Regioselective formation of a 2,3-oxaphosphabicyclo[2.2.2] octene 3-oxide in baeyer-villiger type oxidation; a dual pathway for its fragmentation

Article abstract of DOI:10.1002/hc.20213

The O-insertion reaction of a 7-phosphanorbornene (3) unsubstituted on the double-bond gave the corresponding 2,3-oxaphosphabicyclo-[2.2.2]octene oxide (4a) in a regioselective manner that was useful in the fragmentation-related phosphonylation of alcohol

Full text of DOI:10.1002/hc.20213

Heteroatom Chemistry
Volume 17, Number 5, 2006
Regioselective Formation of a
2,3-Oxaphosphabicyclo[2.2.2]octene 3-oxide
in Baeyer–Villiger Type Oxidation; a Dual
Pathway for Its Fragmentation
Stefan Jankowski,1 Ja´nos Kova´cs,2 Krzysztof Huben,1
3
3
2
ꢀ
ꢀ
ꢀ
Michal Blaszczyk, Marek Glo´wka, and Gyo¨rgy Keglevich
1Institute of Organic Chemistry, Department of Chemistry, Technical University of Lodz,
Zeromskiego 116, 90-924 Lodz, Poland
2Department of Organic Chemical Technology, Budapest University of Technology and Economics,
1521 Budapest, Hungary
3Institute of General and Ecological Chemistry, Department of Chemistry, Technical University of
Lodz, Zeromskiego 116, 90-924 Lodz, Poland
Received 17 November 2005; revised 16 December 2005
in Wiley InterScience (www.interscience.wiley.com). DOI
ABSTRACT: The O-insertion reaction of a 7-phos-
10.1002/hc.20213
phanorbornene (3) unsubstituted on the double-
bond gave the corresponding 2,3-oxaphosphabicyclo-
[2.2.2]octene oxide (4a) in a regioselective manner
INTRODUCTION
that was useful in the fragmentation-related phospho-
nylation of alcohols. Both the UV-light mediated and
the thermoinduced phosponylation accomplished on
the bridged P-heterocycle (4a) were found to be sensi-
tive toward steric factors, suggesting that beside the
well-known elimination–addition reaction path tak-
ing place via metaphosphonate (11), a competitive
novel addition–elimination route involving an inter-
mediate with a pentacoordinated P-atom (12) is also
present. This was confirmed by the kinetic considera-
The Baeyer–Villiger reaction has been applied for the
oxidation of ketones with peroxides into esters or
lactones for over a hundred years [1]. For acyclic
substrates, a general preference for the migration of
the most electron-rich alkyl group (more substituted
carbon) was established from the literature exam-
ples. The steric effects can, however, compete with
electronic effects in the oxidation of norbornanone
derivatives [2–4].
The 7-phosphanorbornene oxides 1 can be eas-
ily transformed by mCPBA into oxaphosphabicy-
clo[2.2.2]octenes 2 [5]. The peroxyacid attacks the
phosphorus atom from both sides affording the
products as two regioisomers (Scheme 1). The ratio
of regioisomers 2 depends on the P-substituent
(Y = EtO [6], Et2N [7], MesNH [8], and P-aryl
[9]). Recently, the steric effect of the P-aryl sub-
stituents (Y = 2,4,6-tri-isopropylphenyl, mesityl or
ꢀ
C
tion of our experimental data. 2006 Wiley Periodicals,
Inc. Heteroatom Chem 17:369–375, 2006; Published online
Correspondence to: Stefan Jankowski; e-mail: jankowsk@p.
lodz.pl.
Contract grant sponsor: OTKA.
Contract grant number: T 042479.
Contract grant sponsor: Technical University of Lodz.
Contract grant number: D.S. I-18/10.
