Coordination catalysts, ring-opening

In the case of lactone polymerization with the (TPP)AIOR catalyst, ring-opening via C(0)-0 bond cleavage involves the participation of two catalyst molecules these can be presented schematically for the polymerization of 5-lactone, as in Scheme 9.20 [60, 61]. The polymerization of BL in the presence of (TPP)AICI, (TPP)A10C(0)R or (Sal)AlCl catalysts, which involves cleavage of the Cp-O bond, can be explained in terms of the nucleophilic attack being carried out with the participation of another catalyst molecule for coordinating the monomer (Scheme 9.21) [62-65]. 

The ring opening of cycloolefins is also possible with certain coordination catalysts. This simplified example shows cyclopentene undergoing a first-step formation of the dimer cyclodecadiene, and then incorporating additional cyclopentene monomer units to produce the solid, rubbery polypentamer ... 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

Group 4 metal complexes of the dianion [ BuNP( -N Bu)2PN Bu] polymerize ethylene in the presence of a co-catalyst, but they are readily deactivated [10,14]. This behaviour is attributed to coordination of the lone-pair electrons on the phosphorus(III) centers to Lewis acid sites, which initiates ring opening of the ligand [15]. 

Moreno described the cycloaddition of 2,5-dimethylfuran (42) catalyzed by silica-supported Lewis acids under solvent-free conditions in closed Teflon vessels using a commercial microwave oven (Scheme 9.11) [28, 52]. Under these conditions coordination of the silica-supported catalyst with the oxygen bridge favors ring opening, thus leading to the aromatic compounds in one step. The use of Si (71) gave the best results for aromatic compounds. 

Rhodium catalysts have also been used with increasing frequency for the allylic etherification of aliphatic alcohols. The chiral 7r-allylrhodium complexes generated from asymmetric ring-opening (ARO) reactions have been shown to react with both aromatic and aliphatic alcohols (Equation (46)).185-188 Mechanistic studies have shown that the reaction proceeds by an oxidative addition of Rh(i) into the oxabicyclic alkene system with retention of configuration, as directed by coordination of the oxygen atom, and subsequent SN2 addition of the oxygen nucleophile. 

Five-coordinate aluminum alkyls are useful as oxirane-polymerization catalysts. Controlled polymerization of lactones102 and lactides103 has been achieved with Schiff base aluminum alkyl complexes. Ketiminate-based five-coordinate aluminum alkyl (OCMeCHCMeNAr)AlEt2 were found to be active catalyst for the ring-opening polymerization of -caprolactone.88 Salen aluminum alkyls have also been found to be active catalysts for the preparation of ethylene carbonate from sc C02 and ethylene oxide.1 4 Their catalytic activity is markedly enhanced in the presence of a Lewis base or a quaternary salt. 

Polyethers are prepared by the ring opening polymerization of three, four, five, seven, and higher member cyclic ethers. Polyalkylene oxides from ethylene or propylene oxide and from epichlorohydrin are the most common commercial materials. They seem to be the most reactive alkylene oxides and can be polymerized by cationic, anionic, and coordinated nucleophilic mechanisms. For example, ethylene oxide is polymerized by an alkaline catalyst to generate a living polymer in Figure 1.1. Upon addition of a second alkylene oxide monomer, it is possible to produce a block copolymer (Fig. 1.2). 

Olefin metathesis catalysts based on ruthenium have been shown to exhibit a quite good tolerance to a variety of functional groups. The ring opening metathesis polymerization of strained, cyclic olefins initiated by group VIII salts and coordination complexes in aque-... 

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). 

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