Halogen acceptors

A number of halogen acceptors, such as alkali and alkaline hydroxides, aceutes, ethoxides, amines, and ammonia, have been used successfully. The pyrida2ine 9 was prepared via dehydrohalogenation of 8 using sodium acetate as acceptor (37). 

Bases function in a more complex way than simply by acting as a halogen acceptor, for they cannot always be used interchangeably, and the product may depend on the base used (58,68,69). Diamines, such as ethylene-, propylene-, or hexamethylene diamines may function differently than monoamines. Diamines were thought to function through chelate formation (32). Loven and Speckamp (40) concluded that selectivity in reduction of a... 

Successful Rosenmund reductions have been carried out in the presence of halogen acceptors, such as anhydrous sodium acetate 67, dimethylaniline 23), ethyidiisopropylamine (55), or 2,6-dimethylpyridine 10). 

Bordwell and coworkers63 87 have studied the reaction of 9-fluorenyl carbanions (9-RFP) with a series of electron acceptors and in particular a-halosulfones and sulfoxides, in dimethyl sulfoxide solution. The overall reaction is characterized by the formation of the 9,9 -bis-fluorenyl derivative and the reduction of the halogenated acceptor. A family of 9-substituted fluorenyl carbanions covering a basicity range of 9.1 pKa units was used and... 

Some particular features should be mentioned. Instead of Michael additions, a-nitroolefins are reported to yield allyl sulfones under Pd catalysis (equation 21). Halogenated acceptor-olefins can substitute halogen P to the acceptor by ipso-substitution with sulfinate (equation 22 , equation 23 ) or can lose halogen a to the acceptor in the course of a secondary elimination occurring P to the introduced sulfonyl groups (equation 24). On the other hand, the use of hydrated sodium sulfinates can lead to cleavage at the C=C double bond (equation 25). 

A. Kovacs and Z. Varga, Halogen acceptors in hydrogen bonding. Coord. Chem. Rev. 250,710... 

The recent time-resolved spectroscopic studies described above (Sections 2 and 3) identify the charge-transfer excitation (/n cr) of aromatic EDA complexes with various types of acceptors (A) to their ion-radical pairs [ArH+-,A ] (Mataga, 1984 Hilinski et al., 1984 Jones, 1988). Such electronic transitions in weak EDA complexes, like those of the halogen acceptors, are mainly associated with the excited states, such as in (32), since the variations in the ground state are minor owing to formation constants K that are not strongly dependent on the arene donor (Briegleb, 1961, pp. 106 ff.). 

The triatomic cations are usually prepared from the corresponding interhalogen, by reaction with a halogen acceptor molecule. This is impossible for some of the more exotic species which have no interhalogen precursor. For example, the [Br3]+ cation has been characterized20 in [Br3]+[AsF6], formed from the interaction of arsenic pentafluoride and a bromine/bromine pentafluoride mixture and the [Cl FJ cation results from the interaction of antimony pentafluoride with chlorine monofluoride.21... 

In the absence of an organic halogen acceptor, the oxidized bromine intermediate is reduced by a second equivalent of dihydrogen peroxide, producing bromide and dioxygen [52]. The net reaction is the disproportionation of dihydrogen peroxide to dioxygen and water... 

It is not possible to establish whether amines are preferentially brominated over organic substrates in the V-BrPO reactions. Bromamine buildup is not observed under V-BrPO-catalyzed conditions when an appropriate halogen acceptor is present, either because the organic substrate is brominated before bromamine formation or because the bromamine concentration never builds up to detectable levels because of very rapid substrate bromination by the bromamine. If bromination of mcd is carried out with peracetic acid as the source of peroxide and in the presence of 3 mM tris buffer, Br-tris is only observed after the mcd has been... 

The mercury compounds HgFe(CO)4 and Fe(CO)4(HgX)2 (X = C1, Br, I), which were the first representatives of non-ionic metal derivatives of iron carbonyl hydrides, were discovered by Hock and Stuhlmann (V, 36). During investigations into the preparation of cobalt carbonyls from cobalt halides under CO pressure, in the presence of another metal as a halogen acceptor, we discovered the mixed metal carbonyls M[Co(CO)4]2 (M = Zn, Cd, Hg, Sn) and M[Co(CO)4]3 (M = In, Tl) (44), e.g.,... 

In 1943, Hieber and Lagally reported that the reaction of anhydrous rhodium trichloride with carbon monoxide at 80°C, under pressure, and in the presence of silver and copper as halogen acceptors, gave a black crystalline product which, on the basis of elemental analysis, was formulated as Rh4(CO)n 75). The exact nature of this compound was established 20 years later by Dahl using three-dimensional X-ray analysis which led to its reformulation as Rh6(CO)i6 53). This discovery can be regarded as the birthday of the chemistry of high nuclearity clusters. 

With halogen-acceptor molecules such as boron trifluoride, arsenic pentafluoride, or... 

At higher temperatures the methylchlorosilanes themselves appear to react with aluminum, furnishing methyl groups for the further alkylation of some molecules at the expense of others which are reduced to free silicon. This actually is a form of disproportionation, in which organic groups are stripped from some silicon atoms and attached to others. In this and the similar reactions zinc or aluminum acts as halogen acceptor. 

The cationic complexes [Ir(CO)2(SbPh3)3]+ and [Ir(CO)2(PR3)2]+ (R = Ph, C6Hn) have been synthesized from iridium carbonyl halide derivatives with halogen acceptors in the presence of CO.109... 

The action of light on an R2PX molecule in the presence of an activated alkene as halogen acceptor can lead to R2P radicals, namely,... 

Dodecacarbonyltetrarhodium(O), Rh4(CO)i2, is prepared from RhCfr and CO at 200 atm in the presence of a halogen acceptor, such as copper, silver, cadmium, or zinc. The nature of the products depends on the temperature at 50-80 °C the tetranuclear compound is mainly formed, whereas at... 

In a systematic study of the carbonylation of nickel halides, NlX2, in the presence of copper or silver as halogen acceptors (10 h, 250 °C, 200 atm of CO pressure), conversion to Ni(CO)4 decreases from X = I (quantitative) to Br (1%) and Cl (2%). Carbonylation at atmospheric pressure of an aqueous solution of a nickel(II) salt in the presence of a reducing agent such as [S204]2- gives M(C0)4. ... 

NOF, NOCl, and NOBr form NO+ salts in reactions with halogen acceptors ... 

CycIobutadienyl derivatives of the transition metals are prepared by dehalogena-tion of 3,4-dihalocyclobutene with a zero-valent metal carbonyl. The zero-valent metal can act as a halogen acceptor and a mole of metal halide is obtained along with the cyclobutadienylcarbonyl-metal complex ... 

The hexanuclear carbonyl Rh6(CO)ie was first prepared by treating anhydrous rhodium trichloride with carbon monoxide at 200 atm for several hours in the presence of a halogen acceptor such as cadmium, copper powder, silver, or zinc at temperatures of 80-230°.i At temperatures of 50-80°, the main product was Rh4(CO)i2. Optimum yields (80-90 %) of Rh6(CO)i6 are obtained by allowing methanolic solutions of rhodium trichloride trihydrate to react with carbon monoxide at 40 atm and 60°. Recently, however, there have been reports - of new high-yield syntheses of rhodium cluster carbonyls which require only ambient pressures of carbon monoxide. Chini and Martinengo have obtained Rh6(CO)i6 and Rh4(CO)i2 in high yield from the reaction of Rh2(CO)4Cl2 with carbon monoxide at atmospheric pressure and room temperature. 

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