An oxidation induced by potassium metal. Studies on the anionic cyclodehydrogenation of 1,1′-binaphthyl to perylene

Article abstract of DOI:10.1021/jo101635z

Oxidative cyclization of 1,1′-binaphthyl (1) to perylene (2) can be achieved in essentially quantitative yield by the action of three or more equivalents of potassium metal in hot tetrahydrofuran. An overall reaction mechanism is proposed that accounts for all of the experimental observations reported by previous investigators and those from the present studies. The trans-6a,6b-dihydroperylene dianion (6^2-) is believed to be the pivotal intermediate from which H₂ is lost. A radical chain reaction involving free hydrogen atoms (H?) in the two-step propagation cycle is proposed to explain the formation of H₂ from 6^2-. Anionic cyclodehydrogenations of this sort are complementary to those performed under strongly acidic/oxidizing conditions, photochemically, or thermally (flash vacuum pyrolysis), and a better understanding of how they occur, together with the optimized synthetic protocol reported here, should encourage their wider use in organic synthesis.

Full text of DOI:10.1021/jo101635z

pubs.acs.org/joc
An Oxidation Induced by Potassium Metal. Studies on the Anionic
Cyclodehydrogenation of 1,10-Binaphthyl to Perylene
Michel Rickhaus,†,‡ Anthony P. Belanger,† Hermann A. Wegner,‡ and
Lawrence T. Scott*,†
†Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill,
Massachusetts 02467-3860, United States, and Department of Organic Chemistry,
University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
‡
lawrence.scott@bc.edu
Received August 18, 2010
Oxidative cyclization of 1,10-binaphthyl (1) to perylene (2) can be achieved in essentially quantitative
yield by the action of three or more equivalents of potassium metal in hot tetrahydrofuran. An overall
reaction mechanism is proposed that accounts for all of the experimental observations reported by
previous investigators and those from the present studies. The trans-6a,6b-dihydroperylene dianion
(62-) is believed to be the pivotal intermediate from which H2 is lost. A radical chain reaction involving
free hydrogen atoms (H•) in the two-step propagation cycle is proposed to explain the formation of H2
from 62-. Anionic cyclodehydrogenations of this sort are complementary to those performed under
strongly acidic/oxidizing conditions, photochemically, or thermally (flash vacuum pyrolysis), and a
better understanding of how they occur, together with the optimized synthetic protocol reported here,
should encourage their wider use in organic synthesis.
Introduction
homocoupling of unfunctionalized arenes, on the other
hand, can often be achieved by the oxidative dimerization
of electron-rich arenes, thanks to the pioneering work of
Scholl, Kovacic, and others,2,3 and intramolecular adapta-
tions of these oxidative methods have been pushed to
Methods for the selective formation of unsymmetrical
aryl-aryl bonds between reaction partners that require no
functionality other than C-H bonds at the carbon atoms to
be joined constitute some of the most exciting new advances
in the field of transition metal catalyzed cross-coupling reac-
tions.1 The formation of symmetrical aryl-aryl bonds by
€
spectacular heights by Mullen et al. for the synthesis of very
large graphene substructures.4 Other noncatalytic, intramo-
lecular methods include the oxidative photocyclization of
stilbenes to phenanthrenes (the Mallory reaction)5 and, in
special circumstances, thermal cyclodehydrogenations by
(1) (a) Stuart, D. R.; Fagnou, K. Science 2007, 316, 1172. (b) Campeau,
L.-C.; Stuart, D. R.; Fagnou, K. Aldrichimica Acta 2007, 40, 35. (c) McGlacken,
G. P.; Bateman, L. M. Chem. Soc. Rev. 2009, 38, 2447.
€
(4) (a) Watson, M. D.; Fechtenkoetter, A.; Mullen, K. Chem. Rev. 2001,
€
101, 1267. (b) Wu, J.; Mullen, K. In Carbon-Rich Compounds; Haley, M. M.,
Tykwinski, R. R., Eds.; Wiley-VCH: Weinheim, 2006; p 90. (c) Mueller, S.;
(2) (a) Musgrave, O. C. Chem. Rev. 1969, 69, 499. (b) Kovacic, P.; Jones,
M. B. Chem. Rev. 1987, 87, 357. (c) McKillop, A.; Turrell, A. G.; Taylor,
E. C. J. Org. Chem. 1977, 42, 764. (d) McKillop, A.; Turrell, A. G.; Young,
D. W.; Taylor, E. C. J. Am. Chem. Soc. 1980, 102, 6504.
€
Mullen, K. Philos. Trans. R. Soc., A 2007, 365, 1453. (d) Wu, J.; Pisula, W.;
€ €
Mullen, K. Chem. Rev. 2007, 107, 718. (e) Zhi, L.; Mullen, K. J. Mater. Chem.
