Tetra dimers

Oxidative cleavage of the complex 549 with CuCri affords 2,3-bis(chloro-methyl)-1,3-butadiene (550) and regenerates PdCri. Thus the preparation of this interesting dimerization product 550 can be carried out with a catalytic amount of PdCl2 and two equivalents of CuCb in MeCN[495], Similarly, treatment of allene with PdBr2 affords the dimeric complex 551. Treatment of this complex with 2 equiv, of bromine yields the dibromide 552. The tetra-bromide 553 is obtained by the reaction of an excess of bromine[496]. Similarly,... 

Double alkoxides of zirconium with alkah metals of the type MZr2(OR) have been obtained by reaction of alkah metal alkoxides with zirconium alkoxides (220). Although these usually are monomeric derivatives, the reaction between zirconium tetra-/-butoxide [1071 -76-7] and sodium /-butoxide was found (221) to form dimeric NaZr(OC(CH2)2) ]2. 

Fig. 7.1. Crystal structures of phertyllithium (A) dimeric structure incorporating tetra-methylethylenediamine (B) tetrameric structure incorporating dietl l ether., (Reproduced ftom Refs. 28 and 29 with permission of Wiley-VCH and the American Chemical Society.)...

Methylpyrrole gives only amorphous polymeric material with HCl or picric acid in ether.In contrast with the pyrroles just mentioned, 2,5- and 3,4-dimethyl-, 2-methyl-5-ethyl-, 3-methyl-4-ethyl-, and 2-methyl-4-ethyl-pyrrole do not form crystalline salts, either monomeric or dimeric, and 2,4-dimethyl- and 4-methyl-2-ethyl-pyrrole form only monomeric salts. The dialkylpyrroles in this latter group all dissolve in aqueous sulfuric acid to form relatively stable solutions (e.g., references 14 and 15). The various tri- and tetra-akylpyrroles are likewise soluble in aqueous mineral acid to form stable solutions, and either do not react with HCl or picric acid... 

The reaction investigated by Diels and Alder in 1928 was not new, examples had been known for several years [6]. Early work on the dimerization of tetra-chloropentadienone was conducted by Zincke in 1893 and 1897. In 1906, Albrecht described the product of addition of p-benzoquinone to one or two molecules of cyclopentadiene. Albrecht assigned erroneous formulas to these addition products, but they were later shown to be typical products of the diene synthesis by Diels and Alder. Ruler and Josephson reported the addition products formed by iso-prene and 1,4-benzoquinone in 1920. This research laid the ground work for Diels and Alder. 

Hydroxyaminoquinoxaline self-condensed with loss of 2 x (H2O) to afford a product, initially formulated as the pentacyclic near-dimer, [ 1,2,4,5]-tetra-... 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

With radical ions, the dimerization equilibrium is strongly influenced by the solvation and association of radical ions with counter ions. It has been shown that the free ions dimerize much more slowly than do the respective contact ion pairs e.g., the quinoline radical anion does not dimerize in the powerfully solvating hexamethylphosphoramide, but it does dimerize rapidly in tetra-hydrofuran (160). Thus, two equilibria should be distinguished (160), viz. 

Trimethylsilyl esters of tris(thio)phosphonic acids 2070 are readily oxidized by DMSO in toluene at -30 °C to give the dimeric tetra(thia)diaphosphorinanes 2071 and HMDSO 7 [208] (cf. also the oxidation of silylated thiophenol via 2055 to diphenyl disulfide). The polymeric Se02 is depolymerized and activated by reaction with trimethylsilyl polyphosphate 195 to give the corresponding modified polymer... 

Intrigued by the finding that Eca PLs exhibit notable differences in their kinetics, HPAEC analyses were carried out to examine the products from the depolymerization of PGA and 31% esterified pectin. After 18 h of incubation with PGA, PL1 and PL2 had produced mainly di- and trimers. Similariy, main products of PL3 action were trimers, followed by dimers. Moreover, it was the only enzyme found to produce monomers from unesterified substrates with a degree of polymerization >3. Using 31% esterified pectin as a substrate, similar end products were released by the PLs as from PGA. In addition to the products described, traces of tetra- up to octamers were detectable. While PL1 and PL2 released di- and trimers at almost... 

It was not until Cazin and co-workers introduced the well-defined dimer complex [Pd( J,-Cl)(Cl)(SIPr)]2 that the Kumada-Tamao-Corriu cross-coupling between severely sterically hindered partners was pursued, allowing the synthesis of tri- and tetra-orf/io substituted biaryls [81] (Scheme 6.20). 

In a few cases, the synthesis was directed towards well-defined oligomers (dimers, trimers, etc.). The synthesis of bis(5,7,3, 4 -tetra-0-benzyl)-EC 4/1,8-dimer from 5,7,3, 4 -tetra-0-benzyl-EC and 5,7,3, 4 -tetra-0-benzyl-4-(2-hydroxyethoxy)-EC was described by Kozikowski et al. [41]. This compound exhibited the ability to inhibit the growth of several breast cancer cell fines through the induction of cell cycle arrest in the Gq/Gi phase. Analogously, procyanidin-B3, a condensed catechin dimer, has been obtained through condensation of benzylated catechin with various 4-0-alkylated flavan-3,4-diol derivatives in the presence of a Lewis acid. This reaction led to protected procyanidin-B3 and its diastereomer. In particular, octa-O-benzylated procyanidin-B3 has been produced with high levels of stereoselectivity and in excellent isolation yields [42]. 

Hydroxyaminoquinoxaline self-condensed with loss of 2 x (H20) to afford a product, initially formulated as the pentacyclic near-dimer, [l,2,4,5]-tetra-zino[l,6-a 4,5-a ]diquinoxaline (170),982,992 but subsequently (after X-ray analysis) as the isomeric 2,2 -azoquinoxaline (170a)62 838 (CoC12-6H20 as... 

Interpreting these results on a detailed molecular basis is difficult because we have at present no direct structural data proving the nature of the split Co(IIl/lI) voltammetry (which seems critical to the electrocatalytic efficacy). Experiments on the dissolved monomeric porphyrin, in CH-C solvent, reveal a strong tendency for association, especially for the tetra(o-aminophenyl)porphyrin. From this observation, we have speculated (3) that the split Co(III/II) wave may represent reactivity of non-associated (dimer ) and associated forms of the cobalt tetra(o-aminophenyl)porphyrins, and that these states play different roles in the dioxygen reduction chemistry. That dimeric cobalt porphyrins in particular can yield more efficient four electron dioxygen reduction pathways is well known (24). Our results suggest that efforts to incorporate more structurally well defined dimeric porphyrins into polymer films may be a worthwhile line of future research. 

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