Comparative spectroscopic and theoretical studies on the conformation of some α-diethoxyphosphoryl carbonyl compounds and their α-ethylsulfonyl analogues

Article abstract of DOI:10.1039/b005501h

Comparative v_co IR analysis of β-carbonylphosphonates [XC(O)CH₂P(O)(OR)₂: X = Me 1, Ph 2, OEt 3, NEt₂ 4 and SEt 5; R = Et] (series I) and β-carbonylsulfones [XC(O)CH₂SO₂R: X = Me 6, Ph 7, OEt 8, NEt₂ 9 and SEt 10; R = Et] (series II) along with ab initio 6-31 G** calculations on la and 6a (R = Me) suggest the existence of only a single gauche conformer for series I. The negative carbonyl frequency shifts for both series follow approximately the electron-affinities of the π*_co orbital of the parent compounds MeC(O)X 11-15. The less positive asymmetric sulfonyl frequency shifts (Δv_SO2) for II in relation to the phosphoryl frequency shifts (Δv_PO) for I and the larger negative carbonyl frequency shifts for II with respect to the corresponding values for I are in line with the upfield ¹³C NMR chemical shifts of the carbonyl carbon for II compared to I. These trends agree with the shorter O_(SO2) ...C_(CO) contact in comparison with the O_(PO) ... C_(CO) one and are discussed in terms of O_lp→π*_co charge transfer and electrostatic interactions, which are stronger for series II than for I, indicating that the sulfonyl oxygen atom is a better electron donor than the phosphoryl oxygen atom. Intrinsic geometrical parameters of O=S-CH₂ and O=P-CH₂ moieties seem to be responsible for this behaviour as indicated by X-ray study and ab initio calculations of dialkyl (methylsulfonyl)methylphosphonate MeSO₂CH₂P(O)(OR)₂(R = Et l8, Me 18a). The Royal Society of Chemistry 2001.

Full text of DOI:10.1039/b005501h

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Comparative spectroscopic and theoretical studies on the
conformation of some ꢀ-diethoxyphosphoryl carbonyl compounds
and their ꢀ-ethylsulfonyl analogues1
2
Paulo R. Olivato,*a Rubens Ruiz Filho,a Julio Zukerman-Schpector,b Maurizio Dal Collec
and Giuseppe Distefano*c
a Instituto de Química, Universidade de São Paulo, C.P. 26077 - CEP 05513-970, São Paulo,
SP Brazil. E-mail: prolivat@iq.usp.br; Fax: ϩ55 11 3815 5579
b Departamento de Química, Universidade Federal de São Carlos, Brazil
c Dipartimento di Chimica, Università di Ferrara, Via Borsari, 46, 44100, Ferrara, Italy
Received (in Cambridge, UK) 10th July 2000, Accepted 31st October 2000
First published as an Advance Article on the web 11th December 2000
Comparative νCO IR analysis of β-carbonylphosphonates [XC(O)CH2P(O)(OR)2: X = Me 1, Ph 2, OEt 3, NEt2 4 and
SEt 5; R = Et] (series I) and β-carbonylsulfones [XC(O)CH2SO2R: X = Me 6, Ph 7, OEt 8, NEt2 9 and SEt 10; R = Et]
(series II) along with ab initio 6-31G** calculations on 1a and 6a (R = Me) suggest the existence of only a single
gauche conformer for series I. The negative carbonyl frequency shifts for both series follow approximately the
electron-affinities of the π*CO orbital of the parent compounds MeC(O)X 11–15. The less positive asymmetric
sulfonyl frequency shifts (∆νSO ) for II in relation to the phosphoryl frequency shifts (∆νPO) for I and the larger
2
negative carbonyl frequency shifts for II with respect to the corresponding values for I are in line with the upfield 13
C
NMR chemical shifts of the carbonyl carbon for II compared to I. These trends agree with the shorter O(SO ) ؒ ؒ ؒ C(CO)
2
contact in comparison with the O(PO) ؒ ؒ ؒ C(CO) one and are discussed in terms of Olp→π*CO charge transfer and
electrostatic interactions, which are stronger for series II than for I, indicating that the sulfonyl oxygen atom is
a better electron donor than the phosphoryl oxygen atom. Intrinsic geometrical parameters of O᎐S–CH and
᎐
2
O᎐P–CH moieties seem to be responsible for this behaviour as indicated by X-ray study and ab initio calculations
᎐
2
of dialkyl (methylsulfonyl)methylphosphonate MeSO2CH2P(O)(OR)2 (R = Et 18, Me 18a).
