The synthesis and molecular structures of 2,5-diarylpyrroles, [pyr Ar2]H (Ar = Ph, 2,4-xyl)

Article abstract of DOI:10.1023/A:1020239919672

2,5-Di(2,4-xylyl)pyrrole, [pyr^2,5-Xyl2]H, has been synthesized by reaction of the diketone [XylC(O)CH₂]₂ with NH ₄OAc in AcOH; the diketone itself is obtained by condensation of XylC(O)Me with XylC(O)CH₂

Full text of DOI:10.1023/A:1020239919672

C
Journal of Chemical Crystallography, Vol. 32, No. 7, July 2002 ( 2002)
The synthesis and molecular structures
of 2,5-diarylpyrroles, [pyrAr2]H (Ar = Ph, 2,4-Xyl)
Joseph M. Tanski(1) and Gerard Parkin(1)
Received October 4, 2001
2,5-Di(2,4-xylyl)pyrrole, [pyr2,5-Xyl ]H, has been synthesized by reaction of the diketone
2
[XylC(O)CH2]2 with NH4OAc in AcOH; the diketone itself is obtained by condensation of
XylC(O)Me with XylC(O)CH2Br in the presence of Ti(OPri)4. The molecular structures of
2,5-Xyl2
both [pyr2,5-Ph ]H and [pyr
]H have been determined by single crystal X-ray diffrac-
2
tion, thereby demonstrating that an ortho methyl substituent increases the dihedral angle
2,5-Xyl2
between the aryl and pyrrolyl groups from 14.5 in [pyr2,5-Ph ]H to 24.1 in [pyr
]H.
2
KEY WORDS: pyrroles; interplanar angles; dihedral angles.
Introduction
Although much less ubiquitous than cyc-
Results and Discussion
It has been proposed that bulky substituents
in the 2- and 5-positions of the pyrrolyl ligand
favor 5-coordination over 1-coordination, since
the latter coordination mode would force the
substituents directly toward the metal center.1,3,4
lopentadienyl, pyrrolyl ligands [pyrRn ](2) have
applications in both main group and transition
metal chemistry.1 Pyrrolyl ligands may bind to
a metal center by a variety of means, which in-
clude 1-, 5- and bridging coordination modes.1,2
Substituents on the pyrrolyl ring play a key role
in influencing the binding mode of the ligand,
with bulky groups in the 2,5-positions increasing
the relative stability of 5-coordination. We are
presently interested in the application of pyrrolyl
ligands that feature aryl substituents, and in this
paper we report on the synthesis and molecular
For example, the 2,5-di-t-butylpyrrolyl ligand
t
2
[pyr2,5-Bu ] has been crystallographically iden-
tified to coordinate in an 5-mode to elements
as diverse as titanium5 and the lanthanides.6
In contrast, incorporation of substituents into
the ortho positions of aryl groups of 2,5-
diarylpyrrolyl ligands [pyr2,5-Ar ] may, however,
2
have the potential of promoting 1-coordination
as a result of destabilizing a conformation in
which the pyrrolyl and aryl rings are coplanar
(Fig. 1). For example, the phenyl-substituted
structures of the pyrrole compounds, [pyr2,5-Ar ]H
2
(Ar = Ph, 2,4-Xyl).
complex [pyr2,5-Ph ]Zr(NMe2)3 exhibits
-
5
2
(1) Department of Chemistry, Columbia University, New York, New
York 10027.
(2) Pyrrolyl ligands, (C5RnH4 nN) are represented by the abbrevia-
tion [pyrRn ], where the superscript indicates the number and type
of substituents.
coordination, whereas the xylyl counterpart
[pyr2,5-Xyl ]Zr(NMe2)3 exhibits 1-coordination.7
2
To address further this issue, we sought to
determine the influence of an ortho methyl
substituent on the interplanar angle of the parent
To whom correspondence should be addressed. E-mail: parkin@
chem.columbia.edu.
185
C
1074-1542/02/0700-0185/0 2002 Plenum Publishing Corporation