2006 Wiley Periodicals, Inc.
c
ꢀ
369

370 Jankowski et al.
SCHEME 2
SCHEME 1
in comparison with the oxidation of P-phenyl
7-phosphanorbornene derivative with a methyl
group attached to the double bond in position 3 were
observed. The O-insertion reaction was about five
times slower, and only a single product was detected
by 31P NMR. 13C NMR spectrum of the product sug-
gested structure 4a revealing a 2 J(PC) coupling con-
stant of 0.6 Hz on the skeletal methyl group of the
product that is in agreement with literature exam-
ple [14]. This coupling could also be a 3 J(PC) in the
other isomer (4b) [15]. Single crystal X-ray analy-
sis of the product confirmed, however, the structure
represented by 4a (Scheme 2, Fig. 1).
Theoretically, two possible intermediates of
the oxidation of 3 are 5a or 5b, possessing one
of the P C bonds and the peroxy group in apical
positions. The migration of the peroxy group into
equatorial position (as in 6a and 6b) is followed
by the insertion of the oxygen atom into the P C
bond that is in apical position (Scheme 3). Our
observation that only product 4a was formed is on
the contrary to the rule of the preference for the
higher bonding carbon migration. In the Baeyer–
Villiger type oxidation of 7-phosphanorbornene
3 the steric effect dominates. In pentacovalent
intermediate 6a, the more crowded carbon atom
prefers the equatorial position and is intact against
the attack of the peroxy group. Faster migration
of the less substituted carbon atom during the
4-methylphenyl) of 7-phosphanorbornene 7-oxides
on reaction with mCPBA was studied in detail [9].
When a 7-phosphanorbornene labeled with O-18
in the P O group was treated with mCPBA, the
product of O-insertion possessed the heavy atom
in the phosphoryl group [9]. The oxygen atom in
the bridge came from the peracid, as was observed
in tracer experiments involving the peracetic acid
oxidation of ketones [10,11]. This observation is con-
sistent with the involvement of the “Criegee” mech-
anism [12].
As a continuation of our studies on the Baeyer–
Villiger oxidation of 7-phosphanorbornene 7-oxides,
we decided to examine the effect of substitution of
carbon atoms adjacent to the phosphorus atom. For
this, 7-phosphanobornene model compound 3, bear-
ing one methyl group attached to the carbon atom
adjacent to the phosphorus atom and synthesized
from a 2-methyl-2,5-dihydro-1H-phosphole oxide,
was chosen to be the starting material [13].
RESULTS AND DISCUSSION
O-Insertion into 7-Phosphanorbornene
Framework
The O-insertion into 7-phosphanorbornene 3 by
means of mCPBA was carried out according
to a known procedure [6–9]. Two differences
FIGURE 1 The view of molecular structure of 4a in solid state.
Heteroatom Chemistry DOI 10.1002/hc

Regioselective Formation of a 2,3-Oxaphosphabicyclo[2.2.2]octene 3-oxide in Baeyer–Villiger Type Oxidation 371
SCHEME 3
Baeyer–Villiger oxidation of trans-2,2,3-trimethyl-1-
phenylphosphetane 1-oxide and 5-isopropyl-2,6-di-
methyl-6-phosphabicyclo[3.1.1]hept-2-ene 6-oxide
(7) was observed previously by Quin et al. [16]. The
result of the second example is shown in Scheme 4.
Compound 7 is similar to 3, but as the epoxidation
occurred faster than the oxygen insertion, the
product of the reaction with mCPBA was identified
as 8.
The necessary time for the consumption
of the oxaphosphabicyclooctene oxide (4a) in-
creased when methanol was replaced by tert-
butanol regardless of the method (thermal or
photochemical) applied. Thermolysis of 4a was
about twice as fast as that of P-phenyl-2,3-
oxaphosphabicyclo[2.2.2]octene derivative with a
methyl group attached to the double bond in posi-
tion 3 [9].
Regarding the photolysis, the fragmentation-
related phosphorylation was ca. 3.5 times slower us-
ing tert-butanol instead of methanol. Photolysis of
substrate 4a in the presence of an equimolar mix-
ture of methanol and tert-butanol led to a mixture
containing the corresponding phosphonic acid es-
ters (9, R = Me and t-Bu) in a ratio of 77/23. Using
tert-butanol in a twofold quantity, the ratio of the
methyl and tert-butyl esters was 67/33. It can be seen
that the steric hindrance brings about a significant
discrimination between the alcohols.
