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An Ab Initio MO Study on Stabilizing Interactions in Dinuclear ZrAl Complexes with a Planar Tetracoordinate Carbon Center.

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without significant loss of optical purity (ee > 96.6%). This
simple separation providing both enantiomers in high yield
and optical purity can be performed in the laboratory easily
with amounts between 0.5 and 1 kg per run.
analyzed by gas chromatography (Chiral-XE-60-S-Val). The enantiomer ratio
found was S : R = 98.3:1.7
Received: December 12. 1992 [Z5750IE]
German version: AngeH,. Cbem. 1993, 105, 785
111 P. C. Belanger. H. W. R. Williams, Con. J. Chem. 1983, 61, 1383-1386.
[21 0. Cervinka, 0. Bajanzulyn. A. FBbryovP. A. Safkus, Collect. Czech.
Cl7em. Commun. 1986, 51. 404-407.
13) H. Nodaira, 0. Osada, M. Kondo, A. Takebayashi, JP-P 01 216962,1989,
Chem. Ahsrr. 1990. 112, 158037h.
141 a) G . Claeson, H.-G. Jonsson, Ark. Kemi 1967.26.247-257; b) G . Stork,
A. F. Kreft 111. J. Am. Cliem. SUC.1977. 99, 3851 -3853.
[S] a) H. Merz. K. Stockhaus. H. Wick, J. Med. Chem. 1977. 20. 844-846;
b) V. Valenta, J. Holubek, E. Svatek, V. Miller, M. Vlkova, M. Protiva,
Cullcxt. Czech. Chem Cornmiin. 1987. 52, 2534-2544.
[6] Reviews: a) D. Descours. D. Festal, J. M. Leger, A. Carpy. Nelv. Chim.
Acfa 1991.74.1757 1763; b) 2. L. Chang, J. F. Bauer, Anal. Profiles Drug
Suhst. 1991. 20, 693-727; c) S. Titmarsh, J. P. Monk, Drugs 1987. 33,
461 - 477.
171 W. Hemmerling. H.-R. Diibal, C. Escher, G. Man, Y Inoguchi, I. Miiller,
M. Murakami. D. Ohlendorf, R. Wingen (Hoechst AG) DE-A 3827600,
1990, C h e m Ahstr. 1990, 113, P06578m.
181 (S)-Wine was obtained from Degussa. The optical purity of the methyl
ester was determined by gaschromatography[l3] after it had been converted into the trifluoroacetate.
191 Menthyl, bornyl, fenchyl, and 2-butyl2-tetrahydrofurancarboxylates
a number of (S)-lactates were examined.
[I01 a) K. Freudenberg. W. Kuhn, I. Bumann, Bpi. Dfsch. Chem. Ges. 1930.63,
2380-2390; b) R. Schwyaer, B. Iselm, H. Kappeler. B. Riniker, W. Rittel,
H. Zuber. Helv. Cbim. Acru 1958, 41. 1272.
1111 D. J. Pasto. M. P. Serve. J. A m . Chem. Soc. 1965, 87, 1515-1521.
[12] a) D. M. Roush, E. M. Prie. L. K. Templeton, D. H. Templeton, C. H.
Heathcock. J. Am. Cliem. So(. 1979, lUf, 2971-2981; b ) G . B. Brown,
C. W. H. Partridge. hid. 1944, 66. 839. c) H. T. Clarke, E. R. Taylor, Orgunk Synrheses, Collective Volume I, 2nd ed.. Wiley. New York, 1941,
pp. 115-116.
