Beyond the Roger Brown rearrangement: Long-range atom topomerization in conjugated polyynes

Article abstract of DOI:10.1021/ja0260115

Long-range carbon atom topomerization in a 1,3-diyne has been demonstrated for the first time. 1-Phenyl-4-p-tolyl-1,3-butadiyne, ¹³C-enriched at C-1, was synthesized and subjected to flash vacuum pyrolysis. At 800 °C and 0.01 Torr, this resulte

Full text of DOI:10.1021/ja0260115

Published on Web 05/11/2002
Beyond the Roger Brown Rearrangement: Long-Range Atom
Topomerization in Conjugated Polyynes
John Mabry and Richard P. Johnson*
Contribution from the Department of Chemistry, UniVersity of New Hampshire,
Durham, New Hampshire 03824
Received February 22, 2002
Abstract: Long-range carbon atom topomerization in a 1,3-diyne has been demonstrated for the first time.
1-Phenyl-4-p-tolyl-1,3-butadiyne, 13C-enriched at C-1, was synthesized and subjected to flash vacuum
pyrolysis. At 800 °C and 0.01 Torr, this resulted in nearly complete 13C label equilibration between C-1 and
C-2, as seen by NMR analysis. Pyrolysis at 900 °C further led to ca. 35% of the label migrating about
equally to C-3 and C-4. These results demonstrate that both intrabond and interbond atom exchange
processes are operative, with the former having a lower activation barrier. DFT and Moller-Plesset
calculations support a mechanism that passes through Brown rearrangement (1,2-shift), closure to trialene
(bicyclo[1.1.0]-1,3-butadiene), bond-shift isomerization to exchange C-2 and C-3, and ring opening. The
resulting vinylidene can rearrange to a butadiyne with the isotopic label at C-3 or C-4. Consistent with
earlier calculations, trialene is predicted to have alternating peripheral bonds, with a weak central σ bond
and significant diradical character. Trialene is predicted [(B3LYP/6-311+G(2d,p)] to lie 64.6 kcal/mol above
butadiyne, with barriers of 2.2 and 4.4 kcal/mol, respectively, for ring opening or bond-shift isomerization.
Other potential rearrangement mechanisms which pass through tetrahedrene (Erel ) 167.2 kcal/mol) or
1,2,3-cyclobutatriene (Erel ) 161.1 kcal/mol) lie at much higher energies.
Introduction
provide a straightforward mechanism for intrabond atom
scrambling, but it is easy to conceive of several mechanisms
The thermal interconversion of alkynes (1) and vinylidenes
(2), also known as the Roger Brown rearrangement, is a reaction
of great mechanistic and synthetic importance.1 This rearrange-
ment was first demonstrated in 1974 by Roger Brown and co-
workers, who observed carbon atom transpositions in isotopi-
cally labeled phenylacetylene.2 An organometallic version of
this reaction forms the basis of a common synthesis of alkynes.3
Vinylidenes clearly represent very shallow energy minima and
even the simplest members of this series still pose many
unanswered questions.4
for interbond atom transposition. In principle, a linear diyne
might fold like an accordion into a symmetrical cyclic structure
and then open with the central atoms having exchanged
positions. We present here the first experimental evidence for
such a process. Computational studies provide support for a
remarkable rearrangement that passes through intermediate
alkynylvinylidenes (4) and trialenes (5).
Our interest in pericyclic routes to highly strained reactive
intermediates5 led us to investigate whether more complex atom
transpositions might occur in a 1,3-butadiyne. In conjugated
polyynes, the sequential 1,2-shifts of Brown rearrangement
* Corresponding author. E-mail: rpj@cisunix.unh.edu.
(1) Recent examples: (a) Review: Brown, R. F. C. Eur. J. Org. Chem. 1999,
3211. (b) Schulz, K.; Hofmann, J.; Zimmermann, G. Eur. J. Org. Chem.
1999, 12, 3407. (c) Scott, L. T.; Cheng, P.-C.; Hashemi, M. M.; Bratcher,
M. S.; Meyer, D. T.; Warren, H. B. J. Am. Chem. Soc. 1997, 119, 10963.
(d) Scott, L. T.; Necula, A. Tetrahedron Lett. 1997, 38, 1877. (e) Synthetic
applications: Kirmse, W. Angew. Chem., Int. Ed. Engl. 1997, 36, 1165.
