A 1-Methyl-Phosphininium Compound: Synthesis, X-Ray Crystal Structure, and DFT Calculations

Article abstract of DOI:10.1002/anie.200352230

A tetravalent phosphorus atom in an aromatic system! A 1-methylphosphininium tetrachlorogallate is synthesized and fully characterized (see structure). This compound has aromatic properties like those of phosphinine, pyridine, and pyridinium salts. These

Full text of DOI:10.1002/anie.200352230

Communications
synthesis. Herein, we report the characterization by NMR
spectroscopy and X-ray crystallography of a 1-methylphos-
phininium gallium tetrachloride adduct and its electronic
structure.
l5-1-methyl-1-chlorophosphinines 5 and 6 were chosen as
starting precursors for this study. Their syntheses rely on the
oxidation of anions 3 and 4 that were conventionally
produced by treating methyllithium with phosphinines 1[7]
and 2,[8] respectively (Scheme 1).[9]
Phosphininium Compounds
A 1-Methyl-Phosphininium Compound:
Synthesis, X-Ray Crystal Structure, and DFT
Calculations**
Audrey Moores, Louis Ricard, and Pascal Le Floch*
It is well established that the replacement of nitrogen by
phosphorus in aromatic structures results in profound mod-
ifications of electronic properties.[1] One of the most repre-
sentative examples is provided by phosphinines, the phos-
phorus equivalent of pyridines. In these heterocycles, the lone
pair at the phosphorus center, which features a significant
amount of 3s character (63.8% vs 29.1% in pyridine), is only
weakly basic.[2] Conversely, phosphinines are relatively inert
towards electrophilic attacks at the phosphorus atom. An
illustration of this peculiar electronic situation is given by
several unsuccessful attempts to produce 1-R-phosphininium
(R = alkyl, aryl, or H). For example, direct protonation of
phosphorus with CF3SO3H did not yield the expected 1-H-
phosphininium and the gas-phase proton affinity of phosphi-
nine, determined by ion-cyclotron resonance techniques, was
found to be slightly higher than that of PH3.[3,4]
Scheme 1. Synthesis of compounds 5 and 6. TMS=SiMe3.
We tried to duplicate the strategy reported by Dimroth
and co-workers, and therefore compounds 5 and 6 were
treated with AlCl3 in CH2Cl2 at room temperature. In both
cases, a reaction took place and the 31P NMR spectrum
showed broad signals at d = 133.5 and 137.0 ppm, respectively.
Suspecting that an exchange could occur between the
À
counterion AlCl4 and the phosphininium, we investigated
the use of different reagents as chloride abstractor (AgBF4,
AgBPh4, TlBPh4 and GaCl3). Whereas no reaction occurred
when 5 or 6 were treated with tetraphenylborate salts salts,
treatment of 5 with AgBF4 resulted in the exchange of the
halogen atom at phosphorus to afford the fluoro derivative 7.
More convincing results were obtained when 5 and 6 were
treated with GaCl3 in CH2Cl2 at room temperature. In both
cases, a clean reaction took place to produce 1-methyl-
phosphininium compounds 8 and 9 which were isolated as
moisture sensitive powders (Scheme 2).
Though phosphininiums were thought to be involved as
intermediates in many transformations leading to l5-phosphi-
nines, only a few data are available on their structure,
reactivity, and electronic properties.[5] In 1984, Dimroth and
co-workers reported the successful preparation of a 1-phenyl-
phosphinium, from the reaction of AlCl3 with a l5-1-phenyl-1-
fluorophosphinine, but this compound was only partially
Both compounds were successfully characterized by NMR
spectroscopy and elemental analysis. The formation of the
1
characterized (31P and H NMR spectroscopy).[6] In view of
the potential synthetic utility of phosphininium compounds in
heterocyclic phosphorus chemistry, we reinvestigated their
[*] Dr. P. Le Floch, A. Moores, Dr. L. Ricard
Laboratoire “HØtØroØlØments et Coordination” UMRCNRS 7653
(DCPH)
DØpartement de Chimie, Ecole Polytechnique
91128 Palaiseau cedex (France)
Fax: (+33)1-6933-4570
E-mail: lefloch@poly.polytechnique.fr
[**] This work was supported by the CNRS and the Ecole Polytechnique.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 2. Synthesis of compounds 7, 8, and 9.
