Alkynylcyclohexanol chairs and twist-boats: Co2(Co)6 as a conformational switch

Article abstract of DOI:10.1021/jo0108820

Treatment of 1-[axial]-(trimethylsilylethynyl)cyclohexan-1-ol with dicobalt octacarbonyl results in a conformational ring flip such that the bulky dicobalt-alkyne cluster moiety now occupies the favored equatorial site. However, when a 4-tert-butyl substi

Full text of DOI:10.1021/jo0108820

J . Org. Chem. 2001, 66, 8585-8591
8585
Alk yn ylcycloh exa n ol Ch a ir s a n d Tw ist-Boa ts: Co
Con for m a tion a l Sw itch
2 6
(CO) a s a
Nicole M. Deschamps, J ohn H. Kaldis, Philippa E. Lock, J ames F. Britten, and
Michael J . McGlinchey*
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
mcglinc@mcmaster.ca
Received August 27, 2001
Treatment of 1-[axial]-(trimethylsilylethynyl)cyclohexan-1-ol with dicobalt octacarbonyl results in
a conformational ring flip such that the bulky dicobalt-alkyne cluster moiety now occupies the
favored equatorial site. However, when a 4-tert-butyl substituent is present, the coordinated alkynyl
group retains its original axial or equatorial position. Complexation of trans-[diaxial]-1,4-bis-
(
triphenylsilylethynyl)cyclohexane-1,4-diol brings about a chair-to-chair conformational inversion
such that both cluster fragments now occupy equatorial sites. In contrast, cis-1,4-bis(triphenyl-
silylethynyl)cyclohexane-1,4-diol reacts with Co (CO) to yield the twist-boat conformer in which
the two axial hydroxy substituents exhibit intra-molecular hydrogen bonding. Likewise, the
corresponding reaction of cis-1,4-bis(trimethylsilylethynyl)cyclohexane-1,4-diol with Co (CO) leads
2
8
2
8
to a twist-boat, but in this case, the molecules are linked through inter-molecular hydrogen bonds.
Eight of these cobalt clusters have been characterized by X-ray crystallography, and the potential
use of twist-boats in synthesis is discussed.
In tr od u ction
Several years ago, we reported the X-ray crystal
structures of 2-methyl-1-(phenylethynyl)cyclopentanol, 1,
and of the corresponding dicobalt hexacarbonyl adduct,
2. In the former case, the alkynyl anion attacks the
precursor ketone in a pseudoaxial fashion, cis to the
Although the ability of the alkynyl dicobalt hexacar-
bonyl moiety to stabilize a neighboring carbocationic site
4
is very well established,1 its use as a stereocontrol
element has been less widely exploited. We note that
2 6
methyl group. Addition of the Co (CO) unit brings about
Melikyan and Nicholas have rationalized the Mn(OAc)
mediated oxidative cycloaddition of â-dicarbonyl com-
pounds with Co (CO) -complexed 1-alken-3-ynes in terms
3
-
a conformational flip such that the methyl and hydroxyl
substituents become pseudoaxial and the cluster occupies
a pseudoequatorial position, as in Scheme 1.
We here describe an extension of this concept whereby
several cyclohexanones have been treated with alkynyl
anions of increasing steric demand. The structures of
these alkynols are compared with the conformations of
the corresponding dicobalt hexacarbonyl complexes, eight
of which have now been characterized in the solid state
by X-ray crystallography.
2
6
of a transition state whereby the bulky cluster unit is
disposed pseudoequatorially; this favors a pseudoaxial
approach by the incoming nucleophile. Similarly, the
stereospecificity of the Friedel-Crafts cyclization of
cobalt-stabilized propargyl cations onto suitably activated
2
3
arenes has been explained by invoking a transition state
in which the (-C≡CH)Co
hindered pseudoequatorial site.
It is particularly interesting to note that Isobe has
reported the facile epimerization of a series of Co (CO)
2 6
(CO) fragment adopts the less
4
Resu lts a n d Discu ssion
2
6
-
complexed alkynyl sugars such that the cluster adopts
an equatorial rather than an axial site. We shall discuss
the mechanism of this process presently.
The conformation adopted by a cyclohexane rings
whether it be a chair, twist-boat, etc.sis critically de-
pendent on the steric interplay between the substituents.
5
6
The proclivity for a particularly bulky group, such as tert-
*
To whom correspondence should be addressed. Phone: (905) 525-
140 ext 27318. Fax: (905) 522-2509.
1) (a) Nicholas, K. M. Acc. Chem. Res. 1986, 20, 207. (b) Caffyn, A.
9
butyl, to occupy an equatorial site and thus “lock in” a
(
7
given conformation is well known. In contrast, a needle-
J . M.; Nicholas, K. M. In Comprehensive Organometallic Chemistry
II; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, U.K., 1995; Vol. 12, Chapter 7.1, pp 685-702. (c) McGlinchey,
M. J .; Girard, L.; Ruffolo, R. Coord. Chem. Rev. 1995, 143, 331. (d) El
Amouri, H.; Gruselle, M. Chem. Rev. 1996, 96, 1077. (e) Melikyan, G.