€
2008, 18, 1472. (f) Feng, X.; Pisula, W.; Mullen, K. Pure Appl. Chem. 2009,
81, 2203.
(5) (a) Mallory, F. B.; Mallory, C. W. Org. React. 1984, 30, 1. (b) Mallory,
F. B.; Butler, K. E.; Evans, A. C.; Brondyke, E. J.; Mallory, C. W.; Yang, C.;
Ellenstein, A. J. Am. Chem. Soc. 1997, 119, 2119. (c) Mallory, F. B.; Butler,
K. E.; Berube, A.; Luzik, E. D.; Mallory, C. W.; Brondyke, E. J.; Hiremath,
R.; Ngo, P.; Carroll, P. J. Tetrahedron 2001, 57, 3715.
(3) See also: (a) Rempala, P.; Kroulik, J.; King, B. T. J. Am. Chem. Soc.
2004, 126, 15002. (b) Rempala, P.; Kroulik, J.; King, B. T. J. Org. Chem.
2006, 71, 5067. (c) King, B. T.; Kroulik, J.; Robertson, C. R.; Rempala, P.;
Hilton, C. L.; Korinek, J. D.; Gortari, L. M. J. Org. Chem. 2007, 72, 2279.
(d) Ormsby, J. L.; Black, T. D.; Hilton, C. L.; Bharat; King, B. T. Tetra-
hedron 2008, 64, 11370. (e) Zhai, L.; Shukla, R.; Rathore, R. Org. Lett. 2009,
11, 3474. (f) Zhai, L.; Shukla, R.; Wadumethrige, S. H.; Rathore, R. J. Org.
Chem. 2010, 75, 4748.
7358 J. Org. Chem. 2010, 75, 7358–7364
Published on Web 10/06/2010
DOI: 10.1021/jo101635z
r
2010 American Chemical Society

Rickhaus et al.
JOCArticle
making it available for a broader scope of substrates and
facilitating its application. Herein, we describe studies on the
archetypical system in Figure 1 that have provided deeper
mechanistic insights and an optimized synthetic protocol.
Background
The cyclization of 1,10-binaphthyl (1) to perylene (2) by
alkali metals was discovered accidentally in 1967 by Solo-
dovnikov et al. during their attempts to record the ESR
spectrum of the 1,10-binaphthyl radical anion in solution.7,8
Because the work was published originally in Russian
and the later English translation is not cited by Chemical
Abstracts, the experimental data described therein, unfortu-
nately, have been mostly overlooked for the last 40 years; the
paper has been cited only three times. To rectify this situa-
tion, we summarize here some of the key findings that we
have gleaned from our own translation of the early Russian
paper; the summary in Chemical Abstracts is not entirely
accurate.
Solodovnikov et al. reduced 1,10-binaphthyl (1) in 1,2-
dimethoxyethane (0.1 M) with an excess of potassium metal
under vacuum at room temperature. The ESR signal for the
1,10-binaphthyl radical anion (1•-) grew in rapidly during the
first hour but then began to diminish slowly, disappearing
essentially completely over a period of 48 h. After 72 h, the
contents of the ampule were exposed to oxygen, and perylene
(2) was isolated in 39% yield. Some other hydrocarbon
products, detected by TLC and tentatively identified as
partially hydrogenated 1,10-binaphthyl, were also obtained,
but the amount of recovered 1,10-binaphthyl, if any, was not
specified. When the experiment was repeated and the reac-
tion mixture was filtered to remove the potassium metal after
1 h, the point at which the ESR signal had reached its maxi-
mum intensity, immediate exposure of the filtrate to oxygen
produced only a 0.3% yield of perylene (2). On the other
hand, allowing the filtrate to stand at room temperature for
72 h, after removal of the potassium at the 1 h point, pro-
duced a 10% yield of perylene (2). These results indicate that
the cyclization and rearomatization process is slow at room
temperature and that it does not require the continued pres-
ence of potassium metal, once the solution contains a
significant concentration of organic radical anions. The yield
of perylene after 72 h was lower, however, if the solution was
not left in contact with excess potassium after the first hour
(10% vs 39%).
