Tetrakis-(N-pyrrolyl)methane: Structurally Important Member of the C(NR2)4 Family

Article abstract of DOI:10.1002/jhet.2233

The structural motif C(NR₂)₄ is rare in chemistry. The formation, NMR spectroscopic characteristics, and X-ray structure are reported for a structurally interesting member of this family, namely tetrakis-(N-pyrrolyl)methane.

Full text of DOI:10.1002/jhet.2233

Month 2014
Tetrakis-(N-pyrrolyl)methane: Structurally Important Member
of the C(NR2)4 Family
Guanxing Sun, Douglas N. Butler, Ronald N. Warrener, and Davor Margetić
a
a
a
b
*
aCentre for Molecular Architecture, Central Queensland University, North Rockhampton, 4702, Queensland, Australia
bDivision of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička c. 54,10000, Zagreb, Croatia
*
E-mail: margetid@irb.hr
Received September 20, 2013
DOI 10.1002/jhet.2233
Published online 00 Month 2014 in Wiley Online Library (wileyonlinelibrary.com).
The structural motif C(NR2)4 is rare in chemistry. The formation, NMR spectroscopic characteristics, and X-ray
structure are reported for a structurally interesting member of this family, namely tetrakis-(N-pyrrolyl)methane.
J. Heterocyclic Chem., 00, 00 (2014).
INTRODUCTION
of this class, namely tetrakis-(N-pyrrolyl)methane 4, com-
parable with the time-honored tetraphenyl analogue and
prototype for a large family of tetrakis-N-[5-ring]heteroaryl
methanes of which tetrakis-N-pyrazolyl methane 2 [5]
appears to be the only other known member.
It has been predicted that the polymorphic forms of
carbon nitride are harder than diamond [1], and this auspi-
cious speculation has prompted huge efforts to reliably pre-
pare C3N4 [2]. Recently, there have been a number reports
in surface science literature about the existence of carbon
nitride phases as thin layers [3]. Carbon nitride is expected
to have each nitrogen atom connected to three carbon
atoms and each carbon atom connected to four nitrogen
atoms. Although tertiary amines (NC3) are very common,
compounds containing C(NR2)4 units are rare in organic
chemistry, and very few examples have appeared in the
literature (e.g., structures 1 [4], 2 [5], and 3 [6], Fig. 1).
In this communication, we describe an interesting member
RESULTS AND DISCUSSION
In the course of our studies of pyrrole cycloaddition
reactions [7], tetrakis-(N-pyrrolyl)methane 4 has been
obtained as an unprecedented product in the reaction of po-
tassium pyrrole 5 in THF with triphosgene 6, a scheme
designed to produce N,N′-dipyrrolyl ketone 7 (Scheme 1).
This ketone has earlier been reported to be formed in low
© 2014 HeteroCorporation

G. Sun, D. N. Butler, R. N. Warrener, and D. Margetić
Vol 000
at δ 121.91, 110.87, and 94.20 ppm. The resonance of
the central quaternary carbon atom at 94.20 ppm is an
important reference value to compare with the four
members of this family thus far reported as having their
methane 13C NMR resonances at δ 92.1, 97.5, 101.7,
and 102.0 for NR2═NMe2, NEt2, pyrrolidinyl, and pip-
eridinyl, respectively [6].
Figure 1. Compounds containing C(NR2)4 units.
Scheme 1. Preparation of 7 and 4.
The X-ray structure of 4 reveals a tetragonal crystal sys-
tem belonging to the I41/a space group. All four core C–N
bond lengths have an average value of 1.4615 ( 4) Å
(Fig. 2). The N–C–N angles at the central carbon atom
are all close to the tetrahedral standard of 109°28′. There
are four slightly larger angles of 110.28 (5)° and two
smaller ones of 107.87 (9)°. This slight difference between
the large and small angles around the central carbon atom
is at variance with the data presented by Schmidbaur [6].