G.; Bright, S.; Monroe, T.; Hardcastle, K. I.; Ciurash, J . Angew. Chem.,
Int. Ed. Engl. 1998, 37, 161. (f) Ruffolo, R.; Brook, M. A.; McGlinchey,
M. J . Organometallics 1998, 17, 4992. (g) Guo, R.; Green, J . J . Chem.
Soc., Chem. Commun. 1999, 2503 and references therein.
like substituent, such as an alkynyl moiety, imposes
minimal steric interactions within the system and thus
offers little opportunity for conformational control. As
previously noted, treatment of the alkynylcyclopentanol
1
with dicobalt octacarbonyl yields the cluster complex
(
2) Melikyan, G. G.; Vostrowsky, O.; Bauer, W.; Bestmann, H. J .;
Khan, M.; Nicholas, K. M. J . Org. Chem. 1994, 59, 222.
3) (a) Grove, D. D.; Miskevich, F.; Smith, C. C.; Corte, J . R.
(5) (a) Tanaka, S.; Tsukiyama, T.; Isobe, M. Tetrahedron Lett. 1993,
34, 5757. (b) Tanaka, S.; Isobe, M. Tetrahedron 1994, 50, 5633.
(6) Weiser, J .; Golan, O.; Fiyjer, L.; Biali, S. E. J . Org. Chem. 1996,
61, 8277 and references therein.
(7) (a) Eliel, E. L. Stereochemistry of Carbon Compounds; McGraw-
Hill: New York, 1962; pp 208-221. (b) Kellie, G. M.; Riddell, F. G.
Top. Stereochem. 1974, 8, 224-269. (c) Bucourt, R. Top. Stereochem.
1974, 8, 159-224.
(
Tetrahedron Lett. 1990, 6277. (b) Grove, D. D.; Corte, J . R.; Spencer,
R. P.; Pauly, M. E.; Rath, N. P. J . Chem. Soc., Chem. Commun. 1994,
4
9.
(
4) Malisza, K. L.; Girard, L.; Hughes, D. W.; Britten, J . F.;
McGlinchey, M. J . Organometallics 1995, 14, 4676.
1
0.1021/jo0108820 CCC: $20.00 © 2001 American Chemical Society
Published on Web 11/15/2001

8
586 J . Org. Chem., Vol. 66, No. 25, 2001
Deschamps et al.
Sch em e 1
Sch em e 2
Sch em e 3
2
in which the formerly pseudoaxial alkyne linkage now
4
occupies a pseudoequatorial site. However, the confor-
mational flexibility of five-membered rings8 is much
greater than is found in cyclohexane systems for which
the chair is generally favored.
When cyclohexanone was allowed to react with (tri-
methylsilylethynyl)lithium and then hydrolyzed, the
resulting alkynol, 3, was characterized by NMR spec-
troscopy as the conformer in which the entering nucleo-
F igu r e 1. Molecular structures of (A) 11 and (B) 12 (50%
thermal ellipsoids).
9
phile occupies an axial site. After identification of all
1
butyl versus an (RC≡C)Co (CO) substituent to maintain
the proton and carbon resonances by use of standard H-
2
6
1
1
13
its equatorial alignment. To this end, trans-4-tert-butyl-
H COSY and H- C shift-correlated techniques, obser-
1
-(phenylethynyl)cyclohexan-1-ol, 7, was treated with
vation of nuclear Overhauser interactions between the
hydroxyl proton and the hydrogens only at C-2 and C-6
clearly established the equatorial positioning of the OH
Co (CO) to furnish 8, whose solid-state structure was
2
8
1
0
determined by X-ray diffraction. It is evident that the
equatorial preference of the tert-butyl group remains
dominant; the axially disposed cluster moiety in 8 causes
the cyclohexane ring to flatten only marginally and the
planar phenyl substituent is oriented so as to minimize
steric interactions with the tert-butyl group at the 4-posi-
tion.
2 6
group. Subsequent incorporation of the Co (CO) unit
yielded the complex 4 in which the cyclohexane ring has
flipped to allow the cluster fragment to occupy the
equatorial position, as illustrated in Scheme 2. The
structure of 4 was unequivocally determined by X-ray
diffraction.10
In an attempt to increase the steric bulk of the alkynyl
substituent, 4-tert-butylcyclohexanone was treated with
It is clearly relevant to note that Braga, Grepioni, and
their co-workers have reported the X-ray crystal struc-
tures of both 1-(tert-butylethynyl)cyclohexan-1-ol, 5, and
of its nickel and platinum complexes 6a ,b, shown in
Scheme 3. As with the conversion of 3 to 4, the alkynyl
substituent in 5 has moved from an axial to an equatorial
site in 6; however, the authors made no mention of this
conformational change since their primary focus was on
the intermolecular hydrogen bonding patterns in the
(
trimethylsilylethynyl)lithium; subsequent hydrolysis
yielded two alkynols that were readily separable by
column chromatography on silica gel. The isomers with
axial and equatorial alkynyl groups, 9 and 10, respec-
tively, were formed in a 5:2 ratio (Scheme 4).
The direction of attack by alkynyl anions on alkyl-
substituted 5- and 6-membered ring systems (initially
1
2
crystal lattice.1
1
investigated by Cadiot and Chodkiewicz ) cannot be
1
3
predicted simply from Cram’s rule. The torsional strain
transition-state model proposed by Felkin,14 and subse-
quently supported by Anh and Eisenstein,15 has been
Alkyn ols Der ived fr om 4-ter t-Bu tylcycloh exan on e.
