Weak basic catalysts

Acylation of pyridazinones and related compounds in the presence of weakly basic catalysts such as pyridine or sodium acetate produces IV-acylated products, while O-acylated products are obtained under strongly basic conditions. However, the reaction between 6-chloropyridazin-3(2//)-one with chlorocarbonates and that of maleic hydrazide with unsaturated acid chlorides or chloromethylsulfonyl chloride gives preferentially N-substituted products. 

Alcoholysis of ester and epoxide with various basic catalysts including alkaline earth metal oxides and hydroxides was reported recently by Hattori et alF61 Various alcohols were transesterified with ethyl acetate at 273 K. The results show that in the presence of strongly basic catalysts such as CaO, SrO and BaO, propan-2-ol reacted much faster than methanol, whereas in the presence of more weakly basic catalysts such as MgO, Sr(0H)2-8H20 and Ba(0H)28H20, methanol reacted faster than propan-2-ol. When the alcoholysis was performed with propene oxide, alkaline earth metal oxides were found to be more reactive than hydroxides the reactivity of the alcohols was in the order methanol > ethanol > propan-2-ol > 2-methylpropan-2-ol, regardless of the type of catalyst. 

A weakly basic catalyst is needed here because we want to discriminate between the primary and secondary alcohols in the diol. Imidazole is too weak (pKaH 7) to remove protons from an alcohol (p-Ka 16) but it can remove a proton after the OH group has attacked the silicon atom. 

If a zeolite with a low Si/AI ratio is exchanged with a large monovalent cation, e.g. Cs+ in zeolite X, one obtains a weakly basic catalyst (CsX). The basic sites in this case are the oxygen framework atoms of the zeolite itself. Much stronger basic sites are formed when a zeolite or MCM-41 support is overexchanged or impregnated with, e.g. Cs acetate, and subsequently calcined (34). This results in formation of Cs20 oxidic particles. 

Relatively weakly basic catalysts, such as hydroxide ion or cyanide ion, may be used to initiate polymerization of monomers, which have strongly electron-withdrawing substituents on the vinyl group, such as a nitrile or carbonyl. Thus, suitable monomers would be acrylonitrile or methylmethacrylate. 

Try to appreciate that different methods will be appropriate at different times. If you want to make a few milligrams of a complex ester, you are much more likely to work with a reactive acyl chloride or anhydride, using pyridine as a weakly basic catalyst, than to try and distil out a minute quantity of water from a reaction mixture containing a strong acid that may destroy the starting material. On the other hand, if you are a chemist making simple esters such as those in Chapter 2, p. 34) for the flavouring industry on a scale of many tons, you will prefer the cheaper option of carboxylic acid plus HCI in alcohol solution. 

As an alternative to the highly specific catalysis indicated by formulas I, II, and III, it is possible that the metal chelate compound merely participates in a generalized type of acid-base catalysis. Thus, the function of the metal would be to increase the acidity of the substrate through molecular association and thereby increase its susceptibility toward attack by other bases present such as hydroxide ion or water molecules. Under these conditions the diaquo chelate A would be an acid catalyst, the monohydroxy chelate Bi would be considered to be bifunctional in its effect, and the dihydroxy chelate B2 would probably be a weak basic catalyst. 

PREPARATIVE TECHNIQUES Synthesized by step-growth polymerization between butylene glycol and terephthalic acid. PBT is often synthesized by ester-exchange polymerization using weak basic catalysts such as alkanoates, hydrides, and alkoxides of sodium, lithium, zinc, calcium, magnesium, titanium, etc. PBT is formed by the reaction of dimethyl terephthalate with l,4-butanediol at 0.020 atm and 160-230°C. Final reaction occur at 260-300°C under vacuum at 0.001 atm. 

Brensted p for bases whose conjugate acids have > 7 is 0.95 and represents nucleophilic catalysed hydrolysis (Proctor and Page, 1979). Evidence for an intermediate ester formed during the reaction has been obtained with alkoxide ions and phosphate dianion (Proctor and Page, 1979 Bundgaard and Hansen, 1981). Weakly basic catalysts probably act as general base catalysts. The reaction with acetate exhibits a solvent isotope effect of 2.1, as expected for general base catalysis but not for a... 

The large sensitivity of the rate constants to base strength for weakly basic catalysts indicates that the catalyst resembles its conjugate acid in the transition state, i.e. there is a large amount of, or complete, proton transfer to the catalyst in the transition state. For strongly basic catalysts the small sensitivity of the rate constants upon base strength suggests that the catalyst resembles its free unprotonated basic form in the transition state. 

For Michael additions of CH acidic compounds (e.g. diethyl malonate with a,3-unsaturated ketones) the following recommendations are given "When possible, relatively weak basic catalysts such as piperidine... should be selected. If stronger bases are required, it is normally appropriate to use only 0.1 to 0.3 equivalent of the base." The analogy of these conditions to those specified by our rule A is obvious (concentration control). On the other hand, preformed carbanions (organometallies) are usually employed when the addend is more basic than the enolate produced by attack at the unsaturated carbonyl compound. Though the nature of the metal ion plays a crucial rule in many "carbanionic" addition reactions, a first understanding of the principles involved can be... 

Polymerization of PET is different from the polyolefins described previously. It is formed by a condensation reaction between two monomers dimethyl terephthalate (DMT) and ethylene glycol (EG). The polymerization process involves a two-step process ester interchange (Eq. [2.3]) and polymerization (Eq. [2.4]). Weak basic catalysts, usually metal salts, are used to accelerate polymerization and control the molecular weight and stoichiometry of the polymer. The first step is carried out at temperatures from 150 to 200°C and the methanol is continuously distilled off. The reaction is a solution polymerization. The temperature in the second step is increased to 260—290°C, at which melt polymerization occurs (Odian, 1981). 

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