Origin of the Preferential Formation of Helicenes in Mallory Photocyclizations. Temperature as a Tool to Influence Reaction Regiochemistry

Article abstract of DOI:10.1021/acs.joc.8b02671

The regiochemistry of four bis-Mallory photocyclization substrates has been examined from experimental and computational perspectives. Formation of all three possible regioisomers was only observed in the reaction of one of the substrates. In the other three substrates, only the two C₂-symmetric products, but not the C₁ product, were formed. In the three reactions that only formed two products, the photocyclization temperature could be used to select for exclusive formation of one or the other regioisomer. The use of temperature to select between two regioisomers also worked in the photocyclization of the substrate that formed three products. However, no temperature was located for exclusive formation of the third component, one of the C₂-symmetric products, which always formed alongside either one or both of its regioisomers. B3LYP/6-311+G(2d,p) calculations were used to determine the energies of all of the dihydrophenanthrene (DHP), tetrahydrophenanthrene (THP), and mono-Mallory photocyclization intermediates. The oscillator strengths of the DHP precursors to the helicene products were a factor of 4.8-9.2 smaller than those of competitively formed DHPs. This observation suggests that establishment of a photostationary state is responsible for the preferential formation of helicenes that has been observed as a unique and useful feature of many Mallory photocyclizations.

Full text of DOI:10.1021/acs.joc.8b02671

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Article
Origin of the Preferential Formation of Helicenes in Mallory Photocyclizations.
Temperature as a Tool to Influence Reaction Regiochemistry.
Jacob Weber, and Edward L. Clennan
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02671 • Publication Date (Web): 12 Dec 2018
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Origin of the Preferential Formation of Helicenes in Mallory
Photocyclizations. Temperature as a Tool to Influence Reaction
Regiochemistry.
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J. Weber, E. L. Clennan*
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Department of Chemistry, University of Wyoming, Laramie WY, 82071
clennane@uwyo.edu
ABSTRACT
The regiochemistry of four bis-Mallory photocyclization substrates have been examined
from experimental and computational perspectives. Formation of all three possible
regioisomers was only observed in the reaction of one of the substrates. In the other three
substrates only the two C2-symmetric, but not the C1 product, were formed. In the three
reactions that only formed two products the photocyclization temperature could be used to
select for exclusive formation of one or the other regioisomer. The use of temperature to
select between two regioisomers also worked in the photocyclization of the substrate that
formed three products. However, no temperature was located for exclusive formation of
the third component, one of the C2-symmetric products, which always formed alongside
either, one, or both of its regioisomers. B3LYP/6-311+G(2d,p) calculations were used to
determine the energies of all of the dihydrophenanthrene (DHP), tetrahydrophenanthrene
(THP), and mono-Mallory photocyclization intermediates. The oscillator strengths of the
DHP precursors to the helicene products were a factor of 4.8 to 9.2 smaller than those of
competitively formed DHPs.
This observation suggests that establishment of a
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photostationary state is responsible for the preferential formation of helicenes that has
been observed as a unique and useful feature of many Mallory photocyclizations.
INTRODUCTION
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The Mallory photocyclization (Scheme 1) was first observed in 1934 and provided, at the
time, an unknown by-product during the photochemically initiated cis-trans isomerization of
stilbene, 1t1c.1 This byproduct was subsequently identified by Parker and Spoerri2 in 1950 as
phenanthrene. This reaction, although studied sporadically, was more or less dormant until
Mallory and coworkers3 reported in 1962 its development into a preparatively useful oxidative
photocyclization by the use of I2, rather than O2, as the oxidant (Scheme 1). The key step in the
mechanism of the Mallory photocyclization is the widely recognized formation of a trans-
dihydrophenanthrene (DHP) intermediate4-7 by an orbital symmetry allowed conrotatory
electrocyclic closure. (Scheme 1) This skeletal rearrangement in many diaryl-ethylenes is
photochromic and consequently has been utilized in the construction of a large number of
photoswitching devices.8
The dihydrophenanthrene, DHP, however, in the Mallory
photocyclization is an intermediate, not an end-product, that is subsequently converted to the
phenanthrene end-product by abstraction of hydrogen and formation of HI.
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A Mallory photocyclization was used during the first synthesis of [7]helicene9 and opened
the door for decades of study of these fascinating polycyclic aromatic hydrocarbons (PAHs).10-13
In addition, the importance of the Mallory photocyclization has subsequently been amply
14-19
demonstrated by the publication of several reviews,
by the development of useful
modifications,20-26 and most recently by its use in the construction of carbon nanomaterials.27
The sentence that appears in the elegant review of Morin, Daigel, and Desroche;27 “The
photochemical dehydrogenation, or Mallory reaction, is probably the most widely spread
photochemical method for the preparation of carbon nanomaterials and PAHs.” is not hyperbole,
and given the seemingly endless applications of PAHs (polycyclic aromatic hydrocarbons)
underscores the value of further studies to understand the intimate details of this important
reaction.
The mechanistic details of the photochemical interconversion of the cis-stilbene, 1c, to
the dihydrophenanthrene, DHP, (Scheme 1) step of the Mallory photocyclization has been
extensively examined from both experimental28-30 and theoretical31-34 perspectives. A two-
dimensional slice through the currently accepted potential energy surfaces (PES) for this reaction
step is depicted in Figure 1. Upon irradiation of either 1c or DHP an allowed transition to the
corresponding Franck-Condon PES, FCstilbene or FCDHP, occurs. Rapid movement on the Franck-
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Condon PES brings the vibrationally excited state into proximity to a conical intersection, CIFC,
where picosecond or sub-picosecond internal conversion to S1 takes place. Finally, both
vibrationally excited states move on the S1 PES until they reach the critical conical intersection
region, CI, where ultrafast deactivation forms either the cis-stilbene or the DHP on the S0 PES.