He has observed in two cases of C(NR2)4 molecules that
the N–C–N angles at the central carbon atom deviate
strongly from the standard tetrahedral geometry. Thus,
two large angles [117.02 (5)°] and four smaller ones
[105.76 (10) to 105.88 (5)°] are reported for C(NMe2)4
and again two large ones [117.57 (12)° and 117.74 (12)°]
and four small ones [105.48 (14)° to 105.60 (14)°] for
structure 3 [6]. In addition, differences of the central angles
are also predicted for C(NH2)4 and Si(NH2)4 by DFT
calculations [11] and have been found in the experimen-
tally determined gas-phase structures of C(OMe)4 [12]
and Si(OMe)4 [13]. Thus, this structure determination of
4 is important in this context because the results show
distinctly smaller deviations from tetrahedral symmetry
than the comparable compounds 3 and C(NMe2)4. The
structure of 2 has been also determined by XRD [14]. The
C1 atom presents a distorted sp3 geometry, with N–C–N
angles closer to tetrahedral standard and ranging from
108.7 (2)° to 110.4 (2)°. On the other hand, ab initio study
of the molecular structure of tetrakis(dimethylamino)
methane at the HF/6-31G** level of theory showed larger
variations in N–C–N angles than 4 (from 106.5° to
115.5°) [15]. The equilibrium structure of C(NMe2)4 shows
a spiro connection of two U- and W-shaped arrangements
of the nitrogen lone pairs with D2d symmetry.
yield (4%) by the reaction of N-pyrrolylmagnesium bro-
mide with ethyl chlorothiolformate; however, no formation
of 4 was mentioned [8]. Cycloaddition properties of 7 are
described in the final section of this account.
We have improved the yield of 7 by the reaction of 5 and
triphosgene 6, and after chromatography, along with ketone
7 (27%), a small amount (4%) of a crystalline compound
assigned structure 4 (mp 130–131°C) was isolated.
Although our aim was to prepare 7 in better yield, the isola-
tion of sufficient amounts of 4 prompted the present inves-
tigation of its structural impact. The mechanism for the
formation of 4 is suggested to be through intermediates 11
and 13 and with 11 reacting further with 5 to give 4
(Schemes 2 and 3) [9].
This compound has been published previously; how-
1
ever, aside from its H NMR spectra, other spectroscopic
data were not given [10]. In an original article, 4 was pre-
pared by the reaction of [CF3S(NMe2)2]+[SCF3]À with
trimethylsilylpyrrole in 90% yield. In addition, a crystal
structure of 4 was obtained and structural data were
deposited in the Cambridge Structural Database, again no
comments on the structure were given in the paper. A
comparison with our structure revealed that the two
structures are identical. Because this structure is known,
our crystallographic data that are discussed herein were
not deposited in the Cambridge Structural Database.
The 1H NMR spectrum of 4 in CDCl3 showed only two
signals at δ 6.63 ppm (t, J = 2.2 Hz, 8H) and 6.23 ppm
(t, J = 2.2 Hz, 8H) corresponding to the pyrrole ring pro-
tons. The 13C NMR spectrum exhibited three resonances
The bond lengths and angles of pyrrole itself have been
obtained by electron diffraction and microwave spectros-
copy [16]. For one of the parental arene heterocycles, crys-
tal data for pyrrole ring parameters are less common than
might be expected. Therefore, the comparative information
provided by the X-ray structure of 4 will also enrich
pyrrole ring data. The N–C2 and N–C5 bonds (1.39 Å)
are shorter than normal N–C bonds (1.45 Å); the C3–C4
bond (1.42 Å) is considerably shorter than a normal C–C
bond (1.54 Å), whereas the C2–C3 and C4–C5 bonds
(1.36 Å) are almost the same as normal C═C bonds
(1.35 Å). The geometry at the nitrogen atom is essentially
trigonal with the sum of the three C–N–C angles around
Scheme 2. Proposed mechanism for the formation of 7.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2014
Tetrakis-(N-pyrrolyl)methane
Scheme 3. Proposed mechanism for the formation of 4.
cations [m/z = 144 (4.6%) and 78 (13.3%), respectively]
were also seen. Formally, the lone pairs of electrons of
the three nitrogen atoms can contribute to the stability of
this special ion 16 with its 18 π-electron aromatic system.
