Degree of alkylation

The first step in this preparation, the epoxidation of 1,4,5,8-tetra-hydronaphthalene, exemplifies the well-known selectivity exerted by peracids in their reaction with alkenes possessing double bonds that differ in the degree of alkyl substitution.12 As regards the method of aromatization employed in the conversion of ll-oxatricyclo[4.4.1.01-6]-undeca-3,8-diene to l,6-oxido[10]annulene, the two-step bromination-dehydrobromination sequence is given preference to the one-step DDQ-dehydrogenation, which was advantageously applied in the synthesis of l,6-metliano[10]annulene,2,9 since it affords the product in higher yield and purity. 

These reagents are prepared by hydroboration of the appropriate alkene, using control of stoichiometry to terminate the hydroboration at the desired degree of alkylation. 

Even the addition of 4 equiv. of dimethylzinc to a bis(dicarbollide)tantalum dichloride, Scheme 23, produced only the monomethyl complex 26 in an isolated yield of 75%.63 In these reactions, the oxidation state of the transition metal and the steric bulk of its ligands obviously play a role in the degree of alkylation. 

For example, the mechanical properties—hardness and elastic modulus—of ORMOSIL can be tuned by varying the degree of alkylation and thus the fraction of six- and four-member siloxane rings in the organosilica matrix. This enables fine tuneability of parameters of crucial practical importance (Figure 4.2).2... 

Figure 4.2 Loading curves for ORMOSIL thin films with varying degrees of alkylation of the silica matrix. (Reproduced from ref. 2, with permission.)...

Measurements of the surface tension of aqueous solutions of various sulphonated and unsulphonated phenylazonaphthol dyes showed that the degree of surface activity (that is, the lowering of surface tension) tended to increase progressively with the degree of alkyl substitution in the series of dyes [7]. The surface-active behaviour of such alkylated dye ions ensures that they become more concentrated at the interface between the dyebath and the fibre surface, just as they do at the air-water interface of the dye solution. Foaming of dyebaths can be a serious practical problem with relatively hydrophobic dye structures solubilised by means of a single ionised group. 

The reduction of dienes by diimide depends on the nature of the substitution of the diene. Several studies of relative reactivity have been carried out and they indicated that an increasing degree of alkyl substitution on the double bond results in decreasing reactivity82. In the case of allenes, the reduction of the less substituted allenic double bonds and the formation of the thermodynamically less stable cis olefin can be explained by the steric control of the approach of the diimide (equation 23)83. 

Cathodic elimination can remove a variety of vicinal nucleophiles (X = Br [140], Cl [141], S-C6H5 [142], or oxalate [143]) to form a double bond. Controlled potential cathodic reduction allows the selective elimination of vic-dihalides depending on the degree of alkylation [144]. Chemically, these reductions are more limited in scope they can be conducted with 1 in DMF, with Zn, Mg, or Cr + [145]. 

Aromatics react with olefins very readily when potassium is used as a catalyst. However, potassium is less selective in catalyzing alkylation than sodium because an additional reaction yields indans, as reported by Schaap and Pines 24). Therefore, the products consist of mixtures of arylalkanes and indans the relative amount of each depends largely on the aromatic used (as shown in Table II). The degree of alkyl substitution of the o -carbon... 

Chemical modification of proteins has been extensively studied over the years to identify which amino acids are involved in catalysis. Much less work has been carried out on its influence on enzyme stability. Chemical modification of proteins may yield stabilization, destabilization or no effect at all. Martinek and Berezin (1978) reported the dependence of the thermostability of chymotrypsin on the degree of alkylation of its amino groups up to 30% alkylation the stability rose slightly at 90% substitution stability increased markedly, with a maximum (110-fold) at 95% stability fell to nearly initial values when 100% amino groups were modified. (With these modifications, the optimum pH of the errzyme can change and one must therefore be cautious in comparing two different... 

