ABSTRACT:Reaction of poly[(S )-1-oxo-2-methylpropylene] (1) with a dizinc-titanium reagent, CH 2 (ZnI) 2 -TiCl 3 , afforded the corresponding completely methylenated polymer, poly[(S )-1-methylene-2-methylpropylene] (3), in 78% isolated yield. It was confirmed that no significant epimerization took place under the reaction conditions by comparing 13 C NMR spectrum of 3 with an atactic analog. Also, poly[(S )-1-oxo-2-butylpropylene] (2) was converted to the methylenated polymer 4. The exo-methylene/ketone ratio of 4 did not exceed 85/15 even by the repeated treatment with CH 2 (ZnI) 2 -TiCl 3 , possibly due to the steric hindrance of side-chain butyl group. In polymer 4, the main chain was suggested to be less flexible based on the smaller peak area and shorter relaxation time (T 1 ) of the main chain protons in 1 H NMR.KEY WORDS Enantioselective Alternating Copolymerization / Polyketone / Methylenation / gem-Dizinc Reagent / Tebbe Reagent / Hydrocarbon Polymer / Asymmetric Center / Physical properties of synthetic polymers are highly dependent upon stereoregularity of the main chain. Since Natta synthesized isotactic polypropylene, 1 many researchers have taken interest in stereocontrol of the main chain, 2 and efforts have been devoted not only to tacticity control but also to chirality control of chirotopic centers in the main chain. 3 However, appeared are few examples of syntheses of optically active hydrocarbon polymers with main chain chirality, the only four examples are, asymmetric polymerization of 1,3-pentadiene, 4 asymmetric cyclopolymerization of 1,5-hexadiene, 5 syntheses of optically active polystyrenes with chiral templates, 6 and polymerization of optically active allene. 7 These methods can be classified as asymmetric polymerization of prochiral monomers or polymerization of optically active monomers. Here we report another method of producing optically active hydrocarbon polymers with main chain chirality, methylenation of optically active polyketones. Polyketones 1 and 2 are given by asymmetric alternating copolymerization of carbon monoxide with olefins, propene and 1-hexene, respectively (Scheme 1). These are γ-polyketones which have an asymmetric carbon at the α-position of each carbonyl group. [8][9][10][11] The carbonyl groups can be transformed into various functional groups, and therefore chemical modification of the carbonyl groups would provide new classes of chiral polymers. 9,12 Although many examples were reported for the functionalization of polyketones, mainly poly(ethene-co-CO) or achiral poly(propene-alt-CO), no report has ever appeared for the nucleophilic addition of carbanions to the carbonyl groups. 13 Carbanions may possibly cause side reactions, such as the enolate formation and the following aldol reaction, due to their strong basicity. In spite of its nucleophilic nature, however, methylenation of carbonyl groups is known to proceed without any enolate formation using appropriate reagents. 14,15 In such a case, the methylenation of optically active polyketones with one ...
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