DOI: 10.1002/anie.200352230 Angew. Chem. Int. Ed. 2003, 42, 4940 –4944
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phosphininium is evidenced by a strong downfield shift in
31P NMR (from d = 63.7 in 5 to d = 160.2 ppm for 8 and from
d = 64.7 in 6 to d = 156.9 ppm for 9). The aromaticity of the
ring is also apparent in the 13C NMR spectra from the
chemical shifts of the C2 and C4 carbon atoms that are
strongly deshielded (from d = 88.8 in 5 to d = 140.3in 8 for C2
and from d = 117.4 to d = 134.7 for C4). The formulation of 8
was definitively established by an X-ray crystal-structure
analysis.[10] A view of one molecule of 8 is presented in
Figure 1. The structure consists of two discrete units contain-
Scheme 3. Reactivity of phosphoninium 8.
of Ib was found to be close to that of the experimental
=
structure. The shortening of the two P C bonds (1.698 Š in Ib
vs 1.743Š in the parent phosphinine Ic; Scheme 4) as well as
the widening of the internal angle C2-P1-C2’ angle (110.678 in
Ib vs 100.028 in Ic) are reproduced.[15] An NBO analysis[16]
reveals that the phosphorus atom bears a significant positive
charge (1.017 in Ia and 1.285 in Ib vs 0.642 in Ic) and has
gained a substantial sp2 character. Thus the contribution of
Figure 1. Molecular structure of 8 without hydrogen atoms. Selected
bond lengths [Š] and angles [8]: P1-C2 1.697(2), C2-C3 1.415(3), C3-C4
1.407(2), Si1-C2 1.921(2), P1-C5 1.790(3); C2-P1-C2’ 117.7(1), C2-P1-
C5 120.80(6), P1-C2-C3 113.1(1), C2-C3-C4 24.7(2), C3-C4-C3’ 126.5(2).
À
ing the phosphininium cation, the GaCl4 ion and two
molecules of THF. One THF molecule is located far away
above the ring (P–O separation: 3.174(5) Š), the oxygen atom
pointing in the direction of the phosphorus atom. The
phosphinine ring is not rigorously planar and both phospho-
rus and the C4 carbon atoms escape from the mean plane (C2-
C3-C3’-C2’) by 3 .88 and 1.78, respectively. But, the most
significant data are given by the internal bond distances which
are significantly modified in comparison to those of a l3-
À
phosphinine: the two internal P C bond lengths are signifi-
cantly shortened from 1.734(5) Š in the reference com-
pound[11] to 1.697(2) Š in 8,[12] whilst the internal C2-P1-C2’
angle changes from 106.3(2)8 to 117.7(1)8. These two pieces of
data clearly suggest that the phosphorus atom has gained a
significant amount of sp2 character. This result is peculiar as it
has always been believed that phosphorus could not undergo
such a rehybridization within a six-membered ring, which
accounted for the low basicity of phosphinine moieties.[4,13]
Phosphininium intermediates are known to be highly
reactive towards nucleophiles and we found that compound 8
rapidly reacts with methyllithium in THF at low temperature
to afford the l5-dimethylphosphinine 10 (Scheme 3). We also
found that the dienic character of the ring is significantly
enhanced compared to l3-phosphinines. Reaction of 8 with 4-
octyne at room temperature for 24 h cleanly yielded the
phosphabarrelenium salt 11 which was isolated as beige solid
and fully characterized by NMR spectroscopy and elemental
analysis (Scheme 3).
Scheme 4. Structures used as the basis for calculations and the aro-
matic stabilization energy equation; LP=lone pair.
