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Effect of Torsional Strain and Electrostatic Interactions on the Stereochemistry of Nucleophilic Additions to Cyclohexanone and Related Systems.

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[9] The "BNMR signal (CD,Cl,, BF,-etherate as external standard) of the
host 2b (a = 30) is shifted upfield (6 = 9) by the addition of benzylamine.
In the l3CNMR spectrum of the adduct 9, the signals of the benzylic C
atoms, for example, are shifted down field, cf. [4].
[lo] Crystallographic data for 9 . OStoIuene (C,,H,,BNO, . 0.5C,H8. M ,
567.5): triclinic, space group PT [no. 21, a = 10.648(1), b = 11.760(1), c =
13.755(2)A, a = 67.66(1), =73.60(1), 7 = 87.29(1)", Z = 2 , pcalEd
=
1.24 gcm-3, p(MoK,) = 0.80 cm-'. Measurements weremade withan Enraf-Nonius CAD4 diffractometer (Mo,, radiation, = 0.71069 A, graphite
monochromator, T = 297 K); 9275 reflections, 8825 independent, 5907
with I > 2 4 1 ) considered as observed. Solution by direct methods and
refinement with SHELX-76, R = 0.074, R , = 0.094 (M' =1/u2(F)); all
non-hydrogen atoms isotropic; hydrogen atoms attached to N and 0
atoms localized and refined (isotropic), the others calculated and fixed.
Further details of the crystal structure determination are available on
request from the Fachinformationszentrum Karlsruhe, Gesellschaft for
wissenschaftlich-technische Information mbH, D-W-7514 EggensteinLeopoldshafen 2 (FRG), on quoting the depository number CSD-56138,
the names of the authors, and the full journal citation.
[I 11 For the generation of the space-filling presentation given in Figure 1, the
crystallographic data were employed on a Silicon Graphics computer.
[I21 The equilibrium constants were obtained mathematically by a nonlinear
tn'o parameter fit of chemical shift (6) and the logarithm of the binding
constants ( I g K ) according to the method of least squares. We thank Dr.
H.-J. Buschmann, Textilforschungsanstalt Krefeld, for providing a computer program to evaluate the titration results.
[13] In a typical titration experiment 500 pL of a 0.15 M solution of the host in
CCI, (with 5vol.% C,D,) was prepared, and various equivalents of
benzylamine stock solution (in the same solvent mixture) added in portions. Eight measurements were made in the range guest:host = 0.2-6.
The duration of the experiments did not exceed 2 h, because in some cases
slow deborylation of the host molecule was observed with a large excess of
amine after ca. 6 h. The values of K given arise from the shifts of the
'HNMR signals of the glycol units attached to boron. Inaccuracies arise
in particular in the determination of the concentration of the host. An
uncertainty of + 5 % leads to a maximum derivation of f 10% in 1gK.
[I41 The ammonia-borane adduct H,N' BH, forms complexes with various
crown ethers: H. Shahriari-Zavereh,J. F. Stoddart, M. K. Williams. B. L.
Allwood, D. J. Williams, .
I
Inclusion Phenom. 1985, 3, 355-377.
Effect of Torsional Strain and Electrostatic
Interactions on the Stereochemistry of Nucleophilic
Additions to Cyclohexanone and Related Systems **
Kobayashi et al. observed that nucleophilic additions to
1,3-dioxan-5-one (1) occur more by axial attack than those
to cyclohexanone, while nucleophilic additions to 1,3-dithian-5-one (7) proceed predominantly by equatorial attack.L6.'] These results were rationalized by Felkin's torsional effects based on structural features of these ketones,[*]an
interpretation which was supported by force-field modeling
of the transition state.[4b.
We have now calculated the axial and equatorial transition
structures for the reactions of lithium hydride with 1 and 7.
The structures were fully optimized with the 3-21G or the
3-21G* (d orbitals on sulfur) basis
and the energies
were further evaluated with MP2/6-31G*(~p)['~]
calculations (Table 1).
Table 1. Absolute [Hartree] and relative energies [kcalmol-'1 of the transition
structures of the reactions of 1,3-dioxan-5-one (l),1,3-dithian-5-one (7), and
cyclohexanone with lithium hydride.
