Structures of zinc ketone enolates

Article abstract of DOI:10.1021/om060367y

Solution- and solid-phase structural characterizations of a series of simple zinc ketone enolates are presented. The new enolate derivatives were prepared (isolated yields 38-69%) by the deprotonation of ketones by the bis(arylzinc) species ^RLZn₂Ph₂ (1) (where ^RL^2- are bis(amidoamine) ligands). Ketones used include 2′,3′,4′,5′,6′-pentamethylacetophenone, cyclohexanone, 2,2-dimethylcyclopentanone, 2,4-dimethyl-3-pentanone, and isobutyrophenone. These ketones were chosen because they have varying degrees of substitution at the acidic carbon (i.e., α to the carbonyl group). Thus, the effect of sterics on the structural preferences of zinc enolates was examined. In all cases, both solid- and solution-phase structural data indicate that the anionic enolate ligands have isolated (and uncoordinated) carbon-carbon double bonds. The enolate anions bind to the Zn centers with O-bridged structures. Single-crystal X-ray diffraction data are reported for ^MeLZn₂[OC(=CMe₂)ⁱPr]₂ (2a), ^iPrLZn₂[OC(=CMe₂)ⁱPr] ₂ (2b), and ^iPrLZn₂[OC(=CH₂)C ₆Me₅]₂ (4b). The Zn₂O₂ core of 2a is symmetrical with the four Zn-O bond lengths, varying only slightly (Zn-O_enolate = 1.975(2)-1.982(2) A). In contrast, the structures of 2b and 4b, which are supported by the more sterically hindered ligand ^iPrL^2-, display parallelogram-shaped Zn ₂O₂ cores and longer intermetal distances. These structural distortions are accompanied with changes in the degree of polarization of the carbon-carbon double bonds of the enolate ligands. This is observed by considering the shift difference between the two ¹³C resonances arising from the carbons of the enolate C-C double bond. The general trend is that the more hindered supporting ligand, ^iPrL^2-, forms enolate derivatives with less polarized carbon-carbon double bonds. As expected, the Zn enolates also display carbon-carbon double-bond polarizations that are intermediate to those of closely related Li and Si enolates.

Full text of DOI:10.1021/om060367y

Organometallics 2006, 25, 3501-3507
3501
Structures of Zinc Ketone Enolates
Mark L. Hlavinka and John R. Hagadorn*
Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215
ReceiVed April 27, 2006
Solution- and solid-phase structural characterizations of a series of simple zinc ketone enolates are
presented. The new enolate derivatives were prepared (isolated yields 38-69%) by the deprotonation of
R
R
ketones by the bis(arylzinc) species LZn2Ph2 (1) (where L2- are bis(amidoamine) ligands). Ketones
used include 2′,3′,4′,5′,6′-pentamethylacetophenone, cyclohexanone, 2,2-dimethylcyclopentanone, 2,4-
dimethyl-3-pentanone, and isobutyrophenone. These ketones were chosen because they have varying
degrees of substitution at the acidic carbon (i.e., R to the carbonyl group). Thus, the effect of sterics on
the structural preferences of zinc enolates was examined. In all cases, both solid- and solution-phase
structural data indicate that the anionic enolate ligands have isolated (and uncoordinated) carbon-carbon
double bonds. The enolate anions bind to the Zn centers with O-bridged structures. Single-crystal X-ray
diffraction data are reported for MeLZn2[OC(dCMe2)iPr]2 (2a), iPrLZn2[OC(dCMe2)iPr]2 (2b), and
iPrLZn2[OC(dCH2)C6Me5]2 (4b). The Zn2O2 core of 2a is symmetrical with the four Zn-O bond lengths,
varying only slightly (Zn-Oenolate ) 1.975(2)-1.982(2) Å). In contrast, the structures of 2b and 4b,
which are supported by the more sterically hindered ligand iPrL2-, display parallelogram-shaped Zn2O2
cores and longer intermetal distances. These structural distortions are accompanied with changes in the
degree of polarization of the carbon-carbon double bonds of the enolate ligands. This is observed by
considering the shift difference between the two 13C resonances arising from the carbons of the enolate
C-C double bond. The general trend is that the more hindered supporting ligand, iPrL2-, forms enolate
derivatives with less polarized carbon-carbon double bonds. As expected, the Zn enolates also display
carbon-carbon double-bond polarizations that are intermediate to those of closely related Li and Si
enolates.
these species have been hindered by their tendencies toward
aggregation, disproportionation, and rapid self-condensation
processes.7
Metal enolates are known to adopt a variety of structural
motifs (Chart 1), which are dependent on the choice of metal,
solvent, and steric bulk of the ligands.8 Generally, enolates of
electropositive metals prefer O-bound structures. For example,
the structurally characterized ketone enolate derivatives of the
Introduction
Main-group and transition-metal enolates are carbon nucleo-
philes of importance in both organic syntheses1 and metal-
mediated polymerizations of polar monomers.2 In this context,
alkali and alkaline-earth enolates are most frequently used, but
there is growing interest in the use of zinc enolates due to their
ideal combination of good reactivity and tolerance of many
functional groups.3 To date, zinc enolates have been used in
aldol additions to carbonyls,4 additions to activated and simple
alkenes,5 and a variety of transmetalation reactions.6 Surpris-
ingly, despite the importance of these reagents there is very
little structural information available that can be used to shed
insight into their reactivity patterns and properties. This is
because prior attempts to isolate and structurally characterize
(4) (a) Drake, N. L.; Eaker, C. M.; Shenk, W. J. Am. Chem. Soc. 1948,
70, 677-680. (b) Hansen, M. M.; Bartlett, P. A.; Heathcock, C. H.;
Organometallics 1987, 6, 2069-2074. (c) Trost, B. M.; Ito, H. J. Am. Chem.
Soc. 2000, 122, 12003-12004. (d) Trost, B. M.; Silcoff, E. R.; Ito, H. Org.
Lett. 2001, 3, 2497-2500. (e) Trost, B. M.; Yeh, V. S. C. Angew. Chem.,
Int. Ed. 2002, 41, 861-863. (f) Ocampo, R.; Dolbier, W. R.; Abboud, K.
A.; Zuluaga, F. J. Org. Chem. 2002, 67, 72-78. (g) Ross, N. A.; Bartsch,
R. A. J. Org. Chem. 2003, 68, 360-366. (h) Palomo, C.; Oiarbide, M.;
Laso, A. Angew. Chem., Int. Ed. 2005, 44, 3881-3884. (i) Andres, J. M.;
Pedrosa, R.; Perez-Encabo, A. Tetrahedron 2000, 56, 1217-1223.
