Solution and solid-state studies of a chiral zinc- sulfonamide complex relevant to enantioselective cyclopropanations

Article abstract of DOI:10.1002/(SICI)1521-3773(19980504)37:8<1149::AID-ANIE1149>3.0.CO;2-F

A highly distorted tetrahedron formed by the four nitrogen atoms around zinc in the crystalline zinc-sulfonamide complex 1 may explain its catalytic activity in asymmetric cyclopropanations. The agent is formed by deprotonation of (R,R)-N,N'-cyclohexane-1,2-diyl)bis(n-butanesulfonamide) with diethylzinc and addition of 2,2'-bipyridyl.

Full text of DOI:10.1002/(SICI)1521-3773(19980504)37:8<1149::AID-ANIE1149>3.0.CO;2-F

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1617, l ˆ 71.069 pm, Tˆ 293 K, m(MoKa) ˆ 0.462 mm 1, crystal dimen-
sions 0.4 Â 0.3 Â 0.25 mm3, 2 ꢀ 2q ꢀ 508; of 7153 measured reflections,
6525 were unique; 462 parameters refined in full matrix, R1 ˆ 0.039
with 3411 reflections (I > 3s(I)), wR2 ˆ 0.041, w ˆ 1, min./max.
residual electron density 290/480 enm 3. (Ph4P)-4: C42H35O6PSiW,
Mr ˆ 878.644, monoclinic, space group P21/c (no. 14); a ˆ 1089.2(2),
Solution and Solid-State Studies of a Chiral
Zinc ± Sulfonamide Complex Relevant to
Enantioselective Cyclopropanations**
Scott E. Denmark,* Stephen P. OꢁConnor,
and Scott R. Wilson
b ˆ 1744.0(4), c ˆ 1989.7(3) pm, b ˆ 100.05(1)8, Vˆ 3.722(2) nm3, Z ˆ
3
4, 1calcd ˆ 1.57 Mgm
,
F(000) ˆ 1747, l ˆ 71.069 pm, Tˆ 293 K,
m(MoKa) ˆ 3.29 mm 1, crystal dimensions 0.25  0.25  0.15 mm3, 2 ꢀ
2q ꢀ568; of 7144 measured reflections, 6519 were unique; 462
parameters refined in full matrix, R1 ˆ 0.032 with 4031 reflections (I >
3s(I)), wR2 ˆ 0.037, w ˆ 1, min./max. residual electron density 910/
790 enm 3. (Ph4P)2-5: C72H60O13P2Si2Mo2, Mr ˆ 1443.261, monoclinic,
space group P21/c (no. 14); a ˆ 1305.0(2), b ˆ 1560.2(3), c ˆ
Sulfonamide derivatives of chiral amines and diamines have
recently emerged as an important class of auxiliaries and
catalysts in enantioselective transformations.[1] Chiral sulfon-
amides serve admirably as ligands for a number of different
metals and have been employed in a wide variety of syntheti-
cally useful reactions, for example, cyclopropanation (Zn),[2]
addition of dialkylzinc to aldehydes (Ti),[3±6] Mukaiyama aldol
reaction (lanthanides),[7] enantiotopic group differentiation
(Li),[8] Diels ± Alder reactions (Al),[9] and enantioselective
allylation of aldehydes (B).[10] In each of these systems, it is
generally assumed that the metal is covalently bound to
sulfonamide nitrogen atoms. Indeed, Corey et al. have
provided an X-ray crystal structure of a chiral diazaalumino-
lidine prepared from N,N'-(1,2-diphenylethylene)bis(trifluoro-
methanesulfonamide) and trimethylaluminum, which clearly
shows that the aluminum atom is bound in just such a
manner.[11] Our interest in this area derives from the
demonstration that chiral bis(sulfonamide)s which, when
pretreated with diethylzinc, are effective catalysts for the
enantioselective cyclopropanation of allylic alcohols.[12] Here-
in we provide reaction data, elemental analyses, spectroscopy
studies, and an X-ray crystal structure in support of a catalyst
structure which, among other interesting features, contains a
zinc atom bound to both sulfonamide nitrogen atoms.
