Phospholes with reduced pyramidal character from steric crowding. 2. Photoelectron spectral evidence for some electron delocalization in 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole

Article abstract of DOI:10.1021/jo9606416

Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra had shown no difference in ionization energies (IE) for

Full text of DOI:10.1021/jo9606416

7808
J . Org. Chem. 1996, 61, 7808-7812
P h osp h oles w ith Red u ced P yr a m id a l Ch a r a cter fr om Ster ic
Cr ow d in g. 2. P h otoelectr on Sp ectr a l Evid en ce for Som e Electr on
Deloca liza tion in
1-(2,4-Di-ter t-bu tyl-6-m eth ylp h en yl)-3-m eth ylp h osp h ole
La´szlo´ Nyulaszi,† Gyo¨rgy Keglevich,‡ and Louis D. Quin*,§
Departments of Inorganic Chemistry and Organic Chemical Technology, Technical University of
Budapest, 1521 Budapest, Hungary, and Department of Chemistry, Box 34510, University of
Massachusetts, Amherst, Massachusetts 01003-4510
Received April 8, 1996X
Photoelectron spectroscopy has been explored as a tool to measure the flattening of the phosphorus
pyramid in a phosphole as caused by a large, sterically demanding P-substituent. Earlier PE spectra
had shown no difference in ionization energies (IE) for simple phospholes and their tetrahydro
derivatives (both around 8.0-8.45 eV). Calculations of the Koopmans IE at the Hartree-Fock
6-31G* level for 1-methylphospholane showed that, as is known for nitrogen, planarization at
phosphorus markedly reduced the ionization energy value (8.74 to 6.29 eV). A reduction in IE
also occurred on planarizing 1-methylphosphole, but to a lesser extent, being offset by increased
electron delocalization (8.93 to 7.16 eV). This suggests that experimental comparison of IE for the
unsaturated and saturated systems could be used to detect the presence of electron delocalization
in the former. The IE experimentally determined for the crowded 1-(2,4-di-tert-butyl-6-methylphe-
nyl)-3-methylphosphole was 7.9 eV, the lowest ever recorded for a phosphole. The corresponding
phospholane had IE 7.55 eV. The difference in the values is attributed to electron delocalization
in the phosphole. Calculations performed on the related model 1-(2-tert-butyl-4,6-dimethylphenyl)-
phosphole showed that the P-substituent adopted an angle of 55.7° (DFT/6-31G* level; 57.6° at the
HF/6-31* level) with respect to the C2-P-C5 plane (for P-phenyl, 67.1° and 68.3°, respectively).
In the preceding paper of this issue,1 we described the
synthesis and NMR characterization of a phosphole (1)
with the sterically demanding 2,4-di-tert-butyl-6-meth-
ylphenyl substituent at phosphorus. This compound was
prepared for the purpose of determining if a large
P-substituent, through steric interactions with the phos-
phole ring, could cause flattening of the normally pyra-
midal2,3 phosphorus, with the consequence of enhancing
cyclic electron delocalization in the ring system. Com-
putations clearly indicated that large aryl substituents
such as this could cause the desired effect, so that the
normal angle (designated R; see structure 1A) by which
a P-substituent would fall below the plane of the C-P-C
moiety of the phosphole ring (about 68° in 1-methylphos-
phole) would be reduced by as much as 10-15°. Both
in Table 1, all of the lone-pair ionization energy values4
are higher than are those for the corresponding tetrahy-
dro derivatives,5 and this provides a measure of the
interaction of the lone pair with the π-system. It is the
interaction with the unoccupied orbitals, rather than with
the occupied orbitals,6 that is responsible for aromatic
stabilization,7 and indeed thiophene, accepted to be the
most aromatic compound of this series,8 has the largest
lone pair stabilization energy. However, with a trivalent
heteroatom, as in the case of N in pyrrole and P in
phosphole, the geometry of the heteroatom must be taken
into consideration. Thus, N is planar in pyrrole, but
pyramidal in pyrrolidine, and a comparison of the ioniza-
1
the 13C and H NMR spectra of 1 showed that the aryl
substituent adopted a conformation in which the two ring
planes were perpendicular; the conformation (1A) with
the aryl methyl group, rather than the tert-butyl group
(as in 1B), positioned under the plane of the phosphole
ring was preferred. Even at low temperatures (-50 °C)
there were no spectral indications of conformer 1B; if a
conformational equilibrium is present, the contribution
of form 1B must be very small.