À
the 3s orbital in a P C bond changes from 19.42% in Ic to
33.30% in Ib. Useful data were obtained by calculating the
ASE (aromatic stabilization energy) using the equation
depicted in Scheme 4.[17] To draw an efficient comparison,
similar calculations were also carried out on the 1-H IIa, 1-Me
IIb pyridinium cations and pyridine IIc. Nuclear independent
chemical shift (NICS; N-iodosuccinimide) at 1 Š above the
ring were calculated at the 6-311 + G** level (Table 1).[18] The
aromaticity of the phosphininium is close to that of the free
phosphinine (94.3% in Ia and 99.2% in Ib). On the contrary,
the two pyridiniums were found to be slightly more aromatic
than pyridine (109.3% in IIa and 105.9% in IIb). NICS values
do not reproduce exactly the same trends, but confirm the
strong aromaticity of the six molecules. In view of these data,
it seems evident that the high reactivity of 8 towards 4-octyne
does not result from a disruption of aromaticity within the
ring. Therefore, the reaction paths of the [4+2] cycloaddition
A theoretical study was undertaken; calculations were
carried out on the unsubstituted 1-H Ia and 1-Me- Ib
derivatives by using a combination of the B3LYP functional
with the 6-311 + G(d,p) basis set.[14] The calculated geometry
Angew. Chem. Int. Ed. 2003, 42, 4940 –4944
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Table 1: Calculated aromatic stabilization energy of compound Ia–c, IIa–c.
Compound
Ia
Ib
Ic
IIa
32.29
À10.7212
IIb
31.29
À10.7446
IIc
29.55
À11.1321
ASE [kcalmolÀ1] ZPE corrected
NICS (1 Š)
26.5
À11.6128
27.87
À11.4749
28.09
À10.7827
process of Ib and Ic with acetylene (to give IIIb and IIIc) were
modeled (Figure 2). These calculations which were conducted
at the B3LYP6-311 + G(d,p) level of theory indicate that the
formation of the phosphabarrelenium is strongly exothermic
(À23.03 kcalmolÀ1 and only requires a weak activation energy
Figure 3. Shape and level of frontier orbitals of phosphinine,
phosphininium, and acetylene.
Figure 2. Calculated reaction profile of Diels–Alder reaction on phos-
phinine or phosphininium (energies, zero-point energy (ZPE)
corrected, are in kcalmolÀ1).
in THF (2 mL) at À788C. The solution turned from colorless to red
and was warmed to room temperature. Completion of the reaction
was checked by 31P NMR spectroscopy. Hexachloroethane (30 mg,
0.127 mmol) was added at À788C. The solution was warmed to room
temperature, solvent was removed in vacuo, and the resulting brown
solid was extracted with hexane (3” 2mL). The product was
recovered as a yellow oil, yield 49 mg (87%); 5 was too air sensitive
to give satisfactory elemental data. Selected data: 1H NMR
(300 MHz, C6D6, 258C): d = 0.14 (s, 18H, Si(CH3)3), 2.51 (d,
2J(H,P) = 15.7 Hz, 3H, PCH3), 6.07 (d, 4J(H,P) = 4.7 Hz, 1H, H4),
7.01–7.30 ppm (m, 10H, Ph); 31P NMR (121.5 MHz, C6D6, 258C, 85%
H3PO4 as external standard): d = 63.7 ppm (s).
(ETSb = 17.32 kcalmolÀ1; TS = transition state). In good agree-
ment with experimental data, the formation of the phospha-
barrelene from Ic is only weakly exothermic and involves a
high activation barrier (ETSc = 31.68 kcalmolÀ1).
These results are fully consistent with the relative energies
of frontier orbitals in Ib and Ic (calculated at the MP2/6-311 +
G(d,p) level of theory using B3LYP geometries). As
expected, the introduction of a methyl group at the phospho-
rus atom does not modify the shape of frontier orbitals, which,
having a p symmetry, can not interact with methyl group
molecular orbitals (MOs). On the contrary, energies of the
MOs are significantly lowered because of orbital contraction.
As can be seen in Figure 3, reaction of Ic with acetylene is a
classical [4+2] cycloaddition process in which the phosphinine
acts as the diene and reacts through its HOMO. On the
contrary, in phosphininium Ib its low-lying LUMO reacts with
the HOMO of the alkyne (inverse electron demand process).
In conclusion, we have established the existence of 1-
methyl-phosphininium compounds. We believe that these
cations will find interesting applications in the synthesis of
new phosphorus heterocycles.
6: A solution of MeLi in diethyl ether (100 mL; 0.149 mmol, 1.6m)
was added to a solution of phosphinine (40 mg 0.149 mmol) in hexane
(2 mL) at À788C. The solution turned from colorless to bright yellow.
