Theoretical and experimental studies on the structure and isomerization of isocyano and cyano cyclopolyenes

Article abstract of DOI:10.1134/S1070428008100096

Quantum-chemical calculations in terms of the density functional theory showed that cyclopolyenyl isocyanides RNC are considerably less stable than the corresponding cyanides and that they are capable of undergoing RNC → RCN isomerization according to bot

Full text of DOI:10.1134/S1070428008100096

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 10, pp. 1451–1463. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © O.I. Mikhailova, G.A. Dushenko, I.E. Mikhailov, R.M. Minyaev, V.I. Minkin, 2008, published in Zhurnal Organicheskoi Khimii,
2
008, Vol. 44, No. 10, pp. 1475–1487.
Theoretical and Experimental Studies on the Structure
and Isomerization of Isocyano and Cyano Cyclopolyenes
a
b
b
a
a
O. I. Mikhailova , G. A. Dushenko , I. E. Mikhailov , R. M. Minyaev , and V. I. Minkin
a
Institute of Physical and Organic Chemistry, Southern Federal University,
pr. Stachki 194/2, Rostov-on-Don, 344090 Russia
e-mail: mikhail@ipoc.rsu.ru
b
Southern Research Center, Russian Academy of Sciences, ul. Chekhova 41, Rostov-on-Don, 344006 Russia
Received May 24, 2007
Abstract—Quantum-chemical calculations in terms of the density functional theory showed that cyclopolyenyl
isocyanides RNC are considerably less stable than the corresponding cyanides and that they are capable of
undergoing RNC→RCN isomerization according to both 1,2-shift mechanism (cyclopropenyl and cyclopenta-
≠
dienyl isocyanides; ΔE = 35.0 and 37.5 kcal/mol, respectively) and previously unknown 2,5-sigmatropic shift
≠
mechanism (cycloheptatrienyl isocyanide, ΔE = 26.4 kcal/mol). Migration of cyano group in the cyclopenta-
diene and cycloheptatriene systems follows the 1,5-sigmatropic shift pattern. The activation barrier to 1,5-shift
of cyano group around the cycloheptatriene ring was estimated by dynamic NMR in deuterated nitrobenzene
≠
(
ΔG190°C = 26.5 kcal/mol).
DOI: 10.1134/S1070428008100096
Organic cyanides and isocyanides play an important
migrating group [4]. Cyanocyclopolyene derivatives
role in organic and organometallic synthesis and are
used in the design of new materials and medicines.
Isocyanides constitute a rare class of organic com-
pounds in which carbon atom possesses unshared elec-
tron pair; therefore, they are widely used in coordina-
tion chemistry, e.g., for the preparation of polynuclear
complexes based on isocyanoferrocene [1–3]. Taking
into account the above stated, studies on the structure
and properties of isocyano- and cyano-substituted cy-
clopolyenes as potential intermediate products for fine
organic synthesis and preparation of metal complexes
seem to be important. In addition, cyclopolyene deriv-
atives in which circumambulatory migration of organic
or organometallic groups is possible are now consid-
ered to be promising models of molecular motors
where the mode and rate of circular motion could be
controlled via proper choice of the cyclic system and
exist mainly as nitrile structures I–III [5–8].
3
-Isocyano-1,2,3-triphenylcyclopropene was de-
tected experimentally in trace amount (0.8%) in the
gas phase during fragmentation of the corresponding
3
-isothiocyanato derivative; the mass spectrum con-
+
tained a ion peak with m/z 293 [M – S] [9] (Scheme 1).
Scheme 1.
Ph
NCS
Ph
Ph
NC
–S
Ph
Ph
Ph
Isocyanocyclopentadiene (V) was reported for the
first time only in 2003; it was generated from 6-azido-
fulvene by photolysis in methanol or pyrolysis in
chloroform at 50°C; the process was accompanied by
·
·
C
··
C
H
CN
H
N
R
CN
H
CN
N
Y–
R
R
I
II
III
VI
VIA
R = H, Ph.
Y = O, NR.
1
451

1
452
MIKHAILOVA et al.
Scheme 2.
NC
NC
N
H
NC
5
0°C
+
–
N2
N3
V
NC
NC
CN
2
70°C
+
+
transformation into isomer mixture via 1,5-hydrogen
shifts [5]. Flash vacuum pyrolysis of 6-azidofulvene at
cyanopolyenes I–VI correspond to minima on the
potential energy surfaces (PES) and cyano isomers
I–III are more stable than isocyano isomers IV–VI by
20.2, 23.2, and 20.9 kcal/mol, respectively (Table 1).
We also calculated the energy barriers to the trans-
formations of cyclopropenyl, cyclopentadienyl, and
cycloheptatrienyl isocyanides IV–VI into the corre-
sponding cyano derivatives according to different
mechanisms (Table 2).
2
70°C was shown to involve isomerization of iso-
cyanide V into the corresponding cyanide in which hy-
drogen shifts occurred (Scheme 2).
Isocyanocycloheptatriene (VI) was not reported so
far, though isocyanotropones and tropone imines like
VIA were described in [10].
