Cycloaddition-based formal C-H alkynylation of isoindoles leading to the synthesis of air-stable fluorescent 1,3-dialkynylisoindoles

Article abstract of DOI:10.1021/ol401051d

Reaction of N-alkylisoindoles with (bromoethynyl)triisopropylsilane afforded 1,3-bis(triisopropylsilylethynyl) isoindoles in high yields. The formal C-H alkynylation proceeds under transition-metal-free conditions through [4 + 2] cycloaddition of the pyrrole ring of isoindole with bromoalkyne followed by ring-opening of the product.

Full text of DOI:10.1021/ol401051d

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Cycloaddition-based Formal CꢀH
Alkynylation of Isoindoles Leading
to the Synthesis of Air-stable
Fluorescent 1,3-Dialkynylisoindoles
Toshimichi Ohmura,* Akihito Kijima, Yusuke Komori, and Michinori Suginome*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
ohmura@sbchem.kyoto-u.ac.jp; suginome@sbchem.kyoto-u.ac.jp
Received April 16, 2013
ABSTRACT
Reaction of N-alkylisoindoles with (bromoethynyl)triisopropylsilane afforded 1,3-bis(triisopropylsilylethynyl) isoindoles in high yields. The formal
CꢀH alkynylation proceeds under transition-metal-free conditions through [4 þ 2] cycloaddition of the pyrrole ring of isoindole with bromoalkyne
followed by ring-opening of the product.
Increasing effort has been devoted to the development of
new aromatic and heteroaromatic compounds bearing
alkynyl groups, because introduction of alkynyl groups
often significantly changes their properties and stability.
Indeed, aromatic compounds bearing two alkynyl groups
on their aromatic ring have been used as high-performance
electronic materials,1 a component of molecular wires,2
and starting substrates for conjugated polymers,3 etc. For
further exploration of new alkyne-conjugated functional
molecules, it is highly desirable to find new aromatic core
units along with their efficient synthesis.
We have focused on the synthesis and properties of
isoindole derivatives, which are a less-explored motif in
the development of organic materials, even though they
generally show strong fluorescence.4,5 One of the reasons
for the limited attention is the lack of efficient reactions
for direct functionalization of isoindoles.4b,6 Recently, we
established palladium-catalyzed CꢀH borylation and
arylation of isoindoles for the synthesis of 1-boryl- and
1-arylisoindoles.7 Based on the interesting reactivity of
isoindoles in catalytic CꢀH functionalization,7 as well as
the recent great progress on a transition-metal-catalyzed
CꢀH alkynylation of heterocyclic compounds,8,9 we
(1) (a) Anthony, J. E. Chem. Rev. 2006, 106, 5028. (b) Anthony, J. E.
Angew. Chem., Int. Ed. 2008, 47, 452.
(2) (a) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (b) James, D. K.;
Tour, J. M. Top. Curr. Chem. 2005, 257, 33.
(3) (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605. (b) Matsumi, N.;
Chujo, Y. Polym. J. 2008, 40, 77.
(4) For reviews on isoindole, see: (a) Jones, G. B.; Chapman, B. J. In
Comprehensive Heterocyclic Chemistry II; Katrizky, A. R., Rees, C. W.,
Scriven, E. F. V., Eds.; Pergamon: Oxford, 1996; Vol 2, p 1. (b) Donohoe,
T. J. In Science of Synthesis; Thomas, E. J., Vol. Ed.; Georg Thieme Verlag:
Stuttgart, 2000; Vol 10, p 653. (c) Heugebaert, T. S. A.; Roman, B. I.;
Stevens, C. V. Chem. Soc. Rev. 2012, 41, 5626.
