Initiators bimetallic

Coates and Darensbourg have separately reported that salen metal catalysts undergo bimetallic initiation, followed by a monometallic propagation [123, 129, 155,163]. In contrast, a theoretical study by Rieger and co-workers predicted chain growth to take place via the attack of the metal-bound alkyl carbonate on a metal-coordinated epoxide [103]. Related bimolecular processes have also been observed by Jacobsen et al. for the asymmetric ring opening of epoxides [164, 165]. Some of the mechanistic routes reported with metal salen catalysts are depicted in Fig. 24. 

SCHEME 23.8 Generation of an active bimetallic initiator system for the polymerization of MMA from the (poorly active) cationic enolate complex 26. 

Pt/Pd bimetallic nanoparticles can be prepared by refluxing the alcohol/water (1 1, v/v) solution of palla-dium(II) chloride and hexachloroplatinic(IV) acid in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) at ca. 95 °C for Ih [15,16,48]. The resulting Pd/Pt nanoparticles have a Pt-core/Pd-shell structure with a narrow size distribution and the dispersion is stable against aggregation for several years. The core/shell structure was confirmed by the technique of EAXFS. Composition of Pt/Pd nanoparticles can be controlled by the initially feed amount of two different metal ions, i.e., in this case one... 

Kitamura and Noyori have reported mechanistic studies on the highly diastere-omeric dialkylzinc addition to aryl aldehydes in the presence of (-)-i-exo-(dimethylamino)isoborneol (DAIB) [33]. They stated that DAIB (a chiral (i-amino alcohol) formed a dimeric complex 57 with dialkylzinc. The dimeric complex is not reactive toward aldehydes but a monomeric complex 58, which exists through equilibrium with the dimer 57, reacts with aldehydes via bimetallic complex 59. The initially formed adduct 60 is transformed into tetramer 61 by reaction with either dialkylzinc or aldehydes and regenerates active intermediates. The high enantiomeric excess is attributed to the facial selectivity achieved by clear steric differentiation of complex 59, as shown in Scheme 1.22. 

Initiation of MMA polymerization by complexes such as (192) was shown to proceed via a bimetallic bis(enolate) intermediate, arising from the dimerization of a radical anion.478" 80 Such a mechanism481,482 explains why efficiencies with such initiators (calculated from polymer molecular weights) are always <50%. Using a similar methodology, the bimetallic bisallyl complex (198) was shown to polymerize MMA in a living fashion (Mw/Mn 1.1) and triblock copolymers with methacrylate and acrylate segments have been prepared. 

Prior to the development of the aluminum initiators described above, a series of bimetallic /x-oxo alkoxides, such as (269) and (270) were examined as lactone polymerization initiators.811 812 At 10 °C, (269) polymerizes CL in a moderately controlled manner (60 equivalents, t1/2 = 23min), as shown by a linear DP vs. [M]0/[I]o plot and Mw/Mn values between 1.3 and 1.5. Mn data is consistent with only one of the terminal alkoxides initiating the ring opening, although in the presence of "BuOH, which is known to dissociate (269), all four alkoxides are active. 

Initiators such as (306) initiate the ROP of CL to form telechelic triblock diols.478 Molecular weights approach theoretical values with polydispersities <1.3 and no significant level of transesterification was detected at up to 95% conversions. Alternative bimetallic samarium initiators have been used to synthesize aromatic, cumulene and amine/imine link-functionalized poly(lactones).479... 

The use of group 4 metallocene alkyne complexes924 and bimetallic aluminum derivatives925 as CL polymerization initiators has also been described. These catalysts generally exhibit poor control with Mn values much lower than expected and Mw/Mn= 1.4-2.6. End groups have not... 

The reduction steps on active Co sites are strongly affected by activated hydrogen transferred from promoter metal particles (Pt and Ru). Several indications for the existence and importance of hetero-bimetallic centers have been obtained.63 [Cp Co(CO)2] in the presence of PEt3 and Mel catalyzes the carbonylation of methanol with initial rates up to 44 mol L 1 h 1 before decaying to a second catalytic phase with rates of 3 mol L 1 h-1.64 HOAc-AcOMe mixtures were prepared by reaction of MeOH with CO in the presence of Co(II) acetate, iodine, and additional Pt or Pd salts, e.g., [(Ph3P)2PdCl2] at 120-80 °C and 160-250 atm.65... 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

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