A topochemical rearrangement with multiple inversions of configuration

Article abstract of DOI:10.1139/v01-099

Crystalline 2-benzyloxypyridine-1-oxide rearranges slowly at room temperature to crystalline 1-benzyloxy-2-pyridone. No intermediates are detected when the process is followed by solid-state ¹³C NMR. The crystal structure of the pyridine-1-oxid

Full text of DOI:10.1139/v01-099

1272
A topochemical rearrangement with multiple
inversions of configuration
Saul Wolfe, Yih-Huang Hsieh, Raymond J. Batchelor, Frederick W.B. Einstein,
and Ian D. Gay
Abstract: Crystalline 2-benzyloxypyridine-1-oxide rearranges slowly at room temperature to crystalline 1-benzyloxy-2-
pyridone. No intermediates are detected when the process is followed by solid-state 13C NMR. The crystal structure of
the pyridine-1-oxide strongly suggests that a topochemically controlled intramolecular process, in which the benzyl
group migrates with retention of configuration, is not feasible. On the other hand, although somewhat disfavoured by
initial solid-state O···C···O angles significantly less than the ideal 180°, intermolecular topochemically controlled pro-
cesses can be envisaged that lead, with multiple inversions of configuration, either to net retention of configuration or
to net inversion of configuration in the benzyl group. In contrast to the 50–80% inversion observed in solution, in the
solid state only inversion is observed experimentally when chirally labelled α-deuteriobenzyloxypyridine-1-oxide is al-
lowed to rearrange.
Key words: X-ray crystallography, solid-state 13C NMR, benzyl-α-D-alcohol, 2-benzyloxypyridine-1-oxide, 1-benzyloxy-
2-pyridone.
Résumé : Le 1-oxyde de 2-benzyloxypyridine à l’état cristallin se réarrangement lentement, à la température ambiante,
pour conduire à la 1-benzyloxypyrid-2-one. Lorsqu’on a suivi cette transformation par RMN du 13C à l’état solide, on
n’a pas pu détecter d’intermédiaires. La structure cristalline du 1-oxyde de pyridine suggère fortement qu’il n’est pas
possible de suggérer l’existence d’un processus intramoléculaire contrôlé de façon topochimique dans lequel le groupe
benzyle migrerait avec rétention de configuration. Par ailleurs, même s’il est légèrement défavorisé à l’état solide par
des angles initiaux O···C···O inférieurs à la valeur idéale de 180°, il est possible d’envisager des processus intermolécu-
laires contrôlés de façon topichimique qui conduiraient après de multiples inversions de configuration à une rétention
ou à une inversion nette de configuration dans le groupe benzyle. Par opposition aux degrés d’inversion de 50 à 80%
observés expérimentalement en solution, le réarrangement du 1-oxyde de l’α-deutérobenzyloxypyridine à l’état solide
ne donne lieu qu’à de l’inversion.
Mots clés : diffraction des rayons X, RMN du 13C à l’état solide, alcool α-D-benzylique, 1-oxyde de 2-
benzyloxypyridine, 1-benzyloxypyrid-2-one.
[Traduit par la Rédaction] Wolfe et al. 1277
We recently reported (1) that the Tieckelmann reaction,
crossover and with net retention of configuration, by a dou-
ble inversion of one alkyl group, or with crossover and net
inversion of configuration, by single inversions of two alkyl
groups.
Although four channels are, in principle, possible for the
second step, only two have so far been observed. Based on
kinetic studies, crossover experiments and measurement of
the secondary H/D isotope effect, in dimethylformamide
(DMF) at 140°C 1 (R = methyl) reacts exclusively via chan-
nel A. In the case of 1 (R = benzyl), crossover experiments
and kinetic and stereochemical studies showed that channels
A and B compete in DMF at 140°C, with channel A com-
prising about 80% of the reaction. In chloroform at 140°C,
channels A and B compete equally.
the conversion of a 2-alkoxypyridine-1-oxide (1) into a 1-
alkoxy-2-pyridone (2) (2), is not an intramolecular [1s,4s]
sigmatropic rearrangement (3), as some workers had previ-
ously believed (4), but a two-step intermolecular process. In
the first step (see Fig. 1), an alkyl group R* is transferred,
with inversion of configuration, from one molecule of 1 to
the N-oxide of a second molecule, to form the anion of 2-
hydroxypyridine-N-oxide (3) and a 1,2-dialkoxypyridinium
cation (4). In the second step, the nature of which depends
upon the alkyl group and the solvent, one of the alkyl groups
of 4 is transferred, also with inversion of configuration, to
one of the oxygens of 3. The product is then formed, without
Ollis and co-workers (5) have made the interesting obser-
vation that 5 undergoes quantitative Tieckelmann rearrange-
ment to 6 in the solid state during 13 months at –15°C. The
topochemical nature (6) of this reaction was not examined,
but there are numerous related examples of intermolecular
methyl transfer which proceed under topochemical control
in the solid state (7). In each case, the crystal structure ex-
hibits an intermolecular X···CH3···Y angle close to linearity
Received January 29, 2001. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on August 22, 2001.
S.Wolfe,1 Y.-H. Hsieh, R.J. Batchelor, F.W.B. Einstein,
and I.D. Gay. Department of Chemistry, Simon Fraser
University, Burnaby, BC V5A 1S6, Canada.
1Corresponding author (telephone: (604) 291-5972; fax:
(604) 291-5973; e-mail: swolfe@sfu.ca).
Can. J. Chem. 79: 1272–1277 (2001)
DOI: 10.1139/cjc-79-8-1272
© 2001 NRC Canada