(2) Brown, R. F. C.; Eastwood, F. W.; Harrington, K. J.; McMullen, G. L.
Aust. J. Chem. 1974, 27, 2393.
C4H2 Chemistry. Present knowledge of the C4H2 potential
surface is sparse; butadiyne (3) is the sole shelf-stable structure6
and few other isomers have been investigated. Butatrienylidene
(6) has been studied by computation7 and generated in the
laboratory;8 the spectroscopic signature of this carbon-rich
(3) Eisler, S.; Tykwinski, R. R. J. Am. Chem. Soc. 2000, 122, 10736 and
references therein.
(4) Recent studies: (a) Hayes, R. L.; Fattal, E.; Govind, N.; Carter, E. A. J.
Am. Chem. Soc. 2001, 123, 641. (b) Sander, W.; Kotting, C.; Hubert, R. J.
Phys. Org. Chem. 2000, 1310, 561-568. (c) Schork, R.; Koppel, H. Chem.
Phys. Lett. 2000, 326, 277-282. (d) Grafenstein, J.; Cremer, D. Phys. Chem.
Chem. Phys. 2000, 2, 2091. (e) Jursic, B. S. THEOCHEM 1999, 488, 87.
(f) Schork, R.; Koeppel, H. Theor. Chem. Acc. 1998, 100, 204. (g) Reed,
S. C.; Capitosti, G. J.; Zhu, Z.; Modarelli, D. A. J. Org. Chem. 2001, 66,
287).
(5) (a) Zheng, M.; DiRico, K. J.; Kirchhoff, M. M.; Phillips, K.; Cuff, L. M.;
Johnson, R. P. J. Am. Chem. Soc. 1993, 115, 12167 (b) Burrell, R. C.;
Daoust, K. J.; Bradley, A. Z.; DiRico, K. J.; Johnson, R. P. J. Am. Chem.
Soc. 1996, 118, 4218. (c) Bradley, A. Z.; Johnson, R. P. J. Am. Chem.
Soc. 1997, 119, 9917.
(6) Review: Maretina, I. A.; Trofimov, B. A. Russ. Chem. ReV. 2000, 69, 591.
9
10.1021/ja0260115 CCC: $22.00 © 2002 American Chemical Society
J. AM. CHEM. SOC. 2002, 124, 6497-6501
6497

A R T I C L E S
Mabry and Johnson
structure has been observed in interstellar gas clouds.9 The
geometry and spectroscopic properties of cyclopropenylidene-
methylene (7) have been predicted,7b,10 but this interesting
carbene remains unknown. Bicyclo[1.1.0]buta-1,3-diene (5),
more commonly known as trialene or propalene, presents an
unusual π bond topology that has invited a variety of theoretical
studies,11 beginning with Hu¨ckel theory.11a,b Trialene was used
as an example in the classic 1961 HMO text by J. D. Roberts.11a
Nevertheless, its existence and involvement in C4H2 chemistry
have received almost no serious consideration. Baird and Dewar
first used the semiempirical MNDO method to predict a C2h
symmetric trialene structure, with alternating single and double
bonds.11c These authors further suggested that trialene might
be made by matrix photolysis of 3. Schleyer and co-workers
later predicted11g that 5 should easily convert to ethynyl-
vinylidene (4), itself lying in a shallow energy minimum.11g,h,12
Simkin predicted that bond shift isomerization in 5 will have a
low barrier, proceeding through a D2h structure.11h
Scheme 1. Synthesis and Pyrolysis of Labeled
1-Phenyl-4-p-tolyl-1,3-butadiyne
Table 1. 13C Label Distributions from NMR Integration
carbon no. (δ ppm)
C-1
C-2
C-3
C-4
(δ 81.87)
(δ 81.22)
(δ 74.05)
(δ 73.29)
unreacted 12
pyrolysis at 800 °C
pyrolysis at 900 °C
20.8
11.6
8.79
0.72
7.37
8.76
0.88
1.16
3.99
1.40
0.89
3.01
recently described calculations on 9 and the related biscarbene
11; these are predicted to be high-energy species.15
Results and Discussion: Experimental Studies
We investigated butadiyne thermal rearrangements through
preparation and pyrolysis (Scheme 1) of 13C-labeled 1-phenyl-
4-p-tolyl-1,3-butadiyne.16 This substance was chosen because
the aryl groups differentiate the sp carbons and also serve as
end-caps. This should allow C4 chain chemistry to occur, without
the fragmentations that are generally observed for alkyl-
substituted alkynes.17 The four sp hybridized carbons of this
diyne were found to be well-resolved in 13C NMR. Control
experiments demonstrated its thermal stability in flash pyrolysis
up to 950 °C. Diphenylbutadiyne rearranges to give polycyclic
aromatics above 1100 °C.18
The results of our experimental studies, including label
distributions and chemical shifts for sp-hybridized carbons, are
summarized in Scheme 1 and Table 1. A sample of diyne that
was ca. 20% 13C enriched at C-1 (isotopomer 12) was
synthesized from 13C-labeled benzaldehyde as shown. This level
of isotopic substitution was chosen so that all the sp carbons
could readily be observed and integrated in each spectrum.