1131 Chiral-XE-60-S-Val, 50 m, quartz capillary (Chromopdk, Munich).
IRI-1 or
IS)- 1
Scheme 1
As shown in Table 1, the methyl and ethyl esters of ( S ) alanine and (S)-leucine can be used instead of (S)-valine
derivatives. Besides the heterocyclic carboxylic acids 1 a-t
previously known in optically active form, the two enantiomers of 2-tetrahydrothiopyrancarboxylicacid 1d (X = S,
n = 2) could be isolated by this procedure. Of course, the
optical purity achieved by the separation of diastereomers
can be improved by increasing the reflux ratio and the number of theoretical plates. The effort required for the distillation can be estimated from the difference in the GC retention
In contrast to the amides described here, the diastereomeric esters prepared from heterocyclic carboxylic acids and
various optically active alcohols[91 have minimal boiling
point differences ( < 1 K). This illustrates that in analogy to
resolution by fractional crystallization, various derivatives
must be tested to determine whether they are suitable for
separation by distillation. In contrast to the traditional
method, however, simple GC analysis of the product mixture
gives a quick and reliable assessment of the separation, as the
difference in the retention times reflects the difference in
boiling points.
Experimental Procedure
Heterocyclic carboxylic acid amides 2: To a stirred suspension of 0.98 mol
(S)-amino acid ester hydrochloride[l0] in 350 mL toluene heated a t 50-7O‘C
was added dropwise 0.89 mol heterocyclic carboxylic acid chloride 3 (obtained
from carboxylic acids l a , lb[4a], Ic[ll], or l d [ l 2 ] by reaction with thionyl
chloride). The reaction mixture was stirred and heated at this temperature until
the evolution of HCI had ceased (ca. 20 h). The reaction mixture was washed
with water and distilled. Representativeyie1ds:Za 94%. Za”87%, 2 b 79%, 2 c
80%, 2d 73% (R = CH(CH,),, R = CH,). The diastereomeric pairs were
separated by distillation, see Table 1
(R)-1a: A mixture of 100 g (0.437 mol) (R,S)-Za (diastereomer ratio
= 98.7:1.3) and 3 g (22 mmol) ZnCI, in 400 mL 1 M HCI was heated a t 100 C for 4 d. The reaction mixture was concentrated to dryness and the
residue extracted with methyl terr-butyl ether (MTBE). The filtered MTBE
extract was distilled giving 36 g (78%) (R)-1 a with b.p. 83 ‘C/O.3Torr (ref.[lO]
b.p. 97-1OO’C/1.05Torr); [a];’ = + 33.3 ( ~ = 1 . 2 3 ,CHCI,; ref.[l]: -30.1
( c = 1.21. CHCI,) for the (S) enantiomer), [I]&’
33.5, [a]:’ = 35.1
( c =1.07. CHCI,; ref.111: + 30.4 ( c = 1.01. CHCI,)). For the determination of
enantiomer ratio the product was reduced with LiAIH, to give the tetrahydrofurfurylalcohol which was then analyzed by gas chromatography[l3]. The ratio
found was R : S = 98.7: 1.3. The residue filtered off the MTBE phase consisting
of (5’)-valine hydrochloride was dried in V ~ C U Oand weighed (43 g, 70% recovery). For the determination of the optical purity of the recovered valine a
portion was treated with methanol/HCl and trifluoroacetic anhydride and then
f> VCH Erlogsgesellscbafi mhH. W-6940 Weinheim, 1993
An Ab Initio MO Study on Stabilizing Interactions
in Dinuclear Zr/Al Complexes with a Planar
Tetracoordinate Carbon Center**
By Rolf’ Gleiter,* Isabella Hyla-Kryspin, Shugiang Niu,
and Gerhard Erker
In their seminal work, Hoffmann et al. considered the possibility of stabilizing planar
They concluded that
the distortion of tetrahedral methane to the square-planar
CH, isomer leaves only six electrons available to form four
C-H bonds and a doubly occupied 2p orbital at the carbon
center (Scheme 1). This high-energy structure can be stabilized by o-donor and x-acceptor substituents X. These
thoughts were substantiated by Schleyer et al. by a b initio
calculations.[21Only a few structures have been determined
so Far in which a carbon atom is surrounded by four substituents in a single
Scheme 1.
