Three significant modifications to existing methods for the preparation of the important 5,10,15,20-tetrakisarylporphyrins have improved isolated yields, simplified work-up and made large-scale synthesis feasible. Two teuakisalkylporphyrins were also produced. A two-stage approach using hydrogen peroxide in acetic acid as second stage oxidant gave good yields but for ease of isolation and convenience in worldng on a large scale, the one-pot approach is preferred. No one method appears to be suitable for all such tetrak~arylporphyrins and, for best yields, the method of preparation needs to be chosen carefully. Application of statistical opthisation techniques (factorial two design and simplex operation) led to considerably enhanced yields for the one-pot method. For one of the two-stage modifications, significant amounts of chlorins were observed, sometimes of such magnitude as to make it suitable as a method for their preparation.

The spontaneous hydrolysis of p‐nitrobenzyl cellulose xanthate (CelXNB) with a degree of substitution (DS) in the range 2–9 was studied in 10% aqueous ethanol at pH 10, and was followed spectrophotometrically by the appearance of p‐nitro‐α‐toluenethiol, in a continuous‐flow system where the reactor was shaken. CelXNB was characterized by solid‐state 13C NMR spectra. The reaction occurs through two parallel processes due to two xanthate ester groups with different reactivities. The fast hydrolysis was ascribed to the reaction of the C‐2 + C‐3 isomers, whereas the slow hydrolysis was due to the C‐6 isomer. The percentage of the latter is much higher than C‐2 + C‐3. The solvent isotope effect of the fast hydrolysis (k′ H 2O/)k′ H 2D 11 was 2·22 ± 0·16 and the proton inventory indicated that there is only one proton transfer involved in the transition state, where a second water (or a neighbouring OH group) acts as a general base. The entropy of activation of the fast hydrolysis was only 3·3 ± 0·8 e.u., suggesting that the water molecules involved are highly oriented with respect to the coordinates required to reach the transition state. It is proposed that they form part of the three‐dimensional hydrogen‐bonded ice‐like structure that involves the cellulose matrix.

The hydrolysis of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) was studied in 10% v/v aqueous ethanol at 25°C and μ = 0.1 (KCl). The water-catalyzed hydrolysis showed that, as for p-nitrobenzyl cellulose xanthate, it occurs through two parallel reactions with rate constants k'H2O = 4.40 x 10-3 s-1 for the fast hydrolysis, and k''H2O = 6.90 x 10-5 s-1 for the slow hydrolysis. The entropy of activation of the fast hydrolysis was 0.7 ± 1.8 cal K-1 mol-1. External nucleophiles such as hydroxide and simple amines show simple first-order kinetics. The spontaneous hydrolysis of CelXDNP in acetone-water mixtures indicates that the fast reaction does not occur through water polymers and that for water molarity higher than 30 M there are no acetone molecules (or very few) in the highly ordered cybotactic region of cellulose. The spontaneous hydrolysis of methyl 4,6-O-benzylidene- α -D-glucopyranoside 3-(S-p-nitrobenzyl-xanthate) although is faster than the 6-isomer, it is slower than the fast hydrolysis of p-nitrobenzyl cellulose xanthate (CelXNB). Also Δ Sdouble dagger is highly negative (-41.0 cal K-1 mol-1), as it is for alkyl and sugar analogues. Only for the fast hydrolyses of CelXDNP and CelXNB is the entropy of activation almost zero. It is concluded that there is no neighbouring OH effect on the fast hydrolysis of cellulose xanthate esters. Key words: hydrolysis, water catalysis, cellulose xanthate esters, methyl glucose, xanthate esters, neighbouring OH effect.

The hydrolysis and aminolysis of a series of S-substituted O-alkylxanthate esters was studied in 20% v/v aqueous methanol at 35°C. The pH-rate profiles of the hydrolyses showed water and hydroxide-ion-catalyzed reactions. The reaction of 2,4-dinitrophenyl cellulose xanthate (CelXDNP) and p-nitrobenzyl cellulose xanthate (CelXNB) with polyalanine and lysozyme produced a covalent bond between the polypeptide and the cellulose matrix, as shown by solid-state 13C NMR. However, the nature of the bonding could not be identified. The reaction of nucleophiles (H2O, OH-, RNH2) and xanthic esters was consistent with an addition-elimination mechanism through a tetrahedral intermediate. Brønsted plots against the pKa of the nucleophile (βnu) or the nucleofuge of the substrate (βlg) were used to characterize the rate-determining step. The pKa values of the nucleophiles ranged between -1.74 and 15.74, and for the nucleofuges, they were in the range of 10.50-0.92. For nucleophiles with pKa values up to about 10, βlg was 0.10-0.15, and βnu changed from 0.48 to 0.35 for the strongest electron-withdrawing nucleofuge. It was concluded that the water-catalyzed hydrolyses, and also aminolyses with moderately basic amines, occur with rate-determining formation of the tetrahedral intermediate. For strong bases such as hydroxide ion, the disappearance of the intermediate becomes the slowest step. The reaction of cellulose xanthic esters with external nucleophiles as hydroxide ion and amines shows simple first-order kinetics and is slower than alkyl or sugar xanthates, probably due to the diffusion effect through the tight cybotactic region of cellulose. Key words: hydrolysis, aminolysis, alkyl xanthic esters, cellulose xanthic esters, sugar xanthic esters.

The hydrolysis of ethyl N‐p‐substituted arylthioncarbamates was studied at 100 °C in the pH range 6.5–12.5. No general catalysis was found, and the presence of an isothiocyanate intermediate was detected, indicating that the alkaline hydrolysis occurs by an E1cb mechanism. From the pH–rate profiles, the first‐order rate constants kE for the elimination step of the thioncarbamate anion forming the isothiocyanate intermediate were determined. The alkaline hydrolysis of p‐substituted arylisothiocyanates was studied at 25 °C in 0.1–0.3 M solutions of NaOH and in 0.1–0.3 M aqueous ethanol solutions, at different concentrations of NaOH. The second‐order rate constants for the addition reaction with hydroxide (kOH) and ethoxide (kA) ions were obtained. Leffler plots for the elimination of the ethoxide ion from the arylthioncarbamate anion and for the addition of the ethoxide ion to the arylisothiocyanate were linear. From Leffler's equation, with the sole condition that dαL/dΔG should be constant, a modified Marcus equation (MME) was obtained, where a parameter p (or q for the reverse reaction) defined the asymmetry of the intrinsic barrier. (When p = 1/2 the barrier is symmetric and the MME becomes the Marcus equation in the usual form.) For the addition–elimination reaction studied, both Leffler plots were adjusted to MME with the asymmetric parameter p = 0.694 ± 0.002 for the addition and q = 0.307 ± 0.002 for the elimination reaction. The intrinsic barrier was $\Delta G_{0}^{\neq} = 24.75 \pm 0.02\,\hbox{kcal mol}^{-1}$ and |ΔGmax| = 438 ± 4 kcal mol−1 (1 kcal = 4.184 kJ). The addition reaction was exoergic and, as expected from the high intrinsic barrier, αL changed very little in the series (0.679–0.683); the transition state was product‐like, and it moved towards the reagents with increasing exoergicity. Copyright © 2002 John Wiley & Sons, Ltd.