The next question concerns the relative ability of a tert-
(
8) (a) Reference 7a, pp 248-252. (b) Malloy, T. B., J r.; Bauman, L.
E. Top. Stereochem. 1979, 11, 97.
9) The synthesis of 1-(trimethylsilylethynyl)cyclohexan-1-ol has
been reported previously, but only low-field NMR data are available:
a) Wenkert, E.; Leftin, M. H.; Michelotti, E. L. J . Org. Chem. 1985,
(12) (a) Battioni, J .-P.; Chodkiewicz, W.; Cadiot, P. C. R. Seances
Acad. Sci. Ser. C 1967, 264, 991. (b) Battioni, J .-P.; Capmau, M.-L.;
Chodkiewicz, W. Bull. Soc. Chim. Fr. 1969, 976. (c) Battioni, J .-P.;
Chodkiewicz, W. Bull. Soc. Chim. Fr. 1969, 981.
(
(
5
1
0, 1122. (b) Iritani, K.; Yanagihara, N.; Utimoto, K. J . Org. Chem.
986, 51, 5499.
(13) Cram, D. J .; Abd Elhafez, F. A. J . Am. Chem. Soc. 1952, 74,
5828.
(
10) X-ray crystal structures of molecules 4, 8, and 26 are shown in
the Supporting Information.
11) Braga, D.; Grepioni, F.; Walther, D.; Heubach, K.; Schmidt, A.;
Imhof, W.; G o¨ rls, H.; Klettke, T. Organometallics 1997, 16, 4910.
(14) (a) Cherest, M.; Felkin, H.; Prudent, N. Tetrahedron Lett. 1968,
2201. (b) Cherest, M.; Felkin, H. Tetrahedron Lett. 1968, 2205.
(15) (a) Anh, N. T.; Eisenstein, O. Nouv. J . Chim. 1977, 1, 61. (b)
Anh, N. T. Top. Curr. Chem. 1980, 88, 145.
(

Co
2
(CO)
6
as a Conformational Switch
J . Org. Chem., Vol. 66, No. 25, 2001 8587
Sch em e 4
Sch em e 5
used to rationalize the experimentally observed stereo-
selectivities of nucleophilic additions to cyclic ketones.
In this model, it is suggested that the direction of
nucleophilic attack is controlled not only by steric effects
but also by the torsional strain imposed on the system
in the transition state. In the case of cyclohexanones, this
involves a distorted chair conformation whereby the axial
transition state is perfectly staggered, whereas equatorial
attack proceeds through a transition state that affords
partial eclipsing.16
The X-ray crystal structures of the dicobalt hexacar-
bonyl clusters 11 and 12 (derived from 9 and 10,
respectively) show that the conformations of the cyclo-
hexane rings in these complexes are not markedly
perturbed. However, the data do allow a comparison of
the solid-state structures of these isomers (Figure 1).
They reveal that the conformation of the cyclohexane
chair at the tert-butyl end of the molecule remains
reasonably constant; the angle between the planes de-
fined by C(3)-C(4)-C(5) and C(2)-C(3)-C(5)-C(6) is 53°
( 0.5°. There is slightly more variation at the cluster end
whereby the interplanar angle between C(2)-C(1)-C(6)
and C(2)-C(3)-C(5)-C(6) ranges from 44° to 47°.
The X-ray crystallographic characterizations of the
cobalt clusters 11 and 12 allowed the unequivocal as-
signment of their precursors 9 and 10 as possessing,
respectively, axial and equatorial alkynyl substituents.
The 13C NMR spectra of the alkynols show clear chemical
shift differences: in 9, the C(1) absorption is found at δ
6
9.7, while in 10 the C(1) peak occurs δ 66.0, suggesting
that for systems of this type the C(1) resonance of the
isomer with the equatorial alkynyl moiety is approxi-
mately 4 ppm more shielded than its axial counterpart.
(16) For more recent analyses of this concept, see: (a) Wu, Y.-D.;
Tucker, J . A.; Houk, K, N. J . Am. Chem. Soc. 1991, 113, 5018. (b)
Frenking, G.; K o¨ hler, K. F.; Reetz, M. T. Angew. Chem., Int. Ed. Engl.
Replacement of the trimethylsilylethynyl substituent
by a triphenylsilylethynyl group again yields a mixture
of 4-tert-butyl-(1-alkynyl)cyclohexan-1-ols; in this case,
1
1
991, 30, 1146. (c) Wu, Y.-D.; Houk, K, N. Angew. Chem., Int. Ed. Engl.
992, 31, 1019.

8
588 J . Org. Chem., Vol. 66, No. 25, 2001
Deschamps et al.
F igu r e 2. Molecular structures of (A) 18 and (B) 20 (30% thermal ellipsoids).
the axial isomer, 13, is more favored than its equatorial
partner, 14, by a 4:1 ratio. These assignments are based
on the C(1) NMR absorptions which again differ by ∼4
ppm. Although both 13 and 14 yield crystalline complexes
2 8
(15 and 16, respectively) when treated with Co (CO) , it
has not yet proven possible to acquire acceptable X-ray
data sets on these cobalt clusters. Nevertheless, it is
evident that increasing the bulk of the attacking nucleo-
phile disfavors the formation of axial isomers.
