Aliphatic halides, oxidative addition

Oxidative addition occurs readily with allylic halides. Donor ligands (tertiary phosphines, bipyridyl, halide ions) and anionic complexes are required for activation of aromatic and vinyl halides (4, 70). Certain aliphatic halides are also reactive. The intermediate species R—Ni—X... 

The possibility that substitution results from halogen-atom transfer to the nucleophile, thus generating an alkyl radical that could then couple with its reduced or oxidized form, has been mentioned earlier in the reaction of iron(i) and iron(o) porphyrins with aliphatic halides. This mechanism has been extensively investigated in two cases, namely the oxidative addition of various aliphatic and benzylic halides to cobalt(n) and chromiumfn) complexes. 

The oxidative addition of aliphatic organic halides to stannous chloride has long been of interest for the preparation of monoorganotin trihalides ... 

These reactions are considered to involve insertion of the unsaturated compounds to arylpalladium species followed by the formation of palladacycle intermediates. Oxidative addition of another halide molecule to them leads to the products. In the reaction with norbornene [105 -108] and diphenylacety-lene [109],the corresponding 3 1 and4 1 products and 3 1 product,respectively, are also formed under somewhat different conditions. The mechanisms to account for the formation of these unusual products involving multiple C-H cleavage steps have been proposed. It is noted that, in contrast to Eq. (49), treatment of aryl bromides with aliphatic internal alkynes gives allene derivatives (Eq.50) [110]. 

A rather different approach to carbonylation of aliphatic halides involves the carbonyl anion [Co(CO)4] which, being a kinetically stable 18-electron species, attacks the halide by nucleophilic displacement rather than by undergoing oxidative addition. A probable catalytic cycle for cobalt-catalyzed carbonylation of benzyl chloride is shown in Scheme 6. ... 

Heck carbonylation involving the oxidative addition of aryl halides is not applicable to aliphatic halides, since alkyl halides react directly with nucleophiles. Tsuji developed a process of carbonylating allyl carbonates to form carboxylic esters by palladium-catalyzed carbonylation that is applicable to aliphatic substrates [60]. The process probably involves (a) the oxidative addition of allyl carbonates to Pd(0) species to form r/ -allyl palladium species, (b) CO insertion into the allyl-Pd bond to give acylpalladium species, (c) decarboxylation of the carbonate ligand to give alkoxide, and (d) liberation of butenoate esters by combination with the alkoxides as shown in Scheme 1.21. 

The carbonylation of aliphatic halides to form ketones is less common than the carbo-nylation of aryl and vinyl halides. However, examples of this process are known, and one example conducted under photochemical conditions to accelerate the oxidative addition of the alkyl halide via radical intermediates is shown in Equation 19.87. ... 

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. ... 

C-S bond formation through reaction between aryl halide (Cl, Br, I) and aromatic or aliphatic thiols has been catalysed by a Ni-NHC complex under basic conditions. The reaction is of wide scope and high efficiency. Vinyl sulfides have been synthesized by hydrothiolation of alkynes catalysed by Rh-NHC complexes. Mechanistic investigations have established that the catalysis proceeds via an oxidative addition of the S-H bond to Rh(I) followed by alkyne insertion and reductive elimination. Interestingly, the regioselectivity could be controlled by the nature of the complex (mono or dinuclear precursors), and the use of a strong donor such as the NHC prevents the deactivation of the catalyst. 

The current working model for the beneficial role of protic and halide additives in reactions with aliphatic amines is outlined in Scheme 9.10 [11]. In these reactions, the rhodium dihalide dimer 30 is proposed to enter two different catalytic cycles. In the productive catalytic cycle, the dimer is cleaved by solvation or binding with the substrate to give 31. Oxidative insertion followed by pro to nation of the rhodium alkoxide... 

The quaternary iodides are crystalline compounds, form platini-chlorides, and with chlorine yield iododicMorides of the type R4ASI.CI2. Some aliphatic-aromatic arsonium iodides combine with metallic salts such as mercwic halides, auric chloride, and cadmiumiodide, whilst iodoform also forms addition compounds with some derivatives. Treatment of the iodides with a boiling suspension of silver chloride replaces the iodine by chlorine, giving R4ASCI. The latter also results when hydrochloric add is added to hydroxides of the type R4AS.OH. The hydroxides are derived from the iodides by boiling the latter with a suspension of silver oxide in water or alcohol. It is not always possible to isolate a crystalline product from this reaction, and the syrups often isolated soon absorb carbon dioxide from the air. 

Aliphatic primary halides—chlorides, bromides, and especially iodides—are converted into aldehydes by treatment with dimethyl sulfoxide [998, 999, 1000] or trimethylaniine oxide [993], The reactivity of alkyl chlorides and bromides is increased by converting them in situ to alkyl iodides by the addition of sodium iodide into the reaction mixtures [999] (equation 188). 

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