In the present work we performed a theoretical
It was found that all isocyanocyclopolyenes IV–VI
are converted into cyanides I–III via 1,2-shift through
transition states (TS) VII, IX, and XI without appreci-
able disturbance of the ring π-system (the endocyclic
double bonds are localized, and the length of the car-
bon–nitrogen bond in the migrating group approaches
that of triple C≡N bond); the corresponding energy
(
DFT) study at the B3LYP/6-31G(d,p) level [11, 12]
on the electronic and steric structure and relative
stability of different isomers of compounds I–VI and
intramolecular sigmatropic shifts and isomerizations in
the series of isocyano- and cyano-substituted cyclo-
polyenes, as well as experimental study on previously
unknown migrations of the cyano group in 7-cyano-
cycloheptatriene.
≠
barriers are ΔE = 35.0, 37.5, and 29.9 kcal/mol, re-
spectively (Table 2, Schemes 3–5). Figures 1–3 show
the structures and geometric parameters of the transi-
tion states for all migration mechanisms.
According to the calculations, the structures of
three-, five-, and seven-membered cyano- and iso-
Table 1. Calculated (B3LYP/6-31G**) total and relative energies of compounds I–VI and their isomers XVIII–XXII in the
a
gas phase
≠
≠
–1
Compound no.
Etot, a.u.
ΔE , kcal/mol
ΔEZPE, kcal/mol
1
ω , cm
I
–208.86737
–208.83475
–208.86918
–286.34335
–286.30575
–286.35797
–286.35468
–286.32952
–363.75386
–363.75656
–363.72018
–363.76383
0.
0.
20.2
00.8
0.
218
190
200
121
125
161
161
140
091
110
083
114
IV
20.5
XVIII
II
0–1.1–
0.
V
23.4
23.2
0–9.2–
0–7.3–
08.4
0.
XIX
XX
XXII
0–9.2–
0–7.1–
08.7
III
-
-ax
0.
III-eq
VI-ax
XXI
0–1.7–
21.1
0–1.8–
20.9
0–6.2–
0–6.3–
a
Hereinafter, Etot is the total energy, 1 a.u. = 627.5095 kcal/mol; ΔEZPE is the relative energy calculated with account taken of zero-point
–
1
harmonic vibration energy; and ω
1
, cm , is the least harmonic vibration frequency.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 10 2008

THEORETICAL AND EXPERIMENTAL STUDIES ON THE STRUCTURE
1453
Table 2. Calculated (B3LYP/6-31G**) total energies of transition states VII–XIV and energies of activation for the trans-
formation of isocyano-substituted cyclopolyenes into the corresponding cyano-substituted isomers
≠
≠
≠
a
–1
Transition state no.
Etot, a.u.
ΔEZPE, kcal/mol
ΔH , kcal/mol
ΔG298, kcal/mol
ω, cm
VII
VIII
IX
–208.77636
–208.68991
–286.24358
–286.24179
–363.67010
–363.67690
–363.60812
–363.70571
35.0
88.7
37.5
38.8
29.9
26.4
68.1
13.0
35.0
88.4
37.4
38.4
30.3
25.6
67.7
13.5
35.1
89.2
37.4
39.5
28.9
27.7
68.7
10.8
410
760
374
484
198
388
430
076
X
XI
XII
XIII
b
XIV
a
ω is the only imaginary harmonic vibration frequency.
Structure VI-ax (PCM, DMSO): Etot = –363.72821, ZPE = 0.12614 a.u.
b
Transition states VII and IX for the cyclopropene
tional electrostatic interaction between the ring and
migrating group is possible, which reduces the energy
of TS-XI compared to VII and IX. The high energy
barrier to alternative shift through TS-VIII for cyclo-
propenyl isocyanide (IV) makes this process impos-
sible under thermal conditions (Scheme 3).
and cyclopentadiene systems are characterized by
a small charge separation between the ring and migrat-
ing group, while it is significant in TS-XI for the
cycloheptatriene system (the total charge on the iso-
cyano group is –0.535 e, and the dipole moment is μ =
7
.2 D). The migrating group in TS-XI is remote from
Cyclopentadienyl and cycloheptatrienyl isocyanides
can also be transformed into the corresponding cya-
nides along a different path, 2,5-sigmatropic shift
(transition states X and XII; Figs. 2, 3) with a similar
energy of activation. Unlike TS-XI, transition states X
the ring to distances of 2.322 (N) and 2.310 Å (C),
and, unlike structures VII and IX, the carbon atom
therein appears above the planar seven-membered ring:
7
1
the dihedral angle CNC C is 70.3°; therefore, addi-
Scheme 3.
N
C
N
H
N
··
H
C
C
VIII, TS, ΔE = 88.7 kcal/mol
IV, E = 0
N
C
I, ΔE = –20.2 kcal/mol
1
,2
H
VII, TS, ΔE = 35.0 kcal/mol
Scheme 4.
N
C
2
1
,5
,2
N
H
N
··
H
C
C
X, TS, ΔE = 38.8 kcal/mol
N
H
C
V, E = 0
II, ΔE = –23.2 kcal/mol
IX, TS, ΔE = 37.5 kcal/mol
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1
454
MIKHAILOVA et al.
