Experimental and theoretical studies on the conjugation of the phosphorus-carbon double bond with a cyclopropyl group

Article abstract of DOI:10.1021/jo034648g

X-ray structural analysis for (Z)-2-cyclopropyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaethene (2) was performed to confirm that the cyclopropyl group largely interacts with the P=C group compared with its carbon analogue, vinylcyclopropane (1). Absorption spectrum and redox properties of 2 were also studied to prove the conjugation. Theoretical investigation for nonsubstituted derivatives (4) indicated conjugative interaction between the P=C and cyclopropyl groups and revealed the physicochemical similarities between the P=C and C=C bonds.

Full text of DOI:10.1021/jo034648g

and we reported various phosphaethenes bearing the
2,4,6-tri-tert-butylphenyl (hereafter Mes*) group.9 The Pd
C skeleton is nearly apolar as a result of the small
difference in the electronegativities of phosphorus and
carbon atoms, and it behaves like an alkene, revealing
the conjugation with the π-electron systems.8,10 Recently
we reported a 1-phosphaallene [sPdCdC<] bearing a
cyclopropylidene group and showed an interaction of the
PdC moiety with the three-membered skeleton.11 Ad-
ditionally, we described preparation of (Z)-2-cyclopropyl-
1-(2,4,6-tri-tert-butylphenyl)-1-phosphaethene (2) by γ-
elimination with potassium tert-butoxide.11 Compound 2
is a suitable derivative to estimate the conjugation of the
PdC and cyclopropyl moieties.
We report here the X-ray crystallography of 2 together
with the absorption spectrum and redox properties,
indicating considerable interaction of the cyclopropane
ring with the PdC moiety. We also describe the results
of ab initio calculation to evaluate this cyclopropyl
conjugation with the results of other conjugated systems.
Exp er im en ta l a n d Th eor etica l Stu d ies on
th e Con ju ga tion of th e
P h osp h or u s-Ca r bon Dou ble Bon d w ith a
Cyclop r op yl Gr ou p
Shigeo Kimura,† Shigekazu Ito,†
Masaaki Yoshifuji,*,† and Tama´s Veszpre´mi‡
Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba, Sendai 980-8578, J apan, and
Department of Inorganic Chemistry, Technical University of
Budapest, Budapest 1521, Hungary
yoshifj@mail.cc.tohoku.ac.jp
Received May 15, 2003
Abstr a ct: X-ray structural analysis for (Z)-2-cyclopropyl-
1-(2,4,6-tri-tert-butylphenyl)-1-phosphaethene (2) was per-
formed to confirm that the cyclopropyl group largely inter-
acts with the PdC group compared with its carbon analogue,
vinylcyclopropane (1). Absorption spectrum and redox prop-
erties of 2 were also studied to prove the conjugation.
Theoretical investigation for nonsubstituted derivatives (4)
indicated conjugative interaction between the PdC and
cyclopropyl groups and revealed the physicochemical simi-
larities between the PdC and CdC bonds.
The cyclopropane ring is an organic functional group
bearing the smallest ring skeleton; it displays unique
properties such as conjugative interaction with π-electron
systems.1 For example, structural elucidation of vinyl-
cyclopropane (1) revealed that the cyclopropane ring has
a bisected conformation due to the cyclopropyl conjuga-
tion with the neighboring CdC group, showing an
alternation of two bond lengths in the cyclopropane ring.2
This conjugation involving the cyclopropyl moiety was
established by photoelectron spectrum analysis,3,4 as well
as theoretical investigation.5 Correspondingly, cyclopro-
pyl conjugations with several unsaturated systems were
found6 and have been used for construction of several
extended π-electron systems.7
As described in our previous report, 2 was prepared
from (Z)-5-bromo-1-(2,4,6-tri-tert-butylphenyl)-1-phos-
phapentene and potassium tert-butoxide11 and crystal-
lized from ethanol. A single crystal was employed for
X-ray structural determination. Figure 1 displays the
molecular structure of 2, and Table 1 lists the bond
lengths and angles. Two independent molecules were
assigned in the crystal lattice, and one of them is shown
in Figure 1.