(5) (a) Menicagli, R.; Samaritani, S. Tetrahedron 1996, 52, 1425-1432.
(b) Ding, Y.; Zhao, Z.; Zhou, C. Tetrahedron 1997, 53, 2899-2906. (c)
Laroche, M.; Belotti, D.; Cossy, J. Org. Lett. 2005, 7, 171-173. (d)
Nakamura, E.; Kubota, K. J. Org. Chem. 1997, 62, 792-793. (e) Nakamura,
M.; Hatakeyama, T.; Hara, K.; Nakamura, E. J. Am. Chem. Soc. 2003, 125,
6362-6363. (f) Nakamura, M.; Hatakeyama, T.; Nakamura, E. J. Am. Chem.
Soc. 2004, 126, 11820-11825.
(6) Uses in Pd-mediated coupling reactions: (a) Hama, T.; Liu, X.;
Culkin, D. A.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 11176-11177.
(b) Bentz, E.; Moloney, M. G.; Westaway, S. M. Tetrahedron Lett. 2004,
45, 7395-7397. In Cu-mediated couplings: (c) Yokomatsu, T.; Suemune,
K.; Murano, T.; Shibuya, S. J. Org. Chem. 1996, 61, 7207-7211. (d) Zhang,
X.; Burton, D. J. Tetrahedron Lett. 2000, 41, 7791-7794.
(7) For descriptions and examples of condensation processes of Zn
enolates, see: (a) Dekker, J.; Schouten, A.; Budzelaar, P. H. M.; Boersma,
J.; van der Kerk, G. J. M. J. Organomet. Chem. 1987 320, 1-12. (b) Goel,
S. C.; Chiang, M. Y.; Buhro, W. E. J. Am. Chem. Soc. 1991, 113, 7069-
7071.
(1) For reviews of metal enolates and their uses in organic syntheses,
see: (a) Heathcock, C. H. In ComprehensiVe Organic Synthesis; Trost, B.
M., Fleming, I., Heathcock, C. H., Eds.; Pergamon Press: Oxford, 1991;
Vol. 2, Chapter 1.6. (b) Kim, B. M.; Williams, S. F.; Masamune, S. In
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Heathcock,
C. H., Eds.; Pergamon Press: Oxford, 1991; Vol. 2, Chapter 1.7. (c) Rathke,
M. W., Weipert, P. In ComprehensiVe Organic Synthesis; Trost, B. M.,
Fleming, I., Heathcock, C. H., Eds.; Pergamon Press: Oxford, 1991; Vol.
2, Chapter 1.8. (d) Paterson, I. In ComprehensiVe Organic Synthesis; Trost,
B. M., Fleming, I., Heathcock, C. H., Eds.; Pergamon Press: Oxford, 1991;
Vol. 2, Chapter 1.9 (e) Caine, D. In ComprehensiVe Organic Synthesis;
Trost, B. M., Fleming, I., Heathcock, C. H., Eds.; Pergamon Press: Oxford,
1991; Vol. 3, Chapter 1.1.
(2) Boffa, L. S.; Novak, B. M. Chem. ReV. 2000, 100, 1479-1493.
(3) (a) Organozinc Reagents in Organic Synthesis; Erdik, E., Ed.; CRC
Press: Boca Raton, 1996. (b) Knochel, P.; Singer, R. D. Chem. ReV. 1993,
93, 2117-2188. (c) Fu¨rstner, A. Synthesis 1989, 571-590.
(8) Seebach, D. Angew. Chem., Int. Ed. Engl. 1988, 27, 1624-1654.
10.1021/om060367y CCC: $33.50 © 2006 American Chemical Society
Publication on Web 05/27/2006

3502 Organometallics, Vol. 25, No. 14, 2006
HlaVinka and Hagadorn
maintained in common solvents.20-22 Following those studies
Boersma proposed that Zn ketone enolates adopt analogous C,O-
bridging structures.7a Others, however, have proposed that Zn
ketone enolates are O-bound on the basis of IR absorption data.23
To resolve this issue, additional solution- and solid-phase
structural information is needed. Currently, such data are limited
to one Na zincate species24 and two chelate-stabilized Zn
enolates (â-amino-substituted enolates).25 Here we describe the
preparation and first structural characterizations of a series of
simple Zn ketone enolates supported by binucleating bis-
(amidoamine) ligands.26 These studies indicate a preference for
O-zincation without any Zn-C interactions.
Chart 1
Experimental Section
group 1-4 metals9 and Ln10 (Ln ) lanthanides) nearly always
feature terminal or bridging O-bound structures. In contrast, the
related derivatives of soft metals, such as Hg,11 Au,12 Tl,13 and
Pb,14 strongly prefer terminal C-bound (2-oxoalkyl) structures.
A few C,O-bridging structures have also been observed for
derivatives of Mg,15 Ru(II),16 and Pd(II).17 Importantly, these
structural preferences are generally thought to correlate with
reactivity. Thus metal enolates with O-bound ground states are
usually more reactive in nucleophilic additions with carbonyls
and nitriles. Mechanistically these species are thought to bind
the substrates to the metal center so that the C-C bond forming
step occurs via a cyclic, chairlike transition state. Empirical
observations and calculations also suggest that metal enolates
with C-bound ground states rearrange to O-bound structures
prior to undergoing addition reactions.18
General Considerations. Standard Schlenk-line and glovebox
techniques were used unless stated otherwise. ZnPh2,27 MeLH2,26b
26b
iPrLH2,26b and MeLZn2Ph2 (1a) were prepared as described in
published procedures. 2,4-Dimethyl-3-pentanone, cyclohexanone,
and isobutyrophenone were Kugelrohr distilled from CaH2 prior
to use. 2,2-Dimethylcyclopentanone and 2′,3′,4′,5′,6′-pentamethy-
lacetophenone were purchased from Sigma-Aldrich and were used
as received. Hexanes, Et2O, toluene, tetrahydrofuran (THF), and
CH2Cl2 were passed through columns of activated alumina and
sparged with N2 prior to use. C6D6 and C7D8 were vacuum
transferred from Na-benzophenone ketyl. CD2Cl2 and CDCl3 were
vacuum transferred from CaH2. Elemental analyses were determined
by Desert Analytics.
MeLZn2[OC(dCMe2)iPr]2 (2a). Toluene (20 mL) was added to
ZnPh2 (0.273 g, 1.24 mmol) and MeLH2 (0.212 g, 0.622 mmol) in
a 100 mL round-bottomed flask. The clear, colorless solution was
stirred for 2 h, during which time a white precipitate formed. 2,4-
Dimethyl-3-pentanone (0.88 mL, 6.2 mmol) was added, and the
mixture was heated to 75 °C. After 3 days, the clear solution was
concentrated to 10 mL under reduced pressure. Cooling to -40 °C
The structural preferences of zinc enolates remain uncertain.