The activation barrier, TS1 on the S1 PES, which was predicted theoretically, provides an
explanation for both the observed temperature,35 and wavelength,36 dependence of the
cycloreversion quantum yield for the DHP.
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Figure 1.
Potential Energy Surfaces Involved in the Reversible cis-Stilbene DHP
Interconversion.
The detailed understanding of the cis-stilbene/DHP interconversion step of the Mallory
photocyclization, however, stands in stark contrast to our fundamental lack of understanding, of
perhaps the most unusual and distinguishing feature of the Mallory reaction, which is its
propensity to form helicenes even when competing photocyclizations to form more
thermodynamically stable, sterically less encumbered, planar PAHs are available. This
phenomenon is illustrated with the reactions of 2,37 and 3,16 which exclusively produced
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[6]helicene, 4, and benzo[b]hexahelicene, 5, without a trace of the alternative photocyclization
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products. (Scheme 2)
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Scheme 2. Regioselectivity in Mallory Photocyclizations.
Laarhoven and coworkers38,39 used the Coulson free valence numbers40 to devise a set of
rules to predict the regiochemical outcome of Mallory photocyclizations. These free valence
∑
3
numbers,
푟∗푠, are measures of the “residual affinity” for bond formation and are given by
퐹
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― 푃
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is the sum of the bond orders for the 3 bonds attached to
푃
for aromatic carbons where
the aromatic carbon in the excited state. According to the Laarhoven rules when the sum of the
free valences of the two carbon atoms that form the new bond in the Mallory reaction are less
∗
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than 1 (i.e.
photocyclization does not occur. This reactivity parameter has been
퐹푟푠 < 1.0)
remarkably successful, however, other experimental parameters in addition to the identity of the
substrate also influence the extent of regioselectivity including, the concentrations of the oxidant
(e.g. I2, O2) and substrate, the identity of the solvent, and the temperature.41 Despite the
predictive power of the Laarhoven rules they do not provide a satisfying rationale or a
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framework to control the unusual regiochemistry observed in many of these important
photocyclizations.
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In this manuscript we describe an experimental and computational study of the bis-
Mallory photocyclizations of 6, 7, 8, and 9 (Chart 1) focused on understanding the
regioselectivity observed in these reactions. The experimental study reveals that the product
regiochemistry in the reactions of 6 and 9 responds very differently to changes in experimental
conditions and that the Mallory photocyclizations of 7 and 8 are intermediate in their behavior.
The computational results are used to argue that the photostationary state established between
competing DHPs plays an important role in these reactions. In addition, in those reactions that
produce the helicene as the major product the photostationary state lies on the side of the DHP
precursor to the helicene (DHP’ Chart 1). These results provide valuable insight into Mallory
photocyclizations that can be used to design new efficient photocyclization reactions.
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Chart 1.
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RESULTS
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Experimental Studies. Bis-Mallory photocyclization substrates 8 and 9 were synthesized from
3,6-dibromophenanthrenequinone in straightforward two-step procedures as outlined in Scheme
3. The key steps were the Mizoroki-Heck reactions42 which had previously been successfully
used with 3,6-dibromophenanthrene and 3,6-dibromo-9,10-dimethoxyphenanthrene to make 6
and 7, respectively.43 Unfortunately, 3,6-bis-styrenylphenanthrene quinone, despite the fact that
is readily accessible from a Mizoroki-Heck reaction, could not be used in this study. It is
completely, unreactive under our Mallory photocyclization conditions.
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Scheme 3. Synthesis of bis-Mallory photocyclization substrates 8 and 9.
The bis-Mallory photocyclizations were conducted by irradiation (600 W medium
pressure mercury vapor lamp) of a 0.5 mM toluene solution of 6, 7, 8, or 9 through the walls of a
Pyrex vessel containing 1.1 mM iodine, and 25 mM propylene oxide.20 At room temperature 9
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reacts to give a mixture of all three Mallory photocyclization products, the benzo[k]naphtho[1,2-
a]tetraphene, 10d, the [7]helicene, 11d, and the dibenzo[c,m]pentaphene, 12d. (Chart 2) The
benzo[k]naphtho[1,2-a]tetraphenes, 10a,b,c however, were conspicuously absent from room
temperature irradiations of 6, 7 and 8. However, oxidation products 15b and 15c are formed in
approximately 40% yield during bis-Mallory photocyclizations of substrates, 7 and 8,
respectively.44 In addition, di-ester, 16,45 is also formed in approximately 5% yield in the
reactions of 7. On the other hand, no oxidative cleavage of the styrenyl double bond was
observed in any of the phenanthrenyl substrates, 6-9. This suggest that singlet oxygen formed by
self- sensitization is involved in these reactions since the singlet triplet energy gap is 5.4
kcal/mol smaller46 and the intersystem crossing rate constant47 to the triplet nearly 3 times larger
in the [5]helicene core48 of 13b,c, that is oxidatively cleaved to form 15b,c, than in the
phenanthrene core of 6-9, which are oxidatively inert. The formation of di-ester 16 can be
attributed to the well-established enhanced reactivity of singlet oxygen49,50 with electron rich
enol ethers and to the even larger intersystem crossing rate constant of [7]helicene in comparison
to [5]helicene.47 The amount of these oxidation products, however, can be substantially reduced
by exclusion of oxygen from the reaction mixtures. Products from further complications such as
hydrogen shifts in the dihydrophenanthrene, DHP, and bis-dihydrophenanthrene, bis-DHP,
intermediates, which have been observed in other systems were not detected in the reactions of
6-9.16
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Chart 2.
