Oxygen-carbon bonds lithium metal

As unidentate ligands, carboxylates are expected to (i) lose the equivalence of the two carbon- oxygen bonds found in the anion and (ii) have one metal-oxygen distance considerably shorter than the next shortest M—O contact. Lithium acetate dihydrate exemplifies this14 with C—O distances of 133 and 122 pm and Li—O distances of 227 and 257 pm. Most examples of unidentate carboxylate complexes have this classical configuration of M(0—C) and C=0 respectively so certainly the presence of features (i) and (ii) unambiguously determine this mode of coordination. 

Treatment of substituted phthalans 1172 with lithium metal in the presence of catalytic quantities of naphthalene leads to reductive cleavage of the arylmethyl carbon-oxygen bond to form a stable dilithium compound 1173, which upon trapping with carbon dioxide furnishes isochroman-3-ones 1174 (Scheme 289) <1996JOC4913>. 

Metal enolates may have structures with either a metal-oxygen (3) or a metal-carbon (4) bond. Lithium enolates typically have oxygen-bonded structures,while mercury enolates are usually assumed to have carbon-bonded structures. ... 

The cleavage of carbon-oxygen bonds from alkenyl or aryl phosphates can be accomplished under reductive conditions with a low valent metal. As vinyl phosphates can be formed readily from ketones, this procedure provides a method to convert a ketone to an alkene. For example, the alkenyl phosphate 74 was prepared by trapping the enolate formed on reduction of the enone 73 and was converted into the alkene 75 (7.55). The chemistry therefore provides a method to prepare structurally specihc alkenes. Low-valent titanium (prepared for example by reduction of titanium(III) chloride with potassium metal) is a convenient alternative to lithium or sodium in liquid ammonia or an amine for the reductive cleavage of alkenyl or aryl phosphates. 

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 details of the mechanism are poorly understood, though the oxygen of the alcohol is certainly attacking the carbon of the isocyanate. Hydrogen bonding complicates the kinetic picture. The addition of ROH to isocyanates can also be catalyzed by metallic compounds, by light, or, for tertiary ROH, by lithium alkoxides ° or n-butyllithium. ° ... 

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