We recently reported that Zn amide enolates adopt C,O-bridging
structures with extensive delocalization of the anionic charge.19
These results were similar to those reported earlier by Boersma
and co-workers for ester enolates. These species were shown
by X-ray diffraction to adopt C,O-bridging structures that are
1
yielded 2a as colorless crystals (0.265 g, 61.2%). H NMR (CD2-
Cl2): δ 7.05 (t, J ) 7.6 Hz, 2H), 6.86 (dd, J ) 1.2, 7.6 Hz, 2H),
6.34 (dd, J ) 1.2, 7.6 Hz, 2H), 3.30 (t, J ) 5.6 Hz, 4H), 2.76 (t,
J ) 5.6 Hz, 4H), 2.74 (sept, J ) 6.8 Hz, 2H), 2.47 (s, 12H, NMe2),
1.94 (s, 6H), 1.59 (s, 6H), 0.64 (d, J ) 6.8 Hz, 12H). 13C{1H}
NMR (CD2Cl2): δ 157.2 (OC(dCMe2)iPr), 145.3, 143.8, 124.7,
124.2, 105.8, 102.9, 102.1 (OC(dCMe2)iPr), 62.5, 45.9, 43.3, 30.3,
20.2, 20.0, 18.8. IR: 2954 (s), 2923 (s), 2854 (s), 1654 (w), 1620
(m), 1597 (w), 1577 (m), 1490 (m), 1465 (s), 1420 (m), 1377 (m),
1363 (s), 1339 (m), 1259 (w), 1203 (w), 1174 (w), 1149 (m), 1102
(w), 1087 (w), 1055 (m), 1038 (w), 1024 (m), 952 (w), 934 (w),
833 (w), 787 (w), 758 (w), 752 (m), 722 (m). Anal. Calcd (found)
for 2a, C34H52N4O3Zn2: C, 58.71 (58.63); H, 7.54 (7.37); N, 8.05
(8.21).
(9) Selected examples of derivatives of group 1: (a) Henderson, K. W.;
Dorigo, A. E.; Liu, Q.-Y.; Williard, P. G.; Schleyer, P.; Bernstein, P. R. J.
Am. Chem. Soc. 1996, 118, 1339-1347. (b) Williard, P. G.; Carpenter, G.
B. J. Am. Chem. Soc. 1985, 107, 3345-3346. (c) Williard, P. G.; Hintze,
M. J. J. Am. Chem. Soc. 1990, 112, 8602-8604. (d) Pospisil, P. J.; Wilson,
S. R.; Jacobsen, E. N. J. Am. Chem. Soc. 1992, 114, 7585-7587. (e) Veya,
P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1994, 13,
214-223. Group 2: (f) Allan, J. F.; Henderson, K. W.; Kennedy, A. R.;
Teat, S. J. Chem. Commun. 2000, 1059-1060. (g) Alexander, J. S.;
Ruhlandt-Senge, K. Chem. Eur. J. 2004, 10, 1274-1280. Group 3: (h)
Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Organometallics
2003, 22, 4705-4714. (i) Evans, W. J.; Dominguez, R.; Hanusa, T. P.
Organometallics 1986, 5, 1291-1296. Group 4: (j) Agapie, T.; Diaconescu,
P. L.; Mindiola, D. J.; Cummins, C. C. Organometallics 2002, 21, 1329-
1340. (k) Curtis, M. D.; Thanedar, S.; Butler, W. M. Organometallics 1984,
3, 1855-1859. (l) Howard, W. A.; Parkin, G. J. Am. Chem. Soc. 1994,
116, 606-615.
(20) Dekker, J.; Budzelaar, P. H. M.; Boersma, J.; van der Kerk, G. J.
M.; Spek, A. L. Organometallics 1984, 3, 1403-1407.
(21) In the presence of strong Lewis bases (e.g., hexamethylphosphora-
mide) mononuclear species form, which presumably feature terminal
O-bound or C-bound enolate ligands. See ref 20.
(22) A C,O-bridging structure has been observed for a zinc enolate
derivative of a chiral N-substituted â-carbonyl sulfoximine ([iPr2NC(O)d
C(H)S(O)(NMe)Ph]ZnEt]2): Bolm, C.; Mu¨ller, J.; Zehnder, M.; Neuburger,
M. A. Chem. Eur. J. 1995, 1, 312-317.
(23) Meyer, R.; Gorrichon, L.; Maroni, P. J. Organomet. Chem. 1977,
129, C7-C10.
(24) Hevia, E.; Honeyman, G. W.; Kennedy, A. R.; Mulvey, R. E. J.
Am. Chem. Soc. 2005, 127, 13106-13107.
(10) Wu, Z.; Xu, Z.; You, X.; Zhou, X.; Huang, X. J. Organomet. Chem.
1994, 483, 107-113.
(11) Bebout, D. C.; Bush, J. F., II.; Crahan, K. K.; Bowers, E. V.;
Butcher, R. J. Inorg. Chem. 2002, 41, 2529-2536.
(12) Ito, Y.; Inouye, M.; Suginome, H.; Murakami, M. J. Organomet.
Chem. 1988, 29, C41-C44.
(13) Vicente, J.; Abad, J.; Cara, G.; Jones, P. G. Angew. Chem., Int. Ed.
1990, 29, 1125-1126.
(14) Morgan, J.; Buys, I.; Hambley, T. W.; Pinhey, J. T. J. Chem. Soc.,
Perkin Trans. 1993, 1677-1682.
(15) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.;
Williams, D. J. Chem. Commun. 2002, 1208-1209.
(16) Catalano, V. J.; Craig, T. J. Inorg. Chem. 2003, 42, 321-334.
(17) (a) Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organome-
tallics 1993, 12, 4899-4907. (b) Ruiz, J.; Rodriguez, V.; Cutillas, N.; Pardo,
M.; Perez, J.; Lopez, G.; Chaloner, P.; Hitchcock, P. B. Organometallics
2001, 20, 1973-1982.
(25) (a) van Vliet, M. R. P.; van Koten, G.; Buysingh, P.; Jastrzebski,
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Chiang, M. Y.; Buhro, W. E. Inorg. Chem. 1990, 29, 4646-4652.
(26) (a) Hlavinka, M. L.; Hagadorn, J. R. Chem. Commun. 2003, 2686-
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1232.