–
0.444
N
–
0.295
1
.195
0
.105
0
.011
N
1
.175
1
.330
1
.920
–0.324
N
1
.820
1
.474
1
1
.800
2
.470
1
.460
2
.470
.530
1
.340
1
.350
1.800
1
.300
1
.370
TS-XVII, C
s
TS-VII, C
1
TS-VIII, C
1
Fig. 1. Calculated (B3LYP/6-31G**) structures of transition states VII, VIII, and XVII for the CN group shifts and isocyanide–
cyanide isomerization in the cyclopropene system. Interatomic distances (Å) and Mulliken charges (italicized numbers, a.u.)
are given.
–
0.430
–
0.060
N
1
.176
1.190
0
.254
.670
–0.273
N
1
1
.800
–
0.039
–
0.138
1
.510
1
.500
1
.420
1.400
1
.400
1
.390
TS-XXIII, C
s
TS-XXIV, C
s
–
0.273
0.109 1.210
–0.305
–
0.055
N
N
1
.770
1
.870
.510
1
.840
1.730
1
1
.540
1.410
1
.490
1
.340
1
.420
1
.390
1
.340
1
.470
1
.400
TS-IX, C
1
TS-X, C
1
Fig. 2. Calculated (B3LYP/6-31G**) structures of transition states IX, X, XXIII, and XXIV for shifts of cyano and isocyano groups
and isocyanide–cyanide isomerization in the cyclopentadiene system. Interatomic distances (Å) and Mulliken charges (italicized
numbers, a.u.) are given.
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THEORETICAL AND EXPERIMENTAL STUDIES ON THE STRUCTURE
1455
1
.190
–
0.417
–0.118
.310
N
1
.400
1
.380
2
2
.322
–
0.303 0.090
1
.430
1
.450
N
1
.212
124.8°
1
.430
1
.600 1.770
1
.500
1.370
1
.510
1
.370
1
.420
1
.330
TS-XII, C
1
TS-XI, C
1
–
0.320
–
0.548
–
0.295
N
N
1
.270
1.180
1
.430
0
.145
3.125
3.230
1
.480
1
.550
1
.390
1
.950
1
.390
1.390
1
.360
1
.390
1.390
1
.390
1.390
TS-XIII, C
1
1
XIV, C (DMSO)
Fig. 3. Calculated (B3LYP/6-31G**) structures of transition states XI–XIV for the isocyanide–cyanide isomerization in the
cycloheptatriene system in the gas phase and DMSO solution. Interatomic distances (Å) and Mulliken charges (italicized numbers,
a.u.) are given.
and XII are characterized by close arrangement of the
ring and migrating group and by a weak charge sep-
aration. Thus cyclopentadienyl isocyanide is capable
of being converted into cyanide II along two paths
with similar energy barriers (TS-IX, TS-X; Scheme 4).
The 2,5-sigmatropic shift in the cycloheptatriene sys-
tem (TS-XII) was found to be the lowest-energy proc-
quent ring closure via formation of new C–C bond
(TS-XIII); however, high energy barrier to this process
makes it noncompetitive (Scheme 5, Table 2). The
heights of the energy barriers to the reverse processes
(cyanide → isocyanide) indicate their low probablity.
We previously performed calculations on the iso-
merization of methyl isocyanide into acetonitrile and
found that the process involves formation of a π-com-
plex like structures VII, IX, and XI [13]. As shown
in [14], rearrangements of primary, secondary, and
tertiary alkyl, substituted benzyl, methoxycarbonyl-
methyl, and triphenylmethyl isocyanides into the cor-
responding cyanides in such nonpolar solvents as
≠
ess among the examined ones (ΔE = 26.4 kcal/mol).
In contrast to XI, transition state structure XII is not
strained; the seven-membered ring therein is not
planar: two double-bonded carbon atoms deviate from
the plane formed by the remaining five carbon atoms,
while the latter participate in electron density delocal-
ization. 2,5-Sigmatropic shifts have not been described
in earlier experimental and theoretical studies.
dodecane and mesitylene are characterized by activa-
≠
50°C
tion barriers ΔG2
ranging from 35 to 37 kcal/mol;
Analysis of the potential energy surface for the
isomerization of cycloheptatrienyl isocyanide VI into
cyanide III revealed one more path involving rupture
of C–C bond in the seven-membered ring and subse-
i.e., the nature of the fragment linked to the isocyano
group weakly affects the process. This was rationalized
in terms of formation of a tight hypervalent three-
membered cyclic transition state.
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MIKHAILOVA et al.
Scheme 5.
N
C
XIII, TS, ΔE = 68.1 kcal/mol
N
C
·
·
C
N
N
H
C
1
,2
H
H
VI, E = 0
III, ΔE = –21.1 kcal/mol
XI, TS, ΔE = 29.9 kcal/mol
N
C
2
,5
XII, TS, ΔE = 26.4 kcal/mol
Conjugated cyclopolyenes give rise to alternative
solvents is likely to be responsible for the fact that
cycloheptatrienyl isocyanide (VI) could not be isolated
in polar solvents.
isomerization paths, among which 2,5-sigmatropic
shift turned out to be the most probable process for the
cycloheptatriene system with three double bonds in the
ring. The calculated activation barrier to 2,5-sigma-
Migrations of the cyano group and competing
hydrogen shifts were found experimentally for some
cyano-substituted benzocyclopolyenes [6, 16, 17]. Our
experiments revealed previously unknown circumam-
bulatory migration of the cyano group in 7-cyanocy-
cloheptatriene (III) having no other substituents on the
ring. With a view to elucidate possible mechanisms of
such rearrangements we analyzed different paths of
migration of cyano and isocyano groups around the
ring in three-, five-, and seven-membered cyclic
polyenes.
