Intermediate-valent halides

Structural details and phase equilibria of numerous intermediate valent halides were reviewed in this handbook (Haschke 1979). Intermediate valent compounds remain an area of exceedingly high activity. Lattice parameters of numerous phases which contain either a lanthanide or an actinide in an intermediate oxidation state, or an alkaline earth substituted for one of these elements, are listed in table 2. 

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

As complex 67 outperforms the Fe(0)-ate complexes in rate and yield, shown in the reaction of cyclooctenyl bromide with PhMgBr (full conversion complex 67 <20 min, 81% yield, 38 18 h, 39% yield), it was shown that both Fe(0)-ate and Fe (—2)-ate complexes should be intermediates in cross-coupling reactions, but the major contribution should be made by the route emanating from Fe(—2)-com-plexes. The superiority of Fe(—2)-ate complexes was also shown in the stoichiometric insertion of 67 into allylic halides, which proceeded much faster (<5 min) than with any higher valent iron complex (hours or days). 

Vinylation or arylation of alkenes with the aid of a palladium catalysts is known as the Heck reaction. The reaction is thought to proceed through the oxidative addition of an organic halide, RX onto a zero-valent [PdL2] species followed by coordination of the olefin, migratory insertion of R, reductive elimination of the coupled product and dehydrohalogenation of the intermediate [HPdXL2] (Scheme 6.1). 

The reaction between a low-valent Group VIII metal complex and an alkyl halide belongs to the class known as oxidative addition and has attracted much study and controversy as to the mechanism. Recent evidence suggests free radicals as intermediates in many cases. The oxidative-addition reaction is of widespread occurrence and importance in transition metal chemistry, due in part to its use in synthesis and to its implication in many catalytic systems. In one of its forms it is described by... 

The most prominent reactions catalyzed by low-valent iron species involving radical intermediates are cross-coupling reactions of alkyl halides (recent reviews [32-35]) and atom transfer radical reactions. In cross-coupling reactions the oxidation state of the catalytically active species can vary significantly depending on the reaction conditions very often it is not known exactly. To facilitate a summary, all iron-catalyzed cross-coupling reactions are treated together and involved oxidation states, where known, are mentioned at the example. In contrast, iron-catalyzed Kharasch reactions will be treated at the oxidation state of the iron precursors. 

Oxidative addition of C2 - H bonds of imidazolium salts to low valent metals was first observed by Nolan and coworkers in 2001, who proposed a NHC - Pd - H intermediate in the catalytic cycle of the dehalogenation of aryl halides with Pd(dba)2 in the presence of imidazolium salts [154]. More direct evidence of this process was described by Crabtree and coworkers two years later [155]. The reaction between a pyridine-imidazolium salt and Pd2(dba)3 afforded the preparation of bis-NHC - Pd(II) complexes by C2 - H oxidative addition (Scheme 40). The presumed Pd - H intermediates were not detected. The authors proposed a mechanism via two successive C - H oxidative additions followed by reductive elimination of H2 [ 155]. 

Thiocarbonyl derivatives of 1,3-dioxolanes and 1,3-oxathiolanes are readily isomerized to the 2-carbonyl compounds as shown in Scheme 20. Alkylation of the sulfur atom with alkyl halides usually leads to ring-opened products (Scheme 21) (69JOC3011). Most of the other chemistry of the sulfur derivatives has focused on desulfurization and subsequent generation of alkenes. The reaction is shown in equation (20) and proceeds with cis elimination via carbene intermediate (see Section 4.30.2.2.5) and is usually carried out with a phosphine (73JA7161) or a zero-valent nickel complex (73TL2667). 

The Michaelis-Arbuzov reaction is the most used and well-known method for the synthesis of phosphonates and their derivatives and may also be used to synthesize phosphinates and tertiary phosphine oxides. The simplest form of the Michaelis-Arbuzov reaction is the reaction of a trialkyl phosphite, 3, with an alkyl halide, 4, to yield a dialkyl alkylphosphonate, 6, and new alkyl halide, 7 (Scheme 2). During this transformation the phosphorus atom of a ter-valent phosphorus(III) species (3) acts as a nucleophile resulting in the formation of an intermediate alkoxy phosphonium salt 5, containing a new [P—C] bond. The precise structure of the intermediates 5 is a subject of debate—as reflected by common reference to them as pseudophosphonium salts —with a penta-coordinate species (containing a [P—X] bond) being proposed and detected in some cases.18 Decomposition (usually rapid under the reaction conditions) of the intermediate 5 by nucleophilic attack of X- on one of the alkyl groups R1, with concomitant formation of a [1 =0] bond yields the product pentavalent phosphorus(V) compound (6) and the new alkyl halide, 7. 

The chemistry of phases with intermediate compositions REX (2.0 < n < 3.0), which are complex mixed valent RE(II)/RE(III) halides, reminds on the AX2/REX3 systems (A = Ca, Sr, Ba). In fluorides REF (n 2.0-2.2) divalent ions and trivalent ions occnpy the sites within a flnorite-type variant, in which, for charge compensation, interstitieUhahdes are incorporated into the primitive anion snblattice and solid solutions REX2/REX3 are observed. For higher n, the anions form clusters and line phases are formed which crystallize as so-called anion ordered excess flnorite-type variants or in other complicated structure types. 

Zero-valent nickel complexes are known to reduce 1,2-dihalides to olefins and to mediate C,C-coupling reactions of vinyl halides. Based on these facts, lyoda and coworkers developed a two-step, one-pot synthesis of alkyl-substituted [4]radialenes which starts from 2,3-dihalo-l,3-butadienes and 1,4-dichloro-2-butyne derivatives and circumvents the isolation of the butadiene intermediates. Furthermore, the synthesis can be made catalytic in nickel when the Ni(0) complex is generated from NiBr2(PPh3)2 with a more than stoichiometric quantity (based on the dihalide) of zinc. Again, the formation of radialene 94 must compete with that of 95 and 96. With preformed Ni(PPh3)4 and Ni(PBu3)4, the [4]radialene is normally favored in benzene solution, but formation of 95 and/or 96 becomes important in the more polar solvents THF and DMF. With a catalyst... 

The chemical reduction of the higher Mo and W halides provides good yields of the octahedral clusters, but the mechanism is obscure. By contrast, chemical oxidation of zero-valent Mo and W leads to the bromo and iodo cluster species in poor yields but provides considerable insight into the formation of these cluster compounds. The reaction of the hexacarbonyls Mo(CO)6 or W(CO)6 with I2 at moderately low temperatures produces a mixture of metal halide phases (25, 16). In the reaction W(CO)6 with I2, lower nuclearity clusters have been isolated as reaction intermediates that lead to W6 species. The tri-, tetra-, and pentanuclear tungsten iodide species are obtained from W(CO)6 and I2 by varying reaction times and temperatures, and the... 

---