Structures of Zinc Ketone Enolates
Organometallics, Vol. 25, No. 14, 2006 3503
iPrLZn2[OC(dCMe2)iPr]2 (2b). Toluene (15 mL), ZnPh2 (0.152
g, 0.692 mmol), and iPrLH2 (0.156 g, 0.342 mmol) were combined
in a 100 mL round-bottomed flask to form a clear, colorless
solution. The solution was heated to 55 °C for 18 h. 2,4-Dimethyl-
3-pentanone (0.49 mL, 3.5 mmol) was then added, and the solution
was heated to 75 °C for 2 days. The clear solution was cooled to
-40 °C to yield 2b as colorless crystals (0.172 g, 61.5%). 1H NMR
(CD2Cl2): δ 7.04 (t, J ) 7.6 Hz, 2H), 6.84 (dd, J ) 1.2, 7.6 Hz,
2H), 6.32 (dd, J ) 1.2, 7.6 Hz, 2H), 3.35 (t, J ) 5.6 Hz, 4H), 3.27
(sept, J ) 6.8 Hz, 4H, N(CHMe2)2), 3.13 (t, J ) 5.6 Hz, 4H), 2.53
(sept, J ) 7.2 Hz, 2H, OC(dCMe2)CHMe2), 1.65 (s, 6H), 1.50 (s,
6H), 1.31 (br, 24H, N(CHMe2)2), 0.58 (d, J ) 7.2 Hz, 12H, OC-
(dCMe2)CHMe2). 13C{1H} NMR (CD2Cl2): δ 157.5 (OC(dCMe2)i-
Pr), 145.2, 143.8, 124.5, 124.1, 106.1, 105.9 (OC(dCMe2)iPr),
103.0, 54.6, 53.6, 47.1, 34.9, 22.2, 20.9, 20.5, 18.6. IR: 2953 (s),
2923 (s), 2854 (s), 1652 (w), 1623 (m), 1601 (w), 1577 (w), 1492
(w), 1462 (s), 1422 (m), 1376 (m), 1364 (m), 1342 (w), 1209 (w),
1184 (w), 1151 (m), 1130 (w), 1099 (m), 1058 (w), 1047 (w), 1024
(w), 753 (w), 726 (m). Anal. Calcd (found) for 2b, C42H68N4O3-
Zn2: C, 62.45 (62.44); H, 8.48 (8.28); N, 6.94 (6.87).
J ) 6.4 Hz, 6H), 0.72 (d, J ) 6.8 Hz, 6H). 13C{1H} NMR (CD2-
Cl2, -20 °C): δ 160.4 (OC(dCH2)Ar), 145.8, 143.8, 138.6, 134.5,
133.3, 132.8, 132.6, 132.0, 124.2, 123.8, 105.9, 102.6, 95.4 (OC-
(dCH2)Ar), 57.4, 51.8, 48.0, 46.2, 24.9, 24.5, 21.0, 18.6, 17.8, 17.0,
16.8, 16.5 (One expected resonance is not observed, presumably
due to overlap). IR: 2954 (s), 2923 (s), 2854 (s), 1620 (m), 1609
(w), 1579 (m), 1491 (w), 1462 (s), 1422 (m), 1376 (m), 1364 (m),
1340 (m), 1303 (w), 1271 (w), 1192 (w), 1177 (w) 1149 (w), 1025
(w), 949 (w), 818 (w), 752 (w), 724 (w). Anal. Calcd (found) for
4b, C54H76N4O3Zn2: C, 67.56 (67.35); H, 7.98 (7.71); N, 5.84
(5.86).
MeLZn2(OC7H11)2 (5). Toluene (10 mL) was added to ZnPh2
(0.211 g, 0.961 mmol) and MeLH2 (0.164 g, 0.480 mmol) in a 100
mL round-bottomed flask. The clear, colorless solution was stirred
for 2 h, during which time a white precipitate formed. 2,2-
Dimethylcyclopentanone (0.18 mL, 1.44 mmol) was added, and
the mixture was heated to 55 °C. After 2 days, the clear solution
was concentrated to 5 mL under reduced pressure. Cooling to -40
1
°C yielded 5 as colorless crystals in 37.8% yield. H NMR (CD2-
Cl2): δ 7.06 (t, 7.6 Hz, 2H), 7.86 (dd, J ) 1.2, 7.6 Hz, 2H), 6.36
(dd, J ) 1.2, 7.6 Hz, 2H), 4.44 (t, J ) 2.5 Hz, 2H, OC(dCH)),
3.33 (t, J ) 5.6 Hz, 4H), 2.83 (t, J ) 5.6 Hz, 4H), 2.58 (s, 12H,
NMe2), 1.99 (dt, J ) 2.5, 6.0 Hz, 4H), 1.59 (t, J ) 6.0 Hz, 4H),
0.83 (s, 12 H). 13C{1H} NMR (CD2Cl2): δ 169.4 (OC(dCH)),
145.7, 143.7, 124.7, 124.2, 105.8, 103.1, 99.2 (OC(dCH)), 62.1,
46.5, 43.3, 43.2, 39.8, 26.3, 24.5. IR: 2953 (s), 2924 (s), 2854 (s),
1619 (m), 1603 (w), 1580 (m), 1491 (m), 1465 (s), 1420 (m), 1377
(m), 1363 (s), 1337 (m), 1294 (w), 1283 (w), 1234 (m), 1205 (w),
1171 (w), 1146 (m), 1135 (w), 1096 (w), 1057 (w), 1022 (w), 955
(w), 931 (w), 832 (w), 786 (w), 751 (m), 721 (m). Anal. Calcd
(found) for 5, C54H76N4O3Zn2: C, 59.05 (59.26); H, 7.00 (7.18);
N, 8.10 (8.35).
MeLZn2[O(dCMe2)Ph]2 (3). Isobutyrophenone (15.6 µL, 0.103
mmol), 1a (32.2 mg, 0.0516 mmol), and C6D6 (0.5 mL) were
combined in a NMR tube. The clear, colorless solution was heated
1
to 75 °C for 2 days. Analysis by H NMR spectroscopy indicated
1
>95% conversion to 3. H NMR (C6D6): δ 7.46 (t, 7.6 Hz, 2H),
7.43 (m, 4H, OC(dCMe2)Ph), 7.35 (dd, J ) 1.2, 7.6 Hz, 2H), 6.96
(m, 4H, OC(dCMe2)Ph), 6.87 (m, 2H, OC(dCMe2)Ph), 6.62 (dd,
J ) 1.2, 7.6 Hz, 2H), 3.11 (t, J ) 5.6 Hz, 4H), 2.12 (t, 5.6 Hz,
4H), 1.76 (s, 12H, NMe2), 1.74 (s, 6H, OC(dCMe(Me))Ph), 1.59
(s, 6H, OC(dCMe(Me))Ph). 13C{1H} NMR (C6D6): δ 150.9 (OC(d
CMe2)Ph), 146.2, 143.6, 142.2, 129.4, 128.9, 127.0, 125.6, 124.6,
107.8 (OC(dCMe2)Ph), 106.8, 104.7, 62.1, 45.2, 43.4, 21.2, 19.0.