Our theoretical calculations at the B3LYP/6-31G* level
indicate it to be propeller-shaped with adjacent pyrrole
rings at a twist angle (N3–C1–N2–C4) of 31.8° and the
central carbon and the three nitrogen atoms connected to
it in a plane (Fig. 3).
A well-known ion with similar structural features is the
triphenylmethyl cation 17 [17] with a calculated dihedral
angle (C3–C1–C2–C4) of the phenyl rings out of the cen-
tral carbon plane of 33.4° (X-ray dihedral angle is 33°).
The comparative stability of cation 16 (first proposed here)
relative to cation 17 is an important question. There is less
H–H steric interaction between pyrrole rings resulting in
smaller dihedral angles (i.e., 16 is slightly more planar
than 17) with the consequent possibility of greater reso-
nance energy of delocalization. Also, the nitrogen atom
lone pairs can better stabilize a carbocation than can carbon
in spite of the increased electronegativity of nitrogen. On
this latter point, the calculated B3LYP/6-31G* difference
[18] in energy between the N-pyrrylmethyl and benzyl
cations obtained from an isodesmic exchange with their
respective conjugate acids (Scheme 4a) favors the N-pyrryl
ion by 8.8 kcal/mol (this preference is corroborated at the
MP2/6-31G* level, by 9.5 kcal/mol) (Scheme 4a). From
our calculations (B3LYP/6-31G*), the isodesmic exchange
of the two ions 16 and 17 with their respective conjugate
acids lies only 4.25 kcal/mol in favor of the trityl cation side
Figure 2. X-ray structure of compound 4.
the pyrryl nitrogens equal to 358.5°, and the pyrrole rings are
completely planar with the sum of inner angles being 540.0°.
The electron impact mass spectrum of 4 showed a low-
intensity parent molecular ion (1.7%) at m/z = 276.1390
(required 276.1375) with the de-pyrrolated cation 16
appearing as the base peak at m/z = 210.1030 as required
for C13H12N3 = 210.1031. The di- and mono-pyrrolyl
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

G. Sun, D. N. Butler, R. N. Warrener, and D. Margetić
Vol 000
Figure 3. Optimized geometries of cations 16 and 17 (B3LYP/6-31G*). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
Scheme 4. Calculated isodesmic reaction energy differences.
(Scheme 4b). Cation 16 thus presents itself as a valid re-
search entity well worthy of experimental investigation
and may be considered the archetype of polyaza variants
of this family.
tube. The product consists of two signals (δ 5.30 and
7.18 ppm) in the H NMR spectrum, and HRMS showed
1
the bis-adduct 19b. On the other hand, the reaction of 7
with DMAD gave the mixture of 1:1 and 1:2 of adducts
19a and 20a. Although electron-withdrawing substituents
enhance the reactivity of pyrroles [20], no intramolecular
cycloadduct of type 21a was observed (Scheme 5). A
possible reason could be the molecular rigidity of 7
introduced by a carbonyl group disfavoring intramolecular
addition. Indeed, our DFT calculations (B3LYP/6-31G*)
indicate a larger activation energy for this process, as
compared with the cycloaddition of N,N′-dipyrrolylmethane
Cycloadditions.
With N,N′-dipyrrolyl ketone 7 in
hand, its reactivity in Diels–Alder cycloadditions was
also studied. The results of the cycloaddition reactions of
7 with hexafluoro-2-butyne (HFB) and dimethylacetylene
dicarboxylate (DMAD) are in variance to products
21a/21b obtained by Battiste in the cycloadditions of
N,N′-dipyrrolylmethane 18 [19]. A crystalline compound
was obtained from the reaction of 7 with HFB in a sealed
Scheme 5. Cycloadditions of di-(N-pyrrolyl)methanes 7 and 18.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2014
Tetrakis-(N-pyrrolyl)methane
1.333 mg/m3; selected bond lengths (Å): N1–C2 1.3922 (18),
N1–C5 1.3941 (18), C1–N1═C1–N1#1═C1–N1#2═C1–N1#3═
1.4216 (11), C2–C3 1.361 (2), C4–C5 1.362 (2), C(3)–C4
1.421 (2); selected angles (°): N1#1–C1–N1#2═N1#1–C1–
N1═N1#2–C1–N1#3═N1–C1–N1#3═110.28 (5), N1#2–C1–
N1═N1#1–C1–N1#3═107.87 (9), C2–N1–C5 108.24 (12),
C2–N1–C1 125.32 (11), C5–N1–C1 124.93 (11), C3–C2–N1
108.00 (13), C2–C3–C4 107.93 (14), C5–C3–C4 107.84 (14),
C4–C5–N1 107.96 (13).