Thermolysis of cycloadduct 4a in the presence of
a 1:2 mixture of methanol and tert-butanol afforded
a mixture containing the corresponding phosphonic
acid esters (9, R = Me and t-Bu) in a nearly equal
Fragmentation Reactions of
2,3-Oxaphosphabicyclo[2.2.2]octane 2-oxide 4a
in the Presence of Alcohols
The fragmentation of 2,3-oxaphosphabicyclo[2.2.2]-
octane 4a was accomplished by thermolysis in
toluene at 110˚C, in the presence of methanol or tert-
butyl alcohol, or by irradiation at 254 nm in acetoni-
trile or in 1,2-dichloroethane in the presence of an
alcohol to give phosphonic acid ester 9. Esterifica-
tion of 9 with diazomethane gave phosphonate 10
(Scheme 5).
SCHEME 4 [15]
SCHEME 5
Heteroatom Chemistry DOI 10.1002/hc

372 Jankowski et al.
TABLE 1 Photolysis of 4a in the Presence of Alcohol ROH
in Acetonitrile (Irradiation Time 40 min)
Yield a of 9 (%)
Alcohol Concentration (M)
R = Me
71
R = t-Bu
0.68
0.75
0.90
1.13
1.50
2.25
44
73
61
74
74
76
aEstablished on the basis of relative 31P NMR intensities of product
9 and substrate 4a.
obtained in reaction with methanol was constant
(ca. 73%) over the concentration range of 0.68–
1.5 M. In the presence of a sterically hindered
alcohol, such as t-BuOH, the product percentage was
found to increase up till 74% in the cases, when
the concentration of alcohol was 1.5 M or higher
(Table 1). This experience is in accord with
the involvement of two mechanisms (elimination–
addition and addition–elimination). The proposed
kinetic scheme is outlined in Scheme 7.
According to Scheme 7, light absorption by 4a
produces an excited molecule 4a*, which is frag-
mented to metaphosphonate 11 (k1), deactivated
(k−1) or reacts with alcohol (k3) to form pentacoordi-
nated intermediate 12. Application of the stationary
steady-state approximation for 4a*, 11, and 12 gives
the rate of product formation as shown by Eq. (5):
SCHEME 6
ratio. This means that the phosphorylation of tert-
butanol is significantly slower than that of methanol.
Mechanism of the Fragmentation-Related
Phosphorylations of Alcohols by 2,3-Oxaphos-
phabicyclo[2.2.2]octene 2-oxide (4a)
It is known that the fragmentation-related phos-
phorylation of a nucleophile by an oxaphosphabi-
cyclo[2.2.2]octene oxide takes place according to an
elimination–addition (EA) mechanism, which is not
sensitive to the structure and concentration of the
alcohol (Scheme 6). Our experience that the reac-
tion is significantly slower using tert-butanol instead
of methanol may, however, suggest the involvement
of the addition–elimination (EA) bimolecular reac-
tion path via an intermediate with a pentacoordi-
nate phosphorus atom formed by the addition of the
alcohol on the P O group (Scheme 6). The sensi-
tivity of the reaction rate to the steric effect of an
alcohol may be in accord with the novel AE mecha-
nism. As a matter of fact, it seems to be probable that
the fragmentation-related phosphorylation proceeds
according to concurrent EA and AE reaction paths.
This situation has recently been substantiated for
the fragmentation-related phosphorylations of nu-
cleophiles by 2-phosphabicyclo[2.2.2]octene 2-oxide
derivatives, where the AE components have never
been assumed before [9].
d[9]/dt = I(k1 + k3[ROH])/(k−1 + k1 + k3[ROH]) (5)
where I is the rate of light absorption.
If the alcohol concentration is low enough to al-
low for k−1 + k1 ꢁ k3[ROH], Eq. (5) is simplified to
Eq. (6):
d[9]/dt = I(k1 + k3[ROH])/(k−1 + k1)
(6)
According to Eq. (6), the rate of photolysis depends
on the rate constants of uni- and bimolecular pro-
cesses, and increases with the alcohol concentration.
(1)
(2)
The effect of alcohol concentration on the yield
of photolysis was examined in detail in small scale.