Theoretical level [a]
3-21G//3-2 1G
6-31G*//3-21G
6-31G*//6-31G'
MP2/6-31G*//3-21 G
7
3-21G*//3-21G*
6-31G*//3-21Gt
MP2j6-3 1G*//3-21G +
Cyclohexanone 3-21G//3-21G
6-31G*//3-21G
MP2/6-31G*//3.21G
1
axial
E
equatorial
E
EW, bl
385.36311
387.52178
387.52148
388.57545
1027.819891
1032.83476
1033.79098
314.15521
315.90968
316.88236
385.36494
387.52010
387.52596
388.57784
1027.83148
1032.84559
1033.80353
314.15372
315.90700
316.88028
-1.1
1.1
1.O
- 1.5
-7.9
- 6.8
-7.9
0.9
1.7
1.3
[a] The 6-31G* basis set here is the 6-31G* with a set of diffuse sand p orbitals
on the hydride ion. [b] Always based on the transition structure of the axial
attack.
Figure 1 shows the optimized structure of 1, and axial and
equatorial transition structures (3 and 5, respectively) for
LiH addition. The six-membered ring of 1 is quite flat, as
indicated by a flap angle of 156" and a ring 0-C-C-C dihedral angle of 27". The corresponding values for cyclohex-
By Yun-Dong Wu, Kendall N . Houk,*
and Michael N. Paddon-Row *
Recently, Frenking, Kohler, and Reetz['' offered a frontier molecular orbital rationalization of the well-known preference for small nucleophiles to attack cyclohexanones from
the axial direction.['] We show here that the torsional effects
identified by Felkin et al.I3I provide an explanation not only
of the stereoselectivity of nucleophilic addition to cyclohexanone, but to other cyclic ketones as well. The LUMO
extensions identified by Frenking et al. are a simple consequence of the geometry of the transition structure, and are
maximized in the direction trans to the bond best eclipsed
with the C-0 E orbitals. In addition, we show that electrostatic interactions can also have a profound effect on
stereoselectivity when polar substituents are present.14,1'
[*I Prof. Dr. K. N. Houk, Prof. Dr. Y-D. Wu
Department of Chemistry and Biochemistry
University of California, Los Angeles
Los Angeles, CA 90024 (USA)
Prof. Dr. M. N. Paddon-Row
Department of Organic Chemistry
University of New South Wales, Kensington NSW, 2033 (Australia)
[**I This work was supported by the U.S. National Science Foundation and
the Australian Research Council. We thank the Pittsburgh Supercomputer
Center for a grant of computing time.
AnRew. Chem. Inl. Ed. Engl. 1992, 31, No. 8
0 VCH
0 4 7 0 ~
,
2271
5
%__
i
6
Fig. 1. Calculated structures of 1,3-dioxan-5-one (l), the axial (3) and equatorial (5) transition structures of the addition of lithium hydride to l, and
Newman projections of the same structures (2,4, and 6, respectively) along the
C4-C5 bonds. Important structural parameters are also shown.
Verlagsgesellschajt mbH, W-6940 Weinheim,1992
0570-0833~92/0808-10~9
$3.50+.25/0
1019
anone are 130" and 54", re~pectively.[~"]
This flattening is
caused by the short ring 0-C bonds. The axial transition
structure 3 achieves a staggered conformation about the C4CS bond without ring distortion. However, a partially
eclipsed arrangement of the substituents exists in the equatorial transition structure 5 despite major ring distortions, as
indicated by a 27" increase in the ring 0-C-C-C dihedral
angle.
The structures of 7 and the axial (9) and equatorial (11)
transition structures for LiH addition are given in Figure 2.
The long S-C bonds and small C-S-C angles cause the sixmembered ring to be significantly puckered (flap angle 120",
S-C-C-C dihedral angle 68". As a result, the O=C-C-He,
0
7
9
10
.
b',1.733
i
11
12
,
.,
O,."
Fig. 2. Calculated structures of 1,3-dithian-S-one (7), the axial (9) and equatorial (11) transition structures of the addition of lithium hydride to (7), and
Newman projections of the same structures (8, 10, and 12, respectively) along
the C4-CS bonds. Important structural parameters are also shown.
moiety is distorted on the opposite face (compare to 1). This
calculated structure is very similar to the crystal structure of
2-phenyl-1,3-dithian-S-one
determined by X-ray structural
analysis."] While there is no ring distortion in the equatorial
transition structure, significant ring flattening occurs in the
axial transition structure, as indicated by a 20" decrease in
the S-C-C-C dihedral angle.