186
Tanski and Parkin
Fig. 2. Molecular structure of [pyr2,5-Ph ]H.
2
Fig. 1. Relative stability of 1- and 5-pyrrolyl coordination
3. Selected bond lengths and angles are listed
in Tables 1 and 2. The metrical parameters
are unremarkable, with the dimensions of the pyr-
role ring being very similar to those of 2,5-di-tert-
butylpyrrole.2 For example, the average N C
as a function of the aryl/pyrrolyl dihedral angle.
pyrrole derivatives, [pyr2,5-Ar ]H (Ar = Ph,
2
2,4-Xyl).
The
compound
2,5-diphenylpyrrole
0
0
C
C
and C
C
bond lengths of 2,5-di-
[pyr2,5-Ph ]H is readily obtained by the reac-
tion of the 1,4-diketone [PhC(O)CH2]2 with
NH4OAc in AcOH.8 Similarly, the corresponding
reaction of [XylC(O)CH2]2, obtained by the con-
densation of XylC(O)Me with XylC(O)CH2Br
promoted by Ti(OPri)4,9 with NH4OAc in AcOH
2
˚
˚
tert-butylpyrrole are 1.377(9) A, 1.365(5) A,
˚
and 1.420(3) A, respectively; furthermore, these
values are very similar to that of pyrrole itself
˚
˚
˚
[1.365(2) A, 1.357(2) A, and 1.423(3) A, respecti-
vely].10 The crystal structures of [pyr2,5-Ph ]H
2
and [pyr2,5-Xyl ]H differ from pyrrole, how-
2
yields [pyr2,5-Xyl ]H (Scheme 1).
2
The molecular structures of [pyr2,5-Ph ]H and
ever, in terms of their molecular packing.
Specifically, pyrrole exhibits intermolecular
2
[pyr2,5-Xyl ]H were determined by single crystal
2
N H
-system hydrogen bonding interac-
X-ray diffraction, as illustrated in Figs. 2 and
tions involving the C C bond. Examination of
the molecular packing diagrams demonstrates that
no such interactions are present in [pyr2,5-Ph ]H
2
and [pyr2,5-Xyl ]H (Figs. 4 and 5).
2
The most noteworthy aspect of the structures
2,5-Xyl
of [pyr2,5-Ph ]H and [pyr
]H in the present
2
2
study is concerned with the dihedral angle be-
tween the pyrrolyl and aryl groups. Specifically,
the xylyl rings are twisted out of the pyrrolyl plane
Fig. 3. Molecular structure of [pyr2,5-Xyl ]H.
2
Scheme 1.

2,5-diarylpyrroles
187
˚
˚
Table 1. Selected Bond Lengths (A) and Angles (deg) for
Table 2. Selected Bond Lengths (A) and Angles (deg) for
[pyrr2,5-Ph2 ]H
[pyrr2,5-Xyl ]H
2
Bond lengths
Bond lengths
N
C(1)
1.372(3) N C(4)
1.384(4) C(3) C(4)
1.408(4) N H(1)
1.459(3) C(4) C(11)
1.380(3)
1.381(4)
0.99(4)
N
C(1)
1.379(2) N C(4)
1.378(2) C(3) C(4)
1.402(2) N H(1)
1.465(2) C(4) C(5)
1.380(2)
1.379(2)
0.86(2)
C(1) C(2)
C(2) C(3)
C(1) C(21)
Bond angles
C(1) C(2)
C(2) C(3)
C(1) C(13)
Bond angles
N(1) C(1) C(2)
C(2) C(3) C(4)
1.455(4)
1.470(2)
N
C(1) C(2)
C(1) C(2) C(3)
C(3) C(4)
C(2) C(1) C(21)
C(1) C(21)
106.5(2)
C(2) C(3) C(4)
C(1) C(4)
C(3) C(4) C(11) 130.4(3)
C(4) C(11) 123.3(2)
108.3(3)
111.0(2)
105.6(1)
C(1) C(2) C(3)
C(3) C(4)
C(2) C(1) C(13) 133.8(1)
C(1) C(13) 120.6(1)
108.7(1)
105.7(1)
108.0(2)
106.2(2)
130.0(2)
123.5(2)
N
108.4(1)
111.5(1)
133.9(1)
120.3(1)
123.2(1)
N
N
C(1)
C(3) C(4) C(5)
C(4) C(5)
C(4) C(5) C(6)
N C(4)
N
N
N
C(4) C(11) C(12) 122.7(2)
C(4) C(11) C(16) 120.1(3)
N
C(1) C(13) C(14) 123.4(1)
C(1) C(13) C(20) 119.0(1)
C(1) C(21) C(22) 122.5(2)
C(1) C(21) C(26) 119.9(3)
C(4) C(5) C(12) 119.3(1)
of [pyr2,5-Xyl ]H (24.1 )(3) to a greater degree than
2
are the phenyl rings of [pyr2,5-Ph ]H (14.5 )(4) as il-
2
lustratedinFig. 6;forcomparison, thedihedralan-
gle in simple polyphenyl complexes is ca. 13.3 .11
Thus, steric interactions between the ortho methyl
groups and the -hydrogens of the pyrrolyl ring
increase the orthogonality of the aryl and pyrrolyl
groups, but the steric interaction is insufficient
to overcome the completely planar structure that
is favored by electronic delocalization across the
three aromatic rings.
Experimental Section
General Considerations
NMR spectra were recorded on Bruker
Avance 300wb DRX, Bruker Avance 400 DRX,
and Bruker Avance 500 DMX spectrometers. 1H
and 13C chemical shifts are reported in parts
per million (ppm) relative to SiMe4 ( = 0) and
were referenced internally with respect to the pro-
tio solvent impurity or the 13C resonances, re-
spectively. All coupling constants are reported
in hertz (Hz). Mass spectra were obtained us-
ing a JEOL LC Mate mass spectrometer in
APCI+ ionization mode using a methanol/water
solution (80/20). C, H, and N elemental analyses
(3) The individual dihedral values for [pyr2,5-Xyl2 ]H are 21.5 and
26.7 .
(4) The individual dihedral values for [pyr2,5-Ph2 ]H are 13.7 and
15.2 .
Fig. 4. Packing diagram of [pyr2,5-Ph ]H (down b axis).
2