The solution was warmed to room temperature and completion of the
reaction was checked by 31P NMR spectroscopy. Hexachloroethane
(35 mg, 0.149 mmol) was added at À788C. The solution was warmed
to room temperature, solvent was removed in vacuo, and the solution
filtered to remove the salts. The product was recovered from the
filtrate as a yellow oil, yield 38 mg (80%); 6 was too air sensitive to
1
give satisfactory elemental data. Selected data: H NMR (300 MHz,
C6D6, 258C): d = 0.39 (s, 18H, Si(CH3)3), 2.12 (d, 4J(H,P) = 0.9 Hz,
6H, C3-CH3), 2.24 (d, 2J(H,P) = 16.3Hz, 3H, PCH 3), 5.67 ppm (d,
4J(H,P) = 5.0 Hz, 1H, H4); 31P NMR (121.5 MHz, C6D6, 258C, 85%
H3PO4 as external standard): d = 64.69 ppm (s).
7: AgBF4 (22 mg, 0.111 mmol) was added to a solution of 5
(0.111 mmol, 49 mg) in THF (2 mL) at room temperature. This
solution was stirred for 24 h. Completion of the reaction was checked
by 31P NMR spectroscopy. Solvent was removed in vacuo and the
resulting brown solid was extracted with hexane (3” 2mL). The
product was recovered as a pale brown oil, yield 35 mg (74%); 7 was
too air sensitive to give satisfactory elemental data. Selected data: 1H
NMR (300 MHz, C6D6, 258C): d = 0.11 (s, 18H, Si(CH3)3), 1.96 (d,
Experimental Section
All work was carried out under nitrogen or argon using Schlenk
techniques. The solvents used were freshly distilled, dried, and
saturated with nitrogen or argon.
5: A solution of MeLi in diethyl ether (80 mL, 0.127 mmol, 1.6m)
was added to a solution of 50 mg of phosphinine (1 or 2, 0.127 mmol)
4
5
2J(H,P) = 14.7 Hz, 3H, PCH3), 5.90 (dd, J(H,P) = 7.0 Hz, J(H,P) =
0.6 Hz, 1H, H4), 7.04–7.31 ppm (m, 10H, Ph); 31P NMR (121.5 MHz,
4942
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Angewandte
Chemie
THF, 258C 85% H3PO4 as external standard): d = 80.09 ppm (d,
1J(P,F) = 1043.2 Hz).
Heterocyclic Chemistry: The Rise of a New Domain (Ed.: F.
Mathey), Pergamon, Oxford, 2001, pp. 485 – 533.
[3] H. Oehling, A. Schweig, Phosphorus Relat. Group V Elem. 1971,
1, 203– 205; R. V. Hodges, J. L. Beauchamp, A. J. Ashe, W. T.
Chan, Organometallics 1985, 4, 457 – 461.
8: A solution of GaCl3 (21 mg, 0.119 mmol) in CH2Cl2 (1 mL) was
added to a solution of 5 (48 mg, 0.108 mmol) in CH2Cl2 (2 mL) room
temperature. After stirring for 5 min, solvent was removed and, the
product was recovered as a beige powder, yield 62 mg (93%).
Crystallization of 8 at À188C in a mixture of CH2Cl2 and hexane
afforded colorless crystals. Elemental analysis (%) calcd: C 46.55, H
[4] A. J. Ashe, Acc. Chem. Res. 1978, 11, 153– 157.
[5] C. C. Price, T. Parasaran, T. V. Lakshminarayan, J. Am. Chem.
Soc. 1966, 88, 1034 – 1037; G. Märkl, A. Merz, Tetrahedron Lett.
1969, 1231 – 1234; G. Märkl, A. Merz, H. Rausch, Tetrahedron
Lett. 1971, 2989 – 2992.
[6] T. N. Dave, H. Kaletsch, K. Dimroth, Angew. Chem 1984, 96,
984 – 985; Angew. Chem. Int. Ed. Engl. 1984, 23, 989 – 990.
[7] N. Avarvari, P. Le Floch, L. Ricard, F. Mathey, Organometallics
1997, 16, 4089 – 4098; N. Avarvari, P. Le Floch, F. Mathey, J. Am.
Chem. Soc. 1996, 118, 11978 – 11979.
