3,5-Pyridyne-A Heterocyclic meta-Benzyne Derivative

Article abstract of DOI:10.1021/ja039142u

3,5-Pyridyne (3) has been generated by flash vacuum pyrolysis of 3,5-diiodopyridine (20) and 3,5-dinitropyridine (21) and characterized by IR spectroscopy in cryogenic argon matrices. The aryne can clearly be distinguished from other side products by its photolability at 254 nm, inducing a rapid ring-opening presumably to (Z)-1-aza-hex-3-ene-1,5-diyne. As byproducts of the pyrolysis, HCN and butadiyne were identified, together with traces of acetylene, cyanoacetylene, (E)-1-aza-hex-3-ene-1,5-diyne, and the 3-iodo-5-pyridyl radical (from 20). Several pathways for rearrangements and fragmentations of 3 and of the parent meta-benzyne (1) have been explored computationally by density functional theory and ab initio quantum chemical methods. The lowest energy decomposition pathway of biradicals 1 and 3 is a ring-opening process accompanied by hydrogen migration, leading to (Z)-hex-3-ene-1,5-diyne [(Z)-10] and (Z)-3-aza-hex-3-ene-1,5-diyne [(Z)-24], respectively. Both reactions require activation energies of 45-50 kcal mol ^-1. Mechanisms leading from (Z)-24 or directly from 3 to the experimentally observed byproducts are discussed. Upon replacement of the C(5)H moiety by N in meta-benzyne, high-level calculations predict a modest shortening of the interradical distance by 5-7 pm and a reduction of the singlet-triplet energy splitting by 3 kcal mol^-1, in good agreement with isodesmic equations, according to which the singlet ground state of 3 is destabilized relative to 1 by 3-4 kcal mol^-1. In contrast to 3,5-borabenzyne (2), which is found to be doubly aromatic, nucleus-independent chemical shifts of 3 are almost identical to that of pyridine, indicating the absence of paramagnetic ring current effects that may be associated with in-plane antiaromaticity . As compared with 1, the overall perturbation caused by the nitrogen atom in 3 is weak, and four electron, three center interaction is of minor importance in this molecule.

Full text of DOI:10.1021/ja039142u

Published on Web 04/22/2004
3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
Michael Winkler, Bayram Cakir, and Wolfram Sander*
Contribution from the Lehrstuhl fu¨r Organische Chemie II der Ruhr-UniVersita¨t Bochum,
UniVersita¨tsstrasse 150, 44780 Bochum, Germany
Received October 21, 2003; E-mail: Wolfram.Sander@rub.de
Abstract: 3,5-Pyridyne (3) has been generated by flash vacuum pyrolysis of 3,5-diiodopyridine (20) and
3,5-dinitropyridine (21) and characterized by IR spectroscopy in cryogenic argon matrices. The aryne can
clearly be distinguished from other side products by its photolability at 254 nm, inducing a rapid ring-
opening presumably to (Z)-1-aza-hex-3-ene-1,5-diyne. As byproducts of the pyrolysis, HCN and butadiyne
were identified, together with traces of acetylene, cyanoacetylene, (E)-1-aza-hex-3-ene-1,5-diyne, and the
3-iodo-5-pyridyl radical (from 20). Several pathways for rearrangements and fragmentations of 3 and of
the parent meta-benzyne (1) have been explored computationally by density functional theory and ab initio
quantum chemical methods. The lowest energy decomposition pathway of biradicals 1 and 3 is a ring-
opening process accompanied by hydrogen migration, leading to (Z)-hex-3-ene-1,5-diyne [(Z)-10] and (Z)-
3-aza-hex-3-ene-1,5-diyne [(Z)-24], respectively. Both reactions require activation energies of 45-50 kcal
mol-1. Mechanisms leading from (Z)-24 or directly from 3 to the experimentally observed byproducts are
discussed. Upon replacement of the C(5)H moiety by N in meta-benzyne, high-level calculations predict a
modest shortening of the interradical distance by 5-7 pm and a reduction of the singlet-triplet energy
splitting by 3 kcal mol-1, in good agreement with isodesmic equations, according to which the singlet ground
state of 3 is destabilized relative to 1 by 3-4 kcal mol-1. In contrast to 3,5-borabenzyne (2), which is found
to be doubly aromatic, nucleus-independent chemical shifts of 3 are almost identical to that of pyridine,
indicating the absence of paramagnetic ring current effects that may be associated with “in-plane
antiaromaticity”. As compared with 1, the overall perturbation caused by the nitrogen atom in 3 is weak,
and four electron, three center interaction is of minor importance in this molecule.
Introduction
cordingly, the structure and reactivity of nitrogen-containing 1,4-
arynes and their corresponding enediyne precursors have been
investigated in some detail.7-13 Heterocyclic meta-benzyne
derivatives, on the other hand, have not yet received the attention
they deserve, and up to now, even the chemistry of the parent
meta-benzyne (1) is not completely understood, especially with
regard to its reactivity toward electrophiles, nucleophiles, and
radicals.14
Arynes1 are useful reactive intermediates in synthesis,2 and
they also play a crucial role in a number of important processes,
among them the biological activity of cytostatics,3 combustion
reactions,4 and heterogeneous catalysis.5 Interest in heterocyclic
arynes6 increased recently due to their potential application as
more selective warheads in endiyne antitumor agents.7-9 Ac-
(1) (a) Wenk, H. H.; Winkler, M.; Sander, W. Angew. Chem. 2003, 115, 518;
Angew. Chem., Int. Ed. 2003, 42, 502. (b) Sander, W. Acc. Chem. Res.
1999, 32, 669. (c) Hoffman, R. W. Dehydrobenzene and Cycloalkynes;
Academic Press: New York, 1967.
(2) For example, see (a) Pellissier, H.; Santelli, M. Tetrahedron 2003, 59, 701.
(b) Grundmann, C. In Houben-Weyl Methoden der Organischen Chemie;
Grundmann, C., Ed.; Thieme: Stuttgart, 1981; Vol. 5.
(3) See, for example, (a) Thorson, J. S.; Sievers, E. L.; Ahlert, J.; Shepard, E.;
Whitwam, R. E.; Onwueme, K. C.; Ruppen, M. Curr. Pharm. Des. 2000,
6, 1841 and references therein. (b) Endiyne Antibiotics as Antitumor Agents;
Borders, D. B., Doyle, T. W., Eds.; Marcel Dekker: New York, 1995. (c)
Nicolaou, K. C.; Smith, A. L. Acc. Chem. Res. 1992, 25, 497. (d) Nicolaou,
K. C.; Dai, W.-M. Angew. Chem. 1991, 103, 1453; Angew. Chem., Int.
Ed. Engl. 1991, 30, 1387.
(4) (a) Wang, H.; Laskin, A.; Moriarty, N. W.; Frenklach, M. Proc. Combust.
Inst. 2000, 28, 1545. (b) Moskaleva, L. V.; Madden, L. K.; Lin, M. C.
Phys. Chem. Chem. Phys. 1999, 1, 3967. (c) Madden, L. K.; Moskaleva,
L. V.; Kristyan, S.; Lin, M. C. J. Phys. Chem. 1997, 101, 6790.
(5) (a) Yamagishi, S.; Jenkins, S. J.; King, D. A. J. Chem. Phys. 2002, 117,
819. (b) Johnson, K.; Sauerhammer, B.; Titmuss, S.; King, D. A. J. Chem.
Phys. 2001, 114, 9539 and literature cited therein.
The structure of meta-benzyne has been clarified recently;
on the basis of high-level calculations, it has been shown that
1 adopts a monocyclic structure in which the distance of the
(9) (a) Kraka, E.; Cremer, D. J. Comput. Chem. 2001, 22, 216. (b) Kraka, E.;
Cremer, D. J. Am. Chem. Soc. 2000, 122, 8245. (c) Kraka, E.; Cremer, D.
J. Mol. Struct. (THEOCHEM) 2000, 506, 191.
(10) (a) Feng, L.; Kerwin, S. M. Tetrahedron Lett. 2003, 44, 3463. (b) Feng,
L.; Kumar, D.; Kerwin, S. M. J. Org. Chem. 2003, 68, 2234. (c) Nadipuram,
A. K.; David, W. M.; Kumar, D.; Kerwin, S. M. Org. Lett. 2002, 4, 4543.
(d) David, W. M.; Kerwin, S. M. J. Am. Chem. Soc. 1997, 119, 1464.
