From 1,2-dialkoxyalkanes to 1,4-dioxanes. A transformation mediated by NbCl5via multiple C-O bond cleavage at room temperature

Article abstract of DOI:10.1039/b804432e

The formation of 1,4-dioxanes and alkyl chlorides from the reactions of polyethers (dme, diglyme, 1,2-diethoxyethane, 1,2-dimethoxypropane) with NbCl₅ at room temperature is described; as far as dme is concerned, the reaction mainly occurs in two steps, consisting of (i) cleavage of one O-CH₃ bond, followed by (ii) cleavage of the second O-CH₃ bond and one of the two O-CH₂ linkages. The characterization of intermediate complexes and the isolation of NbOCl₃(dme) are reported. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804432e

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From 1,2-dialkoxyalkanes to 1,4-dioxanes. A transformation mediated
by NbCl5 via multiple C–O bond cleavage at room temperaturew
a
Fabio Marchetti,z Guido Pampaloni*a and Stefano Zacchinib
Received (in Cambridge, UK) 14th March 2008, Accepted 28th April 2008
First published as an Advance Article on the web 9th June 2008
DOI: 10.1039/b804432e
The formation of 1,4-dioxanes and alkyl chlorides from the
reactions of polyethers (dme, diglyme, 1,2-diethoxyethane,
1,2-dimethoxypropane) with NbCl5 at room temperature is
described; as far as dme is concerned, the reaction mainly occurs
in two steps, consisting of (i) cleavage of one O–CH3 bond,
followed by (ii) cleavage of the second O–CH3 bond and one of
the two O–CH2 linkages. The characterization of intermediate
complexes and the isolation of NbOCl3(dme) are reported.
the crystalline compound NbOCl3(dme), 2, has been isolated
and characterized by X-ray structural analysis,7 see Fig. 1. The
complex displays a distorted octahedral geometry, with the
Nb(V) centre bound to three chlorides, one oxide and the
oxygen atoms of one dme unit. The Nb(1)–O(1) interaction,
at 1.6939(18) A, is consistent with a niobium–oxygen double
bond.8 The oxygen atoms of dme respectively occupy trans and
cis positions with regard to the oxide. A similar arrangement of
the ligands has been found in other NbOCl3L2 complexes, such
as NbOCl3(HMPA)2, HMPA = hexamethylphosphoramide,9
and NbOCl3(CH3CN)2.10
The preparation of functionalized species starting from readily
available materials via fast and selective reactions represents
one of the most appealing goals for the synthetic chemist. The
breaking of the C–O bond of ethers is a key step in several
organic syntheses:1 for instance, the acylative cleavage of
ethers, mediated by Lewis acids or low-valent transition metal
complexes, is an attractive reaction to obtain esters.2
Noteworthy is the fact that the formation of compound 2
occurs under mild conditions (i.e. room temperature or
below), in spite of the fact that all the known oxygen abstrac-
tions from ethers by metal complexes generally require high
temperatures.11–13
The formation of 2, accompanied by the production of
methyl chloride and 1,4-dioxane (detected by GC/MS), can
be represented by eqn (1):
On the other hand, the direct combination of ethers with
oxophilic metal complexes (lanthanides, early transition ele-
ments) generally proceeds with formation of stable metal
alkoxides.3 In these cases, the starting ether does not trans-
form into functionalized species, but breaks down into smaller
fragments. For example, 1,2-dimethoxyethane, dme, that is
widely used as a solvent in synthetic chemistry, can react with
transition metal compounds3f,g,4 undergoing degradation.
This latter occurs with cleavage of one or two O–CH2 bonds,
and affords methoxy-derivatives3f,g,4 and, in some cases,
free3f,4c,d or coordinated ethylene.4e Otherwise, the cleavage
of the dme O–CH3 bonds is known to be performed by some
aluminium derivatives under mild conditions.5
NbCl5 þ 2C4H10O2 ! NbOCl3ðC4H10O2Þ þ2CH3Cl
2
1
2
þ C4H8O2
ð1Þ
Compound 2 has been prepared in optimal yield by reacting
NbCl5 with two equivalents of dme, coherently with the
In the framework of our research on the activation of
oxygenated species by Group 5 metal pentahalides, MX5,6
we were eager to investigate the reactivity of MX5 (M = Nb,
Ta, X = F, Cl, Br, I) with dme and similar molecules. Here,
we report a study of the reaction of NbCl5, 1, with
1,2-dialkoxyalkanes, showing the formation, inter alia, of
1,4-dioxanes.
NbCl5 rapidly dissolves in a large excess of dme at room