(5) For study on utilization of isoindoles as fluorescent and electro-
luminescent materials, see: (a) Zweig, A.; Metzler, G.; Maurer, A.;
Roberts, B. G. J. Am. Chem. Soc. 1967, 89, 4091. (b) Ding, Y.; Hay,
A. S. J. Polm. Sci. Part A: Polym. Chem. 1999, 37, 3293. (c) Gauvin, S.;
Santerre, F.; Dodelet, J. P.; Ding, Y.; Hlil, A. R.; Hay, A. S.; Anderson,
J.; Armstrong, N. R.; Gorjanc, T. C.; D’Iorio, M. Thin Solid Films 1999,
353, 218. (d) Mi, B.-X.; Wang, P.-F.; Liu, M.-W.; Kwong, H.-L.; Wong,
N.-B.; Lee, C.-S.; Lee, S.-T. Chem. Mater. 2003, 15, 3148. (e) Jiao, L.;
Yu, C.; Lin, M.; Wu, Y.; Cong, K.; Meng, T.; Wang, Y.; Hao, E. J. Org.
Chem. 2010, 75, 6035. For study on isoindoles as bioactive molecules,
see: (f) Diana, P.; Martorana, A.; Barraja, P.; Montalbano, A.; Carbone,
A.; Cirrincione, G. Tetrahedron 2011, 67, 2072.
(6) For examples of electrophilic aromatic substitution reaction of
isoindoles, see: (a) Winn, M.; Zaugg, H. E. J. Org. Chem. 1969, 34, 249.
(b) Simons, S. S., Jr.; Ammon, H. L.; Doherty, R.; Johnson, D. F. J. Org.
Chem. 1981, 46, 4739. (c) Bonnett, R.; North, S. A.; Newton, R. F.;
Scopes, D. I. C. Tetrahedron 1983, 39, 1401. For nucleophilic substitu-
tion reaction of 1-haloisoindoles, see ref 5f.
(7) (a) Ohmura, T.; Kijima, A.; Suginome, M. J. Am. Chem. Soc.
2009, 131, 6070. (b) Ohmura, T.; Kijima, A.; Suginome, M. Org. Lett.
2011, 13, 1238.
r
10.1021/ol401051d
XXXX American Chemical Society

envisioned that CꢀH alkynylation would be useful for
converting isoindoles to alkynylated isoindoles. Herein, we
describe a synthesis of 1,3-dialkynylisoindoles via CꢀH
alkynylation of isoindoles with 1-bromo-1-alkynes. It is
interesting to note that the CꢀH alkynylation can be
carried out under transition-metal-free conditions.10
During our studies on transition-metal-catalyzed CꢀH
alkynylation of 2-methylisoindole (1a) with (bromoethynyl)-
triisopropylsilane (2a), we performed the reaction in the
absence of any transition metal catalyst at 110 °C (entry 1,
Table 1).11 We unexpectedly found formation of 1,3-bis-
(triisopropylsilylethynyl)isoindole 3aa in 20% yield,12
indicating that the CꢀH alkynylation of isoindole need no
transition metal catalysts.10 The yield of 3aa was improved to
85% when the reaction was carried out in the presence
of K3PO4 (4 equiv) (entry 2). The CꢀH alkynylation of 1a
also took place in the reaction of tert-butyldimethylsilyl-
substituted alkyne 2b (entry 3). In sharp contrast, phenyl-
and alkyl-substituted 2cꢀe did not give the CꢀH alkynyla-
tion product 3cꢀe at all, but instead gave [4 þ 2] cycloaddi-
tion products 4cꢀe (entries 4ꢀ6).11
Table 1. Reaction of Isoindole 1a with 1-Bromo-1-alkynes 2a
entry
1-bromo-1-alkyne
yield of 3 (%)b
yield of 4 (%)b
1c
2
3
4
5
6
2a [R = Si(i-Pr)3]
2a
20 (3aa)
85 (3aa)
84 (3b)
0 (3c)
0 (4a)
0 (4a)
2b [R = SiMe2(t-Bu)]
2c (R = Ph)
0 (4b)
56 (4c)
76 (4d)
81 (4e)
2d (R = n-C6H13
)
0 (3d)
2e (R = t-Bu)
0 (3e)
a 1a (0.20 mmol), 2 (0.50 mmol), and K3PO4 (0.80 mmol) were
reacted in 1,4-dioxane (0.3 mL) at 110 °C for 48 h. b Isolated yield based
on 1a. c In the absence of K3PO4.