Rearrangements in cyclopolyene systems can
follow several paths. Cyanocyclopropene can undergo
rearrangements via 1,3-sigmatropic shifts of the cyano
group or dissociation of the C–C bond with formation
of ion pair; recombination of the latter could involve
equally probable covalent bonding with any ring
carbon atom (random mechanism). Five- and seven-
membered systems give rise to greater diversity of pos-
sible migration mechanisms, including 1,3-, 1,5-, and
1,7-sigmatropic shifts of the cyano group and random
mechanism. In addition, concurrent 1,j-sigmatropic
shifts of hydrogen may occur.
≠
tropic shift is ΔE = 26.4 kcal/mol (for the gas phase),
indicating that isocyanide VI can be converted into cy-
anide III in several hours at room temperature. Insofar
as the reaction of cycloheptatrienyl bromide with KCN
was carried out in strongly polar media [15] and the
only product was cyanide III, the energy barrier to the
isomerization VI → III was calculated for the DMSO
medium in terms of the polarization continuum model
(
PCM). The barrier along the 2,5-signatropic shift path
≠
was almost similar (ΔE = 27.1 kcal/mol, DMSO),
while that corresponding to the 1,2-shift was consider-
≠
ably lower (TS-XIV, ΔE = 13.0 kcal/mol, DMSO,
≠
ΔΔE = 16.9 kcal/mol; Table 2). The calculated en-
≠
thalpy of activation (ΔS = 9.1 e.u., TS-XIV) is fairly
large and positive, which is typical of ion pair-like
transition states. The structure of TS-XIV (Fig. 3) is
characterized by very strong charge separation be-
tween the seven-membered ring and the isocyano
group (–0.874 a.u. on NC), large dipole moment (μ =
2
0.0 D), and long distance between the ring carbon
atoms and NC group. The total energy of structure
XIV is less than the sum of the energies of tropylium
cation (XV, Etot = –270.75760 a.u.) and cyanide ion
According to the results of quantum-chemical cal-
culations, 1,3-sigmatropic shifts of the cyano group in
unsubstituted cyanocyclopropene in the gas phase re-
quire high energy (ΔG298 = 70.2 kcal/mol) (Scheme 6,
(
XVI, E = –92.94626 a.u.) in DMSO. The low calcu-
tot
lated barrier to the isomerization VI → III in polar
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THEORETICAL AND EXPERIMENTAL STUDIES ON THE STRUCTURE
1457
Table 3. Calculated (B3LYP/6-31G**) total energies of transition states XVII, XXIII, XXV, XXVI, XXVIII, and XXX and
energies of activation for shifts of the cyano group and hydrogen atoms in cyano-substituted cyclopolyenes
≠
≠
≠
a
–1
Transition state no.
Etot, a.u.
ΔEZPE, kcal/mol
ΔH , kcal/mol
ΔG298, kcal/mol
ω, cm
XVII
–208.74790
–286.28683
–363.67125
–363.69229
–363.69989
–455.92181
72.2
34.4
49.9
37.4
32.5
32.4
72.9
34.1
50.5
36.8
32.2
32.1
70.2
34.9
47.8
38.3
32.9
33.5
0603
0676
0394
0432
1489
0456
XXIII
XXVI
XXV
XXVIII
b
XXX
a
ω is the only imaginary harmonic vibration frequency.
XXIX: Etot = –455.97549, ZPE = 0.12448 a.u.
b
2
Table 3) and are forbidden by the symmetry rules.
Figure 1 shows the structure of polar transition state
XVII for this process, which is characterized by ap-
preciable charge separation (–0.63 a.u. on the migrat-
ing CN group), long distance (2.470 Å) between the
ring and the CN group, and hence large dipole moment
where the cyano group is attached to sp -hybridized
carbon atom; these structures can be formed via hydro-
gen shifts (Table 1). The energy of cyclopentadienyli-
denemethylidenamine (XXII) having an NH group
(ΔEZPE = 8.4 kcal/mol, Table 1) is also similar to that
of cyanocyclopentadiene II. In fact, compounds XIX–
XXI were isolated as individual substances [5, 6], and
structural analog of isomer XXII was reported for pen-
tacyanocyclopentadiene [18]; it exists in the polymeric
form with N–H···N hydrogen bonds [19].
(
μ = 8.8 D).
Scheme 6.
CN–
NC
H
The calculated structures of transition states for
1
,5-sigmatropic shifts of cyano and isocyano groups in
cyclopentadiene (Scheme 7; Fig. 2; Tables 3, 4), which
are allowed by the orbital symmetry rules, are typical
of sigmatropic shifts: the charge separation between
the ring and migrating group is insignificant, and the
distances between the ring and migrating group are
I
XVII, TS
H
·
· ·
CN
1
.670 (XXIII) and 1.800 Å (XXIV).
