Containing metal-oxygen bonds esters

In the early 1970s, Carraher and coworkers developed a number of different organometallic polymers containing metal-oxygen, metal-sulfur, and metal-nitrogen bonds in the polymer backbone. For example, a number of polyethers, " thioethers, amines, " esters, and oximes of titanium, zirconium, and hafnium were prepared. Scheme 1 shows the synthesis of haftiium, zirconium, and... 

Just before we go any further we need a quick note on nomenclature. We see below a lithium enolate 29 and a titanium enolate 30 of an ester. By definition, the lithium enolate 29 contains a Z double bond (lithium is lighter than carbon) whereas the titanium enolate 30 contains an E double bond (titanium is heavier than carbon). But to call one E and the other Zis plainly crazy. The important thing, as far as we are concerned, is that the substituent R is trans to the reactive oxygen. In order to avoid confusion we shall refer to both of these as trans enolates regardless of the metal 31. This is not universally done (some chemists will be outraged by such simplicity) but is what we shall be doing in this book. 

The transition-metal-catalyzed reaction of diazo compounds 702, which have a N—H or O—H bond at an appropriate position, gives nitrogen- and oxygen-containing heterocycles 703 (Scheme 2 1 8).293 Wang and Zhu demonstrated a convenient synthesis of the polyfunctionalized /Tfluoropyrroles by the rhodium-catalyzed intramolecular N—H insertion reaction.2933 The reaction of d-amino-y,y-difluoro-a-diazo-/Tketo esters 704 in the presence of Rh2(OAc)4 gave the... 

What is the mechanism of this transformation The initial reaction of the ir bond with osmium tetroxide constitutes a concerted (Section 6-4) addition in which three electron pairs move simultaneously to give a cyclic ester containing Os(VI). This process can be viewed as an electrophilic attack on the alkene Two electrons flow from the alkene onto the metal, which is reduced [Os(VIII) Os(VI)]. For steric reasons, the product can form only in a way that introduces the two oxygen atoms on the same face of the double bond—syn. This intermediate is usually not isolated but converted upon reductive work-up into the free diol. 

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