Dimerization halide displacement

A series of complexes (Chart 5) have been reported that comprise both a poly(pyrazolyl)borate ligand and an intramolecularly chelating, monoanionic coligand. These are typically obtained via halide displacement and dimer cleavage by the respective sodium or potassium poly(pyrazo-lyl)borate salt, a means by which 462—466,45 467—469,143 and 470 479144 145 were each prepared in good yield, while the related poly (pyrazolyl)methane complexes 480+— 483+ were isolated as their perchlorate salts in the presence of NaClO 145... 

The first palladium alkenyls, pzTpPd C,N-C(Cl) CHCMe2NMe2 (484)144 and the 3-oxo-hexenyl complex 493,160 were obtained systematically by halide displacement and dimer cleavage (Scheme 36). In common with alkyl and aryl systems (462—479, Section III.C.3), the pzTp ligand was in each case concluded to adopt a -coordination mode in solution, on the basis of (i) spectroscopic data, (ii) literature precedent, and (iii) the assumption that the Pd(II) centers in these complexes were too electron rich to permit coordination of the third pyrazole no solid-state data were reported. Both materials are fluxional in solution, and for 484 the slow-exchange limit was attained at —30 °C, with equilibration of the pyrazolyl environments becoming rapid at 79 °C, though the fast exchange limit... 

The only isolated and characterized fused-ring thiirenes are those resulting from [4 + 2] cycloaddition chemistry as discussed in Section 1.06.6.3. Both photolysis (or thermolysis) of benzo-1,2,3-thiadiazolines and internal halide displacement from 2-halothiophenoxides have previously been reported <83HC(42/l)333, 84CHEC-i(7)i3i> to give dimeric bis-sulfides via initial formation of a fused-ring thiirene. 

The complex (328) has been obtained from /rans-cyanohalogeno-bis-(triphenylphosphine) Pt(II) by halide displacement with azide ion followed by cycloaddition of acetonitrile.619 Above 150°C fluxional exchange of the coordinated N atom around the tetrazole ring was observed.619 New dimeric zr-allyl complexes of Pd and Pt with tetrazolate groups as bridging ligands bonded at the 2-N and 3-N atoms (329) have... 

Hauser et were able to isolate the intermediate dimeric halide when 9-chlorofluorene was allowed to react with only one equivalent of sodium amide in liquid ammonia. In t-butyl alcohol containing its potassium salt or a dilute aqueous solution of benzyltrimethylammonium hydroxide, the rate of formation of bifluorenylidene is second-order in 9-bromofluorene and first-order in the basicity of the reaction medium as measured by the ionisation of nitroaniline indicators . Under the same reaction conditions, protium exchange of 9-deutero-9-bromofluorene and elimination from 9-bromo-9,9 -bifluorenyl are much faster reactions than the conversion of 9-bromofluorene into bifluorenylidene. These facts are consistent with the displacement mechanism. 

With sodamide in liquid ammonia, dimeric halides (20b) were isolated in good yield. Experiments with optically active a-phenylethyl chloride indicated that the displacement reaction (20b) proceeds with Walden inversion. Clearly, the bimolecular displacement mechanism operates in the benzyl and benzhydryl series at low temperatures. 

Use of strongly anionic nucleophiles such as alkylmagnesium halides and lithium alkyls leads to reductive dimerization or displacement of the olefin. It is quite possible however that alkylation of olefin-iron cations (IX) would be feasible with alkyl derivatives of zinc, cadmium, or tin reagents. Alkylation of dienyliron cations with dialkylzinc and dialkylcadmium reagents has recently been demonstrated (Section III,A,4). Alkylation of olefin-iron cations can be achieved under mild conditions with cr-bonded allyliron compounds [Eqs. (9) and (10), (Rosan et ai, 1973)]. 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

Alkynes react with haloethenes [38] to yield but-l-en-3-ynes (55-80%), when the reaction is catalysed by Cu(I) and Pd(0) in the presence of a quaternary ammonium salt. The formation of pent-l-en-4-ynes, obtained from the Cu(I)-catalysed reaction of equimolar amounts of alk-l-ynes and allyl halides, has greater applicability and versatility when conducted in the presence of a phase-transfer catalyst [39, 40] although, under strongly basic conditions, 5-arylpent-l-en-4-ynes isomerize. Symmetrical 1,3-diynes are produced by the catalysed dimerization of terminal alkynes in the presence of Pd(0) and a catalytic amount of allyl bromide [41]. No reaction occurs in the absence of the allyl bromide, and an increased amount of the bromide also significantly reduces the yield of the diyne with concomitant formation of an endiyene. The reaction probably involves the initial allylation of the ethnyl carbanion and subsequent displacement of the allyl group by a second ethynyl carbanion on the Pd(0) complex. 

Alternatively, the rhodium dimer 30 may be cleaved by an amine nucleophile to give 34. Since amine-rhodium complexes are known to be stable, this interaction may sequester the catalyst from the productive catalytic cycle. Amine-rhodium complexes are also known to undergo a-oxidation to give hydridorhodium imine complexes 35, which may also be a source of catalyst poisoning. However, in the presence of protic and halide additives, the amine-rhodium complex 34 could react to give the dihalorhodate complex 36. This could occur by associative nucleophilic displacement of the amine by a halide anion. Dihalorhodate 36 could then reform the dimeric complex 30 by reaction with another rhodium monomer, or go on to react directly with another substrate molecule with loss of one of the halide ligands. It is important to note that the dihalorhodate 36 may become a new resting state for the catalyst under these conditions, in addition to or in place of the dimeric complex. 

Another possible precursor to conduct free radical reactions is the glycosyl-cobait(III) dimethylglyoximato complex 33 [22,23], These organometallic compounds can readily be prepared by the displacement of the halide atom in 17 with the highly nucleophilic cobalt(I) anion 32. The latter can be generated from the dimeric Co(II) complex 31 under reducing conditions. 

In addition to displacement of halides, ligands can also be displaced by metal carbonyl anions, as shown in equation (45). These reactions need not be restricted to formation of dimers. Incorporation of the anions into existing polymetahic complexes has proven to be a general means of expanding the cluster size. 

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