Asymmetric synthesis and photophysical properties of benzopyrano- or naphthopyrano-fused helical phosphafluorenes

Article abstract of DOI:10.1021/ol100227k

"Chemical Equation Presented" Enantioenriched benzopyrano- and naphthopyrano-fused helical phosphafluorenes have been synthesized by the rhodium-catalyzed enantioselective double [2 + 2 + 2] cycloaddition of dialkynyl phosphorus compounds with phenol- or naphthol-linked tetraynes. Photophysical properties of these phosphafluorenes are also disclosed.

Full text of DOI:10.1021/ol100227k

ORGANIC
LETTERS
2010
Vol. 12, No. 6
1324-1327
Asymmetric Synthesis and
Photophysical Properties of
Benzopyrano- or Naphthopyrano-Fused
Helical Phosphafluorenes
Naohiro Fukawa,† Takuya Osaka,† Keiichi Noguchi,‡ and Ken Tanaka*,†
Department of Applied Chemistry, Graduate School of Engineering, and
Instrumentation Analysis Center, Tokyo UniVersity of Agriculture and Technology,
Koganei, Tokyo 184-8588, Japan
tanaka-k@cc.tuat.ac.jp
Received January 28, 2010
ABSTRACT
Enantioenriched benzopyrano- and naphthopyrano-fused helical phosphafluorenes have been synthesized by the rhodium-catalyzed
enantioselective double [2 + 2 + 2] cycloaddition of dialkynyl phosphorus compounds with phenol- or naphthol-linked tetraynes. Photophysical
properties of these phosphafluorenes are also disclosed.
Phosphafluorenes have attracted much attention as interesting
candidates for organic semiconducting materials.1 To syn-
thesize a structurally diverse array of densely substituted
phosphafluorenes, the synthetic methods that allow regiose-
lective functionalization of the phosphafluorene core are
highly attractive.1,2 The most frequently employed method
for their synthesis is biaryl coupling leading to 2,2′-
dihalobiphenyls followed by the reaction with dichlorophos-
phines using a strong base and subsequent oxidation (Scheme
1).1,2a-d However, this method requires prior preparation of
regioselectively substituted arenes and harsh reaction condi-
tions.
Scheme 1
† Department of Applied Chemistry.
‡ Instrumentation Analysis Center.
(1) For selected recent examples, see: (a) Geramita, K.; McBee, J.; Tilley,
T. D. J. Org. Chem. 2009, 74, 820. (b) Chen, R.-F.; Zhu, R.; Fan, Q.-L.;
Huang, W. Org. Lett. 2008, 10, 2913. (c) Geramita, K.; McBee, J.; Tao,
Y.; Segalman, R. A.; Tilley, T. D. Chem. Commun. 2008, 5107.
(2) For selected recent examples, see: (a) Widhalm, M.; Aichinger, C.;
Mereiter, K. Tetrahedron Lett. 2009, 50, 2425. (b) Chen, R.-F.; Fan, Q.-
L.; Zheng, C.; Huang, W. Org. Lett. 2006, 8, 203. (c) Dubrovina, N. V.;
Jiao, H.; Tararov, V. I.; Spannenberg, A.; Kadyrov, R.; Monsees, A.;
Christiansen, A.; Bo¨rner, A. Eur. J. Org. Chem. 2006, 3412. (d) Leroux,
F.; Mangano, G.; Schlosser, M. Eur. J. Org. Chem. 2005, 5049. (e) Tsuji,
H.; Komatsu, S.; Kanda, Y.; Umehara, T.; Saeki, T.; Tamao, K. Chem.
Lett. 2006, 35, 758. (f) Fadhel, S.; Szieberth, D.; Deborde, V.; Lescop, C.;
On the other hand, our research group demonstrated that
cationic rhodium(I)/biaryl bisphosphine complexes are highly
effective catalysts for the [2 + 2 + 2] cycloaddition to
produce substituted arenes under mild reaction conditions.3-5
(3) For our account, see: (a) Tanaka, K. Synlett 2007, 1977. (b) Tanaka,
K.; Nishida, G.; Suda, T. J. Synth. Org. Chem. Jpn. 2007, 65, 862.
Nyula´szi, L.; Hissler, M.; Re´au, R. Chem.sEur. J. 2009, 15, 4914
.