188
Tanski and Parkin
Synthesis of [XylC(O)CH2]2
A mixture of 2,4-XylC(O)Me (9.8 g,
66 mmol) and (2,4-Xyl)C(O)CH2Br (10.0 g,
44 mmol) in dry CCl4 (50 mL) was treated
with freshly distilled Ti(OPri)4 (20.8 mL, 20.0 g,
70 mmol) and stirred at room temperature un-
der an inert atmosphere for 1 h. The mixture
was allowed to stand for 1 week, over which
period white crystals were deposited. The flask
was cooled to 0 C, and the product was iso-
lated by filtration, washed successively with
cold (0 C) CCl4, cold (0 C) HClaq (10%), and
cold (0 C) methanol, and dried in vacuo, giv-
ing [XylC(O)CH2]2 (4.6 g, 35%—unoptimized).
Analysis calcd. for C20H22O2: C, 81.6%; H, 7.6%.
Found: C, 79.8%; H, 7.6%. LC/MS(MeOH/H2O):
1
m/z = 295.4 (MH+, 100%). H NMR (CDCl3):
2.34 [s, 2 CH3 of 2 Xyl], 2.46 [s, 2 CH3 of 2 Xyl],
3.30 [s, (CH2)2], 7.01 7.08 [m, 4 m-H of 2 Xyl],
3
1
7.72 [d, JH-H = 7.8, 2 o-H of 2 Xyl]. 13C{ H}
NMR (CDCl3): 21.4 [2 CH3 of 2 Xyl], 21.5 [2
CH3 of 2 Xyl], 35.4[(CH2)2], 126.3 [2 C of 2 Xyl
(CH)], 129.1 [2 C of 2 Xyl (CH)], 132.8 [2 C of
2 Xyl (CH)], 134.8 [2 C of 2 Xyl (quarternary)],
138.6 [2 C of 2 Xyl (quarternary)], 141.8 [2 C of
2 Xyl(quarternary)], 202.0 [CO].
Fig. 5. Packing diagram of [pyr2,5-Xyl ]H (down b axis).
2
were measured using a Perkin-Elmer 2400 CHN
Elemental Analyzer. 2,4-Dimethylacetophenone
[(2,4-Xyl)C(O)Me] was obtained from Aldrich,
2-bromo-20,40-dimethylacetophenone [(2,4-Xyl)
C(O)CH2Br] was obtained from Maybridge
Chemical Company, Cornwall, UK (via Ryan
Scientific), and Ti(OPri)4 was obtained from
Synthesis of [pyr2,5-Ph ]H
2
A mixture of [XylC](O)CH2]2 (4.5 g,
15 mmol) and NH4OAc (15.9 g, 206 mmol)
was refluxed in AcOH (50 mL) overnight un-
der an inert atmosphere. The reaction mixture
was allowed to cool to room temperature and
poured into ice-cold water (1.5 L). The mix-
ture was stirred for several hours, after which
the white precipitate was isolated by filtration,
washed with cold water, and dried in vacuo giving
Aldrich. [pyr2,5-Ph ]H was prepared by the liter-
2
ature method.8
[pyr2,5-Xyl ]H (3.4 g, 81%). Analysis calcd. for
2
C20H21N: C, 87.2%; H, 7.7%; N, 5.1%. Found: C,
87.4%; H, 7.8%; N, 4.6%. LC/MS(MeOH/H2O):
m/z = 276.4 (MH+, 100%). H NMR (CDCl3):
1
Fig. 6. Comparison of the aryl/pyrrolyl dihedral angles of
2.33 [s, 2 p-CH3 of 2 Xyl], 2.47 [s, 2 o-CH3 of 2
Xyl], 6.36 [d, 4 JH-H = 2.7, 2 H of N (C4Xyl2H2),
2,5-Xyl2
[pyr2,5-Ph ]H (upper) and [pyr
]H (lower).
2