Wolfe et al.
1273
Fig. 1. Rearrangement of 1 (R = benzyl) to 2 (R = benzyl) with net inversion of configuration in the benzyl group (channel A) or with
net retention of configuration in the benzyl group (channel B).
and an X···Y distance of 4 to 5 Å, and the reaction occurs
exclusively, or is much faster, in the solid state than in solu-
tion or in the melt.
tabular prisms were employed for the determination of the
crystal structure. Figure 2 shows a portion of this crystal
structure, including relevant geometrical parameters. The
molecules in the set that includes A, B, and C are mutually
related by simple translations in the x-direction, as are the
molecules of the crystallographically distinct set that in-
cludes D and E. The figure displays an edge-viewed cross
section of a sheet structure, two-dimensionally extended par-
allel to the xy-plane, which is the result of the formation of
chains cross-linked by hydrogen bonds from water mole-
cules to nitroxide oxygen atoms.
Since the methyl group is positioned anti to the C—N
bond,2 Fig. 2 strongly suggests that an intramolecular trans-
fer of this group in the solid state is improbable. However,
there are several potential pathways for an intermolecular,
topochemically controlled Tiekelmann rearrangement: for
example, a transfer of methyl groups from A to B and from
B to C would convert B to product. Alternatively, a transfer
of methyl groups from A to D and from D to B, or from A
to D and D to E, etc., would convert D to product. The rele-
vant O···O distances in the crystal, which range from
4.322(2)–4.451(2) Å, are close to the calculated gas phase
value of 4.32 Å for the bimolecular complex, shown in
Fig. 5 of ref. 1; however, the O···C···O angles in the crystal,
which range from 107.5(2)–121.6(2)°, deviate substantially
from linearity and from the calculated gas phase angle of
174.2°, also shown in Fig. 5 of ref. 1. This is one possible
reason for our inability to observe an intermolecular solid-
state reaction (9). A second is that, in the solid, each
nitroxide oxygen is hydrogen-bonded to one or two water
molecules, and it is known that the solvation of a
nucleophile by just one water molecule greatly reduces the
rate of an intermolecular gas-phase SN2 reaction (12).
A distinction between these possibilities became possible
when one of us (Y.H.) discovered that a sample of crystal-
line 2-benzyloxypyridine-1-oxide (1, R = benzyl), which had
been maintained at room temperature for some time, had un-
dergone partial rearrangement to 2 (R = benzyl).
There is also an example of an intermolecular solid state
rearrangement that is observable within 1 week at 40°C, but
which, because of the molecular packing in the crystal, can-
not proceed under topochemical control (8). This is the cele-
brated reaction 7 → 8 which, as a paradigm of linear methyl
transfer (9), had previously been found to be intermolecular
in solution (10). To account for the non-topochemical nature
of the solid state reaction, Venugopalan et al. (8) have sug-
gested that intermolecular methyl transfer can take place at
defects such as microcavities, surfaces, and other irregulari-
ties in the ordered crystal arrangement.
2-Methoxypyridine-1-oxide (1, R = methyl) is reported to
crystallize in two forms (11): anhydrous needles, mp 78 to
79°C, and hydrated prisms, mp 66.5 to 67.5°C. In our hands,
despite considerable effort, only a hydrate, mp 73–76°C,
was observed (1), as needles from ethyl acetate, and as tabu-
lar prisms after boiling in toluene, followed by cooling. The
Figure 3 shows a portion of the crystal structure of 1 (R =
benzyl).
It can be seen that, except for the absence of water mole-
cules and the substitution of a phenyl group for one of the
methyl hydrogens, there is a similar arrangement of the mol-
2 The N-C-O-C torsion angles are 178.9(2)° and –178.5(3)°.
© 2001 NRC Canada