I'
Scheme 7.
Isomeric cyanocyclopropenes I and XVIII in which
the CN group is attached, respectively, to sp - and
sp -hybridized carbon atoms are similar in stability
3
H
X
X
2
~1,5-X
(
ΔEZPE = 0.8 kcal/mol), whereas more stable isomers
of cyanocyclopentadiene and cyanocycloheptatriene
have structures XIX–XXI (ΔEZPE = 6–9 kcal/mol)
II, V
XXIII, XXIV
H
X
CN
CN
CN
II, V
II, XXIII, X = CN; V, XXIV, X = NC.
XVIII
XIX
XX
NH
C
The calculated energies of activation for cyano
NC
group migration (Table 3) approach those found ex-
perimentally for analogous migration in the 1,3-di-
methyldibenzo[e,g]indene system (Scheme 8), where
the rate constant of the overall process including shifts
H
H
–
5
–1
XXI
XXII
of the CN group and H-shift is k = 19.0×10
s
in
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 10 2008

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MIKHAILOVA et al.
Scheme 8.
Me
Me
H
CN
Me
CN
H
CN
Me
Me
Me
Scheme 9.
N
C
N
7
~1,5-CN
C
3
7
H
2
1
N
6
C
7
4
5
3
3
4
III'-ax
2
1
H
XXV
N
C
5
6
N
C
III-ax
4
5
~1,7-CN
1
3
2
H
1
7
6
7
III''-ax
XXVI
2
1
6
3
2
~1,5-H
3
4
1
N
7
4
5
C
H
5
6
7
NC
H
H
H
CN
III-eq
XXVIII
XXI
≠
diphenyl ether at 250°C (ΔG250°C = 40.1 kcal/mol); i.e.,
it exceeds those reported for 1,3-dimethylindenes by
about 2 orders of magnitude [16, 17].
Analysis of the potential energy surfaces for migra-
tions of cyano and isocyano group in the seven-mem-
bered ring revealed transition states for 1,5-sigma-
tropic shifts of the cyano group (XXV) and 1,7-sigma-
tropic shifts of the cyano (XXVI) and isocyano groups
(XXVII) (Tables 3, 4; Schemes 9, 10; Fig. 4). Allowed
1,5-sigmatropic shifts (according to the Woodward–
Hoffmann rules) of the cyano group in cyanocyclo-
heptatriene are more energetically favorable than the
corresponding 1,7-sigmatropic shifts by 12.5 kcal/mol,
and their energies of activation approach those calcu-
lated for cyanocyclopentadiene (II). This may be
rationalized in terms of the structure of transition state
XXV for 1,5-shifts, where two ring carbon atoms devi-
Probable paths of migration of the cyano group in
compound III were revealed by DFT calculations; in
addition, paths of migration of the isocyano group in
VI were calculated for comparison. The results showed
Table 1) that the energy difference ΔEZPE between pos-
sible conformers III-eq and III-ax with equatorial and
axial orientation of the cyano group is 1.8 kcal/mol,
which is very consistent with the experimental data
= 0.8 kcal/mol) [20]. Suprafacial migrations
of the CN group in III-ax and suprafacial hydrogen
shifts in III-eq are possible (Scheme 9).
(
0
–
(
ΔG
155°C
Table 4. Calculated (B3LYP/6-31G**) total energies of transition states XXIV and XXVII and energies of activation for
shifts of the isocyano group in isocyano-substituted cyclopolyenes
≠
≠
≠
a
–1
Transition state no.
Etot, a.u.
ΔEZPE, kcal/mol
ΔH , kcal/mol
ΔG298, kcal/mol
ω, cm
XXIV
–286.23035
–363.66751
45.9
31.3
45.8
31.8
46.0
29.8
774
185
XXVII
a
ω is the only imaginary harmonic vibration frequency.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 10 2008

THEORETICAL AND EXPERIMENTAL STUDIES ON THE STRUCTURE
1459
Scheme 10.
·
·
··
C
C
C
N
N
7
N
1
3
4
4
5
2
1
3
2
H
H
1
7
· · ·
5
6
6
7
VI
XXVII
VI'-ax
ate by an angle of 67.9° from the plane formed by the
remaining five carbon atoms; as a result, they are not
involved in delocalization of π-electron density. Struc-
ture XXV is characterized by insignificant charge sep-
aration between the ring and migrating group, and the
corresponding distance is 1.701 Å (Fig. 4).
with the calculated data. The calculated energy barrier
ΔG298 for suprafacial 1,5-hydrogen shifts in isomer
≠
III-eq is 32.5 kcal/mol (TS-XXVIII, Scheme 9,
Table 3), i.e., it is similar to that calculated for the
1,5-CN shifts, indicating that these processes may
compete with each other. This conclusion does not
contradict the experimental data [6].
Comparison of the calculated gas-phase energy bar-
riers (Table 3) with the experimental value determined
≠
Scheme 11.
in deuterated nitrobenzene (ΔG190°C = 26.5 kcal/mol)
counts in favor of 1,5-sigmatropic shift. Somewhat
larger height of the calculated barrier suggests specific
effect of such solvent as C D NO on the rate of mi-
N
CN
C
CN
CN
6
5
2
gration; PCM calculations give a value which is lower
≠
by only 1.2 kcal/mol (MeNO , ΔE = 36.2 kcal/mol).