Solodovnikov et al. also report the formation of hydrogen
gas in this reaction. They monitored the production of H2 by
GC analysis and the formation of perylene by UV-vis
spectroscopy. Figure 2 reproduces the graph they published
in 1968 that shows the percent yields of H2 gas and perylene
(2), as well as the growth and decline of the ESR signal, as a
function of time. It is noteworthy that (i) the amount of H2
detected was always significantly lower than the amount of
perylene and (ii) the formation of both products leveled off
with time; after 72 h, the yield of H2 reached 24.6%, whereas
the yield of perylene (2) reached 39%.
As the reaction proceeded, color changes were observed
that gave a crude indication of the dominant species present.
In the beginning, a green color developed that corresponded
to the radical anion of 1,10-binaphthyl (1•-); the radical
anion of naphthalene itself is also green. The solution then
FIGURE 1. Anionic cyclodehydrogenation of 1,10-binaphthyl (1)
to perylene (2) by alkali metals.
flash vacuum pyrolysis.6 Less well-known than all of these
methods, however, is the anionic cyclodehydrogenation of
aromatic hydrocarbons, for which the cyclization of 1,10-
binaphthyl (1) to perylene (2) by alkali metals serves as the
classic example (Figure 1).7,8
None of the other methods mentioned above convert 1,10-
binaphthyl (1) to perylene (2) very efficiently, if at all, and this
makes the anionic cyclodehydrogenation method unique,
complementary, and worthy of attention.9 Most intriguing is
the seemingly incongruous fact that the starting material (1 =
C20H14) is being oxidized (2 = C20H12), whereas the reagents
used to induce this oxidation are alkali metals, which arguably
qualify as some of the strongest reducing agents known. The
alkali metal is also being oxidized during the course of this
reaction, so what is being reduced? The inescapable conclusion
is that the two lost hydrogen atoms must end up reduced to a
lower oxidation state, either H2 or hydride.
Almost nothing is known about the mechanistic details of
this reaction.10,11 Even the stoichiometry is uncertain, so the
metal is generally just used in large excess. Because this
reaction is so poorly understood and the yields rarely exceed
50%,11 the method remains underutilized in synthesis. The
most prominent application is probably the syntheses of
€
12
rylenes by Mullen et al., in which potassium metal was
used to “zip up” specifically designed oligomers of 1,4-linked
naphthalenes. A knowledge of the mechanism operating, the
intermediates involved, and the optimum conditions to use
would allow chemists to tame this rather obstreperous reaction,
(6) (a) Clar, E. Polycyclic Hydrocarbons; Academic Press: New York,
1964. (b) Scott, L. T.; Bratcher, M. S.; Hagen, S. J. Am. Chem. Soc. 1996, 118,
8743. (c) Hagen, S.; Bratcher, M. S.; Erickson, M. S.; Zimmermann, G.;
Scott, L. T. Angew. Chem., Int. Ed. 1997, 36, 406. (d) Scott, L. T.; Bronstein,
H. E.; Preda, D. V.; Ansems, R. B. M.; Bratcher, M. S.; Hagen, S. Pure Appl.
Chem. 1999, 71, 209. (e) Boorum, M. M.; Vasil’ev, Y. V.; Drewello, T.; Scott,
L. T. Science 2001, 294, 828. (f) Scott, L. T.; Boorum, M. M.; McMahon,
B. J.; Hagen, S.; Mack, J.; Blank, J.; Wegner, H.; de Meijere, A. Science 2002,
295, 1500. (g) Scott, L. T. Angew. Chem., Int. Ed. 2004, 43, 4994.
(h) Tsefrikas, V. M.; Scott, L. T. Chem. Rev. 2006, 106, 4868. (i) Xue, X.;
Scott, L. T. Org. Lett. 2007, 9, 3937.
(7) (a) Solodovnikov, S. P.; Zaks, Y. B.; Ioffe, S. T.; Kabachnik, M. I.
Radiospektrosk. Kvantovokhim. Metody Strukt. Issled. 1967, 106; Chem.
Abstr. accession no. 1969:28235; CAN 70:28235. (b) Solodovnikov, S. P.;
Ioffe, S. T.; Zaks, Y. B.; Kabachnik, M. I. Izv. Akad. Nauk SSSR, Ser. Khim.
1968, 442; Chem. Abstr. accession no. 1968:476168; CAN 69:76168; English
translation Bull. Acad. Sci. USSR, Div. Chem. Sci. 1968, 442.
(8) See also: Gilman, H.; Brannen, C. G. J. Am. Chem. Soc. 1949, 71, 657.
