mon target value during the least squares refinement; a similar
=
constraint was applied to the 2 C C distances (in all, 2 extra
least squares parameters, final values 1.501(6) and
1.352(11)).**
In the double dehydration of 3 and 4 to give 7 and 8 only
the carbocationic centre in the first proposed intermediate A
is adjacent to a Co2(CO)6 moiety (Scheme 3). It could be
argued that the first dehydration occurs at the Co2(CO)6 pro-
tected propargyl alcohol site and that the single Co2(CO)6 moi-
ety then switches to the uncoordinated triple bond, with the
second dehydration occurring in a stepwise manner. Although
we cannot entirely exclude this possibility, the near quantita-
tive yields in which 7 and 8 are obtained do not seem consistent
with this, since the reactive Co2(CO)6 fragment might be
expected to decompose rapidly in the acidic reaction media.
It seems more likely that the dehydration of the cycloalkyl sub-
Scheme 2 Formation of 5 and 6.
stituent at the carbon atom separated by the uncoordinated
2
=
=
=
C C bond from the [Co (CO) -m-Z –C C] moiety is due to
=
2
6
stabilisation of the second proposed intermediate carbocation
B by the remote dicobalt hexacarbonyl fragment. The rigidity
=
of the system enforced by the uncoordinated C C triple bond
=
means that such stabilisation must be a through-bond rather
than a through-space phenomenon.
In conclusion, we have demonstrated that the dehydration
of both alkyl substituents at the propargyl sites of the diyne-
diols studied here requires the coordination of only one
Co2(CO)6 moiety. This implies that the carbocationic inter-
mediates presumably involved in the acid-catalysed dehydra-
tion of diynediols under mild conditions can be stabilised by
a non-adjacent Co2(CO)6 moiety. Such remote stabilisation
of a carbocation is unprecedented.
Fig. 1 Molecular structure and atom numbering scheme for
˚
2
=
[{Co (CO) -m-Z -(C H )C C-} ] 5 with selected bond distances (A)
=
2
6
5
7
2
and angles (ꢀ). C7–C11 1.355(4), C24–C20 1.325(4), C11–C10
1.477(5), C24–C23 1.492(4), C20–C21 1.500(4), C21–C22 1.502(5),
C22–C23 1.521(5), C10–C9 1.515(5), C9–C8 1.507(5), C8–C7
1.511(5), C11–C12 1.445(4), C13–C26 1.433(4), C25–C24 1.447(4),
C7–C11–C12 126.1(3), C12–C11–C10 123.3(3), C7–C11–C10
110.6(3), C8–C7–C11 111.8(3), C11–C10–C9 104.1(3).
Acknowledgements
We thank the Cambridge Overseas Trust and Schlumberger
Cambridge Research for a Cambridge Overseas Trust Scho-
larship and the Committee of Vice-Chancellors and Princi-
pals of the Universities of the United Kingdom for an
Overseas Research Students Award to VBG, also St Cathar-
ine’s College, Cambridge for the award of a Research Fel-
lowship to ADW. We acknowledge the financial support of
the EPSRC for the purchase of the Nonius Kappa CCD dif-
fractometer. We thank Dr. J. E. Davies for determining the
crystal structures of 5 and 9 and the EPSRC National Mass
Spectrometry Service Centre, Swansea for providing FAB
(LSIMS) spectra.
dicobalt hexacarbonyl fragment may be transmitted through
an uncoordinated alkyne to a remote carbocationic centre.
Although double-dehydration of uncoordinated diynediols
is known, it requires very harsh conditions,7 whereas in the
current case the reaction takes place under very mild condi-
tions (catalytic amounts of HBF4 , < 0 ꢀC).Although we were
unable to obtain X-ray quality crystals of either 7 or 8, defini-
tive evidence as to their structures is provided by the crystal
2
=
=
=
structure of [Co (CO) dppm{m-Z -(C H )C C–C C(C H )}]
=
2
4
5
7
5
7
9, obtained by substitution of two molecules of CO in 7 by
bis(diphenylphosphino)methane (dppm).k This substituted
product crystallised readily (Fig. 2). The analogous substitu-
tion of 8 by dppm led to the complex [Co2(CO)4dppm{m-Z2-
=
=
(C H )C C–C C(C H )}] 10. The bond lengths and angles
=
=
6
9
6
9
** Crystal data for 9: C43H36Co2O4P2 , M ¼ 796.52, triclinic, space
within the Co2C2 cores are unremarkable in both 5 and 9.6,8
˚
˚
ꢀ
˚
3
¯
group P1, a ¼ 12.2764(4) A, b ¼ 15.3093(6) A, c ¼ 21.7463(8) A,
˚
˚
The short C(14)–C(13) bond length in 9 [1.435(6) A],
a ¼ 104.381(2), b ¼ 90.547(2), g ¼ 106.997(2) , V ¼ 3771.2(2) A ,
T ¼ 180(2) K, Z ¼ 4, m ¼ 1.006 mmꢁ1, Dc ¼ 1.403 Mg mꢁ3, 30 195
reflections collected, 13 151 independent reflections (Rint ¼ 0.0826),
goodness-of-fit on F2 0.975, final R indices [I > 2s(I)]: R1 ¼ 0.0495,
although typical of such enyne separations,9 implies a degree
of conjugation in the uncoordinated enyne unit which might
aid charge delocalisation and promote the stability of the pro-
posed intermediate carbocation from which 7 is derived. The
two five-membered rings are not well resolved, hence the C–C
single bond distances in these rings were restrained to a com-
wR2 ¼ 0.1021, Largest diff. peak 0.433 and hole ꢁ0.487 e Aꢁ3, 921
˚
parameters, 20 restraints. Compound 9 contains two crystallographi-
cally independent molecules within its asymmetric unit: two of these
possess a strict crystallographic inversion centre. The bond lengths
and angles within all three molecules are similar and thus only one
molecule is discussed. Data for 5 and 9 were collected using a Nonius
Kappa CCD diffractometer equipped with an Oxford Cryosystems
cryostream. Cell refinement, data collection and data reduction were
performed with the programs DENZO10 and COLLECT11 and
multi-scan absorption corrections were applied to all intensity data
with the program SORTAV.12 Both structures were solved and refined
with the programs SHELXS97 and SHELXL9713 respectively. CCDC
suppdata/nj/b2/b206295j/ for crystallographic data in CIF or other
electronic format.
k To a solution of the complexes (7–8) 0.5 mmol in toluene (50 ml) was
added dppm (0.5 mmol) and the mixture was heated to 70 ꢀC for one
hour. Chromatography on silica gel (hexane:dichloromethane 4:1 v/v)
afforded two fractions. The first and major one proved to be the
mono(dicobalt tetracarbonyl) dppm-coordinated complexes (9–10),
whereas the minor second fraction corresponded to the bis(dicobalt
tetracarbonyl) dppm-coordinated complexes.
New J. Chem., 2002, 26, 1706–1708
1707