temperature and, working under strictly anhydrous conditions,
a Dipartimento di Chimica e Chimica Industriale, University of Pisa,
Via Risorgimento 35, I-56126 Pisa, Italy. E-mail:
pampa@dcci.unipi.it; Fax: +050-2219-246; Tel: +050-2219-219
b Dipartimento di Chimica Fisica e Inorganica, University of Bologna,
Viale Risorgimento 4, I-40136 Bologna, Italy
w Electronic supplementary information (ESI) available: Analytical
data and experimental details of the X-ray crystal structure of
compound 2. CCDC 679568. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/b804432e
z Born in Bologna, Italy, in 1974.
Fig. 1 ORTEP drawing of NbOCl3(dme), 2. Displacement ellipsoids
are at the 30% probability level. Selected bond lengths (A) and angles
(1): Nb(1)–O(1) 1.6939(18), Nb(1)–Cl(2) 2.3721(6), Nb(1)–Cl(3)
2.3777(6),
Nb(1)–Cl(4)
2.3334(7),
Nb(1)–O(2)
2.1879(17),
Nb(1)–O(3) 2.3801(18); O(1)–Nb(1)–O(2) 92.57(8), O(1)–Nb(1)–Cl(4)
103.22(7), O(1)–Nb(1)–Cl(2) 97.73(6), O(1)–Nb(1)–Cl(3) 97.48(6).
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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
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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.
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¨
Scheme 1
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6 (a) F. Marchetti, G. Pampaloni and S. Zacchini, Dalton Trans.,
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14 A suspension of NbCl5, 1 (0.220 g, 0.814 mmol), in CH2Cl2
(20 mL), was treated with dme (0.18 mL, 1.73 mmol), and the
resulting mixture was stirred at room temperature for 5 h, during
which progressive dissolution of the solid was noted. The volatile
materials were removed in vacuo and recrystallization of the
residue from CH2Cl2–pentane at room temperature gave pale
yellow crystals of 2. Yield: 0.174 g (70%).
7 Crystal data for 2: C4H10Cl3O3Nb: M = 305.38, T = 100(2) K,
orthorhombic, space group Pbca, a = 7.3270(7), b = 11.9741(11),
c = 23.187(2) A, V = 2034.2(3) A3, Z = 8, Dc = 1.994 g cmꢁ3
,
m = 1.932 mmꢁ1, graphite-monochromatized Mo-Ka radiation (l
= 0.71073 A). Final R indices were R1 = 0.0239 and wR2 =
0.0562 for 2150 independent reflections having I 4 2s(I) (Rint
=
0.0323). The diffraction experiments were carried out on a Bruker
APEX II diffractometer equipped with a CCD detector and using
Mo-Ka radiation. Data were corrected for Lorentz polarization
and absorption effects [empirical absorption correction SADABS
(G. M. Sheldrick, SADABS, Program for empirical absorption
15 CH2Cl2 (0.0289 mL, 0.451 mmol) and dme (0.0465 mL, 0.447
mmol) were introduced into a NMR tube containing a suspension
of compound 1 (0.0600 g, 0.222 mmol) in CDCl3 (0.80 mL). The
tube was sealed, and a dark solution formed in 3 h. Subsequent 1H
and 13C NMR analyses (recorded at 243 and at 298 K) revealed the
presence of CH2Cl2, compound 2, CH3Cl [1H NMR: 3.00 ppm
(C. J. Pouchert and J. Behnke, The Aldrich Library of 13C and
1H FT NMR Spectra, Aldrich Chemical Company Inc., Milwau-
kee, USA, 1st edn, 1993)], 1,4-dioxane, 1,2-dichloroethane and the
known complexes NbCl5(1,4-dioxane) (see ref. 6b) and
[NbCl3(OCH3)2]2 (see ref. 16), in ca. 20 : 9 : 17 : 2 : 1 : 2 : 1 ratio.
GC/MS analysis of the solution, filtered on alumina to remove the
inorganic materials, indicated the presence of dme, CH3Cl, 1,4-
dioxane and 1,2-dichloroethane.
¨ ¨
correction, University of Gottingen, Gottingen, Germany, 1997)].
Structures were solved by direct methods and refined by full-matrix
least-squares based on all data using F2 [G. M. Sheldrick,
SHELX97, Program for crystal structure determination, University
of Gottingen, Gottingen, Germany, 1997]. H-atoms were placed in
¨
¨
calculated positions and treated isotropically using 1.2 ꢂ Uiso value
of the parent atom except methyl protons, which were assigned 1.5
ꢂ Uiso value of the parent C-atom. All non-hydrogen atoms were
refined with anisotropic displacement parameters.
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12 The room-temperature generation of oxychlorides from the reac-
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functionalities (E = C, P, As, S) under mild conditions (see ref. 13)
is probably due to the presence of adventitious water rather than to
21 S. F. Pedersen, J. B. Hartung, Jr, E. J. Roskamp and P. S.
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