Beta position

The piefeiied route depends upon the avajlabihty of a hydrogen atom in the beta-position to the thiol group. In other words, a-toluenethiol (in toluene) decomposes to give 1,2-diphenylethane and hydrogen sulfide, via the homolytic route, whereas 2-methyl-2-propanethiol decomposes to give 2-methyl-1-propene and hydrogen sulfide. 

The distribution of chlorine atoms along the polymer chain has been studied in great detail. The distribution in various functional types is shown in Table 4 (18). High density polyethylene chlorosulfonated to 35% G1 and 1% S has been found to contain only 1.7% highly active chlorines, ie, reactive to weak bases. AH of these are attributed to the chlorine in the sulfonyl chloride group and those in beta position to SO2GI. No vicinal chlorides groups were found (19). 

Radical decomposition is one of the most important types of reactions. In this case, a larger radical decomposes to an olefin and a smaller radical. Radicals usually decompose at the beta position of the radical center where the C—C bond is the weakest. In the case of naphthenes and aromatics this may not be the case, and C—H bond may be the weakest. Radical isomerization frequently occurs for large radicals, and explains to a large extent the observed product distribution. 

The carbocation may rearrange, eliminate a proton to produce an olefin, or crack at a beta position to yield an olefin and a new carbocation. Under an atmosphere of hydrogen and in the presence of a catalyst with hydrogenation-dehydrogenation activity, the olefins are hydrogenated to paraffinic compounds. This reaction sequence could be represented as follows ... 

Now let s consider the stereochemical outcome. In this case, the beta position (where the reaction is taking place) has only one proton ... 

Figure 13.10.2 The structure of a beta hydroxy acid, a substituted carboxylic acid in which the hydroxyl substituent is attached to the beta position, i.e., at a carbon atom two positions away from the carboxylic acid functionality.

This survey has highlighted the importance of cycloaddition reactions as powerful tools for the functionalisation of meso and beta-positions of a porphyrin macrocycle. 

As Table 2 indicates, ligand 23c proved effective for substrates with large, branched groups on the beta positions. The best selectivity was obtained for 54c, which was substituted with aromatic groups on both sides of the substrate, and a large branched alkyl attached to the p position [62]. Entries 54e and 54f are encouraging and may indicate that this method could be extended to more broadly useful compounds. The investigators performed a solvent study and determined the reaction to be equally selective in toluene and dichloromethane so the former was used presumably for its industrial attractiveness. 

The heterocycle itself and the position of the nitrogen atoms within the ring. For example, in Table 1 we showed that a,0f-diphenylmethyl-3-pyridazine was inactive despite the presence of a beta positioned nitrogen. We supposed that the presence of the alpha nitrogen atom adjacent to the beta nitrogen was responsible for the lack of activity. 

Khan AT, Parvin T, Choudhury LH (2008) Effects of Substituents in the beta-Position of 1,3-dicarbonyl compounds in bromodimethylsulfonium bromide-catalyzed multicomponent reactions a facile access to functionalized piperidines. J Org Chem 73 8398-8402... 

The increased ionic freedom between the propagating polymer ion and its gegen ion occurs concurrently with increased space separation between the two ion species. The studies of Schuerch and co-workers and of Yoshino and co-workers (98) with deuterated acrylates and by Natta and co-workers (99) with sorbic esters show that this increased separation allows trans addition to mono olefins and 1,4 trans addition to conjugated dienes before complete loss of isotactic steric control at the end of the chain. The increased freedom between the propagating ion and the less closely associated gegen ion appears to result in a distortion of the cyclic transition state which permits backside attack at the beta position of the incoming acrylate monomer and 1,4 attack on the incoming sorbate monomer. 

Specific deuteration within the incipient decyl radical (alpha and beta positions). 

When the double bond of an unsaturated acid is farther down the carbon chain than between the alpha and beta positions, the so-called conjugate addition is not possible. Nonetheless, the double bond and carboxyl group frequently interact in the presence of acidic catalysts because the carbocation that results from addition of a proton to the double bond has a built-in nucleophile (the... 

The formation of 5-hexenal (reaction 18) is believed to be an intramolecular rearrangement since the addition of oxygen does not cause its suppression. At least in a methyl substituted cyclohexanone the analogous process has been shown to occur by the transfer of a hydrogen atom from the beta position to the carbonyl group before the fission of the six-membered carbocyclic ring (29) as only one of the two possible isomeric heptenals is formed. 

R1 = H R2 = PNB) in a C6H6/H20/MeCN/Ac0H(6 3 1 l)-KSeCN-(Pt) system at a current density of 7 mA/cm2 yields methyl 26eta-cyanoselenomethyl-penicillin 17 (Y = SeCN) in 90% yield. Similarly, thiocyano and azide groups can be introduced at the C-2 beta position of penams the representative electrolysis conditions are shown in Table 2.3. 

A plot of A/(Sz) versus D/(S2) for the H2 and H3 protons of quinoline adducts with Ln(dpm)3 is shown in Fig. 10.8. The H2 proton that is in beta position shows some deviations from the model while the H3 and other nuclei conform to the model well. 

The higher reactivity of 1-butanol gamma site [87] is in line with the results of Wallington and Kurylo [90] who have proven that in ketones there is a significant enhancement of the group reactivity toward OH for both CH3 and CH2 when moved from the alpha to beta positions. This has been taking into account in the Structure-Activity Relationship (SAR) method [91,92] by factors of F[-CH2C(0)] = f [>CHC(0)] = f [>CC(0)] = 3.9 [22]. In this improved version of... 

The experimental evidence [22,89,90,110-115] suggests that ketones react with OH radicals via a hydrogen abstraction mechanism, leading to a water molecule and a new radical. Nevertheless, there is a peculiarity in the ketones -I- OH reactions hydrogen atoms attached to carbon atoms in beta positions to the carbonyl group are the most likely to be abstracted [110-113,115]. However, if the beta carbon is a primary carbon, its contribution to the total reaction is much less important. The contribution is about 66% [116] to 67% [117] for secondary beta carbons, while it is only about 11% [116] to 17% [117] for primary ones. To explain the large contribution of the beta abstractions, Wallington and Kurylo [90] have proposed a complex... 

Lam, T.B.T. Kadoya, K. Liyama, K. 2001. Bonding of hydroxycinnamic acids to lignin ferulic and />-coumaric acids are predominantly linked at the benzyl position of lignin, not tbe beta-position, in grass cell walls. Ph54ochem. 57 987-992. 

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