Acetonitrile solutions of 4a with different concen-
trations of alcohols were irradiated in a quartz NMR
tube placed in Rayonet reactor for 40 min. The rel-
ative amount of phosphonic derivative 9, R = Me
(3)
(4)
SCHEME 7
Heteroatom Chemistry DOI 10.1002/hc

Regioselective Formation of a 2,3-Oxaphosphabicyclo[2.2.2]octene 3-oxide in Baeyer–Villiger Type Oxidation 373
When the alcohol concentration is high enough
it is thermo- or photoinduced. The involvement of
the concurrent EA and AE mechanisms is supported
by the experimental data.
that the ratio (k1 + k3[ROH])/(k−1 + k1 + k3[ROH]) be-
comes close to unity, Eq. (5) is simplified to
d[9]/dt = I. The rate of product formation becomes
constant and is limited only by the rate of light ab-
sorption. The light absorption-controlled value is
reached faster in the case of methanol than with
tertiary butanol, as in the bimolecular reaction (4)
one can expect that MeOHk3 > t-BuOHk3. This means that
indeed concurrent EA and AE mechanisms oper-
ate, and the model can be adequately described by
Eq. (5).
EXPERIMENTAL
NMR spectra were recorded on a Bruker Avance DPX
250 spectrometer at 101.20 MHz (31P) and 250.13
MHz (1H), 62.86 MHz (13C) in CDCl3, using tetra-
methylsilane as internal and 85% H3PO4 as exter-
nal standard. Solvents were dried as follows: CH3CN
and 1,2-dichloroethane by distillation from P2O5;
toluene, by distillation over Na in the presence of
benzophenone.
Photolysis of 4a in 1,2-dichloroethane was car-
ried out in the Rayonet reactor. The photochemi-
cally mediated fragmentation afforded phosphonic
acid esters 9 in lower yield (50–60%) than achieved
in acetonitrile. Two additional signals in 31P NMR
spectra were observed in the ratio of about 1:3. The
minor signal appeared at δ 78.9 (C2H4Cl2) that was
independent on the alcohol used. The other signals
were recorded at δ 45.9 (methanol, C2H4Cl2) and
δ 38.5 (tert-butanol, C2H4Cl2). The rearrangement of
2,3-oxaphosphabicyclo[2.2.2]octenes (1-adamantyl,
neopentyl, ethyl as O-substituents, phenyl) accompa-
nying usual metaphosphate extrusion was observed
at −75◦C and at room temperature with very low
yield [17,18]. It was concluded that after the C O
bond cleavage, the oxygen atom can be transferred to
the adjacent sp2 carbon resulting in a cyclopropane
ring. According to this mechanism 4a may rearrange
to 14 (Scheme 8), in which the cyclic phosphinate
ring can be opened in the presence of an alcohol as
it is in 15. The downfield shift at δP 78.9 signal seems
to be appropriate for structure 14 and the upfield
signals at δP 45.9 (R = Me) or 38.5 (R = t-Bu) to 15.
The chemical shift of δP 65.6 measured in propioni-
trile at −75◦C for compound 16 [17] supports our
assignment in regard to structure 14. The suggested
rearrangement needs further examination.
7-Methyl-2,8-diphenyl-3a,4,7,7a-tetrahydro-1H-
4,7-(epoxyphosphano)isoindole-1,3-dione
8-oxide (4a)
2.5 g (purity 77%, 8.7 mmol) of mCPBA was added to
the 20 cm3 of dry CHCl3 solution of 0.80 g (2.2 mmol)
of 7-phosphanorbornene 3 [13]. The solution was
stirred at room temperature and monitored by 31P
NMR. After the competition of the reaction, 2.5 g
(43.1 mmol) of KF was added and the mixture was
stirred at room temperature for 2 h. The suspension
was filtered off (Celite 500), and the volatile compo-
nents were removed. The crude product was purified
by crystallization from AcOEt to give product 4a in
0.56 g (67%).