18 (25.8 kcal/mol). No reactions of tetrapyrrolylmethane 4
with HFB (60°C) or with DMAD (14 kbar, 60°C) were
observed. The recovery of 4 indicated its stability under
such conditions.
CONCLUSION
Molecule 4 thus represents a structurally interesting entry
in the C(NR2)4 family, and we agree with the opinion [6] that
it is an open question if a study of the structure and properties
of tetra(amino)methanes will help to solve the problems
associated with carbon nitride. Nevertheless, it is an impor-
tant addition to the data on molecular systems where carbon
is connected to four tricoordinate nitrogen atoms.
Synthesis of 19a and 20a. A solution of ketone 7 (2.0 g,
12.5 mmol) and DMAD (1.8 g, 12.5 mmol) in benzene (20 mL)
was stirred at 55°C for 1 week. The reaction mixture was
allowed to cool to RT, and crystals were separated from the
black solution. Recrystallization from CH2Cl2/petrol afforded
the bis-adduct 20a. The starting material 7 and mono-adduct
19a were separated from the mother liquor by column
chromatography (silica gel, Et2O/petrol 1:1).
1
Product 19a (gum, 200 mg, 11%). H NMR (CDCl3) δ 3.79
(6H, s), 5.67 (2H, t, J = 1.5 Hz), 6.23 (2H, t, J = 2.3 Hz), 7.06
(2H, t, J = 2.3 Hz), 7.20 (2H, t, J = 1.5 Hz); 13C NMR (CDCl3) δ
52.8, 70.1, 112.5, 120.1, 143.1, 152.1, 151.1, 163.1; HRMS
C15H14N2O5 requires m/z 302.0903, found 302.0898.
EXPERIMENTAL
All reagents were purchased from Aldrich (St. Louis, MO) and
used as received. Melting points, which are uncorrected, were
obtained on a Gallenkamp melting point apparatus (Gallenkamp,
UK). 1H NMR spectra were recorded with Bruker AMX-300 and
Bruker Avance DPX 400 NMR spectrometers (Vienna, Austria).
13C NMR spectra were recorded by using an inverse gated
sequence at 75.4 MHz using CDC13 solutions with TMS as an
internal standard. Radial chromatography was carried out with a
Chromatotron, Model No. 7924T, using 1-mm plates coated with
Merck silica gel 60F254. Mass spectra were obtained by EI or
Electrospray Mass Spectrometry (ESMS) on a Micromass Platform
II single quadrupole mass spectrometer (Manchester, UK).
Synthesis of 7 and 4. To a solution of pyrrole (5.0 g, 74.6 mmol) in
dry THF (30 mL) at 0–5°C under N2 were added small pieces of
potassium (2.9 g, 74.6 mmol), and the mixture was allowed to warm
to RT with stirring overnight. A solution of triphosgene (11 g,
37.3 mmol) in THF (30 mL) was added over 20 min to the
suspension of potassium pyrrole at 0°C. The reaction was stirred at
RT under N2 overnight. CH2Cl2 (50 mL) and water (100 mL) were
then added, and the organic phase was separated. The aqueous layer
was extracted with additional CH2Cl2 (30 mL × 2), and the
combined organic extracts were concentrated and chromatographed
(Et2O/petrol 1:1) to afford the crude product 7 containing
approximately 5% of 4. This mixture was subjected to another
radial chromatography (Et2O/petrol 1:1) to yield separated products:
Product 20a (colorless crystals, 570 mg, 10%, mp 160–161°C).