10.1021/ol100227k  2010 American Chemical Society
Published on Web 02/24/2010

Furthermore our research group demonstrated that the
rhodium-catalyzed double [2 + 2 + 2] cycloaddition is an
effective method for the synthesis of symmetric biaryls,6
spiranes,7 and helicene-like molecules.8 We anticipated that
the rhodium-catalyzed double [2 + 2 + 2] cycloaddition
between dialkynyl phosphine oxides9,10 and tetraynes would
be useful for the synthesis of symmetric and regioselectively
substituted phosphafluorenes11 because of the facile prepara-
tion of dialkynyl phosphine oxides from the corresponding
terminal alkynes and dichlorophosphine oxides (Scheme 2).
tetrayne 2a leading to phosphafluorene 3aa by using a
cationic rhodium(I)/H8-BINAP [2,2′-bis(diphenylphosphino)-
5,5′,6,6′,7,7′,8,8′-octahydro-1,1′-binaphthyl] complex as a
catalyst. We were pleased to find that the desired reaction
proceeded at room temperature to give 3aa in 38% isolated
yield, although excess 2a (2.5 equiv) was required due to
the rapid homo-[2 + 2 + 2] cycloaddition of 2a (Scheme
3).
Scheme 3
Scheme 2
We next examined the synthesis of helical phosphafluo-
renes by the rhodium-catalyzed enantioselective double
[2 + 2 + 2] cycloaddition (Table 1).12,13 After optimization
of reaction conditions, the reaction of dialkynyl phosphinate
1a and phenol-linked terminal tetrayne 2b proceeded at room
temperature for only 1 h to give benzopyrano-fused helical
phosphafluorene 3ab in 40% isolated yield with 73% ee by
using a cationic rhodium(I)/(R)-tol-BINAP [2,2′-bis(di-p-
tolylphosphino)-1,1′-binaphthyl] complex as a catalyst (entry
1). In this reaction, the homo-[2 + 2 + 2] cycloaddition of
2b proceeded slowly, therefore a slight excess of 2b (1.2
equiv) was sufficient. Thus the scope of helical phosphafluo-
rene synthesis by the double [2 + 2 + 2] cycloaddition was
examined. Dialkynyl phosphine oxides 1b,c could react with
In this paper, we disclose the synthesis of benzopyrano- and
naphthopyrano-fused helical phosphafluorenes by the rhodium-
catalyzed enantioselective double [2 + 2 + 2] cycloaddition.
Photophysical properties of these phosphafluorenes are also
disclosed.
We first examined the rhodium-catalyzed double [2 + 2
+ 2] cycloaddition of dialkynyl phosphinate 1a and terminal
(4) For a review of rhodium-catalyzed [2 + 2 + 2] cycloadditions, see:
Fujiwara, M.; Ojima, I. In Modern Rhodium-Catalyzed Organic Reactions;
Evans, P. A., Ed.; Wiley-VCH: Weinheim, Germany, 2005; Chapter 7, p
129
.
(5) For recent reviews of transition metal-catalyzed [2 + 2 + 2]
cycloadditions, see: (a) Galan, B. R.; Rovis, T. Angew. Chem., Int. Ed.
2009, 48, 2830. (b) Tanaka, K. Chem. Asian J. 2009, 4, 508. (c) Varela,
J. A.; Saa´, C. Synlett 2008, 2571. (d) Shibata, T.; Tsuchikama, K. Org.
Biomol. Chem. 2008, 1317. (e) Heller, B.; Hapke, M. Chem. Soc. ReV. 2007,
36, 1085. (f) Agenet, N.; Buisine, O.; Slowinski, F.; Gandon, V.; Aubert,
C.; Malacria, M. Organic Reactions; RajanBabu, T. V., Ed.; John Wiley &
Sons: Hoboken, NJ, 2007; Vol. 68, p 1. (g) Chopade, P. R.; Louie, J. AdV.
Synth. Catal. 2006, 348, 2307. (h) Gandon, V.; Aubert, C.; Malacria, M.
Chem. Commun. 2006, 2209. (i) Kotha, S.; Brahmachary, E.; Lahiri, K.
Eur. J. Org. Chem. 2005, 4741. (j) Gandon, V.; Aubert, C.; Malacria, M.
Curr. Org. Chem. 2005, 9, 1699. (k) Yamamoto, Y. Curr. Org. Chem. 2005,
9, 503.
(11) Syntheses of heterofluorenes by transition metal-catalyzed [2 + 2
+ 2] cycloadditions of heteroatom-bridged 1,6-diynes with monoynes have
been reported. Carbazoles: (a) Witulski, B.; Alayrac, C. Angew. Chem.,
Int. Ed. 2002, 41, 3281. (b) Alayrac, C.; Schollmeyer, D.; Witulski, B. Chem.