1676.9(6) pm, b ˆ 103.90(2)8, Vˆ 3.314(1) nm3, Z ˆ 2, 1calcd
ˆ
3
1.45 Mgm
,
F(000) ˆ 1469, l ˆ 71.069 pm, Tˆ 293 K, m(MoKa) ˆ
1.025 mm 1, crystal dimensions 0.5 Â 0.6 Â 0.2 mm3, 2 ꢀ 2q ꢀ568; of
8624 measured reflections, 7973 were unique; 503 parameters refined
in full matrix, R1 ˆ 0.045 with 4724 reflections (I > 3s(I)) and wR2 ˆ
0.047, w ˆ 1, min./max. residual electron density 640/870 enm 3. The
data were collected on a Enraf-Nonius CAD4 four circle diffractom-
eter (w-2q scans). All measurements were made at room temperature;
two standard reflections showed no significant variation in intensity.
Corrections were made for Lorentzian and polarization effects; an
extinction correction was also applied;[19] an empirical absorption
correction on the basis of Y scan data was introduced. The structures
were solved by direct methods (SHELXS-86 program)[20] and
subsequent Fourier difference techniques. All non-hydrogen atoms
were refined anisotropically. All hydrogen atoms were found on
difference Fourier maps; their positions were not refined and they
were given one overall isotropic thermal parameter. Refinements
were carried out by minimizing the function Sw( j FO
j
j FC j )2, where
FO and FC are the observed and calculated structure factors (program
CRYSTALS).[21] Crystallographic data (excluding structure factors)
for the structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Center as supplementary publica-
tion no. CCDC-100828. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
Although our previous studies have shown (R,R)-N,N'-
cyclohexane-1,2-diyl)bis(methanesulfonamide) (1) to be the
optimal cyclopropanation catalyst (Scheme 1), we selected
the analogous and equipotent bis(n-butanesulfonamide) 2 for
these structural studies owing to its superior solubility (e.g. in
[12] a) H. Mimoun in The Chemistry of Functional Groups, Peroxides (Ed.:
S. Pataï), Wiley, New York, 1983, chap. 15; b) K. A. Jùrgensen, Chem.
Rev. 1989, 89, 431 ± 458; c) A. Butler, M. J. Clague, G. E. Meister, ibid.
1994, 94, 625 ± 638; d) M. H. Dickman, M. T. Pope, ibid. 1994, 94,
569 ± 584.
[13] a) D. C. Crans, A. D. Keramidas, H. Hoover-Litty, O. P. Anderson,
M. M. Miller, L. M. Lemoine, S. Pleasic-Williams, M. Vandenberg,
A. J. Rossomando, L. J. Sweet, J. Am. Chem. Soc. 1997, 119, 5447 ±
5448; b) R. E. Drew, F. W. B. Einstein, Inorg. Chem. 1972, 11, 1079 ±
1083.
1
CH2Cl2: 1: ꢁ 1 mgmL 1 CH2Cl2; 2: >40 mgmL ).
We first established the chemical composition of the
catalyst, which according to earlier studies requires the
combination of 1 with diethylzinc.[12c] Therefore, a solution
of
2 in dichloromethane was treated with diethylzinc
(1.0 equiv, room temperature), and the solvent removed in
[14] a) R. Murugavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Chem.
Rev., 1996, 96, 2205 ± 2236; b) N. M. Rutherford, Dissertation,
University of California, Berkeley (USA), 1987.
Â
[15] J.-Y. Piquemal, L. Salles, C. Bois, J.-M. Bregeault, New J. Chem. 1998,
1) ZnI2
Ph
in press.
NHSO2R
Ph
(1.0 equiv)
Â
[16] J.-M. Bregeault, R. Thouvenot, S. Zoughebi, L. Salles, A. Atlamsani,
+
+
Et2Zn
2) Zn(CH2I)2
(1.0 equiv)
0 ˚C/15 min
E. Duprey, C. Aubry, F. Robert, G. Chottard, New Developments in
Selective Oxidation II (Eds.: V. Cortes Corberan, S. Vic Bellon),
NHSO2R
0.1 equiv
OH
HO
Á
Á
Â
1.1 equiv
3
Elsevier, Amsterdam, 1994, pp. 571 ± 581.
1 R=Me
2 R=n-Bu
99% (89% ee)
98% (84% ee)
[17] V. Conte, F. Di Furia, S. Moro, J. Mol. Catal. 1997, 117, 139 ± 149.
[18] G. I. Harris, J. Chem. Soc. 1963, 5978 ± 5982.
[19] A. C. Larson in Crystallographic Computing (Eds.: F. R. Ahmed, S. R.