These reactions were typically conducted overnight and the crude product mixtures
analyzed by NMR spectroscopy. Products 11a, 11b, and 16 are known, and products 10d, 11c,
12c, 12d, 15b, and 15c were isolated, purified, and fully characterized by H and 13C NMR
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spectroscopy (with the exception of 12c whose limited solubility precluded collection of its C
NMR spectrum), and by high-resolution mass spectrometry. Diagnostic peaks in the NMR
spectra of these conclusively identified products were then used to identify their homologues
(11d, 12a, 12b, and 12c) formed in the other Mallory photocyclization reactions. Stacked
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comparator H NMR plots used in this analysis are provided in the Supporting Information for
the 11, and 12 homologous series. A diagnostic ddd for proton H2 (Chart 2) showed up in the 11
comparator spectra at approximately 6.4 ppm in all of the [7]helicenes 11a, b, c, and d. This
proton was upfield of all other peaks in the reaction mixtures because it is located in the
shielding cone of the terminal ring on the other end of the helicene. The
dibenzo[c,m]pentaphenes, 12, were also easily identified because Mallory photocyclization
conditions (vide infra) are available where these compounds are the major or exclusive product
of the reaction. Their most upfield peaks, between 7.6 and 7.7 ppm are also ddd’s and are
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readily assigned to H4 and H5. The cis- and trans-3-strenyl[5]helicenes, 13 and 14, were
identified as fleeting intermediates in the crude reaction mixtures by their characteristic doublet-
of-doublets for the styrenyl double bond. (See Supporting information)
The product compositions at approximately 40oC during Mallory photocyclizations of 6-9
as a function of irradiation time are given in Figure 2. The mono-Mallory products 13 and 14
form rapidly followed by slower formation of the [7]helicenes 11. In addition, the yields of the
[7]helicenes decrease in the order 6 > 7 > 8 > 9. The photocyclization of 9 (Figure 2) was
complete in approximately 5 h in comparison to the 10-12 hours of irradiation needed to produce
the final products in the reactions of 6, 7, and 8. The photocyclization of 9 also produced 10d
under these conditions as the dominant product in approximately 85% yield. In contrast, 10a,b,c
were not observed at any point during the photocyclizations of 6, 7, or 8.
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Figure 2. Reaction Compositions as a Function of Time at 40oC.
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The effect of temperature on the Mallory photocyclizations was examined by running
each of the reactions in a pyrex vessel submerged in an appropriate temperature-controlled bath.
The products were then analyzed by NMR spectroscopy and their relative yields plotted versus
temperature as depicted in Figure 3. The [7]helicenes, 11a, 11b, and 11c were the exclusive
products in the reactions of 6, 7, and 8 at low temperatures (< 0oC). On the other hand, the
[7]helicene, 11d, is observed, but its % yield peaks at approximately 0oC and never becomes the
major product at any temperature between -30oC and +150oC. In all four Mallory
photocyclizations the thermodynamically most stable product (vide infra), the planar
dibenzo[c,m]pentaphene, 12, is the dominant product at high temperature.
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Figure 3. Relative product composition as a function of temperature for the Mallory
photocyclizations of 6, 7, 8, and 9. All reactions were run under an atmosphere of
air, however, the oxygenated products are not included in the plots.
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Computational Studies. The mechanisms of the bis-Mallory photocyclizations of 6-9 are
considerably more complex than the simple photocyclization of cis-stilbene shown in Scheme 1.
The bis-Mallory substrates undergo two photocyclizations and each photocyclization can occur
at two different sites on the phenanthrene core to give two different DHPs. This leads to eleven
different possible closed shell intermediates on these very complicated reaction surfaces in
addition to the three possible products, 10, 11, and 12. The structures of all the potential
intermediates are depicted in Chart 3.
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Chart 3. Potential intermediates in the Mallory bis-photocyclization of 6.
In order to facilitate the consideration of the computational results and the upcoming
consideration of potential mechanisms in the discussion section of the manuscript, these
intermediates are organized using the interconversion diagram shown in Scheme 4. However, it
is important to recognize that this complicated interconversion diagram is itself a simplification
of the actual potential energy surface. Both diastereomers of the DHP regioisomers depicted in
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Chart 3, which are generated by competing clockwise and counterclockwise ring closures, are
not explicitly included in the interconversion diagram. This is not a serious omission. A single
diastereomer thermodynamically dominates in most cases (Table 1) but even in those cases
where two stereoisomers might be simultaneously present (e.g. Etct and Ettt in Table 1; see
Supporting Information for detailed structural information) it will not change the mechanistic
conclusions discussed below. Consequently, all the Tables in this manuscript, except Table 1,
contain only those values calculated using the thermodynamic data for the most stable
diastereomer of each of the DHP regioisomers shown in Chart 3.
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The A’s in the interconversion diagram represent the cis-cis (cc), cis-trans (ct), and trans-
trans (tt) isomers of the bis-styrenyl starting materials, 6-9, and the subscripts represent the total
number of Clar sextets51 (i.e. the number of six membered rings with a localized aromatic
6 cyclic array of electrons). The remaining letters (See Chart 3 and Scheme 4) represent either
dihydrophenanthrenes, B, C, I, J, K, and L, each with 2 Clar sextets, or bis-
dihydrophenanthrenes, D, E, and F, with 1, 0, and 0 Clar sextets, respectively, or mono-Mallory
photocyclized intermediates, G and H, containing 4 Clar sextets. To the left, emanating from Act
is an identical interconversion diagram differing only from the one on the right by the trans-
configuration of the styrenyl double bond in intermediates B, C, G and H. The mechanistic
conclusions that can be deduced from either wing of the interconversion diagram are identical so
we will use the expanded right hand wing for the remaining discussion. Intermediates in
different colored regions of the diagram are not interconvertible, either thermally or
photochemically, because it would require reversible loss of H2, which is highly improbable.
The *’s in the diagram are for microscopic steps that can occur either thermally by a stepwise, or
concertedly by a controtatory photochemically allowed process.