Commun 2009, 1464. Silafluorenes: (c) Matsuda, T.; Kadowaki, S.; Goya,
T.; Murakami, M. Org. Lett. 2007, 9, 133. Dibenzofurans and azadiben-
zofurans: (d) Komine, Y.; Kamisawa, A.; Tanaka, K. Org. Lett. 2009, 11,
2361
.
(6) For examples by our research group, see: (a) Nishida, G.; Suzuki,
N.; Noguchi, K.; Tanaka, K. Org. Lett. 2006, 8, 3489. (b) Nishida, G.;
Ogaki, S.; Yusa, Y.; Yokozawa, T.; Noguchi, K.; Tanaka, K. Org. Lett.
2008, 10, 2849. For examples by other research groups, see: (c) Doherty,
S.; Knight, J. G.; Smyth, C. H.; Harrington, R. W.; Clegg, W. Org. Lett.
2007, 9, 4925. (d) Doherty, S.; Smyth, C. H.; Harrington, R. W.; Clegg,
W. Organometallics 2008, 27, 4837.
(12) For enantioselective syntheses of helicenes and helicene-like
molecules by transition metal-mediated [2 + 2 + 2] cycloadditions, see:
ˇ
ˇ
(a) Stara´, I. G.; Stary´, I.; Kolla´rovicˇ, A.; Teply´, F.; Vyskocˇil, S.; Saman,
D. Tetrahedron Lett. 1999, 40, 1993. (b) Teply´, F.; Stara´, I. G.; Stary´, I.;
ˇ
ˇ
Kolla´rovicˇ, A.; Saman, D.; Vyskocˇil, S.; Fiedler, P. J. Org. Chem. 2003,
68, 5193. (c) Caeiro, J.; Pen˜a, D.; Cobas, A.; Pe´rez, D.; Guitia´n, E. AdV.
Synth. Catal. 2006, 348, 2466. (d) Tanaka, K.; Kamisawa, A.; Suda, T.;
Noguchi, K.; Hirano, M. J. Am. Chem. Soc. 2007, 129, 12078. See also ref
8.
(7) Wada, A.; Noguchi, K.; Hirano, M.; Tanaka, K. Org. Lett. 2007, 9,
1295.
(8) For the rhodium-catalyzed enantioselective double [2 + 2 + 2]
cycloaddition of dialkynylketones with phenol- or naphthol-linked tetraynes,
see: Tanaka, K.; Fukawa, N.; Suda, T.; Noguchi, K. Angew. Chem., Int.
Ed. 2009, 48, 5470.
(13) For a recent review of helicene synthesis, see: Urbano, A. Angew.
Chem., Int. Ed. 2003, 42, 3986
.
(14) Nonphotochemical syntheses of higher order (g[8]) helicenes or
helicene-like molecules have been reported in a limited number; see: (a)
Fox, J. M.; Katz, T. J. J. Org. Chem. 1999, 64, 302. (b) Norsten, T. B.;
Peters, A.; McDonald, R.; Wang, M.; Branda, N. R. J. Am. Chem. Soc.
2001, 123, 7447. (c) Han, S.; Bond, A. D.; Disch, R. L.; Holmes, D.;
Schulman, J. M.; Teat, S. J.; Vollhardt, K. P. C.; Whitener, G. D. Angew.
Chem., Int. Ed. 2002, 41, 3223. (d) Han, S.; Anderson, D. R.; Bond, A. D.;
Chu, H. V.; Disch, R. L.; Holmes, D.; Schulman, J. M.; Teat, S. J.; Vollhardt,
K. P. C.; Whitener, G. D. Angew. Chem., Int. Ed. 2002, 41, 3227. (e)
Miyasaka, M.; Rajca, A.; Pink, M.; Rajca, S. J. Am. Chem. Soc. 2005, 127,
(9) Enantioselective synthesis of P-stereogenic alkynylphosphine oxides
by the rhodium-catalyzed [2 + 2 + 2] cycloaddition of dialkynyl phosphine
oxides with 1,6-diynes was reported;see: Nishida, G.; Noguchi, K.; Hirano,
M.; Tanaka, K. Angew. Chem., Int. Ed. 2008, 47, 3410.
(10) For transition metal-catalyzed [2 + 2 + 2] cycloadditions involving
alkynyl phosphorus compounds, see: (a) Heller, B.; Gutnov, A.; Fischer,
C.; Drexler, H.-J.; Spannenberg, A.; Redkin, D.; Sundermann, C.; Sunder-
mann, B. Chem.sEur. J. 2007, 13, 1117. (b) Nishida, G.; Noguchi, K.;
Hirano, M.; Tanaka, K. Angew. Chem., Int. Ed. 2007, 46, 3951. (c) Kondoh,
A.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc. 2007, 129, 6996. See
also refs 6b,c, and d.
