Dimerization suppressing

The polymerization activities of the CD dimers depend on linker length. Linkers with the appropriate lengths displayed high levels of polymerization. If a linker was too short, the CD dimer suppressed monomer recognition. In contrast, if the linker was too long, the CD dimer could not guide the growing polymer chain. 

The use of pyrrole and N-methylpyrrole was found to be preferable. Through the addition of N-methylpyrrole, all cationic side reactions could be effectively suppressed, and only dimerization products produced by Ni-catalysis were obtained. In this case the dimer selectivity was as high as 98 %. Scheme 5.2-21 shows the catalytic system that allowed the first successful application of [(H-COD)Ni(hfacac)] in the biphasic linear dimerization of 1-butene. 

It is known that Na2Fe(CO)4 can be silylated twice to form cri-[(H3C)3Si]2Fe(CO)4 [109]. Also the reaction of Na2Fe(CO)4 with 1.1-dichlorosilanes has been described and leads exclusively to the dimeric compounds [110, 111], In polar solvents the formation of dimers can be suppressed and monomeric base-stabilized compounds are obtained. A very elegant procedure is the in-situ generation of the carbonylate anions in solution by deprotonation of H2Fe(CO)4. 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

This behaviour was rationalised by a stepwise reduction mechanism, in which a high catalyst or KOH concentration gives a high hydride concentration and leads to the aniline formation and suppression of intermolecular reactions to the dimeric azo-compound. 

Since BamH I binds as a dimer to the palindromic sequence of 5 -GGATCC-3, two GG sites in the sequence should be equally insulated from one electron oxidation. In the absence of the protein, both G16G17 in ODN 35 (Fig. 8a, lane 2) and G8G9 in ODN 2 (Fig. 8b, lane 2) showed similar oxidization patterns under the irradiation conditions. In contrast, cleavage bands at both GG sites completely disappeared in the presence of BamH I (1.2 U/pL) (lane 3 in Fig. 8a,b). Simultaneous suppression of oxidation at both GG sites shows that insulation of both GG sites from one electron oxidation is due to the binding of BamH I to the recognition sequence. 

The photocycloaddition of maleic anhydride to acenaphthylene has been studied by Hartmann and Heine.(107a> Irradiation of acenaphthylene in the presence of maleic anhydride in light-atom solvents (dioxane, acetone, or acetonitrile) yields only dimers or copolymers of acenaphthylene. In heavy-atom solvents (dichloromethane, dibromomethane, or iodomethane), however, dimerization is suppressed and cycloaddition with maleic anhydride predominates ... 

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

The common by-products obtained in the transition-metal catalyzed reactions are the formal carbene dimers, diethyl maleate and diethyl fumarate. In accordance with the assumption that they owe their formation to the competition of olefin and excess diazo ester for an intermediate metal carbene, they can be widely suppressed by keeping the actual concentration of diazo compound as low as possible. Usually, one attempts to verify this condition by slow addition of the diazo compound to an excess (usually five- to tenfold) of olefin. This means that the addition rate will be crucial for the yields of cyclopropanes and carbene dimers. For example, Rh6(CO)16-catalyzed cyclopropanation of -butyl vinyl ether with ethyl diazoacetate proceeds in 69% yield when EDA is added during 30 minutes, but it increases to 87 % for a 6 h period. For styrene, the same differences were observed 65). 

Occurence of olefins which are, formally speaking carbene dimers, as well as of similar products (R2C=N—N=CR2, R2CH—CHR2) represents an usually unwanted side-reaction which the chemist endeavors to suppress as far as possible. Nevertheless, conditions for high-yield synthesis of carbene dimers from several diazo compounds have been reported in the past13,141. Some novel examples, published since the last review14) was written, are listed in Table 22. 

On the basis of their results the authors postulated two distinct pathways for the electroreduction of C02 by the rhenium catalyst, with the intermediates being (a) (Bipy)Re[CO]3 and (b) the radical (Bipy)Re[CO]3 (see Scheme 3.1). The former is indicated by the fact that production of the green dimer is completely suppressed in CO2-saturated electrolyte, as a result of the C02 intercepting the active species present. 

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