Measurement of quantitative 13C spectra for the sp resonances
was facilitated by addition of chromium acetonylacetonate to
the sample.19
Several higher energy C4H2 structures are relevant to the
present work; these include 1,2,3-cyclobutatriene or cy-
clobutenyne (8a or 8b), tetrahedrene (9), and 1,3-butadiene-
1,4-diylidene (10).7a All of these substances are unknown. In
principle, 7 and 8 might interconvert by a 1,2-shift pathway,
which is similar to that predicted for cyclobutyne.13 Structures
8a and 8b represent the smallest homologues in the cyclic
butatriene or cyclic enyne series.14 Sauer and Harris very
(7) (a) Dykstra, C. E.; Parsons, C. A.; Oates, C. L. J. Am. Chem. Soc. 1979,
101, 1962-5. (b) Apeloig, Y.; Schrieber, R.; Stang, P. J. Tetrahedron Lett.
1980, 21, 411. (c) Maluendes, S. A.; McLean, A. D. Chem. Phys. Lett.
1992, 200, 511-17. (d) Fischer, G.; Maier, J. P. Chem. Phys. 1997, 223,
149-158. (e) Oswald, M.; Botschwina, P. J. Mol. Spectrosc. 1995, 169,
181-4.
(8) (a) Stang, P. J.; Fisk, T. E. J. Am. Chem. Soc. 1979, 101, 4772-3. (b)
Goldberg, N.; Suelzle, D.; Schwarz, H. Chem. Phys. Lett. 1993, 213, 593-
6. (c) Travers, M. J.; Chen, Wei; Novick, Stewart E.; Vrtilek, J. M.; Gottlieb,
C. A.; Thadedeus, P. J. Mol. Spectrosc. 1996, 180, 75-80.
(9) (a) Cernicharo, J.; Gottlieb, C. A.; Guelin, M.; Killian, T. C.; Thaddeus,
P.; Vrtilek, J. M. Astrophys. J. 1991, 368, L43-L45. (b) Kawaguchi, K.;
Kaifu, N.; Ohishi, M.; Ishikawa, S.; Hirahara, Y.; Yamamoto, S.; Saito,
S.; Takano, Shuro; Murakami, A. Publ. Astron. Soc. Jpn. 1991, 43, 607-
19 (c) Turner, B. E.; Herbst, Eric; Terzieva, R. Astrophys. J., Suppl. Ser.
2000, 126, 427-460.
Flash vapor pyrolysis of 13C-labeled diyne 12 at 800 °C/0.01
Torr led to nearly complete exchange between C-1 and C-2, as
shown by NMR integration (Table 1), but with good recovery
of starting material. Lower temperature reaction resulted only
in partial equilibration. Pyrolysis of samples at 900 °C similarly
exchanged C-1 and C-2 but also reproducibly resulted in ca.
(10) Collins, C. L.; Meredith, C.; Yamaguchi, Y.; Schaefer, H. F., III J. Am.
Chem. Soc. 1992, 114, 8694.
(11) Previous theoretical studies on trialene: (a) Roberts, J. D. Notes on
Molecular Orbital Calculations; W. A. Benjamin, Inc.: New York, 1962;
pp 120-126. (b) Heilbronner, E.; Staub, H. Huckel Molecular Orbitals;
Springer-Verlag: New York, 1966, (c) Baird, N. C.; Dewar, M. J. S. J.
Am. Chem. Soc. 1967, 89, 3966. (d) Toyota, A.; Nakajima, T. Theor. Chim.