Hall, C. P. Huber), Munksgaard, Copenhagen, 1970, pp. 291 ± 294.
[20] G. M. Sheldrick, SHELXS-86, Program for the Solution of Crystal
Structures, Universität Göttingen, 1985,
[21] D. T. Watkin, J. R. Carruthers, P. W. Betteridge, CRYSTALS, An
Advanced Crystallographic Program System 1989, Chemical Crystal-
lography Laboratory, University of Oxford.
[22] D. T. Watkin, C. K. Prout, L. J. Pearce, CAMERON 1996, Chemical
Crystallography Laboratory, University of Oxford.
Scheme 1. Bis(sulfonamide)-catalyzed enantioselective cyclopropanation.
[*] Prof. Dr. S. E. Denmark, S. P. OꢁConnor, Dr. S. R. Wilson
Department of Chemistry
University of Illinois
Urbana, IL 61801 (USA)
Fax: (‡1)217-333-3984
E-mail: sdenmark@uiuc.edu.
[**] We are grateful to the Pharmacia and Upjohn Company for generous
support. S.P.O. also thanks Chevron, the University of Illinois, and the
Deptartment of Energy for Fellowships.
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vacuo. The resulting off-white powder provided a correct
elemental analysis (C, H, N, S, Zn)[13] for the formula
C14H28N2O4S2Zn, which corresponds to the simple zinc salt
of 2 (i.e., [Zn2‡(2 2H)2 ]). That deprotonation at the
sulfonamide nitrogen atom had occurred was confirmed by
1H NMR spectroscopy. A comparison of the spectra acquired
before and after the addition of diethylzinc (1.0 equiv) to 2 in
CDCl3 showed clearly that the sulfonamide NH protons (d ˆ
4.78) had disappeared (Figure 1a and b). Interestingly, the
SO2n-Bu
NH
1) ZnI2
(1.0 equiv)
N
N
Ph
Ph
+
+
+
Et2Zn
2) Zn(CH2I)2
(1.0 equiv)
NH
HO
HO
SO2n-Bu
1.1 equiv
3
2 (0.1 equiv)
0.1 equiv
76-84% ee
Scheme 2. Cyclopropanation with 2 in the presence of bipy.
(0.1 equiv) was formed separately and then treated with bipy
(0.1 equiv). The solution of this preformed ternary complex
([Zn2‡(2 2H)2 (bipy)]) was then used for the cyclopropa-
nation of cinnamyl alcohol. The product was obtained with a
slightly lower enantiomeric excess (76% ee). Taken together,
these experiments reveal that [Zn2‡(2 2H)2 ] retains its
catalytic activity upon complexation with bipy, and that bipy is
a weakly competitive ligand for the catalyst in place of the
substrate and reagents in the cyclopropanation.
The most compelling evidence concerning the nature of
the zinc ± sulfonamide species is provided by the
solid-state structure (Figure 2).[14] Single crystals of
[Zn2‡(2 2H)2 (bipy)] suitable for X-ray diffraction analysis
Figure 1. 1H NMR spectra of a) 2, b) 2 ‡ Et2Zn (1.0 equiv), c) 2 ‡ Et2Zn
(1.0 equiv) ‡ bipy (1.0 equiv).
remaining signals became very broad, which suggested that
the zinc ± sulfonamide species was aggregated. Furthermore,
the solution had gelled within 15 min of the addition of
diethylzinc, thus providing additional evidence that the zinc ±
sulfonamide species was strongly self-associating. The gel
could be solubilized by the addition of pyridine (2 equiv),
1,10-phenanthroline (1 equiv), or 2,2'-bipyridyl (bipy,
1
1 equiv). The H NMR spectrum of [Zn2‡(2 2H)2 ] which
had been treated with one equivalent of bipy displayed very
sharp resonances (Figure 1c). Presumably, the bidentate
ligand was capable of disrupting the intermolecular zinc ±
sulfonamide interactions to give a monomeric complex.
Having demonstrated that deprotonation of 2 to form a zinc
complex proceeds efficiently under the chosen reaction
conditions, we next sought to establish that [Zn2‡(2 2H)2 ]
is a kinetically significant species. Therefore, the zinc ±
sulfonamide complex was formed from 2 (0.1 equiv) and
diethylzinc (0.1 equiv) under the same conditions as for the
NMR experiment, and this solution was then added to
ethylzinc cinnamyl alcoholate followed by the addition of
zinc iodide (1.0 equiv) and di(iodomethyl)zinc (1.0 equiv).