2
XXIX
XXX
In fact, the results of additional calculations on the
1
,5-CN shifts in dicyanocycloheptatriene XXIX
≠
NC
CN
[ΔG (gas) = 33.5 kcal/mol, TS-XXX; Table 3] are in
298
≠
good agreement with the experimental data (ΔG298
1.0 kcal/mol in chloroform) [6] (Scheme 11).
,5-Sigmatropic shift of the CN group in cyano-
substituted benzoheptene is observed at 130–160°C
=
H
3
1
According to the orbital symmetry conservation
rules, 1,7-sigmatropic shifts of isocyano group in the
cycloheptatriene system are forbidden (Table 3, 4). The
calculated rate of 1,7-NC shift is much higher than that
of the 1,7-CN shift (the energy of activation is lower
by 18.5 kcal/mol), in contrast to 1,5-shifts in the cyclo-
≠
≠
(
C D , ΔH = 27.2 kcal/mol, ΔS = –12.7 e.u.) [17] and
6 6
is accompanied by 1,5-H shift; furthermore, in going
from benzene to methanol the rate of migration in-
creases only twofold, which provides a support for the
concerted mechanism of the process and is consistent
–
0.446
–0.420
1.187
.51
–0.172
N
–0.492
N
1
.170
N
1
1.19
–
0.134
0
.316
–0.413
1.44
2
.44
2
.514
1
.701
1
.400
1.41
1
.390
1
.400
1.393
1.39
1
.390
1
.39
1.39
1
.390
TS-XXVI, C
s
TS-XXV, C
s
TS-XXVII, C
s
Fig. 4. Calculated (B3LYP/6-31G**) structures of transition states XXV–XXVII for shifts of cyano and isocyano groups in the
cycloheptatriene system. Interatomic distances (Å) and Mulliken charges (italicized numbers, a.u.) are given.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 10 2008

1
460
MIKHAILOVA et al.
7
pentadiene system. In the latter case, the energy barrier
to migration of isocyano group is higher by 11.5 kcal×
with the difference that the 7-H (δ 2.39 ppm, t) and C
signals (δ 29.65 ppm) of III appeared in a stronger
C
–
1
mol than the energy barrier to migration of cyano
group). While considering the calculated geometric
parameters of transition states for the 1,7-shifts, strong
charge separation between the planar seven-membered
ring and migrating group (–0.582 a.u. on CN and
field. The observed upfield shift is likely to be related
to the presence of electron-withdrawing cyano group
in position 7 of molecule III.
Taking into account the C symmetry of molecule
III, its H NMR spectrum contained four multiplets
from protons in the cycloheptatriene ring. The most
upfield triplet with an intensity corresponding to one
proton was assigned to 7-H, and the other three signals
δ, ppm: 4.84 m (1-H, 6-H), 5.73 m (2-H, 5-H), and
.23 m (3-H, 4-H)] were assigned by analysis of the
H– H coupling constants using the double resonance
technique (successive irradiation at a resonance fre-
quency of each proton, leading to change in the multi-
plicity of signals of the other protons). In keeping with
the H NMR data, 7-cyanocyclohepta-1,3,5-triene (III)
exists mainly as a boat conformer with quasiequatorial
orientation of the cyano group. This follows from
the coupling constant between 7-H and 1(6)-H ( J =
.15 Hz), which considerably differs from the corre-
sponding coupling constant for triphenylstannyl deriv-
ative of cycloheptatriene (C H SnPh , J = 8.0 Hz)
where quasiaxial orientation of the Ph Sn group was
s
1
–
0.664 a.u. on NC), long distances between the ring
and migrating group (2.514 Å in XXVI and 2.440 Å in
XXVII), and large dipole moments (9.4 and 9.2 D for
TS-XXVI and TS-XXVII) must be noted. Structures
[
7
1
XXVI and XXVII differ by dihedral angles C C CN
6
7
1
and C C NC (132.6 and 99.5°, respectively). It follows
that the isocyano group in TS-XXVII hangs over the
cycloheptatriene ring (Fig. 4), providing the possibility
for additional interaction between π-orbitals of the ring
and migrating group, which should stabilize the transi-
tion state. As shown in previous theoretical studies,
just 1,7-shifts forbidden by the Woodward–Hoffmann
rules are typical of migrations of chlorine [21] and
1
1
1
3
BAlk groups [22] in the cycloheptatriene system. This
2
6
was rationalized in terms of Coulomb interaction be-
tween the system and migrating atom in the first case
3
7
7
3
1,7
[21] and Möbius type interaction in the second [22].
3
Previously, migrations of various substituents
determined by X-ray analysis [24].
