Synthesis alkoxide initiator

Conventional sodium-, potassium-, or cesium-based initiators in an ether solvent, such as tetrahydrofuran (THF), or in polar media, such as dimethyl sulfoxide (DMSO) or hexamethylphosphortriamide (HMPTA), afford a controlled polymerization allowing the synthesis of end-functional PEO. The chemical mechanism of EO polymerization is relatively simple in view of the well-known stability of alkoxide growing chains toward termination and transfer reactions. The standard method for producing low-MW PEO (PEG) is based on controlled addition of EO to water or alcohols in the presence of alkaline catalysts. The kinetics of EO polymerization, initiated by sodium methoxide in the presence of a small excess of methanol in dioxane involves a contact ion pair of the initiator. The reaction rate in the case of alkaU/alkoxide initiation alone is slow, but increases with the concentration of excess alkanol. Within the variation of excess alkanol concentration, the MW distribution (MWD) of the polymers produced remains narrow. Formation of an alkanol-alkoxide complex as an effective initiator is envisaged. The complex formation loosens the bonding of the tight alkoxide ion pair. 

This group has also developed two ring-contraction systems of potential use in crown synthesis. In the first of these, extrusion of a phenylphosphine oxide unit results from treatment with alkoxide ion. In the second, similar conditions initiated decarbonyla-tion of a bis-pyridyl ketone Despite the apparent potential of these methods for crown synthesis, direct formation of crowns by processes which involve them do not appear to have enjoyed great success thus far. 

For the classical Williamson synthesis an alcohol is initially reacted with sodium or potassium to give an alkoxide, e.g. 1. Alternatively an alkali hydroxide or amide may be used to deprotonate the alcohol. Phenols are more acidic, and can be converted to phenoxides by treatment with an alkali hydroxide or with potassium carbonate in acetone. ... 

A highly modified methyl testosterone derivative also exhibits antiandrogenic activity. One synthesis of this compound Involves initial alkylation of methyl testosterone (3 ) by means of strong base and methyl iodide to afford the 4,4-dimethyl derivative Formylation with alkoxide and... 

The most studied catalyst family of this type are lithium alkyls. With relatively non-bulky substituents, for example nBuLi, the polymerization of MMA is complicated by side reactions.4 0 These may be suppressed if bulkier initiators such as 1,1-diphenylhexyllithium are used,431 especially at low temperature (typically —78 °C), allowing the synthesis of block copolymers.432,433 The addition of bulky lithium alkoxides to alkyllithium initiators also retards the rate of intramolecular cyclization, thus allowing the polymerization temperature to be raised.427 LiCl has been used to similar effect, allowing monodisperse PMMA (Mw/Mn = 1.2) to be prepared at —20 °C.434 Sterically hindered lithium aluminum alkyls have been used at ambient (or higher) temperature to polymerize MMA in a controlled way.435 This process has been termed screened anionic polymerization since the bulky alkyl substituents screen the propagating terminus from side reactions. 

Taramasso et al. (5) had originally reported two methods for the hydrothermal synthesis of TS-1. The first method (mixed alkoxide method) involves the preparation of a solution of mixed alkoxides of titanium and silica (preferably ethoxides) followed by hydrolysis with alkali-free solution of tetrapropylammonium hydroxide (TPAOH), distillation of the alcohol and crystallization of the resulting gel at 448 K. In the second method (dissolved or hydrolyzed titanium method) a soluble tetrapropylammonium peroxo-titanate species was prepared initially and then colloidal SiC>2 (Ludox AS-40) was added. This entire operation had to be carried out at 278 K. The TS-1 samples obtained by these two synthesis routes differed, particularly because of the presence of impurities such as Al3+ usually present in colloidal silica (33). 

Simple examples shown above are the base-catalysed formation of oxygen- and nitrogen-containing ring systems. We have shown base-initiated ionization of the alcohol to an alkoxide anion in epoxide formation the anion is a better nucleophile than the alcohol. For pyrrolidine synthesis, the amino group is sufficiently nucleophilic for reaction to occur, but base is required to remove a proton from the first-formed intermediate. 

Multifunctional initiators made up of metal alkoxides rather than alcohols have been less used for the synthesis of star-shaped polyesters than have the tin (II) bis-(2-ethylhexanoate)/alcohol system. Nevertheless, Kricheldorf initiated the polymerization of sCL using a spiro-cyclic tin(IV) aUcoxide to obtain a tin-containing height-shaped polyester whose final hydrolysis resulted in the formation of a star-shaped polyester (Fig. 35) [25, 159-161]. 

Fig. 39 Synthesis of macrocyclic aliphatic polyester by ring-expansion polymerization initiated by cyclic tin(IV) alkoxides...

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