Bimolecular coupling

The extension of the cyclization from tetrahydrofurans and pyrrolidines to car-bocycles leads to a sharp decrease in the yield of cyclized product. This is due to the slower cyclization rate of 5-hexenyl radicals compared to 5-(3-oxahexenyl) radicals, which favors the competing bimolecular coupling to the acyclic product. Three measures help to increase the yield in these cyclizations. 

It should be noted that the degree of polymerization in an emulsion polymerization is synonymous with the kinetic chain length. Although termination is by bimolecular coupling, one of the radicals is a primary (or oligomeric) radical, which does not significantly contribute to the size of a dead polymer molecule. The derivation of Eq. 4-7 assumes the absence... 

The theoretical molecular weight distributions for cationic chain polymerizations are the same as those described in Sec. 3-11 for radical chain polymerizations terminating by reactions in which each propagating chain is converted to one dead polymer molecule, that is, not including the formation of a dead polymer molecule by bimolecular coupling of two propagating chains. Equations 2-86 through 2-89, 2-27, 2-96, and 2-97 withp defined by Eq. 3-185... 

Excess ground state MB deprotonates the amine cation radical (eq. 11) to afford the a-amino radical. Bimolecular coupling with MB- yields a reduced, leuco dye eq. 12. 

In addition to azobenzene, coumarin-modified mesoporous networks have been used to create light-induced controlled release.63,64 The action of coumarin differs from that of azobenzene in that illumination drives a reversible bimolecular coupling reaction, creating a cyclobutane dimer that physically blocks pore egress. 

The growing polymer chains (radicals) are terminated by a bimolecular coupling reaction between two radicals. There are two possible terminations combination and disproportionation. In combination, two radicals react to form a single bond by pairing the electrons ... 

The chemical or electrochemical reduction of [(arene)Mn(CO)3]+ (23+) can lead to bimolecular coupling through the arene rings, CO loss followed by dimerization, or arene ring slippage.134 Which mode of decomposition is followed depends on many factors, especially the nature of the arene. In the presence of P(OBu)3, these decomposition pathways are short circuited in favor of ETC substitution to give [(arene)Mn(CO)2P(OBu)3]+. For example, Fig. 6 shows that application of a reducing current for a... 

Surfaces with four-fold coordinate Ti cations are capable of bimolecular coupling of surface formate to form formaldehyde these sites can be created by more severe thermal faceting [1,3,23]. Reaction (5) illustrates this, where R may be a hydrogen atom or an alkyl group, and where Os denotes surface lattice oxygen. 

This bimolecular coupling is a structure-sensitive reaction, and it illustrates a key characteristic of metal oxides multiple coordinative unsaturation of surface metal cations may facilitate coupling of ligands in a manner similar to that for unsaturated metal complexes in solution. Examples of other coupling reactions... 

It should not be surprising that, except in those cases where the coordination environment around the cation is suitable for bimolecular coupling reactions, UHV decomposition reactions tend toward unimolecular processes. The bimolecular dehydrogenation process that occurs during steady-state reaction on the single crystal TiOafOOl) surface should serve as a reminder that steady-state processes involve interaction between the surface, surface species, and vapor phase components, and that it is not appropriate to attribute product selectivity exclusively to oxide characteristics. 

Acetic acid decomposed on the (114[-faceted of the TiOi (001) surface to produce ketene as well as acetone [44]. The acetone generated arose from bimolecular coupling of pairs of surface acetates at four-fold coordinate cations this is analogous to the production of formaldehyde from surface formate on identically prepared surfaces. The reaction of propionic acid corresponded directly to the reaction of acetic acid, producing methyl ketene and 3-pentanone [46]. 

Qualitatively similar results were obtained for reaction and desorption of normal and iso-propanol on the 011 [-faceted TiO2(001) surface. In the case of normal propanol, almost half of the molecules initially adsorbed desorbed as the parent molecule at 370 K, while half of the remaining surface species reacted to form propanol at 580 K. The ratio of propene to propionaldehyde generated at 580 K was 10 1. Desorption of isopropanol quantitatively mirrored the desorption of normal propanol in two desorption states at 365 and 512 K. Isopropanol did not generate any dehydrogenation products (e.g., acetone), and the surface did not generate any bimolecular coupling products for any of the probe alcohol molecules. The absence of ether formation on the (Oil [-faceted surface is consistent with the need for double-coordination vacancies to facilitate that reaction, and the absence of such sites on this surface of titanium dioxide [80]. 

Previous studies on amide bond formation via conversion of the bimolecular coupling reaction (see Scheme 1) into an intramolecular reaction by grafting the carboxy and amino component on a template has clearly demonstrated the strong entropic effect, i.e. the high effective local concentration on the subsequent base-catalyzed intramolecular acyl transfer reaction. 

To determine the proportions of unimolecular (chain transfer) and bimolecular (coupling) termination, let y fraction of molecules be terminated by the former mechanism. Therefore,... 

Usually, as the formation of a radical-cation from a neutral substrate is associated with an increase in its acidity, facile deprotonation can take place [6, 7]. In a majority of instances, proton transfer takes place between radical-cation/radical-anion pairs, with the net result being the formation of two radicals and consequently a bimolecular coupling product (Scheme 3). This process is encountered in benzyl radical-cations, olefin radical-cations, and amine radical-cations. 

The current density controls the concentration of radicals at the electrode surface. This concentration, depending on the rate constants of the follow-up reactions of the radicals, is 10 to 10 times higher than in homogeneous reactions. A low current density favors the monomolecular cyclization against the bimolecular coupling to an acyclic product. A decrease in the current density by a factor of 30 increased the proportion of cyclized product tenfold (Eq. 15) [133]. 

Elongation of acyclic dimer 47 at both ends, using the method encountered so frequently in Section 9-2, followed by oxidative cyclization gave the 16-membered ring hexayne 70 in 14% yield (Fig. 9-20) [22]. An alternative synthesis of the same macrocycle via a bimolecular coupling between the bis-cuprate of 37 and dibromo compound 67, according to the method in Fig. 9-19, worked in only 2.6% yield [22]. 

With the idea of avoiding the potential bimolecular coupling reaction of the radical centers in the solution-phase chemical oxidation reactions, a photochemical approach was adopted. Diazo compounds 10 and 12 were treated with the Rh catalyst under basic conditions to give the poly(acetylene)s 43 [R and m- /7-(/i-C,9H39)C6H4]C(N2)] of 200000 [8]. While photolysis of the diazo groups proceeded smoothly on neat films at 2 K and broad EPR si-... 

Tandem Bimolecular Coupling Followed by Intramolecular Cyclization to Form a Foldable Phenylacetylene Macrotetracycle... 

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