substituted 4-penten-l-oxyl radicals are faster and frequently more selective than those of terminal unsubstituted derivatives [48, 53]. This finding is in accord with the electrophilic nature of alkoxyl radicals in addition reaction to C-C double bonds. 5-Alkyl- or 5-phenyl-substituted 4-pentenoxyl radicals, such as intermediates 25 or 26, were generated from a number of different sources. For example, alkyl nitrites [39], A-alkoxypyridine-2(l//)-thiones [46], and A-alkoxy-(/ -chlorophenyl)thiazole-2(3/f)-thiones [54] in photochemically induced reactions, and A-alkoxyphthalimides [55] or type IV radical precursors [53] in thermally initiated reactions have been applied for this purpose (Scheme 6). 

One of the most recent examples of C-H activation comes from the Hartwig laboratories. The complex [C5(CH3)5]W(C0)3[B(1,2-02C6H2-3,5-dimethyl)] undergoes a photochemically induced reaction with alkane solvents to form alkylboronate esters, thus activating the C-H and functionalizing the alkyl groups. Various metal-carbonyl-boryl complexes undergo this reaction. 

Reactions with Radicals. Isoquinoline does react with radicals to give addition to the 1-position with subsequent loss of hydrogen. The Minisci reaction is a well-known example of this transformation. It is an effective reaction for the addition of hydroxymethyl, alkyl, and acyl radicals. Formyl and carbamoyl groups have also be added using this method. Acidic conditions help to promote the reaction. Alkylation is effective when carried out with alkyl iodides or alkyl xanthates. Hydroxymethyla-tion can also be carried out. Photochemical-induced free radical reactions have also been reported. Alkylation is possible using ethanol or propanoic acid but low yields are often obtained. Addition of a phenyl group can be achieved in 81% yield. ... 

Hu, S. K. and Neckers, D. C., Photochemical reactions of mercapto/amino substituted alkyl phe-nylglyoxylates induced by intramolecular electron transfer, Tetrahedron, 53, 2751,1997. 

The mechanism of the photochemical alkylation shows particular characteristics as regards the formation of alkyl radicals, the reaction of these radicals with the heteroaromatic substrates, and the rearomatization of the intermediate products. A variety of alkylating agents (hydrocarbons, alcohols, amines, carboxylic acids, amino acids) have been used for photochemical and y-ray-induced alkylation. " ... 

As has already been indicated, a thorough discussion of the mechanism of metal, metal alkyl-, oxygen- or peroxide-induced polymerizations is beyond the scope of this article. Chiefly for purposes of comparison, it seems worthwhile to present a brief outline of the free radical type of mechanism which best explains these reactions and which probably is valid also for the photochemical and thermal polymerizations, particularly in the lower temperature range. 

The first step in the peroxide-induced reaction is the decomposition of the peroxide to form a free radical. The oxygen-induced reaction may involve the intermediate formation of a peroxide or a free radical olefin-oxygen addition product. (In the case of thermal and photochemical reactions, the free radical may be formed by the opening up of the double bond or, more probably, by dissociation of a carbon-hydrogen bond in metal alkyl-induced reactions, decomposition of the metal alkyl yields alkyl radicals.)... 

These conclusions were anticipated by product studies. Alkyl azides are readily available and their thermal and photochemical decomposition reactions have been studied. In general, light and heat induced decomposition of methyl azide does not produce a MeN species that can be intercepted in respectable yields with a bimolecular trap. For example, attempts to trap MeN with cyclohexane as solvent produced only a 0.4% yield of adduct. Photolysis of CH3N3 or CD3N3 at cryogenic temperatures fails to produce an IR spectrum attributable to triplet methylnitrene. The IR spectrum of CH2=NH (or CD2=ND) is observed instead. " ... 

The mechanism of aminyl radical generation from PTOC carbamates follows closely the radical chain mechanism of alkyl radical generation from PTOC esters. The chain reaction sequence involves the series of steps shown in Scheme 8. Several methods for inducing N —O bond cleavage are possible. Photochemical decomposition of 29 via visible light irradiation is used to initiate the chain reaction sequence at ambient or subambient temperature reactions have been run as low as -78°C (91TL6493). 

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