(9) For related anionic cyclodehydrogenations, see: (a) Tamarkin, D.;
Benny, D.; Rabinovitz, M. Angew. Chem. 1984, 96, 594. (b) Tamarkin, D.;
Cohen, Y.; Rabinovitz, M. Synthesis 1987, 196. (c) Rabinovitz, M.; Tamarkin,
D. Synth. Met. 1988, 23, 487. (d) Deselets, D.; Kazmaier, P. M.; Burt, R. A.;
Hamer, G. K. Can. J. Chem. 1995, 73, 325. (e) Yao, J. H.; Chi, C.; Wu, J.; Loh,
K.-P. Chem.-Eur. J. 2009, 15, 9299.
(10) (a) Hnoosh, M. H.; Zingaro, R. A. J. Am. Chem. Soc. 1970, 92, 4388.
(b) Eisenstein, O.; Mazaleyrat, J. P.; Tordeux, M.; Welvart, Z. J. Am. Chem.
Soc. 1977, 99, 2230.
(11) (a) Michel, P.; Moradpour, A. Synthesis 1988, 894. (b) Benshafrut,
€
R.; Hoffman, R. E.; Rabinovitz, M.; Mullen, K. J. Org. Chem. 1999, 64, 644.
€
€
(12) (a) Bohnen, A.; Koch, K. H.; Luttke, W.; Mullen, K. Angew. Chem.,
Int. Ed. 1990, 29, 525. (b) Scherf, U.; Muellen, K. Synthesis 1992, 23.
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FIGURE 3. Anionic cyclodehydrogenation of [5]helicene (3) by
alkali metals.14
FIGURE 2. Change in ESR signal intensity (relative units) and
yields of perylene (2) and H2 over time in the cyclodehydrogenation
of 1,10-binaphthyl (1) by potassium metal at room temperature.7b
step, in which two hydrogen atoms are lost, could be acceler-
ated by warming the reaction mixture to 25-30 °C.
Two important additional conclusions can be drawn from
this report: (i) the two C-H bonds remain intact until after
the C-C bond has been formed and two electrons have been
added, and (ii) rearomatization by rupture of the two C-H
bonds is slower than the initial cyclization (at least in this
loaded case) but occurs under an inert atmosphere, before
the solution is exposed to oxygen. Against this background,
we now add our observations.
turned a wine-red color, which Solodovnikov et al. attributed
to the dianion of 1,10-binaphthyl (12-); however, the ESR
spectrum of the 1,10-binaphthyl radical anion (1•-) could also
still be recorded at this stage. After 1 d, the solution became
purple, which the authorsattributed tothe dianionof perylene
(22-); by this time, the ESR signal had dropped to 10% of its
maximum amplitude. A UV-vis absorption spectrum of the
purple solution matched that reported in the literature for the
dianion of perylene (22-).13 Heating the wine-red solution
from 20 to 100 °C caused it to turn purple and caused the ESR
spectrum to change from that of the 1,10-binaphthyl radical
anion (1•-) to that of the perylene radical anion (2•-). Expos-
ing purple solutions of the perylene dianion (22-) to oxygen
causedthecolor tochangefirst toblue(perylene radicalanion,
Results and Discussion
From the outset, we reasoned that this reaction should be
conducted at elevated temperatures so that it could be pushed
to completion in less than a day. We began by screening a
few solvents, using both sodium metal and potassium metal.
Owing to the high reactivity of alkali metals, only ethers,
hydrocarbons, and tertiary amine solvents were examined.
Thus, we heated 1,10-binaphthyl (1) overnight in various
solvents with excess metal and compared the yields of perylene
(2) formed under the different conditions. Potassium metal in
THF at 80 °C in a pressure vessel proved far superior to all of
the other combinations examined (73% yield),15 so we settled
on potassium and THF as the metal and solvent for further
studies. An operating temperature above the melting point of
potassium metal (63 °C) in a pressure vessel with constant
stirring ensures a large area of clean metal surface throughout
the course of the reaction, and this undoubtedly contributes to
the success of these conditions. On a small scale, the anionic
products were oxidized to the neutral hydrocarbons by ex-
posure to oxygen in the workup, but elemental iodine was
used as the oxidizing agent for larger scale reactions.
2
•-) and then to yellow (neutral perylene, 2). It seems safe to
conclude that electron transfer reactions among the species
present inthesolution atvariousstages allow interconversions
between the radical anion and dianion of 1,10-binaphthyl
(1•-þ e- / 12-) and likewise between the radical anion and
dianion of perylene (2•- þ e- / 22-).