1
5.21; found: C 46.53, H 5.22; H NMR (300 MHz, C6D6, 258C): d =
0.21 (s, 18H, Si(CH3)3), 3.20 (d, 2J(H,P) = 20.8 Hz, 3H, PCH3), 7.32–
7.48 (m, 10H, Ph), 7.63ppm (d, 4J(H,P) = 6.1 Hz, 1H, H4); 13C NMR
(75.5 MHz, C6D6, 258C): d = 1.68 (d, 3J(C,P) = 3.4 Hz, Si(CH3)3),
12.77 (d, 1J(C,P) = 51.5 Hz, PCH3), 128.46 (s, o-C of Ph), 128.54 (s, m-
C of Ph), 129.40 (s, p-C of Ph), 134.72 (d, 3J(C,P) = 51.3Hz, C4),
1
2
140.34 (d, J(C,P) = 22.4 Hz, C2-TMS), 142.29 (d, J(C,P) = 18.9 Hz,
C3), 162.45 ppm (d, 3J(C,P) = 14.9 Hz, Cipso of Ph); 31P NMR
(121.5 MHz, THF, 258C, 85% H3PO4 as external standard): d =
160.23ppm (s).
[8] L. Cataldo, S. Choua, T. Berclaz, M. Geoffroy, N. Mezailles, L.
Ricard, F. Mathey, P. Le Floch, J. Am. Chem. Soc. 2001, 123,
6654 – 6661.
9: A solution of GaCl3 (23mg, 0.131 mmol) in CH 2Cl2 (1 mL) was
added to a solution of 6 (38 mg, 0.119 mmol) in CH2Cl2 (2 mL) at
room temperature. After stirring for 5 min, solvent was removed and
the product was recovered as a light brown powder, yield 52 mg
(88%). Elemental analysis (%) calcd: C 33.97; H 5.70; found: C 33.93,
H 5.68; 1H NMR (300 MHz, C6D6, 258C): d = 0.55 (s, 18H, Si(CH3)3),
2.71 (d, 2J(H,P) = 2.4 Hz, 3H, PCH3), 3.03 (d, 2J(H,P) = 20.9 Hz, 3H,
PCH3), 7.50 ppm (d, 4J(H,P) = 6.1 Hz, 1H, H4); 13C NMR (75.5 MHz,
C6D6, 258C): d = 0.97 (d, 3J(C,P) = 3.7 Hz, Si(CH3)3), 10.91 (d,
[9] A. Moores, L. Ricard, P. Le Floch, N. MØzailles, Organometallics
2003, 22, 1960 – 1966.
[10] Crystal data for 8; C32H48Cl4GaO2PSi2; Mr = 763.37; 0.20 ” 0.20 ”
0.20 mm; orthorhombic; space group Cmc21; a = 15.9430(10),
b = 12.8210(10), c = 18.7420(10) Š; V= 3831.0(4) Š3; Z = 4;
calcd = 1.324 gcmÀ3; qmax = 30.028; MoKa radiation (0.71069 Š);
f and w scans; T= 150.0(1) K; 8344 measured, 4963 unique, 4612
used reflections, criterion I > 2s(I); Refinement: least-squares
(full matrix) on F2; F(000) = 1592, m = 1.128 cmÀ1, KappaCCD
diffractometer; Rint = 0.0226; Absorption corrections : multiple
scans, 0.8058 min, 0.8058 max; 235 parameters refined; R1 =
0.0271; wR2 = 0.0751, GoF = 1.084; Flack's parameter 0.001(7);
difference peak/hole: 0.426(0.047)/À0.358(0.047) eŠÀ3. CCDC-
212930 (8) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
1
1J(C,P) = 53.6 Hz, PCH3), 26.92 (d, J(C,P) = 17.1 Hz, PCH3), 134.72
(d, 3J(C,P) = 53.2 Hz, C4), 136.46 (d, 2J(C,P) = 30.2 Hz, C2),
2
158.94 ppm (d, J(C,P) = 15.4 Hz, C3); 31P NMR (121.5 MHz, THF,
258C, 85% H3PO4 as external standard): d = 156.86 ppm (s).
