Lactone oligomerization

Early-on it was discovered that these Salen compounds, and the related six-coordinate cations [6], were useful as catalysts for the polymerization of oxiranes. These applications were anticipated in the efforts of Spassky [7] and in the substantial work of Inoue [8]. Subsequently, applications of these compounds in organic synthesis have been developed [9]. Additional applications include their use in catalytic lactide polymerization [10], lactone oligomerization [11], the phospho-aldol reaction [12], and as an initiator in methyl methacrylate polymerization [13]. 

The nature of the product strongly depends on the length of the hydroxy acid generally when the hydroxyl group is remote the yield of lactone drops significantly. For example, 10-hydroxydecanoic acid [1679-53-4] does not produce any decanoUde instead, the reaction proceeds by intermolecular oligomerization, and a complex mixture of di-, tri-, tetra-, and pentalactones results (90). However, when Pseudomonas sp. or Candida iylindracea]i 2Lses are incubated with 16-hydroxyhexadecanoic acid [506-13-8] hexadecanoUde is the predorninant product (91). 

Lipase-catalyzed intermolecular condensation of diacids with diols results in a mixture of macrocycUc lactones and liuear oligomers. Interestingly, the reaction temperature has a strong effect on the product distribution. The condensation of a,(D-diacids with a,(D-dialcohols catalyzed by Candida glindracea or Pseudomonas sp. Upases leads to macrocycUc lactones at temperatures between 55 and 75°C (91), but at lower temperatures (<45°C) the formation of oligomeric esters predorninates. Optically active trimers and pentamers can be produced at room temperature by PPL or Chromobacterium viscosum Upase-catalyzed condensation of bis (2,2,2-trichloroethyl) (+)-3-meth5ladipate and 1,6-hexanediol (92). 

Hodge, P Ji-Long, J., Owen, G. J. and Houghton, M. P., Reactions of polymer-supported co-bromoalkylcarboxylates formation of lactones versus oligomerization, Polymer, 37, 5059 (1996). 

TMSCl as internal electrophile. Otherwise, only dimerization was observed. The diastere-omeric epoxy lactone 195 undergoes mainly oligomerization under the same conditions. 

A variety of anionic initiators, both ionic and covalent, have been used to polymerize lactones [Duda and Penczek, 2001 Jedlinski, 2002 Jerome and Teyssie, 1989 Penczek and Duda, 1993]. Much of the more recent activity involves the use of anionic covalent (coordination) initiators such as alkylmetal alkoxides and metal alkoxides such as R2A OR and Al(OR)3, metal carboxylates such as tin(II) 2-ethylhexanoate, metalloporpyrins (VI), and aluminox-anes such as oligomeric [A1(CH3)0] [Biela et al., 2002 Duda et al., 1990 Endo et al., 1987a,b Gross et al., 1988 Kricheldorf et al., 1990 Penczek et al., 2000a,b Sugimotoa and Inoue, 1999]. 

Reduction of [Mo(CO)(Bu C=CH)2Cp] + BF4 with KBHBu3(s) at — 78°C in an atmosphere of carbon monoxide yields a complex of a vinyl substituted y-lactone linked tj3 t]2 (220). The allylidene ruthenium complex 64, obtained by photochemical addition of one alkyne molecule to a /x-carbene derivative, is transformed into pentadienylidene complexes 65 and 66 on photolysis with more alkyne substrate. These reactions show clearly the stepwise growth of chains in alkyne oligomerizations at dimetal centers [Eq. (31)] (221). Similar reactions are also known for dinuclear iron (222), molybdenum (223), and tungsten (224) complexes. 

Table 3. Preparation of optically active cyclic tetramer (44S) and pentamer (45S) by the oligomerization of bicyclic lactone (41) of lower optical purity , [49]...

As the rate of the intramolecular reaction decreases, the formation of oligomeric materials may become predominant. Thus, the attempt to form the ten-membered lactone 307 directly from the hydroxy acid 308, under conditions optimal for the formation of a five- or six-membered lactone, would invariably lead to the formation of the oligomeric material 309 as the major product. 

Prochiral y-hydroxy diesters underwent enantioselective lactonization with PPL to afford the (S)-lactone in a highly enantioselective fashion (eq 17). Formation of macrocyclic lactones by the condensation of diacids or diesters with diols, leading to mono- and dilactones, linear oligomeric esters, or high molecular weight optically active polymers, depending upon type of substrates as well as reaction conditions, has also been described. 

The carbonyl ir-bond has been found to add chemo- and regio-selectively across the alkenic ir-bond of ketenes. Thus diphenylketene readily reacts with benzoquinone to yield a stable [2 + 2] adduct (equation 1). With an excess of diphenylketene the bis-adduct is formed, which decomposes into tetraphe-nylquinodimethane and carbon dioxide (equation 2). With the less stable ketene, thermal [2 + 2] cycloadditions are observed with highly electrophilic carbonyl compounds (equation 3). With unactivated aldehydes and ketones, yields are much lower due to a faster oligomerization of the ketene reagent. However, in the presence of a Lewis acid catalyst, most aldehydes or ketones form P-lactones with ketene (equation 4). Cycloadditions with ketones usually require more active catalysts than with aldehydes. The catalyzed reaction of ketene with methyl vinyl ketone is chemoselective, yielding a 10 1 ratio of [2 + 2] versus [4 + 2] adducts (equation 5). In the absence of catalyst, methyl vinyl ketone reacts with ketene to give exclusively the [4 + 2] adduct. 

A bis(dicyclohexylphosphino)butane (DCPB)-based palladium catalyst was found to catdyze the analogous reaction between isoprene and CO2 [11]. A mixture of lactones 7 and 8 is obtained but the yield of co-oligomerization products is significantly lower (8%) than for the reaction of 1,3-butadiene. 

Thus, the ring opening of /3-proplolactone takes place almost exclusively at the alkyl-oxygen bond to form a porphlnatoalumlnum carboxylate (Equation 2a) (7). The same conclusion was obtained for the oligomerization of 0-proplolactone and butyrolactone as a result of 1H-NMR and 13C-NMR spectroscopy. As expected, the equimolar reaction product of TPPAlEt (1e) and carboxylic acid (cf. Equation 3) Is a good Initiator for the polymerization of lactone and yields a polymer with narrow molecular weight distribution. 

Enzymatic polymerization and oligomerization can be used to make polyesters, polypeptides, polysaccharides, polymers from phenols, polymers from anilines, and many others. This approach could lead to fewer side reactions, higher regio- and stereoselectivity, under milder conditions. Oligomeric polyesters can be prepared from lactones. Caprolactone can be polymerized in bulk with lipases (9.43) to polymers with molecular weights of 7000.305... 

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