1.1- Dihalides dimerization

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... 

A mechanism that accounts for the oxidative addition of halocarbons has been proposed for the two d8-d8 dimers (Figure 4) (23). The mechanism involves the oxidative quenching of the triplet excited state of the metal dimer as the primary photoprocess. This gives a radical anion species that dissociates a halide, thereby producing an organic radical. The dissociated halide adds to the partially oxidized metal dimer to form a mixed valence Ir -Ir -X or Pt 1-Pt -X intermediate. This intermediate reacts further with the remaining organic radical (presumably in a second, thermal electron transfer step) to form the final d2-d2 dihalide dimer. 

By the radical pathway l, -diesters, -diketones, -dienes or -dihalides, chiral intermediates for synthesis, pheromones and unusual hydrocarbons or fatty acids are accessible in one to few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four units can be coupled in one step. By way of intramolecular addition unsaturated carboxyhc acids can be converted into five raembered hetero- or carbocyclic compounds. These radical reactions are attractive for synthesis because they can tolerate polar functional groups without protection. 

The utility of thallium(III) salts as oxidants for nonaromatic unsaturated systems is a consequence of the thermal and solvolytic instability of mono-alkylthallium(III) compounds, which in turn is apparently dependent on two major factors, namely, the nature of the associated anion and the structure of the alkyl group. Compounds in which the anion is a good bidentate ligand are moderately stable, for example, alkylthallium dicar-boxylates 74, 75) or bis dithiocarbamates (76). Alkylthallium dihalides, on the other hand, are extremely unstable and generally decompose instantly. Methylthallium diacetate, for example, can readily be prepared by the exchange reaction shown in Eq. (11) it is reasonably stable in the solid state, but decomposes slowly in solution and rapidly on being heated [Eq. (23)]. Treatment with chloride ion results in the immediate formation of methyl chloride and thallium(I) chloride [Eq. (24)] (55). These facts can be accommodated on the basis that the dicarboxylates are dimeric while the... 

Examinations of the far-infrared spectra of solutions of Zr (allyl) 3CI and Zr (allyl) 2CI2 (9) suggest that the former exists in solution as the dimer (X), whereas the latter has the monomeric structure (XI). A broad intense peak at 244 cm-1 can be assigned to zirconium-bridging chlorine stretching mode. This band is completely absent from the spectrum of the dihalide and is replaced by a very strong band at 342 cm-1 due to the nonbridging chlorine. 

Alkali metal reduction of dihalide precursors is shown to be a valuable route to various silole and germole dianions in solution. Crystal structure determinations for [K(18-crown-6)+h(C4Me4Si2 ] and the dimer [K4(18-crown-6)3][C4Me4Ge]2 are consistent with the presence of delocalized jr-systems and with -bonding modes for all of the potassium cations. 

This binuclear photooxidative addition reaction is general for a number of halocarbons (Figure 3). While DCE and 1,2-dibromoethane react cleanly to give the dihalide metal dimers and ethylene, substrates such as bromobenzene or methylene chloride react through an alkyl or aryl intermediate. This intermediate reacts further to yield the dihalide d2-d2 metal complexes. 

A widely exploited procedure for bringing about carbenoid reactions of organic mono- and fifem-dihalides is by use of lithium alkyls. Examples are given in equations (11) and (12). Dimeric olefin formation, stereospecific cyclopropane formation from olefins, and insertion into carbon-hydrogen bonds have all been observed in suitable cases, together with further reactions of these products with excess of the lithium alkyl. 

A proposed mechanism resulting in inversion of configuration at tin in dihalides 273 and 274 includes an achiral dimeric intermediate 275 with hexacoordinate tin centers (equation 63)653. Evidence for such an intermediate is the observation of two Sn—Me resonances in the XH NMR spectrum of a mixture of dichloride 273a and dibromide 273b in toluene- at < — 80 °C, while only one resonance pattern is observed at higher temperatures. 

Crown ethers948 and their sulfur equivalents949,950 may be synthesized by reaction of diols (or dithiols) with a, co-dihalides in the presence of a base. Similar reactions have been used to prepare heterocyclic bissulfides951. Thianthrenes have been synthesized by dimerization of or/Zzo-chlorothiophenols in HMPA in the presence of triethylamine952. The same targets may also be prepared by reaction of ortho-dihalo aromatic compounds with aryl dithiols with photostimulation275,953. 

The high yield reduction of 1,2-dihalides to produce olefins has been employed to advantage to prepare reactive olefins. Electron transfer in electrochemistry is proportional to the diffusion coefficient, which is related in a much less sensitive way to temperature changes than is chemical reactivity. Thus it may become possible to synthesize and study electro-chemically species whose chemical reactivity is high by working at low temperatures. Electroreduction of 1,2-dibromobenzocyclobutene (144) in acetonitrile or butyroni-trile/TEAP or chemical reduction using the biphenyl radical anion resulted in the formation of benzocyclobutadiene (145)128. Efforts to observe the electrochemically generated anion radical or dianion of benzocyclobutadiene indicated that dimerization to 146 was faster than further reduction (equation 84). 

Two proposed mechanisms for the polymerization of a,a -bis(dialkyl-sulfonio)-p-xylene dihalides (463) are shown in Fig. 70 [302]. Both mechanisms begin with the abstraction of an a-proton to produce ylid 464. The 1,6-elimination of dialkyl sulfide produces the p-xylylene pseudo-diradical 465. One mechanism involves the formation of polymer from this species via the dimerization of 465 to give the dication diradical 466. This species was proposed to grow rapidly to high molecular weight polymer (467) by head-to-tail additions to both ends [301]. An alternative mechanism involving polymer formation from 465 via a putative anionic mechanism has been proposed [302]. There were two main factors in the proposal of... 

Dimers of M centers occur in the mixed valent compounds GayXs (X = Br, I) = (Ga" )2 [GayXe] " which contain ethane-type [GayXe] " units with Ga-Ga single bonds. As in the dihalides, mixtures of complex and simple ions combine also in InsXy = (In+)3[In2Br6]Br, which contains the ethane-like units with M-M single bonds as weU. ... 

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