Acta 1979, 53, 297. (e) Doehnert, D.; Koutecky, J. J. Am. Chem. Soc. 1980,
102, 1789. (f) Agranat, I.; Hess, B. A, Jr.; Schaad, L. J. Pure Appl. Chem.
1980, 52, 1399. (g) Andrade, J. G.; Chandrasekhar, J.; Schleyer, P. v. R.
J. Comput. Chem. 1981, 2, 207. (h) Simkin, B. Y.; Glukhovtsev, M. N.
Zh. Org. Khim. 1982, 18, 1337. (i) Jug, K. J. Org. Chem. 1983, 48, 1344.
(j) Sinanoglu, O. Tetrahedron Lett. 1988, 29, 889-92. (k) Alexander, S.
A.; Klein, D. J. J. Am. Chem. Soc. 1988, 110, 3401. (l) Aoki, Y.; Imamura,
A.; Murata, I. Tetrahedron 1990, 46, 6659. (m) Toyota, A.; Koseki, S. J.
Phys. Chem. 1996, 100, 2100.
(14) (a) Johnson, R. P. Chem. ReV. 1989, 89, 1111. (b) Meier, H. AdV. Strain
Org. Chem. 1991, 1, 215. (c) Hopf, H. Classics in Hydrocarbon Chemistry;
Wiley-VCH Verlag GmbH: Weinheim, Germany, 2000.
(15) Sauers, R. R.; Harris, J. S. J. Org. Chem. 2001, 66, 7951.
(16) (a) Schroth, W.; Dunger, S.; Billig, F.; Spitzner, R.; Herzschuh, R.; Vogt,
A.; Jende, T.; Israel, G.; Barche, J. Tetrahedron 1996, 52, 12677. (b)
Nakasuji K.; Akiyama, S.; Akashi, K.; Nagakawa, M. Bull. Chem. Soc.
Jpn. 1970, 43, 3567.
(17) Brown, R. F. C. Recl. TraV. Chim. Pays-Bas 1988, 107, 655.
(18) Brown, R. F. C.; Eastwood, F. W.; Wong, N. R. Tetrahedron Lett. 1993,
34, 3607.
(19) (a) El-Shahed, F.; Doerffel, K.; Radeglia, R. J. Prakt. Chem. 1979, 321,
859. (b) Levy, G. C.; Edlund, U. J. Am. Chem. Soc. 1975, 97, 4482. (c)
Shoolery, J. N. Varian Instrum. Appl. 1976, 10, 10.
(12) Collins, C. L.; Hu, C. H.; Yamaguchi, Y.; Schaefer, H. F., III Isr. J. Chem.
1993, 33, 317.
(13) Johnson, R. P.; Daoust, K. J. J. Am. Chem. Soc. 1995, 117, 362.
9
6498 J. AM. CHEM. SOC. VOL. 124, NO. 22, 2002

Long-Range Atom Topomerization in Conjugated Polyynes
A R T I C L E S
Scheme 2. Potential Reaction Mechanism
Scheme 3. Alternate Reaction Mechanisms
35% of the label migrating to C-3 and C-4, as indicated by
formation of isotopomers 14 and 15 in nearly equal proportions.
Minor amounts of other products were observed at the higher
temperature. Pyrolysis at 1000 °C resulted in more complex
mixtures, consistent with Brown’s previous studies on diphen-
ylbutadiyne.18
The isotopic distributions observed from pyrolyses at 800 or
900 °C demonstrate that both intrabond and interbond exchange
process are operative, with the former passing through a lower
activation barrier.
Potential Reaction Mechanisms. Scheme 2 presents a
potential mechanism that passes through alkynylvinylidenes and
trialenes. Initial 1,2-shift can lead to carbene 17; a second shift
of the opposite σ bond then affords 18. This Roger Brown
rearrangement1,2 easily explains the intrabond atom transposition
that is observed from pyrolysis of 12 at 800 °C. Alternatively,
higher temperature closure of the carbene to trialene 19 might
be followed by bond-shift isomerization (19 f 21) via transition
state TS20 and subsequent reopening to carbene 23. This
intermediate will rearrange to either 22 or 24, thus accomplish-
ing full transit across the four-carbon chain. This process is
nearly complete after pyrolysis at 900 °C. In general, 1,2-shifts
should result only in alkyne intrabond atom interconversions,
while trialene provides a simple, if somewhat exotic, mechanism
for interbond atom rearrangement.
