Synthesis and aldol reactivity of O- and C-enolate complexes of nickel

Article abstract of DOI:10.1021/ja028711f

The often facile C-/O-tautomerization of transition metal enolates is severely hindered in the cyclic Ni complexes 1 and 2, allowing the study of their individual reactivities. At room temperature only the O-bound tautomer, 2, reacts with aldehydes, giving rise to the corresponding addition products. Copyright

Full text of DOI:10.1021/ja028711f

Published on Web 01/18/2003
Synthesis and Aldol Reactivity of O- and C-Enolate Complexes of Nickel
Juan Ca´mpora,*,† Celia M. Maya,† Pilar Palma,† Ernesto Carmona,† Enrique Gutie´rrez-Puebla,‡ and
Caridad Ruiz‡
Instituto de InVestigaciones Qu´ımicas, UniVersidad de SeVilla,
Consejo Superior de InVestigaciones Cient´ıficas, 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 Cient´ıficas, 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
Figure 1. Structure of the complex 1.
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
Scheme 1
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
O-coordination are encountered in the corresponding Ni derivatives,7b,9
enolates, we set out to prepare σ-coordinated enolate complexes
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
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
slow conversion (ca. 12 h) to equilibrium mixtures of 1 and the
enolate complex 1 and its thermal equilibration with its isomeric
C-enolate 2 (Scheme 1). The isomer ratio varies very little in the
C-bound enolate 2, as well as their reactivity toward enolizable
solvents used (2/1 ) 0.30 in THF; ca. 0.60 in C6D6 or cyclohexane)
and nonenolizable aldehydes (MeC(O)H and PhC(O)H).
Treatment of a THF solution of Ni(C6H4-o-C(O)CH3)(Cl)(dippe)
with 1 equiv of KOtBu allows the preparation of the nickel enolate8
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 13C{1H} NMR spectrum
of the mixture, which displays a characteristic doublet of doublets
at 47.7 ppm (2JCP ) 40, 16 Hz), due to the metal-bound CH2 group
of 2. Kinetic measurements carried out in C6D6 between 52 and 92
1 in good isolated yields (ca. 60%). O-Coordination of the enolate
fragment can be proposed on the basis of the NMR spectra. Thus,
1
the terminal methylene group gives rise to two signals in the H
spectrum, at δ 4.62 and 4.79 that correlate (1H-13C HETCOR
experiment) with a 13C resonance at 75.9 ppm which exhibits no
°C showed that the equilibration process follows first-order kinetics,
coupling to phosphorus. In addition, the formulation of 1 has been
with ∆Hq ) 18.5(3) kcal mol-1, ∆Sq ) -22(1) cal mol-1 K-1
,
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
and ∆Gq(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 η3-oxoallyl transition state, seems likely. η3-Oxoallyl
complexes have been proposed before as intermediates in the
interconversion between the C-and O-coordination modes of
enolates.4c
Ru(OC(dCH2)-o-C6H4)(PMe3)4.4c
Even if C-enolate coordination is prevalent among compounds
of the heavier group 10 elements Pd and Pt,5 both C- and
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
† Universidad de Sevilla.
‡ Instituto de Ciencia de Materiales de Madrid.
9
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J. AM. CHEM. SOC. 2003, 125, 1482-1483
10.1021/ja028711f CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
to investigate this and related reactivity, in particular that of cyclic
enolates in Michael additions to R,â-unsaturated carbonyl com-
pounds.
Acknowledgment. Financial support from DGESIC (Project
BQU2000-1169) is gratefully acknowledged. C.M.M. thanks the
Direccio´n General de Ensen˜anza Superior for a PFPI studentship.
Supporting Information Available: Synthetic procedures and
spectroscopic and analytical data for compounds 1-7 and X-ray
crystallographic data (cif) for 1. This material is available free of charge
via the Internet at http//pubs.acs.org.
References
(1) (a) Paterson, I. In ComprehensiVe Organic Synthesis, Vol. 2; Trost, B.
M., Fleming, I., Heathcock, C. H., Eds.; Pergamon Press: Oxford, 1991;
pp 301-319. (b) Hamann, B. C.; Hartwig, J. F. J. Am. Chem. Soc. 1997,
119, 12382-12383. (c) Palucki, M.; Buckwald, S. L. J. Am. Chem. Soc.