MeLZn2(OC6H9)2 (6). Cyclohexanone (35.3 µL, 0.340 mmol),
1a (106 mg, 0.170 mmol), and CH2Cl2 (5 mL) were combined in
a 25 mL round-bottomed flask. The clear, colorless solution was
stirred overnight. Removal of the volatiles under reduced pressure
yielded 6 as a colorless solid (0.11 g, 90% yield). Attempts to
crystallize this product failed. 1H NMR analysis of the crude solid
MeLZn2[OC(dCH2)Ar]2 (Ar ) C6Me5) (4a). Toluene (25 mL)
was added to ZnPh2 (0.464 g, 2.11 mmol) and MeLH2 (0.360 g,
1.06 mmol) in a 100 mL round-bottomed flask. The clear, colorless
solution was stirred for 2 h, during which time a white precipitate
formed. 2′,3′,4′,5′,6′-Pentamethylacetophenone (0.503 g, 2.64 mmol)
in toluene (15 mL) was added, and the mixture was heated to 75
°C for 40 h. Colorless crystals of the product formed as the solution
slowly cooled to room temperature. Further cooling to -15 °C
1
indicated approximately 90% purity. H NMR (C6D6): δ 7.46 (t,
7.6 Hz, 2H), 7.36 (dd, J ) 1.2, 7.6 Hz, 2H), 6.71 (dd, J ) 1.2, 7.6
Hz, 2H), 4.94 (t, J ) 4.0 Hz, 2H, OC(dCH)), 3.22 (t, J ) 5.6 Hz,
4H), 2.29 (t, J ) 5.6 Hz, 4H), 2.09 (s, 12H, NMe2), 1.98 (t, J )
6.8 Hz, 4H), 1.87 (m, 4H), 1.33 (m, 4H), 1.24 (m, 2H), 1.13 (m,
2H). 13C{1H} NMR (C6D6): δ 157.1 (OC(dCH)), 146.7, 143.8,
125.7, 124.8, 107.0, 105.0, 99.6 (OC(dCH)), 62.2, 45.7, 43.6, 32.0,
24.9, 24.2, 23.4.
1
yielded additional 4a (0.663 g, 69.4% yield). H NMR (CD2Cl2):
δ 7.11 (t, J ) 7.6 Hz, 2H), 6.89 (dd, J ) 1.0, 7.6 Hz, 2H), 6.35
(dd, J ) 1.0, 7.6 Hz, 2H), 4.46 (d, 2JHH ) 0.5 Hz, 1JCH ) 153 Hz,
2
1
2H, OC(dCHH)Ar), 3.57 (d, JHH ) 0.5 Hz, JCH ) 159 Hz, 2H,
OC(dCHH)Ar), 3.17 (t, J ) 5.6 Hz, 4H), 2.19 (s, 6H), 2.1 (br,
28H), 2.01 (s, 12H, NMe2). 13C{1H} NMR (CD2Cl2): δ 162.8
(OC(dCH2)Ar), 146.6, 143.9, 139.7, 134.5, 132.6, 132.4, 124.7,
124.3, 105.8, 103.0, 90.8 (OC(dCH2)Ar), 61.2, 45.7, 43.4, 18.3,
16.9, 16.7. IR: 2954 (s), 2923 (s), 2853 (s), 1637 (w), 1619 (m),
1598 (m), 1580 (m), 1491 (m), 1463 (s), 1423 (m), 1378 (m), 1366
(m), 1338 (m), 1307 (w), 1274 (w), 1198 (w), 1171 (w), 1145 (w),
1096 (w), 1058 (w), 1022 (w), 975 (w), 956 (w), 934 (w), 832
(w), 804 (w), 759 (w), 751 (w), 721 (m). Anal. Calcd (found) for
4a‚(toluene)1.2, C54.4H69.6N4O3Zn2: C, 68.18 (68.15); H, 7.32 (7.24);
N, 5.85 (5.69).
NMR Spectroscopy. NMR spectra were measured on a Varian
1
Inova-400 NMR spectrometer, at a H observation frequency of
400.16 MHz. Chemical shifts (δ) for 1H NMR (400 MHz) spectra
are given relative to residual protium in the deuterated solvent at
7.16, 5.32, 7.27, and 2.09 ppm for C6D6, CD2Cl2, CDCl3, and C7D8,
respectively.
X-ray Crystallography. Table 1 lists a summary of crystal data
and collection parameters for all crystallographically characterized
compounds. Additional data are presented as Supporting Informa-
tion.