The product cyclopropane 3 was formed rapidly with 83% ee.
This verified that the active form of the sulfonamide must be
the deprotonated species produced upon treatment of 2 with
diethylzinc.
Figure 2. SHELXTL representation of the crystal structure of
[Zn2‡(2 2H)2 (bipy)] (35% thermal ellipsoids). Selected bond lengths
[Š] and angles [8]: Zn(1) N(1) 1.942(8), Zn(1) N(8) 2.047(8), S(1) N(1)
1.569(7), S(1) O(1) 1.461(8), S(1) O(2) 1.446(8); N(1)-Zn(1)-N(1A)
85.8(4), N(8)-Zn(1)-N(8A) 80.0(5), N(1)-Zn(1)-N(8) 124.8(3), N(1)-Zn(1)-
N(8A) 123.4(3), Zn(1)-N(1)-S(1) 119.8(4); torsional angles [8]: C(4)-S(1)-
N(1)-Zn(1) 119.0(10), S(1)-C(4)-C(5)-C(6) 161(2).
could be obtained by treatment of racemic 2 in CHCl3 with
diethylzinc (1.0 equiv) and then bipy (1.0 equiv) at room
temperature followed by cooling to 20 to 258C.[15] The
compound crystallized as a tris(chloroform) solvate.
In addition, we also demonstrated that the zinc ± sulfon-
amide moiety remains intact and catalytically active upon
complexation with bipy. Two different protocols were used to
generate this species (Scheme 2). In the first reaction, all three
components (cinnamyl alcohol, 2, and bipy) were combined
prior to treatment with diethylzinc. The product was obtained
with the same selectivity (84% ee) as in the absence of bipy
(see Scheme 1). In the second reaction, [Zn2‡(2 2H)2 ]
The crystal structure confirmed that the zinc atom is
bonded to both sulfonamide nitrogen atoms. The Zn ± N bond
lengths are both 1.942(8) Š, as the molecule resides on a
twofold symmetry axis. The distances between zinc and the
bipy nitrogen atoms are 2.047(8) Š; the four nitrogen atoms
around zinc form a highly distorted tetrahedron. The sulfon-
amide N-Zn-N angle is contracted from the ideal tetrahedral
angle of 109.58 to 85.8(4)8, whereas the N-Zn-N angle for the
1150
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analysis: calcd for C14H28N2O4S2Zn1 (417.89): C 40.24, H 6.75, N 6.70, S
15.34, Zn 15.65; found: C 40.39, H 6.91, N 6.66, S 14.88, Zn 15.83.
bipy ligands is 80.0(5)8 owing to the long Zn ± N bonds in each
of the two five-membered rings. Another noteworthy feature
is the pyramidality of the sulfonamide nitrogen atoms (sum of
angles at N 352.38). This places the sulfonyl groups in pseudo-
equatorial orientations and clearly results from the minimi-
zation of nonbonded interactions with the cyclohexane back-
bone. The differential disposition of the sulfonamide groups
plays a critical role in our model for asymmetric induction.[12c]
The chelation of zinc by the Lewis basic bipy nitrogen
atoms suggests that the active catalyst may function in
solution as a divalent Lewis acid. With our transition state
model[12c] for the enantioselective cyclopropanation, we
propose that the coordination sites at the zinc atom of
[Zn2‡(2 2H)2 ] are occupied by the oxygen atom of the
alkoxide and the zinc-bound iodine atom of the reagent,
iodomethylzinc iodide. The crystal structure of [Zn2‡(2
2H)2 (bipy)] and the data described herein provide addi-
tional support for this dual mode of action of a zinc-containing
Lewis acid. The simultaneous coordination of two reactive
species, namely the substrate ethylzinc alkoxide and the
reagent iodomethylzinc iodide, by the zinc atom in [Zn2‡(2
2H)2 (bipy)] implies that it acts as a focal point, or organiza-
tional center, and at the same time enhances the reactivity of
the reagent by virtue of the electron-deficient and geometri-
cally distorted zinc atom.
[Zn2‡(2 2H)2 (bipy)]: To a solution of 2 (355 mg, 1.00 mmol) in CHCl3
(8.0 mL) in a 35-mL two-neck round-bottom flask fitted with a rubber
septum and a ground glass gas inlet adapter but no stirbar was added
diethylzinc (104 mL, 1.02 equiv) at room temperature. This was done while
gently swirling the flask to mix the contents. After about 10 min the
solution became very viscous.
