Bifunctional carriers of alkali-metal enolates: The use of zirconium meso-octaethylporphyrinogen in aldol condensation reactions

Article abstract of DOI:10.1021/om9609325

The (meso-octaethylporphyrinogenato)zirconium(IV) species [(Et₈N₄)Zr(thf)] (1) binds the acetophenone potassium enolate [PhCOCH₂K] (2) in its ion-pair form, [(η⁵:η¹:η¹-Et₈N ₄)Zr{PhC(CH₂)O}K(thf)₃] (3), and thus drives the aldol condensation reaction with acetophenone. The resulting aldolate, which occurs in a metallacyclic form due to the salvation of potassium by a phenyl ring, remains η¹ (O)-bonded to zirconium, [(η⁵:η¹:η¹:η¹- Et₈N₄)Zr{PhC(CH₂)OC(O)C(Me)Ph}K] _n(4).

Full text of DOI:10.1021/om9609325

508
Organometallics 1997, 16, 508-510
Bifu n ction a l Ca r r ier s of Alk a li-Meta l En ola tes: Th e Use
of Zir con iu m m eso-Octa eth ylp or p h yr in ogen in Ald ol
Con d en sa tion Rea ction s
Giovanna Solari, Euro Solari, and Carlo Floriani*
Institut de Chimie Mine´rale et Analytique, BCH, Universite´ de Lausanne,
CH-1015 Lausanne, Switzerland
Angiola Chiesi-Villa and Corrado Rizzoli
Dipartimento di Chimica, Universita` di Parma, I-43100 Parma, Italy
Received November 4, 1996X
Summary: The (meso-octaethylporphyrinogenato)zir-
conium(IV) species [(Et8N4)Zr(thf)] (1) binds the ac-
etophenone potassium enolate [PhCOCH2K] (2) in its ion-
pair form, [(η5:η1:η1:η1-Et8N4)Zr{PhC(CH2)O}K(thf)3] (3),
and thus drives the aldol condensation reaction with
acetophenone. The resulting aldolate, which occurs in
a metallacyclic form due to the solvation of potassium
by a phenyl ring, remains η1 (O)-bonded to zirconium,
[(η5:η1:η1:η1-Et8N4)Zr{PhC(CH2)OC(O)C(Me)Ph}K]n (4).
sium enolates and in the aldol condensation reaction,
as reported in Scheme 1.
The employment of early transition metals to influ-
ence the reactivity of enolates is a commonly used
strategy in organic synthesis.5 Although mediation by
titanium(IV)6 and, in some cases, zirconium(IV)7 is
commonly encountered,8 very rarely have these inter-
mediates been isolated and characterized.9 In addition,
they have never been identified as zirconium-alkali-
metal enolate adducts. To this end we wish to report a
zirconium complex acting as a carrier of the alkali-metal
enolate and the derived alkali-metal aldolate in their
ion-pair forms.
Bifunctional coordination compounds, which have as
a part of their structure two complementary (i.e. electron-
poor and electron-rich) reactive sites, display the ability
to carry polar functionalities in their tight or separated
ion-pair form. This unique behavior is exemplified by
organocuprates,1 which act as carriers of lithium orga-
nometallics and exhibit a bifunctional nature, by which
both copper and lithium can intervene in the reaction
with a substrate. Only a few other examples can be
mentioned, such as the alkyl and aryl derivatives of
transition metals in their “ate” form.2 We should
emphasize the relevance of using “carriers” [LnM-
R-Li+] rather than neutral organometallic functional-
ities [LnM-R], since the former is more likely to convert
a stoichiometric into a catalytic metal-promoted reac-
tion.
Complex 1 displays its bifunctional character by
binding the potassium enolate 2 in the tight ion-pair
(5) Paterson, I. In Comprehensive Organic Synthesis; Heathcock, C.
H., Ed.; Pergamon: Oxford, U.K., 1991; Vol. 2, p 301.
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Wiley: London, 1994; p 195. Duthaler, R. O.; Hafner, A. Chem. Rev.
1992, 92, 807.
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1986.