(11) (a) Debbert, S. L.; Cramer, C. J. Int. J. Mass Spectrom. 2000, 201, 1. (b)
Cramer, C. J.; Debbert, S. Chem. Phys. Lett. 1998, 287, 320. For an earlier
computational study of pyridynes, see (c) Nam, H. H.; Leroi, G. E.;
Harrison, J. F. J. Phys. Chem. 1991, 95, 6514. See also Adam, W.;
Grimison, A.; Hoffmann, R. J. Am. Chem. Soc. 1969, 91, 2590.
(6) For general reviews on heterocyclic arynes, see (a) Reinecke, M. G.
Tetrahedron 1982, 38, 427. (b) Kaufmann, T.; Wirthwein, R. Angew. Chem.
1971, 83, 21; Angew. Chem., Int. Ed. Engl. 1971, 10, 20.
(7) (a) Hoffner, J.; Schottelius, M. J.; Feichtinger, D.; Chen, P. J. Am. Chem.
Soc. 1998, 120, 376. See also (b) Chen, P. Angew. Chem. 1996, 108, 1584;
Angew. Chem., Int. Ed. Engl. 1996, 35, 1478.
(12) Cioslowski, J.; Szarecka, A.; Moncrieff, D. Mol. Phys. 2003, 101, 839.
(13) Halter, R. J.; Fimmen, R. L.; McMahon, R. J.; Peebles, S. A.; Kuczkowski,
R. L.; Stanton, J. F. J. Am. Chem. Soc. 2001, 123, 12353.
(8) Cramer, C. J. J. Am. Chem. Soc. 1998, 120, 6261.
9
10.1021/ja039142u CCC: $27.50 © 2004 American Chemical Society
J. AM. CHEM. SOC. 2004, 126, 6135-6149
6135

A R T I C L E S
Winkler et al.
Scheme 1
Scheme 2
the authors in terms of a rapid equilibrium between ortho-,
meta-, and para-benzyne via quantum mechanical tunneling of
H atoms, from which the para-isomer 11 reacts to the entropi-
cally favored enediyne 10.21 According to B3LYP/6-31G**
calculations, however, a different mechanism, in which ring-
opening of 1 and hydrogen migration proceed in one step,
appears to be more likely (Scheme 2).18,22
Aza-analogues of the benzynes have been studied computa-
tionally by Cramer and Debbert,11 Ciosloswski et al.,12 and
Kraka and Cremer.9 Similar to 1, the calculated equilibrium
structures (monocyclic vs bicyclic) of the meta-pyridynes show
a pronounced dependence on the level of theory employed. The
electronic effects of the nitrogen lone pair have been interpreted
in terms of a σ-allylic four electron, three center interaction,
which destabilizes the singlet ground states of 3,5-pyridyne (3)
and in particular that of 2,6-pyridyne (12), the lowest singlet
and triplet states of which are almost degenerate. In contrast,
the 2,4-isomer 13 is slightly stabilized relative to 1 [based on
calculated biradical stabilization energies (BSE), vide infra] by
virtue of strong contributions of a nitrilium ion structure to the
resonance hybrid.11 Compounds 12, 3, and 13 are less stable
than the lowest energy 3,4-pyridyne (14) by 17.5, 14.7, and
1.2 kcal mol-1, respectively [CAS(8,8)PT2/cc-pVDZ//CAS(8,8)-
SCF/cc-pVDZ].11
radical centers is around 205 pm, while a bicyclic anti-Bredt
form 1b does not occupy a stationary point on the C6H4 potential
energy surface, which, nonetheless, is extremely flat in the
region between both structures.15 Several computational methods
that work well in other cases fail in this respect and overestimate
bonding between the radical sites.16
The computational predictions have been firmly established
by comparison of the calculated and measured IR spectra.17,18
The latter have been obtained in cryogenic argon matrices, where
1 was generated from four different precursors, both under flash
vacuum pyrolysis (FVP) conditions (from 6 to 8) and photo-
chemically (from 9) in situ (Scheme 1).18
In accord with earlier investigations,19 the enediynes (Z)-10
and (E)-10 are formed as side products during pyrolysis of 7
and 8 at temperatures above 500 °C, and the ratio of 10:1
increases with temperature, suggesting a rearrangement 1 f
10 that requires a change of the CH connectivity.20 Several
pathways for this reaction have been discussed in the literature.
Investigations by Zewail et al. using femtosecond time-resolved
spectroscopy with mass spectrometric detection on the photolysis
of 1,2-, 1,3-, and 1,4-dibromobenzene have been interpreted by
(14) For interesting studies on the reactivity of charged meta-benzyne derivatives
employing the distonic ion approach, see (a) Price, J. M.; Nizzi, K. E.;
Campbell, J. L.; Kentta¨maa, H. I.; Seierstad, M.; Cramer, C. J. J. Am. Chem.
Soc. 2003, 125, 131. (b) Nelson, E. D.; Artau, A.; Price, J. M.; Tichy, S.
E.; Jing, L.; Kentta¨maa, H. I. J. Phys. Chem. A 2001, 105, 10155. (c) Nelson,
E. D.; Artau, A.; Price, J. M.; Kentta¨maa, H. I. J. Am. Chem. Soc. 2000,
122, 8781. (d) Thoen, K. K.; Kentta¨maa, H. I. J. Am. Chem. Soc. 1999,
121, 800.
(15) (a) Winkler, M.; Sander, W. J. Phys. Chem. A 2001, 105, 10422. (b) Kraka,
E.; Anglada, J.; Hjerpe, A.; Filatov, M.; Cremer, D. Chem. Phys. Lett. 2001,
348, 115.
(16) (a) Hess, B. A., Jr. Eur. J. Org. Chem. 2001, 2185. See refs 15 and 18 for
a correction of the conclusions drawn in that work. The related but formally
symmetry-forbidden cyclization of para-benzyne to butalene has been
investigated using density functional theory and coupled cluster methods.
(b) Warner, P. M.; Jones, G. B. J. Am. Chem. Soc. 2001, 123, 10322 and
ref 15b.
(17) Marquardt, R.; Sander, W.; Kraka, E. Angew. Chem. 1996, 108, 825; Angew.
Chem., Int. Ed. Engl. 1996, 35, 746.
(18) Sander, W.; Exner, M.; Winkler, M.; Balster, A.; Hjerpe, A.; Kraka, E.;
Cremer, D. J. Am. Chem. Soc. 2002, 124, 13072.
(19) Fisher, I. P.; Lossing, F. P. J. Am. Chem. Soc. 1963, 85, 1018.
(20) Sander, W.; Marquardt, R.; Bucher, G.; Wandel, H. Pure Appl. Chem. 1996,
68, 353.
In contrast to the four electron, three center interaction, a
two electron, three center system in the plane of the aromatic
ring is strongly stabilizing, e.g., in the 3,5-didehydrophenyl
cation (4) (D3h), the classic example23 of “in-plane aromaticity”,
(21) Diau, E. W.-G.; Casanova, J.; Roberts, J. D.; Zewail, A. H. Proc. Natl.
Acad. Sci. U.S.A. 2000, 97, 1376.
(22) The formation of vinylidene intermediates from various meta-benzyne
derivatives has been proposed by Brown et al., e.g., (a) Brown, R. F. C.
Eur. J. Org. Chem. 1999, 3211, 1 and references therein. (b) Brown, R. F.
C.; Eastwood, F. W. Pure Appl. Chem. 1996, 68, 261. For a rearrangement
of 1,3-didehydronaphthalene to diethynylbenzene under high-temperature
conditiones, see (c) Gru¨tzmacher, H.-F.; Lohmann, J. Liebigs Ann. Chem.
1975, 2023.
(23) Chandrasekhar, J.; Jemmis, E. D.; Schleyer, P. v. R. Tetrahedron Lett. 1979,
3707.
