Metal diolates

In 1977, Sharpless proposed the intermediacy of a four-membered metal-lacycle in the oxidation of cyclododecene by Cr02Cl2.60 This was an attempt to explain a minor but primary product, in which an oxygen and a chlorine were added in a syn fashion to one face of the /3-bond. Later, this type of intermediate was incorporated into a new mechanism for osmylation,61 in which an initial [2 + 2] cycloaddition led to an osmaoxetane which in a second step underwent ring expansion to form the observed metal diolate product. At the time, the [2 + 2] process was viewed as a violation of orbital symmetry rules, and it was not until the extensive work in the 1970s and 1980s on cycloadditions to metal carbenes that a theoretical basis for such a process allowed its broader acceptance. 

The literature system most closely related to equation 5 is the two electron reduction of two CO molecules by alkali metals which is reported to form an insoluble alkyne diolate (Equation 6) (60). The relationship between reactions 5 and 6 can be seen by comparing... 

Molybdenum catalysts that contain enantiomerically pure diolates are prime targets for asymmetric RCM (ARCM). Enantiomerically pure molybdenum catalysts have been prepared that contain a tartrate-based diolate [86], a binaph-tholate [87], or a diolate derived from a traris-1,2-disubstituted cyclopentane [89, 90], as mentioned in an earlier section. A catalyst that contains the diolate derived from a traris-1,2-disubstituted cyclopentane has been employed in an attempt to form cyclic alkenes asymmetrically via kinetic resolution (inter alia) of substrates A and B (Eqs. 45,46) where OR is acetate or a siloxide [89,90]. Reactions taken to -50% consumption yielded unreacted substrate that had an ee between 20% and 40%. When A (OR=acetate) was taken to 90% conversion, the ee of residual A was 84%. The relatively low enantioselectivity might be ascribed to the slow interconversion of syn and anti rotamers of the intermediates or to the relatively floppy nature of the diolate that forms a pseudo nine-membered ring containing the metal. 

The LIC-KOR reagent consisting of stoichiometrically equal amounts of butyllithium ( LIC ) and potassium feri-butoxide ( KOR ) was conceived in Heidelberg and optimized in a trial-and-error effort . The fundamental idea was simple. To activate butyllithium optimally by deaggregation and carbon-metal bond polarization, a ligand was required that would surpass as an electron donor any crown ether but not suffer from the drawback of the latter, i.e. its proneness to /3-elimination. Whereas pinacolates and other v/c-diolates proved too labile to be generally useful, potassium terf-butoxide or any other bulky, hence relatively soluble, potassium or cesium alkoxide was found to serve the purpose. ... 

Recently, dithio-diolate ligands have been employed for construction of group 4 metal catalysts for the ROP of lactide. These metal dithiolate complexes form mononuclear species of the type [(OSSO)M(OR)2] with an octahedrally coordinated metal center. These fluxional compounds acted as highly active catalysts in the ROP of L- and rac-lactide. Hafnium complexes were also introduced as initiators for the ROP of L-lactide and rac-lactide (vide infra) in very limited cases. To our knowledge, the hafnium derivative 146 displayed the highest activity among the group 4 catalysts reported to date (complete conversion of 300 equiv. of... 

N Strater, WN Lipscomb (1995) Two-metal ion mechanism of bovine lens leucine aminopeptidase active site solvent structure and binding mode of L-leucinal, a gem-diolate transition state analogue, by X-ray crystallography, Biochemistry 34 14792-14800... 

In both the case of epoxidations via j2-peroxides and that of diolate cycloreversion, chemists must exercise caution that the mechanistic lessons learned for rhenium may not translate to processes mediated by other metals, ligands, or oxidation states. This has always been a particular problem in mechanistic organometallic chemistry. However, it is likely that systems can be developed based on the observations described here that can discern the structural features that control competing mechanistic manifolds. 

Several cases of bridging groups of the type (11-V) are known. In (Me3SiCH2)6(0)2Ru2, Ru=0 is 1.733 A and O — Ru is 2.208 A. In [Os02(cyclohex-ane-l,2-diolate)quinuclidene]2 there is dimerization only via oxygen bridging (1.78, 2.22 A), but other compounds have metal-metal bonds also. 

In general, the dissolving metal or electrochemical reductions of acyl halides or acyclic anhydrides are not useful for the preparation of primary alcohols. Such reductions invariably provide acyloin esters, ene-diolate diesters or related species. Cyclic anhydrides may be reduced to give lactones. For example, the reduction of phthalic anhydride at a mercury cathode has been used in the synthesis of phthalide (90%). In general, however, such reduction are not widely employed in synthesis. 

Since the diolate is not formed in toluene, the THF solvent is its most likely source. Metal-coordinated alcohol molecules frequently accompany the isolated alkoxides, and the rate and possible the yield of reactions are dependent on the cleanliness of the surface of the metal (the use of activated metals can be helpful), the acidity of the alcohol, and the solubility of the resulting... 

The main compound released in the gas phase during TPD is the symmetric olefin (RHC=CHR). The reaction may occur via an intermediate containing both C-0 and C-C bonds such as a diolate (7.9). It is not known if this reaction occurs via two metal... 

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