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1982, 104, 5528. Brenet, B.; Bishop, P. M.; Caron, M.; Kawamato, T.;
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Chem. 1985, 63, 2810. Oertkle, K.; Beyler, H.; Duthaler, R. O.;
Lottenbach, W.; Riediker, M.; Steiner, E. Helv. Chim. Acta 1990, 73,
353. Evans, D. A.; Clark, J . S.; Metternich, R.; Novack, V. J .; Sheppard,
G. S. J . Am. Chem. Soc. 1990, 112, 866. Evans, D. A.; Gage, J . R.;
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1993, 34, 4393. Oppolzer, W.; Rodriguez, I. Helv. Chim. Acta 1993,
76, 1282. Paterson, I.; Perkins, M. W. Tetrahedron Lett. 1992, 33, 801.
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(9) Cozzi, P. G.; Veya, P.; Rotzinger, F. P.; Floriani, C.; Chiesi-Villa,
A.; Rizzoli, C. Organometallics 1995, 14, 4092-4100 and references
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Villa, A.; Rizzoli, C. Organometallics 1995, 14, 4101-4108 and
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(10) Procedure for 3: KH (0.232 g, 5.79 mmol) was added at room
temperature to a colorless THF (150 mL) solution of freshly distilled
CH3COPh (0.69 g, 5.8 mmol). At the end of the gas evolution, the
suspension of a white solid in a pale yellow solution was warmed to
50 °C for 2 h. 1 (4.06 g, 5.79 mmol) was added, yielding a yellow
solution. The solvent was removed in vacuo, toluene was added to the
residue, and the product, 3, was collected and dried in vacuo (3.2 g,
70%). Crystals suitable for X-ray analysis were grown in a mixture of
toluene and THF. Anal. Calcd for C52H75KN4O3Zr: C, 67.09; H, 7.63;
N, 6.02. Found: C, 67.82; H, 7.20; N, 6.43. 1H NMR (pyridine-d5, 200
MHz, 298 K): δ 7.91 (m, 2H, Ar H), 7.22 (m, 3H, Ar H), 6.45 (s, 8H,
C4H2N), 5.00 (s, 1H, CdCH2), 4.51 (s, 1H, CdCH2), 2.24 (q, J ) 7.31
Hz, 8H, CH2), 2.04 (q, J ) 7.25 Hz, 8H, CH2), 0.96 (t, J ) 7.31 Hz,
12H, CH3), 0.86 (t, J ) 7.25 Hz, 12H, CH3). The crystals used for the
X-ray analysis contain a THF molecule of crystallization.
The bifunctional (meso-octaethylporphyrinogenato)-
zirconium species [Et8N4Zr(THF)] (1), which displays
carrier properties toward alkali-metal alkyls, aryls, and
hydrides,3,4 has been engaged in reactions with potas-
* To whom correspondence should be addressed. E-mail:
carlo.floriani@icma.unil.ch.
X Abstract published in Advance ACS Abstracts, J anuary 15, 1997.
(1) Lipshutz, B. H.; Sengupta, S. In Organic Reactions; Paquette,
L. A., Ed.; Wiley: New York, 1992; Vol. 41, Chapter 2. Lipshutz, B. H.
In Organometallics in Synthesis: A Manual; Schlosser, M., Ed.;
Wiley: New York, 1994; Chapter 4.
(2) Reetz, M. T. In Organotitanium Reagents in Organic Synthesis;
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Peter, R.; Wenderoth, B. Chem. Ber. 1985, 118, 1441. Reetz, M. T.;
Wenderoth, B. Tetrahedron Lett. 1982, 23, 5259. Morris, R. J .;
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G. C.; Kiely, A. F.; Schaefer, W. P.; Bercaw, J . E. J . Am. Chem. Soc.
1984, 116, 4489.
(3) Floriani, C. Pure Appl. Chem. 1996, 68, 1-8.
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Chem. Soc. 1993, 115, 3595. (b) J acoby, D.; Isoz, S.; Floriani, C.;
Schenk, K.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1995, 14,
4816-4824. (c) J acoby, D.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J .