iPrLZn2[OC(dCH2)Ar]2 (Ar ) C6Me5) (4b). The compound
was prepared using a method analogous to that used to prepare 2b
except that 2.5 equiv of 2′,3′,4′,5′,6′-pentamethylacetophenone was
used in place of 5 equiv of 2,4-dimethyl-3-pentanone. The product
was isolated as colorless crystals from the toluene solution at
ambient temperature. Additional crystalline product formed as the
General Procedure. A crystal of appropriate size was mounted
on a glass fiber using hydrocarbon oil (Paratone-N), transferred to
a Siemens SMART diffractometer/CCD area detector, centered in
the beam (Mo KR; λ ) 0.71073 Å; graphite monochromator), and
cooled to approximately -127 °C by a nitrogen low-temperature
apparatus. Preliminary orientation matrix and cell constants were
determined by the collection of 60 frames, followed by spot
integration and least-squares refinement. A minimum of a hemi-
sphere of data was collected using 0.3° ω scans. The raw data were
integrated and the unit cell parameters refined using SAINT. Data
analysis was performed using XPREP. Absorption correction was
1
mother liquor cooled to -15 °C (total yield: 56.7%). H NMR
(CD2Cl2, -20 °C): δ 7.08 (t, J ) 7.6 Hz, 2H), 6.88 (d, J ) 7.6
1
Hz, 2H), 6.30 (d, J ) 7.6 Hz, 2H), 4.46 (s, JCH ) 154 Hz, 2H),
3.63 (s, 1JCH ) 159 Hz, 2H), 3.29 (br, 4H), 2.94 (br, 4H), 2.25 (s,
6H), 2.19 (s, 6H), 2.14 (s, 6H), 2.08 (s, 6H), 1.7 (s, 6H), 1.35 (br,
4H), 1.30 (d, J ) 6.8 Hz, 6H), 1.26 (d, J ) 6.4 Hz, 6H), 1.21 (d,

3504 Organometallics, Vol. 25, No. 14, 2006
HlaVinka and Hagadorn
4b‚(toluene)0.5
Table 1. Crystallographic Data and Collection Parameters
2a
2b
formula
C34H52N4O3Zn2
695.54
P2(1)/c (#14)
147
10.2301(4)
19.8950(7)
17.3484(6)
90
94.168(1)
90
C42H68N4O3Zn2
807.74
Pna2(1) (#33)
147
18.6748(5)
13.3529(4)
16.9723(4)
90
C57.5H80N4O3Zn2
1005.99
fw (g‚mol-1
space group
temp (K)
a (Å)
)
C2/c (#15)
145
32.949(1)
19.2436(7)
19.7699(7)
90
121.5010(10)
90
8
10687.9(7)
1.250
2.01-27.88
0.944
0.50 × 0.30 × 0.25
40 653
12 741/30/615
0.0564
0.0874, 0.1619
1.030
1.088, -0.533
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
Z
90
90
4
4
V (Å3)
3521.5(2)
1.312
1.56-27.88
1.399
0.45 × 0.35 × 0.08
28 239
8407/0/400
0.0515
0.0932, 0.1238
1.005
0.546, -0.494
4232.3(2)
1.268
1.87-27.88
1.174
0.25 × 0.20 × 0.20
33 162
10 087/1/464
0.0527
0.0837, 0.1187
1.007
0.557, -0.567
d
calc (g‚cm-3
)
θ range (deg)
µ (mm-1
)
cryst size (mm)
no. of reflns collected
no. of data/restraints/params
R1 (for Fo > 4σFo)
R1, wR2 (all data)
GOF
largest peak, hole (e‚Å-3
)
applied using SADABS. The data were corrected for Lorentz and
polarization effects, but no correction for crystal decay was applied.
Structure solutions and refinements were performed (SHELXTL-
Plus V5.1) on F2.28 Notable details of each data collection and
refinement are described below.
equal within a standard deviation of 0.02. The six 1,3 carbon-
carbon distances were restrained to be equal within a standard
deviation of 0.04. The largest peak in the difference Fourier was
located near the disordered toluene molecule (1.8 Å from H55a).
Structure of MeLZn2[OC(dCMe2)iPr]2 (2a). Crystals that were
suitable for X-ray diffraction studies were grown at ambient
temperature by the vapor diffusion of hexanes into a C6D6 solution.
Preliminary data indicated a primitive monoclinic cell. Systematic
absences indicated space group P2(1)/c (#14). This choice of space
group was confirmed by the successful solution and refinement of
the structure. The asymmetric unit contains a single molecule of
2a. All non-H atoms were refined anisotropically. Hydrogens were
placed in idealized positions and were included in structure factor
calculations but were not refined.
Structure of iPrLZn2[OC(dCMe2)iPr]2 (2b). Crystals that were
suitable for X-ray diffraction studies were grown at ambient
temperature from a toluene solution. Preliminary data indicated a
primitive orthorhombic cell. Systematic absences were consistent
with space groups Pna2(1) (#33) and Pnam (#62). The choice of
Pna2(1) was confirmed by the successful solution and refinement
of the structure. Pnam failed to yield a successful solution. The
asymmetric unit contains a single molecule of 2b. All non-H atoms
were refined anisotropically. Hydrogens were placed in idealized
positions and were included in structure factor calculations but were
not refined. The Flack parameter refined to 0.05(1), indicating that
the correct absolute structure was refined. Inversion of the structure
led to an increase in R1 (all data) to 0.0953 and a Flack parameter
of 0.88(2).
Structure of iPrLZn2[OC(dCH2)Ar]2‚(toluene)0.5 (4b). Crystals
that were suitable for X-ray diffraction studies were grown at -5
°C from a toluene solution. Preliminary data indicated a C-centered
monoclinic cell. Systematic absences were consistent with space
groups C2/c (#15) and Cc (#9). The choice of C2/c was confirmed
by the successful solution and refinement of the structure. Cc failed
to yield a successful solution. The asymmetric unit contains a
molecule of 3b and a half-occupancy cocrystallized toluene that is
located on a crystallographic inversion center. All non-H atoms
not associated with the disordered toluene molecule were refined
anisotropically. Hydrogens were placed in idealized positions and
were included in structure factor calculations but were not refined.
The carbons of the cocrystallized toluene were modeled isotropi-
cally. The six 1,2 carbon-carbon distances were restrained to be
Results and Discussion
Synthesis. Zinc ketone enolates are commonly prepared in
situ by reaction of simple ketones with Zn alkyls or amidos.
Typically these generated enolates are then rapidly quenched
by reaction with an electrophilic carbonyl or other substrate. In
our initial attempts to prepare Zn enolate species, the dinuclear
organozincs 1a and 1b (Scheme 1) were reacted with acetone
in CD2Cl2 solution. After 12 h at 20 °C a mixture of products
was observed (1H NMR), likely due to self-condensation of the
acetone. The use of acetophenone in place of acetone gave
similar results.
The use of more sterically hindered ketones, however, led to
the successful formation of isolable Zn ketone enolates. For
example, heating a C6D6 solution of 1a with excess 2,4-
dimethyl-3-pentanone at 75 °C for 3 days cleanly formed (by
1H NMR) the enolate derivative 2a. Conveniently, it was found
that 1a could be formed in situ and reacted with ketones without
significant reduction in isolated yields. Thus, a suspension of
1a in toluene was generated by reaction of MeLH2 with 2 equiv
of ZnPh2 for 2 h. The addition of 10 equiv of 2,4-dimethyl-3-
pentanone to the solution followed by heating to 75 °C for 3
days formed a clear, colorless solution, from which 2a was
crystallized in 61% yield. Repeating the synthesis using the more
hindered ligand iPrLH2 in place of MeLH2 formed 2b, which was
isolated in 62% yield. In an analogous fashion a C6D6 solution
of 1a reacted with 2 equiv of isobutyrophenone at 75 °C over
2 days to form the enolate product 3 in over 95% yield, as
1
determined by H NMR spectroscopy relative to an internal
standard.
The aforementioned Zn enolates were each formed by the
deprotonation of a tertiary carbon located R to the carbonyl
functionality. In the interest of increasing the scope of this study,
we have also explored enolates formed by the deprotonation of
primary and secondary ketones. For example, heating a toluene
solution of in situ formed 1a with 2.5 equiv of 2′,3′,4′,5′,6′-
pentamethylacetophenone to 75 °C for approximately 2 days
formed a clear, colorless solution, from which 4a crystallized
upon cooling. The product was isolated in 69% yield. Using
(28) Sheldrick, G. M. SHELXTL-Plus, A Program for Crystal Structure
Determination, Version 5.1; Bruker AXS: Madison WI, 1998.