Prof. Dr. R. Gleiter, Dr. I. Hyla-Kryspin, S.Niu
Organisch-chemisches Institut der Universitat
Im Neuenheimer Feld 270. D-W-6900 Heidelberg (FRG)
Telefax: Int. code+(6221) 564205
Prof. Dr G. Erker
Organisch-chemisches Institut der Universitit
Corrensstrasse 40, D-W-4400 Miinster (FRG)
This research was supported by the Volkswagen-Stiftung, the Deutsche
Forschungsgemeinschaft (SFB 247). the Fonds der Chemischen Industrie,
and BASF AG.
0570-0833193/0505-0754 $ 10.00f.25/0
Angew. Cbem. Inr. Ed. Engl. 1993, 32, No. 5
Prompted by the experimental work of Erker et aLL4]on
zirconocene complexes such as 1 we present here a theoretical model based on ethylene. Bending a =CH, unit of
ethylene into a T-shape (Fig. 1) stabilizes the C-C (3 orbital
but destabilizes two C-H G orbitals (denoted as P + and P-).
To test our qualitative arguments presented for a distorted
ethylene molecule as a model for constructing complexes
containing planar tetracoordinate carbon and to probe the
acceptor and donor capabilities of a ZrCp, moiety we have
carried out model calculations on the neutral dimetallic complexes 4-8.
E lev1
H P - H
-1 3
-1 4
-1 6
6: X=AI
7: X=B
The calculations were carried out with the Gaussian 86
programL8' using STO-3G basis sets for all atoms (4-7)[91
and a split valence basis set for Zr[Io1in the case of 8. In
addition we also carried out Extended Huckel (EH) calculations[L'lon these systems using standard parameters for all
atoms.'I21 In our model calculations we replaced the C p
groups of the real molecules by chloro ligands which has
been shown in other studies to provide a good theoretical
substitute for the actual bent metallocene system.r131
b: X=AI
5: XB
€?= - 4 5 1 4 . 6 0 0 2
Fig. 1. Correlation diagram for the protonation ofdistorted T-shaped ethylene.
As a result this distorted ethylene is about 2 eV higher in
energy than the normal structure. When a suitable 0-acceptor orbital is offered (e.g. the Is orbital of a proton, Fig. 1 )
the highest occupied CY MO may be stabilized considerably.
Further stabilization may be achieved by introducing an additional z-acceptor substituent. A group that fulfills both
conditions is a do zirconocene fragment, as it provides both
G- and x-accepting energy levels.[51
Our previous investigations on the nature of agostic interactions of alkenyl moieties bound to a Cp,ZrX (2) o r a
Cp,Zr+ (3) fragment, respectively, have shown that in the
case of 2 a specific combination of steric and electronic effects must be met[61for an agostic interaction, whereas in the
case of 3 electronic factors dominate.[']
154 29
6 ET=
7 Er=
- 4 5 1 4 7087
la: X=H
lb: X=CI
A n g m C h n . Inr. Ed. Engl. 1993, 32. Nu. 5
E r = . -5202.2674
8a ET-
- 5 2 0 2 2124
Fig. 2. Geometrical parameters and total energies [Hartree] for 4 -8.
ii", VCH Verlu~.~gesrll.~lhufr
mbH, W-6940 Weinhein?,1993
0570-0H33/93/0505-0755$ /O.OO+ .Z5/0
The fully optimized geometries of 4-8 are shown in Figure 2. The “inside” structures 6 and 7 were found to be
more stable than the “outside” congeners 4 and 5 by 91 and
68 kcal mol- I , respectively. The geometrical parameters calculated for 6-8 are remarkably close to those reported from
X-ray investigations on
The geometry optimization under the constraint that both
olefinic carbon atoms maintain normal sp2 hybridization
but leaving all other parameters relaxed, leads to the “standard structures” 6a-8a. As an example, we have depicted
the constrained structure of 8a at the bottom of Figure 2.