Among alternative mechanisms, structures 3 and 5 might
interconvert directly through a synchronous accordion-like [π2s
+ π2a] mechanism that by-passes the carbene. Entirely different
routes pass through other high-energy structures (Scheme 3) in
which the central sp carbons of the diyne become structurally
equivalent. Tetrahedrene 25 and cyclobutenyne 26 both possess
the requisite symmetry for interbond atom transposition. Either
structure might be formed directly from the diyne through an
electrocyclic process. Reversible interconversion of 25 with
Figure 1. B3LYP/6-311+G(2d,p) stationary points.
vinylidenecyclopropene 27 might be expected if their energetics
are favorable. Alternatively, a biscarbene such as 10 might result
from two sequential shifts or a dyotropic process.
Results and Discussion: Computational Studies
We employed a variety of wave functions to assess the
energetics of potential C4H2 reaction mechanisms. Figure 1
shows structures for selected B3LYP/6-311+G(2d,p) stationary
points, while the energetics from B3LYP and MP4//MP2 levels
of theory are summarized in Table 2. In general, these two
methods provided very similar structures and relative energetics.
Selected CASSCF(8,8)/6-31G* calculations also were per-
formed. TS1 connects 4 and 5 while TS2 describes bond-shift
isomerization in 5. Relative energies are summarized in Figure
2.
As with parent vinylidene4 (2), theory provides equivocal
predictions on the existence of ethynylvinylidene (4); as a
consequence, the surface for 1,2-shifts is quite dependent on
the computational level. Calculations with HF, TCSCF,
CASSCF, and B3LYP/6-31G(d) theories all located a minimum
for the carbene structure 4.11,12 We observe that MP2 and larger
basis set B3LYP calculations did not; in these cases, the
optimization proceeded steeply downhill to 3, even with
analytical calculation of the gradient at each point. Thus it is
unclear if ethynylvinylidene represents a true stationary point.
In the present work, it suffices to know that the energy of 4
should lie slightly below the transition state (TS1) for closure
to trialene.
Consistent with previous lower level calculations,11 trialene
is predicted at all levels of theory to have alternating peripheral
9
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A R T I C L E S
Mabry and Johnson
Table 2. Summary of Computational Resultse
a
b
c
b
structure
B3LYP
E
rel
NIMAG
MP4//MP2
E
rel
NIMAG
1,3-butadiyne (3)
ethynylvinylidene (4)
closure TS (TS1)
singlet trialene (5)
bond-switch TS (TS2)
tetrahedrene (9)
1,2,3-cyclobutatriene (8)
cyclopropenylidene (7)
butatrienylidene (6)
triplet trialene
-153.534 12
-153.450 87d
-153.427 69
-153.431 13
-153.424 10
-153.267 62
-153.277 41
-153.419 68
-153.468 68
-153.403 69
-153.375 97
-153.414 30
0.00
52.2
0
0
1
0
1
0
1
0
0
0
0
0
-153.140 16
0.00
0
66.8
64.6
69.0
167.2
161.1
71.8
41.0
81.8
99.2
75.2
-153.037 39
-153.042 95
-153.032 46
-152.814 56
-152.909 07
-153.020 43
-153.063 94
64.5
61.0
67.6
204.3
145.0
75.1
1
0
1
0
0
0
0
47.8
biscarbene (10)
bicyclic biscarbene (11)
-152.976 63
-153.018 82
102.6
76.1
1
0
a B3LYP ) B3LYP/6-311+G(2d,p)//B3LYP/6-311+G(2d,p). b NIMAG ) number of imaginary vibrational modes from B3LYP or MP2 calculation.
c MP4SDTQ/6-311+G(2d,p)//MP2(FC)/6-311+G(2d,p). d At B3LYP/6-31G* optimized geometry. See text. e Relative energies in kcal/mol, uncorrected for
zero-point differences.
to be nonplanar, with C2h symmetry and equal C-C bond
lengths around the ring. Efforts to locate a synchronous transition
state directly connecting 3 and 5 afforded much higher barriers
and no true stationary points.