Am. Chem. Soc. 1993, 115, 7025. (d) J acoby, D.; Isoz, S.; Floriani, C.;
Chiesi-Villa, A.; Rizzoli, C. J . Am. Chem. Soc. 1995, 117, 2793. (e)
Floriani, C. In Stereoselective Reactions of Metal-Activated Molecules;
Werner, H., Sundermeyer, J ., Eds.; Vieweg: Wiesbaden, 1995; pp 97-
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Am. Chem. Soc. 1995, 117, 2805.
S0276-7333(96)00932-6 CCC: $14.00 © 1997 American Chemical Society

Communications
Organometallics, Vol. 16, No. 4, 1997 509
Sch em e 1
F igu r e 1. SCHAKAL drawing of complex 3. Selected bond
distances (Å) are as follows: Zr1-O1, 2.041(3); Zr1-O2,
2.400(3); Zr1-N1, 2.239(5); Zr1-N2, 2.255(4); Zr1-N3,
2.233(5); Zr1-N4, 2.429(4); Zr1-Cp4, 2.249(4); K1-O1,
2.886(4); K1-O3, 2.760(4); K1-O4, 2.795(6); K1-O5, 2.772-
(4); K1-Cp2, 2.907(4); O1-C37, 1.337(5); C37-C38, 1.346-
(8); C37-C39, 1.494(8). Cp2 and Cp4 refer to the centroids
of the pyrrole rings containing N2 and N4, respectively.
the formation of a metallacyclic structure derived from
the complexation of the aldolate by the potassium cation
(see Scheme 1 and Figure 213 ). Such a metallacycle
formation mimics, at least structurally, the various
proposed cyclic transition states for the aldol condensa-
tion reaction.9,14 The present metallacycle has, in
addition, some peculiarities when it is compared to the
early reports on the use of transition metals in enolate
chemistry.9 First of all, the transition metal is not part
of the metallacycle but is supporting a unique structur-
ally identified alkali-metal metallacycle. Second, one
form, with zirconocene taking care of the oxygen, while
potassium is η5-bonded to one of the N2, C6, ..., C9
pyrrolyl anions (see complex 310 in Scheme 1 and Figure
111); K-N and K-C distances range from 3.122(5) to
3.255(5) Å. During this reaction the porphyrinogen
changes its bonding mode from η5:η1:η5:η1 to η5:η1:η1:
η1, having the pyrrolyl ring (N4, C16, ..., C19) nearly
perpendicular to the N4 core; the dihedral angle is 81.0-
(2)°. The η5:η1:η1:η1 bonding mode of the porphyrinogen
cannot be revealed in solution, because of its very low
solubility in solvents allowing NMR experiments at low
temperature. The N4 core shows tetrahedral distortions
(from -0.075(4) to +0.079(1) Å), Zr protruding by 0.519-
(2) Å in the O1 direction. The porphyrinogen assumes
a flattened saddle-shaped conformation, which conse-
quently allows a suitable binding site for the K cation.
The structural parameters of the enolato functionality,
with significant CdC double-bond character (C37-C38
) 1.346(8) Å) and a CsO single bond (O1-C37 ) 1.337-
(5) Å), are quite close to those we observed in other η1-
(O)-bonded zirconium enolates.
(12) Procedure for 4: Freshly distilled PhCOCH3 (0.52 g, 4.3 mmol)
was added dropwise to a pale yellow THF (200 mL) solution of 3 (3.4
g, 4.3 mmol). The resulting light yellow solution was stirred at room
temperature for 12 h. After removal of the solvent, toluene was added
to the residue and a yellow solid, 4, was collected (3.04 g, 77%).
Recrystallization from THF/toluene gave crystals suitable for X-ray
analysis. They contain a toluene molecule of crystallization. Anal. Calcd
for C52H63KN4O2Zr: C, 68.91; H, 7.01; N, 6.18. Found: C, 68.49; H,
7.19; N, 6.00. 1H NMR (pyridine-d5, 200 MHz, 298 K): δ 8.15 (dd, J o
) 8 Hz, J m ) 1.6 Hz, 2H, OdCAr H), 7.85 (dd, J o ) 8 Hz, J m ) 0.8 Hz,
2H, -OCAr H), 7.35 (m, 3H, Ar H), 7.21 (m, 2H, Ar H), 7.07 (t, J o ) 8
Hz, 1H, -OCAr H), 6.40-6.36 (two singlets overlapping with a pair of
doublets, 8H, C4H2N), 4.04 (d, J ) 16.4 Hz, 1H, CH2), 3.77 (d, J )
16.4 Hz, 1H, CH2), 2.37 (m, 4H, CH2), 2.29 (m, 4H, CH2), 2.22 (s, 3H,
CH3), 2.08 (q, J ) 7.2 Hz, 8H, CH2), 1.03 (t, J ) 7.2 Hz, 12H, CH2),
0.95 (t, J ) 7.2 Hz, 12H, CH3).