The Mes* group is perpendicular to the PdC bond to
protect sterically the inherently unstable multiple bond
of trivalent phosphorus. The cyclopropane ring is nearly
perpendicular to the P-C1-C2 plane, taking the anti
conformation. The PdC length is very similar to that of
2-phenyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaethene
(3a ) [1.674(2) Å]12 The bond lengths of C2-C3 and C2-
In contrast to hydrocarbon compounds, a number of
phosphorus-carbon double-bonded compounds have been
synthesized by using the kinetic stabilization method,8
† Tohoku University.
‡ Technical University of Budapest.
(1) (a) de Meijere, A. Methoden der Organischen Chemie (Houben-
Weyl), Vol. E17: Carbocyclic Three- and Four-Membered Ring Com-
pounds; Thieme: Stuttgart, Germany, 1996. (b) de Meijere, A. Angew.
Chem., Int. Ed. 1979, 18, 809.
(2) Nijveldt, D.; Vos, A. Acta Crystallogr., Sect. B 1988, 44, 281.
(3) (a) Bruckmann, P.; Klessinger, M. Chem. Ber. 1974, 107, 1108.
(b) Askani, R.; Gleiter, R.; Heilbronner, E.; Hornung, V.; Musso, H.
Tetrahedron Lett. 1971, 4461. (c) Harada, Y.; Seki, K.; Suzuki, A.;
Inokuchi, H. Chem. Lett. 1973, 893.
(4) Rademacher, P. Chem. Rev. 2003, 103, 933.
(5) (a) Hoffmann, R. Tetrahedron Lett. 1970, 2907. (b) Gu¨nther, H.
Tetrahedron Lett. 1970, 5173.
(6) (a) Haumann, T.; Boese, R.; Kozhushkov, S. I.; Rauch, K.; de
Meijere, A. Liebigs Ann./Recueil 1997, 2047. (b) de Meijere, A. Chem.
Ber. 1974, 107, 1684.
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J .; J ohnels, D.; Scott, L. T. J . Am. Chem. Soc. 1991, 113, 3935. (b)
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Soc. 1999, 121, 9994.
(8) (a) Regitz, M.; Scherer, O. J . Multiple Bonds and Low Coordina-
tion in Phosphorus Chemistry; Thieme: Stuttgart, 1990. (b) Dillon, K.
B.; Mathey, F.; Nixon, J . F. Phosphorus: The Carbon Copy; Wiley:
Chichester, 1998.
(9) (a) Yoshifuji, M.; Shima, I.; Inamoto, N.; Hirotsu, K.; Higuchi,
T. J . Am. Chem. Soc. 1981, 103, 4587; 1982, 104, 6167. (b) Yoshifuji,
M. J . Chem. Soc., Dalton Trans. 1998, 3343. (c) Yoshifuji, M. J .
Organomet. Chem. 2000, 611, 210. (d) Yoshifuji, M.; Ito, S. Top. Curr.
Chem. 2003, 223, 67.
(10) Nyula´szi, L.; Veszpre´mi, T.; Re´ffy, J . J . Phys. Chem. 1993, 97,
4011.
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(12) Yoshifuji, M.; Toyota, K.; Inamoto, N.; Hirotsu, K.; Higuchi, T.
Tetrahedron Lett. 1985, 26, 6443.
10.1021/jo034648g CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/26/2003
6820
J . Org. Chem. 2003, 68, 6820-6823

F IGURE 2. Geometric isomers and relative energies of 4.
F IGURE 1. ORTEP drawing for one molecule of 2 with 50%
probability ellipsoids. Hydrogen atoms other than H1 are
omitted for clarity.
cases of hydrocarbon derivatives, such as spiro[2.5]oct-
4-ene.6b Additionally, we analyzed 2 using cyclic volta-
mmetry to understand the redox potentials of the system.