Bis(trialkylsilylethynyl)isoindoles 3aa and 3b showed
remarkable stability under air, which is in sharp contrast
to the relatively oxygen-sensitive isoindoles such as 1a and
1,3-diarylisoindoles.13 Isoindoles 3aa and 3b could be
purified by column chromatography on silicagel under air.
The reaction of 1a with 2a was investigated in detail
(eqs 1ꢀ3). When the reaction was carried out at lower
temperature (60 °C), 4a was formed via [4 þ 2] cycloaddi-
tion (42%, eq 1). The isolated 4a was then heated at 110 °C
in the presence of K3PO4 (eq 2), resulting in ring-opening
of 4a with elimination of HBr to give 1-alkynylisoindole 5
in 73% yield. These results indicated that the CꢀH alky-
nylation of isoindole proceeds through [4 þ 2] cycload-
dition followed by ring-opening with elimination of HBr
(Scheme 1).14 The reaction also gave a small amount of
3aa (4%, eq 2), indicating that competitive retro-[4 þ 2]
cycloaddition gave 2a under these conditions (Scheme 1).15
The isoindole 5 reacted with 2a at 110 °C in the presence
of K3PO4 to give 3aa in 99% yield (eq 3). In the second
alkynylation, the [4 þ 2] cycloadduct was not observed,
even in the reaction at lower temperature.
(8) For reviews on transition-metal-catalyzed CꢀH alkynylation of
aromatic and heteroaromatic compounds, see: (a) Dudnik, A. S.;
Gevorgyan, V. Angew. Chem., Int. Ed. 2010, 49, 2096. (b) Messaoudi,
S.; Brion, J.-D.; Alami, M. Eur. J. Org. Chem. 2010, 6495.
(9) For examples of transition-metal-catalyzed alkynylation of
heterocyclic C(sp2)ꢀH bond, see: (a) Seregin, I., V.; Ryabova, V.;
Gevorgyan, V. J. Am. Chem. Soc. 2007, 129, 7742. (b) Matsuyama,
N.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2009, 11, 4156. (c)
Gu, Y.; Wang, X. Tetrahedron Lett. 2009, 50, 763. (d) Brand, J. P.;
Charpentier, J.; Waser, J. Angew. Chem., Int. Ed. 2009, 48, 9346. (e)
Besselievre, F.; Piguel, S. Angew. Chem., Int. Ed. 2009, 48, 9553. (f) de
Haro, T.; Nevado, C. J. Am. Chem. Soc. 2010, 132, 1512. (g) Kim, S. H.;
Chang, S. Org. Lett. 2010, 12, 1868. (h) Yang, L.; Zhao, L.; Li, C.-J.
Chem. Commun. 2010, 46, 4184. (i) Brand, J. P.; Waser, J. Angew. Chem.,
Int. Ed. 2010, 49, 7304. (j) Matsuyama, N.; Kitahara, M.; Hirano, K.;
Satoh, T.; Miura, M. Org. Lett. 2010, 12, 2358. (k) Berciano, B. P.;
Lebrequier, S.; Besselievre, F.; Piguel, S. Org. Lett. 2010, 12, 4038. (l)
Kim, S. H.; Yoon, J.; Chang, S. Org. Lett. 2011, 13, 1474. (m) Brand,
J. P.; Waser, J. Synthesis 2012, 44, 1155. (n) Ackermann, L.; Kornhaass,
C.; Zhu, Y. Org. Lett. 2012, 14, 1824. (o) Shibahara, F.; Dohke, Y.;
Murai, T. J. Org. Chem. 2012, 77, 5381. (p) Brand, J. P.; Chevalley, C.;
Scopelliti, R.; Waser, J. Chem.;Eur. J. 2012, 18, 5655. (q) Tolnai, G. L.;
Ganss, S.; Brand, J. P.; Waser, J. Org. Lett. 2013, 15, 112.