The standard structures are 46.4 (6a), 47.5 (7a), and 34.0
(8a) kcalmol-’ less stable than 6-8. In order to find out
why the geometries of structures 6-8 are preferred, we calculated the differences in the overlap populations AOP and the
differences in the electron densities Aq of 6-8 and 6a-8a
(Table 1). An analysis of the data shows that the structures
Table 1. Differences in the overlap populations (AOP) and in the electron densities (Aq) of fully optimized (6-8) and standard structures (6a-8a).
0 1981
0 0585
0 0901
0 0597
-0 1235
-0 1459
Aq* Pal
[a] Positive sign of Aq means an increase in the electron density
with the square-planar carbon atom (6-8) are stabilized by
a strong Zr-C, bonding interaction and to lesser extent by
Zr-Al/B interactions. This can be visualized by comparison
of the contour plots of the HOMO-I of 8 and the HOMO of
its congener 8 a (Fig. 3). The MO plots are based on EH
calculations on 8 with application of experimental X-ray
structure data, and on 8a with sp2-hybridized C, and C,.1141
It is evident that there is a strong o-bonding interaction
between the 4d orbital at Zr and the 2p orbital at C, in 8.
This interaction is absent in 8a. Our analysis shows furthermore that any direct rc(C,)41,(Zr) interaction is negligible,
although some Zr-C, R interaction may be present.’3s41
To conclude we can say that the planar tetracoordinate
geometry at the C, center of 1 is based on electronic effects.
It is not stabilized by additional (C,-Zr) delocalization of the
olefinic n electrons or the good donor properties of both cis
metal substituents. The major factors contributing to its stabilization are the “in-plane’’ o-acceptor property of the Zr
center which stabilizes the high-lying P- MO of the distorted
ethylene, and the good donor properties of C,, Al, and R .
Received: December 11.1992 [Z 5744IEl
German version: Angew. Chem. 1993, 105, 753
[I] R. Hoffmann. R. W. Alder, C. F. Wilcox Jr. J. Am. Chem. So<. 1970, 92,
4992; R. Hoffmann Pure Appl. Chem. 1971,28, 181.
[2] J. B. Collins. J. D. Dill, E. D. Jemmis, Y. Apeloig, P. von R. Schleyer, R.
Seeger, J. A. Pople, J. Am. Chem. Soc. 1976, 98, 5419;
131 Reviews: G. Erker, Nuchr. Chem. Tech. Lub. 1992, 40, 1099: G. Erker
Comments Inorg. Chem. 1992, 13, 111.
[41 G. Erker, R. Zwettler, C. Kriiger. R. Noe, S. Werner J. Am. Chem. SOC.
1990, 112,9620;G. Erker, M. Albrecht, S. Werner, M. Nolte, C. Kriiger,
Chem. Ber. 1992, 125, 1953; G. Erker, M. Albrecht, C. Kriiger, S. Werner
Organometollics 1991, 10, 3791; G. Erker. M. Albrecht, C. Kriiger, S.
Werner, P. Binger, F. Langhauser ibid. 1992, 11, 3517; M. Albrecht, G
Erker, M. Nolte, C. Kriiger J. Orgunomet. Chem. 1992,427, C1; G. Erker,
M. Albrecht, C. Kriiger. S. Werner J. Am. Chem. Soc. 1992, 114, 8531;
A. D. Horton, A. G. Orpen Angen. Chem. 1992,104,912; Angew. Chem.
In/. Ed. Engl. 1992, 31, 876.