Alk yn ols Der ived fr om Cycloh exa n e-1,4-d ion e.
The observation that a triphenylsilylethynyl group could
adopt either an axial or an equatorial position when a
bulky substituent was already present at C(4) suggested
that both cis and trans isomers of 1,4-bis(triphenylsilyl-
ethynyl)cyclohexane-1,4-diol might be readily available.
Assuming that the initial attack occurred at an axial site,
to give 17, the second alkynyl unit could then adopt either
an axial, 18, or an equatorial, 19, orientation, as depicted
in Scheme 5. Treatment of cyclohexane-1,4-dione with 2
equiv of (triphenylsilylethynyl)lithium yielded three prod-
ucts; the trans dialkynol 18 was obtained as colorless
crystals and characterized by X-ray crystallography. The
solid-state structure of 18 appears as Figure 2A and
illustrates clearly the diaxial nature of the alkynyl
substituents. The molecule has an inversion center, the
crystal packing is apparently dominated by the bulk of
the triphenylsilyl groups, and hydrogen bonding does not
seem to play a significant role in this particular case.
The other two products from this reaction were identi-
fied spectroscopically as the isomeric disubstituted mate-
rial 19, along with a small quantity of the monoalkyne
F igu r e 3. Molecular structures of (A) 21 and (B) 25 (30%
thermal ellipsoids).
1
7. However, since these latter two materials could not
C(3A) planes is ∼33°, and the distance between O(1) and
O(1A) is 2.705(10) Å, appropriate for intramolecular
hydrogen bonding. The net result of converting two
alkyne substituents into cobalt clusters has been to
provide a convenient route to a twist-boat cyclohexane-
1,4-diol; decomplexation with iodine allows facile reversal
of this conformational change.
The analogous reaction of cyclohexane-1,4-dione with
2 equiv of (trimethylsilylethynyl)lithium again afforded
the trans- and cis-dialkynols, 22 and 23. Treatment with
be cleanly separated by chromatography, they were
treated with dicobalt octacarbonyl and the separation
achieved at the cluster stage. Similarly, the diaxial
2 8
isomer 18 was allowed to react with Co (CO) to give the
corresponding trans cluster 20 whose solid-state struc-
ture is shown in Figure 2B. The most obvious result is
that the cyclohexane ring has undergone a conforma-
tional flip whereby addition of a dicobalt hexacarbonyl
moiety to each of the axial alkynyl units in 18 has
resulted in the formation of the diequatorial isomer 20.
The most dramatic effects are found for the cis complex
2 8
Co (CO) gave the bis-complexed trans- and cis-dialkynols
24 and 25, respectively, together with a small quantity
of the monocomplexed trans-dialkynol 26, which was also
characterized by X-ray crystallography.10 The ring-flipped
nature of complex 26 further illustrates the tendency of
the cluster moiety to favor an equatorial site.
2
1, the X-ray structure of which appears as Figure 3A.
The molecule adopts a well-defined twist-boat conforma-
tion with overall C molecular symmetry. The angle
between the C(2)-C(1)-C(3) and C(2)-C(3)-C(2A)-
2

Co
2
(CO)
6
as a Conformational Switch
J . Org. Chem., Vol. 66, No. 25, 2001 8589
Ta ble 1. Id ea lized a n d Exp er im en ta l Tor sion An gles in Cycloh exa n e Tw ist-Boa ts
idealized
torsion angle
deg)
a
Bertib Dieksc Biali 1,2,3,4d
Biali 1,2,4,5e cobalt-TPS (1)f cobalt-TPS (2)f cobalt-TMSg
(
twist-boat
C6-C1-C2-C3
C1-C2-C3-C4
C2-C3-C4-C5
C3-C4-C5-C6
C4-C5-C6-C1
C5-C6-C1-C2
+65
-31
-31
+65
-31
-31
+63.5
-36.5
-20.6
+61.8
-37.6
-25.4
+61.8
-42.4
-17.5
+61.8
-42.3
-17.8
+60.1
-28.5
-24.0
+48.8
-16.1
-37.2
+64.5
-24.7
-37.1
+64.5
-24.7
-36.9
+57.8
-37.8
-17.6
+56.1
-37.1
-20.9
+58.2
-38.0
-21.3
+60.0
-40.1
-18.5
+61.2
-29.4
-30.2
+61.2
-29.4
-30.2
a
From ref 7c. b Data from ref 17. Data from ref 20. d Data from ref 18. e Data from ref 19. f This work; there are two independent
c
g
molecules of 21 in the unit cell. This work; molecule 25.
Sch em e 6
Gratifyingly, the bis-cluster complex 25, derived from
the cis-dialkynol, yielded a twist-boat conformation with
overall C molecular symmetry, as shown in Figure 3B.
2
In contrast to the triphenylsilylated twist-boat, 21, the
O(1)‚‚‚O(4) distance in 25 is now 3.099(12) Å, probably
too long to support intramolecular hydrogen bonds.
However, in the solid state, the molecules of 25 pack in
pairs such that the four oxygens are in a pseudotetra-
hedral arrangement with intermolecular O‚‚‚O distances
of approximately 2.776(11) Å, indicative of hydrogen
bonding. One can readily envisage silicon, germanium,
or a transition metal occupying the cavity in such a
tetrahedral coordination site!