Our previous spectroscopic (IR, 13C NMR, UV and UPS), X-
ray diffraction and theoretical studies of some β-carbonyl-
sulfides and -sulfones XC(O)CH2SOnR2–13 (X = Me, Ar, NR2,
OR and SR; R = Me, Ar; n = 0 and 2) indicated that these com-
pounds in the gas phase, in solution and in the solid state (for
the β-carbonylsulfones) prefer a gauche conformation between
compounds (EtO)2P(O)CH2C(O)X (X = Me 1, Ph 2, OEt 3,
NEt2 4 and SEt 5) by means of IR and 13C NMR spectroscopies
and ab initio calculations in order to compare these data with
those previously reported for the corresponding α-ethylsulfonyl
carbonyl compounds2,9,11 EtSO2CH2C(O)X (X = Me 6, Ph 7,
OEt 8, NEt2 9 and SEt 10). This paper also reports the X-ray
diffraction data and the results of ab initio calculations on
dialkyl (methylsulfonyl)methylphosphonates (EtO)2P(O)CH2-
SO2R (R = Et 18, Me 18a), necessary to obtain the experi-
mental geometric parameters of the (EtO)2P(O)CH2– group
which cannot be easily obtained from the liquid compounds
1–5. Moreover, compound 18 allows a comparison of the
relative electron-donor abilities of the sulfonyl and phosphoryl
oxygen atoms.
the C᎐O and CH –S bonds. For the corresponding β-carbonyl-
᎐
2
sulfoxides XC(O)CH2S(O)R2,5,9,13–15 the gauche rotamer is
also the more stable, but the cis one prevails for some X or R
substituents due to electronic16,17 or steric17 factors.
In general, the stability of the gauche rotamers of β-carbonyl-
sulfides, -sulfoxides and -sulfones has been ascribed to π*CO
/
σC–S and πCO/σ*C–S orbital interactions. However, in the case
of β-carbonyl-sulfones10,11 and -sulfoxides,14,15 additional
stabilisation of the gauche (or cis) rotamer derives from
crossed electrostatic and charge transfer interactions between
oppositely charged atoms i.e. O(SO )→C(CO) and (or) O(CO)→S(SO
Experimental
Materials
)
n
2
(n = 1 and 2).
The relevant electronic properties of closely related mole-
cules differing only in the nature of their third row element,
such as P or S, are quite similar. In fact, the ionisation energies
of the outermost MO (oxygen lone pair) for dimethyl sulfone
Me2SO2 (10.65 eV)18–20 and dimethyl methylphosphonate
(MeO)2P(O)Me (10.71 eV)21 are almost identical. Similarly, the
field-inductive parameters for the ethylsulfonyl EtSO2– and
diethoxyphosphoryl (EtO)2P(O)– groups are equal (F ≡ 0.55).22
In addition, the attachment energy (i.e. the negative of the
electron affinity) values for electron capture into the σ*C–S
and σ*C–P orbitals of Me2S (3.25 eV)8 and Me3P (3.10)23,24 are
similar, and the σC–S and σC–P ionisation energies (12.78 and
11.325 eV, respectively) are not very different. Therefore, it
was interesting to study the α-diethoxyphosphoryl carbonyl
All solvents for IR measurements were spectrograde and were
used without further purification. The already known α-di-
ethoxyphosphoryl carbonyl compounds (EtO)2P(O)CH2C(O)X
(X = Me 1,26 Ph 2,27 OEt 3,28 NEt2 428,29 and SEt 530) were
prepared by an adaptation of the method described for 5,
i.e., by reacting equimolar quantities of triethyl phosphite with
the corresponding α-bromocarbonyl compound in benzene
at room temperature, followed by reflux for 8 hours. The
α-diethoxyphosphoryl carbonyl compound was chromato-
graphically separated from isomeric diethyl 1-substituted-vinyl
phosphate H C᎐C[X][OP(O)(OEt) ] through a silica gel column
᎐
2
2
using hexane–acetone as eluent. After solvent evaporation, the
liquid α-diethoxyphosphoryl carbonyl compounds 1–5 were
DOI: 10.1039/b005501h
J. Chem. Soc., Perkin Trans. 2, 2001, 97–102
This journal is © The Royal Society of Chemistry 2001
97

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Table 1 Frequencies of the carbonyl stretching bandsa in the IR
obtained in a pure form by distillation under reduced pressure.