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Scheme 4. Interconversion diagram for intermediates in the Bis-Mallory Photocyclizations of 6,
7, 8, and 9. * h or heat; cc cis-cis and ct cis-trans; subscripts on all numerical and
letter compound designations refer to its number of Clar sextets.
DFT based-methods with large basis sets have been shown to perform well in
thermochemical studies of large polyaromatic hydrocarbons and in studies of delocalized
radicals.52 Consequently, we used the B3LYP/6-311+G(2d,p) computational method to optimize
and determine the energies of all the diastereomers for the regioisomeric intermediates located
on the right hand side of Scheme 4, including the products 10, 11, and 12. The energies are
given in Table 1 and the optimized geometries are provided in the supporting information.
The stabilities of the C30H22 isomers (for 6) in the blue region and the C30H20 isomers in
풄풄
the green/red regions of Scheme 4 decrease with the decreasing number of Clar sextets (blue:
푨ퟒ
> B2  C2 > Eo  F0 and green/red: H4  G4 > I2  J2  K2  L2). Superimposed on these
primarily aromaticity driven stability sequences there is also a strain/steric contribution. In the
reactions of 6 and 7 this internal strain raises the energy of the most stable isomer of the
tetrahydro[7]helicene D with one Clar sextet above that of both E and F with zero Clar sextets.
In the reactions of all the bis-Mallory substrates, 6-9, the embedded [5]helical structure in F
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raises its energy by 6 or more kcal/mol above that of E despite the fact that both contain the same
number of Clar sextets (zero). The internal strain energy imparted by the helical architecture, and
the greater strain imparted by the [7]- relative to the [5]helical architecture, is also expressed in
the relative energies of the final bis-Mallory products, (11 > 10 > 12) all of which can be drawn
with 4 Clar sextets. (Table 1)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
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53
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Table 1. Relative Free Energiesa of bis-Mallory Reaction Stationary Points with Zero
Negative Frequencies.a
CXHY+4 (Blue Region)b Bis-Mallory Reaction Intermediatesc
6(parent)
7(di-MeO)
8(Naphtho)
9(Bis-Ketal)
A
B
Ba
C
Diooi
Doiio
Doioi
Etct
Ettt
Ftct
Ftcta
Fttt
Fttta
0
0
0
0
52.4
48.1
39.1
107.4
96.7
101.3
78.7
78.1
85.2
49.8
44.2
46.1
101.7
84.3
89.9
85.9
85.2
93.7
97.1
99.2
92.3
50.7
44.8
45.8
102.8
85.0
90.7
85.6
85.8
93.9
97.5
99.4
92.1
52.8
46.0
42.8
105.5
90.6
97.0
84.1
83.4
89.4
93.7
96.1
89.0
88.7
90.5
85.5
CXHY+2 (Red/Green Region)b Bis-Mallory Reaction Intermediatesc
6(parent)
13.4
0
7(di-MeO)
13.2
0
8(Naphtho)
9(Bis-Ketal)
G
H
I
Ia
J
K
Ka
L
13.7
0
63.7
60.8
42.9
53.2
46.4
57.8
12.4
0
63.4
62.2
39.5
52.0
47.8
52.0
60.9
57.4
45.0
50.4
44.7
61.1
58.0
61.7
58.2
44.5
51.2
45.0
61.1
58.0
La
55.3
50.6
CXHY (Black)b Final Reaction Productsc
10
11
12
13.2
24.2
0
13.7
24.4
0
13.3
25.5
0
11.9
23.4
0
a. B3LYP/6-311+G(2d,p) sum of electronic and thermal Free Energies in kcal/mol. b. See Scheme 4 for region
colors. c. X = number of carbons in 6-9; Y = number of hydrogens in the final reaction products, 10-12, of bis-
Mallory photocyclization/oxidation of 6-9.
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The dihydrophenanthrenes, B and C, are key intermediates in the blue region (Scheme 4)
6
7
8
of the bis-Mallory photocyclizations of 6-9.
The most stable isomers of the bis-
9
dihydrophenanthrenes, D, E, and F, (Table 1) formed in the blue region are approximately 42.6 ±
6.0, 36.6 ± 6.6, and 43.2 ± 5.8 kcal/mol higher in energy than the least stable
dihydrophenanthrene and are unlikely to be involved in the reactions. The B3LYP/6-
311+G(2d,p) optimized structures of B and C formed in the photocyclization of 6 are shown in
Scheme 5 and have the trans-configuration of the hydrogens at the dihydrophenanthrene ring
closure junctions as dictated by the allowed conrotatory closure. The two conrotatory closures
in B lead to diastereomers B and Ba. Diastereomer B (PRR/MSS configuration) is 5.6 kcal/mol
less stable than diastereomer Ba (PSS/MRR configuration) as a result of the 0.36 Å closer
approach (1.765 Å vs. 2.128 Å) of the blue and the red hydrogens (Scheme 5). The two
conrotatory closures to form the C intermediates are very close in energy at the B3LYP/6-31G(d)
computational level (Go(RB3LYP) = 0.4 kcal/mol) so only the most stable diastereomer was
optimized at the higher B3LYP/6-311+G(2d,p) computation level and included in Table 1 and
Scheme 5.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ph
H
H
B
B
Ba
Ph
H
H
C
C
Scheme 5. Structures of the dihydrophenanthrene intermediates, B and C, formed in the bis-
Mallory photocyclization of 6.