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Can. J. Chem. Vol. 79, 2001
Fig. 3. The crystal structure of 1 (R = benzyl), showing
Fig. 2. A portion of the crystal structure of 1 (R = methyl),
showing intermolecular O···O distances and O···C···O angles.
intermolecular O···O distances and O···C···O angles.
A
B
C
A
B
C
O---O = 4.36
O---C---O = 121.58
O---O =
4.2 Å
C-C--O = 115.5°
C-C-O-C = -76.1°
C-C--O-N = 25.5°
O---O = 4.32
O---C---O = 107.52 O---C---O = 111.82
O---O = 4.45
D
E
O---O = 4.38
O---C---O = 119.45
D
E
A
B
C
ecules of the methyl and benzyl compounds. Consequently,
the possibilities for intermolecular topochemical rearrange-
ment are also similar.
O---O = 4.1 Å
C-C--O = 108.7°
Referring to Fig. 3, molecules A, D, B, E, and C are se-
quentially related by a 21-screw operation. There is a second
crystallographically unique molecule in the structure which
shows a closely similar arrangement about an alternate
screw axis. One can visualize numerous combinations of rel-
atively small rotations of molecules and (or) internal rota-
tions about CH2—O, CH2—C, or Cring—O bonds which
would produce the 180° angle required for the concerted
transfer of a benzyl group, with inversion of configuration.
For example: (i) transfer from A to B and B to either C or E,
or from A to D and D to either B or E could produce 2 (R =
benzyl) with net inversion of configuration in the benzyl
group; alternatively, (ii) transfer from A to D, with inversion
of configuration, followed by counterclockwise rotation of
A, and return of the benzyl group to A with inversion of
configuration will produce 2 with net retention of configura-
tion in the benzyl group.
C-C--O-N = 90.9°
D
E
Fig. 4. The crystal structure of 2 (R = benzyl).
Figure 4 shows a portion of the crystal structure of 2 (R =
benzyl).
Molecules, related by simple translations in x, are shown,
and the nearest approaches between neighbouring NO and
methylene groups are indicated. The N-O-C-C dihedral an-
gle is 175.4(5)°. It is noteworthy that the much smaller
intermolecular C-C···O-N torsion angles listed in Fig. 3 per-
mit neither (i) nor (ii) above to produce this conformation
initially, so that subsequent conformational rearrangement of
2 (R = benzyl) within the host crystal of 1 (R = benzyl) may
be expected to lead to degradation of the crystal as the solid-
state reaction proceeds.
The foregoing analyses of the two crystal structures per-
mit the following tentative predictions: (i) the initial step of
a solid-state Tieckelmann rearrangement of 1 (R = methyl,
benzyl) will be slow, because of the substantial deviation of
the entering and leaving groups from linearity; (ii) in the
case of 1 (R = methyl), rearrangement is slowed further by
O---O = 3.986
O--C--O = 106.64
the presence of the water molecules;3 (iii) depending on the
nature of the intramolecular and intermolecular librations in
the crystal, the solid-state rearrangement of 1 (R = benzyl)
could potentially produce 2 with net inversion of configura-
tion, or with net retention of configuration; (iv) since the
antiperiplanar N-O-C-C conformation of 2 (R = benzyl) can
probably not be accommodated within the host lattice of 1
(R = benzyl), the reaction likely proceeds at domain bound-
aries or packing defects and will be accompanied by a loss
3 The intermolecular RCH2···O distances are from 0.15–0.36 Å larger in 1 (R = methyl, hydrate) than in 1 (R = benzyl).
© 2001 NRC Canada