ˇ
13806. (f) Sehnal, P.; Stara´, I. G.; Saman, D.; Tichy´, M.; M´ısˇek, J.; Cvacˇka,
J.; Rul´ısˇek, L.; Chocholousˇova´, J.; Vacek, J.; Goryl, G.; Szymonski, M.;
C´ısaˇrova´, I.; Stary´, I. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 13169.
Org. Lett., Vol. 12, No. 6, 2010
1325

4- and 5-positions of the phosphafluorene core is of interest
for further extension of the conjugation. Therefore the
reaction of dialkynyl phosphinate 1e and styryl-substituted
internal tetrayne 2e leading to 2,7-styryl-substituted phos-
phafluorene 3ee was investigated. Although the product yield
was low, the desired reaction proceeded at room temperature
by using the cationic rhodium(I)/(R)-tol-BINAP catalyst
(entry 9).
Table 1. Rh-Catalyzed Asymmetric Synthesis of Benzopyrano-
or Naphthopyrano-Fused Helical Phosphafluorenesa
In our previous report, gradual racemization of a ben-
zopyrano-fused helical fluorenone proceeded in CH2Cl2
solution at room temperature.8 On the contrary, the ben-
zopyrano-fused helical phosphafluorenes of Table 1 possess
stable helical chirality in CH2Cl2 solution at room temper-
ature presumably due to the longer length of the C-P bond
than the C-C bond. Indeed, the ORTEP diagram of
benzopyrano-fused helical phosphafluorene (()-3bb revealed
that two 2-alkoxyphenyl rings of 3bb largely overlapped each
other (Figure 1).
Figure 1
. ORTEP diagram of helical phosphafluorene (()-3bb (top
view) with ellipsoids at 30% probability.
a Reactions were conducted with [Rh(cod)2]BF4 (20 mol %), ligand (20
mol %), 1a-e (1 equiv), and 2a-e (1.2 equiv) in (CH2Cl)2 at rt for 1 h.
Ligand: (R)-tol-BINAP (entries 1-4 and 9), (R)-H8-BINAP (entries 5-8).
b Isolated yield. c For 6 h. d A mixture of olefin geometric isomers. e ee
value of the major olefin geometric isomer.
Figure 2. ORTEP diagram of helical phosphafluorene (()-3bb (side
view) with ellipsoids at 30% probability.
tetrayne 2b to yield the corresponding phosphafluorenes in
higher yields than that from dialkynyl phosphinate 1a,
although lower enantioselectivities were observed (entries 2
and 3). Not only phenyl-substituted dialkynyl phosphinate
1a but also the methyl-substituted one 1d could participate
in this reaction albeit in low yield (entry 4). With respect to
tetraynes, naphthol-linked terminal tetrayne 2c could also
react with 1a to yield more overcrowded naphthopyrano-
fused helical phosphafluorene 3ac, possessing nine successive
rings,15 by using (R)-H8-BINAP as a ligand (entry 5). Not
only the phenol-linked terminal tetrayne 2b but also the
internal tetrayne 2d could participate in this reaction by using
(R)-H8-BINAP as a ligand (entries 6-8). The introduction
of the π-component at the 2- and 7-positions as well as the
To compare the photophysical properties between 4,5- and
3,6-di(2-alkoxyphenyl)phosphafluorenes, the synthesis of 3,6-
ˇ
(15) For selected examples, see: (a) Adriaenssens, L.; Severa, L.; Sa´lova´,
ˇ
T.; C´ısaˇrova´, I.; Pohl, R.; Saman, D.; Rocha, S. V.; Finney, N. S.; Posp´ısˇil,
L.; Slav´ıcˇek, P.; Teply´, F. Chem.sEur. J. 2009, 15, 1072. (b) Graule, S.;
Rudolph, M.; Vanthuyne, N.; Autschbach, J.; Roussel, C.; Crassous, J.;
Reau, R. J. Am. Chem. Soc. 2009, 131, 3183. (c) Takenaka, N.; Sarangthem,
R. S.; Captain, B. Angew. Chem., Int. Ed. 2008, 47, 9708. (d) Misek, J.;
Teply, F.; Stara, I. G.; Tichy, M.; Saman, D.; Cisarova, I.; Vojtisek, P.;
Stary, I. Angew. Chem., Int. Ed. 2008, 47, 3188. (e) Nakano, K.; Hidehira,
Y.; Takahashi, K.; Hiyama, T.; Nozaki, K. Angew. Chem., Int. Ed. 2005,
44, 7136. (f) Wigglesworth, T. J.; Sud, D.; Norsten, T. B.; Lekhi, V. S.;
Branda, N. R. J. Am. Chem. Soc. 2005, 127, 7272. (g) Phillips, K. E. S.;
Katz, T. J.; Jockusch, S.; Lovinger, A. J.; Turro, N. J. J. Am. Chem. Soc.
