Insertion metals into halogen-carbon

The classical Reformatsky reaction consists of the treatment of an a-halo ester 1 with zinc metal and subsequent reaction with an aldehyde or ketone 3. Nowadays the name is used generally for reactions that involve insertion of a metal into a carbon-halogen bond and subsequent reaction with an electrophile. Formally the Reformatsky reaction is similar to the Grignard reaction. 

Yanagisawa, A., Habaue, S., Yamamoto, H. Direct insertion of alkali (alkaline earth) metals into allylic carbon-halogen bonds avoiding stereorandomization. J. Am. Chem. Soc. 1991,113, 5893-5895. 

Whereas osmium dithiocarboxylato complexes are very rare, some dithiocar-boxylato iron and ruthenium complexes have been synthesized by substituting into the metal-halogen complex with lithium dithiocarboxylate [77] and by inserting CS2 into the carbon-metal bond [78,79]. The reaction of a vinylidene complex 32 with CS2 and NaOMe has also been reported to undergo a facile loss of HCl, followed by insertion of CS2 to give the dithiocarboxylato complex 33 (Scheme 7) [80]. 

Few examples of this mechanism have been clearly demonstrated because of tfie difficulty in establishing that this path occurs from experimental data. The most well-established examples are reactions of nickel complexes with aryl halides Studied by Tsou and Kochi. The rate of the reaction of Ni(PEt3)jWith aryl halides was shown to be first order in nickel and in ArX and retarded by added PEtj, Ortho-methyl substituents had little effect on the rate. Because of the lack of steric effect, electron transfer was proposed to occur after formation of a TT-complex between Ni(PEt3)j and ArX, rather than by direct insertion of the metal into the carbon-halogen bond by a three-centered mechanism. Moreover, the products of the reaction included the Ni(I) species L3NiX and arene. Tliese products are likely to result from the pathway in Scheme 7.4, involving electron transfer from Ni(0) to the aryl halide and escape of the aryl radical from the solvent cage. Other studies of oxidative additions of aryl halides and sulfonates to Ni(0) complexes have been reported. " ... 

There are three classes of reaction that can generate the reactive haloalkylzinc species (1) Oxidative addition of zinc metal into a carbon-halogen bond, (2) alkyl group exchange between and organozinc reagent and a dihaloalkane, and (3) the insertion of a diazoalkane into a zinc iodide bond. 

A very promising synthesis of /3-lactones has been recently reported, involving the palladium-catalyzed carbonylation reaction of halogeno alcohols. For example, 3-phenyl-2-oxetanone was obtained in 63% yield from 2-phenyl-2-bromoethanol in DMF solution at room temperature under 1 atmosphere pressure of carbon monoxide (equation 115). A proposed mechanism, in which palladium metal inserts into the carbon-halogen bond, followed by insertion of a molecule of carbon monoxide into the carbon-palladium bond and then ring closure, fits kinetics data (80JA4193). 

Three approaches to zinc enolates are commonly adopted the process associated to the classical Reformatsky reaction is based on the insertion of Zn(0) into the carbon—halogen bond of an a-haloester. Two additional routes involve (i) transmetallation of a lithium enolate with a Zn(II) salt (Section V.A) and (ii) the transition-metal-catalysed conjugate addition of diethylzinc to Michael acceptors (Section V.B). 

In a general sense, the Reformatsky reaction can be taken as subsuming all enolate formations by oxidative addition of a metal or a low-valent metal salt into a carbon-halogen bond activated by a vicinal carbonyl group, followed by reaction of the enolates thus formed with an appropriate electrophile (Scheme 14.1).1-3 The insertion of metallic zinc into a-haloesters is the historically first and still most widely used form of this process,4 to which this chapter is confined. It is the mode of enolate formation that distinguishes the Reformatsky reaction from other fields of metal enolate chemistry. 