Wolfe et al.
1275
1
Fig. 5. (a) The solid-state 13C NMR spectrum of a fresh sample
of 1 (R = benzyl); (b) the spectrum after 10 months; (c) the
spectrum after 25 months; (d) the spectrum after 36 months;
(e) the spectrum of an authentic sample of 2 (R = benzyl).
Fig. 6. Deuterium-decoupled H NMR spectra of the benzyl pro-
tons of the Mosher esters of: (a) chirally labelled benzyl-α-D-al-
cohol; (b) chirally labelled benzyl-α-D-alcohol recovered from 1
(R = benzyl), after partial rearrangement to 2; (c) chirally la-
belled benzyl-α-D-alcohol recovered from 2 after partial rear-
rangement.
of crystallinity. Indeed, the crystal of 1 (R = benzyl) used
for the X-ray structure determination decayed on a time
scale of months; remeasurement of selected reflections
showed that all had dropped to 28% of their original intensi-
ties after 130 days.
The conversion of 1 to 2 (R = benzyl) was also followed
at room temperature by solid-state 13C NMR, and was also
found to occur on a time scale of months. Figure 5 shows:
(a) the spectrum of a fresh sample of 1; (b) the spectrum af-
ter 10 months; (c) the spectrum after 25 months; (d) the
spectrum after 36 months; (e) the spectrum of an authentic
sample of 2. While not all carbon resonances are resolved in
the solid, the occurrence of the reaction is easily seen from
the decay of the peaks at 70, 110, and 118 ppm, and the
growth of peaks at 79, 106, 132, and 135 ppm. There is a
steady conversion of 1 to 2, and no peaks corresponding to
intermediates are observed. Unfortunately, 1 has a long spin-
lattice relaxation time, necessitating a repolarization delay of
90 s. This makes it impractical to signal average to a signifi-
cantly better signal-to-noise ratio than shown. Consequently
the presence of intermediates at levels up to a few percent of
the total cannot be excluded.
Assuming that the rearrangement occurs in more than one
step, with each step slow for the reasons already stated, tran-
sient formation of 3 and 4 can be expected, but these would
only have been observable in the initial stages of the reac-
tion.
To determine the stereochemical course of the rearrange-
ment, a sample of optically active 2-(α-deuteriobenzyloxy)-
pyridine-1-oxide was synthesized as described previously (1,
13), carefully crystallized from ethyl acetate, and allowed to
stand for 11 months. At this time the rearrangement was
found to be 85% complete. The reactant and product were
separated, benzyl alcohol was released from each, by treat-
ment with sodium methoxide in the case of 1, and by treat-
ment with zinc and acetic acid in the case of 2, and the
alcohols were converted to their Mosher esters (1, 14) for
benzyl-α-d-alcohol employed to prepare the initial sample of
1; (B) the benzyl-α-D-alcohol recovered from 1 following re-
arrangement; (C) the benzyl-α-D-alcohol recovered from 2
following rearrangement. Samples (A) and (B) are identical;
sample (C) is the epimer of (A).
It follows that the solid-state rearrangement of 1 (R =
benzyl) proceeds with essentially complete inversion of con-
figuration, in contrast to the 50–80% inversion observed in
1
determination of their H NMR spectra.
1
Figure 6 shows the deuterium-decoupled H NMR spectra
of the benzyl protons of the Mosher esters of: (A) the
© 2001 NRC Canada