2001, 123, 11899, and references cited therein. See also refs 14a,b, and
14e.
1326
Org. Lett., Vol. 12, No. 6, 2010

Scheme 4
di(2-alkoxyphenyl)phosphafluorene 3cf by the reaction of
dialkynyl phosphine oxide 1c and phenol-linked terminal
tetrayne 2f was investigated. The desired reaction proceeded
at room temperature by using the cationic rhodium(I)/tol-
BINAP catalyst to yield 3cf in good yield (Scheme 4).
Table 2. Photophysical Properties of Representative
Phosphafluorenesa
Figure 3. (a) UV-vis spectra and (b) emission spectra of 3aa, 3ab,
UV absorption
λmax (nm)b
emission
λmax (nm)
3ac, 3ee, and 3cf in CHCl3 (1.0 × 10-5 M).
b c
,
d
entry
3
[R]25
D
1
2
3
4
5
6
7
8
9
3aa
259, 341
288, 341
288, 344
289, 337
281, 338
308, 386
285, 343
281, 348e
275
375
477
471
474
469
490
464
482e
406
(-)-3ab
(-)-3bb
(-)-3cb
(-)-3db
(+)-3ac
(-)-3ad
(+)-3ee
3cf
699
546
636
584
420
586
279e
substitution (3ee, entry 8) showed a small shift of emission
maxima compared with 3ab (entry 2) and 3db (entry 5),
respectively, while the emission intensity of 3ee significantly
increased (Figure 3). Optical rotation values of representative
helical phosphafluorenes are also shown in Table 2. Interest-
ingly, the more overcrowded naphthopyrano-fused phos-
phafluorene 3ac (entry 6) exhibits a smaller optical rotation
value than that of the less overcrowded benzopyrano-fused
phosphafluorene 3ab (entry 2).
a Measured in CHCl3. b Concentration: 1.0 × 10-5 M. c Excited at 280
nm. d Values are calculated as 100% ee. e Values were measured with use
of a mixture of olefin geometric isomers.
In conclusion, we have successfully synthesized enan-
tioenriched benzopyrano- or naphthopyrano-fused helical
phosphafluorenes by the rhodium-catalyzed enantioselective
double [2 + 2 + 2] cycloaddition of dialkynyl phosphorus
compounds with phenol- or naphthol-linked tetraynes. We
believe that the compounds synthesized in this paper are
fascinating examples of helicene-like species containing
heteroatoms in the helical backbone.15 Future work will focus
on application of this methodology to the synthesis of various
heterofluorenes.
Absorption and emission data of representative phos-
phafluorenes are shown in Table 2 and Figure 3. As expected,
4,5-di(2-alkoxynaphthyl)phosphafluorene 3ab (entry 2) showed
large red shifts of absorption and emission maxima compared
with phosphafluorene 3aa, which contained no 2-alkoxyphe-
nyl group (entry 1). Further red shifts were observed in 4,5-
di(2-alkoxynaphthyl)phosphafluorene 3ac (entry 6). On the
other hand, 3,6-di(2-alkoxyphenyl)phosphafluorene 3cf showed
smaller red shifts compared with 3aa (entry 9). The substit-
uents on the phosphorus appeared to have a small impact
on absorption and emission maxima (entries 2-4). As shown
in the ORTEP diagram of phosphafluorene (()-3bb (Figure
2), two phenyl rings at the 1- and 8-positions and the
phosphafluorene core are in the twisted conformation, which
would inhibit the conjugation. Indeed, 1,8-diphenyl- (3ab,
entry 2) and 1,8-dimethylphosphafluorenes (3db, entry 5)
showed close absorption and emission maxima. With respect
to the substituents at the 2- and 7-positions of the phos-
phafluorene core, 2,7-dimethyl (3ad, entry 7) and 2,7-distyryl
Acknowledgment. This work was supported partly by a
Grant-in-Aid for Scientific Research (No. 20675002) from
MEXT, Japan. We thank Takasago International Corporation
for the gift of H8-BINAP and tol-BINAP, and Umicore for
generous support in supplying a rhodium complex.
Supporting Information Available: Experimental pro-
cedures, compound characterization data, and an X-ray
crystallographic information file. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL100227K
Org. Lett., Vol. 12, No. 6, 2010
1327