1997, 119, 11108-11109. (d) Satoh, T.; Kawamura, Y.; Miura, M.;
Nomura, M. Angew. Chem., Int. Ed. Engl. 1997, 36, 1740-1742. (e)
Ahman, J.; Wolfe, J. P.; Troutman, M. V.; Palucki, M.; Buchwald, S. L.
J. Am. Chem. Soc. 1998, 120, 1918-1919. (f) Fuji, A.; Hagiwara, E.;
Sodeoka, M. J. Am. Chem. Soc. 1999, 12, 5450-5458. (g) Fujimura, O.
J. Am. Chem. Soc. 1998, 120, 10032-10039. (h) Kru¨ger, J.; Carreira, E.
M. J. Am. Chem. Soc. 1998, 120, 837-838. (i) Pagenkopf, B. L.; Kru¨ger,
J.; Stojanovic, A.; Carreira, E. M. Angew. Chem., Int. Ed. 1998, 37, 3124-
3126.
substituted enolate ligand. For instance, they display a singlet
resonance in their H NMR spectra (δ 5.48, 4; 4.73, 5) which is
1
assigned to an olefinic methyne proton. The corresponding carbon
atom resonates at 97.4 and 97.3 ppm for 4 and 5, respectively (1JCH
1
ca. 140 Hz). The hydroxyl group can be detected both in the H
NMR (4, 6.91; 5, 5.77 ppm) and in the IR spectra (ca. 3200 cm-1).
The two products display low thermal stability. This has prevented
us from gathering good analytic data for 5, but both 4 and 5 are
quantitatively carbonylated to the stable lactones 6 and 7, which
have been isolated and characterized. It is noteworthy that
compounds 4-7 are selectively obtained as a single isomer that
displays a Z double-bond substitution pattern. This is unambiguously
established from their 2D NOESY spectra which exhibit clear NOE
cross-peaks between the olefin proton and the H 5′ aromatic
resonance. Although 4 and 5 retain an enolate functionality, they
do not react further with aldehydes. As the presence of substituents
on the double bond is not expected to decrease the nucleophilic
character of the enolate, we assume that this lack of reactivity is
due to steric effects.
(2) (a) Culkin, D. A.; Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 5816-
5817. (b) Slough, G. A.; Bergman, R. G.; Heathcock, C. H. J. Am. Chem.
Soc. 1989, 111, 938-949. (c) Slough, G. A.; Hayashi, R.; Ashbaugh, J.
R.; Shamblin, S. L.; Aukamp, A. M. Organometallics 1994, 13, 890-
898.
(3) (a) Veya, P.; Cozzi, P. G.; Floriani, C.; Rotzinger, F. P.; Chiesi-Villa, A.;
Rizzoli, C. Organometallics 1995, 14, 4101-4108. (b) Veya, P.; Floriani,
C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1994, 13, 208-213. (c)
Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1993,
12, 4892-4898. (d) Cozzi, P. G.; Veya, P.; Floriani, C.; Rotzinger, F. P.;
Chiesi-Villa, A.; Rizzoli, C. Organometallics 1995, 14, 4092-4100. (e)
Evans, D. A.; McGee, L. R. Tetrahedron Lett. 1980, 21, 3975-3978. (f)
Yamamoto, Y.; Maruyama, K. Tetrahedron Lett. 1980, 21, 4607-4610.
(g) Manriquez, J.; Fagan, P. J.; Marks, T. J. J. Am. Chem. Soc. 1978,
100, 7112-7114. (h) Sonnenberger, D. C.; Mintz, E. A.; Marks, T. J. J.
Am. Chem. Soc. 1984, 106, 3484-3491. (i) Mukaiyama, T.; Banno, K.;
Narasaka, K. J. J. Am. Chem. Soc. 1974, 96, 7503-7509. (j) Stille, J. R.;
Grubbs, R. H. J. Am. Chem. Soc. 1983, 105, 1664-1665. (k) Curtis, M.
D.; Thanedar, S.; Butler, W. M. Organometallics 1984, 3, 1855-1859.
To compare the reactivity of 1 and 2 toward aldehydes, ca. 2:1
mixtures of 1 and 2 have been reacted, at room temperature, with
PhC(O)H and MeC(O)H. Monitoring these chemical changes by
31P{1H} NMR spectroscopy over a period of 24 h shows full
consumption of 1, whereas 2 remains unaltered. This result evinces
that under the above conditions only the O-bound enolate has
enough nucleophilic character to add to aldehydes, while the
C-bound enolate lacks this reactivity.