1276
Can. J. Chem. Vol. 79, 2001
solution. Based on our earlier examination of Fig. 3, it can
be concluded that the topochemical rearrangement is an
intermolecular process that proceeds by transfer of a benzyl
group from A to B and B to C, etc., or by transfer from A to
D, D to B, B to E, and E to C, etc., with inversion of config-
uration in every step.
sample was undergoing dehydration in the glove box to pro-
duce a lower melting substance or mixture. Full-matrix
least-square refinement, on F, of 254 parameters for 1558
data (Io > 2.5 σ(Io)) included independent coordinates for all
atoms, anisotropic displacement parameters for the non-
hydrogen atoms, and a single isotropic thermal parameter for
each of the following three groups of hydrogen atoms: those
on the C5N-rings, those of the methyl groups, and those of
the water molecules.
Experimental
A crystal of 1 (R = benzyl)5 obtained from ethyl acetate
solution was mounted on a glass fiber using epoxy adhesive.
While less than 1% decays of the intensity standards were
observed during data acquisition, remeasurement of three of
the stronger reflections showed a systematic decay over a
period of 130 days, by which time the net intensities had all
dropped to 28% of their original values. This net loss of in-
tensity appeared to apply equally to all the reflections tested.
A peak search at the end of that time revealed only reflec-
tions indexed on the original cell and orientation. Full-
matrix least-squares refinement, on F, of 273 parameters for
1728 data (Io > 2.5σ(Io)) included independent coordinates
and anisotropic displacement parameters for the non-
hydrogen atoms. Hydrogen atoms were riding in calculated
positions (C—H, 0.95 Å) and were initially assigned isotro-
pic thermal parameters proportionate to the equivalent iso-
tropic parameter of the respective carbon atoms.
Subsequently, all hydrogen atom isotropic thermal parame-
ters were constrained to have equivalent shifts.
NMR Studies
The sample used for NMR experiments was supplied as a
powder of fine crystals. This was loosely packed into a seg-
ment of 5 mm NMR tubing, and remained in this tube over
the course of all measurements. The sample had a tendency
to “clump” over time, and was occasionally stirred with a
spatula and repacked. During the whole course of the NMR
experiments, the sample was exposed to the atmosphere, and
was maintained at ambient laboratory temperature, 23 ±
5°C. Probe temperatures during NMR experiments were also
within this range.
Solid-state 13C NMR spectra were recorded at 37.56 MHz
on a 3.5T instrument. H–13C cross-polarization was used,
1
with a contact time of 2 ms and a relaxation delay of 90 s.
The sample was spun at the magic angle at 2.4 kHz; at this
speed and field, small spinning sidebands are seen for the sp
carbons. These were suppressed, in the spectra shown, by a
phase-cycled TOSS pulse sequence. Proton decoupling fields
of 50–85 kHz were used, the higher fields giving a slight im-
provement in line width. Resonances were referred to TMS
using solid adamantane as a secondary standard, taking the
shift of the high-frequency line to be 38.56 ppm.
A fragment cleaved from a crystal of 2 (R = benzyl)6 ob-
tained from hexane solution was mounted on a glass fiber
using epoxy adhesive. Full-matrix least-squares refinement,
on F, of 138 parameters for 570 data (Io > 2.5σ(Io)) was con-
ducted analogously to that of 1 (R = benzyl).
Stereochemical studies
All data were acquired at room temperature using an
Enraf–Nonius CAD-4F diffractometer and with graphite-
monochromatized Mo Kα radiation. Complex scattering fac-
tors for neutral atoms (15) were used in the calculation of
structure factors. The programs used for data reduction,
structure solution, and graphical output were from the
NRCVAX Crystal Structure System (16). The program suite
CRYSTALS (17) was employed in the refinement. Full de-
tails of the X-ray crystallographic structure determinations
have been deposited as hardcopy and in CIF format.7
Full details of the synthesis of chirally deuterated 1, of the
recovery of benzyl-α-D-alcohol from 1 and 2, of the conver-
sion of these alcohols to Mosher esters, and of the NMR
analyses of these esters can be found in ref. 1.
X-ray crystallography
One large crystal of 1 (hydrate, R = methyl)4 (obtained
from toluene solution and believed to be anhydrous) was ex-
posed to a dry nitrogen atmosphere in a crystallographic
glove box while it was cleaved to produce the fragment used
for crystallographic analysis. When the crystal was exposed
to the dry atmosphere it appeared to begin to liquefy. None-
theless a fragment was successfully sealed in a capillary
tube, after which time it ceased to “melt” while the remain-
ing fragments liquefied completely. It seems likely that the
Acknowledgement
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC).
4 Crystal structure of 1-Me-hydrate: colourless prism, C6H7NO2·1.5H2O, triclinic, space group P1, Z = 4, a = 6.5109(8) Å, b = 6.8620(13) Å,
c = 17.4633(23) Å, α = 79.511(13)°, β = 88.944(11)°, γ = 73.374(12)°, V = 734.64 Å3, T = 293 K, RF = 0.028, GoF = 1.49.
5 Crystal structure of 1-Bz: colourless prism, C12H11NO2, monoclinic, space group P21/a, Z = 8, a = 16.4480(20) Å, b = 6.3356(7) Å, c =
20.941(3) Å, β = 111.597(9)°, V = 2029.0 Å3, T = 293 K, RF = 0.041, GoF = 1.86.
6 Crystal structure of 2-Bz: colourless prism, C12H11NO2, orthorhombic, space group Pc21b,; Z = 4, a = 5.8898(5) Å, b = 13.0782(14) Å, c =
13.3848(12) Å, ν = 1031.00 Å3, T = 293 K, RF = 0.040, GoF = 1.59.
7 Full details of the crystallographic structure determinations, displacement ellipsoid plots of the molecular structures, and a diagram of the ex-
tended H-bonding in 1-Me-hydrate, tables of crystallographic details, atomic coordinates, distances and angles, anisotropic displacement pa-
rameters, and least-squares planes are available as supplementary data, and may be purchased from the Depository of Unpublished Data,
Documentary Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0S2, Canada (http:\\www.nrc.ca\cisti\irm\unpub_e.shtml
for information on ordering electronically). The CIF file is also available. Tables of crystallographic details, atomic coordinates, and dis-
tances and angles have also been deposited with the Cambridge Crystallographic Data Centre. Copies of the data can be obtained free of
charge, on application to the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (Fax: 44-1223-336033 or e-mail: Structure fac-
tors are no longer being deposited and may be obtained from the author.
© 2001 NRC Canada

Wolfe et al.
1277
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9. S. Wolfe, C.-K. Kim, K. Yang, N. Weinberg, and Z. Shi. Can.
J. Chem. 76, 359 (1998).
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