1
1
around the seven-membered ring in η -cycloheptatri-
ene compounds were studied by experimental meth-
ods. It was shown that their energy barriers change
over a wide range, depending mainly on the nature of
the central atom in the migrating group. For example,
high-energy 1,5-sigmatropic shifts of hydrogen [23, 6]
and methoxy group [6], relatively fast 1,5-shifts of
Bushweller et al. [20] found by dynamic H NMR
spectroscopy that 7-cyanocyclohepta-1,3,5-triene (III)
exists as two rapidly interconverting (via ring inver-
sion) boat conformers with quasiequatorial (III-eq)
and quasiaxial (III-ax) orientation (Scheme 9). In the
1
H NMR spectrum (60 MHz, CH CHCl) recorded at
2
–
155°C, conformers III-eq and III-ax are observed
0
Ph Sn [4, 24], Me Sn [22], and PhS groups [25],
3
3
separately at a ratio of 28:1 (ΔG
= 0.78 kcal ×
–
155°C
–
1
1
,7-sigmatropic shifts of ruthenium [26] and rhenium
mol ), while their signals coalesce at –120°C and be-
residues [27], chlorine atom [21], and Pr B group [22],
2
come narrower on further raising the temperature; the
≠
3
1
,3-displacement of azido group accompanied by its
,7-sigmatropic shifts [28], fast 3,7-hetero-Cope rear-
rangements of SC(R)=S groups [23, 29], and shifts of
NCO [30, 31], NCS [30, 31], NCSe [23, 31], and
phthalimido groups [32] according to the random
mechanism were described.
activation barriers ΔG–
for the interconversion
120°C
were estimated at ~8 (III-eq→III-ax) and ~9.5 kcal ×
–
1
mol (III-ax→III-eq). The authors believed that the
conformer ratio weakly depends on the temperature,
for the Gibbs entropy of activation in such processes is
close to zero. No signals assignable to norcaradiene
1
isomer were detected in the H NMR spectrum of
7
-Cyanocyclohepta-1,3,5-triene (III) was synthe-
cycloheptatriene III.
sized by us from tropylium bromide and potassium
cyanide according to the procedure reported in [15].
1
The H NMR spectra of 7-cyanocycloheptatriene
1
13
The H and C NMR data showed that compound III
III showed temperature dependence in the range from
has covalent structure where the cyano group is at-
120 to 180°C (nitrobenzene-d ): reversible asynchro-
5
3
tached to the sp -hybridized carbon atom in the seven-
nous broadening of signals from protons in the cyclo-
heptatriene ring was observed. Here, the 7-H and
3(4)-H signals turned wider first, and then the others
1
13
membered ring. The H and C NMR spectra of III
1
were similar to those reported for η -cycloheptatriene
1
13
derivatives C H SC(OEt)=S [29] and C H SnPh [24]
became broader. In the 2D (EXSY) H and C NMR
7
7
7
7
3
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 10 2008

THEORETICAL AND EXPERIMENTAL STUDIES ON THE STRUCTURE
1461
spectra of 7-cyanocyclohepta-1,3,5-triene (III) (Fig. 5)
cross peaks corresponding to 7–3,4, 3,4–1,6, and
,6–2,5 correlations appeared at 120°C. This spectral
pattern results from intramolecular circumambulatory
,5-sigmatropic migration of the cyano group
Scheme 9). Analysis of the shape of signals in the
1
1
(
3
4
0
0
1
dynamic H NMR spectrum of cyanocycloheptatriene
≠
III allowed to estimate the energy barrier ΔG190°C to
the cyano group shift at 26.5 kcal/mol (nitroben-
–
1
zene-d , k
= 2.8 s ).
5
190°C
1
1
1
1
00
10
20
30
After heating a solution of cycloheptatriene III in
nitrobenzene-d for 0.5 h at 130°C, an additional group
5
1
of signals appeared in the H NMR spectrum due to
isomer XXI, the equilibrium III-to-XXI ratio being
0
1
3
.59 : 0.41. Isomer XXI is formed as a result of
,5-sigmatropic H-shift from the 7-position to position
or 4 in the cycloheptatriene ring (Scheme 9). The
energy barriers for the 1,5-H shift in 7-substituted
cyclohepta-1,3,5-trienes were determined previously
C
δ ,
ppm
1
30 120 110 100
, ppm
40
30
δ
C
[
6]; they range from 30.2 to 33.6 kcal/mol [6].
1
3
Fig. 5. Two-dimensional (EXSY) C NMR spectrum of
cyclohepta-2,4,6-triene-1-carbonitrile (III) in C NO at
20°C. Solvent signals are cut off.
Thus the calculations showed that cyanocyclopoly-
6
D
5
2
enes are considerably more stable than their isocyano
isomers. This implies irreversible transformation of
isocyanide into cyanide according to the 1,2-shift
mechanism for cyclopropene and cyclopentadiene de-
1
forbidden 1,7-shifts of the NC group were revealed at
the DFT level for isocyanocycloheptatriene, which
may be due to electrostatic interaction between the
seven-membered ring and migrating group.
≠
rivatives (ΔE = 35.0 and 37.5 kcal/mol, respectively)
and previously unknown 2,5-sigmatropic shift for
isocyanocycloheptatriene. The barrier to the isomeriza-
≠
tion of the latter in the gas phase (ΔE = 26.4 kcal/mol)
EXPERIMENTAL
suggests that the transformation of the isocyanide into
cyanide is possible to be complete in several hours at
room temperature and that the isomerization in strong-
ly polar solvents should occur at a high rate through
a ion pair-like transition state (ΔE = 13.0 kcal/mol,
DMSO).