Butatrienylidene (6) is the second lowest energy C4H2
structure but there seems to be no facile connection with its
isomers. The other exotic intermediates shown in Scheme 3 offer
more tempting alternative mechanisms. In principle, 3 might
undergo pericyclic closure to 8, followed by rearrangement
through cyclopropenylidenemethylene (7). At ca. 70 kcal/mol
above 3, carbene 7 might be accessible energetically, especially
if it could derive from 1,2-shifts in 8. The optimized structure
Figure 2. B3LYP energetics along the trialene pathway.
for 8 displays one imaginary mode with B3LYP/6-311+G(2d,p)
theory but none at MP2/6-311+G(2d,p). However, in either
ring bonds with an elongated central bond. The central σ bond
case, the energy of this structure still is >70 kcal/mol above
length in 5 is sensitive to the level of theory, presumably because
that of carbene 7; this places 8 out of consideration as an
of the strong contribution from resonance structure 5d, but
calculations indicate it is not a true diradical. In the CASSCF-
(8,8)/6-31G* wave function, the σ bonding and antibonding
orbitals which describe the transannular bond have occupation
numbers of 1.88 and 0.126, respectively; for a pure diradical
these values would be equal.
explanation for our experimental results. Interestingly, B3LYP
bond lengths in 8 support its characterization as a cyclic
cumulene rather than an enyne. As recently noted by Sauers,15
and observed independently by us, both DFT and MP2 theories
predict the highly pyramidalized tetrahedrene (9) to be an energy
minimum. While 9 presumably has a low barrier for rearrange-
ment to butadiyne, this structure lies >100 kcal/mol above 5,
which excludes it as a likely intermediate.
Several biscarbenes represent high-energy stationary points
on the C4H2 surface. According to our DFT calculations,
biscarbene 10 is planar and lies about 30 kcal/mol above 5.
Once again, this seems too high for serious consideration. MP2
optimization on this same structure led to the transition state
for 1,2-migration of both hydrogens. Sauers has recently
Other spectroscopic properties for trialene are of inherent
interest. Using the GIAO method20 and a B3LYP wave function,
1
we calculate a H NMR chemical shift of 3.79 ppm, perhaps
described calculations on the interesting biscarbene 11.15 This
consistent with a modest degree of antiariomaticity, and 13C
structure is predicted to lie at very high energy relative to
chemical shifts of 87.9 (C-2 methine carbon) and 150.18 ppm.
butadiyne. In neither case is it clear how C-2 to C-3 atom
RTD-B3LYP/6-311+G(2d,p) calculations21 predict the lowest
transposition might occur.
electronic excitation to occur at 3.41 eV (363 nm) with a modest
oscillator strength. Triplet trialene is predicted (Table 2) to lie
17 kcal/mol higher in energy and thus should not play a role in
this chemistry.
Our calculations predict trialene (5) to lie in a shallow
minimum 61-65 kcal/mol above butadiyne. Low-energy paths
exist for both ring opening of 5 to 4 via transition state TS1 or
for bond-switch isomerization through TS2. TS2 is predicted
Conclusions
The Roger Brown rearrangement, which interconverts alkynes
and vinylidenes, plays a central role in diverse high-temperature
chemistry.1,2 Our experimental and computational results provide
the first evidence for a further link to long-range atom
topomerization in a polyyne chain. We observe that carbon
atoms can migrate from one end of a 1,3-diyne to the other in
a process that clearly passes through sequential stages of
intrabond and interbond atom exchange. DFT and Moller-
Plesset computations provide consistent predictions for a likely
(20) (a) Wolinski, K.; Hilton, J. F.; Pulay, P. J. Am. Chem. Soc. 1990, 112,
8251. (b) Ditchfield, R. Mol. Phys. 1974, 27, 789.
(21) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys.
1998, 108, 4439.
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Long-Range Atom Topomerization in Conjugated Polyynes
A R T I C L E S
Synthesis of 13C-Labeled 1-Phenyl-4-p-tolyl-1,3-butadiyne. 13C-
enriched benzaldehyde was purchased from Aldrich and diluted to ca.