One final aspect of the reactivity of 1 which is worthy of note
concerns the nature of the aldehyde reaction products 4 and 5. It is
widely accepted that in aldol reactions induced by transition metal
compounds, aldehyde coordination precedes the C-C bond-forming
step.2b,c,3a-f As the metallacyclic nature of enolate 1 would impose
considerable strain to achieve the C-C bond-making transition state,
its reactions proceed otherwise. Hence the products 4 and 5 are
not classical aldolates but instead new enolate derivatives that may
result from aldehyde attack by the nucleophilic enolate carbon,
followed by a proton shift (rather than Ni2+ shift) in the resulting
dipolar intermediate 3 of Scheme 2. Another consequence of the
cyclic structure of 1 is its ability to react with enolizable carbonyl
compounds, a reaction that is often hampered by acid-base
exchange of the added reagent and the coordinated enolate
ligand.2b,7b,10
(4) (a) Hartwig, J. F.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc.
1990, 112, 5670-5671. (b) Hartwig, J. F.; Bergman, R. G.; Andersen, R.
A. Organometallics 1991, 10, 3344-3362. (c) Hartwig, J. F.; Bergman,
R. G.; Andersen, R. A. Organometallics 1991, 10, 3326-3344.
(5) (a) Albe´niz, A. C.; Catalina, N. M.; Espinet, P.; Redo´n, R. Organometallics
1999, 18, 5571-5576. (b) Vicente, J.; Abad, J. A.; Bergs, R.; Jones, P.
G.; Bautista, D. J. Chem. Soc., Dalton Trans. 1995, 3093-3095. (c)
Vicente, J.; Arcas, A.; Ferna´ndez-Herna´ndez, J. M.; Bautista, D. Organo-
metallics 2001, 20, 2767-2774. (d) Veya, P.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. Organometallics 1993, 12, 4899-4907. (e) Henderson,
W.; Fawcett, J.; Kemitt, R. D. W.; McKenna, P.; Russell, R. R. Polyhedron
1997, 16, 2475-2481. (f) Falvello, L. R.; Garde, R. Miqueleiz, E. M.;
Tomas, M.; Urriolabeitia, J. P. Inorg. Chim. Acta 1997, 297-302. (g)
Wanat, R. A.; Collum, D. B. Organometallics 1986, 5, 120-127. (h)
Green, M.; Howard, J. A. K.; Mitrprapachon, P.; Pfeffer, M.; Spencer, J.
L.; Woodward, P. J. Chem. Soc., Dalton Trans. 1979, 306-314. (i)
Russell, D. R.; Tucker, P. A. J. Chem. Soc., Dalton Trans. 1975, 2222-
2225. (j) Bennett, M. A.; Robertson, G. B.; Whimp, P. O.; Yoshida, T. J.
Am. Chem. Soc. 1973, 95, 3028-3030.
(6) Stack, J. G.; Doney, J. J.; Bergman, R. G.; Heathcock, C. H. Organo-
metallics 1990, 9, 453-466.
(7) (a) Burkhardt, E. R.; Doney, J. J.; Bergman, R. G.; Heathcock, C. H. J.
Am. Chem. Soc. 1987, 109, 2022-2039. (b) Burkhardt, R. R.; Bergman,
R. G.; Heathcock, C. H. Organometallics 1990, 9, 30-44.
(8) This compound is prepared by reaction of 2-chloroacetophenone with
Ni(dippe)(cod). Full details on synthetic procedure and spectroscopic data
will be published elsewhere.
In summary, we have shown that the metallacyclic oxygen-bound
nickel enolate 1 can be prepared in a straightforward manner and
can be thermally isomerized to the corresponding C-bound enolate.
Both tautomers display different reactivity, with only the former
being capable of undergoing aldol additions to enolizable and
nonenolizable aldehydes. Research in our laboratories continues
(9) (a) Amarasinghe, K. K. D.; Chowdhury, S. K.; Heeg, M. J.; Montgomery,
J. Organometallics 2001, 20, 370-372. (b) Mindiola, D. J.; Hillhouse,
G. L. J. Am. Chem. Soc. 2002, 124, 9976.
(10) Sun, C.; Tu, A.; Slough, G. J. Organomet. Chem. 1999, 582, 235.
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