Allyl halides catalytic reduction

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

The Barhier-type reaction of aldehydes and ketones with allyl halides (485) in the presence of Sml2, leading to homoallyl alcohols (486), has received recent interest as a one-step alternative to the Grignard reaction. However, the reactions require the use of stoichiometric amounts of the reducing Sm(III) species. Recently, the electroreductive Barhier-type allylation of carbonyl compounds in an SmH-mediated reaction has been developed [569]. The electrolysis of (485) is carried out in a DMF-SmCl3-(Mg/Ni) system in an undivided cell to give the adduct (486) in 50 85% yields (Scheme 168) [569]. Electrosynthesis of y-butyrolactones has been achieved by the reductive coupling of ethyl 3-chloropropionate with carbonyl compounds in the presence of a catalytic amount of SmCfi [570]. 

The concept of CPTC has been applied in a large number of catalytic reactions such as reduction of allyl chlorides with HCOONa, carbonylation of aryl and allyl halides, allylation of aldehydes, cyanation of aryl halides etc.214 For example, Okano et a/.215 reduced l-chloro-2-nonene to afford 1-nonene and... 

Allylation of imines.1 A low-valent Ti(0) species generated by reduction of TiCl4 with aluminum foil in THF can effect allylation of imines with allyl bromide, even when used in catalytic amounts (0.05 equiv.). This combination of a catalytic amount of TiCU with 1 equiv. of aluminum presumably generates Al(III) and Ti(0), which reacts with the allyl halide to form an allyltitanium, the reactive species. 

A particularly simple variation of this reaction has been developed (47, 48) in which the catalytic nickel species is formed in situ by reduction of nickel chloride with a manganese -iron alloy in the presence of thiourea. Allyl halide is added and at the same time acetylene and carbon monoxide are bubbled through the methanolic solution. Conversion is almost complete and yields of ar-methyl-2,5-dienoate of up to 80% have been claimed. 

In 1993, Molander found that in the presence of catalytic Fe(III) salts, Sml2 mediates intramolecular Barbier additions to esters to give cyclic ketones (or cyclic hemiketals, if they prove to be stable).135 Double addition to the ester is not observed, nor is reduction of the cyclic ketone product. This suggests that the tetrahedral intermediate, a samarium alkoxide of a cyclic hemiketal, is partially stable to the reaction conditions and the ketone group is not revealed until work-up. Molander found that both alkyl and allyl halides could be used in the additions (Scheme 5.83).135... 

Allyl halides have been reduced with electrogenerated tris(bipyridine)cobalt(I) to afford 1,5-hexadiene [369,370]. Some of the earliest work with cobalt(I) salen involved its use for the catalytic reduction of bromoethane [371], bromobenzene [371], and /er/-butyl bromide and chloride [372]. More recently. Fry and coworkers examined the cobalt(I) salen-cata-lyzed reductions of benzal chloride [373-375] and of benzotrichloride [376], and the catalytic reductions of 1-bromobutane [377,378], 1-iodobutane [378], 1,2-dibromobutane [378], benzyl and 4-(trifluoromethyl)benzyl chlorides [379], iodoethane [380], diphenyl disulfide [381], 1,8-diiodooctane [382], and 3-chloro-2,4-pentanedione [383] have been investigated. 

Although the reaction mechanism is not clear at present, the intermediate formation of allylbismuth species through the oxidative addition of allylic halide to Bi(0) generated in situ has been proposed [90BCJ1738], The generation of metallic bismuth by the reduction of bismuth chloride with metallic zinc is known [58DOK(122)614]. A plausible catalytic cycle with... 

Although nickel-catalyzed carbonylative Pauson-Khand cycloadditions have not been broadly developed, related doubly carbonylative cycloadditions involving allyl halides have been demonstrated as an entry to functionalized cyclopentenones. In recent catalytic versions, iron powder was used as the terminal reductant and dehalogenating agent (Scheme 3-38). [Caution Ni(CO)4, which could potentially be liberated in this reaction, is a highly toxic gas.]... 

A pathway for the catalytic reduction of allyl halides is formulated as involving homolytic cleavage of the substrate by pentacyanocobaltate(II), with subsequent cleavage of the allyl complex so formed by hybrido complex ... 

An attractive pathway with a lot of potential uses the transition metal mediated reaction of organic halides with carbon monoxide. Suitable substrates are organic halides capable of oxidative addition to low-valent transition metal compounds. Insertion of carbon monoxide and reductive elimination of an acid halide will complete the catalytic cycle. In tins way it was shown tiiat allyl chloride yields butenoic acid chloride in >80% yield accor g to equation 22)P As well as palladium, rhodium and iridium also act catalytically. It is of no surprise that allylic halides, benzylic halides and aryl halides in particular are readily converted to acid halides. Simple aliphatic halid undergo the oxidative addition step more slowly and, if they cany hydrogen atoms on an sf hybridized C atom in the -position to the halogen atom, may give alkenes via 3-hydrogen elimination. Alkenes can also be converted to acid halides widi carbon monoxide in the presence of transition metal catalysts in solvents such as methylene chloride or tetrachloromethane. ... 

The bipyridyl complexes of Co(ID showed electrocatalytic activity for reduction of allyl halides to 1,5-hexadiene in micellar media. Catalysis lowered the overpotential for reduction of allyl chloride in 0.1 M SDS and CTAB by 1.4 V compared to direct reduction. Yields of about 60% of l,5 hexadiene were obtained from electrolyses at carbon felt electrodes. Small micellar enhancements of reaction rates were found for tris 2,2 bipyridyl)cobalt(II) (Table 1). Catalytic efficiency followed the order CTAB > SDS = acetonitrile. Preliminary work with the long chain derivative bis(2,2 -bipyridyl)(4,4 -hexadecyl-2,2 -bipyridyl)cobalt(II)... 

The second pathway is represented by Eqs. (8)—(11). These reactions involve reduction of the Nin halide to a Ni° complex in a manner similar to the generation of Wilke s bare nickel (37, 38) which can form a C8 bis-77-alkyl nickel (17) in the presence of butadiene [Eq. (9)]. It is reasonable to assume that in the presence of excess alkyaluminum chloride, an exchange reaction [Eq. (10)] can take place between the Cl" on the aluminum and one of the chelating 7r-allyls to form a mono-77-allylic species 18. Complex 18 is functionally the same as 16 under the catalytic reaction condition and should be able to undergo additional reaction with a coordinated ethylene to begin a catalytic cycle similar to Scheme 4 of the Rh system. The result is the formation of a 1,4-diene derivative similar to 13 and the generation of a nickel hydride which then interacts with a butadiene to form the ever-important 7r-crotyl complex [Eq. (11)]. 

The prime functional group for constructing C-C bonds may be the carbonyl group, functioning as either an electrophile (Eq. 1) or via its enolate derivative as a nucleophile (Eqs. 2 and 3). The objective of this chapter is to survey the issue of asymmetric inductions involving the reaction between enolates derived from carbonyl compounds and alkyl halide electrophiles. The addition of a nucleophile toward a carbonyl group, especially in the catalytic manner, is presented as well. Asymmetric aldol reactions and the related allylation reactions (Eq. 3) are the topics of Chapter 3. Reduction of carbonyl groups is discussed in Chapter 4. 

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