Structures of Zinc Ketone Enolates
Organometallics, Vol. 25, No. 14, 2006 3505
Figure 1. Molecular structures of 2a, 2b, and 4b drawn with 50% thermal ellipsoids. Hydrogens are omitted in all structures. Cocrystallized
toluene and the methyls of the enolate ligands are omitted in 4b.
Scheme 1
analogous reaction conditions, the related enolate product 4b
was prepared in 56% yield from the ketone and 1b. Access to
enolates of secondary ketones was achieved by the use of cyclic
ketones to prevent the formation of mixtures of E and Z isomers.
Heating a toluene solution of in situ formed 1a with 3 equiv of
2,2-dimethylcyclopentanone to 55 °C for 2 days formed 5, which
was isolated as colorless crystals in 38% yield following cooling
to -40 °C. In a similar fashion 6 was formed in 90% yield (by
1H NMR) by reaction of 1a with cyclohexanone in CH2Cl2 at
ambient temperature.
bonds varying only slightly (Zn-Oenolate ) 1.975(2)-1.982(2)
Å). Bond lengths and angles indicate the enolate ligands have
isolated carbon-carbon double bonds with an average bond
length of 1.36 Å. The enolate ligands are oriented approximately
perpendicular to the Zn-Zn axis. This geometry appears to
minimize the steric repulsions between the hindered enolates
and the bis(amidoamine) ligand. Overall the structure of 2b is
similar to that of 2a with a few exceptions. Most notable, the
Zn2O2 core of 2b is parallelogram shaped with alternating short
and long Zn-O bonds that differ by an average of 0.04 Å.
Accompanying this distortion is a lengthening of the Zn-Zn
distance by 0.05 Å relative to 2a. This modest expansion of
the core likely arises due to increased steric pressures. Geometry
at the enolate oxygens is slightly pyramidalized, with the sum
of the angles at O1 and O2 being 347.7° and 348.7°, respec-
tively.
Structures. The solid-state structures of 2a, 2b, and 4b were
determined by single-crystal X-ray diffraction. These are shown
in Figure 1 as 50% thermal ellipsoid plots. Selected bond lengths
and angles are included in Table 2.
Compound 2a features a pair of four-coordinate Zn centers,
each bound to a chelating amidoamine donor and two O-
bridging enolates. Each amidoamine ligand is coordinated in
an unsymmetrical fashion to a Zn center, with the Zn-Namido
bonds being, on average, 0.18 Å shorter than the dative Zn-
The solid-state structure of 4b differs significantly from those
of 2a and 2b. Most notably, the enolates of 4b are not oriented
perpendicular to the Zn-Zn axis. This reflects the less hindered
profile of the enolate. The Zn2O2 core is parallelogram shaped
with alternating short and long Zn-O bonds that differ by an
average of 0.07 Å. This expansion of the core results in a
relatively long intermetal separation of 2.9610(5) Å. Metrical
parameters of the enolate ligands again indicate the presence
of localized carbon-carbon double bonds (C29-C30, 1.330(4)
Å; C42-C43, 1.331(4) Å).
N
amine bonds. This is typical for this type of ligand. Interestingly,
the enolate oxygens adopt nearly planar geometries.29 The sum
of the angles around O1 and O2 are 358.9° and 359.9°,
respectively. The Zn2O2 core is symmetrical with the four Zn-O
(29) Solid-state structures of related Zn alkoxide derivatives of MeL2-
and iPrL2- all feature pyramidalized µ-alkoxide ligands. See ref 26c.

3506 Organometallics, Vol. 25, No. 14, 2006
HlaVinka and Hagadorn
Table 2. Core Bond Lengths and Angles of Structurally Characterized Zn Enolates
metrical
parameter
2a (R1 ) iPr, R2 ) Me)
2b (R1 ) iPr, R2 ) Me)
4b (R1 ) C6Me5, R2 ) H)
a (Å)
1.980(2)
Zn1-O1a
1.986(3)
Zn1-O1
1.973(2)
2.045(2)
2.047(2)
1.980(2)
1.380(4)
1.391(3)
1.330(4)
1.331(4)
1.489(4)
1.506(4)
94.86(8)
94.71(8)
122.2(2)
123.3(2)
122.0(2)
123.0(2)
2.9610(5)
Zn1-O1
1.982(2)
1.975(2)
1.980(2)
1.384(4)
1.379(4)
1.341(5)
1.334(5)
1.517(5)
1.512(5)
92.77(9)
92.69(9)
132.7(2)
133.4(2)
134.2(2)
133.0(2)
2.8649(5)
Zn1-O2
2.017(3)
2.014(3)
1.983(3)
1.387(5)
1.399(5)
1.336(6)
1.338(6)
1.523(6)
1.523(6)
93.7(1)
Zn1-O2
Zn1-O2
Zn2-O1
Zn2-O1
Zn2-O1
Zn2-O2
Zn2-O2
Zn2-O2
b (Å)
O1-C21
O1-C29
O1-C29
O2-C28
O2-C36
O2-C42
c (Å)
C21-C22
C29-C30
C29-C30
C28-C29
C36-C37
C42-C43
d (Å)
C21-C25
C29-C33
C29-C31
C28-C32
C36-C40
C42-C44
R (deg)
â (deg)
Zn1-O1-Zn2
Zn1-O2-Zn2
Zn1-O1-C21
Zn2-O1-C21
Zn1-O2-C28
Zn2-O2-C28
Zn1-Zn2
Zn1-O1-Zn2
Zn1-O2-Zn2
Zn1-O1-C29
Zn2-O1-C29
Zn1-O2-C36
Zn2-O2-C36
Zn1-Zn2
Zn1-O1-Zn2
Zn1-O2-Zn2
Zn1-O1-C29
Zn2-O1-C29
Zn1-O2-C42
Zn2-O2-C42
Zn1-Zn2
93.7(1)
130.5(2)
123.5(2)
125.8(2)
129.2(2)
2.9183(7)
Zn-Zn (Å)
a Atom numbers correspond to the those shown in Figure 1.
Table 3. 13C NMR Spectroscopic Data for Ketone Enolates
a T ) 253K. b This work. c Reference 31. d Reference 35. e Reference 25a. f Reference 24.
1
NMR Spectroscopy. H and 13C NMR spectroscopic data
be discussed. 13C NMR spectroscopic data for all new Zn
enolates and some related compounds are shown in Table 3.