A solution of 2,2'-bipyridyl (156 mg,
1.00 equiv) in CHCl3 (2.0 mL) was added, and the flask swirled to mix
the contents. The solution was no longer viscous upon complete addition of
the 2,2'-bipyridyl solution. After cooling at 20 to 258C in a freezer, in
the absence of light, a large amount of colorless, long, thick needlelike
crystals had formed. The crystals could be redissolved at 08C and reformed
upon subsequent cooling to 20 to 258C for several hours. A single
crystal was removed and quickly mounted in a stream of N2 at 708C.
Received: October 22, 1997 [Z11064IE]
German version: Angew. Chem. 1998, 110, 1162 ± 1165
Keywords: asymmetric catalysis
sulfonamides ´ zinc
´
cyclopropanations
´
[1] a) J. Seyden-Penne, Chiral Auxiliaries and Ligands in Asymmetric
Synthesis, Wiley-Interscience, New York, 1995, p. 57-67; for corre-
sponding ligands, see b) H. Brunner, W. Zettlmeier, Handbook of
Enantioselective Catalysis, Vol. 2, VCH, Weinheim, 1993.
[2] a) S. Kobayashi, H. Takahashi, M. Yoshioka, M. Ohno, Tetrahedron
Lett. 1992, 33, 2575; b) S. Kobayashi, H. Takahashi, M. Yoshioka, M.
Ohno, M. Shibasaki, N. Imai, Tetrahedron 1995, 51, 12013.
[3] M. Yoshioka, T. Kawakita, M. Ohno, Tetrahedron Lett. 1989, 30, 1657.
[4] M. Ohno, M. Yoshioka, S. Kobayashi, H. Takahashi, T. Kawakita,
Tetrahedron Lett. 1989, 30, 7095.
The structure of the active species in any catalytic process is
difficult to guarantee. This is particularly true for a hetero-
geneous reaction of this complexity with many components,
and [Zn2‡(2 2H)2 ] may represent a precatalyst. Never-
theless, taken together with the results from previous studies,
we can now formulate a more coherent picture for the nature
of the substrate[12c] as well as the structures of the reagent[12d]
and the catalyst in these enantioselective cyclopropanations.
[5] S. Kobayashi, M. Ohno, M. Yoshioka, T. Takahashi, T. Kawakita,
Tetrahedron 1992, 48, 5691.
[6] P. Knochel, R. Singer, Chem. Rev. 1993, 93, 2117.
[7] M. Shibasaki, K. Uotsu, H. Sasai, Tetrahedron: Asymmetry 1995, 6, 71.
[8] T. Kunieda, H. Imado, T. Ishizuka, Tetrahedron Lett. 1995, 36, 931.
[9] E. J. Corey, R. Imwinkelried, S. Pikul, Y. B. Xiang, J. Am. Chem. Soc.
1989, 111, 5493.
[10] E. J. Corey, C.-M. Yu, S. S. Kim, J. Am. Chem. Soc. 1989, 111, 5495.
[11] E. J. Corey, S. Sarshar, J. Bordner, J. Am. Chem. Soc. 1992, 114, 7938.
[12] a) S. E. Denmark, B. L. Christenson, D. M. Coe, S. P. OꢁConnor,
Tetrahedron Lett. 1995, 36, 2215; b) S. E. Denmark, B. L. Christenson,
S. P. OꢁConnor, ibid. 1995, 36, 2219; c) S. E. Denmark, S. P. OꢁConnor,
J. Org. Chem. 1997, 62, 584; d) ibid. 1997, 62, 3390.
Experimental Section
2: To a solution of (R,R)-1,2-cyclohexanediamine (1.097 g, 9.6 mmol) and
triethylamine (2.92 g, 28.9 mmol) in CH2Cl2 (30 mL) at 08C was added n-
butanesulfonyl chloride (3.92 g, 25.0 mmol). The reaction mixture was
warmed to room temperature and stirred for 12 h. The mixture was then
cooled to 08C, and aqueous H2SO4 (3.0n, 30 mL) was added. Extraction of
the aqueous layer with CH2Cl2 (3 Â 20 mL), drying of the combined
extracts (MgSO4), and in vacuo concentration followed by column
chromatography (silica gel, CH2Cl2/CH3OH 19/1) and crystallization from
hexane/CH2Cl2 provided 2.98 g (8.4 mmol, 87%) of 2 as a white crystalline
material. M.p. 137.0 ± 138.08C; 1H NMR (500 MHz): d ˆ 4.78 (d, J ˆ 6.8,
2H, NH), 3.06 (m, 6H, HC(1), H2C(4)), 2.15 (d, J ˆ 12.4, 2H, HeqC(2)), 1.79
(m, 6H, HaxC(2), H2C(5)), 1.46 (m, 4H, H2(6)), 1.32 (m, 4H, H2C(3)), 0.95
(t, J ˆ 7.3, 6H, H3C(7)); 13C NMR (125 MHz): d ˆ 57.42 (C1), 53.84 (C4),
34.58 (C2), 25.60 (C5), 24.63 (C3), 21.54 (C6), 13.59 (C7); MS (EI): m/z
[13] Elemental analysis calcd for C14H28N2O4S2Zn (417.89): C 40.24, H 6.75,
N 6.70, S 15.34, Zn 15.65; found: C 40.39, H 6.91, N 6.66, S 14.88, Zn
15.83.
[14] For a recent compilation of organozinc crystal structures, see a) P.
OꢁBrien in Comprehensive Organometallic Chemistry II, Vol. 3 (Eds.:
E. W. Abel, F. G. A Stone, G. Wilkinson), Pergamon Press, Tarrytown,
1995, chap. 4; for other sulfonamide structures in which zinc is bound
to ntirogen atoms, see b) S. Z. Haider, K. M. A. Malik, M. B.
Hursthouse, S. Das, Acta Crystallogr. Sect. C 1984, 40, 1147; c) U.
Hartmann, H. Vahrenkamp, Inorg. Chem. 1991, 30, 4676.
[15] Crystal structure analysis of [Zn2‡(2 2H)2 (bipy)]: A single crystal
grew from a solution in CHCl3 at 258C. C30H42Cl18N4O4S2Zn, Mr ˆ
1290.27, crystal size 0.3  0.3  1.2 mm, a ˆ 20.8189(6), b ˆ 16.6984(5),
‡
(%): 354 (M , <1), 96 (100); IR: nÄ ˆ 3279 (s), 2957 (s), 2943 (s), 2922 (s),
c ˆ 15.6761(3) Š,
b ˆ 100.2910(10)8,
Vˆ 5362.0(2) Š3,
1calcd
ˆ
1
1451 cm (s); TLC Rf 0.30 (CH2Cl2/CH3OH, 49/1); elemental analysis
1.598 gcm 3, m ˆ 1.470 mm 1, empirical absorption correction, Z ˆ 4,
monoclinic, space group C2/c, Siemens three-circle platform diffrac-
tomer with Mo irradiation (MoKa ˆ 0.71073 Š) and a CCD area
detector, Tˆ 198 K; of 9873 reflections measured( Æ h, Æ k, Æ l), 3493
were independent and 2458 observed with I > 2s(I); R ˆ 0.101 wR ˆ
0.271 (against j F2 j ), residual electron density 1.57eŠ 3; programs
used: SHELXS-86, SHELXL-93. Crystallographic data (excluding
structure factors) for the structure reported in this paper have been
deposited with the Cambridge Crystallographic Data Center as
supplementary publication no. CCDC-100967. Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
calcd for C14H30N2O4S2 (354.53): C 47.43, H 8.53, N 7.90, S 18.09; found: C
47.69, H 8.49, N 7.97, S 18.04.
[Zn2‡(2 2H)2 ]: To a solution of 2 (753 mg, 2.12 mmol) in CHCl3 (20 mL)
at 08C was added diethylzinc (218 mL, 1.00 equiv), and the solution
allowed to warm to room temperature. After about 10 min the solution
became somewhat viscous, and a white, powdery precipitate formed. The
contents of the flask were cooled in a liquid N2 bath, and a vacuum
( ꢁ 0.1 mm) was applied overnight. As the liquid N2 was allowed to
evaporate, the solvent was distilled into a liquid N2 trap to leave an off-
white powdery solid. This material was sealed under vacuum and trans-
ferred to a dry box. Samples were weighed and sealed in tin capsules for
C,H,N elemental analysis and in aluminum capsules for S and Zn elemental
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