(10) Although there have been some reports on transition-metal-free
CꢀH alkynylation, these are limited to the reaction of pyrroles with
3-brormo-1-phenylprop-2-yn-1-one or ethyl 3-bromopropiolate. See:
(a) Trofimov, B. A.; Stepanova, Z. V.; Sobenina, L. N.; Mikhaleva,
A. I.; Ushakov, I. A. Tetrahedron Lett. 2004, 45, 6513. (b) Trofimov,
B. A.; Sobenina, L. N.; Stepanova, Z. V.; Vakul’skaya, T. I.; Kazheva,
O. N.; Aleksandrov, G. G.; Dyachenko, O. A.; Mikhaleva, A. I. Tetra-
hedron 2008, 64, 5541 and references therein.
(11) The pyrrole ring of isoindole has been known as a reactive diene
in [4 þ 2] cycloaddition with dienophiles such as benzyne, dimethyl
acetylenedicarboxylate, and N-methyl maleimide. For examples, see: (a)
LeHoullier, C. S.; Gribble, G. W. J. Org. Chem. 1983, 48, 2364. (b) Chen,
€
Z.; Muller, P.; Swager, T. M. Org. Lett. 2006, 8, 273. (c) Chen, Y.-L.;
Lee, M.-H.; Wong, W.-Y.; Lee, A. W. M. Synlett 2006, 2510. (d) Duan,
S.; Sinha-Mahapatra, D. K.; Herndon, J. W. Org. Lett. 2008, 10, 1541.
ꢀ
(e) Sole, D.; Serrano, O. Org. Biomol. Chem. 2009, 7, 3382. (f) Tong,
We then turned our attention to the synthesis of un-
symmetrical 1,3-dialkynylisoindoles via alkynylation of
monoalkynylated 5 (Table 2).16 The reaction with 2b took
place efficiently in 1,4-dioxane at 110 °C in the presence of
B. M. K.; Hui, B.W.-Q.; Chua, S. H.; Chiba, S. Synthesis 2011, 3552.
(12) The chemical structure of 3aa was confirmed by X-ray crystal-
lographic analysis (see Supporting Information).
(13) For autooxidation of 1,3-diphenylisoindole, see: Ahmed, M.;
Kricka, L. J.; Vernon, J. M. J. Chem. Soc., Perkin 1 1975, 71.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 2. CꢀH Alkynylation of 5 for the Synthesis of
Unsymmetrical 1,3-Dialkynylisoindoles 7a
Scheme 1. Possible Mechanism of CꢀH Alkynylation
K3PO4 (2 equiv) to give 1,3-dialkynylisoindole 7a bearing
two different trialkylsilyl groups at the alkyne termini
(entry 1). Note that the alkynylation of 5 also took place
with phenyl- and tert-butyl-substituted 2c and 2e, which
did not give the alkynylated products in the reaction with 1a
(entries 2 and 3). Unsymmetrical 1,3-dialkynylisoindoles
7d and 7e could also be synthesized via the reaction with
2-thienyl- and 3-pyridyl-substituted 2f and 2g, respectively
(entries 4 and 5). These results indicate that 5 is a more
suitable substrate than 1a for the CꢀH alkynylation.
1,4-Bis(bromoethynyl)benzene (2h) reacted with 2 equiv
of 5 to afford phenylene-linked 7f in good yield (eq 4).
These examples clearly demonstrate the utility of the
cycloaddition-based alkynylation protocol for the synthesis
of unsymmetrically substituted dialkynylated products.
entry
1-bromo-1-alkyne
temp (°C)/time (h)
yield (%)b
1
2b [R = SiMe2(t-Bu)]
2c (R = Ph)
110/48
110/48
135/48
110/24
110/24
81 (7a)
49 (7b)
69 (7c)
60 (7d)
79 (7e)
2c
3c
4
2e (R = t-Bu)
2f (R = 2-thienyl)
2g (R = 3-pyridyl)
5
a 5 (0.15 mmol), 2 (0.18 mmol), and K3PO4 (0.30 mmol) were reacted
in 1,4-dioxane (0.25 mL). b Isolated yield based on 5. c 5 (0.20 mmol),
2 (0.24 mmol), and K3PO4 (0.40 mmol) in 1,4-dioxane (0.3 mL).