9
6136 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
A R T I C L E S
+
Results and Discussion
and the global minimum on the C6H3 potential energy
surface.24 This interesting system has recently been detected
for the first time in a FT-ICR mass spectrometer by Kenta¨maa
et al.25 The corresponding anion 5, which is isoelectronic to 3,
has been characterized in the gas phase by Hu and Squires.26
The observed reactivity of 5 is in agreement with a ground state
singlet anion with a Jahn-Teller distorted C2V structure (see,
however, ref 27). It has been argued that 5 may be considered
to be the antiaromatic analogue of 4, but a justification for that
notion (e.g., based on energetic or magnetic aromaticity criteria)
is not available yet.26,28 More recently, Wenthold et al.
determined the heat of formation of 1,3,5-tridehydrobenzene
(17) (2A1), which takes an intermediate position between 4 and
5.28 The first and third bond dissociation energies (BDE) of
benzene (18) are very similar while the second is considerably
lower; therefore, 17 is best described as a phenyl radical that
interacts only weakly with a meta-benzyne moiety on the
opposite side of the ring.28
Matrix Isolation of 3,5-Pyridyne. In analogy to our previous
matrix isolation study of meta-benzyne,18 we carried out FVP
of diiodopyridine 20 at temperatures around 600 °C in the gas
phase with subsequent trapping of the thermolysis products in
a large excess of argon at 10 K. The IR spectrum of a matrix
containing the FVP products of 20 is shown in Figure 1. Apart
from some unreacted starting material and the usual impurities
(H2O, CO, and CO2) that are always found in FVP experiments,
several new absorptions are detected in the tC-H stretching
region around 3300 cm-1, indicating the formation of ring-
opened, acetylenic byproducts. Another prominent band is
observed at 560.5 cm-1. This IR band and several other, less
intense signals below 2000 cm-1 rapidly decrease upon short
wavelength (254 nm) irradiation. From the decay kinetics of
these bands during photolysis, at least two distinct species can
be deduced. Simultaneously, a new acetylenic compound is
formed that is not present (or only in very small amounts) in
the original matrix after trapping the FVP products of 20.
Strong absorptions around 550 cm-1 are a characteristic
feature in the vibrational spectra of 1 and derivatives substituted
in the 5-position.17,18,30 By comparison with IR spectra calculated
at the BLYP level of theory, the photolabile species are readily
identified as the 3-iodo-5-pyridyl radical (22) and 3,5-pyridyne
(3) (Figure 2).
To further establish these assignments, we also investigated
the FVP of 3,5-dinitropyridine (21). The same photolabile
absorptions assigned to 3 can also be identified in the vibrational
spectrum of the FVP products of 21 (Figure 3 and Table 1),
while those associated with 22 are absent, confirming the
conclusions drawn above.
Structural assignments of the photostable acetylenic byprod-
ucts formed thermally from 20/21 or photochemically from 3
are more challenging. Reasonable candidates involve several
C5H3N isomers, e.g., 23-25, or fragmentation products such
as diacetylene (26) and cyanoacetylene (27).
Despite the fundamental interest in pyridynes and related or
isoelectronic systems, direct spectroscopic information on these
molecules is scarce. The only hetaryne that has so far been
characterized by IR spectroscopy is 3,4-pyridyne (14).29 In this
contribution, we describe the matrix isolation and IR spectro-
scopic characterization of 3,5-pyridyne (3). The article is
arranged as follows: First, we discuss the FVP experiments of
3,5-diiodopyridine (20) and 3,5-dinitropyridine (21). The mea-
sured IR spectrum of 3 is subsequently compared with calculated
vibrational spectra, and complemented by high-level ab initio
calculations, the structure of 3,5-pyridyne is elucidated. The next
part is devoted to the electronic structure of 3 [and comparison
with 3,5-borabenzyne (2)], and the notation of in-plane aroma-
ticity/antiaromaticity is evaluated critically on the basis of
energetic and magnetic arguments. Finally, we discuss the
kinetic stability and intramolecular rearrangements of the title
compound and compare the calculated data to the experimental
findings.
(24) Schleyer, P. v. R.; Jiao, H.; Glukhovtsev, M. N.; Chandrasekhar, J.; Kraka,
E. J. Am. Chem. Soc. 1994, 116, 10129.
By comparison with authentic samples and reference data,
we can unambiguously identify significant amounts of HCN
(25) Nelson, E. D.; Kentta¨maa, H. I. J. Am. Soc. Mass Spectrom. 2001, 12,
258.
(26) Hu, J.; Squires, R. R. J. Am. Chem. Soc. 1996, 118, 5816.
(27) Slipchenko, L. V.; Krylov, A. I. J. Chem. Phys. 2003, 118, 9614.
(28) (a) Lardin, H. A.; Nash, J. J.; Wenthold, P. G. J. Am. Chem. Soc. 2002,
124, 12612. For computational investigations of 17, see (b) Bettinger, H.
F.; Schleyer, P. v. R.; Schaefer, H. F., III. J. Am. Chem. Soc. 1999, 121,
2829 and ref 27.
(29) (a) Nam, H.-H.; Leroi, G. E. J. Am. Chem. Soc. 1988, 110, 4096. Related
attempts to generate and characterize the 2,3-isomer (15) by photolysis of
2,3-pyridinedicarboxylic anhydride in cryogenic nitrogen matrices were
unsuccessful. (b) Nam, H. H.; Leroi, G. E. Tetrahedron Lett. 1990, 31,
4837. (c) Dunkin, I. R.; MacDonald, J. G. Tetrahedron Lett. 1982, 23,
4839. See also (d) Cava, M. P.; Mitchell, M. J.; DeJongh, D. C.; Van
Fossen, R. Y. Tetrahedron Lett. 1966, 26, 2947. (e) August, J.; Kroto, H.
W.; McNaughton, D.; Phillips, K.; Walton, D. R. M. J. Mol. Spectrosc.
1988, 130, 424.
(721.0 and 3306.5 cm-1 31 and 26 (627.5 and 3326.5 cm-1 32
)
)
as well as traces of acetylene (737.0, 3289.0, and 3303.0 cm-1 32
)
and cyanoacetylene (667.5 and 3316.0 cm-1 33
) among the FVP
products. Another characteristic signal at 947.0 cm-1 can be
(30) Sander, W.; Exner, M. J. Chem. Soc., Perkin Trans. 2 1999, 2285.
(31) Abbate, A. D.; Moore, C. B. J. Chem. Phys. 1985, 82, 1255.
(32) (a) Wrobel, R. Ph.D. Thesis, Ruhr-Universita¨t Bochum, 1998. (b) Gan-
tenberg, M. Ph.D. Thesis, Ruhr-Universita¨t Bochum, manuscript in prepara-
tion. (c) Marquardt, R.; Sander, W. Unpublished results.
(33) Guennoun, Z.; Couturier-Tamburelli, I.; Pietry, N.; Aycard, J. P. Chem.
Phys. Lett. 2003, 368, 574.
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6137

A R T I C L E S
Winkler et al.
Figure 1. (a) IR spectrum of 3,5-diiodopyridine (20) (Ar, 10 K). (b) IR spectrum of a matrix containing the FVP products of 20. (c) Spectrum of the same
matrix after irradiation with λ ) 254 nm. (d) Difference spectrum c - b; bands pointing downward decrease in intensity upon irradiation of a matrix
containing the FVP products of 20.
Figure 2. Comparison of measured and calculated vibrational spectra. (a) IR spectrum of a matrix containing the FVP products of 20. (b) Difference
spectrum; bands pointing downward decrease in intensity upon irradiation (λ ) 254 nm) of a matrix containing the FVP products of 20. (c) Calculated IR
spectrum of 3-iodo-5-pyridyl (22) (UBLYP/6-311G**). (d) Calculated IR spectrum of 3,5-pyridyne (3) (BLYP/cc-pVTZ).
assigned to (E)-23, although it should be noted that the
vibrational spectra of 23-25 simulated at the density functional
theory (DFT) level are too similar to allow a rigorous dif-
ferentiation between isomers on the basis of calculations alone
(cf., Figure 1S). However, formation of (E)-23 (the global
minimum on the C5H3N potential energy surface, vide infra)
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6138 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
A R T I C L E S
Figure 3. (a) IR spectrum of a matrix containing the FVP products of 21. (b) Difference spectrum; bands pointing downward decrease in intensity upon
irradiation (λ ) 254 nm) of a matrix containing the FVP products of 21. (c) Difference spectrum; bands pointing downward decrease in intensity upon
irradiation of a matrix containing the FVP products of 20.
Table 1. IR Spectroscopic Data of 3,5-Pyridyne (3)
unscaled at the BLYP/cc-pVTZ level), whereas only one signal
(3394 cm-1) is expected for (Z)-1-aza-hex-3-ene-1,5-diyne. On
ν˜exp
ν˜calcd
Icalcd
mode
symmetry
(cm-1 a
)
Iexp,rel
(cm-1 b
)
(km mol-1 b
)
a
this basis, we tentiatively identify (Z)-23 as the photoproduct
of 3. High-resolution matrix IR data of acyclic C5H3N isomers
(a prerequisite for more reliable assignments), although very
desirable, are not yet available in the literature. Efforts into this
direction are currently under way in our laboratory.