TABLE 1. Bon d Len gth s (Å) a n d An gles (d eg) of 2 a n d 4
ox
An irreversible oxidation potential Ep of 1.68 V and
red
reduction potential Ep of -0.71 V vs Ag/AgCl (in CH2-
Cl2) were observed. The redox potential data of 2 indi-
cated a higher HOMO as well as a lower LUMO com-
pared with 3c (Ep 1.76 V, Ep -0.87 V).14 Thus, these
physical data support the considerable conjugation be-
tween the PdC and cyclopropyl groups.
ox
red
2a (R ) Mes*)
4Bb (R ) H)
We theoretically investigated 2-cyclopropy-1-phospha-
ethene 4 and the related compounds. Figure 2 displays
four different conformations, 4A-D, with the relative
energies calculated at the MP2/6-311+g(d,p) level.15 The
exo conformations (A, B) are more stable than the endo
conformations (C, D). The Z-configuration (B) is slightly
more stable than the E-configuration (A). Photolysis of
(Z)-2 in benzene with a medium-pressure mercury lamp
for 3 h afforded a mixture of 3:1 Z/E isomers. This
Z-configuration preference in 2 can be ascribed to the
CH-π interaction16 between the neighboring tert-butyl
protons of the Mes* group and the cyclopropane moiety.
The calculated geometry of the optimized structure of
4 (B form) is displayed in Table 1, and the bond lengths
of 4B and its related compounds are summarized in Table
2. The optimized structure, 4B, is comparable to the
observed values in 2. The conjugation of a CdC double
bond with a cyclopropane ring causes similar effects: a
C-C bond shortening and a CdX bond elongation with
an elongation of the proximal bonds in the cyclopropyl
P-C1
1.667(3)
1.664(3)
1.456(3)
1.522(4)
1.529(5)
1.489(5)
0.033
1.684
1.469
1.523
1.523
1.502
0.021
0.021
96.2
129.0
115.0
118.8
118.8
59.1
C1-C2
1.452(4)
1.527(4)
1.532(4)
1.489(5)
0.038
C2-C3
C2-C4
C3-C4
∆1c
∆2d
0.043
0.040
R-P-C1
P-C1-C2
H1-C1-C2
C1-C2-C3
C1-C2-C4
C3-C2-C4
C2-C3-C4
C2-C4-C3
103.2(1)
132.3(2)
111(1)
118.9(3)
117.8(3)
58.3(2)
61.1(2)
60.7(2)
103.2(1)
132.8(3)
111(2)
118.7(3)
119.2(3)
58.4(2)
61.0(2)
60.6(2)
60.4
60.4
a
b
X-ray data. MP2/6-311+g(d,p) level. c [C2-C3] - [C3-C4].
d
[C2-C4] - [C3-C4].
C4 are remarkably longer than that of the distal C3-C4
bond. These differences in the bond lengths (∆1 ) [C2-
C3] - [C3-C4], ∆2 ) [C2-C4] - [C3-C4]) are larger
than the corresponding data for 1 (0.015 Å).2,6 Moreover,
the C1-C2 bond is shorter than the corresponding length
for 3a [1.465(3) Å].12 These results suggest that the
interaction of the cyclopropane ring with the PdC system
is stronger than that with the CdC group, and the
quantity is comparable to the cyclopropyl conjugation
with dienyl as observed in spiro[2.4]hepta-4,6-diene.6
This large conjugation of the cyclopropyl group with the
PdC bond might be due to a higher HOMO and a lower
LUMO of PdC, compared with its carbon analogue.
(13) Ito, S.; Toyota, K.; Yoshifuji, M. Chem. Commun. 1997, 1637.
(14) Ito, S.; Yoshifuji, M. Chem. Lett. 1998, 651.
(15) Frisch, M. J .; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J . R.; Zakrzewski, V. G.; Montgomery, J . A.,
J r.; Stratmann, R. E.; Burant, J . C.; Dapprich, S.; Millam, J . M.;
Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J .;
Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo,
C.; Clifford, S.; Ochterski, J .; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J . B.; Cioslowski, J .; Ortiz, J . V.; Baboul, A. G.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.;
Martin, R. L.; Fox, D. J .; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; J ohnson, B.; Chen,
W.; Wong, M. W.; Andres, J . L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J . A. Gaussian 98, revision A.9; Gaussian Inc.:
Pittsburgh, PA, 1998.