In 1992, a careful NMR study of the closely related
cyclization shown in Figure 3 was reported by Ayalon and
Rabinovitz.14 In this case, [5]helicene (3) was reduced in-
dependently with sodium metal and with lithium metal in
THF-d8 at -33 °C over periods of several days to give the
dianion of benzo[ghi]perylene (52-). Exposure of the final
reaction mixture to oxygen gave the neutral hydrocarbon,
benzo[ghi]perylene (5). By continuous NMR monitoring, the
growth and disappearance of even-electron species could be
observed; no ESR or UV-vis absorption spectra were re-
ported. As the first paramagnetic species formed (3•-), the
NMR signals for 3 disappeared. Then, over time, the NMR
signals for 42- grew in; no signals were ever seen for the dianion
of the open [5]helicene (32-). After prolonged exposure of the
solution to the alkali metal, the NMR spectrum of the dihydro-
benzo[ghi]perylene dianion (42-) was slowly replaced by that
of the benzo[ghi]perylene dianion (52-). This rearomatization
With this basic protocol in hand, the optimization was
then turned toward determining the minimum number of
equivalents of potassium metal required to effect complete
conversion. An excess of potassium metal (5 equiv)16 was
(15) Other conditions examined (metal, solvent, temperature) all gave
yields of 10% or less: (a) potassium, 1,2-dimethoxyethane, 80 °C;
(b) potassium, diglyme, 80 °C; (c) potassium, toluene, 80 °C; (d) sodium,
THF, 66 °C; (e) sodium, diglyme, 162 °C; (f) sodium, tetramethylethylene-
diamine (TMEDA), 120 °C; (g) sodium, toluene, 110 °C.
(13) Balk, P.; Hoijtink, G. J.; Schreurs, J. W. H. Recl. Trav. Chim. Pays-
Bas Belg. 1957, 76, 813.
(14) Ayalon, A.; Rabinovitz, M. Tetrahedron Lett. 1992, 33, 2395.
(16) The maximum amount of potassium that could be consumed in this
reaction should be 4 equiv, even if all of the H2 were reduced to KH.17
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TABLE 1. Screening of Different Equivalents of Potassium Used
to Cyclize 1,10-Binaphthyl (1) to Perylene (2) at 85 °C in Tetrahydrofuran
in a Pressure Vessel
TABLE 2. Screening of Different Temperatures for Cyclization
of 1,10-Binaphthyl (1) Using 3 equiv of Potassium in Tetrahydrofuran
time (d)
temp (°C)
conversion (%)a
equiv of potassium
time (d)
conversion (%)a
1
1
1
1
3
1
95b
85b
70b
55b
25
quant
90-quant
45
34
15
8
5.0
3.0
3.0
2.5
2.5
2.2
2.2
2.0
1.8
1.8
1.0
0.5
0.5
2
2
1
7
1
7
4
3
5
4
6
12
7
quant
quant
90-quantb,d
quantb
0-78d
quant
64-70c,d
58
25
aConversion based on NMR integration. bReactions run in a pressure
vessel.
58
48
28-40d
5
0
aConversion based on NMR integration. bQuantitative yield isolated.
c70% yield isolated. dRanges resulting from multiple runs.
chosen as the starting point, and the stoichiometry was
progressively adjusted. With 5 equiv of potassium metal in a
pressure vessel at 85 °C in THF for 2 d, the cyclodehydro-
genation of 1,10-binaphthyl (1) to perylene (2) is remarkably
clean and essentially quantitative; no starting material re-
mains, and no other significant products are detected by
NMR analysis of the crude material obtained after work-
up. Table 1 summarizes the results from lower amounts of
potassium at various reaction times, all in THF at 85 °C in
a pressure vessel.
We find that the conversion of 1 to 2 is still quantitative
with 3 equiv of potassium after 2 d in THF at 85 °C.
Shortening the reaction time to 1 d with 3 equiv of potassium
still gives quantitative conversion most of the time, but small
amounts of unconverted starting material were detected
in some experiments run under these conditions. With lesser
amounts of potassium (2.5 and 2.2 equiv), the conversion
of 1 to 2 can still be pushed to completion after 7 d in THF
at 85 °C in a pressure vessel, but the reaction is definitely
slower than with 3 equiv of potassium under the same
conditions.
Although fewer than 3 equiv of potassium may be im-
practical for this reaction, it is noteworthy that even 0.5 equiv
of potassium metal is enough to produce small amounts
(5%) of 2 from 1 at 85 °C in THF over a period of 12 d.
Finally, the temperature dependence of the reaction was
investigated, all with 3 equiv of potassium metal in THF in a
pressure vessel (Table 2). A temperature of 95 °C was found
to be the most reliable for ensuring 100% conversion of 1 to 2
in 1 d, but the NMR spectrum of the crude product was not
as clean as that from the 85 °C reactions. Like Solodovnikov
et al., we see the cyclodehydrogenation occurring only rela-
tively slowly at room temperature. The conditions we re-
commend as optimum for converting 1 to 2 are 3.0-3.5 equiv
of potassium metal in THF in a pressure vessel at 85 °C for
12 h (see Experimental Section).