1
2
3
4
5
6
7
8
b1
a1
a2
b2
b1
b1
a1
b1
a2
b2
a1
a1
b2
b2
b2
a1
b2
a1
342.5
354.3
537.2
537.6
580.4
782.6
852.8
864.9
870.2
0
26
0
130
22
21
5
17
0
8
0
8
1
1
1
0
47
11
560.5
595.0
790.5
861.5
871.0
100
20
25
5
Molecular Structure of 3,5-Pyridyne. The structure of 3
has been calculated with several DFT methods, and the distances
RC3C5 of the radical centers are summarized in Table 2.
Analogous to 1, hybrid functionals predict a bicyclic form,
whereas pure GGA (generalized gradient approximation) func-
tionals lead to a monocyclic geometry for 3.
10
9
10
11
12
13
14
15
16
17
18
951.0
5
5
936.5
1058.3
1120.1
1180.2
1226.9
1291.9
1373.0
1417.2
1684.4
1141.0
In a previous study, we investigated the structure of 1 by a
series of constraint geometry optimizations with frozen distances
RC1C3 between the radical centers at the B3LYP and CASSCF
levels.15a By varying the separation of the dehydrocarbons (step
size 1-5 pm), potential energy curves for the cyclization reac-
tion were obtained. Subsequently, high-level single point calcu-
lations [MR-CI, CAS-RS2, CAS-RS3, CCSD(T), and BCCD(T),
cf., Experimental Section] have been carried out for a large
number of structures obtained in this way. It has been demon-
strated that the resulting potential energy curves are almost par-
allel for both sets of geometries, with the (U)B3LYP structures
being slightly lower in energy. Following these lines, we inves-
tigated the structure of 3 at the above-mentioned levels of theory
for (U)B3LYP/cc-pVTZ optimized geometries with frozen dis-
tances RC3C5. In all cases, the lowest energy structure is found
at RC3C5 ) 200 ( 5 pm (Figure 4). Thus, C5H by N replacement
in 1 reduces the distance of the dehydrocarbons by 5-7 pm (in
1418.5
10
c
a Argon, 10 K. b BLYP/cc-pVTZ. c Not assigned due to the low signal-
to-noise ratio in this spectral region.
has been reported upon FVP of other pyridine derivatives in
the same temperature range,29d,e and the 947.0 cm-1 absorption
is also observed upon pyrolysis of 2,6-diiodopyridine at 675
°C [Figure 2S; the structurally related (E)-10 has a strong and
characteristic absorption at 942.0 cm-1 that allows us to
distinguish it from the (Z)-isomer]. As expected, all signals
associated with 3 and 22 are absent from the IR spectra of the
pyrolysate of the 2,6-diiodo compound. The species formed
photochemically from 3 and 22 is characterized by a very strong
absorption at 3320.5 cm-1, and the most reasonable candidates
are (Z)-23 or (Z)-24. The latter compound possesses two intense
signals in the tC-H stretching region (3393 and 3406 cm-1
,
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6139

A R T I C L E S
Winkler et al.
Figure 4. Energy profiles for the cyclization of 3,5-pyridyne (3) (energy as a function of the distance of the radical centers) calculated at different levels
of theory.
Table 2. Distances R of the Radical Centers in meta-Benzyne (1) and 3,5-Pyridyne (3) in pm; Values in Parentheses Give Changes in R
Due to CH by N Replacement
1
3
method
B3PW91
B3LYP
BPW91
cc-pVDZ
cc-pVTZ
cc-pVQZ
cc-pVDZ
cc-pVTZ
cc-pVQZ
157.3
161.5
187.8
202.1
211 ( 1
211 ( 1
211 ( 1
213 ( 1
215 ( 1
215 ( 1
156.2
160.3
184.3
199.7
204 ( 1
205 ( 1
205 ( 1
207 ( 1
210 ( 1
210 ( 1
156.2
160.4
182.2
199.5
203 ( 1
203 ( 1
203 ( 1
206 ( 1
209 ( 1
209 ( 1
159.9 (+2.6)
166.3 (+4.8)
176.8 (-11.0)
196.2 (-5.9)
202 ( 1 (-9)
203 ( 1 (-8)
204 ( 1 (-7)
206 ( 1 (-7)
209 ( 1 (-6)
208 ( 1 (-7)
158.3 (+2.1)
163.9 (+3.6)
172.7 (-11.6)
193.9 (-5.8)
198 ( 1 (-6)
198 ( 1 (-7)
199 ( 1 (-6)
202 ( 1 (-5)
206 ( 1 (-4)
204 ( 1 (-6)
158.3 (+2.1)
163.9 (+3.5)
172.0 (-10.2)
193.6 (-5.9)
197 ( 1 (-6)
198 ( 1 (-5)
196 ( 1 (-7)
BLYP
BCCD(T)a
RCCSD(T)a
CAS-RS3a
CAS-CISD+Qa
CAS-CISDa
CAS-RS2a
203 ( 1 (-6)
a Lowest energy structure calculated for (U)B3LYP/cc-pVTZ optimized geometries with frozen distances R. See text for details.
pyridine 31, the corresponding distance is 2-3 pm smaller than
in benzene 18 at all DFT levels considered in this work, cf.
Figure 5).15
for the BLYP functional, the only exception being mode 17, for
which the calculated intensity is higher than the measured one,
and mode 18 that could not be detected in the IR spectra as the
low signal-to-noise ratio in this spectral region hampers the iden-
tification of very weak absorptions. In contrast, the patterns cal-
culated with the other functionals differ considerably from the
observed spectrum and hardly allow any correlation with ex-
periment.
This trend is reproduced correctly at the BLYP level, whereas
the geometries fully optimized with the hybrid methods deviate
even qualitatively and predict a slightly increased separation
and an overall bicyclic structure. Given the flatness of the
potential energy surface along the interradical distance coordi-
nate, a detailed examination of this point might seem irrelevant;
however, small structural changes have been used to rationalize
substituent effects in a series of substituted meta-benzynes;34
therefore, the ability of different methods to account for
systematic changes within this class of compounds is of interest.
The measured IR spectrum of 3 is compared to the vibrational
spectra calculated with different DFT methods in Figure 6. Good
overall agreement between experiment and the theory is found
All in all, the distance of the radical centers in 3 is around
200 pm and only slightly smaller than in the parent meta-
benzyne (1). The BLYP calculated data are in good agreement,
with both the measured IR spectrum and the structure calculated
with highly correlated ab initio methods.
(34) (a) Johnson, W. T. G.; Cramer, C. J. J. Phys. Org. Chem. 2001, 14, 597.
(b) Johnson, W. T. G.; Cramer, C. J. J. Am. Chem. Soc. 2001, 123, 923.
9
6140 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
A R T I C L E S
Figure 5. Structures of biradicals 1-3 and of the corresponding monoradicals and parent arenes calculated at the BLYP/cc-pVTZ level of theory.
Figure 6. Comparison of the measured IR spectrum of 3 with the vibrational spectra calculated with different DFT methods.
Electronic Structure of 3,5-Pyridyne. To clarify the effect
of the nitrogen lone pair on the electronic structure of 3,5-
pyridyne, we calculated the energy differences between the
lowest energy singlet and triplet states (∆EST) of 1 and 3 at
high levels of theory (Table 3). For 1, CASSCF(8,8) predicts a
singlet-triplet splitting considerably smaller (13 kcal mol-1
than the experimentally determined value of 21.0 ( 0.3 kcal
)
mol-1 35
.
Inclusion of dynamic electron correlation stabilizes the
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6141

A R T I C L E S
Winkler et al.
Table 3. Singlet-Triplet Energy Splittings [E0(3B2) - E0(1A1)] in meta-Benzyne (1) and 3,5-Pyridyne (3) in kcal mol-1; Electronic
Contributions [Ee(3B2) - Ee(1A1)] Are Given in Parentheses
1
3
method
B3PW91
B3LYP
BPW91
cc-pVDZ
cc-pVTZ
cc-pVQZ
cc-pVDZ
cc-pVTZ
cc-pVQZ
15.8 (15.5)
14.0 (13.6)
19.7 (19.2)
19.5 (19.3)
11.8 (11.7)
17.0 (16.9)
16.5 (16.3)
16.4 (16.2)
17.3 (17.1)
16.6 (16.4)
14.8 (14.4)
20.1 (19.6)
19.9 (19.7)
12.9 (12.7)
19.5 (19.3)
19.0 (18.9)
18.6 (18.5)
20.0 (19.8)
16.7 (16.5)
15.0 (14.6)
20.2 (19.7)
19.9 (19.7)
13.0 (12.8)
20.2 (20.0)
19.7 (19.5)
19.1 (18.9)
20.5 (20.4)
21.0 ( 0.3
11.1 (10.7)
9.2 (8.6)
12.9 (12.2)
11.5 (11.2)
9.3 (8.9)
12.1 (11.6)
10.2 (9.6)
13.6 (12.9)
12.2 (11.9)
10.3 (9.9)
12.3 (11.8)
10.4 (9.8)
13.7 (13.1)
12.4 (12.0)
10.4 (10.0)
BLYP
CAS-SCFa
CAS-RS2a
CAS-RS3a
CAS-CISDa
CAS-CISD+Qa
expb
13.5 (13.1)
14.4 (14.0)
15.7 (15.4)
17.1 (16.7)
16.2 (15.8)
17.6 (17.2)
NA
a (U)BLYP/cc-pVTZ geometries and ZPVE corrections. b Ref 35.
