Synthesis and Aldol Reactivity of O- and C-Enolate Complexes of Nickel Juan Ca´ mpora,* ,² Celia M. Maya, ² Pilar Palma, ² Ernesto Carmona, ² Enrique Gutie´ rrez-Puebla, ‡ and Caridad Ruiz ‡ Instituto de InVestigaciones Quı ´micas, UniVersidad de SeVilla, Consejo Superior de InVestigaciones Cientı ´ficas, AVda. Ame´ rico Vespucio, s/n, Isla de la Cartuja, 41092 SeVilla, Spain, and Instituto de Ciencia de Materiales de Madrid, Consejo Superior de InVestigaciones Cientı ´ficas, Campus de Cantoblanco, 28049 Madrid, Spain Received September 26, 2002; E-mail: campora@iiq.csic.es The development of transition metal enolates has provided important contributions to organic synthesis. 1 The reactivity of these compounds is often characterized by high levels of selectivity or stereoselectivity, which may be tuned by modifying the nature of the metal center and the ancillary ligands. Obviously, these factors also determine the coordination mode of the enolate fragment, which in turn exerts a major influence on its reactivity. 2 Enolate σ-coordination is predominant, with O-binding being almost the only coordination mode observed for the early transition metals. 3 In contrast, both O- and C-coordination have been ascertained for the middle and late transition metal enolates, 4 the latter being more common for the heavier elements of the last groups. 5 It is frequently observed that C-bound enolates display low enolate-like reactivity and behave instead as sort of stabilized metal alkyls, 5c undergoing typical reactivity such as migratory insertion. 3,6 However, aldol- type additions of C-bound enolates, although rare, are not unknown. 7 In these cases, the participation of undetected O-bound tautomer cannot be ruled out, since the energy difference between isomers is usually small. This prevents establishing unambiguously if the coordination mode of the enolate ligand could have an influence not only in the reaction rate but also in its selectivity. To obtain some clear indications on the relative reactivities of C- and O-bound enolates, we set out to prepare σ-coordinated enolate complexes of nickel, in which the interconversion between the two modes is hindered under normal conditions. To this end, we devised the cyclic complex 1, in which the enolate functionality is part of a rigid metallacyclic structure. Herein we describe the synthesis of the enolate complex 1 and its thermal equilibration with its isomeric C-bound enolate 2, as well as their reactivity toward enolizable and nonenolizable aldehydes (MeC(O)H and PhC(O)H). Treatment of a THF solution of Ni(C 6 H 4 -o-C(O)CH 3 )(Cl)(dippe) with 1 equiv of KO t Bu allows the preparation of the nickel enolate 8 1 in good isolated yields (ca. 60%). O-Coordination of the enolate fragment can be proposed on the basis of the NMR spectra. Thus, the terminal methylene group gives rise to two signals in the 1 H spectrum, at δ 4.62 and 4.79 that correlate ( 1 H- 13 C HETCOR experiment) with a 13 C resonance at 75.9 ppm which exhibits no coupling to phosphorus. In addition, the formulation of 1 has been confirmed by a single-crystal diffraction study, as illustrated in the ORTEP diagram shown in Figure 1. Although the quality of the diffraction data is not high, the molecular structure is well- defined and the bond lengths and angles are comparable to those found in related complexes, particularly in the analogous derivative Ru(OC(dCH 2 )-o-C 6 H 4 )(PMe 3 ) 4 . 4c Even if C-enolate coordination is prevalent among compounds of the heavier group 10 elements Pd and Pt, 5 both C- and O-coordination are encountered in the corresponding Ni derivatives, 7b,9 as expected for a metal center with intermediate hard/soft character. Under the experimental conditions described above, the O-enolate is the major if not the exclusive tautomer that forms, but upon heating at 50 °C, the solutions of 1 in different solvents undergo slow conversion (ca. 12 h) to equilibrium mixtures of 1 and the C-enolate 2 (Scheme 1). The isomer ratio varies very little in the solvents used (2/1 ) 0.30 in THF; ca. 0.60 in C 6 D 6 or cyclohexane) and does not change when the sample is cooled to room temper- ature. Unfortunately, all attempts to separate 2 by fractional crystallization have proved unsuccessful. Despite this, the identity of 2 is unambiguously deduced from the 13 C{ 1 H} NMR spectrum of the mixture, which displays a characteristic doublet of doublets at 47.7 ppm ( 2 J CP ) 40, 16 Hz), due to the metal-bound CH 2 group of 2. Kinetic measurements carried out in C 6 D 6 between 52 and 92 °C showed that the equilibration process follows first-order kinetics, with ΔH q ) 18.5(3) kcal mol -1 , ΔS q )-22(1) cal mol -1 K -1 , and ΔG q (298 K) ) 25.3(3) kcal mol -1 . In view of the negative value of the activation entropy, a concerted mechanism, with a highly ordered η 3 -oxoallyl transition state, seems likely. η 3 -Oxoallyl complexes have been proposed before as intermediates in the interconversion between the C-and O-coordination modes of enolates. 4c Enolate 1 reacts with 1 equiv of PhC(O)H or MeC(O)H at room temperature, giving rise to the condensation products 4 and 5, respectively (Scheme 2). The NMR spectra of these compounds share many features with those of 1 and indicate the presence of a ² Universidad de Sevilla. ‡ Instituto de Ciencia de Materiales de Madrid. Figure 1. Structure of the complex 1. Scheme 1 Published on Web 01/18/2003 1482 9 J. AM. CHEM. SOC. 2003, 125, 1482-1483 10.1021/ja028711f CCC: $25.00 © 2003 American Chemical Society