Diethyl (methylsulfonyl)methylphosphonate 18 (mp 92–93 ЊC,
colourless crystals from chloroform) was synthesised following
a literature procedure.31
spectra of α-diethoxyphosphoryl carbonyl compounds (EtO)2P(O)-
CH2C(O)X 1–5, for the gauche conformerb of the α-ethylsulfonyl
carbonyl compounds EtSO2CH2C(O)–X 6–10 and of the parent
carbonyl compoundsc CH3C(O)–X 11–15
IR measurements
ν/cmϪ1
Com-
pound
The IR spectra were obtained using an FT-IR Nicolet Magna
550 Spectrometer with 1.0 cmϪ1 resolution. For the carbonyl
phosphonates 1–5 the carbonyl stretching region (1800–1600
cmϪ1) was recorded in 2.0 × 10Ϫ2 mol dmϪ3 carbon tetra-
chloride, chloroform and acetonitrile solution, using a 0.519
X
CCl4
CHCl3
CH3CN
1
6d
11
2
Me
1719.3
1720.5
1718.5
1685.0
1680.0
1691.0
1740.8
1738.0
1742.0
1646.5
1650.0
1650.0
1687.2
1678.0
1695.0
1714.7
1720.0
1711.5
1681.9
1679.0
1683.0
1735.0
1739.0
1732.5
1636.8
1644.0
1640.0
1680.6
1677.5
1687.0
1716.1
1723.0
1714.5
1683.4
1682.0
1693.0
1736.6
1743.0
1736.6
1637.4
1645.0
1644.0
1683.7
1681.0
1690.0
mm sodium chloride cell. The phosphoryl (P᎐O) stretching
᎐
Ph
region (1300–1100 cmϪ1) was analysed in carbon tetra-
chloride solution. The existence of a single symmetric carbonyl
stretching band for the series 1–5, in all solvents, was confirmed
by Fourier Self Deconvolution (FSD) and second derivative
analysis performed on each carbonyl band using the OMNIC
Version 1.0 FT-IR software of the Nicolet instrument.
7e
12
3
OEt
NEt2
SEt
8f
13
4
9f
14
5
NMR measurements
10f
15
1H and 13C NMR spectra of 0.5 mol dmϪ3 solutions in chloro-
form with TMS as an internal standard were recorded at 200
and 50 MHz, respectively, using a Bruker AC-200 spectrometer
in the FT mode. The conditions for recording 1H and 13C NMR
spectra have been described elsewhere.12
a Each carbonyl frequency corresponds to the maximum of a single
symmetrical band (see Experimental section). b The gauche conformer
is the more abundant one (conc. >80%). c From ref. 9.d,e, f From refs. 2,
5 and 9, respectively.
Calculations
The most stable conformation, the geometric parameters
and the electron charge distribution at the various atoms of
α-dimethoxyphosphorylacetone 1a, dimethyl (methylsulfonyl-
methyl)phosphonate 18a, α-methylsulfonylacetone 6a, dimethyl
sulfone 19 and dimethyl methylphosphonate 20 were computed
at the HF/6-31G** level using the Gaussian 98 series of pro-
grams.32 To save computer time, the ethyl groups were replaced
by methyl groups. C2v and Cs symmetry was used for 19 and 20,
respectively, while several (gauche and cis) starting geometries
were used for the other compounds.