Removal of LiH from the transition structures and recalculation of energies of the isolated distorted ketones provides a qualitative estimate of the ring strain contribution to
stereoselectivity (see below).['. 41 The ketone moiety of 3 is
2.5 kcalmol-' more stable than that of 5, reflecting the lack
of distortion in 3. The ketone moiety in the axial transition
structure 9 is less stable than that in equatorial transition
structure 11 by 1.2 kcalmol-', reflecting the ring distortion
in 9.[111
Why are the axial transition structures calculated (see
Table 1) for the reactions of 1 and 7 with LiH to be disfavored by 1.5 and 8 kcalmol- respectively? There is significant electrostatic repulsion between the negative hydride ion
and the lone pairs of the respective oxygen and sulfur atoms
1020
0 VCH
VerlagsgeseNsrhaftmbH, W-6940 Weinheim, 1992
in the axial transition structures: the hydridic hydrogen
atom is quite negatively charged (H, -0.5 e, 6-31G*(sp));
the H--0 distances in 3 (and the H--S distances in 9) are
only about 3.3 A; a simple Coulomb's Law calculation using
point charge interactions (- 0.6 e on 0) gives repulsion of
about 3 kcalmol-' for each H--0 interaction. Therefore,
significant electrostatic repulsions are expected in the axial
transition structures. These electrostatic interactions are less
significant with lithium aluminum hydride in solution, and
the axial addition to 1 becomes favorable. In the case of 7,
both torsional and electrostatic effects work against the axial
addition, resulting in a large preference for the equatorial
addition. For the reaction of 2-phenyl-I ,3-dithian-5-0ne,[~I
the two observed products are both likely to be derived from
an equatorial attack of the nucleophile ,the major transition
state having the phenyl substituent equatorial and the less
populated transition state having it axial. The degree of
stereoselectivity is roughly equal to the relative stability of
the two conformations of the reactant.[t2]
Electrostatic effects of the type involved here have also
been shown for other cases. We proposed that electrostatic
effects cause stereochemical variations in nucleophilic additions to cyclohexanone and acetaldehyde derivatives with
polar s u b ~ t i t u e n t s ."I[ ~For
~ ~ example, a fluorine substituent
at C3 of cyclohexanone causes increased axial attack of the
nucleophile due to increased positive charge at the C3 position, which gives stabilizing coulombic interactions with the
nucleophile approaching from the axial direction." 31
It is necessary to reiterate our version of the Felkin torsional strain theory[31for the preference of axial addition to
cyclohexanone by small nu~leophiles,[~'
as Frenking et al.
misinterpreted our argument."' For equatorial addition, in
order to form an ideal staggered transition structure, ring
strain would have to be introduced; to reduce or avoid this
ring strain, the equatorial transition structure is forced to
adapt a partially eclipsed conformation. The overall result is
a balance between the two types of strain. For the cyclohexanone-LiH reaction, calculations give an ideal staggered axial transition structure (13) but a partially eclipsed equatorial
transition structure (14) in which the ring strain is largely
avoided!'41 but only at the expense ofeclipsing strain involving the attacking nucleophile.['']
13
14
Torsional effects and orbital interactions are not mutually
exclusive. Earlier, we described the secondary orbital interactions which cause the attack of nucleophiles, radicals, and
electrophiles on unsaturated centers to occur in a staggered
fashion with respect to allylic bonds.['61 The LUMO extensions in cyclohexanone identified by Frenking et al. are a
direct consequence of geometrical distortion. As shown in
Scheme 1, the LUMO interacts with the most eclipsed allylic
bond in an antibonding fashion. This leads to an extension
of the LUMO in the direction trans to the allylic bond. In
cyclohexanone, the ring distortion causes the C-H bond to
0570-0833/92/0808-1020$ 3 . 5 0 + ,2510
Angew. Chem. Int. Ed. Engl. 1992, 31, No. 8
be more eclipsed with the K orbitals (3:0-C-C-H, 115') than
the C-C bond is (3:0-C-C-C, 127"), resulting in orbital distortion in the axial direction.[1, l 7 I On the other hand, ring
distortions in dithianone and in benzocycloheptenone are
opposite to that in cyclohexanone." 8] This geometrical distortion causes equatorial orbital exten~ion,~''~
and equatorial addition of hydride ion is favored.["]
Scheme 1. The antibonding interaction indicated (left) leads to the extension of
the LUMO in the direction trans to the allylic bond (right).