(4) The π-type p orbital of the heteroatom has the largest contribu-
tion in the uppermost doubly filled b1 orbital: (a) Distefano, G.;
Pignatoro, S.; Innorta, G.; Fringuelli, F.; Marino, G.; Taticchi, A. Chem.
Phys. Lett. 1973, 22, 132. (b) Scha¨fer, W.; Schweig, A.; Gronowitz, S.;
Taticchi, A.; Fringuelli, F. J . Chem. Soc., Chem. Commun. 1973, 541.
(5) Pignatoro, S.; Distefano, G. Chem. Phys. Lett. 1974, 26, 356.
(6) The lone pair orbital of tetrahydrofuran analogues has been
shown to interact with the σ CH2 orbitals.5 This (destabilizing)
interaction was shown to be the most effective for tetrahydrofuran,
thus without this destabilization an even higher lone pair IE should
be expected for the oxygen lone pair.
(7) Epiotis, N. D.; Cherry, W. R.; Bernardi, F.; Hehre, W. J . J . Am.
Chem. Soc. 1976, 98, 4361.
(8) (a) Katritzky, A. R.; Barczynsky, P.; Musumarra, G.; Pisano, D.;
Szafran, M. J . Am. Chem. Soc. 1989, 111, 7. (b) Schleyer, P. v. R.;
Freeman, P. K.; J ias, H.; Goldfuss, B. Angew. Chem., Int. Ed. Engl.
1995, 34, 337. (c) Nyula´szi, L.; Va´rnai, P.; Veszpre´mi, T. THEOCHEM,
in press.
Photoelectron spectroscopy can be used to determine
the stabilization energy of the lone pair in the family of
five-membered heterocycles with divalent atoms. As seen
† Department of Inorganic Chemistry, Technical University of
Budapest.
‡ Department of Organic Chemical Technology, Technical University
of Budapest.
§ University of Massachusetts.
X Abstract published in Advance ACS Abstracts, October 15, 1996.
(1) Quin, L. D.; Keglevich, G.; Ionkin, A. S.; Kalgutkar, R.; Szalontai,
G. J . Org. Chem. 1996, 61, 7801.
(2) Mathey, F. Chem. Rev. 1988, 88, 437.
(3) Hughes, A. N. In Handbook of Organophosphorus Chemistry;
Engel, R., Ed.; Marcell Dekker: New York, 1992.
S0022-3263(96)00641-X CCC: $12.00 © 1996 American Chemical Society

Phospholes with Reduced Pyramidal Character
J . Org. Chem., Vol. 61, No. 22, 1996 7809
Ta ble 1. Heter oa tom Lon e P a ir (n X) Ion iza tion En er gies
(IE, in eV) of Som e Heter ocyclic System s
a
b
Reference 4. Derrick, P. J .; Asbrink, L.; Edqvist, O.; Lind-
holm, E. Spectrochim. Acta 1971, 27A, 2525. c Reference 5.
Yoshikawa, K.; Hashimoto, M.; Morishima, I. J . Am. Chem. Soc.
d
1974, 96, 288.
tion energies of the two molecules will not give a useful
measure of the stabilization energy of the former. In fact
the difference is only 0.45 eV, less than that of any of
the divalent heteroatom systems, yet the aromaticity of
pyrrole is comparable to that of thiophene.8 Flattening
of the N pyramid in an amine is known to reduce the
ionization energy by about 0.9 eV; compare the value of
7.05 eV for manxine, a cage compound with planar N,
with that (7.92 eV) for tri-n-propylamine.9 In a simple
phosphole, P is pyramidal,2,3 as it is in the tetrahydro
derivatives. Early PES studies10,11 showed no difference
in the ionization energies of these molecules, and the
conclusion was drawn that there was little interaction
between the lone pair and the π-system of the phosphole.
Epiotis and Cherry12 later emphasized that the energy
of the phosphorus lone pair orbital for a planar phosphole
would be determined by its interaction with the occupied
and unoccupied π-orbitals; if these interactions happen
to be of equal strength, the orbital energy would remain
at the same value as for the unperturbed lone pair.