The mechanism by which 1,10-binaphthyl (1) is converted
into the dianion of perylene (22-) by potassium metal
requires two single-electron reductions, one C-C bond-
forming step, two C-H bond-breaking steps, and a H-H
bond-forming step. In principle, the two C-H bond-break-
ing steps and the H-H bond-forming step could all be
concerted, but they may not be; the order of the steps has
never been determined.
FIGURE 4. First steps in the mechanism of the anionic cyclodehy-
drogenation of 1,10-binaphthyl (1) to perylene (2) by alkali metals.
One can safely assume that the first step is a single-electron
reduction of 1 to produce the radical anion 1•-. This odd-
electron intermediate might then close spontaneously to the
dihydroperylene anion radical (6•-), or it might live long
enough to accept a second electron and get reduced to the
dianion of 1,10-binaphthyl (12-). Cyclization of 12- would
give the dianion of dihydroperylene (62-), whereas the same
intermediate could be formed alternatively by single-electron
reduction of 6•- (Figure 4).
Whether the C-C bond forming step occurs at the anion
radical stage (path A) or at the dianion stage (path B) is
uncertain. Attempts to answer this question by molecular
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Rickhaus et al.
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FIGURE 5. A plausible mechanism for the trans-to-cis isomeriza-
tion of 62- by two symmetry-allowed suprafacial [1,4]-shifts of
hydrogen.
orbital calculations would likely be futile, because the partially
solvated counterions in such species are bound to play a signi-
ficant role that is difficult to model. Fortunately, from a broad
perspective, the order of the steps for the second electron addi-
tion and the cyclization is relatively inconsequential. The fact
that this reaction can be driven to 100% conversion does indi-
cate, however, that the dianion of 1,10-binaphthyl (12-, pre-
sumed by Solodovnikov to be the red-wine colored species)
cannot be a mechanistic dead end; if 12- is unable to cyclize by
path B, then it must be able to return to the radical anion 1•-
by donating one of its electrons to another species in solution
and cyclize by path A. All of the electron transfer steps and the
cyclization steps in Figure 4 should be reversible, and the
intermediates in this manifold are removed irreversibly only
FIGURE 6. Highest occupied molecular orbital (HOMO, B3LYP/
6-31G*) of perylene dianion (22-).
after C-H bonds begin to be broken as 62- goes to 22-
.
The mechanism by which the cyclized dianion 62- loses the
two hydrogen atoms is more obscure. How and when is the
new H-H bond formed? The direct observation of H2 by
Solodovnikov et al. proves that the two hydrogen atoms are
lost as H2 and not as 2 KH, but there is no evidence that the
two hydrogen atoms in the observed H2 both originated from
the same molecule. Ayalon and Rabinovitz established that
the hydrogen atoms on the newly joined carbon atoms are
oriented trans with respect to one another;14 thus, a concerted
loss of H2 from dianion 62- appears geometrically impossible.
Figure 5 outlines a plausible pathway for the isomerization
of trans-62- to cis-62- by two symmetry-allowed suprafacial
[1,4]-shifts of hydrogen over the allylic anion moiety. A σ2s þ
σ2s pericyclic loss of H2 from cis-62- is symmetry-forbidden
in the ground state, however, because the HOMO of the
perylene dianion is symmetric with respect to the plane
between the two carbon atoms from which the hydrogens
are lost (Figure 6).
FIGURE 7. Two-step propagation cycle in the proposed radical
chain mechanism for conversion of the dihydroperylene dianion
(62-) to the perylene dianion (22-) and H2.
We propose a radical chain mechanism involving free hydro-
gen atoms (H•) to account for conversion of the cyclized dia-
nion 62- to H2 and the perylene dianion (22-). Figure 7 depicts
the two steps of our proposed propagation cycle. Thus, ab-
straction of a hydrogen atom from 62- by a free hydrogen atom
(H•) would result in the formation of a very strong H-H bond
(104 kcal/mol) and the rearomatization of one of the benzene
rings at the expense of a very weak C-H bond (tertiary and
conjugated with a very electron-rich π-system). This step should
be strongly exothermic. Subsequent rearomatization of the
other benzene ring by homolysis of the second (now even
weaker) C-H bond would deliver the final product (22-) and
release a new hydrogen atom (H•) to carry on the next cycle.