In the UV absorption spectrum, 2 revealed a batho-
chromic shift to 3b, indicating an extended conjugation
and suggesting that the cyclopropyl group is more ef-
fectively conjugated with the PdC moiety than the
methyl group in 3b [λmax (log ꢀ) 300 (2.9), 240 nm (4.2)].13
Similar phenomena in UV spectra were observed in some
(16) Tsuzuki, S.; Honda, K.; Uchimaru, T.; Mikami, M.; Tanabe, K.
J . Am. Chem. Soc. 2000, 122, 3746.
(17) Bartell L. S.; Higginbotham, H. K. J . Chem. Phys. 1965, 42,
851.
J . Org. Chem, Vol. 68, No. 17, 2003 6821

TABLE 2. Op tim ized Bon d Len gth s (Å) for 1, 4, a n d th e Rela ted Molecu les; Va lu es in P a r en th eses Ar e Exp er im en ta l
Da ta
a
b
d
Reference 17. Reference 18. c Reference 19. Reference 20. e Reference 21. f Reference 22.
1-phosphabutadiene and c-C3H5CHdPH (4B). The ex-
planation is similar to that for the electronic structure
of butadiene. The interaction of the π-orbitals between
ethene and phosphaethene results in the huge split
between the lowest π1 orbital and the π2 HOMO level.
The similar orbital correlation between phosphaethene
and the saturated cyclopropane ring is clear. In this case
the π-orbital of phosphaethene interacts with the σ ring
orbital of appropriate symmetry (a′′ symmetry in Cs point
group). The correlation diagram also shows the MO
correlation of the related carbon compounds, butadiene
and 1. Thus, the similarities in the conjugative ability of
the CdC and PdC bonds have been established.10
In conclusion, the structure of a stable cyclopropyl-
phosphaethene 2 was unambiguously determined by
X-ray crystallography. The cyclopropane ring interacts
considerably with the PdC bond and the cyclopropyl
conjugation involving the PdC bond is larger than that
observed in its carbon analogue, vinylcyclopropane (1).
The conjugation between the PdC and cyclopropyl groups
was explained by the absorption spectra and cyclic
voltammetry. Theoretical investigation indicated this
cyclopropyl conjugation with the PdC bond and revealed
the similarities in PdC and CdC bonds.
F IGURE 3. Shape of HOMO for 4B and its related mol-
ecules.21
group. The shortest CdX (X ) C, P) bond is observed in
the nonconjugated molecule. Figure 3 shows the shape
of the HOMO for 4B with those for the related species.
The butadiene-like structure of the π orbitals and the
similarity among them are both clear. The shape of the
HOMO for vinylcyclopropane (1) is identical to those
described in the other reports.4
Exp er im en ta l Section
An orbital correlation diagram (see Supporting Infor-
mation) suggests the similarity in the π-conjugation of
P r ep a r a tion of 2. A mixture of potassium tert-butoxide (1.34
mmol) in THF (10 mL) at 0 °C was added to a solution of (Z)-
5-bromo-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaethene11 (280 mg,
0.680 mmol) and warmed to room temperature. The reaction
mixture was stirred for 30 min, and the solvent was removed in
vacuo. The residue was purified by silica gel column chroma-
tography (hexane) to afford 132 mg of 2 (45% yield). Data for 2:
colorless crystals, mp 98-99 °C; 31P{1H} NMR (162 MHz, CDCl3)
δ 232; 1H NMR (400 MHz, CDCl3) δ 7.45 (m, 2H, Mes*), 6.57
(18) Bartell, L. S.; Bonham, R. A. J . Chem. Phys. 1959, 31, 400.
(19) Hopkinson, M. J .; Kroto, H. W.; Nixon, J . F.; Simmons, N. P.
C. J . Chem. Soc., Chem. Commun. 1976, 513.
(20) Almenningen, A.; Bastiansen, O.; Trætteberg, M. Acta Chem.
Scand. 1958, 12, 1221.
(21) Yamamoto, S.; Nakata, M.; Fukuyama, T.; Kuchitsu, K. J . Phys.
Chem. 1985, 89, 3298.
(22) Schaftenaar, G. MOLDEN 3.4; CAOS/CAMM Center: The
Netherlands, 1998.
