Pora-Substitution

Figure 13.15 ATR-IR spectra of various pora-substituted aryl isocyanides adsorbed on a palladium film [26].

Fig. 14 Typical potential dependencies of the heterogeneous rate constant for the electroreduction of pora-substituted diaryl disulfides in DMF/O.IM TBAP. The dashed lines represent the parabolic fitting to the experimental data. The extrapolated standard potential and rate constant values (see text) are indicated.

Table 12. Oxidation potentials, Ep, of pora-substituted 2-aryl-propionaldehydes 88 and their tautomeric enols 89 in acetonitrile/DMSO [171] compared with calculated , 2 (89)- The enol content was taken from Ref. [187]...

This phase can be used in the preparative and analytical stereochemical resolution of a wide variety of compounds, including acetate derivatives of alcohols and diols benzylic alcohol derivatives pora-substituted d-phenyl-d-valerolactones 3,4-dihydro-2H-pyran-2-carboxylic acid derivatives phenylvinylsulfoxide fra s-stilbene oxide (46). 

The mechanism of thyroxine biogenesis was considered by Johnson and Tewkesbury (451), who recognized the similarity between the conversion of diiodotyrosine to thyroxine and the type of oxidation of ortho- and pora-substituted phenols extensively studied by Pummerer (684). They suggested that oxidation gave rise to two types of free radical, one with the electron lost from the para position of the quinonoid form and one from the phenolic oxygen. Addition of these two, as in diagram 15, would giv e... 

Aryl 1-thioglycosides have also been examined for their ease of acid hydrolysis.i Pora-substituted-phenyl 1-thioglycopyranosides of fi-D-glucose, /S-D-galactose, -D-xylose, and a-L-arabinose were hydrolyzed in 3 N hydrochloric acid at 80° the rates are in the order (p-substituents) 0H>0Me>Me>H>Cl>Br>N02. This is the same as the order found for glycosides (see Table XXVI). 

This catalyst is exceptionally active and allows the Heck reaction to be performed on iodobenzene at 30 °C. The catalyst is inhibited by addition of a 300-fold excess of Hg. Hammett correlation of the reaction on pora-substituted bromobenzenes gave a p = 2.7, which is in accord with an oxidative addition on Pd(0).TEM showed the presence of nanoparticles in a solution of the catalyst which was pre-treated with n-butyl acrylate, NaOAc and TBAB. After addition of iodobenzene nanoparticles could no longer be detected showing that the oxidative addition of iodobenzene to the colloids is indeed a very fast reaction. Reaction with haloarenes and acrylate esters attached to a solid support gave the Heck products in excellent yields, proving that the catalyst is not heterogeneous in nature. 

An appreciable range of cis-trans product ratios has been found in ring-opening reactions of meta- and pora-substituted 1-phenylcyclohexene oxides. Suspensions of a series of these compounds (276) in 0.02 mol 1 H2SO4 at 25 °C for 24 h gave products (277) and (278) together with 3 % of minor products, which were shown not to be... 

The eighth item in Table 8.7 is an example of the Fries rearrangement (after K. Fries, vide supra. Scheme 8.42). In this interesting reaction, evidence has accumulated that carboxylic acid esters of phenols, on treatment with aluminum trichloride, reversibly dissociate into two fragments (presumably the aluminum-substituted phenolate and acetyl chloride). The fragments then recombine into the corresponding ortho- and pora-substituted phenolic ketones (Scheme 8.83). 

Thiazetidines and Thiazetes.—Kinetic data for the c cloadditions of dicyclo-hexylcarbodi-imide with pora-substituted phenyl isothiocyanates to give 1,3-thiazetidine derivatives (68) (yields of 31—92%) have been obtained for both... 

A Hammett plot for pora-substituted benzaldehydes showed that electron-rich aldehydes gave higher ees (r = -0.4). As in Shibuya s related results (Section 5.3.3.1 above), this indicates that aldehyde coordination is important in enantiodifferentia-tion, but the lower rvalue (compared to Shibuya s r = -1.30) suggests a weaker electronic influence, probably due to the relative Lewis acidities of A1 and La. For ortho-substituted aldehydes, lower ees were observed, presumably due to steric effects. Although Al-Cl and Al-triflate complexes 29-30a-b did not catalyze the reaction, they... 

Comparing p-silyl-substituted vinyl cations however with different a-aryl substituents shows that the stabilizing effect of a P-silyl groiq> is not constant but is dependent on the electron demand of the carbocation. Fig. 4 shows the pora-carbon C NMR chemical shift difference A8 between the p-silyl and P-unsubstituted vinyl cations (For the a-ferrocenyl substituted cation A8 C3,4 is used to probe the silyl effect). 

It has been shown by Nath and Rydon that, in general, the acid hydrolysis of substituted phenyl /3-i)-glucopyranosides is facilitated by any electron-repelling properties of the substituents, and there seems to be a direct relationship between the Hammett substitution constant velocity constant of the reaction, especially for the meta- and pora-sub-stituted derivatives. For instance, with 0.1 N hydrochloric acid at 60°, the rate of hydrolysis of the p-cresyl (k = 2.32 X 10 min , [Pg.32]

It is worthwhile to note the dependence of " on the adsorption energy of i (j constant). In Fig. 11-2 values of corresponding to the data of Table 11-1 for the / -methyl-substituted i-benzenes are plotted versus ag for adsorption on alumina (a) and silica (b). The data for alumina show a good linear dependence of a on a0 i.e., the change in adsorption energy of i as a result of electronic interaction with a pora-methyl group... 

Arl Ar—Ar. lodobenzene and pora-disubstituted derivatives are converted into biaryls in about 50% yield when treated with a catalytic quantity of pal-ladium(ll) acetate in triethylamine at lOO . Presence of water results in replacement of iodine by hydrogen. The reaction fails or proceeds in low yield with orfho-substituted aryl iodides. During the reaction Pd(0) is deposited. ... 

The 2,5-diaryl-substituted 1,3,4-oxadiazoles 27 were obtained from the anhydrides 2 and 19 and the hydrazides of aromatic acids in acetic acid in the presence of pora-toluenesulfonic acid [14],... 

The relative method of quantification is used in the orphenadrine citrate monograph in order to determine the content of meta- and pora-methylphenyl isomer in the ortho-methylphenyl substituted drug (see Figure 3-3). 

The oxidation of phenols to catechols or hydroquinones by tyrosinase enzymes has been developed for biocatalysis. For example, the ortho-hydroxylation of L-tyrosine 162 (and also substituted variants) to give l-DOPA 163 has been extensively studied due to the importance of l-DOPA in the treatment of Parkinson s disease [92, 93]. An arene hydroxy lating enzyme having a broad substrate scope is 2-hydroxybiphenyl 3-monooxygenase from Pseudomonas azelaica, which is able to oxidize many ortho-substituted phenols 68 to the corresponding catechols 127 [94], as shown in Scheme 32.19. A notable example of an industrial biocatalytic arene hydroxylation that has been employed on very large scale (lOOm fermentation) is the pora-hydroxylation of R)-2-phenoxypropionic acid 164 by whole cells of Beauveria bassiana Lu 700 to give (R)-2-(4-hydroxyphenoxy)propionic acid 165, an important intermediate in herbicide manufacture [95]. 

---