Crystal data
C6H15O5PS, M = 230.21, monoclinic, space group P21/a, a =
10.074(1), b = 8.3570(9), c = 13.991(2) Å, β = 108.46(1)Њ, V =
1117.3(2) Å3, Z = 4, Dx = 1.369 Mg mϪ3, λ(Mo-Kα) = 0.71073
Å, µ = 0.423 mmϪ1, R = 0.0453.†
Data collection and processing
X-Ray diffraction data were collected on a CAD4 Mach3
diffractometer with the θ/2θ scan technique at 293 K; solution
by direct methods (SIR92),33 full-matrix least-squares refine-
ment on F2. 2040 measured reflections (2θmax = 50Њ) yielded
2
1958 unique and 1463 with Fo2 у 4σFo . Anisotropic displace-
ment parameters for all non-H atoms were applied. H atoms
were located on stereochemical grounds and refined with fixed
geometry, each riding on a carrier atom, with an isotropic dis-
placement parameter amounting to 1.5 (for methyl H atoms) or
1.2 (for the other H atoms) times the value of the equivalent
isotropic displacement parameter of the atom to which they
are attached. 121 parameters were refined and the final con-
ventional R was 0.0453. Structure refinement, final geometrical
calculations were carried out with SHELXL97,34 PARST-9535
and WinGX.36 Fig. 1 was produced using ZORTEP.37
Results and discussion
Table 1 collects the carbonyl stretching frequencies for the
α-diethoxyphosphoryl carbonyl compounds 1–5, in carbon
tetrachloride, chloroform and acetonitrile. This table also
includes the carbonyl stretching frequencies of the predomin-
ant gauche conformer of the α-ethylsulfonyl carbonyl com-
pounds2,5,9 6–10 and the corresponding data for the parent
† CCDC reference number 188/279. See http://www.rsc.org/suppdata/
p2/b0/b005501h/ for crystallographic files in .cif format.
98
J. Chem. Soc., Perkin Trans. 2, 2001, 97–102

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a
Table 2 Carbonyl frequency shifts (∆νCO
) for the α-diethoxyphosphoryl (EtO)2P(O)CH2C(O)–X 1–5 and for the α-ethylsulfonyl EtSO2CH2C-
(O)–X 6–10 carbonyl compounds, in CCl4, and the attachment energy values (EA)b for the π*C(O)X orbital of the parent carbonyl compounds
CH3C(O)–X 11–15
Compound
X
∆νCO/cmϪ1
Compound
∆νCO/cmϪ1
EA/eV
1–11
2–12
3–13
4–14
5–15
Me
Ph
OEt
NEt2
SEt
ϩ0.8
6–11
7–12
8–13
9–14
10–15
ϩ2.0
Ϫ11.0
Ϫ4.0
0.0
1.26
Ϫ6.0
Ϫ1.2
Ϫ3.5
Ϫ7.8
<0c
2.09
2.26
0.95
Ϫ17.0
a ∆νCO refers to the difference: νsubstituted
compound Ϫ νparent
.
b From ref. 8. c The value for acetophenone is not detectable by ETS, from
carbonyl
compound
ref. 39.
Table 3 Frequencies of the phosphoryl (νPO) and sulfonyl asymmetric (νSO ) stretching bands in the IR spectra of the α-diethoxyphosphoryl
2
(EtO)2P(O)CH2C(O)–X 1–5 and α-ethylsulfonyl EtSO2CH2C(O)–X 6–10 carbonyl compounds, in CCl4, and the corresponding frequency shifts (∆ν)a
in relation to the parent compounds 16 and 17
Compound
X
νPO/cmϪ1
∆νPO/cmϪ1
ϩ15
ϩ21
ϩ24
ϩ7
Compound
νSO (as)/cmϪ1 b
∆νSO (as)/cmϪ1
2
2
1
2
Me
Ph
OEt
NEt2
SEt
—
1261
1267
1270
1253
1265
1246
6
7
8
9
1331
1332
1335
1325
1335
1321
ϩ10
ϩ11
ϩ14
ϩ4
ϩ14
—
3
4
5
ϩ19
—
10
16c
17d
a Refers to the difference: να-phosphoryl or νsulfonyl compound Ϫ νparent
.
b From ref. 9. c,d Refers to the parent compounds (EtO)2P(O)Me and Et2SO2,
compound
respectively.
Table
4
Carbonyl carbon chemical shifts (δC, ppm) in the 13C NMR spectra of the α-diethoxyphosphoryl carbonyl compounds
(EtO)2P(O)CH2C(O)–X 1–5, α-ethylsulfonylcarbonyl compounds EtSO2CH2C(O)–X 6–10 and of the parent carbonyl compounds CH3C(O)X
11–15, in CDCl3–Me4Si
a
a
X
Compound
δCO
Compound
δCO
∆δCO
Ϫ3.7
Ϫ4.7
Ϫ4.0
Ϫ2.3
Ϫ3.2
Compound
δCO
∆δCO
Ϫ5.7
Ϫ7.6
Ϫ6.7
Ϫ3.4
Ϫ5.4
Me
Ph
OEt
NEt2
SEt
11
12
13
14
15
203.7
196.7
169.8
164.8
193.6
1
2
3
4
5
200.0
192.0
165.8
162.5
190.4
6
7
8
9
198.0
189.1
163.1
161.4
188.2
10
a ∆δCO refers to the difference: δsubstituted carbonyl compound Ϫ δreference compound
.
carbonyl compounds9 11–15. Only a symmetrical carbonyl
band is observed for the whole series 1–5 in all solvents. The
existence of a good linear correlation between the carbonyl fre-
quencies of 6–10 and 1–5 in carbon tetrachloride [r = 0.990;
s = 5.8 cmϪ1] suggests that the latter compounds also exist in
solution in the gauche conformation.38
Table 2 lists the carbonyl frequency shifts (∆νCO/cmϪ1) for 1–5
and 6–10 in relation to the parent compounds 11–15 together
with the attachment energy value for the latter compounds. The
∆νCO values for both series are negative, or slightly positive for
the methyl derivatives 1 and 6. The two series are reasonably
well correlated (r = 0.912) and follow approximately the
electron affinity trend of the parent carbonyl compounds8,39
(except in the case of the methyl derivatives 1 and 6). These
data suggest that the geometry of the α-phosphoryl carbonyl
compounds (structure I) is similar to that of the gauche con-
former of the α-sulfonyl carbonyl compounds (structure II),
whose geometry was previously determined by theoretical
calculations and X-ray diffraction analysis. The trends of
Table 2 are in line with previous propositions2,9,12,13 on β-keto
respect to their respective parent compounds 16 and 17 are
collected in Table 3. All the frequency shifts are positive and the
∆νSO values are ca. 1.7 times smaller than the corresponding
2
∆νPO ones. This behaviour is in line with the absolute carbonyl
gauche shifts for 6–10 being larger than the corresponding
values for 1–5 (Table 2) and strongly suggests that the
Olp→π*CO charge transfer interaction in the gauche rotamer
of β-carbonyl sulfones is stronger than the corresponding
interaction for β-carbonyl phosphonates. In fact, a stronger
O(SO )→π*CO charge transfer than the O(PO)→π*CO one should
lead2 to a large decrease in the bond order of both C᎐O and
᎐
sulfones and suggest that the O(PO)→π*CO and O(SO )→π*CO
O᎐S᎐O oscillators in compounds 6–10 and, therefore, in their
᎐ ᎐
2
charge transfer and π*CO/σC–Het hyperconjugative2,40 inter-
actions are the main factors which stabilise the gauche con-
formation (structures I and II).
frequencies, compared to the C᎐O and P᎐O oscillators for
᎐
᎐
compounds 1–5.
Table 4 shows the carbonyl 13C chemical shifts in deutero-
chloroform for 1–5 and 6–10 together with the differences (∆δ)
between the chemical shift of each α-substituted carbonyl com-
pound and the chemical shift of the corresponding parent
The frequencies of the phosphoryl (νPO) and asymmetric
sulfonyl (νSO ) stretching bands of 1–5 and 6–10 in carbon
2
tetrachloride, and the corresponding frequency shifts with
J. Chem. Soc., Perkin Trans. 2, 2001, 97–102
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Table 5 Relative energy, conformer concentration, dipole moment, selected dihedral angles and interatomic distances of selected non-bonded
atoms for the gauche (g) conformers of (MeO)2P(O)CH2C(O)Me 1a and MeC(O)CH2SO2Me 6a, at the HF/6-31G** level, and the sum of the
relevant van der Waals radii
Dihedral anglesd/Њ
Com-
O(5) ؒ ؒ ؒ C- O(1) ؒ ؒ ؒ P- O(7) ؒ ؒ ؒ C- O(6) ؒ ؒ ؒ C-
pound Conf.a E/kJ molϪ1b p (%)c
µ/D
α
β
γ
γЈ
(2)/Åf
(4)/Åg
(2)/Åf
(2)/Åf
1a
g1
g2
0.0
4.78
87.1
12.9
2.28 82.2
2.41 88.2
45.6
Ϫ165.3
171.6
Ϫ37.6
Ϫ81.8
68.4
3.128
4.057
3.331
3.459
3.976
3.065
3.416
3.234
Dihedral anglese/Њ
O(6) ؒ ؒ ؒ C- O(1) ؒ ؒ ؒ S-
µ/D
α
β
γ
γЈ
(2)/Åf
(4)/Åg
6ah
g
0.0
>99
3.03 78.8
Ϫ70.3
44.9
174.5 2.973
3.298
a Refers to the gauche conformation. b Relative energy. c Molar fraction of each rotamer as a percentage. d α = O(1)–C(2)–C(3)–P(4); β = C(2)–C(3)–
P(4)–O(5); γ = C(2)–C(3)–P(4)–O(7); γЈ = C(2)–C(3)–P(4)–O(6). e α = O(1)–C(2)–C(3)–S(4); β = C(2)–C(3)–S(4)–C(5); γ = C(2)–C(3)–S(4)–O(6);
γЈ = C(2)–C(3)–S(4)–O(7). f Sum of van der Waals radii = 3.22 Å. g Sum of van der Waals radii = 3.32 Å. h The second minimum energy conformation
corresponds to another gauche rotamer whose concentration is less than 1%.
Table 6 Charges (e) at selected atoms for gauche conformers of MeC(O)CH2X: X = P(O)(OMe)2 1a and X = SO2Me 6a by ab initio 6-31G**
computations (a minus sign indicates an excess of negative charge)
e/C
Com-
pound
Conf.
O(5)[PO]
O(6)[OR]
O(7)[OR]
P(4)[PO]
C(2)[CO]
ϩ0.498
ϩ0.512
ϩ0.513
O(1)[CO]
Ϫ0.523
Ϫ0.520
Ϫ0.514
O(6)[SO ]
O(7)[SO ]
S(4)[SO ]
2
2
2
1a
6a
g1
g2
g
Ϫ0.740
Ϫ0.714
Ϫ0.718
Ϫ0.716
Ϫ0.700
Ϫ0.717
ϩ1.577
ϩ1.574
Ϫ0.690
Ϫ0.672
ϩ1.457
compound 11–15. The smaller carbonyl upfield shifts (∆δ) of
ca. 2.0 ppm for the α-phosphoryl derivatives compared to the
α-sulfonyl derivatives, in spite of the quasi equal field-inductive
effect22 for the diethylphosphoryl and the ethylsulfonyl groups,
indicate that the O(PO)→π*CO CT interaction in the gauche
close contact produces significant OδϪPO→Cδϩ Coulombic
CO
and charge transfer interactions. Further stabilisation derives
from the distance (3.331 Å) between the carbonyl oxygen
(qO = Ϫ0.523 e) and the phosphoryl phosphorus (qP = ϩ1.577 e)
which is very close to the sum of the van der Waals radii
(3.32 Å).
rotamers of series 1–5 is weaker than the O(SO )→π*CO CT in
2
the corresponding rotamers of series 6–10, supporting the
IR frequency shift analysis.
The geometry of the gauche conformer of 6a (see Table 5 and
structure V) is very close to that of the g1 conformer of 1a
(structure III). However, the O(6) ؒ ؒ ؒ C(2) and O(1) ؒ ؒ ؒ S(4)
contacts between pairs of oppositely charged atoms are shorter
than the corresponding distances for 1a (g1) (see Tables 5
and 6). Thus, the HF/6-31G** calculations for 1a and 6a
corroborate the IR and 13C NMR data for 1–5 and 6–10,
indicating that both series of compounds exist, in the gas phase
and in solution, in the gauche conformation and that the
OδϪSO →Cδϩ charge transfer and Coulombic interactions in
In order to confirm the gauche conformer assignment of
the single carbonyl band of the diethoxyphosphoryl carbonyl
compounds 1–5, and to have precise geometries for the gauche
rotamer of these compounds, ab initio calculations on α-
dimethoxyphosphorylacetone 1a (chosen as a representative
compound for the whole series) were carried out. The relevant
data are presented in Table 5 along with the corresponding
data for the gauche rotamer of α-methylsulfonylacetone 6a. The
two most stable minima of 1a have the gauche conformation
(Structures III and IV). It seems reasonable, therefore, to decide
that the single carbonyl band observed in solution for the whole
α-diethoxyphosphoryl carbonyl series 1–5 should correspond
to the more abundant g1 rotamer of 1a in the gas phase.
CO
2
β-carbonyl sulfones are stronger than the OδϪPO→CδϩCO CT and
Coulombic interactions in β-carbonyl phosphonates.
In the less stable g2 conformer of 1a the alkoxy oxygens O(6)
and O(7) are the donor atoms (structure IV). Their interatomic
distances from the oppositely charged carbonyl carbon atom
C(2) are close to and shorter than, respectively, the sum of the
van der Waals radii and shorter than the corresponding
distances in the g1 conformer (see Tables 5 and 6). The lower
stability of the g2 with respect to the g1 rotamer is probably
related to the oxygen lone pair IE values which are higher
for the methoxy than for the phosphoryl oxygen (12.0 and
The higher stability of the g1 with respect to the g2 rotamer is
in line with a propitious geometry (structure III) giving an
intramolecular distance (3.128 Å) between the negatively
charged phosphoryl oxygen (qO = Ϫ0.740 e) and the positively
charged carbonyl carbon (qC = ϩ0.498 e), which is shorter than
the sum of the van der Waals radii (3.22 Å) (see Table 5). This
100
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Table 7 Selected dihedral angles and non-bonded distances for the minimum energy conformation of (MeO)2P(O)CH2SO2Me 18a at the HF/6-
31G** level, and the corresponding X-ray data for (EtO)2P(O)CH2SO2Me 18, and the sum of the relevant van der Waals radii
Dihedral anglesa/Њ
Compound
Conf.b
α
β
γ
γЈ
P ؒ ؒ ؒ O(1)/Åc
P ؒ ؒ ؒ O(2)/Åc
S ؒ ؒ ؒ O(3)/Åc
18
q-g
q-g
X-Ray
Ϫ41.8(2)
Ϫ47.5
74.4(2)
69.3
Ϫ42.5(2)
Ϫ46.2
Ϫ170.3(2)
Ϫ175.5
3.180(2)
3.243
4.203(2)
4.242
3.295(2)
3.417
18a
HF/6-31G**
a α = O(3)–P–C(2)–S; β = P–C(2)–S–C(1); γ = P–C(2)–S–O(1); γЈ = P–C(2)–S–O(2). b Refers to the quasi-gauche conformation. c Sum of van der
Waals radii = 3.32 Å.
ethylphosphonate–phenol complex with respect to phenol
(CCl4, ∆νOH = 398 cmϪ1 41 is more than twice the basicity of the
)
oxygen of the sulfonyl group estimated for the dimethyl
sulfone–p-fluorophenol complex (∆νOH = 154 cmϪ1),42 and the
basicity trend is in line with the larger negative charge at O(PO)
in 19 (Ϫ0.743 e) than at O(SO ) in 20 (Ϫ0.678 e). However, in the
2
model compound 18/18a (structure VI), the CH –S᎐O angle
᎐
2
and the S᎐O bond length are smaller, respectively, than the
᎐
Fig. 1 ZORTEP view of compound 18 showing the thermal ellipsoid
at 50% probability and the heavy atom labelling.
CH –P᎐O angle and the P᎐O bond length. Moreover, the
᎐
᎐
2
corresponding parameters O᎐P–CH (118.0Њ) and the P᎐O
᎐
᎐
3
(1.459 Å) for MeP(O)(OMe) 19, and O᎐S–CH (107.8Њ) and
10.71 eV, respectively24 for dimethyl methylphosphonate taken
as a reference compound).
The existence of only the g1 conformer for 1–5 in a low
permittivity solvent such as carbon tetrachloride (Table 1)
can hardly be justified by the small (0.1 D) dipole moment dif-
ference between the two gauche rotamers of 1a. However, a
᎐
2
3
S᎐O (1.435 Å) for Me SO 20 are very close to those computed
᎐
2
2
for 18a. Thus, these intrinsic geometrical parameters, which
allow close contact between oppositely charged atoms in 18,
seem to be responsible for the abnormally stronger electron-
donor ability of the sulfonyl oxygen lone pair nO(SO ) in 6–10
2
than the phosphoryl oxygen lone pair in 1–5.
close inspection of structures III and IV shows that the P᎐O
᎐
and C᎐O dipoles are relatively close to each other in g and
᎐
1
practically directly opposite each other in g2. Therefore, even
the low relative permittivity but polarizable nature of carbon
tetrachloride as solvent would stabilise the g1 to a greater extent
than the g2 conformer.
Conclusions
The IR frequency of the single νCO band of the β-carbonyl-
phosphonates [XC(O)CH2P(O)(OEt)2: X = Me 1, Ph 2, OEt 3,
NEt2 4 and SEt 5] (series I), which correlates well with the
νCO frequencies of the gauche rotamer of the corresponding
β-carbonyl sulfones [XC(O)CH2SO2Et: X = Me 6, Ph 7, OEt 8,
NEt2 9 and SEt 10] (series II) along with ab initio 6-31G**
computations for 1a and 6a (X = Me) indicate the existence of
a single gauche conformer (g1) for series I in solution.
The abnormal negative carbonyl frequency shifts (∆νCO) for
both series approximately follow the electron affinities of
the π*CO orbital of the parent carbonyl compounds MeC(O)X
11–15. These data suggest that the gauche conformations of
series I and II should have similar geometries.
The less positive asymmetric sulfonyl frequency shifts (∆νSO
in comparison with the phosphoryl frequency shifts (∆νPO
and the larger negative carbonyl gauche conformer shifts of β-
carbonyl sulfones 6–10 in relation to the corresponding values
of the β-carbonyl phosphonates 1–5 are in line with the greater
upfield carbonyl 13C chemical shifts for series II with respect to
Direct information about the relative donor/acceptor ability
of the PO and SO2 groups has been obtained from an X-ray
single crystal analysis of diethyl (methylsulfonyl)methyl
phosphonate 18. Fig. 1 and Table 7 show that in the solid state
18 assumes a syn-clinal or quasi-gauche geometry with respect
to both the α (Ϫ41.8Њ) and γ (Ϫ42.5Њ) dihedral angles. More-
over, the O(1) ؒ ؒ ؒ P contact (3.18 Å) is significantly shorter than
the sum of the relevant van der Waals radii (3.32 Å), while the
O(3) ؒ ؒ ؒ S contact (3.295 Å) is only slightly smaller. The most
stable conformer of 18a from HF/6-31G** calculations has
practically the same geometrical parameters as those obtained
by X-ray diffraction for 18. In conclusion, this model com-
pound shows that the O(SO )→P(PO) charge transfer interaction
between the sulfonyl oxyg2en (qO = Ϫ0.700 e) and the phos-
phoryl phosphorus (qP = 1.601 e) occurs over a shorter distance
and is likely to be more pronounced than the interaction
between the phosphoryl oxygen (qO = Ϫ0.743 e) and the sulfonyl
)
)
2
sulfur (qs = 1.459 e), giving some support to the fact that O(SO )
in 6–10 is a better electron donor toward the π*CO orbital than
O(PO) in 1–5.
series I. These trends are in agreement with their O(SO ) ؒ ؒ ؒ C(CO)
2
2
distances which are shorter than O(PO) ؒ ؒ ؒ C(CO) in compounds
6a and 1a, respectively, and are discussed in terms of the
Olp→π*CO charge transfer and electrostatic interactions, which
are stronger for series II than for I. This unexpected behaviour
indicates that the sulfonyl oxygen atom of the SO2R group is a
better electron donor than the phosphoryl oxygen atom of
the P(O)(OR)2 group. The intrinsic geometric parameters of the
The better electron-donor ability of the sulfonyl oxygen lone
pair nO(SO ) than the phosphoryl oxygen lone pair nO(PO)
2
towards the π*CO orbital would appear to be unexpected.
In fact, the basicity of the oxygen atom of the phosphoryl
group evaluated from the νOH frequency shift in the diethyl
J. Chem. Soc., Perkin Trans. 2, 2001, 97–102
101

View Article Online
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᎐
᎐
2
2
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Acknowledgements
The Brazilian authors thank Fundação de Amparo à Pesquisa
do Estado de São Paulo (FAPESP) for financial support and
the concession of the CAD4-Mach 3 X-ray diffractometer,
and the Conselho Nacional de Desenvolvimento Científico e
Tecnológico (CNPq) for a grant (to P. R. O. and J. Z.-S.) and
a scholarship (to R. R. F.). The Italian authors thank the
Ministero della Ricerca Scientifica e Tecnologica for financial
support.
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