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The key dihydrophenanthrene intermediates in the red and green regions (Scheme 4) of
the interconversion diagram, I, J, K, and L, which are formed in the photocyclization of 6, are
shown in Scheme 6. (P)-18a(S), 18b(S)-Dihydro[7]helicene Ia is 3.5 kcal/mol more stable than
its isomer (P)-18a(R), 18b(R)-dihydro[7]helicene I. Isomer Ia is also likely to form faster
because it is formed by photocyclization on the least hindered face of the [5]helicene
intermediate, G. (P)-Naphtho[1,2,b]-14a(R), 14b(R)-dihydro[5]helicene, K, is significantly
destabilized, by 5.7 kcal/mol, relative to (P)-naphtho[1,2,b]-14a(S), 14b(S)-dihydro[5]helicene,
Ka, by the energetically costly 1.78 Å through space H1-H14a distance, which is increased to a
sterically more tolerable 2.23 Å H1-H14b closest approach distance in Ka (H1 blue atom and
H14a,14b red atoms in Scheme 6). (P)-1(R), 8a(S)-dihydronaphtho[1,2,b]-[5]helicene, L, is 3.1
kcal/mol less stable than its isomer (P)-1(S), 8a(R)-dihydronaphtho[1,2,b]-[5]helicene, La, due
in part to the shorter H1-H14 (blue and red atoms in Scheme 6) through space distance (2.60 Å in
L and 2.82 Å in La) and by the increased strain in the helical section of the intermediate as
revealed by a 3.1o increase in the C14a-C14b-C14c-C14d internal dihedral angle (green atoms in L
and La in Scheme 6).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
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37
38
39
40
41
42
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44
45
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51
52
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7
H
H
8
9
I
10
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15
16
17
18
19
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56
57
58
59
60
I
Ia
H H
L
L
La
H H
J
J
H
H
K
K
Ka
Scheme 6. Structures of the dihydrophenanthrene intermediates, I, L, J, and K, formed in the
bis-Mallory photocyclization of 6.
The DHPs shown in Schemes 5 and 6 are subsequently oxidized by stepwise removal of
two hydrogen atoms with photochemically generated iodine atoms. Aromatic resonance energy
is recovered as a result of the second hydrogen abstraction and as a result the first hydrogen
abstraction is likely to be the rate determining step for formation of the fully polyaromatic
hydrocarbon product. The initial hydrogen abstraction can occur from the terminal or internal
퐴 ⋇
퐵 ⋇
series, respectively. The
ring of the DHP leading to two different radicals, the
and
relative energies of these radicals generated by hydrogen abstraction from Ba, C, Ia, J, Ka, and
La in the reactions of 6 and 9 are given in Table 2. Hydrogen abstraction is prohibitively favored
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표
⋇
(
from the terminal ring to give the
series when the DHP is part of
퐴
∆∆퐺 ≥ 14.4 푘푐푎푙/푚표푙)
5
6
⋇
⋇
the helical domain (i.e. B , I , and K ). On the other hand, the energies of the
and
series
퐴
퐵
a
a
a
7
8
9
radicals, formed by hydrogen abstraction from the DHPs that reside in the acene domain (i.e. C,
⋇
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
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44
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47
48
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50
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53
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56
57
58
59
60
J, and L ), are nearly equal. Formation of the
series radicals from the helical embedded
퐴
a
⋇
DHPs open up the jaws of the helicene decreasing steric interactions while formation of the
퐵
series radicals closes the jaws and increases the intra-helicene steric interactions (Scheme 7).
⋇
The formation of the
series radicals in the helical domain embedded DHPs are also likely
퐴
kinetically preferred since the hydrogen on the terminal ring is on the periphery of the helicene
while the internal hydrogen is buried in the jaws of the helical clef. (See Ba in Scheme 5 and the
supporting information).
Table 2. Relative Energies of Radicals formed in Mallory Cyclizations of 6 and 9.a,b
6
9c
A*
1.0
1.4
B*
17.6
0
A*
5.4
0
B*
15.1
2.6
Ba
C
Ia
J
Ka
La
14.2
0.6
1.0
28.6
0
16.4
12.6
19.8
0
5.2
11.3
27.6
2.83
15.2
14.9
12.1
a. Relative B3LPY/6-311+G(2d,p) sum of electronic and thermal Free Energies in kcal/mol for the
퐴 ⋇
radicals generated from the most stable DHP diastereomer (See Table 1). b.
hydrogen abstraction from the terminal ring of DHP;
– radical formed by
퐵 ⋇
– radical formed by hydrogen abstraction
from internal ring of DHP. c. Calculated at the B3LYP/6-31G(d) level.
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6Ia B* series
6Ia A* series
6Ia
⋇
⋇
Scheme 7. Steric Interactions in 6I in comparison to those observed in the
and
series
퐴
퐵
a
hydrogen abstraction products (radicals)
The UV-Vis spectra of the DHPs were calculated using the TD-DFT/6-311+G(2d,p)
computational model. The lowest energy transitions are given in Table 3 along with their
oscillator strengths. The DHPs (i.e. Ba and C, Ia and La, and Ka and J) in the same colored
region of Scheme 4 are thermally and photochemically interconvertible and are placed in the
same rows of Table 3 for comparison. The HOMO-LUMO determinant in all cases was the
contributor to the excited-state wavefunction as determined by taking two times the
≥ 99%
square of the coefficient for the configuration interaction expansion. The absorbance maxima
are all between 580 and 750 nm consistent with the transient formation of a colored intermediate
noted in many Mallory photocyclizations.53 For example, the bathochromic absorption maxima
for DHPs 17, 18, and 19 (Chart 4) are reported at 603 nm, 448 nm, and 530 nm, respectively,
with extinction coefficients of approximately 10,000-12,000 M-1cm-1.54 The oscillator strengths
(f) for the DHPs embedded in helical domains (column 3) were 5 to approximately 9 times
smaller than those embedded in acene domains (column 6, Table 3).