At first, we did not entertain any mechanisms involving
free hydrogen atoms (H•), because we assumed that such a
reactive species would simply abstract an R-hydrogen from
THF. On further consideration, however, we realized that
even the great abundance of THF is probably not enough
to overcome the far greater hydrogen donor ability expected
for 62-. Furthermore, the THF R-radical, if formed, could
likewise abstract a hydrogen atom from 62- and thereby carry
on the chain reaction. As with all radical chain reactions,
chain termination would result only from infrequent radical-
radical encounters.
Initiation of this chain reaction requires a source of either
free hydrogen atoms (H•) or the other chain-carrying inter-
mediate (7•2-). One source of these initiators could be the
thermal homolysis of one of the very weak C-H bonds in
6
2-, a process that would actually give both 7•2- and a free
hydrogen atom (H•), thereby initiating two independent
chain reactions simultaneously.
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Rickhaus et al.
JOCArticle
after 3 d at room temperature drops from 39% to only 10% if
the potassium is removed from the reaction mixture by
filtration after the first hour? According to Solodovnikov’s
ESR spectrum, the concentration of the initial radical anion
(1•-, although possibly already closed, 6•-) had reached its
maximum when the potassium was removed. Before any
C-H bonds could be broken, however, the radical anion
would need to have been converted to 62- by acquiring a
second electron. In the absence of potassium metal, genera-
tion of dianions 12- and 62- could be accomplished by dis-
proportionation of either 1•- or 6•-, and the loss of H2 would
then produce perylene dianion (22-). Conversely, if the
solution were still in contact with potassium metal, all of
the starting material would automatically be reduced to the
closed dianion (62-) by the potassium, thereby accelerating
perylene production, which would then not be dependent on
disproportionation reactions of radical anions to generate
the pivotal intermediate 62-
.
Similar arguments can account for our observation that
1,10-binaphthyl (1) can be cyclized to perylene (2) even with as
little as 0.5 equiv of potassium metal, albeit only in low
conversion at elevated temperatures over many days (Table 1).
One final comment is in order concerning the hydrogen-
abstracting ability of the anionic species involved in this
reaction. When the reactions are run in dry THF under a
nitrogen atmosphere and are then quenched with either dry
oxygen or iodine, we obtain essentially no Birch reduction
products. On the other hand, in reaction mixtures that come
into contact with an adventitious proton source prior to
completion of the oxidation, we see varying amounts of 3,4-
dihydro-1,10-binaphthyl (11),18 unless the starting material
has been converted entirely to perylene dianion (22-). Mass
spectral analysis of the 11 obtained from a reduction of 1,10-
binaphthyl (1) run in THF-d8 showed no evidence for any
deuterium in the 11, so none of the organic intermediates
involved are capable of abstracting hydrogens, either as
protons or as hydrogen atoms, from THF.
FIGURE 8. Published mechanism for the reduction of H2 to NaH
by sodium naphthalenide.17
Once the first H2 is formed, more free hydrogen atoms (H•)
will be generated in this reaction mixture from a background
reaction that must also be mentioned. For many decades, it
has been known that the radical anions of aromatic hydro-
carbons can slowly reduce H2 (e.g., sodium naphthaleninde
1
þ /2H2 f naphthalene þ NaH).17 The mechanism for this
reaction has been studied, and the intermediacy of anion 9-
has been verified experimentally (Figure 8).17 Presumably,
many of the anionic species present in the cyclization of 1 to
22- are also capable of reducing H2, and that background
reaction could provide another source of free hydrogen
atoms (H•) to initiate the radical chain reaction we have
proposed.
The operation of this background reaction can also explain
why the yield of H2 reported by Solodovnikov et al. never
equaled the yield of perylene; as perylene and H2 were being
formed in their experiments, the amount of H2 was concur-
rently being depleted by reduction to KH throughout the
course of the reaction. The fact that the H2 level rises
continuously during this reaction (Figure 2), however, indi-
cates that the reduction of H2 is slow compared to the rate of
cyclization and rearomatization. It is intriguing to note that
H2 is serving as an oxidizing agent in this transformation.
If the reduction of H2 were fast compared to the rate of
cyclization and rearomatization, then H2 would not accu-
mulate, and the overall stoichiometry for the cyclodehydro-
genation would be
4K þ 1 f 22 - =2Kþ þ 2KH
We have found that 2.2 equiv of potassium metal is
sufficient to drive the formation of perylene to completion
(Table 1), and this would not be possible if a 4:1 stoichiom-
etry were required.