2
3
(dd, 1H, J PH ) 38 Hz, J HH ) 11 Hz, PdCH), 1.59 (s, 18H,
6822 J . Org. Chem., Vol. 68, No. 17, 2003

o-C(CH3)3), 1.38 (s, 9H, p-C(CH3)3), 0.85 (m, 1H, CH), 0.69 (m,
2H, CHH), 0.48 (m, 2H, CHH); 13C{1H} NMR (101 MHz, CDCl3)
ratio in 31P NMR spectrum. The Z/E peak ratio did not change
even after irradiation for 24 h.
1
δ 177.7 (d, J PC ) 43 Hz, PdC), 154.6 (s, o-Mes*), 149.9 (s,
Ab in itio Ca lcu la tion . Calculations were carried out using
the Gaussian 98 package15 with the MP2/6-311+g(d,p) level of
theory. The nature of the stationary points was further checked
with second derivative calculations to prove that the optimized
geometries were real minima on the potential energy surface.
To check the reliability of the calculations, several basis sets
were tried at the HF, MP2, and QCISD levels. The results of
calculations were not sensitive to the basis set but were
significantly modified by inclusion of electron correlation. Be-
cause the deviation among the results of higher-level calculations
was small, we ignored the HF calculations and drew conclusions
only from the MP2 results.
1
p-Mes*), 137.0 (d, J PC ) 59 Hz, ipso-Mes*), 121.7 (s, m-Mes*),
4
38.5 (s, o-C(CH3)3), 35.4 (s, p-C(CH3)3), 33.4 (d, J PC ) 7 Hz,
2
o-C(CH3)3), 31.8 (s, p-C(CH3)3), 18.3 (d, J PC ) 21 Hz, CH), 10.6
3
(d, J PC ) 7 Hz, CH2); UV (hexanes) λmax (log ꢀ) 300 (sh, 3.5),
254 nm (sh, 4.3); FAB-MS m/z (rel int) 330 (M+; 45), 315 (M+
CH3; 100). Anal. Calcd for C22H35P: C, 79.95; H, 10.67. Found:
C, 79.71; H, 10.77.
-
X-r a y Cr ysta llogr a p h y for 2. C22H35P, M ) 330.49, triclinic,
P1h(No. 2), a ) 11.2406(7), b ) 20.008(2), c ) 9.3703(9) Å, R )
103.125(2)°, â ) 90.143(3)°, γ ) 89.988(5)°, V ) 2052.3(3), Z )
4, T ) 120 K, 2θmax ) 55.0°, Fcalc ) 1.070 g cm-3, µ(Mo KR) )
0.133 mm-1, 16168 observed reflections, 6267 unique reflections
(Rint ) 0.040), R1 ) 0.064 (I > 2σ(I)), RW(F) ) 0.071 (all data),
S ) 1.50 (695 parameters). Crystallographic data (excluding
structure factors) for the structure reported in this paper have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication no. CCDC-210342.
Cyclic Volta m m etr y of 2. Cyclic voltammetry conditions:
1 mM in dichloromethane; supporting electrolyte, 0.1 M tet-
rabutylammonium perchlorate (TBAP); working electrode, glassy
carbon; counter electrode, platinum wire; reference electrode,
Ag/AgCl [E1/2 (ferrocene/ferricinium) ) +0.60 V] at 20 °C; scan
Ack n ow led gm en t. This work was supported in part
by Grants-in-Aid for Scientific Research (13304049 and
14044012) from the Ministry of Education, Culture,
Sports, Science and Technology, J apan. T.V. received a
grant (OTKA Project T034768) from the National Basic
Program of Scientific Research, Hungary.
Su p p or tin g In for m a tion Ava ila ble: General experimen-
tal procedure, UV spectra for 2 and 3b, cyclic voltammograms
for 2, and Cartesian coordinates and total energies for 4A-D.
This material is available free of charge via the Internet at
http://pubs.acs.org.
rate, 50 mV sec-1
.
P h otolysis of 2. A solution of (Z)-2 (0.015 mmol) in benzene-
d6 (0.35 mL) was irradiated in a NMR tube with a medium-
pressure Hg lamp (100 W) at 5 °C. After irradiation for 3 h, the
peaks due to 2 and the isomer (E)-211 were observed in a 3:1
J O034648G
J . Org. Chem, Vol. 68, No. 17, 2003 6823