Table 4. Different Measures (Enthalpies at 298 K in kcal mol-1
for the Influence of Heteroatomic Substitution on the Stability of 1
)
singlet relative to the triplet state. Particularly good agreement
with experiment is obtained at the MR-CI level (20.5 kcal
mol-1), provided that sufficiently large basis sets (cc-pVQZ)
are used and higher order correlation effects are accounted for
by the Davidson correction scheme. Among the DFT methods,
BPW91 and BLYP give energy differences around 20 kcal
mol-1, whereas admixture of exact exchange (B3PW91, B3LYP)
lowers these values by 4-5 kcal mol-1. For 3, MR-CI predicts
a decrease of the singlet-triplet splitting by 3 kcal mol-1 as
compared with 1, in good agreement with previous investiga-
tions.11,12 All DFT methods perfom somewhat worse in this case
and underestimate ∆EST of 3 by 4 (BPW91) to 8 kcal mol-1
(B3LYP) as compared to the MR-CI benchmark data.
(1)a
(2)a
a
b
system
BDE
BDE
BSE
SE
X ) B
X ) CH
X ) N
97.7
107.0
106.3
71.3
90.4
94.1
26.4
16.6
12.3
28.3
0.0
-3.0
a Cf. eq 2. b According to eq 3.
underlines the significant coupling of the formally unpaired
electrons. According to both schemes described above, 3 is
approximately 3-4 kcal mol-1 less stable than meta-benzyne.
In contrast, 2 benefits from a significant stabilization (SE ) 28
kcal mol-1), and the structure of this molecule is indicative of
strong two electron, three center interaction with very similar
C-C and C-B distances (Figure 5), coming close to the D3h
structure of 4 (RC1C3 ) 200.3 pm).
Whereas the considerable stabilization of 2 may be expressed
by the term in-plane aromaticity, the overall effect of the
nitrogen lone pair on the coupling of the formally unpaired
electrons in 3, as measured by the distance of the radical centers,
the stability, and the singlet-triplet splitting, is rather small.
Analogous to tridehydrobenzene (17),28 3 is best described as
a meta-benzyne moiety interacting only weakly with a nitrogen
atom on the opposite side of the ring. This view is further
supported by comparison of the frontier molecular orbitals of
1-3, shown in Figure 7.
The effect of heteroatomic substitution on the stability of
meta-benzynes can be estimated on the basis of BSE, defined
according to isodesmic eq 2 (cf. refs 36 and 37). These provide
a measure for the stabilization or destabilization involved when
the two radical centers interact within the same molecule. A
positive value indicates stabilization, and a negative value
indicates destabilization with respect to a hypothetical nonin-
teracting biradical model system;36 it is immediately obvious
that the BSE is equivalent to the difference between the first
and the second C-H BDE in the corresponding arene. An
alternative approach that avoids reference to the monoradical
is given by eq 3, defining what we would like to call the
stabilization energy (SE).
The highest occupied (HOMO) and lowest unoccupied
molecular orbitals (LUMO) of 1 and 3 are almost indistinguish-
able in the region of the dehydrocarbons, and the occupation
numbers of the corresponding MCSCF natural orbitals (NOON)
are very similar (note that the distance of the radical centers in
3 is slightly smaller than in 1, leading to larger NOON of the
HOMO and lower NOON of the LUMO of 3). The lone pair,
on the other hand, is largely localized on the nitrogen atom. In
contrast, the frontier molecular orbitals of 2 are those of a
σ-allylic system with significantly increased NOON of the
bonding three center HOMO.
We also investigated the magnetic properties of 1-3 and of
the corresponding parent arenes at the BLYP/cc-pVTZ level.
Nucleus-independent chemical shifts (NICS)38 were calculated
to access the diamagnetic/paramagnetic effects of the ring
currents associated with aromaticity/antiaromaticity.38,39 Abso-
The (U)BLYP/cc-pVTZ calculated BSEs and SEs of meta-
benzyne (1), 3,5-borabenzyne (2), and 3,5-pyridyne (3) are listed
in Table 4. The BSE of 1 amounts to 16.6 kcal mol-1 and
(36) (a) Wierschke, S. G.; Nash, J. J.; Squires, R. R. J. Am. Chem. Soc. 1993,
115, 11958. See also (b) Cramer, C. J.; Nash, J. J.; Squires, R. R. Chem.
Phys. Lett. 1997, 277, 311.
(35) Wenthold, P. G.; Squires, R. R.; Lineberger, W. C. J. Am. Chem. Soc.
1998, 120, 5279.
(37) Lindh, R.; Bernhardsson, A.; Schu¨tz, M. J. Phys. Chem. A 1999, 103, 9913.
9
6142 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
A R T I C L E S
density within the molecular plane. Following the recommenda-
tions mentioned above, we concentrate on NICS(1) here. Almost
identical values between -9.6 and -9.9 are calculated for
benzene (18), borabenzene (30), and pyridine (31). Removing
two hydrogens from 18 leads to slightly more negative NICS
for meta-benzyne (as well as for the ortho- and para-isomers),
as discussed in detail previously.41 Chemical shifts for 3 are
almost identical to that of pyridine, indicating the absence of
any paramagnetic ring currents that may be associated with
antiaromaticity. On the other hand, a large negative NICS for
2 is calculated, justifying the notion of in-plane aromaticity in
this case.
Rearrangements of 3,5-Pyridyne. Rearrangements on the
C6H4 potential energy surface have been investigated in several
studies employing the B3LYP functional.4b,18,21 To compare the
chemistry of benzynes and pyridynes at consistent levels of
theory, we reinvestigated the most important intramolecular
reactions of 1 using the BCCD(T)/cc-pVTZ//(U)BLYP/cc-pVTZ
and CCSD(T)/cc-pVTZ//(U)BLYP/cc-pVTZ methods. A po-
tential energy diagram for the rearrangements considered in this
work is shown in Figure 8.
The meta- to para-benzyne interconversion proceeds by a
nonplanar transition state structure TS1-11 and requires an activa-
tion energy of approximately 60 kcal mol-1 (Figure 3S), which
is similar to the degenerate [1,2]H shift in the phenyl radical.42
A related hydrogen shift mechanism leading to ortho-benzyne
(32) is characterized by an even higher barrier43 (Figure 4S),
but a second transition state with a bicyclic Cs structure could be
located that is less stable than meta-benzyne by 58 kcal mol-1
and leads to fulvenediyl 33 (Figure 5S). This, in turn, rearranges
with a very small barrier to 32.44 In accordance with our pre-
vious discussion on the basis of B3LYP calculations and exper-
imental observations,18 all of these reactions are unlikely to occur
under FVP conditions, however, because a lower energy trans-
ition state TS1-Z10 was identified that leads to (Z)-hex-3-ene-
Figure 7. Frontier molecular orbitals of biradicals 1-3 (CASSCF canonical
orbitals) and occupation numbers of the corresponding natural orbitals.