Quantum-chemical calculations were performed in
the restricted Hartree–Fock (RHF) approximation with
account taken of all (valence and inner-shell) electrons
in terms of the second-order Møller–Plesset perturba-
tion theory [MP2(full)] [11] and density functional
theory (DFT) [11] with three-parameter B3LYP poten-
tial and valence-split 6-31G** basis set (Gaussian-03
software package [12]). All stationary points were
localized by calculating the Hesse matrix. Topological
analysis of total electron density distribution according
to Bader (AIM analysis) [33] was performed using
AIMPAC software package [34]. The molecular struc-
tures shown in Figures 1–4 were plotted using Gauss
View 3 program [35].
≠
The calculated activation barriers for 1,3-sigma-
tropic shifts of the cyano group in cyanocyclopropene
≠
(
ΔE = 72.2 kcal/mol) are so high that these trans-
formations are impossible under experimental condi-
tions. The cyano and isocyano groups in the cyclo-
pentadiene system are capable of undergoing migration
via 1,5-sigmatropic shift in keeping with the Wood-
ward–Hoffman orbital symmetry conservation rules;
here, the calculated activation barrier for the CN group
≠
1
13
migration (ΔE = 34.4 kcal/mol) approaches the ex-
The H and C NMR spectra of compound III
were recorded on Bruker-AM and Varian Unity spec-
trometers at 300 and 75.47 MHz, respectively, using
tetramethylsilane as internal reference. Deuterated sol-
vents were dried over calcined molecular sieves prior
to use. The behavior of III in solution was studied by
perimental value found for dibenzoindene systems.
Both theoretical and experimental data showed that
migration of the cyano group in cyanocycloheptatriene
follows the 1,5-sigmatropic shift pattern typical of
cyclopentadiene derivatives. By contrast, symmetry-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 10 2008

1
462
MIKHAILOVA et al.
1
dynamic one- and two-dimensional (EXSY) H and
C NMR techniques .
mery, J.A., Jr., Vreven, T., Kudin, K.N., Burant, J.C.,
Millam, J.M., Iyengar, S.S., Tomasi, J., Barone, V.,
Mennucci, B., Cossi, M., Scalmani, G., Rega, N., Peter-
sson, G.A., Nakatsuji, H., Hada, M., Ehara, M., Toy-
ota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakaji-
ma, T., Honda, Y., Kitao, O., Nakai, H., Klene, M.,
Li, X., Knox, J.E., Hratchian, H.P., Cross, J.B.,
Adamo, C., Jaramillo, J., Gomperts, R., Strat-
mann, R.E., Yazyev, O., Austin, A.J., Cammi, R.,
Pomelli, C., Ochterski, J.W., Ayala, P.Y., Morokuma, K.,
Vothm, G.A., Salvador, P., Dannenberg, J.J., Zakrzew-
ski, V.G., Dapprich, S., Daniels, A.D., Strain, M.C.,
Farkas, O., Malick, D.K., Rabuck, A.D., Raghava-
chari, K., Foresman, J.B., Ortiz, J.V., Cui, Q., Ba-
boul, A.G., Clifford, S., Cioslowski, J., Stefanov, B.B.,
Liu, G., Liashenko, A., Piskorz, P., Komaromi, I.,
Martin, R.L., Fox, D.J., Keith, T., Al-Laham, M.A.,
Peng, C.Y., Nanayakkara, A., Challacombe, M.,
Gill, P.M.W., Johnson, B., Chen, W., Wong, M.W.,
Gonzalez, C., and Pople, J.A., Gaussian 03, Revision
B.03, Pittsburgh, PA: Gaussian, 2003.
1
3
Cyclohepta-2,4,6-triene-1-carbonitrile (III) was
synthesized by reaction of tropylium bromide with po-
tassium cyanide according to the procedure described
in [15]. H NMR spectrum (300 MHz, C D ), δ, ppm:
.39 t (1H, 1-H), 4.84 m (2H, 2-H, 7-H), 5.73 m (2H,
-H, 6-H), 6.23 m (2H, 4-H, 5-H). C NMR spectrum
1
6
6
2
3
(
1
3
1
2
75.47 MHz, C D ), δ , ppm: 29.65 (C ), 116.20 (C ,
6 6 C
7
3,
6
4
6
C ), 119.54 (CN), 127.28 (C C ), 131.26 (C , C ).
This study was performed under financial support
by the Russian Foundation for Basic Research (project
nos. 07-03-00223, 06-03-32158a), by the Presidium of
the Russian Academy of Sciences (program no. 8,
New Effective Luminophores for Organic Light-
Emitting Diodes”), by the Ministry of Education and
Science of the Russian Federation (project no. RNP.-
“
2
.2.2.2.5592, program for the development of the
Southern Federal University), and by the President of
the Russian Federation (program for support of leading
scientific schools, project no. NSh-363.2008.3).
1
1
3. Minkin, V.I., Olekhnovich, L.P., and Zhdanov, Yu.A.,
Molecular Design of Tautomeric Compounds, Dord-
recht: D. Reidel, 1988.
4. Meier, M., Müller, B., and Rüchardt, C., J. Org. Chem.,
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