Crystallographically characterized cyclohexane twist-
boats are relatively rare. Twenty-five years ago, Berti et
al. reported the structure of 2,3-dibromo-4-tert-butyl-
cyclohexyl p-nitrobenzoate,17 and Biali has recently de-
intramolecular hydrogen bonding, consistent with a
twist-boat conformation ) furnishes the monomer 27 in
22
scribed two other cases, cis,trans,trans-1,2,3,4- and
75% yield. Nevertheless, the product still contains bulky
alkyl groups that would be difficult to remove, whereas
use of cobalt carbonyl clusters to effect a temporary
augmentation of the steric bulk of an appropriately
positioned substituent can be used for stereochemical
control.
cis,syn,cis-1,2,4,5-tetracyclohexylcyclohexane.1
8,19
There
are also several examples in the Cambridge Crystal-
lographic Database of metal complexes containing mul-
tiple polycyclohexyl-phosphines or -silanes in which
severe crowding gives rise to the occasional maverick
nonchair cyclohexyl ring. To our knowledge, the only
crystallographically characterized 1,4-disubstituted twist-
boat cyclohexane, reported by Dieks and co-workers,20
possesses both a porphyrin ring and a 2-methyl-1,4-
naphthoquinone. In that system, the torsion angles
within the cyclohexane ring differ quite substantially
from those expected for an idealized twist-boat geometry.7c
For comparison, the torsion angles for all the previously
reported twist-boats and for the cobalt-containing boats
Ep im er iza tion of Alk yn yl Su ga r s. The elegant work
of Tanaka and Isobe5 on the cobalt cluster-mediated
epimerization of the alkynyl sugar 28 into 29 merits some
comment. As noted by these authors, complexation of the
alkynyl substituent by dicobalt hexacarbonyl and sub-
sequent protonation leads to a metal-stabilized propargyl
cation. However, they did not discuss the mechanism of
epimerization which must be considered in terms of the
structure and dynamics of the cationic intermediate. It
has been previously noted that generation of cobalt-
stabilized cations preferentially proceeds through an
antiperiplanar transition state such that the elimination
2
1 and 25 are listed in Table 1; it is evident that the
structure of the trimethylsilyl-substituted twist-boat 25
is remarkably close to the reported idealized geometry.
It would be interesting to reevaluate the most favored
twist-boat geometry of cyclohexane-1,4-diol using modern
calculation techniques.
2
3
of water is anchimerically assisted by the metal. This
process maintains the stereochemical integrity of the
2
developing sp -carbon center. However, a symmetry-
In principle, the accessibility of a boat conformer of cis-
cyclohexane-1,4-diol can be turned into a synthetic
advantage. For example, it has been reported that
treatment of cis-cyclohexane-1,4-diol with dibutyldichlo-
rogermane yields a mixture of cyclic germanoxane poly-
mers, as depicted in Scheme 6; however, under conditions
of very high dilution, small quantities of the cyclic dimer
are obtained.21 In contrast, use of cis-2,5-di-tert-butylcy-
clohexane-1,4-diol (for which infrared data indicate strong
allowed antarafacial migration from one cobalt vertex to
the other inverts the configuration of the cationic center.
2
4
Scheme 7 depicts such a process for the alkynyl sugar
28 whereby protonation and ring opening affords the
cation 30; antarafacial migration (such that the smaller
substituent maintains the position proximal to the
metal-metal bond) generates the diastereomeric cation
31. Subsequent ring closure from the exo face yields a
(
22) Stolow, R. D. J . Am. Chem. Soc. 1961, 83, 2592.
(
17) Bellucci, G.; Berti, G.; Colapietro, M.; Spagna, R.; Zambonelli,
L. J . Chem. Soc., Perkin Trans. 2 1976, 1213.
18) Columbus, I.; Hoffman, R. E.; Biali, S. E. J . Am. Chem. Soc.
996, 118, 6890.
(23) El Hafa, H.; Cordier, C.; Gruselle, M.; Besace, Y.; J aouen, G.;
McGlinchey, M. J . Organometallics 1994, 13, 5149.
(
(24) (a) Padmanabhan, S.; Nicholas, K. M. J . Organomet. Chem.
1984, 212, C23. (b) Schreiber, S. L.; Sammakia, T.; Crowe, W. E. J .
Am. Chem. Soc. 1986, 108, 3128. (c) Edidin, R. T.; Norton, J . R.; Mislow,
K. Organometallics 1982, 1, 561. (d) Schilling, B. E. R.; Hoffmann, R.
J . Am. Chem. Soc. 1979, 101, 3456. (e) D’Agostino, M. F.; Frampton,
C. S.; McGlinchey, M. J . J . Organomet. Chem. 1990, 394, 145.
1
1
(
(
19) Golan, O.; Cohen, S.; Biali, S. E. J . Org. Chem. 1999, 64, 6505.
20) Dieks, H.; Senge, M. O.; Kirste, B.; Kurreck, H. J . Org. Chem.
997, 62, 8666.
21) Sara, A. N. J . Organomet. Chem. 1973, 47, 331.