Table 5. NICS of 1-3 and of the Parent Hydrocarbons Calculated
at the BLYP/cc-pVTZ Level of Theory at the (3,+1) Ring Critical
Point [NICS(0)] and 100 [NICS(1)] or 200 pm [NICS(2)] above the
Molecular Plane
molecule
NICS(0)
NICS(1)
NICS(2)
meta-benzyne (1)
benzene (18)
3,5-borabenzyne (2)
borabenzene (30)
3,5-pyridyne (3)
pyridine (31)
-11.8
-7.8
-11.1
-9.9
-17.3
-9.6
-9.6
-9.8
-4.8
-5.0
-6.3
-4.1
-4.4
-4.9
#
1,5-diyne 10. This pathway is also favored entropically (T∆S298
-33.4
-12.2
-5.2
at 298 K is 2.62 kcal mol-1 for the 1 f (Z)-10 ring-opening,
but only 0.44 kcal mol-1 for the hydrogen shift 1 f 11 at the
(U)BLYP/cc-pVTZ level), and should, therefore, prevail in high-
temperature processes. Details of this unusual rearrangement
are given in Figure 9, which shows several structures along the
intrinsic reaction coordinate (IRC).45 The potential energy sur-
face in the region of transition state TS1-Z10 is extremely flat,
and the imaginary frequency (148i cm-1) is small. In the early
stage of this process, the ring opens and the acetylenic moiety
forms, whereas the hydrogen migration takes place mainly after
passage through the transition state. In agreement with prior
calculations,18 no vinylidene exists as a stationary point.
Several other mechanisms for the reaction 1 f (Z)-10 can
be envisaged but are less likely due to the high energy of the
-6.6
lute chemical shifts within the plane of cyclic molecules are a
measure of the extent of cyclic electron delocalization only when
the radii of the systems are sufficiently large, so that local
shielding effects are negligible.38a,c It has, therefore, been
recommended to calculate NICS(1) values 100 pm above the
ring plane, where local contributions to the total chemical shift
are considerably weaker than ring current effects.38a,c The
computed NICS(0), NICS(1), and NICS(2) values of 1-3 are
compared to those of the parent systems in Table 5.
NICS(0) refers to the (3,+1) ring critical point40 (instead of
the ring center usually chosen), i.e., the point of lowest electron
(41) De Proft, F.; Schleyer, P. v. R.; Lenthe, J. H. van; Stahl, F.; Geerlings, P.
Chem. Eur. J. 2002, 8, 3402.
(38) (a) Schleyer, P. v. R.; Manoharan, M.; Wang, Z.-X.; Kiran, B.; Jiao, H.;
Puchta, R.; Hommes, N. J. R. van E. Org. Lett. 2001, 3, 2465. (b) Schleyer,
P. v. R.; Jiao, H.; Hommes, N. J. R. van E.; Malkin, V. G.; Malkina, O. L.
J. Am. Chem. Soc. 1997, 119, 12669. (c) Schleyer, P. v. R.; Maerker, C.;
Dransfeld, A.; Jiao, H.; Hommes, N. J. R. van E. J. Am. Chem. Soc. 1996,
118, 6317.
(39) For general reviews on aromaticity criteria, see, for example (a) Special
Issue on Aromaticity. Schleyer, P. v. R., Ed. Chem. ReV. 2001, 101, 1115.
(b) Minkin, V. I.; Glukhovtsev, M. N.; Simkin, B. Y. Aromaticity and
Antiaromaticity; Wiley: New York, 1994.
(42) Brooks, M. A.; Scott, L. T. J. Am. Chem. Soc. 1999, 121, 5444.
(43) In ref 4b, it has been reported that this transition state leads to a high-
energy intermediate [at B3LYP/6-31G(d,p)]. Our IRC calculations reveal,
however, that TS1-32 leads directly to ortho-benzyne at the (U)BLYP/cc-
pVTZ level (cf., Figure 4S) without passage through an additional
intermediate.
(44) There is no experimental evidence for meta- to ortho-benzyne conversion
under FVP conditions. Recently, M. Jones et al. reported the rearrangement
of a substituted ortho-benzyne to the meta-isomer by migration of a phenyl
group. Blake, M. E.; Bartlett, K. L.; Jones, M., Jr. J. Am. Chem. Soc. 2003,
125, 6485.
(45) (a) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523. (b)
Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154.
(40) (a) Popelier, P. Atoms in Molecules; Prentice Hall: Harlow, 2000. (b) Bader,
R. W. F. Chem. ReV. 1991, 91, 893. (c) Bader, R. W. F. Atoms in
Molecules: A Quantum Theory; Clarendon Press: Oxford, 1990.
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6143

A R T I C L E S
Winkler et al.
Figure 8. Potential energy diagram for rearrangements of meta-benzyne calculated at the BCCD(T)/cc-pVTZ//(U)BLYP/cc-pVTZ [CCSD(T)/cc-pVTZ//
(U)BLYP/cc-pVTZ] ((U)BLYP/cc-pVTZ) level of theory. All values include ZPVE corrections ((U)BLYP/cc-pVTZ).
Figure 9. Structures along the IRC for the ring-opening of meta-benzyne (BLYP/cc-pVTZ). Energies (E0) relative to 1 are given in italics.
intermediates involved. The direct formation of fulvene-1,4-
diyls 35 from 1 has been shown to require large activation
energies and is not considered any further here (Scheme 3).18
Given the recent interest in these five-membered biradicals,46
however, it should be mentioned that the (E)-isomer does not
correspond to a stationary point on the UBLYP/cc-pVTZ
potential energy surface and is only found as an intermediate
when small basis sets are employed.46 Formation of the isomeric
fulvene-1,3-diyls (Z)-34 and (E)-34 has not yet been discussed
in the literature. As these species are already 5-6 kcal mol-1
higher in energy than TS1-Z10 (Table 6), they can be excluded
as intermediates in the FVP process.
Another group of rearrangements is initiated by breaking the
C4-C5 bond of meta-benzyne (instead of the C1-C2 bond,
leading to TS1-Z10). These mechanisms all involve high-energy
(46) (a) Prall, M.; Wittkopp, A.; Schreiner, P. R. J. Phys. Chem. A 2001, 105,
9265. (b) Alabugin, I. V.; Manoharan, M. J. Am. Chem. Soc. 2003, 125,
4495.
9
6144 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
A R T I C L E S
Figure 10. Potential energy diagram for rearrangements of 3,5-pyridyne calculated at the BCCD(T)/cc-pVTZ//(U)BLYP/cc-pVTZ [CCSD(T)/cc-pVTZ//
(U)BLYP/cc-pVTZ] ((U)BLYP/cc-pVTZ) level of theory. All values include ZPVE corrections ((U)BLYP/cc-pVTZ).
Figure 11. Structures along the IRC for the ring-opening of 3,5-pyridyne (BLYP/cc-pVTZ). Energies (E0) relative to 3 are given in italics.
structures (e.g., carbene 36, first suggested by Chen in 1996,7b
is 70-80 kcal mol-1 less stable than 1, Scheme 4) and, therefore,
will not be discussed in detail here. An important point, however,
relates to the experimentally observed formation of the (E)-
enediyne. One might argue that the formation of this species at
comparatively low temperatures (550-600 °C) is indicative of
an additional intermediate that, like 36, could lead to (Z)-10
and (E)-10 independently. To clarify this point, we would like
to stress that cis-trans isomerization of pure (Z)-10 takes place
already at temperatures around 500 °C under the conditions
employed in our experiments.32c Thus, (Z)-10 is most likely the
primary product, formed directly from 1, and subsequently
equilibrates with the (E)-isomer.
A potential energy diagram for rearrangements of 3,5-
pyridyne (3) is given in Figure 10. The barriers for the hydrogen
migration mechanisms leading to 14 or 16 are similar to their
all-carbon analogues (Figures 6S and 7S). In line with our earlier
studies on meta-benzyne,18 there are no indications for the for-
mation of the more stable ortho-isomer under FVP conditions
(Table 7). The ring-opening, hydrogen shift mechanism leading
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6145

A R T I C L E S
Winkler et al.