What role, if any, do orbital interactions involving the antiperiplanar C-0, C-S, C-C, and C-H bonds have?'"] The 0
C-0 or fi C-S bonds in the axial transition structures are
about 0.02 A shorter than the corresponding bonds in the
equatorial transition structures. However, the C-C bonds in
the axial transition structure of the cyclohexanone-LiH reaction are also shorter by about 0.02 A. These bond length
variations reflect the geometrical differences in the axial and
equatorial transition structures, and do not support the argument of significant differences in hyperconjugative interactions by antiperiplanar C-0, C-s, C-C, and C-H bonds.
In summary, we demonstrate that torsional strains, and
electrostatic effects in the presence of polar substituents, are
most responsible for the stereoselectivity of nucleophilic additions to cyclohexanone and related systems. Orbital interactions due to the inherent difference in C-C, C-H, C-0,
and C-S bonds are less important. We also point out that
LUMO extension and small pyramidalizations of sp2 cent e r ~ ' ~are
" ] a direct consequence of geometrical distortions.
Received: December 16, 1991
Revised: June 3, 1992 [Z 5074 IE]
German version: Angew. Chem. 1992, 104, 1087.
[l] G. Frenking, K. F. Kohler, M. T. Reetz, Angew. Chem. 1991, 103, 1167;
Angen. Chem. Inl. Ed. Engl. 1991, 30, 1146.
(21 J. R. Boone, E. C. Ashby, Top. Stereochem. 1979, 11, 53.
131 a) M. Cherest, H. Felkin, Tetrahedron Lett. 1968, 2201, 2205; b) N. T.
Anh, Top. Curr. Chem. 1980,88, 145.
[4] a) Y:D. Wu. J. A. Tucker, K. N. Houk, J. Am. Chem. Soc. 1991,113,5018:
b) Y.-D. Wu. K. N. Houk, ibid. 1987, 109,908; c) K. N. Houk, Y.-D. Wu
in Stereochemistry of Organic and Bioorganic Transformations (Eds.: W.
Bartmann, K. B. Sharpless), VCH, Weinheim. 1987. pp. 247-260; d) Y.D. Wu. K. N. Houk, B. M. Trost. J Am. Chem. SOC.1987, 109, 5560;
e) Y:D. Wu. K. N. Houk, J. Florez, B. M. Trost, L Org. Chem. 1991, 56,
3656.
[5] a) S. S. Wong, M. N. Paddon-Row, J Chem. SOC.Chem. Commun. 1990,
456: b) Ibid. 1991,327; c) Aust. J. Chem. 1991,44,765; d) Y-D. Wu, Ph.D.
Thesis. University of Pittsburgh, 1986.
[6]Y. M. Kobayashi, J. Lambrecht, J. C. Jochims, U. Burkert, Chem. Eer.
1978, 111. 3442; J. C. Jochims, Y. Kobayashi, E. Skrzelewski, Tetrahedron
Lett. 1974, 571, 575.
[7] See also: T. Terasama, T. Okada, J. Chem. SOC.Perkin Trans. I 1978.1252.
[XI Y. M. Kobayashi, Y Iitaka, Acra Crystallogr. Sect. B 1977, 33, 923.
191 The calculations were performed with the GAUSSIAN 90 program; M. J.
Frisch, M. Head-Gordon, G. W Trucks, J. B. Foresman, H. B. Schlegel,
K. Raghavachari, M. A. Robb, J. S . Binkley, C. Gonzalez, D. J. Defrees,
D. J. Fox, R. A. Whiteside, R. Seeger, C. F. Melius, J. Baker, R. L. Martin,
L. R. Kahn, J. J. P. Stewart, S. Topiol, J. A. Pople, Gaussian Inc., Pittsburgh, PA. USA. 1990.
Angew. Chem. Int. Ed. Engi. 1992. 31, No. 8
0 VCH Verlag~gesellschajtmbH,
[lo] A set of diffuse s and p orbitals was added to the hydride ion.