2,5-diarylpyrroles
189
3
using direct methods and standard difference
map techniques, and were refined by full-matrix
least-squares procedures on F2 with SHELXTL
(Version 5.03).12 All hydrogen atoms were lo-
cated and refined.
7.04 [d, JH-H = 7.8, 2 m-H of 2 Xyl], 7.08
3
[s, 2 m-H of Xyl], 7.28 [d, JH-H = 7.8, 2 o-
H of 2 Xyl], 8.21 [br s, NH]. 13C{ H} NMR
1
(CDCl3): 21.0 [2 p-CH3 of 2 Xyl], 21.3 [2 o-
CH3 of 2 Xyl], 109.4 [2 C of N(C4Xyl2H2)
(CH)], 126.8 [2 m-C of 2 Xyl (CH)], 127.7
[2 o-C of 2 Xyl (CH)], 129.9 [2 ipso-C of 2 Xyl
(quarternary)], 131.4 [2 C of N(C4Xyl2H2 (quar-
ternary)], 131.9 [2 m-C of 2 Xyl (CH)], 134.7 [2
o-C of 2 Xyl (quarternary)], 136.4 [2 p-C of 2 Xyl
(quarternary)].
Conclusions
2,5-Di(2,4-xylyl)pyrrole, [pyr2,5-Xyl ]H, has
2
been synthesized by reaction of the dike-
tone [XylC(O)CH2]2 with NH4OAc in AcOH.
The molecular structures of [pyr2,5-Ph ]H and
2
[pyr2,5-Xyl ]H indicate that an ortho methyl sub-
2
X-ray Structure Determinations
stituent increases the dihedral angle between
the aryl and pyrrolyl groups from 14.5 in
Crystals of [pyr2,5-Ph ]H and [pyr
]H
2,5-Xyl
2
2
2,5-Xyl
[pyr2,5-Ph ]H to 24.1 in [pyr
]H.
2
2
were obtained from benzene and CDCl3, re-
spectively. X-ray diffraction data were collected
on a Bruker P4 diffractometer equipped with a
SMART CCD detector, and crystal data, data
collection, and refinement parameters are sum-
marized in Table 3. The structures were solved
Acknowledgment
We thank the National Science Foundation
(CHE 99-87432) for support of this research.
Table 3. Crystal, Intensity Collection, and Refinement Data
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[pyr2,5-Ph2 ]H
[pyr2,5-Xyl ]H
2
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Lattice
Formula
CCDC deposit no.
Formula weight
Space group
Monoclinic
Monoclinic
C20H21N
C
16H13
N
CCDC-1003/6141 CCDC-1003/6142
219.17
P21/n
7.475(1)
5.967(1)
26.301(5)
93.715(4)
1170.6(4)
4
233
0.71073
1.244
0.072
28.3
275.38
C2/c
˚
a/A
47.431(3)
5.2104(4)
12.7162(10)
97.817(1)
3113.4(4)
8
243
0.71073
1.175
0.068
28.3
9807
3502
˚
b/A
˚
c/A
/
3
˚
V/A
Z
Temperature (K)
Radiation ( , A)
˚
3
(calcd.), g cm
(Mo K ), mm
max, deg.
Total no. of data
No. of data
Rmerge
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R1
wR2
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7974
2726
0.0682
207
0.0774
0.2115
1.070
0.0276
275
0.0458
0.0993
1.085

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