Although less explicity expressed, a similar observation
can be found in an earlier paper.11
We have performed calculations of the Koopmans
ionization energy of 1-methyl- and 1-phenylphosphole at
the Hartree-Fock 6-31G* level, as a function of the
degree of flattening of the phosphorus pyramid (Table
2). It is clearly seen that, as the angle of deflection (R in
1A) of the methyl substituent in 1-methylphosphole
decreases from the natural pyramidal value of 68.0° to
planarity at 0°, the calculated IE decreases from 8.93 eV
to 7.16 eV, therefore suggesting that the experimental
determination of the ionization energy could provide a
measure of the approach to planarity induced by the bulk
of a substituent on P. In this table, the pyramidalization
angle13 (θσπ - 90, discussed in the preceding paper of this
issue1), is also given. The calculated values for IE of
1-phenylphosphole show the same trend and range from
9.03 eV to 7.18 eV at planarity. Also of importance is
the effect of flattening the P atom in the tetrahydro
derivative; just as is seen in amines,9 flattening of
1-methylphospholane causes a decrease in the ionization
F igu r e 1. Photoelectron spectra of compounds 1 and 2.
energy (Table 3), from 8.74 eV for the natural pyramid
to 6.29 eV in the planar form. On comparing the natural
pyramidal structures for 1-methylphosphole and 1-me-
thylphospholane, it is seen that there is little difference
(0.19 eV) in their np energies, just as had been observed
in early experiments on the PES of unhindered phos-
pholes.10,11 Thus, even though the calculated values do
tend to be on the high side of experimental (1-phe-
nylphosphole, calcd 9.03 and 8.45 ( 0.05 expt10; for
1-methylphosphole, calcd 8.93, compared to expt 8.45 for
1-butylphosphole in the absence of data for 1-methyl;
1-methylphospholane, calcd 8.74, compared to expt 8.2511
for 1-butyl), it can be concluded that flattening of
phosphorus will have a greater effect on the lone pair
ionization energy of the phospholane than on the phos-
phole, there being some offset of the effect of planariza-
tion by electron delocalization. The maximum difference
of 0.87 eV for 1-methylphosphole is achieved at planarity.
A role for PES in the study of partially flattened phos-
pholes is therefore clearly established.
The photoelectron spectra of hindered phosphole 1 and
its tetrahydro derivative 21 are given in Figure 1 and
Table 4, together with the spectrum of the aromatic
hydrocarbon (3,5-di-tert-butyltoluene) corresponding to
the P-substituent. This allows the recognition of the
signals arising from the aryl ring system (8.3-8.4 eV) in
all of the spectra. That this value is the same in the
phosphole implies no π-electron interaction between the
two ring systems, a fact already reported10,11 for the
1-phenyl derivative and consistent with the conforma-
(9) Aue, D. H.; Webb, H. M.; Bowers, M. T. J . Am. Chem. Soc. 1975,
97, 4136.
(10) Scha¨fer, W.; Schweig, A.; Ma¨rkl, G.; Hauptmann, H.; Mathey,
F. Angew. Chem., Int. Ed. Engl. 1973, 12, 145.
(11) Scha¨fer, W.; Schweig, A.; Mathey, F. J . Am. Chem. Soc. 1976,
98, 407.
(12) Epiotis, N. D.; Cherry, W. R. J . Am. Chem. Soc. 1976, 98, 4365.
(13) (a) Haddon, R. C. J . Am. Chem. Soc. 1990, 112, 3385. (b)
Haddon, R. C. Science 1993, 261, 1545.

7810 J . Org. Chem., Vol. 61, No. 22, 1996
Nyulaszi et al.
Ta ble 2. Ca lcu la ted (Koop m a n s) Ion iza tion En er gies [IE, b1/a ′(n p )] in eV a n d Rela tive En er gies (∆E) in k ca l/m ol for
1-Meth ylp h osp h ole
Ta ble 3. Ca lcu la ted (Koop m a n s) Ion iza tion En er gies [IE, b1/a ′(n p )] in eV a n d Rela tive En er gies (∆E) in k ca l/m ol for
1-Meth ylp h osp h ola n e
tional preference for phosphole 1 with perpendicular ring
planes which would prevent such interaction. This
shoulder on the aryl π-band. This is the lowest energy
ever recorded for a phosphole; it is some 0.5-0.6 eV below
that for 1-phenylphosphole, indicating that 1, as pre-
dicted, has partial flattening of the phosphorus pyra-
mid.14 The signal shows a somewhat decreased HeII
intensity in agreement with this involvement of the lone
pair in the R′ MO.15 For the assignment of the photo-
electron spectrum HF/6-31G* calculations were carried
(14) The effect of placing a methyl substituent at the â-position, as
in 1, has not been considered. The effect of this substitution should be
to lower the ionization energy by 0.1-0.2 eV. The observed band
position10 as well as the position of the 7.9 eV shoulder of 1 can be
given with an uncertainty of about 0.05 eV.
(15) The ionization cross section of phosphorus s-type orbitals, which
have a large contribution in the a′ MO involved in the ionization, is
known to decrease under HeII conditions. Eland, J . H. D. Photoelectron
Spectroscopy; 2nd ed.; Butterworths: London, 1983; p 73.
conformational preference is supported by calculations
for 1-phenylphosphole, 1-mesitylphosphole, and 1-(2-tert-
butyl-4,6-dimethylphenyl)phosphole. At the HF/6-31G*
and DFT/6-31G* levels this conformation was the mini-
mum according to the second derivative calculations. The
ionization energy of phosphole 1 is 7.9 eV, seen as a

Phospholes with Reduced Pyramidal Character
J . Org. Chem., Vol. 61, No. 22, 1996 7811
Ta ble 4. Mea su r ed Ion iza tion En er gies a n d
Assign m en ts of Som e P h osp h oles a n d P h osp h ola n es in
eV
delocalization in the phosphole caused by the partial
flattening of the pyramid.
It would be of value to know the degree of flattening
of the phosphorus pyramid in 1A that is associated with
the lowered experimental ionization energy. We have
approached this question by performing ab initio calcula-
tions on the preferred geometry of the phosphole and
from that the ionization energy associated with this
structure. To facilitate these calculations, some simpli-
fications of the structures were made; the 3-methyl group
on the phosphole ring was not included, and the 4-tert-
butyl group of phosphole 1A was replaced by methyl.
None of these simplifications should have significant
effects on the parameters of the phosphole ring; semi-
empirical MNDO calculations on several substituted
phospholes showed that adding a 3-substituent caused
a change in R of only a few tenths of a degree. The results
of calculations for 1H-phosphole, 1-phenylphosphole, and
1-(2-tert-butyl-4,6-dimethyl-phenyl)phosphole are given
in Table 5. The calculated value for R in the case of 1-(2-
tert-butyl-4,6-dimethylphenyl)phosphole at the HF/6-
31G* level is 57.4° (55.7° at the DFT/6-31G* level), and
the calculated value for the ionization energy is 8.19 eV,
in reasonable agreement with the experimental value.
On comparing the value for R to that calculated for
1-phenylphosphole (67.1° and 68.3° at DFT/6-31G* and
HF/6-31G* levels, respectively), the extent of the flat-
tening can be seen clearly. A relationship between the
degree of flattening and electron delocalization was
observed by calculating the full molecular geometry
associated with the structure having the calculated value
for R ) 57.4°. These calculations revealed that replacing
H by phenyl caused practically no change in bond lengths,
even though the values for R differ, but replacing phenyl
by the larger trisubstituted phenyl group caused quite
pronounced changes. The internal P-C bond was short-
ened by 0.02 Å, the C2-C3 double bond was decreased
by 0.009 Å, and the C3-C4 single bond was lengthened
by 0.010 Å. Similar conclusions can be drawn from the
HF/6-31G* calculations (Table 5). All of these bond-
length changes are consistent with increased electron
delocalization in the substituted-phenyl phosphole. Our
conclusion is that the concept of increasing the electron
compound
ionization energy
assignment
1-phenylphospholea
8.45
8.45
9.25
9.25
8.35
9.30
9.30
7.9 (sh)
8.35
8.35
8.35
7.54
8.27
8.27
nP
π (diene)
π (phenyl)
π (phenyl)
nP
π (phenyl)
π (phenyl)
nP
π (diene)
π (phenyl)
π (phenyl)
nP
1-phenylphospholanea
1
2
π (phenyl)
π (phenyl)
a
See ref 10.
out for 1. As a basis for the structure the HF/6-31G*-
optimized geometry of 1-(2-tert-butyl-4,6-dimethyl-
phenyl)phosphole was used. The methyl substituent at
the â-position of the phosphole ring and the methyl
groups of the second t-Bu moiety were attached with
standard geometrical features (CC, 1.54; CH, 1.08 Å;
tetrahedral angles). The HOMO obtained from the
calculations was the np orbital, in agreement with the
HeII intensity. The Koopmans ionization energy was
8.19 eV. This value is lower by 0.8 eV than the one
calculated for 1-phenylphosphole, while the difference in
the observed IE is 0.5-0.6 eV. Thus it seems that the
calculated 10° opening of angle R (vide infra) is reason-
able.
