equation above.14 However, in these conditions, minor
amounts of the known complex NbCl5(1,4-dioxane)6b have
been detected by NMR,15 this compound resulting from the
coordination of 1,4-dioxane to the still unreacted NbCl5.
Moreover, small quantities of 1,2-dichloroethane and of the
chloromethoxy derivative [NbCl3(OCH3)2]216 have been found
(NMR, GC/MS), suggesting that diverse fragmentation path-
ways of dme are operating during the reaction with NbCl5. A
quantification of the relative amounts of the products has been
possible by 1H NMR spectroscopy, by using CH2Cl2 as
reference.15
breakages are partially counterbalanced by the establishment
of one new C–O bond, inside the dioxane unit, thus the driving
force of the overall process is presumably the formation of the
strong niobium–oxygen double bond.20 Actually the forma-
tion of strong Group 5 metal–oxygen bonds may be also
related to the efficiency of MX5 (M = Nb, Ta) as catalysts
for the acylative cleavage of ethers.2
The synthesis of the oxychloride species 2, by combination
of 1 with dme at room temperature, deserves further comment.
In fact, examples of redox processes involving early transition
metal halides and taking place in dme solutions have been
reported; significantly, the reduction of NbCl5 to Nb(III) is
carried out by SnBu3H in dme.21 According to the results
discussed in the present communication, those processes might
proceed via the formation of intermediates generated by
preliminary reaction of the metal complex with dme itself.
The multiple C–O bond activation process described above
occurs also when other 1,2-dialkoxyalkanes, different from
dme, are involved. Thus, preliminary results indicate that the
reactions of NbCl5 with CH3O(CH2)2O(CH2)2OCH3 (diglyme),
or with C2H5O(CH2)2OC2H5 give 1,4-dioxane/CH3Cl or 1,4-
dioxane/C2H5Cl, respectively, in quantities comparable to those
observed for the reaction of 1 with dme. More interestingly, the
reaction of NbCl5 with two equivalents of 1,2-dimethoxypro-
pane has led to the formation of 2,5-dimethyl-1,4-dioxane
(GC/MS), indicating a new potential route for the synthesis
of substituted 1,4-dioxanes.
In order to discover possible intermediates, we carried out the
reaction of 1 with one equivalent of dme, in CDCl3 at ꢁ30 1C.
1H NMR spectroscopy indicated the fast and complete disap-
pearance of the resonances due to free dme and the growth of
two resonances at 4.01 and 3.70 ppm, attributed to coordinated
dme within the ionic species [NbCl4(dme)2][NbCl6], 3 (vide
infra), containing a presumably octa-coordinated niobium cen-
tre. Octa-coordinated niobium adducts have been reported in
the literature,17 the most pertinent example being the ionic
hexachloroniobate [NbCl4(S2R2)2][NbCl6], R = Me, Pri.18
The resonances shown by 3 rapidly disappeared on warming
at room temperature, due to conversion into the neutral
NbCl4[O(CH3)CH2CH2O], 4, with concomitant formation of
one equivalent of methyl chloride. Compound 4 has been
identified by comparison of its NMR data with those collected
for the product prepared by reaction of NbCl5 with one
equivalent of 2-methoxyethanol.19
This work was supported by the Consiglio Nazionale delle
Ricerche (CNR, Roma), and the Ministero dell’Universita
e della Ricerca Scientifica (MIUR, Roma).
Compound 3 was not isolated due to its thermal instability,
but evidence of its ionic character was obtained by monitoring
the conductivity during the reaction carried out in dichlor-
omethane. The conductivity of the starting solution, NbCl5 in
CH2Cl2 at ꢁ30 1C, corresponded to 10 mSi immediately after
the addition of dme, but was raised to 155 mSi after a few
minutes due to the formation of the ionic species 3. When the
solution was allowed to warm up to room temperature (20
min), the conductivity decreased to 11 mSi, in agreement with
the formation of neutral 4, which is stable in the absence of
uncoordinated dme. The successive formation of compound 2,
CH3Cl and 1,4-dioxane (detected by 1H NMR) takes place
upon addition of further dme, see Scheme 1.