Conclusions
Oxidative cyclization of 1,10-binaphthyl (1) to perylene (2)
can be achieved in essentially quantitative yield by heating 1
with three or more equivalents of potassium metal in THF
at 85 °C in a pressure vessel overnight. The reaction can be
driven to completion at the same temperature with as little as
2.2 equiv of potassium, but it then requires much longer
times (4-7 days). The use of sodium or potassium in various
other solvents proved inferior. An overall reaction mechan-
ism is proposed that accounts for all of the experimental
observations reported by Solodovnikov et al., by Ayalon and
Rabinovitz, and from this study. The trans-dihydroperylene
How does our mechanistic proposal account for the ob-
servation by Solodovnikov et al. that the yield of perylene
(17) Bank, S.; Lois, T. A.; Prislopski, M. C. J. Am. Chem. Soc. 1969, 91,
5407 and references therein.
(18) Dal Zotto, C.; Wehbe, J.; Virieux, D.; Campagne, J.-M. Synlett 2008,
2033.
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Rickhaus et al.
JOCArticle
dianion (62-) is believed to be the pivotal intermediate from
which H2 is lost. A radical chain reaction involving free
hydrogen atoms (H•) in the two-step propagation cycle is
to 85 °C; the mixture turned dark purple within 2 h. After 12 h at
85 °C, the reaction mixture was allowed to cool to room
temperature, and the solution was removed from the remaining
potassium by vacuum cannulation into a flame-dried flask
under a nitrogen atmosphere. The pressure vessel was washed
with some dry THF, and the solution again was removed by
vacuum cannulation. The dark purple to blue solution was
quenched by adding a solution of iodine (560 mg, 4.41 mmol)
in ∼6 mL of dry THF over a period of 15 min, which caused the
solution to turn yellow and then dark red. To the solution was
then added slowly ethanol (5 mL) followed by sodium thiosul-
fate (10% in water, ∼50 mL) until the color no longer changed
(typically a yellow solution). The reaction was then exposed to
air, and the organic solvents were removed on a rotary eva-
porator. To the remaining aqueous solution containing preci-
pitates was added dichloromethane (DCM, 100 mL), and the
aqueous layer was extracted 3 times. The combined organic
layers were washed with brine and dried over MgSO4, and the
solvent was removed on a rotary evaporator. The residue was
adsorbed onto silica, and the mixture was subjected to flash
column chromatography (SiO2, 1:15 DCM/hexanes) yielding
219 mg (quantitative) of 2 as a yellow solid. The 1H NMR
spectrum matched that obtained from authentic perylene, with
no more than traces of impurities (<5%). To the residues in the
pressure vessel was added THF (50 mL), and any remaining
potassium was slowly quenched by the slow addition of EtOH
and then water.
proposed to account for the formation of H2 from 62-
.
Anionic cyclodehydrogenations of the sort investigated here
are complementary to those performed under strongly acidic/
oxidizing conditions, photochemically, or thermally (flash
vacuum pyrolysis). It is our hope that a better understanding
of anionic cyclodehydrogenations such as this one will
encourage their wider use in organic synthesis.
Experimental Section
General. The tetrahydrofuran (THF) used in this reaction was
dispensed from a Glass Contour solvent purification system. 1H
NMR spectra were recorded in CDCl3 on a 500 MHz instru-
ment. CAUTION: Glass encased magnetic stirbars should be
used for all reactions involving potassium metal; potassium
reacts with standard Teflon-coated stirbars to produce KF
and highly reactive carbon as a finely divided black powder
that, in one instance, upon exposure to oxygen, resulted in a
vigorous explosion.
Perylene (2) from Binaphthyl (1) by the Action of Potassium
Metal. To a flame-dried and nitrogen-flushed 350-mL pressure
vessel equipped with a glass-encased magnetic stirbar19 was
added dry tetrahydrofuran (THF, 250 mL) and 1,10-binaphthyl
(1) (221 mg, 0.87 mmol). The solvent was degassed for 15 min
while a freshly cut piece of potassium metal20 (108 mg, 2.77
mmol) was rinsed with hexanes and THF and then added to the
reaction mixture. The vessel was sealed and heated in an oil bath
Acknowledgment. We thank Maria Eliseeva for her trans-
lation of the Russian papers in ref 7. This work was sup-
ported by the University of Basel, the Werenfels Fond der
Freiwilligen Akademischen Gesellschaft Basel, the U.S.
National Science Foundation, and the U.S. Department of
Energy.
(19) Note the CAUTION in the General Section.
(20) Not cleanly cutting all sides of the potassium piece generally leads to
a significant drop in yield.
7364 J. Org. Chem. Vol. 75, No. 21, 2010