Alkoxy precursor route

Alkoxy Precursor Route. Sulfonium precursor polymers can be readily converted to methoxy-substituted polymers, which are soluble in organic solvents and which are much more stable than the sulfonium precursor. The alkoxy group can be removed by acid catalysis and/or heating to produce fully converted PPV derivatives [950, 968, 970]. 

The sulfonium precursor route may also be applied to alkoxy-substituted PPVs, but a dehydrohalogenation-condensation polymerization route, pioneered by Gilch, is favored 37]. The polymerization again proceeds via a quinomethide intermediate, but die syndicsis of the conjugated polymer requires only two steps and proceeds often in improved yields. The synthesis of the much-studied poly 2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylene vinylene], MEH-PPV 15 is outlined in Scheme 1-5 33, 35]. The solubility of MEH-PPV is believed to be enhanced by the branched nature of its side-chain. 

PPV derivatives can be prepared by water-soluble precursor route [14-16] involving sulfonium polyelectrolyte or organic-soluble precursor route [3,17,18] involving alkoxy substituted non-ionic form, respectively. The representative synthetic pathways for PPV derivatives are shown in Scheme 1. 

Soluble main chain oxadiazole polymers can be realised using solubilizing side groups such as tert-bu y and long flexible alkoxy or alkyl groups. Alternatively, flexible spacers can be incorporated between aromatic oxadiazole units, and finally a soluble precursor route may be employed. 

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

This route has many ramifications depending on the nature of R and R" in Scheme 2 thus R may be alkyl, aryl, amino (thioamide), alkylthio, or thiol and R" may be alkyl, aryl, amino (amidine), hydroxy (amide), or alkoxy (imino ether), although as yet not all of the possible combinations of these groups has been fully examined. In practice, the utility of the route depends on the availability of the requisite intermediates, and the situation is further complicated by the numerous routes which have been devised to these precursors. 

The Julia olefination reaction is highly regioselective and ( )-stereoselective, providing a valuable alternative to the Schlosser reaction for making rrans -disubstituted olefins. The reaction involves condensation of a metalated alkyl phenyl sulfone with an aldehyde to yield a P-hydroxysulfone, which is then subjected to a reductive elimination to produce the alkene. There are limitations to the preparation of tri- and tetra-substituted alkenes via the sulfone route because the P-alkoxy sulfones derived from addition of the sulfone anion to ketones may be difficult to trap and isolate or they may revert back to their ketone and sulfone precursors. 

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