(

8
590 J . Org. Chem., Vol. 66, No. 25, 2001
Deschamps et al.
Sch em e 7
chair conformation in which the bulky alkynylcobalt
cluster moiety occupies the favored equatorial site.
7.64 mmol) in ether (6 mL) was added dropwise. The solution
was allowed to warm to room temperature, stirred for 24 h,
and quenched with water. After ether extraction, washing with
brine, drying over magnesium sulfate, and removal of solvent
in vacuo, the resulting white solid was flash chromatographed
on silica gel using 50/50 hexane/dichloromethane as eluent to
give trans-4-tert-butyl-1-(trimethylsilylethynyl)cyclohexane-1-
ol, 9 (62%), and the corresponding cis isomer 10 (24%) as
colorless crystals. Subsequently, each alkynol was stirred at
Con clu d in g Rem a r k s
To our knowledge, the present examples are the only
ones in which the chair-twist-boat interconversion can
be controlled by incorporation of a readily removable
functionality. This approach, whereby a dicobalt hexa-
carbonyl moiety can function as a conformational switch,
has considerable potential for the selective manipulation
of other ring systems, and our ongoing studies will be
the basis for future communications.
2
room temperature for 24 h with an equimolar quantity of Co -
(CO) in THF under a nitrogen atmosphere. Removal of the
8
solvent and flash chromatography on silica gel using 50/50
hexane/dichloromethane as eluent yielded dark red crystals
of the clusters 11 and 12, respectively. Crystals suitable for
X-ray diffraction were grown for 4, 8, 11, 12, 18, 20, 21, 25,
and 26 by slow evaporation from a 50/50 solution of diethyl
Exp er im en ta l Section
ether and hexanes.
9
1
-(Tr im eth ylsilyleth yn yl)cycloh exa n -1-ol, 3: colorless
Ma ter ia ls. All syntheses were carried out under a dry
nitrogen atmosphere in an enclosed round-bottom flask, with
constant stirring, and all reagents and products were weighed
in a glovebag. Solvents were dried and distilled according to
standard procedures. All chemicals were purchased and used
as supplied by the Aldrich Chemical Co., with the exception
of dicobalt octacarbonyl, obtained from Strem Chemicals Inc.
Silica gel, 230-400 mesh, was used for flash column chroma-
tography.
1
crystals (70%); mp 72-73 °C; H NMR δ 3.29 (1H, s), 1.70 (2H,
m) 1.57 (2H, m), 1.48-1.35 (5H, m), 1.16 (1H, m), 0.13 (9H,
s); 13C NMR δ 112.0, 86.1, 66.8, 39.7, 24.9, 22.7, 0.1.
1
2 6
-(Tr im et h ylsilylet h yn yl)cycloh exa n -1-ol[Co (CO) ],
1
4
1
2
: dark red crystals (79%); mp 104-105 °C; H NMR δ 1.85-
.57 (11H, m), 0.31 (9H, s); 13C NMR δ 200.5, 122.8, 73.5, 42.1,
5.7, 22.56, 1.1.
tr a n s-4-ter t-Bu tyl-1-(p h en yleth yn yl)cycloh exa n ol, 7:
1
In str u m en ta tion . 1H and 13C solution NMR spectra were
measured on spinning samples by using Bruker AC-200 or AC-
colorless crystals (72%); mp 125-126 °C; H NMR δ 7.36 (5H,
m), 5.47 (1H, s), 1.96 (2H, m), 1.68 (2H, m), 1.38 (4H, m), 1.01
(
9
1H, t of t), 0.84 (9H, s); 13C NMR δ 130.9, 128.6, 128.1, 122.7,
3
00 spectrometers. The NOE experiments for 3 were carried
4.1, 83.7, 67.9, 46.4, 40.2, 31.9, 27.4, 24.3.
out, using a nonspinning sample, on a Bruker DRX 500
spectrometer. All NMR spectra were recorded at ambient
tr a n s-4-ter t-Bu tyl-1-(p h en yleth yn yl)cycloh exa n ol[Co
2
-
1
temperatures and referenced to the residual proton or 13
(CO)
6
], 8: dark red crystals (90%); mp > 156 °C dec; H δ
C
7
1
1
.43 (2H, m), 6.97 (3H, m), 4.30 (1H, s), 2.03 (2H, m), 1.63-
.07 (m, 6H), 0.85 (1H, m), 0.54 (9H, s); 13C NMR δ 130.3,
27.7, 74.1, 69.1, 48.0, 43.2, 32.2, 27.5, 24.3.
solvent signal. Low-resolution mass spectra were acquired with
a Finnigan EI/CI mass spectrometer system, using direct
electron impact and chemical ionization methods. Chemical
tr a n s-4-ter t-Bu tyl-1-(tr im eth ylsilyleth yn yl)cycloh exan -
3
ionization was induced using NH as the collision gas. Elec-
2
5
1
1
-ol, 9: colorless crystals (62%); mp 145 °C (lit. mp 145-
trospray ionization methods were used to acquire the mass
spectra for compounds 15, 16, 18, 20, 21, and 23-26, employ-
ing a Quattro L.C. triple quadrupole mass spectrometer
1
47 °C); H NMR δ 2.60 (1H, s), 1.95-1.32 (9H, m), 0.84 (9H,
s), 0.14 (9H, s); C NMR δ 109.1, 89.6, 69.7, 46.8, 40.2, 32.2,
1
3
2
7.5, 24.7, 0.0.