10: MeLi in diethyl ether (70 mL; 0.111 mmol, 1.6m) was added to
a crude solution of 8 (0.111 mmol) in THFat À788C. The solution was
stirred for 10 min, then allowed to warm to room temperature, solvent
was removed in vacuo, and the product extracted in hexane (2 ” 2mL)
and recovered as a pale yellow powder, yield 41 mg (88%). Selected
data: elemental analysis (%) calcd for: C 71.04, H 8.35; found: C
71.07, H 8.36; 1H (300 MHz, C6D6, 258C): d = À0.06 (s, 18H,
2
4
Si(CH3)3), 1.29 (d, J(H,P) = 12.1 Hz, 6H, PCH3), 5.57 (d, J(H,P) =
1.5 Hz, 1H, H4), 7.02–7.35 ppm (m, 10H, Ph); 31P (121.5 MHz, C6D6,
258C 85% H3PO4 as external standard): d = 10.5 ppm (s).
[11] A silacalix-[4]-phosphinines was taken as a reference compound:
N. Avarvari, N. Mezailles, L. Ricard, P. Le Floch, F. Mathey,
Science 1998, 280, 1587 – 1589.
11: An equimolar amount of dried 4-octyne (17 mL, 0.111 mmol)
was added to a crude solution of 8 (0.111 mmol) in CH2Cl2. The
resulting mixture was stirred at room temperature in the glove box for
24 h. After removing of the solvent in vacuo, the product was
recovered as a beige powder, yield 71 mg (86%). Selected data:
elemental analysis (%) calcd for: C 53.32, H 6.51; found: C 53.28, H
6.51; 1H NMR (300 MHz, CD2Cl2, 258C): d = 0.04 (s, 18H, Si(CH3)3),
À
À
[12] This value is intermediate between P C single bond and P C
double bond in methylene phosphenium: H. Grützmacher, H.
Pritzkow, Angew. Chem. 1992, 104, 92 – 94; Angew. Chem. Int.
Ed. Engl. 1992, 31, 99 – 101; A. Igau, A. Baceiredo, H.
Grutzmacher, H. Pritzkow, G. Bertrand, J. Am. Chem. Soc.
1989, 111, 6853– 6854.
[13] D. J. Berger, P. P. Gaspar, J. F. Liebman, Theochem J. Mol. Struct.
1995, 338, 51 – 70; A. J. Ashe, M. K. Bahl, K. D. Bomben, W. T.
Chan, J. K. Gimzewski, P. G. Sitton, T. D. Thomas, J. Am. Chem.
Soc. 1979, 101, 1764 – 1767.
0.78 (t, 3J(H,H) = 7.3Hz, 3H, CH of propyl), 0.92 (t, 3J(H,H) =
3
7.3Hz, 3H, CH of propyl), 1.12–1.23(m, 2H, CH of propyl),
3
2
1.41–1.50 (m, 2H, CH2 of propyl), 2.34–2.39 (m, 2H, CH2 of propyl),
2.45–2.58 (m, 2H, CH2 of propyl), 2.70 (d, 2J(H,P) = 13.9 Hz, 3H,
PCH3), 5.52 (d, 4J(H,P) = 6.0 Hz, 1H, H4) 6.93–7.00 (m, 5H, Ph),
7.32–7.48 ppm (m, 5H, Ph); 31P (121.5 MHz, CD2Cl2, 258C 85%
H3PO4 as external standard): d = 4.36 ppm (s).
[14] A. D. Becke, Phys. Rev. A 1988, 38, 3098 – 3108; Gaussian98
(RevisionA.11), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A.
Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M.
Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J.
Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.
Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson,
P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B.
Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.
Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres,
M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian, Inc.,
Pittsburgh, PA, 1998; J. P. Perdew, Phys. Rev. B 1986, 33, 8822 –
8832.
Received: June 26, 2003[Z52230]
Keywords: aromaticity · density functional calculations · Diels–
.
Alder reactions · phosphininium compounds · phosphorus
[1] K. B. Dillon, F. Mathey, J. F. Nixon, Phosphorus: The Carbon
Copy, Wiley, Chischester, 1998.
[2] G. Frison, A. Sevin, N. Avarvari, F. Mathey, P. Le Floch, J. Org.
Chem. 1999, 64, 5524 – 5529; P. Le Floch in Phosphorus—Carbon
Angew. Chem. Int. Ed. 2003, 42, 4940 –4944
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Communications
[15] This trend has already been observed for compound Ia at the
HF/4.31G level: L. L. Lohr, H. B. Schlegel, K. Morokuma, J.
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