Colorless solid, mp 182–184◦C (ethyl acetate);
IR, νmax (CCl4) 2928, 1708, 1392, 1232, 1200, 984,
784 cm−1; 31P NMR (CDCl3) δ 37.5; 13C NMR
(CDCl3) δ 14.4 (J = 0.6, C7 − Me), 40.1 (J = 6.3,
C7a), 41.1 (J = 67.9, C7), 47.6 (J = 12.6, C3a), 71.8
a
b
ꢂꢂ
ꢂ
(J = 8.8, C4), 126.2 (C2 ), 128.5 (J = 13.2, C3 ), 128.8
a
ꢂ
ꢂ
ꢂꢂ
ꢂꢂ
(J = 83.0, C1 ), 129.0 (C4 ),129.2 (C3 ), 131.2 (C1 ),
b
ꢂꢂ
ꢂ
131.6 (J = 9.4, C5), 133.2 (C4 ), 134.2 (J = 9.4, C2 ),
138.6 (J = 6.3, C6), 173.0 (C3), 174.5 (J = 15.1, C1);
a,b
1
may be reversed; H NMR (CDCl3) δ 1.72 (3H,
d, J = 17.0 Hz, C7CH ), 3.78 (1H, t, J = 8.2 Hz,
3
C7a H), 4.23 (1H, dd, J = 8.2, 3.8 Hz, C3a H), 5.60
(1H, m, C4 H), 6.17 (1H, t, J = 8.2 Hz, C6 H), 6.68
(1H, dt, J = 8.2, 5.5 Hz, C5 H), 7.19–7.16 (2H, m,
HAr), 7.64–7.37 (6H, m, HAr), 7.76–7.68 (2H, m, HAr);
IR, νmax (CCl4) 2928, 1708, 1392, 1232, 1200, 984,
784 cm−1; MS, (M + H)f+ound = 380.1038, C21H19NO4P
requires 380.1052.
16
Photolysis of 2,3-Oxaphosphabicyclo[2.2.2]-
octene Oxide (4a)
In conclusion, steric reasons allowed the re-
gioselective synthesis of a new 2,3-oxaphosphabi-
cyclo[2.2.2]octenes 3-oxide, a useful precursor in
the phosphonylation of alcohols. An alternative
mechanism of novel type was substantiated for the
fragmentation-related phosphorylation no matter if
1. The solution of 0.3 mmol of cycloadduct 4a in the
mixture of 40 cm3 of acetonitrile and 4 cm3 of al-
cohol was irradiated with a higher light intensity
source 125 W mercury lamp in a photochemical
Heteroatom Chemistry DOI 10.1002/hc

374 Jankowski et al.
SCHEME 8
quartz reactor for 30 min at 26◦C. Volatile compo-
nents were removed in vacuo to give phosphonic
acid-ester 9.
an NMR tube (5 mm) under argon. The sample was
placed in a thermostat set to 110◦C, and the reaction
was monitored by 31P NMR until the signal of sub-
strate (4a) diminished. The reaction time was 25 and
35 min, respectively. The volatile components were
removed, and products were identified as phospho-
nic acid esters 9 (R = Me, t-Bu) by 31P NMR.
9, R = Me: Yield, 92%; 31P NMR (CDCl3) δ 22.0
(δPlit19.6 [19], 20.1 [20]); MS, (M + H)+ = 173.
9, R = t-Bu: Yield, 83%; 31P NMR (CDCl3) δ 17.6
(δlPit 11.6 crude sample in tert-BuOH [19]): MS,
(M + H)+ = 215.
9, R = Me, 31P NMR (CDCl3) δ 18.9 (δlPit 19.6 [19]);
The diethylether solution of phosphonic
acid-ester 9 was reacted with an excess of dia-
zomethane in diethylether at 26◦C. The crude
products obtained after evaporating the volatile
components was purified by flash column chro-
matography on silica gel to furnish 4-methyl-
1,3-dimethylene-2-phenyl-2,3,3a,7a-tetrahydro-1
MS, (M + H)+ = 173.
9, R = t-Bu, 31P NMR (CDCl3) δ 12.3 (δlPit 11.6 [19]);
MS, (M + H)+ = 215.