(Micromass Canada, Inc.).
cis-4-ter t-Bu tyl-1-(tr im eth ylsilyleth yn yl)cycloh exa n -1-
Gen er a l Syn th etic P r oced u r e. Each reaction was con-
1
ol, 10: colorless crystals (24%); mp 68-69 °C; H NMR δ 2.15
ducted at -78 °C in a dry ice/2-propanol bath and monitored
to completion using thin-layer chromatography, before the
mixture was allowed to warm to room temperature and
1
3
(
1H, s), 2.02-1.20 (9H, m), 0.82 (9H, s), 0.12 (9H, s); C NMR
δ 111.4, 85.8, 66.0, 47.2, 39.3, 32.4, 27.4, 21.8, 0.0.
tr a n s-4-ter t-Bu tyl-1-(tr im eth ylsilyleth yn yl)cycloh exan -
2
quenched with H O. In a typical reaction, n-butyllithium (7.64
2 6
-ol[Co (CO)
], 11:26 dark red crystals (63%); mp 89-90 °C;
1
mmol) in ether (30 mL) was added to a cold solution of
trimethylsilylethyne (1.08 mL, 7.64 mmol) dropwise via a
syringe over a 1 h period. After the mixture was stirred for 3
h at -78 °C, a solution of 4-tert-butylcyclohexanone (1.178 g,
1
H NMR δ 3.71 (1H, s), 1.92-1.82 (5H, m), 1.23-1.19 (4H,
(25) Yoshimatsu, M.; Fuseya, T. Chem. Pharm. Bull. 1996, 44, 1954.

Co
2
(CO)
6
as a Conformational Switch
J . Org. Chem., Vol. 66, No. 25, 2001 8591
m), 0.88 (9H, s), 0.31 (9H, s); 13C NMR δ 200.6, 120.9, 79.3,
7.44 (15H, m), 2.76-1.50 (9H, m); 13C NMR δ 210.6, 199.8,
136.2, 133.9, 130.4, 128.1, 2.1, 40.2, 37.3.
7
4.3, 47.9, 42.8, 32.5, 28.0, 24.5, 1.6.
cis-4-ter t-Bu tyl-1-(tr im eth ylsilyleth yn yl)cycloh exa n -1-
cis-1,4-Bis(tr ip h en ylsilyleth yn yl)cycloh exa n e-1,4-d iol-
2
6
1
ol[Co
2
(CO)
6
], 12: dark red crystals (58%); mp 105-106 °C;
[Co (CO) ] , 21: dark red crystals; mp >150 °C dec; H NMR
2
6 2
1
13
H NMR δ 3.72 (1H, s), 1.89-1.49 (9H, m), 0.87 (9H, s), 0.30
δ 7.72-7.33 (30H, m), 1.91-1.55 (8H, m); C NMR δ 199.8,
136.3, 133.9), 130.2, 128.0, 72.9, 37.3.
1
3
(
2
9H, s); C NMR δ 200.5, 122.61, 73.14, 47.8, 42.5, 32.4, 27.5,
3.5, 1.1.
tr a n s-1,4-Bis(t r im et h ylsilylet h yn yl)cycloh exa n e-1,4-
tr a n s-4-ter t-Bu tyl-1-(tr iph en ylsilyleth yn yl)cycloh exan -
1
d iol, 22: colorless crystals (56%); mp 188-190 °C; H NMR δ
1
1
7
-ol, 13: colorless crystals (58%); mp 121-122 °C; H NMR δ
13
2
3
.10-1.89 (10H, m), 0.16 (18H, s); C NMR δ 108.5, 89.0, 68.0,
6.5, -0.1.
.69-7.26 (15H, m), 2.18-1.34 (9H, m), 0.88 (9H, s); 13C NMR
δ 135.5, 133.5, 129.9, 127.9, 113.8, 84.4, 70.1, 47.2, 40.3, 32.3,
cis-1,4-Bis(t r im et h ylsilylet h yn yl)cycloh exa n e-1,4-d i-
2
7.6, 24.4.
1
ol, 23: colorless crystals (21%); mp 146-147 °C; H NMR δ
cis-4-ter t-Bu tyl-1-(tr ip h en ylsilyleth yn yl)cycloh exa n -1-
13
2
8
.58 (2H, s), 1.87 (8H, s), 0.12 (18H, s); C NMR δ 109.1, 108.7,
8.8, 88.0, 67.8, 66.8, 36.4, 35.7, -0.1.
1
ol, 14: colorless crystals (14%); mp 187-188 °C; H NMR δ
7
.69-7.26 (15H, m), 2.17-1.28 (9H, m), 0.89 (9H, s); 13C NMR
tr a n s-1,4-Bis(t r im et h ylsilylet h yn yl)cycloh exa n e-1,4-
δ 135.5, 133.5, 129.9, 127.9, 116.1, 81.1, 66.4, 47.2, 39.3, 32.4,
2 6 2
d iol[Co (CO) ] , 24: dark red crystals (80%); mp >165 °C dec;
2
7.3, 21.8.