20% enrichment with unlabeled material. n-Butyllithium (31.4 mmol)
in hexane was added over 30 min to 1-(4-methylphenyl)-2-propyn-1-
ol25 (2.30 g, 15.7 mol) in THF (17 mL) maintained at -78 °C. The
solution was allowed to warm to room temperature and then was
recooled to -78 °C. Benzaldehyde (20% 13C enriched; 15.0 mmol) in
THF (10 mL) was added over 20 min. After the solution was warmed
to room temperature over 3 h, conventional workup and solvent removal
afforded 1-phenyl-4-p-tolyl-2-butyne-1,4-diol as a viscous oil: 3.24 g
reaction mechanism. At lower energy lies a well-precedented
Roger Brown rearrangement1,2 that accounts for intrabond atom
transpositions. It is uncertain if ethynylvinylidene (4) represents
a true energy minimum on this reaction pathway. Along a
slightly higher energy pathway, closure to singlet trialene (5),
followed by bond-switch isomerization, and ring opening
(Scheme 2) presents what we believe to be the lowest energy
mechanism for interbond carbon scrambling. While our scan
of the C4H2 potential energy surface is hardly exhaustive, the
structural features and energetic similarity of the carbene and
trialene mechanisms provide strong argument that they are
closely connected. Qualitatively, the predicted reaction energet-
ics are consistent with the temperature required for this
rearrangement. Other logical but exotic intermediates such as
8, 9, or 10 lie at much higher energy than the trialene
mechanism; we believe these can be excluded. Preliminary
calculations indicate that the energetics of these processes are
little changed in longer polyynes. Thus, in principle, extensive
carbon scrambling might occur in an sp carbon chain of any
length. One important conclusion is that trialene (5) now seems
to have evolved from a fictional subject of π electron
calculations11a to a real reactive intermediate; we speculate that
other structures with similar bonding schemes should exist.
1
(82%). H NMR (360 MHz, CDCl3) δ 7.12-7.54 (m, 9H), 5.52 (s,
1H), 5.49 (s, 1H), 2.91 (br s, 2H), 2.36 (s, 3H).
Thionyl chloride (30.0 mmol) in THF (10 mL) was added dropwise
over 30 min to a mixture of the diol (2.50 g, 9.91 mmol), pyridine
(2.40 mL), and THF (25 mL) at 0 °C. The reaction mixture was allowed
to warm to room temperature and crushed ice was added. Conventional
workup afforded the dichloride, which was used without further
purification.
A solution of the crude dichloride (1.66 g, 5.74 mmol) in THF (10
mL) was added over 5 min to a suspension of freshly prepared sodium
amide (17.2 mmol) in liquid ammonia (30 mL) at -78 °C. After 1.5
h, ammonium chloride (2 g) was added and the ammonia and solvent
were removed. The residue was extracted with hexanes (50 mL).
Concentrated extracts were purified on silica with hexane as eluent to
1
give 0.30 g (20%) of diyne 12. H NMR (360 MHz, CDCl3) δ 7.57-
7.15 (m, 9H), 2.39 (s, 3H); mp 107-109 °C (lit.16a 110 °C). The 13C
Experimental Section
NMR spectrum is described in the text.
Computational Methods. Structures for reactants and transition
states were created initially with Spartan22 and optimized at the pBP86/
DN* level of theory. Further DFT and Moller-Plesset calculations were
carried out with GAUSSIAN 98.23 Both GAMESS24 and GAUSSIAN
were used for CASSCF calculations. Each optimized structure exhibited
the expected number of imaginary vibrational modes: i.e., zero for
minima and one for saddle points.
Pyrolysis Experiments. The apparatus consisted of a 50 cm
horizontal quartz tube, packed loosely with quartz chips and held in a
tube furnace. Samples of the labeled butadiyne (30-50 mg) were slowly
sublimed into the tube at a pressure of 0.01-0.02 Torr. Products were
collected in a trap cooled with dry ice. Results are described in the
text.
Acknowledgment. This research was supported by the
National Science Foundation. We are grateful to the New
England Board of Higher Education for a fellowship to J.M.
(22) Spartan, Version 5.1.1 (UNIX), and PC Spartan Pro, Version 1.06;
Wavefunction, Inc.: Irvine, CA, 1995-2000.
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
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-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.5;
Gaussian, Inc.: Pittsburgh, PA, 1998.
Supporting Information Available: 13C NMR spectra for
samples before and after pyrolysis; Cartesian geometries,
energies, and thermodynamic data for B3LYP stationary points
(PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
JA0260115
(24) GAMESS: Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.;
Gordon, M. S.; Jensen, J. H.; Koseki; Matsunaga, S. N.; Nguyen, K. A.;
Su, S.; Windus, T. L.; Dupuis, M.; Montgomery J. A. J. Comput. Chem.
1998 14, 1347.
(25) Nakasuji, K.; Akiyama, S.; Akashi, K.; Nakagawa, M. Bull. Chem. Soc.
Jpn. 1970, 43, 3567.
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J. AM. CHEM. SOC. VOL. 124, NO. 22, 2002 6501