To confirm the significance of the observed value of 7.9
eV for phosphole 1, it should be compared to the
experimental value for the tetrahydro derivative 2, which
is 7.55 eV. This value also indicates substantial flatten-
ing of the pyramid in 2. This is the first experimental
observation of a considerable difference between the
energies of a phosphole and its tetrahydro derivative,
clearly pointing to the presence of a stabilizing interac-
tion between the lone pair and the π*-orbital. The
difference in ionization energy (0.35 eV, 8 kcal/mol) for
the 1-phenyl derivative is a measure of the additional
Ta ble 5. Selected Bon d Len gth s for Som e P h osp h oles, a s Op tim ized by th e Den sity F u n ction a l Meth od Usin g th e
B3LYP F u n ction a l a n d th e 6-31G* Ba sis Set (DF T/6-31G*) a n d by th e Ha r tr ee-F ock Meth od (HF /6-31G*), a n d
P yr a m id a liza tion Effects

7812 J . Org. Chem., Vol. 61, No. 22, 1996
Nyulaszi et al.
F igu r e 2. The shape of the phosphorus nonbonding MO for 1-methylphosphole calculated with deflection angles (see 1A) of 68°
(A), 50° (B), and 0° (C), as plotted by the MOLDEN21 program.
functional method with the B3LYP functional of Becke20
delocalization in a phosphole by reducing the pyramidal
(referred to as DFT/6-31G* in the text). For the structures
obtained, second derivatives were calculated. When no struc-
character at phosphorus is valid. It would, of course, be
important to confirm the bond parameters by X-ray
diffraction analysis, but attempts to perform this experi-
ment have been unsuccessful with the crystals of 1
available. We are proceeding to prepare other phospholes
with large P-substituents in hopes of obtaining better
crystals for the X-ray study, and even further flattening
at phosphorus.
tural constraint was imposed during the course of the opti-
mization, all the frequencies were real, while for the structures
with fixed deflection angle (see 1A) a single imaginary
frequency was obtained. Koopmans ionization energies refer
to the optimized structures.
Syn th esis of 1-(2,4-d i-ter t-bu tyl-6-m eth ylp h en yl)-3-m e-
th ylp h osp h ola n e (2). To 1.0 g (0.00314 mol) of 1-(2,4-di-tert-
butyl-6-methylphenyl)-3-methyl-3-phospholene 1-oxide in 30
mL of absolute ethanol was added 0.2 g of 5% palladium on
carbon, and the suspension was then hydrogenated at 450 kPa
and room temperature with 1 equiv of hydrogen. The mixture
was filtered, and solvent was evaporated. The residue was
purified by column chromatography (silica gel, 3% methanol
in chloroform) to afford 0.98 g (98%) of the diastereomers of
phospholane oxide (31P NMR (CDCl3) δ +64.4 (79%) and +63.9
(21%): MS, m/z (rel int) 320 (M+, 19), 305 (100), 250 (63), 57
(36): HR-MS, calcd for C20H33OP: 320.2269; found: 320.2206.
Phenylsilane (0.15 mL, 0.00122 mmol) was added to a solution
of the phospholane oxide in 20 mL of dichloromethane and
the mixture stirred at reflux for 24 h under nitrogen. Evapo-
ration of solvent left 0.88 g (95%) of the phospholane 2 as an
air-sensitive oil: 31P NMR (CDCl3) δ -25.6 (70%), and -25.0
for the minor (30%) diastereomer: MS, m/z (rel int) 304 (M+,
54), 289 (42), 248 (100), 233 (59), 57 (73): HR-MS, calcd for
C20H33P: 304.2320; found: 304.2270.
Finally, we consider the effect of flattening on the
phosphorus lone pair orbital, as depicted in Figure 2 for
deflection angles of 68°, 50°, and 0°. With the largest
angle, which corresponds to the minimum on the poten-
tial energy surface for a 1-alkylphosphole,16 the orbital
is almost entirely localized at phosphorus. As the angle
decreases, the involvement with the pz orbitals on carbon
increases. The shape of the orbital with an angle of 20-
30° is quite similar to that of the planar form and to the
corresponding MO in thiophenes,17 indicating that even
slightly nonplanar phospholes could exhibit chemical
character like that in the planar, aromatic form. The
synthesis of other phospholes with smaller R values than
that for 1 will allow a test of this prediction.
Exp er im en ta l Section
Ack n ow led gm en t. L.N. and Gy.K. gratefully ac-
knowledge the support from OTKA T014955 and
TO14917, respectively, and L.D.Q. thanks the Exxon
Education Foundation for a Research Award that
enhanced this international collaboration.
P h otoelectr on Sp ectr a l Mea su r em en ts. Photoelectron
spectra were recorded at the HeI and HeII resonance lines,
using an instrument described earlier.18 For calibration
purposes, MeI was introduced simultaneously with the sample.
Lone pair ionization [b1/a′(np)] energies were 7.9 eV (sh) (1)
and 7.55 (3); the ionization energy for aryl was 8.35 eV for
both 1 and 2.
J O9606416
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