[ l l ] The 6.31'3: and MP2/6-31G* energies of the ketone moieties are (in
Hartree): 379.51067 and 380.54766 in 3; 379.50198 and 380.54374 in 5;
1024.82766 and 1025.76654 in 9; 1024.82760 and 1025.76839 in 11.
[12] R. J. Abraham, W. A. Thomas, L Chem. SOC.1965, 335.
[13] A thorough discussion can be found in [4a]. Frenking et al. [l] suggested
a change in 2s(C) coefficient in the C - 0 n* orbital by a 3-F substituent at
C3 to be responsible for the increased axial addition, although the change
is very small.
[14] The energy difference between the cyclohexanone moiety in the two cyclohexanone-LiH transition structures is basis set dependent. It is 0.0, 0.8,
-0.3, and 0.2 kcalmol-' with the 3-21G, 6-31G*//3-21G, MP2/6-31G*//
3-21G, and MP3/6-31G*//3-21G calculations. Similar basis set dependence is also found for dioxanone and dithianone systems and for relative
energies of transition structures.
[15] This strain can be roughly estimated by the calculations on the transition
structure of acetone-LiH reaction. A 15" rotation of the two methyl
groups away from the transition structure position in either direction
causes about 1 kcalmol-' increase in energy.
[16] P. Caramella, N. G. Rondan, M. N. Paddon-Row, K. N. Houk, J. Am.
Chem. SOC.1981, 103, 2438.
[17] J. Klein, Gtrahedron Lett. 1983, 24, 4307; 0. Eisenstein, J. Klein, J.-M.
Lefour, Tetrahedron 1979, 25, 225.
[18] D. Mukherjee, Y-D. Wu, F. R. Fronczek, K. N. Houk, J. Am. Chem. SOC.
1988, 110, 3328.
[19] Y. Kurita, C. Takayama, Tetrahedron Lett. 1990, 46, 3789.
[20] A. S. Cieplak, J. Am. Chem. Soc. 1981,103,4540; A. S. Cieplak, B. D. Tait,
C. R. Johnson, ibid. 1989, 111. 8447.
The Reaction of [Cp:Nb,(B,H,),]
(Cp' = tpEtMe,C,) with Sulfur:
Stabilization of the Tetrathioborato Ligand
in Novel Sulfido Niobium Clusters
By Henri Brunner, Giinther Gehart, Bernd Nuber,
Joachim Wachter,* and Manfred L. ZiegIer
The structural chemistry of binary boron-sulfur compounds is characterized by the formation of rings either in
isolation or linked to chains, in which boron usually has the
coordination number three."] In contrast the coordination
number four is not often found. Tetrathioborates with cornerlinked BS, tetrahedra occur in the layer structures of heavy
metal thioborates."] However, monomeric M[H,B(SH),-,I
(n = 0-3) compounds could only be proven by spectroscopy
in solution in the form of their alkali metal salts because they
are easily
Hitherto tetrathioborates have not
been observed as ligands in organometallic chemistry. Herein
we report the first tridentate, triply bridging tetrathioborato
ligand.
The surprisingly stable diboranato complex 1 was chosen
as the starting material. The synthesis of 1 had been mentioned previously ten years ago, however, without specific
details being given.f31We have have recently studied 1 systematically with regard to its method of formation, structure, and spectroscopic properties.[41The two (unstable in
the free state) B,H, dianions, which are arranged as bridges
perpendicular to the (formal) N b N b double bond are characteristic of 1. The reaction of 1 with sulfur in decane at
170 "C affords the neutral, diamagnetic complex 2 in yields
of up to 26%. The boiling temperature of o-xylene (144°C)
[*] Dr. J. Wachter, Prof. Dr. H. Brunner, G. Gehart
Institut fur Anorganische Chemie der Universitit
Universitltsstrasse 31, D-W-8400 Regensburg (FRG)
Dr. B. Nuber, Prof. Dr. M. L. Ziegler (deceased)
Anorganisch-chemisches Institut der Universitat
Im Neuenheimer Feld 270, D-W-6900 Heidelberg (FRG)
W-6940 Weinheim, 1992
0570-0833i92iO808-1021 .%
3.50+.25/0
1021
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