The 1H NMR spectrum of 2a (in CD2Cl2) is consistent with
overall C2V symmetry. Thus, one sharp singlet was observed at
δ 2.47 ppm (12H) for the four Me groups of the two -NMe2
for the Zn enolate complexes 2-6 are consistent with the
structures shown in Scheme 1. Thus, they each have O-bridging
enolate anions and overall C2V symmetry in solution. Due to
the similarity of these complexes, only selected NMR data will

Structures of Zinc Ketone Enolates
Organometallics, Vol. 25, No. 14, 2006 3507
donors.30 Also, only three resonances, with integrated intensities
of 2H each, were observed in the aromatic region for the
dibenzofuran backbone. In addition to the aforementioned
resonances attributable to the MeL2- ligand, a pair of singlets
were also found at δ 1.94 and 1.59 ppm (6H each), indicating
that the two Me groups of the enolate ligand were inequivalent.
Collectively these data suggest that 2a has an O-zincated
structure, almost certainly with the O atoms bridging the two
Zn centers, as was observed in the solid-state structure.
Additional confirmation of O-zincation can be found from the
13C{1H} NMR spectrum of 2a, which features two downfield
resonances at δ 157.2 (CR) and 102.1 (Câ) ppm arising from
the alkene carbons of the enolate ligand (Table 3). These
resonances are comparable to those reported for other O-bound
metal enolates, including the chelate-stabilized zinc enolate 725a
(160.0, 104.6 ppm) and the sodium zincate species 824 (146.6,
106.8 ppm). Likewise, the 13C NMR spectrum of the lithium
enolate {Li[OC(dCMe2)Ph]}n (in D8-THF)31 features olefinic
resonances at δ 155.6 (CR) and 95.6 (Câ) ppm.
also dependent on the degree and type of substitution of the
alkene functionality, meaningful comparisons can only be drawn
from closely related species. The 13C{1H} NMR spectrum of 3
features resonances at δ 150.9 and 107.8 ppm for the alkene
carbons CR and Câ, respectively. The chemical shift difference
between these resonances is 43.1 ppm (Table 3). This value is
significantly less than that reported for the analogous Li enolate
{Li[OC(dCMe2)Ph]}n,31 which has ∆δ(R - â) ) 60.0 ppm.
This is consistent with the Zn enolate being less polarized and
less nucleophilic. Consistent with this general trend, the
analogous Si enolate Me3SiOC(dCMe2)Ph,35 which is only
weakly nucleophilic, has an even smaller ∆δ(R - â) of 30.8
ppm.
The composition of Zn enolate 2b is identical to that of 2a
except that it is supported by the more sterically hindered ligand
iPrL2- (instead of Me 2-). The same relationship exists between
L
4b and 4a. Thus, comparison of the 13C NMR data for these
pairs of molecules allows us to observe the effect of ligand
sterics on enolate polarization. For 2b ∆δ(R - â) ) 51.6 ppm,
while the less hindered 2a has ∆δ(R - â) ) 55.1 ppm. A similar
trend is observed for 4b and 4a. Thus, the more hindered
supporting ligands appear to form enolates with decreased
polarization of the carbon-carbon bonds. This observation can
be rationalized by considering solid-state structural data. The
Zn-Namine bonds of 2b are on average 0.06 Å longer than those
of 2a. Thus, the Zn centers of 2b are expected to form stronger
bonds to the enolate donors, which results in less charge
localization at oxygen. In turn this should yield less polarization
of the carbon-carbon double bond.
The 1H NMR spectrum of 4a in CD2Cl2 solution is consistent
with overall C2V symmetry for the molecule. The spectrum
features two doublets (2J ) 0.5 Hz) at δ 4.46 and 3.57 ppm
(2H each), which are assigned to the two inequivalent vinylic
hydrogens of each enolate. 13C satellites for these two resonances
1
indicate JCH values of 153 and 159 Hz for the resonances at
4.46 and 3.57 ppm, respectively. These values are consistent
with sp2-hybridized vinylic C-H bonds.32 The 1H NMR
spectrum of 4b (CD2Cl2) at ambient temperature displays broad
resonances for the bis(amidoamine) ligand. This is a result of
the -NiPr2 donors undergoing reversible dissociation at a rate
that is similar to that of the NMR data acquisition.33 Cooling
the solution to -20 °C sharpens the spectrum. At this temper-
ature four distinct doublets (6H each) are observed for the four
inequivalent methyl groups of the coordinated -NiPr2 groups.
In addition, the vinylic hydrogens are observed as singlets at δ
4.46 and 3.63 ppm.
Comparison of the 13C NMR resonances arising from the
enolate ligands can provide some insight into bonding and
reactivity. Most notably, the difference in shift between the two
13C resonances arising from the carbon-carbon double bond,
∆δ(R - â), can be used to describe the degree of polarization
of the C-C bond.34 This value should correlate with the
nucleophilicity of the enolate. However, since ∆δ(R - â) is
In conclusion, we have isolated and fully characterized the
first series of zinc enolates derived from simple ketones. In all
cases, O-bridging structures were observed in the crystalline
state. NMR data suggest that these structures are maintained in
solutions of noncoordinating solvents. Chemical shift differences
observed for the enolate ligands indicate that Zn enolates have
C-C bond polarizations that are intermediate to those of related
Li and Si enolates. This polarization is also dependent on the
degree of steric bulk on the supporting ligands. The use of these
and related functionalized organozincs in a variety of bond-
forming processes, including polymerizations, is currently being
explored.
Acknowledgment. We thank the Petroleum Research Fund,
administered by the ACS, for funding. NMR instrumentation
used in this work was supported in part by the National Science
Foundation CRIF program, award #CHE-0131003.
(30) In contrast, the related Zn amide enolate MeLZn2[OC(CH2)NEt2]2
has overall C2 symmetry due to the presence of C,O-bridging [OC(CH2)NEt2]-
groups. This is observed as two inequivalent Me groups in the -NMe2
donors. See ref 19.
(31) Jackson, L. M.; Szeverenyi, N. M. J. Am. Chem. Soc. 1977, 99,
4954-4962.
(32) Carbon-13 NMR Spectroscopy, 3rd ed.; Breitmaier, E., Voelter, W.,
Eds.; Wiley-VCH: Weinheim, Germany, 1989; Table 3.6, p 137.
(33) The related dizinc alkoxide derivative iPrLZn2[OCHPh2]2 was
similarly found to undergo this type of reversible amine dissociation. See
ref 26c.
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.
OM060367Y
(34) Similar analysis of 13C NMR spectroscopic data has been performed
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(35) Eames, J.; Coumbarides, G. S.; Suggate, M. J.; Weerasooriya, N.
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