The complex formation of partially alkylated PVP was reported by Kabanov et a 42 and by us5S. The K value fell by about two orders of magnitude with the degree of alkylation for the PVP derivatives that were alkylated or quaternized by methyl bromide42. This reduction of K was due to electrostatic repulsion on the quaternized PVP chain. On the other hand, Table 5 shows no effect of alkylation on the stability of the polymer chelate when benzyl chloride is used as the alkylating reagent, while the formation constant falls slightly for PVP alkylated by ethyl... 

The telomerization of sucrose with butadiene was catalyzed in aqueous solution by palladium acetate and tppts (102). The sucrose conversion was about 96%, but octadienyl ethers of different degrees of alkylation were also formed. 

Two empirical increment systems (Table 4.22) derived from experimental data as collected in Table 4.23 permit prediction of alkanol carbon-13 shifts. One is related to the shift value B of the hydrocarbon R —H and involves, as usual, addition of the increments Z, = <5,(r oh) — r)(- R > according to eq. (4.8 a) [268]. The other employs a linear equation (4.8 b), correlating the shifts of an alkanol R — OH and the corresponding methyl-alkane R —CH3 by a constant bk and a slope ak, which is 0.7-0.8 for a and about 1 for ji and y positions [269]. Specific parameter sets characterize primary, secondary, and tertiary alcohols (Table 4.22). The magnitudes of Z) increments in eq. (4.8 a) decrease successively from primary to tertiary alcohols (Table 4.22), obviously as a result of reduced populations of conformers with yliauche interactions in the conformational equilibrium when the degree of alkylation increases. 

Carbon-13 shift values of aliphatic amines [337-339] collected in Table 4.42 indicate that the y effect is about — 4.5 ppm for primary, secondary, and tertiary amines. The a effect, however, increases while the ft effect decreases with an increasing degree of alkylation at the amino nitrogen. Analogously to alkanols, two increment systems can be used to predict alkyl carbon shifts of amines according to eq. (4.12 a, b), based on the reference shifts of corresponding alkanes (R —H) or the methyl homologs (R —CH3) [337, 338] (Table 4.43). 

Steroids isolated from sponges frequently contain substantially modified side chains, such as those with high degrees of alkylation or other unusual functionalization. Biosynthetic studies have been performed on the mechanism and scope of sterol side-chain dealkylation in sponges ... 

Source temperature plays an important role in PAH formation. The composition of PAHs varies depending on the combustion temperature. At low to moderate temperatures, such as in a wood stove, or from the combustion of coal, low molecular weight parent PAH compounds are abundant, and display a higher degree of alkylation. On the other hand, at high temperatures, such as in vehicle emissions, HMW parent PAH compounds are dominant, and display a higher number of unsubstituted... 

Derivatized products such as alkyl-, aryl-, alkoxyl- and amino-nanotubes and cross-linked nanotubes are obtained (Scheme 1.2). F-SWCNTs prepared by the HiPCO process exhibited a higher degree of alkylation using alkyllithium reagents than fluorinated SWCNTs from the laser-oven method. Dealkylation occurred at 500 °C. 1-Butene and n-butane were formed during the thermolysis... 

It follows from all the above-mentioned facts that the direct synthesis of methyl-, ethyl and phenylchlorosilanes is a complex heterophase process which depends on many factors and forms a compex reactive mixture. For example, in the direct synthesis of methylchlorosilanes there are about 130 compounds found and characterised. This does not mean, however, that in this or other definite synthesis all the 130 products are formed. The composition of the mixtures formed and the transformation degree of alkyl-chlorides and chlorobenzene in the synthesis of methyl-, ethyl and phenylchlorosilanes depend on the synthesis conditions, the type of the reactor used and many other factors. In spire of the complexity of the process and the variety of its products, the reaction of direct synthesis can nevertheless be directed (towards a preferential formation of a main product), changing the conditions for the preparation of contact mass, introducing various promoters into contact mass and changing the reaction conditions. 

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