Chart 4
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Table 3. Wavelength Maxima and Oscillator Strengths of Low Energy DHP UV/Vis
Absorbances.a
fc
fc
b
b
MAX
MAX
7
8
9
6
Ba
Ia
683
689
727
0.0622
0.0628
0.0527
C
La
J
663
704
662
0.3135
0.3325
0.4857
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
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59
60
Ka
7
Ba
Ia
688
696
732
0.0619
0.0631
0.0514
C
La
J
670
713
670
0.2946
0.3154
0.4500
Ka
8
Ba
Ia
717
730
743
0.0617
0.0629
0.0683
C
La
J
624
667
630
0.4144
0.4358
0.5821
Ka
9
Ba
Ia
713
720
730
0.0877
0.0888
0.1019
C
La
J
585
601
607
0.4764
0.5019
0.6746
Ka
a. Calculated at the TD-DFT/6-311+G(2d,p) level in toluene . b. Wavelength of lowest energy transition.
c. Oscillator strength.
DISCUSSION
As evident from examination of the interconversion diagram in Scheme 4 the yields of
the three products formed in the bis-Mallory photocyclizations are sensitive functions of the
concentrations of the DHPs ([B], [I], etc.) and of the rate constants for hydrogen abstraction,
kDHP (e.g. kI, kB, etc.). This is expressed mathematically in Eqns. 1, 2, and 3. The yield of 11 for
example is the product of the fraction of the precursor B formed in the blue region
and the
∅
퐵
fraction of I formed in the red region . The yield of 10 on the other hand has two terms since it
∅
퐼
can be formed in both the green and red region of Scheme 4.
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1
%ퟏퟎ = [∅퐿∅퐵 + ∅퐾∅퐶] 푥100
5
6
7
8
9
푘퐼[퐼]
푘퐵[퐵]
x100
x100
2
%ퟏퟏ = ∅ ∅ =
(
푘 [퐼] + 푘 [퐿])(푘 [퐵] + 푘 [퐶]
)
퐼 퐵
퐼
퐿
퐵
퐶
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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31
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57
58
59
60
푘퐽[퐽]
푘퐶[퐶]
3
%ퟏퟐ = ∅ ∅ =
(
푘 [퐽] + 푘 [퐾])(푘 [퐵] + 푘 [퐶]
Several experimental and structural variables can influence the concentrations of the
DHPs and the rate constants of hydrogen abstraction, kDHP, whose magnitudes dictate product
formation (Eqns. 1, 2, and 3). These include: (1) The position of the photostationary state
)
퐽 퐶
퐽
퐾
퐵
퐶
established between the two competing DHPs; (2) the relative stability of the competing DHPs;
⋇
(3) the relative stabilities of DHP radicals (
produced during hydrogen abstraction from the
퐴 )
competing DHPs; and (4) experimental variables such as reaction temperatures.
(1) The photostationary states established between the two competing DHPs in the
mutually inaccessible blue, red, and green regions of Scheme 4 are given by equations 4a, 4b,
and 4c, respectively. These equations are analogous to the equation for the photostationary state
reached upon irradiation of cis- and trans-alkenes55 but differs because it is the product of two
photostationary equilibria, for example L  G and G  I in the red region of Scheme 4. The
ratio, of the extinction coefficients, L /I, at the MAX of the HOMO  LUMO absorption peaks
for the DHPs shown in Table 3, are proportional to the ratio of oscillator strengths56 (Table 3)
when the band widths are the same. This relationship also implies that the ratio of DHP
concentrations will change with changing irradiation wavelength. This is consistent with the
observation of Fisher and coworkers53 who observed a strong wavelength dependence on the
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ratio of DHPX/DHPY during the Mallory photocyclization of 1,2-di-(2-naphthyl)cyclopentene.
(Scheme 8)
7
8
9
(
)(
)
)
휀퐶 휙퐴→퐵 휙퐶→퐴
[퐵]
[퐶]
4a
4b
4c
= ∙ (
)(
휙퐵→퐴 휙퐴→퐶
휀퐵
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
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49
50
51
52
53
54
55
56
57
58
59
60
휀퐿 (휙퐺→퐼)(휙퐿→퐺
)
[퐼]
[퐿] = ∙
휀퐼 (휙퐼→퐺)(휙퐺→퐿
)
휀퐽 (휙퐻→퐾)(휙퐽→퐻
)
)
[퐾]
[퐽]
= ∙
휀퐾 (휙퐾→퐻)(휙퐻→퐽
Scheme 8. DHP diastereomers formed during Mallory photocyclization of 1,2-di-(2-
naphthyl)cyclopentene.
(2) The relative stabilities of the competing DHPs in the blue (Ba-C), red (Ia-La), and
green (Ka-J) regions of the mechanism shown in Scheme 4 are compared in terms of both their
free energies in Table 1 and enthalpies in Table 4. This data reveals that the DHPs in the
reactions of 6 and 7 that are embedded in the helical domain, Ba, Ia, and Ka, and are the
precursors for formation of a helical product, are equal or perhaps slightly more stable than the
DHPs embedded in the acene domain, C, La, and J. (Columns 2 and 3 in Table 4). On the other
hand Mallory substrates 8 and 9 exhibit the opposite behavior with their acene-embedded DHPs,
C, La, and J, more stable than their helicene-embedded DHPs, Ba, Ia, and Ka. This stability
preference for C, La, and J in 9 is especially large (7.0-10.1 kcal/mol) and essentially precludes
all but a trace of Ba, Ia, and Ka at thermodynamic equilibrium.