Scheme 3
Table 6. Energies of Various Minima and Transition States on the
C6H4 Potential Energy Surface in kcal mol-1 Relative to
meta-Benzyne; T1 Diagnostics (CCSD), S2 Expectation Values
(UBLYP), and Zero-Point Vibrational Energies (UBLYP) Are Given
for Comparison
Scheme 5
2
a
a
a
structure state
E
(U)BLYP
ZPVE
S
ECCSD(T)
T a
EBCCD(T)
EMRCI
1
1b
(Z)-10
(E)-10
11
32
33
(Z)-34
(E)-34
(Z)-35
36
1A1
1A1
1Ag
1Ag
0.0 44.85 0.000
0.3 42.79 0.000
0.0 0.015
4.3 0.013
3.9 0.013
14.5 0.019
0.0
4.3
3.9
0.0
-0.3 42.67 0.000
13.9 44.52 0.768
14.5
15.1
1A1 -12.3 45.74 0.000 -13.4 0.013 -13.5 -14.0
1A1
1A′
1A′
1A′
1A
21.5 44.10 0.000
55.8 43.18 0.954
56.3 43.03 0.982
52.3 42.47 0.807
69.1 40.20 0.636
48.1 43.82 0.000
48.6 40.66 0.000
62.4 41.03 0.000
71.7 41.73 0.000
61.7 41.72 0.000
29.0 42.56 0.000
51.9 43.21 0.000
22.7 43.73 0.000
17.9 0.016
53.9 0.043
49.4 0.070
25.9 0.107
81.5 0.020
46.9 0.013
51.4 0.014
66.9 0.036
76.8 0.027
61.6 0.016
34.1 0.017
52.1 0.016
19.4 0.013
17.7
56.1
57.5
51.3
81.4
46.8
51.3
67.5
76.7
61.5
34.0
52.0
19.2
18.4
54.2
55.5
51.2
41
1Ag
TS1-Z10 1A′
TS1-11
TS1-32
TS1-33
1A
1A
1A′
TSZ10-11 1A1
TS11-41 1A
TS32-33 1A′
a UBLYP/cc-pVTZ optimized geometries. b Absolute energies for this
row: E(U)BLYP ) -230.885963; ECCSD(T) ) -230.456091; EBCCD(T)
-230.455781; and EMRCI ) -230.409049.
)
Scheme 4
Some plausible mechanisms for the net reaction 3 f 26 + HCN
are given in Scheme 5.
Formation of azabutalene (38) from 16 requires an activation
energy of 52-56 kcal mol-1, considerably more than the 36-
39 kcal mol-1 barrier for the formation of butalene (41) from
11 (Tables 6 and 7).16b In addition, the cyclization of 16 is an
unlikely event, because the pyridyne easily undergoes a retro-
Bergman cyclization to (Z)-23 with a very small and at high
temperatures probably vanishing barrier.8,12 Thus, 16 qualifies
as a precursor for 23 but not for the fragmentation products
HCN and butadiyne. Biradical 39 is a high-energy species (67
kcal mol-1 less stable than 3 at the UBLYP level) and can be
excluded as an intermediate in HCN extrusion. On the basis of
similar arguments, the involvement of aza-fulvenediyls 40 in
the decomposition process of 3 is unlikely, because the former
are already comparable in energy to TS3-Z24 (Table 7), and the
barrier for the 3 f (Z)-40 rearrangement is at least 5 kcal mol-1
higher than for the 3 f (Z)-24 ring-opening. However, all
to (Z)-24 requires an activation energy of approximately 50 kcal
mol-1, so that the kinetic stability of 3 is even slightly higher
than that of 1. This lowest energy decomposition pathway pro-
ceeds similar to the 1 f (Z)-10 rearrangement in a single step
without involvement of a vinylidene intermediate (Figure 11).
One can imagine several pathways for the formation of 26
+ HCN from 3 or (Z)-24. Noteworthy, these compounds are
not observed in detectable amounts upon FVP of 2,6-diiodopy-
ridine (Figure 2S), which gives (E)-23 (the thermodynamic sink)
as the main product (vide supra). Therefore, it seems unlikely
that hydrogen cyanide and butadiyne are formed through
decomposition of 23, as proposed in an early investigation.29d
9
6146 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
A R T I C L E S
Table 7. Energies of Various Minima and Transition States on the C5H3N Potential Energy Surface in kcal mol-1 Relative to 3,5-Pyridyne;
T1 Diagnostics (CCSD), S2 Expectation Values (UBLYP), and Zero-Point Vibrational Energies (UBLYP) Are Given for Comparison
2
a
a
a
structure
3b
12
13
14
15
16
(Z)-23
(E)-23
(Z)-24
(E)-24
(Z)-25
(E)-25
26/HCN
27/C2H2
37
state
E
ZPVE
S
ECCSD(T)
T a
EBCCD(T)
EMRCI
(U)BLYP
1
1A1
0.0
0.8
37.66
38.35
37.94
38.53
38.47
37.71
36.59
36.44
35.45
35.18
36.24
36.07
32.55
32.72
37.16
36.29
36.20
36.11
33.13
33.08
34.40
33.68
35.27
36.41
36.67
35.55
35.77
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
1.001
1.007
0.007
0.000
0.000
0.000
0.474
0.000
0.000
0.000
0.000
0.0
6.6
0.014
0.0
6.5
-10.5
-17.1
-11.3
0.1
-37.0
-37.3
6.7
0.0
5.1
-11.4
-17.6
-11.7
-0.3
1A1
1A′
0.026
0.030
0.017
0.023
0.038
0.014
0.014
0.015
0.015
0.015
0.015
0.014/0.014
0.015/0.013
0.021
0.017
0.026
0.030
0.038
0.033
0.015
0.017
0.053
0.014
0.025
0.023
0.015
-12.9
-18.7
-12.9
-4.9
-38.5
-39.2
-0.2
-0.3
-16.5
-16.9
-1.8
1.5
-11.1
-17.1
-11.4
-0.9
-37.1
-37.5
6.6
1A′
1A′
1A′
1A′
1A′
1A′
1A′
7.0
7.1
1A′
-15.2
-15.2
4.1
-15.1
-15.1
4.3
1A′
1Σg+/1Σ+
1Σ+/1Σg
7.1
7.4
+
1A′
1A′
1A′
1A′
1A
12.4
51.8
49.1
49.9
66.3
61.0
61.2
50.3
11.2
50.5
52.0
53.3
75.9
68.1
61.7
53.6
57.3
16.8
3.2
11.1
50.0
52.0
53.1
76.0
68.6
61.6
53.6
59.6
16.7
3.3
11.7
38
(Z)-40
(E)-40
TS3-14
TS3-16
TS3-37
TS3-Z24
TS3-Z40
TS14-37
TS16-Z23
TS16-Z24
TS16-38
49.7
51.0
1A
1A′
1A′
1A′
1A′
1A′
1A′
1A
58.0
19.8
59.3
-1.9
19.9
53.0
27.5
52.9
27.7
52.9
a UBLYP/cc-pVTZ optimized geometries. b Absolute energies for this row: E(U)BLYP ) -246.932953; ECCSD(T) ) -246.477854; EBCCD(T) ) -246.477537;
and EMRCI ) -246.429420.
attempts to find a feasible route for HCN elimination from 3 or
24 failed so far, and only high-energy structures (more than 60
kcal mol-1 above 3,5-pyridyne) could be located for the reverse
reaction of HCN with 26. Therefore, additional experimental
(e.g., isotopic labeling) and systematic computational efforts will
be necessary, to clarify the mechanism of HCN formation during
the FVP experiments described in this work.
(E)-23 is most likely formed via an aza-Bergman rearrangement
of (Z)-24, followed by cis-trans isomerization of (Z)-23.
Although some mechanisms for HCN extrusion from 3 or (Z)-
24 have been considered, a plausible route for this fragmentation
has not been identified yet and a more detailed investigation of
this point will be the subject of a forthcoming study.
Experimental Section
Conclusions
General. THF was distilled from benzophenone/Na prior to use.
Column chromatography was carried out on ICN silica 32-63 (60 Å).
NMR spectra were recorded on a Bruker DPX 200; chemical shifts
are given relative to TMS: d ) doublet, t ) triplet, m ) multiplet.
Mass spectra were measured on Varian MAT CH 5, and IR spectra
(KBr) were measured on a Perkin-Elmer 841 IR spectrometer. 2,6-
Diiodopyridine was prepared from the corresponding dibromo com-
pound as described previously47 and purified by 2-fold sublimation.
3,5-Diiodopyridine (20).48 A solution of t-BuLi in hexane (11.8 mL,
1.6 N, 20.0 mmol) was added with stirring to a solution of 3,5-
dibromopyridine (1.2 g, 5.0 mmol) in THF (30 mL) at -80 °C in a
dry argon athmosphere. After 1 h, iodine (5.1 g, 20.0 mmol) in THF
(20 mL) was added. After an additional hour of stirring, the resulting
mixture was allowed to warm to room temperature. The solution was
poured into 5% Na2S2O3 (150 mL) and extracted twice with TBME
(200 mL). The combined organic layers were washed with brine and
dried (MgSO4). After removal of the solvent, the product crystallized
in yellowish needles (1.8 g). The crude product was purified chro-
matographically (silica gel; CH2Cl2/pentane 50:50) to furnish 1.2 g (3.6
mmol, 83.5%) of white needles; mp 170 °C. IR (cm-1, Ar, 10 K): 1533,
3,5-Pyridyne, the second least stable among the six pyridynes,
has been generated by FVP of 3,5-diiodopyridine and 3,5-
dinitropyridine. The IR spectrum of 3 is well-reproduced at the
BLYP level, which predicts a monocyclic structure with a
distance between the radical centers close to 200 pm, in good
agreement with high-level ab initio calculations. The nitrogen
atom in 3 causes only a small perturbation on the electronic
structure of meta-benzyne that destabilizes the singlet ground
state of 3 by 3-4 kcal mol-1. From the calculated magnetic
properties and from inspection of molecular orbitals, it can be
inferred that a four electron, three center interaction is insig-
nificant within this molecule and that 3,5-pyridyne is best
described in terms of a separated meta-benzyne moiety and a
nitrogen lone pair on the opposite side of the ring. In contrast,
a strongly stabilizing two electron, three center interaction is
observed for the in-plane aromatic 3,5-borabenzyne.
