Crystal palladium enolates

Scheme 1.6 Rhodium and palladium enolates. Equilibrating O- and C-bound tautomers 14 and 15 rhodium complex 16, characterized by its crystal structure, as an...

Transmetallation of the lithium or potassium enolates is also a reliable method for the preparation of palladium and nickel enolates, as illustrated in Scheme 2.50. Clear evidence for the C-bound structure of enolates 172 and 173 thus prepared was provided by NMR spectroscopy and - for nickel enolate 172 (M = Ni, L = Cp ) - by a crystal structure analysis. The reaction of C-bound nickel and palladium enolates 172 and 173 with aldehydes is much more sluggish and much less uniform than the analogs of that of the polar main-group metals. In addition to P-hydroxyketones or esters, products resulting from a Tishchenko reaction were also observed [164b]. 

Also in the enolates of group 10 metals, the different metalla tautomers were postulated, isolated, and characterized. Generally, it is assumed that in particular palladium - due to its low oxophilicity - favors the C-bound mode, but for this metal too, the O-bound tautomer has been detected. In an early study on nickel and palladium enolates 37, the C-bound structure was assigned on the basis of and C NMR and IR spectroscopy and confirmed for the nickel ester enolate (L = Cp, R = OCMe3) by a crystal structure [73]. The metallacyclic nickel enolates 38, on the other hand, are O-bound tautomers, as unambiguously shown by a crystal structure of the complex 38a, wherein the nickel adopts a slightly distorted square planar coordination. A slow equilibration was observed between the O-bound tautomer 38b and the C-bound isomer 39, with AG = 25.3 kcal mol . Remarkably, the isomerization rate was substantially... 

Various examples of both C- and O-bound tautomers were described for enolates of palladium, and the corresponding complexes were characterized. In addition, both bonding modes were postulated as intermediate palladium enolates in catalyses. For a palladium(II) enolate of acetophenone, the monomeric C-bound form 40, and the dimer 41 were characterized by crystal structure analyses. In both complexes, palladium is tetra-coordinated in a distorted square planar arrangement [75aj. Similarly, palladium enolates derived from acetic esters were isolated and characterized under the form of monomeric enolate 42 as well as bimetallic complex 43 [75b]. The dimeric complexes 41 and 43 feature the structural motif of an C,0-bridging enolate moiety (Scheme 3.14). 

Culkin and Hartwig studied the influence of various parameters on the connectivity in palladium enolates. Evidence based upon NMR spectroscopy and crystal structure analyses was provided for the C-bound enolate 44 and O-bound 45, both derived from aryl alkyl ketones (Figure 3.12). It was deduced that the C-bound tautomer was favored for electronic reasons if the enolate was located trans to a phosphine, and the O-bound isomer was favored if the enolate was oriented trans to an aryl group. In addition, the substitution pattern of the enolate and steric hindrance have an influence on the metalla tautomerism [76]. 

In Fig. (12) keto ester (94) was selected as starting material. It was converted to the formyl derivative (95) which yielded a,P-unsaturated aldehyde (96) by treatment with DDQ. Michael addition of the sodium enolate of tert-butyl- isovalerylacetate to aldehyde (96) afforded the adduct (97) as a mixture of C-ll diastereomers. By fractional crystallization one of the adducts could be separated but for the synthetic purpose the mixture was not separated. Treatment of the adduct (97) with p-toluenesulfonic acid in glacial acetic acid caused t-butyl ester cleavage, decarboxylation and cyclodehydration leading the formation of tricyclic enedione (98) in 80% yield. This approach was previously utilized by Meyer in the synthesis of nimbiol [29], Treatment of (98) with pyridinium bromide perbromide, followed by hydrogenolysis with palladium and carbon caused aromatization of (98) leading the formation of the phenolic ester (99). 

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