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Table 4. Differences in Enthalpies and Free Energies of Formation and for Hydrogen
Abstraction from DHPs with Iodine atom.a
∆∆푯풐
풇 (6)b
∆∆푯풐
풇 (7)b
∆∆푯풐
풇 (8)b
∆∆푯풐
풇 (9)c
∆∆푯풐푹풙풏(ퟔ)풅
+1.21(+1.61)
+2.59(+2.72)
+0.42(+0.63)
∆∆푯풐푹풙풏(ퟗ)풅
-2.29(-2.89)
-2.58(-2.64)
-2.62(-2.59)
-2.6(-1.9)
-1.2(-0.6)
-1.9(-1.0)
-0.6(+0.2)
+2.3(+3.2)
+4.9(+5.5) +10.1(+11.1)
+2.2(+3.5) +7.0(+7.8)
+7.8(+8.3)
(Ba-C)
(Ia-La)
(Ka-J)
∆∆퐺표
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
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49
50
51
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54
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59
60
-1.0(-0.3) -0.3(+0.5)
a.
in parenthesis. b. In kcal/mol calculated at the B3LYP/6-311+G(2d,p), and, c. at the B3LYP/6-
푓
31G(d) computational level. d. Calculated using equation 5.
(3) The relative stabilities of DHP radicals formed in reactions with the iodine
atom/radical can also potentially play an important role in the rates of hydrogen abstraction. The
relative stability’s of radicals generated by abstraction of hydrogen from the terminal unsaturated
⋇
ring of the DHPs (Series
radicals) are given in Table 2. In the reaction of 6 the stability
퐴
differences between radicals formed from competing DHPs, Ba-C, Ia-La, and J-Ka are very small,
-0.4, + 2.1, and -0.4 kcal/mol respectively. In stark contrast, these energy differences, Ba-C, Ia-
La, and J-Ka, +5.4, +8.5, and -5.2 kcal/mol respectively, are much larger in the reaction of 9.
These values can be used in conjunction with equation 5 shown in Figure 9 to generate the
differences in the enthalpies of reaction for abstraction of the first hydrogen from the competing
DHPs with the iodine atom (Table 4 columns 5 and 6). In 6 these endothermic hydrogen
abstractions are energetically more favorable from its acene-embedded DHPs, C, La, and J,
while in substrate 9 they are more favorable from their helicene-embedded DHPs (Table 4
columns 6 and 7). In both cases, hydrogen abstraction is enthalpically more favorable from the
least stable set of DHPs.
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o
(DHP)
(DHP)
DHP
+
I
M HP
+
-
HI
HI
HRxn
o
DHP’
+
I
M HP’
+
HRxn
o (I )
Hf
o
Hof
Hof
(DHP) =
o (MHP ) +
Hf
o (HI)
Hf
(DHP) -
(DHP’) -
-
HRxn
10
11
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o
o (MHP’ ) +
Hf
o (HI)
Hf
o (I )
-
-
(DHP’) =
HRxn
Hf
o (DHP’)
Hf
(DHP)
Hof
=
(DHP’)
o (MHP’ )
Hf
o
HRxn
Hof
(MHP )
o
-
(DHP)
-
-
5
HRxn
DHP = Ba, Ia, and Ka ; DHP’ = C, La, and J
Figure 9. Equation 5 to Determine HoRxn for 6 and 9 (Table 4).
(4) Increasing temperature can: (a) enhance the rate of passage over the TS1 barrier on
the Mallory photocyclization S1 PES, (Figure 1) (b) it can increase the rate of thermal
decompositions of the DHPs, and (c) it can increase the rate of hydrogen abstraction by iodine
atom from the DHPs.
(a) Dulić and coworkers35 have examined the temperature dependence of passage over
the TS1 barrier for a series of cis-stilbene photochromic switches. The rate of approach to the
open/closed photostationary state is temperature independent at temperatures above 0oC where
most Mallory photocyclizations are conducted. At temperatures below 0oC the quantum yields
(e.g. A J , etc.) may exhibit small changes as a function of temperature, however, to a

large extent these changes are likely to cancel each other in the quantum yield ratios (e.g.
[(
)(C )]/[(
)(A C)], etc.) that are directly proportional to the steady state
A
A
A 
concentrations of the competing DHPs (Eqns. 4a, 4b, and 4c). trans-Stilbene but not cis-stilbene
also encounters an activation barrier on the way to the twisted phantom intermediate on its
geometric isomerization energy surface and as a result competitive fluorescence is observed in
solution for the trans- but not the cis-isomer.57 This barrier, however, is only 3.5 kcal/mol58 and,
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consequently, unlikely to influence the cis-/trans-stilbene photostationary state at temperatures
used for Mallory photocyclizations. It is also worthwhile to note that the 13/14 cis/trans
photostationary state is established early on the bis-Mallory photocyclization PES (Figure 2) and
is maintained throughout the reaction.
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(b) DHPs have been directly observed and their thermal decompositions have been
monitored.54 Lifetimes of the DHPs have also been measured but their accuracy, and what is
really being measured, is debatable because of the wide range of processes including oxidation
and rearrangements that contribute to their decompositions. Nevertheless, the thermal
decompositions of the competing DHP isomers in the blue, red, and green regions of the
interconversion diagram (Scheme 4) with the greatest internal energy are expected to be more
facile. The concerted thermal conrotatory openings of the DHPs reported here are not allowed,
however, their high-energy content (> 39 kcal/mol relative to their styrenyl precursor; Table 1)
provides a driving force for their homolytic cleavage initiated decompositions to regenerate the
Mallory substrates.
(c) In DHPs Ba, Ia, and Ka the two hydrogens available for abstraction differ in their
accessibility to the iodine atom and lead to radicals of very different thermodynamic stabilities
(Table 2). In contrast, both hydrogens in DHPs C, J, and La are equally accessible for
abstraction and lead to nearly iso-energetic radicals.
Consequently, the rate constants for
hydrogen abstraction will increase with increasing temperature more rapidly for DHPs C, J, and
La than for DHPs Ba, Ia, and Ka since they enjoy an Rln2 symmetry contribution to the entropy
of activation.
Reaction Mechanisms for Mallory Substrates 6, 7, 8, and 9. The bis-Mallory
photocyclizations of 6, 7, 8, and 9 are very complex reactions, (Scheme 4) and as discussed
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above, depend upon a myriad of experimental conditions and substrate structural features that
can influence their choice of reaction pathway. Detailed photophysical studies will be required
to unravel the precise mechanistic details for each of these bis-Mallory substrates. Nevertheless,
it is useful to qualitatively examine how the factors discussed above can influence the reaction
pathways and product distributions in these substrates.