Notes and references
1 (a) R. L. Burwell, Jr, Chem. Rev., 1954, 54, 615; (b) M. V. Bhatt
and S. U. Kulkarni, Synthesis, 1983, 249; (c) R. C. Larock, Ether
Cleavage in Comprehensive Organic Transformations, Wiley-VCH,
Weinheim, 2nd edn, 1999, p. 1013.
2 Q. Guo, T. Miyaji, R. Hara, B. Shen and T. Takahashi, Tetra-
hedron, 2002, 58, 7327, and references therein.
3 (a) W. J. Evans, T. A. Ulibarri and J. W. Ziller, Organometallics,
1991, 10, 134; (b) C. Eaborn, P. B. Hitchcock, K. Izod and J. D.
Smith, J. Am. Chem. Soc., 1994, 116, 12071; (c) B.-J. Deelman, M.
Booij, A. Meetsma, J. H. Teuben, H. Kooijman and A. L. Spek,
Organometallics, 1995, 14, 2306; (d) F. Preuss, G. Hornung, W.
In conclusion, the reaction of NbCl5 with dme (X2 equiva-
lents) proceeds in two main steps: in the first step one O–CH3
bond breaks giving methyl chloride and the methoxyniobium
species 4, whereas the second step consists of the cleavage of
two C–O bonds (the remaining O–CH3 bond and one of the
two O–CH2 bonds), with consequent formation of the oxo-
derivative 2, 1,4-dioxane and further methyl chloride.
Frank, G. Reiss and S. Muller-Becker, Z. Anorg. Allg. Chem.,
¨
1995, 621, 1663; (e) K. Takaki, M. Maruo and T. Kamata, J. Org.
Chem., 1996, 61, 8332; (f) D. J. Duncalf, P. B. Hitchcock and G. A.
Lawless, Chem. Commun., 1996, 269; (g) M. C. Cassani, M. F.
Lappert and F. Laschi, Chem. Commun., 1997, 1563; (h) C. A.
Bradley, L. F. Veiros, D. Pun, E. Lobkovsky, I. Keresztes and P. J.
Chirik, J. Am. Chem. Soc., 2006, 128, 16600.
4 (a) Y. K. Gun’ko, P. B. Hitchcock and M. F. Lappert, J.
Organomet. Chem., 1995, 499, 213; (b) M. C. Cassani, Y. K.
Gun’ko, P. B. Hitchcock, A. G. Hulkes, A. V. Khvostov, M. F.
Lappert and A. V. Protchenko, J. Organomet. Chem., 2002, 647,
The sequence of reactions reported in Scheme 1 is unprece-
dented, since it takes place through cleavage of three of the
four dme C–O linkages at room temperature. These C–O bond
71; (c) S. Le Caer, M. Heninger, J. Lemaire, P. Boissel, P. Maıtre
¨
and H. Mestdagh, Chem. Phys. Lett., 2004, 385, 273; (d) S. La
Caer, M. Heninger, P. Pernot and H. Mestdagh, J. Phys. Chem. A,
¨
2006, 110, 9654; (e) C. A. Bradley, L. F. Veiros and P. J. Chirik,
Organometallics, 2007, 26, 3191.
¨
5 (a) A. Ecker, R. Koppe, C. Uffing and H. Schnockel, Z. Anorg.
¨
¨
Allg. Chem., 1998, 624, 817; (b) W. Uhl, A. Vester, D. Fenske and
G. Baum, J. Organomet. Chem., 1994, 464, 23; (c) W. Uhl, R.
Gerdin and A. Vester, J. Organomet. Chem., 1996, 513, 163; (d) W.
Uhl, Coord. Chem. Rev., 1997, 163, 11; M. Westerhausen, C. Birg,
H. Noth, J. Knizek and T. Seifert, Eur. J. Inorg. Chem., 1999, 2209.
¨
Scheme 1
ꢀc
This journal is The Royal Society of Chemistry 2008
3652 | Chem. Commun., 2008, 3651–3653