1H NMR (CDCl3) δ 3.81 (12Η, s), 5.35 (4H, s), 7.11 (4H, s); 13
C
NMR (CDCl3) δ 53.6, 70.4, 143.1, 152.4, 156.9, 163.8; Anal.
Calcd for C21H20N2O9: C, 56.76; H, 4.54; N, 6.30. Found: C,
56.75; H, 4.37; N, 6.24%.
Synthesis of 20b.
Hexafluoro-2-butyne (1.6 g, 10 mmol)
was introduced from a gas cylinder to a heavy-walled glass tube
containing a solution of ketone 7 (1.6 g, 10 mmol) in CH2Cl2
(10 mL) cooled to À78°C. The mixture was heated in the sealed
tube at 90°C overnight. The adduct 20b was formed as cloudless
cube crystals on the tube wall, and more was harvested after the
1
evaporation of the solvent (2.0 g, 41%, mp 117–8°C). H NMR
(CDCl3) δ 5.30 (4H, s), 7.18 (4H, s); 13C NMR (CDCl3) δ 69.3,
121.4 (q, J = 268.8 Hz), 143.2, 148.9, 156.3; HRMS (EI)
C17H8H12N2O requires m/z 484.0445, found 484.0456.
Computational details. Quantum chemical calculations were
performed using the Gaussian03 program [21], implemented on a
dual core Opteron 240 personal computer under Linux operating
system within the DFT framework. B3LYP hybrid functional was
used along with the 6-31G(d) basis set. Isodesmic reaction in
Scheme 4a was also calculated by the MP2/6-31G* method. All
the stationary points were characterized by harmonic analysis,
and activation energies were computed including zero-point
vibrational energy corrections.
Product 7 (1.6 g, 27%, mp 56–57°C, from Et2O/petrol). IR
(KBr, cmÀ1) 1724 (C═O); 1H NMR (CDCl3) δ 6.37 (t, J = 2.3Hz,
4H), 7.31 (t, J = 2.3 Hz, 4H); 13C NMR (CDCl3) δ 113.86, 122.58,
148.62 (carbonyl); low resolution mass spectrometry (LRMS)
160.0 (M+, 47.5%), 94.0 (68), 66.0 (100).
Product 4 (160 mg, 4%, mp 130–131°C). 1H NMR (CDCl3) δ
6.23 (t, J = 2.2 Hz, 8H), 6.63 (t, J = 2.2 Hz, 8H); 13C NMR
(CDCl3) δ 94.20, 110.87, 121.91; LRMS 276 (M+, 1.7%), 210
(100), 144 (4.6), 143 (19.8), 78 (13.3), 67 (16.6), 66 (10.7);
HRMS C17H16N4 requires m/z 276.1275, found 276.1290;
(C4H4N)3C requires m/z 210.1030, found 210.1031.
Crystal data for 4: colorless cube; empirical formula: C17H16N4;
Mr = 276.34; temp. 100 (2) K; wavelength 0.71073 Å; crystal size:
0.35 × 0.35 × 0.25 mm3; crystal system: tetragonal; space group
I41/a; unit cell dimensions: a = b = 9.7794 (9) Å, c = 14.403 (2) Å;
V = 1377.5 (3) Å3; α = β = γ = 90o; Z = 4; density (calculated) =
Acknowledgments. This work was financially supported by the
Australian Research Council and the Ministry of Science,
Education and Sport of the Republic of Croatia (project no.
098-0982933-3218).
REFERENCES AND NOTES
[1] (a) Liu, A. Y.; Cohen, M. L. Phys Rev B 1990, 41, 10727; (b)
Liu, A. Y.; Cohen, M. L. Science 1989, 245, 841.
[2] For a review on carbon nitride, see: (a) Malkow, T. Kagaku to
Kogyo (Tokyo) 2000, 53, 1323; (b) Angew Chem Int Ed Engl 1993, 32, 1580.
[3] (a) Fitzgerald, A. G.; Jiang, L.; Rose, M. J.; Dines, T. J Appl
Surf Sci 2001, 175-176, 525; (b) Xu, W.; Fujimoto, T.; Li, B.; Kojima, I.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

G. Sun, D. N. Butler, R. N. Warrener, and D. Margetić
Vol 000
[17] Gomes de Mesquita, A. H.; MacGillavry, C. H.; Eriks, K. Acta
Cryst 1965, 18, 437.