(13) Crystal data for 4: C52H63KN4O2Zr‚C7H8, Mr ) 998.6, mono-
clinic, space group P21/n, a ) 15.015(4) Å, b ) 20.844(6) Å, c ) 16.489-
(3) Å, â ) 102.13(2)°, V ) 5045(2) Å3, Z ) 4, Fcalcd ) 1.315 g cm-3
,
F(000) ) 2112, λ(Cu KR) ) 1.541 78 Å, µ(Cu KR) ) 29.02 cm-1; crystal
dimensions 0.23 × 0.25 × 0.58 mm. The structure was solved by the
heavy-atom method and anisotropically refined for all the non-H atoms.
All the hydrogen atoms were located from difference Fourier maps,
The aldol condensation has been performed by react-
ing 3 with acetophenone in THF at room temperature.
The reaction led to 412 as a yellow crystalline solid in
good yield. Zirconium drives the reaction by assisting
introduced as fixed contributors in the last stage of refinement (Uiso
)
0.05 Å2). For 5350 unique observed reflections (I > 2σ(I)) collected at
T ) 138 K on a Rigaku AFC6S diffractometer (5 < 2θ < 140°) and
corrected for absorption, the final R value is 0.054 (wR2 ) 0.144 for
(11) Crystal data for 3: C60H87KN4O5Zr‚C4H8O, Mr ) 1146.8,
triclinic, space group P1h, a ) 14.189(3) Å, b ) 19.264(4) Å, c ) 12.724-
(2) Å, R ) 103.84(3)°, â ) 115.56(3)°, γ ) 94.08(2)°, V ) 2985.5(15) Å3,
Z ) 2, Fcalcd ) 1.276 g cm-3, F(000) ) 1228, λ(Cu KR) ) 1.541 78 Å,
µ(Cu KR) ) 25.60 cm-1: crystal dimensions 0.18 × 0.25 × 0.44 mm.
The structure was solved by the heavy-atom method and anisotropi-
cally refined for all non-hydrogen atoms. All the hydrogen atoms were
located from difference Fourier maps and introduced as fixed contribu-
tors in the last stage of refinement (Uiso ) 0.08 Å2). For 8755 unique
observed reflections (I > 2σ(I)) collected at T ) 138 K on a Rigaku
AFC6S diffractometer (5 < 2θ < 140°) and corrected for absorption,
the final R value is 0.063 (wR2 ) 0.172 for the 10 471 reflections having
I > 0 used in the refinement). All calculations were carried out on a
Quansan personal computer equipped with an Intel Pentium processor.
Atomic coordinates, bond lengths and angles, and thermal parameters
have been deposited at the Cambridge Crystallographic Data Centre,
University Chemical Laboratory, Lensfield Road, Cambridge CB2
1EW, U.K. See the Supporting Information for more details.
the 8080 reflections having
I
> 0 used in the refinement). All
Quansan personal computer
calculations were carried out on
a
equipped with an Intel Pentium processor. Atomic coordinates, bond
lengths and angles, and thermal parameters have been deposited at
the Cambridge Crystallographic Data Centre, University Chemical
Laboratory, Lensfield Road, Cambridge CB2 1EW, U.K. See the
Supporting Information for more details.
(14) Zimmerman, H. E.; Traxler, M. D. J . Am. Chem. Soc. 1957,
79, 1920. Evans. D. A.; Nelson, J . V.; Vogel, E.; Taber, T. R. J . Am.
Chem. Soc. 1981, 103, 3099. Heathcock, C. H. In Asymmetric Synthesis;
Morrison, D. J ., Ed.; Academic: New York, 1984; Vol. 3, Chapter 2.
Evans, D. A.; Nelson, J . V.; Taber, T. R. In Topics in Stereochemistry;
Eliel, E. L., Wilen, S. H., Eds.; Wiley: New York, 1983; Vol. 13, p 1.
Denmark, S. E.; Henke, B. R. J . Am. Chem. Soc. 1991, 113, 2177 and
references therein. For anti-selective aldol addition with Cp2ZrCl2
enolates see: Sacha, H.; Waldmu¨ller, D.; Braun, M. Chem. Ber. 1994,
127, 1959.

510 Organometallics, Vol. 16, No. 4, 1997
Communications
the N2′ nitrogen atom (the prime indicates the sym-
metry transformation -0.5 + x, 0.5 - y, -0.5 + z) and
in an η5 fashion with the N1′, C1′, ..., C4′ pyrrole ring,
as indicated by the narrow range of the K-N and K-C
distances ranging from 2.981(5) to 3.180(5) Å. In the
crystal the bridging role of potassium gives rise to
infinite chains running along the [101] crystallographic
axis (Figure S1, Supporting Information). The bonding
mode and geometry of the [Zr(Et8N4)] moiety is close to
that observed in 3, the major difference being the out-
of-plane distance of zirconium from the planar N4 core
(0.886(1) vs 0.519(2) Å), which can be attributed to the
stronger Zr-O(alkoxide) interaction. The Zr-O1 line
forms a dihedral angle of 20.3(1)° with the normal to
the N4 core.
Among the structural parameters of the cyclic potas-
sium aldolate, we should mention the very short Zr-
O1 [1.917(4) Å; Zr-O1-C37 ) 169.4(4)°) and the long
O1-C37 (1.430(7) Å) distance and the CdO double bond
F igu r e 2. SCHAKAL drawing of complex 4. Selected bond
distances (Å) are as follows: Zr1-O1, 1.917(4); Zr1-N1,
2.243(4); Zr1-N2, 2.262(4); Zr1-N3, 2.227(4); Zr1-N4,
2.349(5); Zr1-Cp4, 2.305(7); K1-O2, 2.628(4); K1-Cp1′,
2.845(5); K1-N2′, 3.128(4); K1-Cb41, 3.055(6); O1-C37,
1.430(7); O2-C39, 1.225(7); C37-C38, 1.533(9); C37-C40,
1.529(9); C37-C41, 1.541(10); C38-C39, 1.518(8); C39-
C47, 1.499(10). The prime denotes a transformation of -0.5
+ x, 0.5 - y, -0.5 + z. Cp1′ and Cp4 refer to the centroids
of the pyrrole rings containing N1′ and N4, respectively.
Cb41 refers to the centroid of the C41, ..., C46 aromatic
ring.
1
(C39-O2 ) 1.225(7) Å). In the aldol compound the H
NMR spectrum at room temperature reveals the same
η5:η1:η1:η1 bonding mode of the porphyrinogen observed
in the solid state. We are extending this work by using
1 as a carrier for both the starting enolates as well as
for the final alkali-metal aldolate along with investiga-
tions into the removal of the potassium aldolate from 4
to give 1, which can then act as a catalyst in the metal-
assisted aldol condensation reactions.
Ack n ow led gm en t. We thank the Fonds National
Suisse de la Recherche Scientifique, (Bern, Switzerland,
Grant No. 20-40268.94) and Ciba-Geigy SA (Basel,
Switzerland) for financial support.
of the arene rings solvates the potassium cation, the
K‚‚‚C distance ranging from 3.148(6) to 3.547(6) Å for
the C41-C46 ring (K‚‚‚C41-C46(centroid) ) 3.055(6)
Å].15
In addition, the K+ cation acts as a bridge with an
adjacent anion symmetry-related by the crystallographic
n glide plane. The cation interacts in an η1 fashion with
Su p p or tin g In for m a tion Ava ila ble: Figure S1, giving
a SCHAKAL drawing of complex 4, and tables of crystal data,
atomic coordinates, thermal parameters, and bond distances
and angles for complexes 3 and 4 (16 pages). Ordering
information is given on any current masthead page.
(15) Weiss, E. Angew. Chem., Int. Ed. Engl. 1993, 32, 1501.
OM9609325