Stabilization of enols as transition-metal complexes

Simple enols: How rare are they?

“…Dienes form stable complexes with a variety of metals, and advantage has been taken of this property to prepare complexes of ligands which in a formal sense are either enols or keto forms of phenols. For example, treatment of 2-acetoxy-1,3-butadiene with Fe2(CO>9 in benzene gave the yellovy crystalline complex L; the acetoxy group was then converted to a hydroxyl group to give LI, air sensitive but stable in solution (33). Compound LI is a complex of the enol of methyl vinyl ketone.…”

Simple enols

“…For example, treatment of 2-acetoxy-1,3-butadiene with Fe2(CO)g in benzene gave the yellow crystalline complex 191; the acetoxy group was then converted to a hydroxyl group to give 192, air sensitive but stable in solution. 92 to a crystalline benzoate. The pKa of 192 is 9.24 in 48% aqueous ethanol corresponding to about 8.5 in water.…”

“…Similar complexes with the hydroxyl at C3 of the butadiene moiety have also been prepared. 92 At about the same time, the iron tricarbonyl complex of 144, the conjugated tautomer of phenol, was also prepared.93 •94 The vinyl ether 193 (obtained by lithium-ammonia reduction of ani-sóle94) was refluxed with iron pentacarbonyl in di-n-butyl ether to give a mixture of 194 and 195 which was treated with trltyl tetrafluoborate. Hydride abstraction (the other organic product is triphenylmethane) gives a mixture of carbonium ion salts 198 and 197.…”

Stable Transition Metal−Enol Complexes in the Cycloheptatriene Ligand System

“…Stabilization of the enol form of the carbonyl keto−enol tautomeric system via metal complexation has also received attention, recently. To date, the bulk of these studies has dealt with structurally simple η 2 -enol complexes prepared as isolable materials or as intermediates in various reaction pathways. , In contrast, stable metal−enol complexes involving greater hapticity in structurally more elaborate ligand systems appear to be quite rare …”

“…Although a number of metal−enol complexes have been prepared recently, very few reports regarding the chemical reactivity of these species (other than keto−enol tautomerism and H/D exchange) have surfaced. , As a consequence, few data are currently available concerning the utility of most enol complexes for subsequent derivatization of the hydroxyl function. It is noteworthy then that many of the metal−enol complexes reported in this document undergo smooth reaction to afford derivatives without compromising the integrity of the metal complex.…”