[ S ] J. W. Lauher, R. Hoffmann J. Am. Chem. Soe. 1976, 98, 1729
[6] G. Erker, R. Zwettler. C . Kriiger, R. Schlund, 1. Hyla-Kryspin, R. Gleiter
J. Organomet. Chem. 1988, 346, C15; G. Erker, R. Zwettler, C. Kriiger, I.
Hyla-Kryspin, R. Gleiter Ogonomerullics 1990, 9, 524; I. Hyla-Kryspin,
R. Gleiter, C. Kriiger. R. Zwettler, G. Erker Organometallics1990,9, 517.
171 R. Gleiter, I. Hyla-Kryspin, S. Niu, G. Erker, unpublished.
[XI M. J. Frisch. J. S. Binkley, H. B. Schlegel, K. Raghavachari, C. F. Melius,
R. L. Martin, J. J. P. Stewart, F. W. Bobrowicr, C. M. Rohlfing, L. R.
Kahn. D. J. Defrees, R. Seeger, R. A. Whiteside, J. D. Fox, E. M. Fleuder.
J. A. Pople. GAUSSIAN 86, Carnegie Mellon Quantum Chemistry Publishing Unit, Pittsburgh, PA 1984.
[9] W. J. Hehre, L. Radom, P. von R. Schleyer, J. A. Pople, Ab initio Molecular
Orbital Theorv. Wiley, 1986; S. Huzinaga, J. Andzelm, M.Klobukowski,
E. Rddzio-Andselm, Y. Sakai, H. Tatewaki, Gaussian Basis Sets For
Molecular Calculations. Elsevier, Amsterdam, 1984.
1101 K. D. Dobbs, W. J. Hehre J. Comput. Chem. 1987, 8, 880.
[ l l ] R. Hoffmann. W. N. Lipscomb J. Chenz. Plzys. 1962, 36, 2179; 1962, 37,
2872; R. Hoffmann, ibid. 1963, 39, 1397.
[12] R. H. Summerville. R. Hoffmann J. Am. Chem. Soc. 1976, 98, 7240; K.
Tatsumi. A. Nakamura, P. Hofmann, P. Stauffert, R. Hoffmann ibid.
1985. 107,4440
[13] N. Koga, K. Morokuma Chem. Rev. 1991, 91, 823 and references therein;
H. Kawamura-Kuribayashi, N. Koga, K. Morokuma J. A m Chem. Soc.
1992,114.2359; L. A. Castonguay, A. K. Rappe J. Am. Chem. Soc. 1992,
114, 5832.
[14] EH calculations predict that 8 is 13.9 kcalmol-’ more stable than 8a.
Phosphinidenetantalu(v) Complexes of the Type
[ (N,N)Ta=PR] as Phospha-Wittig Reagents
(R = Ph, Cy, tBu; N,N = (Me3SiNCH2CH2),N)**
By Christopher C. Curnmins, Richard R. Schrock,*
and William M . Davis
We wish to report that terminal phosphinidene complexes
of the type [(N,N)Ta=PR] (R = Ph, c-C,H,,(Cy), tBu;
N,N = (Me,SiNCH,CH,),N) are readily prepared and, furthermore, that these complexes react smoothly with aldehydes
to generate phosphaalkenes. Phosphaalkenes[” stabilized either by conjugation[31or by sterically demanding substitu[‘I
Fig. 3. Contour plots of the HOMO of 8 a (standard structure, left), and the
HOMO-1 of 8 (planar structure, right). The values of the contour lines are
t 0.01, k 0.02. f 0.04, k 0.06, t 0.10. k 0.22, t 0.30.
0 VCH Verlugsgesellschafi mbH,
W-6940 Welnheim, 1993
Prof. Dr. R. R. Schrock, C. C. Cummins, Dr. W. M. Davis“’
Department of Chemistry
Massachusetts Institute of Technology
Cambridge, MA 021 39 (USA)
Telefax: Int. code +(617) 253-7670
[‘I X-ray structure analysis.
[**I This work was supported by the National Science Foundation (Grant
CHE 91 22827). C . C . C . thanks the National Science Foundation for a
Predoctoral Fellowship. The term phosphinidene complex is used here
instead of that recommended by IUPAC. A’-phosphanediyl complex.
0570-0833/93jOSO5-0756$10.00 + ,2510
Angew. Chem. I n ! . Ed. Engl. 1993, 32, No. 5
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