Appl Surf Sci 2001, 175-176, 456; (c) Andujar, J. L.; Pino, F. J.; Polo, M. C.;
Bertran, E. Diamond Relat Mater 2001, 10, 1175; (d) Jimenez, I.; Gago,
R.; Albella, J. M.; Terminello, L. J Diamond Relat Mater 2001, 10,
1170; (e) Veprek, S.; Weidman, J.; Glatz, F. J Vac Sci Technol A
1995, 13, 2914.
[4] Quast, H.; Schmitt, E. Chem Ber 1968, 101, 1137.
[5] Julia, S.; del Mazo, J. M.; Avila, L.; Elguero, J. Org Prep Proc
Int 1984, 16, 299.
[6] Jockisch, A.; Schmidbaur, H. Chem Ber 1997, 130, 1739.
[7] (a) Warrener, R. N.; Margetić, D.; Butler, D. N.; Sun, G. Synlett
2001, 202; (b) Warrener, R. N.; Margetić, D.; Sun, G.; Russell, R. A. Aust
J Chem 2003, 56, 263; (c) Margetić, D.; Warrener, R. N.; Sun, G.; Butler,
D. N. Tetrahedron 2007, 63, 4338.
[8] Wang, N.-C.; Anderson, H. J Can J Chem 1977, 55, 4103.
[9] Cotarca, L.; Eckert, H. Phosgenations—A Handbook; Wiley:
Weinheim, 2003; p 44.
[10] Müller, M.; Lork, E.; Mews, R. Angew Chem Int Ed 2001, 41,
1247. CCDC-149967
[11] Albert, K.; Rosch, N. Chem Ber 1997, 130, 1745.
[12] Mijlhoff, J. C.; Geise, H. J.; van Schaick, E. J. M. J Mol Struct
1974, 20, 393.
[13] Boonstra, L. H.; Mijlhoff, F. C.; Renes, G.; Spelbos, A.;
Hargittai, I.; J Mol Struct 1975, 28, 129.
[14] Claramunt, R. M.; Elguero, J.; Fabre, M. J.; Foces-Foces, C.;
Cano, F. H.; Fuentes, I. H.; Jaime, C.; Lopez, C. Tetrahedron 1989, 45, 7805.
[15] Galasso, V.; Jones, D.; Richman, J. E. Theochem-J Mol Struct
1998, 429, 247.
[18] aIsodesmic cations calcs: (a) de Meijere, A.; Faber, D.;
Noltemeyer, M.; Boese, R.; Hauman, T.; Müller, T.; Bendikov, M.;
Matzner, E.; Apeloig, Y. J Org Chem 1996, 61, 8564; (b) Schleyer, P. v.
R.; Jiao, H.; Glukhotsev, M. N.; Chadrasekhar, J.; Kraka, E. J Am Chem
Soc 1994, 116, 10129..
[19] (a) Visnick, M.; Battiste, M. A. J Chem Soc, Chem Commun
1985, 1621; (b) For theoretical treatments of this reaction, see Domingo,
L. R.; Picher, M. T.; Arno, M.; Andres, J.; Safont, V. S. Theochem 1998,
426, 257; (c) Domingo, L. R.; Arno, M.; Andres, J. J Am Chem Soc
1998, 120, 1617.
[20] Warrener, R. N.; Margetić, D.; Sun, G. Tetrahedron Lett 2001,
42, 4263.
[21] Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T. Jr; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.;Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.;
Jaramillo, J.; Gompert, R. S.; Stratmann, R. E.; Yazyev, O.; Austin, A.
J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Glifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,
T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.;
Gill, M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople,
J. A. Gaussian 03, Revision B.03; Gaussian, Inc.: Pittsburgh PA, 2003.
[16] Comprehensive Organic Chemistry: The Synthesis and
Reactions of Organic Compounds; Sammes, P. G., Ed.; Pergamon Press,
Oxford, 1979; Vol 4, p 277.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet