Styrene dimers structures

Terminal olefins appear to be reactive only if they are not allylic in nature (e.g., styrene and 2,4,4-trimethyl-l-pentene). Allylbenzene (3-phenyl-l-pro-pene) is inert toward nitrosyl chloride, whereas propenylbenzene (1-phenyl-l-propene) reacts. The preparations are usually carried out at low temperatures. When molecular weights of the products are determined at 5°C, they correspond to dimeric structures. At the melting point of naphthalene, the products are predominantly monomeric. This observation is reasonably general for nitroso compounds [69]. 

In contrast to the claim of Szwarc and co-workers 291) that the a-methyl(styryl)-sodium active center is stable, measurements from various sources have shown 293 30l) that transformations readily occur. The dimer structure formed froma-methyl styrene and sodium is as follows ... 

The sterically hindered hydride [(lndenyl)2YH]2 [8] is an effective catalyst to accomplish homo- and co-dimerisation of a wide range of terminal olefins CH2=CHR, R=Ph, n-Bu, /-Pr, t-Bu, SiMcj, CH2Ph. The presence of substituents and functionalities in the monomer are usually tolerated. An induction period of ca. 30 min may be necessary to cleave the dimeric structure of the catalyst precursor the yields are nearly quantitative within two days. Co-dimerisation of a-olefins with styrene proceeds via initial insertion of the a-olefin into the Y-H bond, followed by a 2,1-insertion of styrene into the Y-C bond of the alkyl intermediate. Subsequent P-H abstraction leads to the releasing of the dimer (Scheme 16). 

Pure Liquids. - Iwahashi et studied the dynamical dimer structure and liquid structure of fatty acids in their binary liquid mixture. Celebre et alP investigated the planarity of styrene in the liquid phase. The NMR data are consistent with the ring fragment, averaged over the ring-ene rotation, planar, while the ene fragment is not. 

Deactivation. One of the factors that complicates the quantification of active-site concentration (135) is the fact that metallocene cations are subject to equilibria between catalytically active and inactive forms. In situations in which intramolecular coordination of an arene group can occur, this process competes with monomer coordination in styrene (136) and possibly olefin polymerization. Another dormant state invoked to explain catalyst decay is the dimeric structure [Cp2Zr(CH3)(/u.-CH3)Zr(CH3)Cp2]+ in which a methyl group bridges two metallocene fragments. This has been characterized by NMR for the reaction of Cp2Zr( CH3)2 with MAO and other cocatalysts (136). 

The formation of intercalate structures have also been suggested for SPS co-crystals with3-carene (3,7,7-trimethylbicyclo[4.1.0]hept-3-ene) [23], anthracene [23,68], and a styrene dimer (1,4-diphenyl-butane) [24]. 

Figure 4 shows the typical H-NMR spectra of DBV oligomers fractionated from a reaction product obtained at a low conversion. All these spectra are consistent with structure 11, with relative peak intensities being in agreement with the calculated values for 11 (shown in parentheses in Fig. 4). The spectra are very similar to that of the linear styrene dimer 1, except for the virtual absence of vinyl protons. In contrast, the products obtained with BF30Et2 in polar solvents, such as (GHjC j, are cross-linked polymers insoluble in organic solvents.

The variation of the dimer structure with reaction conditions is the same as in dimerization of styrene i.e., the dimer is exclusively linear and unsaturated with AcClO in benzene and exclusively cyclic with Bp30Et2 in a polar solvent, e.g., (CH2C1)2. The geometric isomerism in the linear dimers depends on that of the starting monomer and on reaction temperature... 

A mechanistic study by Haynes et al. demonstrated that the same basic reaction cycle operates for rhodium-catalysed methanol carbonylation in both homogeneous and supported systems [59]. The catalytically active complex [Rh(CO)2l2] was supported on an ion exchange resin based on poly(4-vinylpyridine-co-styrene-co-divinylbenzene) in which the pendant pyridyl groups had been quaternised by reaction with Mel. Heterogenisation of the Rh(I) complex was achieved by reaction of the quaternised polymer with the dimer, [Rh(CO)2l]2 (Scheme 11). Infrared spectroscopy revealed i (CO) bands for the supported [Rh(CO)2l2] anions at frequencies very similar to those observed in solution spectra. The structure of the supported complex was confirmed by EXAFS measurements, which revealed a square planar geometry comparable to that found in solution and the solid state. The first X-ray crystal structures of salts of [Rh(CO)2l2]" were also reported in this study. 

Several extensions of these reactions are possible with respect to the electrophilic reagent as well as to the structure of the adduct. Thus functionalized alkyl groups, such as 2-hydroxyethyl and 2-hydroxy-2-phenylethyl, can be introduced into the pyridine ring at the position by treating l-lithio-2-phenyI-l,2-dihydropyridine with ethylene epoxide and styrene epoxide, respectively.144 When polyhalides such as CF3I are used, bis-(substituted-pyridyl)methanes and the dimeric substituted dipyridyls are obtained along with other products.144... 

Finally, Wenger feels that the dimer formed on addition of styrene to styrene- radical-ion has a head-to-tail structure, and that this structure characterizes the corresponding di-anion, S. S-. This is again erroneous. Such dimers were produced, converted into corresponding dicarboxylic acid and proved to have the structure HOOC. CH(Ph). CH2. CH2. CH(Ph). COOH. (See also refs. 21 and 25.)... 

The dimer cation was supposed to have a sandwich structure in which the orbitals of one molecule overlapped with those of the other molecule. The band at 450 nm (B) is due to the bonded dimer cation (St—St T) the formation of this species corresponds to the initiation step of the polymerization. The bonded dimer cation may be formed by the opening of the vinyl double-bonds. Egusa et al. proposed that the structure was a linked head-to-head type I or II, by the analogy of the dimeric dianions of styrene and a-methylstyrene. Table 1 summarizes the assignment of absorption bands observed in pulse radiolysis of 1,1-diphenylethylene in dichloromethane, which is a compound suitable for studying monomeric and dimeric cations [28],... 

The structure of the dimers of methyl vinyl sulfone, styrene, and a-methylstyrene indicate that the preferred orientation of the ion-radical formed follows what would be expected from polarization of the 7t electrons of the bond by the attached groups. 

Fig. 7-25. Main reactions of the phenolic /8-aryl ether structures during alkali (soda) and kraft pulping (Gierer, 1970). R = H, alkyl, or aryl group. The first step involves formation of a quinone methide intermediate (2). In alkali pulping intermediate (2) undergoes proton or formaldehyde elimination and is converted to styryl aryl ether structure (3a). During kraft pulping intermediate (2) is instead attacked by the nucleophilic hydrosulfide ions with formation of a thiirane structure (4) and simultaneous cleavage of the /3-aryl ether bond. Intermediate (5) reacts further either via a 1,4-dithiane dimer or directly to compounds of styrene type (6) and to complicated polymeric products (P). During these reactions most of the organically bound sulfur is eliminated as elemental sulfur.

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