Metal dialkoxides

Alkoxides of all 3 metals are well characterized but it is those of titanium which are of particular importance. The solvolysis of TiCU with an alcohol yields a dialkoxide ... 

In all of, these cases substitution of the second alkyl can then occur to yield the dialkoxide or diphenoxide. This allowed the isolation of the monomeric beryllium phenoxide Be(OAr )2 (OAr = 2,6-di-t-butylphenoxide).98 The alkyls of the Group IV metals, MR (M = Ti, Zr, Hf), undergo rapid reactions with common alcohols and phenols yielding eventually the corresponding tetra-alkoxides or -phenoxides and four equivalents of alkane.97,100 With very bulky substituted alcohols or phenols the reactivity can be very sluggish, in some cases leading to only partial substitution (equation 28). 66,100... 

When the catalyst is not fully regioselective, chain release by a /3-H transfer after a secondary insertion with formation of internal double bonds is often observed. This has been reported for ethylene/a-olefin co-poly-mers, PP, and other polyolefins, as well as for 1-hexene polymerization with dialkoxide catalysts. The reaction is shown in Scheme 14 for the case of propylene, where kinetic studies have shown it to be a bimolecular process, following the rate law s/J/ -h=s / -h[sZr][m].217,257 [sZr] refers to the concentration of active Zr centers bearing a growing chain having a secondary propylene unit linked to the metal. 

The dialkoxides of the metals Zn, Cu, Cr, Fe, Ni, Co and Mn are non-volatile, insoluble compounds presumably due to their polymeric nature. However, by using a combination of studies of their magnetic properties and reflectance spectral characteristics, some information concerning the stereochemistry about the M " " ion was obtained. The tertiary alkoxides of the Group II metals are more volatile and soluble in organic media. In contrast to the sparingly soluble phenoxides [Be(OPh)2] and [Mn(OPh)2] , the use of the sterically demanding 2,6-di-r-butylphenoxide (OAr ) leads to soluble compounds M(OAr )2 (M = Be, Mn ) which have been shown to be monomeric in benzene solution. 

The high reactivity of certain metal alkyls is well known. The autoxidation of dialkyIzinc was investigated as early as 1864 by several workers and they showed that dialkylzinc is oxidized rapidly to zinc dialkoxides" ... 

Similar behaviour has also been observed in the case of magnesium and other alkaline earth metal alkoxides. The normal dialkoxides of magnesium, calcium, strontium, and barium are insoluble and nonvolatile "" whereas the alkylmagnesium alkoxides are soluble in common organic solvents and some of these (especially methyl and ethyl derivatives) can be volatilized in usually accompanied by... 

Although the transfer of oxygen from an oxo-transition metal to an organic substrate is well known (Scheme 33), the reverse transformation is less common. Lower-valent tungsten halide derivatives, prepared by addition of lithium alkyls to tungsten hexachloride, can transform dialkoxides into olefins (Scheme 33 and bring about reductive coupling of aldehyde... 

Then, the resulting tin(II) mono- and/or dialkoxide initiates polymerization in the same manner as the other metal alkox-ides. However, there was, at that time, no direct proof of such a mechanism and several other mechanisms have been pro-posed. " The most often cited was the trimolecular mechanism in which first the catalyst-monomer complex is formed. This mechanism has conclusively been shown not to operate since it excludes the presence of Sn atoms covalently bonded to the growing macromolecules. The matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectral measurements of the cyclic ester/ROH/Sn(Oct)2 system revealed the presence of tin(II) alkoxides in the growing polyester chains. Moreover, the kinetic studies also clearly supported this sequence of the exchange reactions. ... 

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