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Mallory Photocyclization Substrates 6 and 7. At 0oC and below Mallory substrates 6 and 7
exclusively produce the helicene products 11a and 11b, respectively (Scheme 4). This is
consistent with preferential photostationary formations of B and I, since the ratio C/B and L/I
for 6 and 7, respectively, are equal to the ratios of the oscillator strengths and the smaller
oscillator strength of the DHP precursor to 11a and 11b (e.g. fC/fB(6) = (0.3135/0.0622) = 5.04
and fC/fB(7) = (0.2946/0.0619) = 4.76; Table 3) means that their concentrations build-up because
they absorbs less light. However, in order to attain [B]/[C] and [I][/[L] ratios of 20 or greater
(i.e. exclusive detection of 11a and 11b), it would also require that [(AB)(CA)]/[(BA)(AC)]
and [(GI)(LG)]/[(IG)(GL)] (See Eqn. 4a,b) have values of approximately 4 to 5. We argue
that values of 4 to 5 are reasonable and are consistent with the Laarhoven effect that suggests
(vide supra) that bonding occurs between carbon atoms where the “residual affinity” for bond
formation is the greatest, which in many cases leads to preferential helicene formation.
Consequently, the Laarhoven (kinetic) effect in the photocyclizations of 6 and 7 requires the blue
quantum yields, (AB) and (GI), involving formation or extension of a helical structure to be
larger than the red quantum yields, (BA) and (IG) that involve loss or decrease of helical
structure. This observation, coupled with the fact that the black quantum yields for the photo-
reversible formation of DHPs, that do not involve formation, extension, loss, or decrease in
helical structure, are more comparable in magnitude, (CA) ≈ (AC) and (LG) ≈ (GL) support
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our contention that values of 4 to 5 for the ratio of quantum yields in equations 4a and 4b are
reasonable. As the temperature is raised the photostationary state favoring helicene formation is
not established, instead, for example, in the case of 6, the 3.65 kcal/mol and 1.94 kcal/mol (Table
4) energetically preferred hydrogen abstractions from DHPs C and J, respectively, serve to shift
the BC and KJ equilibrium towards these more stable DHPs and ultimately produce the high
temperature products 12a and 12b.
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Mallory Photocyclization Substrate 8. At 0oC the helicene, 11c, is the exclusive product of the
Mallory photocyclization of 8. However, the concentration of the dibenzopentaphene product,
12c, becomes approximately equal to the concentration of 11c at 30oC to 40oC at lower
temperatures than observed for 6 and 7 (Figure 3). This is consistent with the observation that
DHPs, C and J, on the way to 12c are 3.2 and 3.5 kcal/mol more stable than their competitively
formed DHPs Ba and Ka, respectively (Tables 1 and 4). Consequently, the rates of hydrogen
abstraction, kC[C][I.] and kJ[J][I.], because of the higher concentrations of C and J, can more
effectively compete with establishment of the photostationary state and enhance the amount of
12c formed in the photocyclization. In comparison, in the reactions of 6 and 7, DHPs, C and J,
on the way to 12a and 12b, respectively, are far less stable relative to their competitively formed
DHPs Ba and Ka (Tables 1 and 4). As a consequence formation of 12a and 12b do not compete
as effectively with formation of 11a and 11b and a higher temperature is required for their
concentrations to become equal (i.e. [12a] = [11a] and [12b] = [11b]).
Mallory Photocyclization Substrate 9. Photocyclization of 9 at -30oC unexpectedly generated
benzo[k]naphtho[1,2-a]tetraphene, 10d, as the only product (Figure 3d). In contrast to the other
two possible regioisomers, 11d and 12d, it can form either in the red or green regions of the
interconversion diagram shown in Scheme 4. However, irradiation of 9 is anticipated to
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predominantly generate DHP C because it is 9 kcal/mol more stable than Ba (Table 1).
Nevertheless, at -30oC hydrogen abstraction from DHP C does not enjoy as large a symmetry
advantage (TS = Tln2) as it would have at higher temperatures, and its lifetime is extended
sufficiently to allow formation of the photostationary state and DHP Ba. Hydrogen abstraction to
form G followed by exclusive photocyclization to form La, driven by its very large
thermodynamic stability (11.6 kcal/mol) relative to Ia, (Table 1) subsequently generates the
observed product, 10d. (Note: for comparison Ia and La formed in the photocyclizations of 6 and
7 (Table 1) are nearly identical in energy) This scenario is supported by the fact that at slightly
higher temperatures the less stable Ia is increasingly populated and 11d forms but then decreases
(Figure 3) as it succumbs to the thermodynamic stability’s of DHPs C, and J on the way to the
thermodynamically most stable regioisomer 12d.
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CONCLUSIONS
Mallory substrates, 6, 7, and 8, react at low temperatures to exclusively produce
their helicene products, 11a, 11b, and 11c, despite the fact that their regioisomers, 10a,
10b, and 10c, that can form competitively are 11.0, 10.7, and 12.2 kcal/mol more stable.
This unusual, and synthetically useful, observation can be attributed to three effects: (1)
the energies of the DHP precursors to these two sets of regioisomers are much closer in
energy (
) than the 10.7-12.2 kcal/mol separating the energies of the final
≤ 5.5 푘푐푎푙/푚표푙
regioisomeric products; (2) the extinction coefficients of the DHP precursors to the
helicenes are smaller than the extinction coefficients of DHP precursors to their
regioisomers by a factor of 4.8 to 9.2; and (3) at low temperatures thermal decompositions
of intermediates are suppressed. Consequently, these effects allow attainment of the
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