The title pyridyne is photolabile under matrix isolation
conditions and rapidly rearranges upon 254 nm irradiation,
presumably to (Z)-1-aza-hex-3-ene-1,5-diyne. Among the byprod-
ucts of the FVP experiments (E)-23 as well as HCN and
butadiyne are the most prominent contaminants. Analogous to
the parent meta-benzyne, the lowest energy decomposition
pathway of 3 is a ring-opening reaction, accompanied by
hydrogen migration, leading to (Z)-3-aza-hex-3-ene-1,5-diyne.
1
1409, 1298, 1096, 1004, 871, 730, 627. H NMR (200 MHz, DMSO-
d6): δH 8.77 (d, 2H), 8.59 (t, 1H). 13C NMR (100 MHz, DMSO-d6):
δC 153.97, 151.18, 95.64. MS (EI, 70 eV) (m/z, %): 331 (100) [M+],
204 (44), 177 (18), 165 (4), 127 (14), 77 (39), 50 (80).
(47) (a) Newkome, G. R.; Moorfield, C. N.; Sabbaghian, B. J. Org. Chem. 1986,
51, 953. (b) Newkome, G. R.; Roper, J. M. J. Organomet. Chem. 1980,
186, 147.
(48) Yamamoto, Y.; Yanagi, A. Chem. Pharm. Bull. 1982, 30, 1731.
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6147

A R T I C L E S
Winkler et al.
3,5-Dinitropyridine (21).49 To a stirred solution of 2-chloro-3,5-
dinitropyridine (2.0 g, 10.0 mmol) in CH2Cl2 (30 mL), hydrazinehydrate
(10 mL, 20.0 mmol) was added at 0 °C. After the solution was stirred
for 18 h at room temperature, the precipitate was filtered off and washed
with CH2Cl2 and water. The brownish substance (1.6 g) was heated in
aqueous AgNO3 for 3 h. After it was cooled to room temperature, the
solution was poured into TBME (250 mL). The organic layer was
washed twice with 200 mL of water, twice with 200 mL of aqueous
ammonia (25%), and finally with brine and dried (MgSO4). After the
solvent was removed, the product crystallized in yellowish needles (1.2
g), which were further purified chromatographically (silica gel; CH2Cl2/
pentane 50:50) to furnish 900 mg (5.4 mmol, 54.5%) of 21; mp 106
°C. IR (cm-1, KBr): 3090, 1603, 1589, 1520, 1453, 1339, 1292, 1022,
the well-known instability problems associated with coupled-cluster
calculations including all single, double, and perturbatively estimated
triple excitations [CCSD(T)] for systems with significant multireference
character, BCCD(T) calculations have routinely been carried out for
comparison.59 Brueckner orbitals eliminate contributions from single
excitations in the coupled-cluster ansatz, which often (but not always)59d
cures the shortcomings of the singlereference CCSD(T) approach
[a good example is the energy of (Z)-35, cf. Table 6]. In several cases,
these computations were complemented by multireference methods all
of which are based on complete active space self-consistent field
(CASSCF) wave functions. For benzynes (and related C6H4 isomers),
the eight electron/eight orbital active space covers the six benzene
valence π orbitals and the two formally nonbonding σ orbitals at the
dehydrocarbons. For the pyridynes (C5H3N), this space was extended
by the nitrogen lone pair, resulting in a CASSCF(10,9) wave function.
Dynamic electron correlation was covered by perturbation theory to
second- (CAS-RS2) or third-order (CAS-RS3)60 or by internally
contracted configuration interaction including single and double excita-
tions (CAS-CISD).61 If not mentioned otherwise, MR-CI energies refer
to Davidson-corrected62 values (CAS-CISD+Q). In post-(CAS)SCF
calculations, the core orbitals were kept frozen.
For most computations, Dunning’s correlation consistent cc-pVTZ
basis set with (10s5p2d1f)[4s3p2d1f] contraction for C and N and
(5s2p1d)[3s2p1d] contraction for H was used.63 In selected cases, we
also employed cc-pVDZ and cc-pVQZ basis sets to study the
convergence behavior.64 Vibrational spectra of iodine-containing mol-
ecules had to be calculated with the 6-311G(d,p) Pople type basis set,
as no correlation consistent basis sets are available for I.65
Electron densities for AIM (atoms in molecules) analysis40 have been
recalculated using Cartesian d and f functions (7d, 10f), whereas pure
angular momentum functions are used for all geometry optimizations
and single-point energy calculations. The AIM 2000 program was used
1
832, 722. H NMR (200 MHz, DMSO-d6): δH 9.72 (d, 2H), 9.11 (t,
1H). 13C NMR (100 MHz, DMSO-d6): δC 149.64, 144.23, 127.13. MS
(EI, 70 eV) (m/z, %): 169 (63) [M+], 123 (76), 93 (10), 77 (29), 76
(47), 66 (28), 50 (100), 30 (46).
Matrix Isolation Spectroscopy. Matrix isolation experiments were
performed by standard techniques50 with an APD DE-204SL and an
APD DE-202 Displex closed cycle helium cryostat. Matrices were
produced by codeposition of the compound with a large excess of argon
(Messer Griesheim, 99.9999%) on top of a cold CsI window with a
rate of approximately 0.15 mmol/min. To obtain optically clear matrices,
the spectroscopic window was retained at 30 K during deposition and
subsequently cooled to 10 K. FVP experiments were carried out without
additional heating of the spectroscopic window (temperature around
15 K). Matrix infrared spectra were recorded with Bruker IFS 66 and
Equinox 55 FTIR spectrometers with a standard resolution of 0.5 cm-1
using a N2(l) cooled MCT detector in the range 400-4000 cm-1
.
Irradiation at 254 nm was carried out with a Gra¨ntzel low-pressure
mercury arc lamp.
Computational Methods. Geometries of all species were fully
optimized at the BLYP level51,52 (in some cases, the B3LYP,52,53
BPW91,51,54 and B3PW9153,54 functionals have been used for compari-
son), and analytic second derivatives were calculated to characterize
stationary points as minima or transition states. Tight convergence
criteria for gradients (with maximum residual forces on nuclei below
0.000015 au) and a full (99, 590) integration grid, having 99 radial
shells per atom and 590 angular points per shell, have been used
throughout to obtain accurate values for geometries and low-frequency
vibrational modes. A spin-unrestricted formalism has been used
generally, where for calculations on singlet states the initial guess
frontier orbitals have been mixed to destroy (spin-)symmetry and in
each step during a geometry optimization or IRC calculation a new
initial guess was calculated. Whenever a spin-restricted solution was
obtained, it was further tested for instabilities by calculating the
eigenvalues of the Hermitian stability matrices.55 For general consid-
erations regarding DFT calculations on biradicals, see ref 56 and
literature cited therein. For many transition states localized in this work,
the corresponding IRCs were calculated in mass-weighted coordinates
with a step size of 0.05 amu1/2 bohr.45 All DFT calculations were carried
out with Gaussian 98.57
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A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; 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.; 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.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
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and funded by the U.S. Department of Energy. The Pacific Northwest
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9
6148 J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004

3,5-PyridynesA Heterocyclic meta-Benzyne Derivative
A R T I C L E S
for topological electron density analysis.66 Chemical shifts were
calculated at the BLYP level by the gauge-independent atomic orbital
(GIAO) method.67
Supporting Information Available: Energies and geometries
(Cartesian coordinates) for all minima and transition states,
structures along several IRCs, and additional spectroscopic
details. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. In memoriam Paul Heimbach (1934-
1991). This work was financially supported by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen Indus-
trie. We thank Dr. H. F. Bettinger for stimulating discussions.
JA039142U
9
J. AM. CHEM. SOC. VOL. 126, NO. 19, 2004 6149