1
H NMR δ 2.21 (4H, d), 1.76 (4H, d), 1.36 (1H, s), 0.36 (9H, s);
tr a n s-4-ter t-Bu tyl-1-(tr iph en ylsilyleth yn yl)cycloh exan -
1
3
C NMR δ 200.3, 121.5, 72.9, 38.2, 1.1.
1
2 6
-ol[Co (CO) ], 15: dark red crystals (50%); mp 103-104 °C;
1
cis-1,4-Bis(tr im eth ylsilyleth yn yl)cycloh exa n e-1,4-d iol-
H NMR δ 7.77-7.26 (15H, m), 1.97-1.25 (9H, m), 1.04 (9H,
1
1
3
[Co
2
(CO)
6
]
2
, 25: dark red crystals (53%); mp >129 °C dec; H
s); C NMR δ 200.1, 136.4, 134.0, 130.0), 127.9), 122.9, 74.2,
1
3
NMR δ 2.75 (2H, s), 2.25-1.80 (8H, m), 0.34 (18H, s); C NMR
4
5.0, 41.2, 32.7, 27.7, 23.5.
δ 200.2, 120.4, 78.8, 73.2, 38.0, 1.1.
cis-4-ter t-Bu tyl-1-(tr ip h en ylsilyleth yn yl)cycloh exa n -1-
tr a n s-1,4-Bis(t r im et h ylsilylet h yn yl)cycloh exa n e-1,4-
ol[Co
2
(CO)
6
], 16: dark red crystals (80%); mp 150-152 °C;
1
d iol[Co (CO) ], 26: dark red crystals (18%); mp >154 °C dec;
H NMR δ 7.75-7.29 (15H, m), 1.83-1.28 (9H, m), 0.86 (9H,
2
6
1
13
1
3
H NMR δ 2.12-1.38 (10H, m), 0.32 (9H, s), 0.16 (9H, s);
NMR δ 200.2, 121.7, 110.1, 87.1, 72.5, 65.7, 36.6, 35.0, 1.0,
C
s); C NMR δ 200.6, 136.3, 134.0, 130.0, 127.9, 73.1, 47.4, 41.6,
2.3, 27.5, 23.3.
-Hydr oxy-4-(tr iph en ylsilyleth yn yl)cycloh exan on e, 17,
3
-
0.1.
4
tr a n s-1,4-Bis(tr ip h en ylsilyleth yn yl)cycloh exa n e-1,4-d i-
ol, 18, a n d cis-1,4-Bis(tr ip h en ylsilyleth yn yl)cycloh exa n e-
Ackn owledgm en t. Financial support from the Natu-
1
,4-d iol, 19. Addition of cyclohexane-1,4-dione to triphenyl-
silylacetylene and n-butyllithium in Et O gave a mixture of
7-19. The dialkynol 18 was separated from the mixture as
ral Sciences and Engineering Research Council of
Canada (NSERC) is gratefully acknowledged. J .H.K.
and P.E.L. thank NSERC and the Province of Ontario
for graduate scholarships. Mass spectra were acquired
courtesy of Dr. Kirk Green of the McMaster Regional
Mass Spectrometry Centre. We thank Professors Dario
Braga and Fabrizia Grepioni (University of Bologna,
Italy) for helpful discussions.
2
1
a white crystalline solid, mp 225-226 °C (37%), by flash
column chromatography on silica gel, whereas 17, mp 154 °C,
was only partially isolated and predominantly coeluted with
1
9 from the column. Compound 19 was not isolated from the
mixture but was treated directly with Co (CO)
7: H NMR δ 7.66-7.34 (15H, m), 2.62-2.12 (9H, m); 13C
NMR δ 209.6, 135.4, 132.9, 130.1, 128.1, 111.9, 85.1, 66.8, 38.8,
7.3.
2
8
.
1
1
3
Su p p or tin g In for m a tion Ava ila ble: Spectroscopic and
other characterization data for all molecules. Crystallographic
data (excluding structure factors) for the molecules reported
in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
nos. CCDC-169852 (4), 169853 (8), 169854 (11), 169855 (12),
69856 (18), 169857 (20), 170373 (21), 169858 (25), and 169859
26). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK (fax: (+44)-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
1
8: H NMR δ 7.67-7.34 (30H, m), 2.19-2.01 (10H, m); 13
NMR δ 135.5, 133.2, 130.0, 128.0, 113.3, 84.2, 67.9, 36.1.
tr a n s-1,4-Bis(t r ip h en ylsilylet h yn yl)cycloh exa n e-1,4-
6 2
d iol[Co (CO) ] , 20: dark red crystals (82%); mp >192 °C dec;
13
C
2
1
H NMR δ 7.78-7.36 (30H, m), 2.09-1.50 (10H, m); C NMR
δ 199.8, 136.4), 133.9), 130.0), 127.9, 72.6, 37.1.
1
(
A small quantity of the monocomplexed dialkyne, 4-h y-
2
d r o x y -4-(t r ip h e n y ls ily le t h y n y l)c y c lo h e x a n o n e [C o -
1
(
CO)
6
], was also isolated as a dark red oil: H NMR δ 7.73-
(26) The clusters 11 and 12 have been reported previously, but no
spectroscopic or structural data are available: Daly, S. M.; Armstrong,
R. W. Tetrahedron Lett. 1989, 30, 5713.
J O0108820