Zinc alkoxide catalysts

Depolymerization of macromolecular esters and repolymerization into linear polyesters is reported by Zhang et al. (3) using zinc alkoxide catalyst, (II). 

Menthone transforms into a polymerizable monomer by Bayer-Villiger oxidation to the seven-membered lactone menthide (oxidant w-chloroperbenzoic acid, wCPBA). Controlled ROP of menthide can be achieved using a zinc alkoxide catalyst (see Scheme 8) (toluene, room temperature) to yield aliphatic polyesters with predictable MWs up to 91 kg/mol and narrow MW distributions (PDl 1.1) [96]. 

Scheme 9 Preparation of a mixture of isotactic and syndiotactic poly(cyclohexene oxide) using chiral zinc alkoxide catalysts.

Scheme 9.19 Proposed mechanism for the ROP of BL by (R)-diiminate zinc alkoxide catalyst. (Adapted from Ref [59]).

Ziegler-Natta catalysis, 431, 449 Zinc diacetate catalysts, 71 Zirconium alkoxides, 68... 

Zinc carbonate reacts with epoxide to form zinc alkoxide, which in turn reacts with carbon dioxide to regenerate zinc carbonate. The most effective catalyst systems were the reaction products between diethylzinc and polyhydroxy compounds such as water or pdyhydric phenols243,244. This copolymer is interesting as a biodegradable elastomer248. ... 

Zinc alkoxide and aryloxide complexes have been of particular interest as enzyme models and catalysts. Tetrameric alkyl zinc alkoxides are a common structurally characterized motif.81... 

Prasad and Joshi121 presented a conceptually different catalyst system—zinc amides of oxazolidine. Because the addition of dialkylzinc to aldehyde is known to involve a chiral zinc alkoxide with a coordinately unsaturated tricoordinated center, they anticipated that a zinc amide with dicoordinate zinc should be a better Lewis acid. Examining three different zinc species 128-130, zinc amide derived from the corresponding oxazolidine 130 was found to lead to a very fast reaction (4 hours, 0°C) and 100% ee (Scheme 2-50). The reaction proceeds even faster at room temperature (completed within 1 hour) without significant loss of stereoselectivity. This reaction can provide excellent ee for aromatic aldehydes,... 

The diethylzinc-alcohol (1 2) system was also extensively studied by Tsuruta and his co workers (85,86). Amorphous zinc dialkoxide was concluded to be an active species, because crystalline zinc alkoxide prepared from zinc chloride and lithium alkoxide proved to have only a very small catalytic activity. Based on kinetic studies of the polymerization of propylene oxide with the ZnEt2-CH3OH (1 2) catalyst system, the catalytically active species was concluded to be the complex formed by coordination of one molecule of monomer to the catalyst. In the polymerization of propylene oxide with the catalyst system, it was concluded that the monomer was polymerized by ring opening brought about by cleaving the CH2-0 bond (87). 

Tomoshige,T., Tsuruta,T. Studies on organometallic compounds as polymerization catalysts. III. Catalyst activity and structure of zinc alkoxide for propylene oxide polymerization. MakromoL Chem 120,161 (1968). 

In the course of the continuing study [9a,b] on the enantioselective addition of dialkylzincs to aldehydes by using chiral amino alcohols such as diphenyl(l-methyl-2-pyrrolidinyl)methanol (45) (DPMPM) [48] A. A -dibutylnorephedrine 46 (DBNE) [49], and 2-pyrrolidinyl-l-phenyl-1-propanol (47) [50] as chiral catalysts, Soai et al. reacted pyridine-3-carbaldehyde (48) with dialkylzincs using (lS,2/ )-DBNE 46, which gave the corresponding chiral pyridyl alkanols 49 with 74-86% ee (Scheme 9.24) [51]. The reaction with aldehyde 48 proceeded more rapidly (1 h) than that with benzaldehyde (16 h), which indicates that the product (zinc alkoxide of pyridyl alkanol) also catalyzes the reaction to produce itself. This observation led them to search for an asymmetric autocatalysis by using chiral pyridyl alkanol. 

The conformational distinction between homo- and heterochiral dimers indicates why a bulky dialkylzinc may be important in limiting the scope of amplifying auto catalysis the Soai prescription remains unique. Since it is the product of reaction that is also the catalyst, a further question needs to be addressed. In the conventional Oguni-Noyori reaction discussed earlier [60-71] the zinc alkoxide product normally plays no further part in the proceedings because it forms a stable cubic tetramer [81-87]. There are scattered exceptions in zinc-mediated catalysis, arising when the product structure is conducive to its further involvement [88,89]. 

Aluminium alkoxides (especially aluminium isopropoxide), dialkylalumi-nium alkoxides, yttrium alkoxides, zinc alkoxides, aluminoxanes, zincoxanes, bimetallic -oxoalkoxides, aluminium porphyrins and aluminium Schiff s base complexes are the most representative coordination catalysts, containing multi-nuclear or mononuclear species, for lactone polymerisations (Table 9.5). 

Ni(0) catalyst. A radical 5-exo cyclization to the potentially zinc or nickel-complexed ketone provides an alkoxyl radical that combines with the co-produced Ni(I) species. A transmetalation to a zinc alkoxide regenerates the catalyst and forms the zinc cyclopentoxide, from which products 79 are liberated on hydrolysis. A bimetallic Cu(I)-Mn(II) system provided similar results (see Sect. 8.4). Analogous samarium diiodide-mediated reactions require in contrast stoichiometric amounts of the reducing agent and are less diastereoselective [26, 27],... 

Friedrich et al. [45] discovered that a catalytic amount of titanium(IV) chloride as a Lewis acid greatly facilitates cyclopropanation reactions of alkenes by the system CFl2Br2-Zn-CuCl. The Lewis acid catalyst might bind to the oxygen atom of the allylic alcohol present as the (iodomethyl)zinc alkoxide, and thus increase the electro-philicity of the methylene group [46]. 

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