Monomethyl selectivity, dimethyl

Selectivity is defined as monomethylated products/(monomethylated product + dimethylated product) x 100... 

The selectivity of alkylation of 1,6-anhydrohexopyranoses is lower than drat of acylations. The relative rate-constants for the three hydroxyl groups in 1,6-anhydro-jS-D-glucopyranose (6) during methyla-tion with dimethyl sulfate in 19% sodium hydroxide solution689 are k2 ks k4 = 2.5 1 1.8, so that the formation of pure monomethyl or dimethyl ethers of 6 by this method is of little preparative value (compare Refs. 224 and 671). l,6-Anhydro-/3-D-galactopyranose (12) affords mostly the 2,4-di-O-methyl derivative,690 and 2-acetamido-l,6-anhy-dro-2-deoxy-/3-D-galactopyranose is mainly methylated at 0-4 by the... 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

A unique feature of this procedure is the selective monode-methylation of the dimethyl ether. The scope of this reaction is illustrated6 in part by the preparation in high yield of p-methoxyphenol, guaiacol, and phloroglucinol dimethyl ether from the respective fully O-methylated compounds. An exception is pyrogallol trimethyl ether which affords pyrogallol 1-monomethyl ether in high yield.6... 

The carbon nanotubes can be useful in the separation of molecules not only with different sizes (monomethyl naphthalenes) but also those with different shapes (dimethyl naphthalenes) as demonstrated with a tube of inner diameter of 7.3A. The above results have clearly indicated that the application of carbon nanotubes for gas separation is in very early stage of a technology with far-reaching consequences. The important point brought out is the fact that computational techniques such as MD and CG methods are efficient for screening and designing of carbon nanotubes for selective adsorption and separation of molecules. 

With a primary amine such as n-octylamine, we observe at low conversion (8 %) high selectivity for the N-monoalkylated product whereas 90 % N,N-dimethylated amine are formed at high conversion (94 %). In reductive alkylation, it is even more difficult to obtain the monomethylated product selectively. For example, in the Eschweiler Clarke procedure [17], only the dimethylated amine is formed, even with an amine-to-formaldehyde ratio of 1. In the Hofmann-type reaction, a mixture of mono- and dimethylamine with the corresponding trimethylammonium salt is generally produced. 

Among the different synthetic procedures available for the preparation of hydratropic acids (e.g., indirect methylation of arylacetic acids,4 asymmetric hydroformylation of styrenes,5 rearrangements of a-bromoalkyl aryl ketals,6 etc.), direct methylation of arylacetic acid derivatives seems the most attractive from both economical and synthetic aspects the reagents are easily accessible and a one-pot reaction is involved. Nevertheless, this procedure is seldom used since the yields of the monomethyl derivatives are severely limited by the low selectivity of the reaction. Sizeable amounts of dimethylated by-products form.2 Even under phase-transfer catalysis conditions, high selectivity in monomethylation is elusive.7-8... 

Entry 6 is one of several examples demonstrating enantioselectivity for both the cis and trans isomers of heptane-2,3-epoxide. Entry 7 shows the kinetic resolution of an exocyclic cyclohexane epoxide. The two stereoisomeric monomethyl analogs were only partially resolved and the 3-methyl isomer showed no enantioselectivity. This shows that the steric or hydrophobic effect of the dimethyl substiments is critical for selective binding. 

Some degree of c -selectivity was initially anticipated on the basis of a possible steric interaction between the cyclopentane in 79 (and 81) and a solvated carbonyl of the approaching pyridone. Therefore, it was inexplicable that the cycloaddition is extraordinarily trans-se-lective (Figure 26) This trans-selectivity is solvent-independent for the A A -dimethyl (entries 1,2) and also for a monomethyl analog (entries 3, 4). Only when both IV-methyl groups are absent does the cis/trans-selectivity of the cycloaddition become solvent-dependent. This dramatic effect is illustrated with eleven solvents in reference 60. With benzene (or toluene) as solvent, the stereochemistry is completely reversed and yields the cis isomer 77 exclusively. The cis-... 

Transfer Catalysis (GL-PTC) conditions (/), the reactions of DMC with methylene-active compounds produce monomethylated derivatives, with a selectivity not previously observed. It is worth noting that industrial monomethylation reactions of methylene-active compounds are not a one-step process because the usual methylating agents produce a significant quantity of dimethyl derivatives. 

Using methyl ethers of di- and tri-hydric phenols, selective mono-demethylation occurs, e.g. resorcinol monomethyl ether is obtained from resorcinol dimethyl ether and sodium ethanethiolate in DMF. An exception is pyrogallol trimethyl ether which afforded pyrogaUol 1-monomethyl ether in high yield. Methylene ethers, such as methylenedioxybenzene, can be quantitatively converted to catechol, via the intermediate formation of ethyl o-hydroxyphenoxymethyl sulphide . 

Even though MA will not easily homopolymerize (see Chapter 8), it is known to copolymerize readily with a variety of electron-donor monomers or monomers of opposite polarity to yield alternating copolymers. It is interesting to note that substituted maleic anhydrides, such as monophenyl, monomethyl, dimethyl, monochloro, monobromo, monofluoro, dichloro, and difluoro derivatives, even though studied very little (see Chapter 8), should also perform in some cases as electron-acceptor monomers and undergo alternating copolymerization with select donor monomers. Imide derivatives... 

Compared to the homogeneous reaction, the selectivity of MMM in heterogeneous reaction was lower. This showed that the use of catalyst increases the conversion of monomethyl maleate to dimethyl maleate. 

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