Aldehyde-phenol ratio

Although 4-hydroxybenzaldehyde can be made by the saligenin route, it has been made historically by the Reimer-Tiemann process, which also produces sahcylaldehyde (64). Treatment of phenol with chloroform and aqueous sodium hydroxide results in the formation of benzal chlorides, which are rapidly hydrolyzed by the alkaline medium into aldehydes. Acidification of the phenoxides results in the formation of the final products, sahcylaldehyde and 4-hydroxybenzaldehyde. The ratio of ortho and para isomers is flexible and can be controlled within certain limits. The overall reaction scheme is shown in Figure 1. Product separation is accomphshed by distillation, but this process leads to environmental problems because of the quantities of sodium chloride produced. 

Phenolic oligomers are prepared by reacting phenol or substituted phenols with formaldehyde or other aldehydes. Depending on the reaction conditions (e.g., pH) and the ratio of phenol to formaldehyde, two types of phenolic resins are obtained. Novolacs are derived from an excess of phenol under neutral to acidic conditions, while reactions under basic conditions using an excess of formaldehyde result in resoles. 

The polyoxyalkylene units in the copolymer have a molecular weight below 500, and the polysiloxane units have 3 to 50 silicon atoms. The resin has a phenol/aldehyde ratio of 2 1 to 1 5 and an average molecular weight of 500 to 20,000 Dalton. The composition shows synergistic demulsification activity when compared with the individual components. The siloxane units can be either in blocks [979,980] of the polyoxyalkylene-polysiloxane copolymer or randomly distributed [728,729]. 

It is highly probable that since a similar ratio is found regarding the identifiable products (phenolic aldehydes and ketones) obtained by the alkaline oxidation of these species, that the same readily releasable fraction of the original lignin is involved in each case. 

The enol tautomers of many ketones and aldehydes, carboxylic acids, esters and amides, ketenes, as well as the keto tautomers of phenols have since all been generated by flash photolysis to determine the pH rate profiles for keto-enol interconversion. Equilibrium constants of enolization, KB, were determined accurately as the ratio of the rate constants of enolization, kE, and of ketonization, kK, Equation (1). 

Figure 9.47 Concentrations of acid aldehyde ratios in vanillyl (Ad/Al)v and syringyl (Ad/Al phenols during the subaqueous decay of lignin in vascular plants in a 4-year laboratory incubation study. (Modified from Opsahl and Benner, 1995.)...

Titanosilicalite (TS-1)[165,166], a highly siliceous MFI type zeolite in which 0.1 to 2.5% of the Si atoms are replaced by Ti, is the most successful example for the use of isomorphously substitited zeolites. As a consequence of the high Si/Al ratio of TS-1 the material contains only a negligible concentration of strong Bronsted acid sites. In fact, the presence of acid sites is detrimental to the selectivity of the catalysts, as discussed below. TS-1 has been found to be a selective oxidation catalyst for a wide variety of reactions such as the conversion of alkenes to epoxides [167], alcohols to aldehydes [168], alkanes to secondary alcohols and ketones [169,170], phenol to hydroquinone and catechol [171] and amines to hydroxylamines [ 172]. A schematic representation of the chemistry is given in Fig. 7 which is adapted from ref [17]. 

Table XII. The ratio of acid to aldehyde for vanillyl and syringyl phenols from stream, foam, and foam-extract humic substances from Como Creek and the Suwannee River.

The hydroformylation of the orf/zo-prop-2-enylphenol 81 which contains no benzylic hydroxy group gives a mixture of the open-chain aldehyde 82 and the seven-membered cyclic hemiacetal 83 in a 82 83 ratio of approximately 40 60 (equation 34) °. The ben-zofuran epoxide 85 and its valence-isomeric quinone methide 86, both readily obtainable from benzofuran 84, rearrange thermally above —20°C to form the allylic alcohol 87 and the tautomeric phenol 88 (equation 35) . ... 

There are significant differences between the CP-MAS spectra of the cured resins with a formaldehyde/phenol (FP) ratio = 1.2 and FP = 1.8. There is no peak at 110 ppm in the FP = 1.8 resin, indicating complete substitution at the ortho position , unlike in the FP = 1.2 resin. The cured FP = 1.8 resin contains methyl (10 ppm), phenoxy (150 ppm) and carbonyl groups (190 ppm) while the cured FP = 1.2 resin does not. Interrupted decoupling identifies the carbonyl in this material as an aldehyde. 

In a typical synthesis, a mixture of C13H12O2 bisphenols was prepared in 80% yield by slow addition of a solution of trioxane (0.036 mole) in benzene (over 1.75 hour) to a stirred, liquid phase suspension of phenol (0.64 mole) and HY zeolite (5 gm) at 182°. The ratio of the 2,2, 2,4, and 4,4 isomers was 1.3 1.8 1.0. This technique, which afforded very high instantaneous ratios of phenol to aldehyde, prevented rapid catalyst aging. Generally, high yields were observed for carbonyl reactants with no a-hydrogens, since competitive intracrystalline aldol condensation reactions were eliminated. 

Compound 47 from the former scheme was also used for the synthesis of the narciclasine alkaloids (Scheme 5). Its treatment with excess ethanethiol and magnesium bromide afforded the dithioacetal 54 in 86% yield. Protection of the hydroxyl groups in 54 followed by hydrolysis of the dithioacetal afforded the corresponding aldehyde, which was treated with nitromethane to give a mixture of diastereomers 55 (1.8 1 ratio) in 80% yield. Treatment of the mixture with excess TBSOTf resulted in the silylation of the hydroxyl group. Subsequent selective deprotection of the phenolic TBS group afforded 56. Oxidation of the mixture of diastereomers 56 with silver(I) oxide afforded 57, whose treatment with DMAP afforded 60 and 61 in 29 and 57% yield, respectively. The minor product 60 possesses five of the six stereogenic centers of pancratistatin (3). 

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