The dichloromethane solution of the phospho-
nic acid ester (9) was treated with an excess of dia-
zomethane in diethylether at 0◦C. After completion
of the esterification, the solution was evaporated to
dryness. The crude product so obtained was purified
by flash column chromatography (as above) to afford
phosphonic ester 10.
H-isoindole
13
(chloroform/hexane,
2:1)
and phosphonic ester 10 (3% methanol in
chloroform).
10, R = Me: Yield, 89%; 31P NMR (CDCl3) δ 22.3
(δlPit 22.5 [21]); MS, (M + H)+ = 187.
10, R = Me: Yield, 90%; 31P NMR (CDCl3) δ 22.0
10, R = t-Bu: Yield, 74%; 31P NMR (CDCl3)
δ 16.4; 13C NMR (CDCl3) δ 30.4 (C(CH3)),
52.2 (J = 5.7, O CH3), 83.5 (J = 8.1, C(CH3)),
(δlPit 22.5 [21]); MS, (M + H)+ = 187.
10, R = t-Bu: Yield, 81%; 31P NMR (CDCl3)
δ 16.1; MS, (M + H)+ = 229.
ꢂ
ꢂ
128.4 (J = 15.4, C3 ), 131.6 (J = 9.9, C2 ), 132.2
(J = 6.7, C4 ); 1H NMR (CDCl3) δ 1.58 (s,
ꢂ
Crystal Data of 4a
9H, C(CH )3), 3.67 (d, J = 11.0, 3H, O CH ),
3
3
7.32–7.61 (m, 3H, Ar H), 7.75–7.86 (m, 2H,
Ar H); m/z (FAB/NBA) 229 (3, MH+), 173
(100), 133 (17), 107 (3).
C21H18NO4P, M = 379.33, colorless prism, mon-
oclinic, P21/c, Z = 4, a = 11.150(2), b= 6.876(1),
◦
3
˚
˚
c= 25.877(3) A, β = 113.42(1) , V = 1820.6(4) A ,
13, 120.5–121.5◦C (lit. [22] mp 120–123◦C); 1H
T = 291 K, μ (Mo Kꢀ) = 0.178 mm−1, R = 0.0736 for
1
NMR (CDCl3) δ 2.11 (s, 3H, CH ), 3.68 (d,
3
1660 observed, wR2 = 0.139 for all 3198 reflections
collected, S= 1.063. Data collection: KM-4 diffrac-
tometer with CCD detector. Hydrogen atoms were
refined in riding positions with free isotropic tem-
perature parameters.
1H, J = 12.5 Hz, H-C3a), 3.95 (dt, 1H, J = 12.5,
2.8 Hz, H-C7a), 5.72 (dd, 1H, J = 9.5, 3.9 Hz, H-
C7), 5.80 (d, 1H, J = 5.9 Hz, H-C5), 5.98 (ddd,
1H, J = 9.5, 5.9, 2.8 Hz, H-C6), 7.23–7.53 (m,
5H, Ph).
Crystallographic data for the structural
analysis has been deposited with Cambridge
Crystallographic Data Centre, CCDC No. 284210
for compound 4a. Copies of this information
can be obtained free of charge from: The
Director CCDC, 12 Union Road, Cambridge,
CB2 1EZ UK. Fax. (Int code) +44(1223)336-033
or Email: deposit@cccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk.
2. In photolysis carried out in small scale, we applied
the Rayonet reactor fitted with eight lamps, and
the procedure was reported recently [9].
Thermolysis of 2,3-Oxaphosphabicyclo[2.2.2]-
octene Oxide (4a)
A solution (1 cm3) of 4a (0.02 mmol) and MeOH
or tert-BuOH (2 mmol) in dry toluene was sealed in
Heteroatom Chemistry DOI 10.1002/hc

Regioselective Formation of a 2,3-Oxaphosphabicyclo[2.2.2]octene 3-oxide in Baeyer–Villiger Type Oxidation 375
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ACKNOWLEDGMENTS
Authors are thankful to Dr. Tomasz Nonas (TU of
Lodz) for his assistance in the preliminary experi-
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Heteroatom Chemistry DOI 10.1002/hc