Comproportionation halides

An important series of reactions, which illustrates the diversity of behaviour to be expected, is the comproportionation of halates and halides. Bromides are oxidized quantitatively to bromine and iodides to iodine, this latter reaction being much used in volumetric analysis ... 

A microwave-assisted three-component reaction has been used to prepare a series of 1,4-disubstituted-1,2,3-triazoles with complete control of regiose-lectivity by click chemistry , a fast and efficient approach to novel functionalized compounds using near perfect reactions [76]. In this user-friendly procedure for the copper(l) catalyzed 1,3-dipolar cycloaddition of azides and alkynes, irradiation of an alkyl halide, sodium azide, an alkyne and the Cu(l) catalyst, produced by the comproportionation of Cu(0) and Cu(ll), at 125 °C for 10-15 min, or at 75 °C for certain substrates, generated the organic azide in situ and gave the 1,4-disubstituted regioisomer 43 in 81-93% yield, with no contamination by the 1,5-regioisomer (Scheme 18). 

With the tripod ligand np3 an entire series of compounds with tbp structures Ni(np3)X (X = Cl, Br, I, CN, CO, and H) can be prepared by reaction of nickel halides with the np3 ligand in the presence of borohydride, followed, if necessary, by metathesis with other X groups. The comproportionation of the Ni complex Ni2(CO)2(/i.-CO)(/i.-dppm)2 with NiG2(dppm)2 (dppm = Ph2PCH2PPh2) gives the iluxional A-frame compound (17-G-XIX).18... 

Reactions of basic lithium compounds LiR or LiER with halides [19,37,59], pseudohalides [62] or unsaturated acceptors [63-65] were frequently reported to involve SET steps. The strongly basic character of such lithium compounds may, however, cause the observation of apparent one electron-reduced species such as ketyls or semiquinones via deprotonation of hydrolysis products and ensuing comproportionation reactions (Eq. (9)) [58]. 

The hydrides R2SnHX (X = e.g. halide, carboxylate, sulphonate, alkoxyl) can be formed by the comproportionation between diorganotin dihydrides, R2SnH2, and the compounds R2SnX2. 

Mono-Cp complexes are generally prepared by the same routes as are the bis-Cp versions (see under preparation), but in addition they may be synthesized via comproportionation (CP 2MX2 -I- MX4 = 2 Cp MXa X = halide) (67) or by reaction of Cp salts with titanium compounds having only one substituent that is readily exchanged (68), such as ClTi(OR)3. 

The dipaUadium(I) compound 112 with a neutral arene bridging a Pd(I)-Pd(I) bond was synthesized from comproportionation from Pd(0) and Pd(II) precursors, and using a sterically demanding phosphine ligand (Scheme 10.55) [76]. In the compound, the phenyl ring is in a p -t r hapticity, and the Pd(I)-Pd(I) bond length is 2.5627(8) A (Entry 22, Table 10.7). Complex 112 was reported to be an active catalyst for amination of aryl halides. 

Another common class of dimercury(l) salts is those of the oxo-acids Hg2(YO ,) . Like the mercury halides, Hg2(YO ,) could be prepared by different methods such as metathesis (anion exchange), comproportionation, or treatment of mercury with the corresponding acid. The anions could be either inorganic such as NOj", COj ", HSO, , XOj" (X = C1, Br, 1), N02 , H2P04" [9], or... 

The comproportionation route (Corbett, 1983a, 1991) is widely used and is very efficient when pure phases are desired, especially when the phase relationships are known or can be anticipated. It led to a great variety of reduced rare-earth halides, binary, ternary, and higher, simple, and complex salts, and such that incorporate metal clusters interstitially stabilized by a non-metal atom or by a (transition) metal atom, for example,... 

It gave birth to a new approach to reduced rare-earth metal halides that were previously synthesized only by the comproportionation route or by reduction with hydrogen, viz.,... 

Without additional reagents Selenous acid ester halides from selenous acid esters by comproportionation... 

Detailed kinetic and mechanistic studies show that alkyl halides react preferentially with the soluble Cu(I)/Me6TREN complex, due to its very high ATRP activity rather than with solid Cu(0) that has relatively small surface area. Cu(0) serves as the reducing agent and comproportionates with Cu(II) formed as a persistent radical in the radical termination process, but it also slowly reacts directly with alkyl halides and acts as a supplemental activator. Only 10% of CuBr/MeeTREN disproportionates in DMSO, even less in mixtures containing monomer, MA. 

Disproportionation/comproportionation is relatively slow rather than instantaneous. Therefore, disproportionation is only partial and, in most systems, comproportionation dominates. Cu(I) is always present in the system and is the predominant activator. The faster rate of polymerization in DMSO is due to a larger ATRP equilibrium constant in DMSO than in other solvents.Moreover, OSET process is much slower than the ISET process, due to very slow electron transfer to alkyl halides. Thus the proposed SET-LRP mechanism is not possible, since it violates the principle of microscopic reversibility. " "... 

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