Scheme 1 depicts some of the outcomes possible (5) when a metal atom reacts with an organic molecule, which might be a monomer or a substituent on a polymer. Some of the more common reactions are generalized. Oxidative addition is relevant to insertion of a metal atom into a carbon-halogen bond, such as might be found in polyvinyl chloride or the monomer, allyl chloride, or... 

A long-standing success in transition metal catalysis is the carbonylation reaction [66], in particular the synthesis of acetic acid [67]. Formally this is the insertion of CO into another bond, in particular into a carbon-halogen bond. After the oxidative addition to the transition metal (the breaking of the carbon-halogen bond), a reaction with a CO ligand takes place. This reaction is often called an insertion. Mechanistic studies have, however, shown that the actual reaction... 

Fluorovinylcopper reagents are prepared by metathesis reactions, and the corresponding zinc or cadmium species are the most convenient metalated precursors, e.g. formation of 1. Note that attempted insertion of copper metal into the vinyl carbon — halogen bond fails to afford the fluorovinylcopper reagent and instead a dimerization reaction occurs.Fluorovinylcopper reagents exhibit excellent thermal stability in the absence of oxygen or moisture. The thermal... 

The effect of fluorinated systems on the reaction process shown in Figure 10.67 is of interest [139] it is to be noted that insertion of the palladium catalyst into the carbon-halogen bond may be considered as a nucleophihc attack by the palladium centre, albeit a soft nucleophile, which prefers to attack C—Br over C—F [138]. This process is, of course, aided by the presence of electron-withdrawing groups (EWG) in the organic system. It is likely, however, that co-ordination of the other reactant, e.g. alkyne, to the palladium is the rate-determining step [137], but this will be aided by EWGs attached to the metal. 

In certain instances (Ir, Pt, Au), the carbon-transition-metal-mercury complexes are actually isolable. They also decompose in the manner indicated. When organomercury halides are employed in such reactions, similar oxidation-reduction processes occur, but evidence suggests that the transition metal inserts primarily into the mercury-halogen bond followed by loss of mercury " ... 

In the nickel(ll)-catalyzed NHK reaction, the first step is the reduction of Ni " to Ni that inserts into the halogen-carbon bond via an oxidative addition. The organonickel species transmetallates with Cr " to form the organochromium(lll) nucleophile, which then reacts with the carbonyl compound. To make the process environmentally benign, a chromium-catalyzed version was developed where a chlorosilane was used as an additive to silylate the chromium alkoxide species in order to release the metal salt from the product. The released Cr " is reduced to Cr " with manganese powder. 

Biintercalation, an interleaving of different inserted species, has been studied in the case of inserted halide species " . Halogen-containing solvents, with metal chlorides, can also lead to ternary phases, such as graphite-FeCl3-C2H4Cl2 . A different approach to inserted halides is the intercalation compound with trimethyltin chloride . Finally, the intercalation of halides into carbon fibers is an area of increasing interest . ... 

The reaction scheme is easy enough to draw, but what is the mechanism Overaii it invoives an insertion of magnesium into the carbon—halogen bond. There is also a change in oxidation state of the magnesium, from Mg(0) to Mg(II). The reaction is therefore known as an oxidative insertion or oxidative addition, and is a general process for many metals such as Mg, Li (which we meet shortly), Cu, and Zn. Mg(II) is much more stable than Mg(0) and this drives the reaction. 

As with other transition metal-catalyzed reactions (Ziegler-Natta polymerization of alkenes, olefin metathesis), the mechanism of the Heck reaction is complicated. In brief, the species that reacts with the aryl halide is I Pd, where L is a ligand such as tiiphenylphosphine. By a process known as oxidative addition, palladium inserts into the carbon-halogen bond of the aryl halide. 

Isocyanide insertion reactions into heteroatom—hydrogen, carbon-halogen, carbon—hydrogen bonds, and metal carbenes 13CSR5257. Last advances in synthesis of added value compounds and materials by laccase-mediated biocatalysis 12COC2508. 

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