step.17 We found that a dba- and P(t-Bu)3-free palla-
dium precursor, for example, Pd(OAc)2, was suitable
for this one-pot conversion (entries 1ꢀ9). In the CꢀH
alkynylation step, isoindoles bearing N-Et, N-i-Pr, and
N-t-Bu groups required prolonged reaction time or higher
reaction temperature for high-yield formation of 3ab, 3ac,
and 3ad (entries 2ꢀ4). The CꢀH alkynylation was appli-
cable to the isoindoles bearing methoxy, trifluoromethyl,
or methyl groups in the six-membered ring (entries
5ꢀ7). It should be noted that sterically hindered 4,7-
diphenylisoindole and 4,5,6,7-tetraphenylisoindole un-
derwent CꢀH alkynylation to give 3ah and 3ai in high
yields (entries 8 and 9). Along with 3aa, 3abꢀ3ai were
stable under air and could be purified by column chroma-
tography on silica gel.
The triisopropylsilyl group of 3aa could be derivatized
with retention of the isoindole structure (Scheme 2).
Treatment of 3aa with Bu4NF in THF resulted in 1,3-
diethynylisoindole 8 in 93% yield (left).18 Desilylative
Sonogashira coupling of 3aa with aryl iodides afforded
1,3-di(arylethynyl)isoindoles 9aꢀ9c in high yields in the
presence of AgF (right).19
To enhance the synthetic utility of the CꢀH alky-
nylation, we then examined a one-pot synthesis of
1,3-dialkynylisoindoles from stable and easily available
isoindolines 6 via palladium-catalyzed dehydrogenation
(Table 3). As we established in previous studies,7b the
corresponding isoindoles 1 were obtained in high yields
via dehydrogenation of 6 in 1,4-dioxane at 110 °C in the
presence of cyclohexene as a hydrogen acceptor and a
catalytic amount of Pd(dba)2 or Pd[P(t-Bu)3]2. How-
ever, one-pot CꢀH alkynylation of 2a (2.5 equiv) in the
presence of the dba- and P(t-Bu)3-based palladium
catalysts suffered from low yields under these condi-
tions because of a side reaction of 2a in the second
Characteristic photophysical properties of the alky-
nylated isoindoles 3a, 5, 7, and 9 are summarized in Table 4.
The 1,3-bis(triisopropylsilylethynyl)isoindoles 3aaꢀ3ai
showed two strong absorption maxima centered at
375ꢀ393 (λab1) and 394ꢀ409 nm (λab2). Some of these
compounds showed two emission maxima centered at
407ꢀ422 (λem1) and 426ꢀ441 nm (λem2). As expected, red
(14) The ring-opening with elimination of HBr probably accelerated
by β-effect of the silyl group. For β-effect of silyl group in elimination of
sulfenic acid from β-silylvinyl sulfoxide, see: (a) Nakamura, S.; Kusuda,
S.; Kawamura, K.; Toru, T. J. Org. Chem. 2002, 67, 640. For silicon-
assisted elimination of HBr from β-silylvinyl bromide, see: (b) Jacobi,
P. A.; Tassa, C. Org. Lett. 2003, 5, 4879.
(15) For examples of retro-[4 þ 2] cycloaddition of isoindole adducts,
see: (a) Bornstein, J.; Remy, D. E. J. Chem. Soc. Chem. Commun. 1972,
1149. (b) Kadzimirsz, D.; Hildebrandt, D.; Merz, K.; Dyker, G. Chem.
Commun. 2006, 661.
(16) A modified reaction conditions was established for preparation
of 5. 1a was reacted with 2a (1.0 equiv) in 1,4-dioxane at 110 °C in the
presence of K3PO4 (2.0 equiv). After 2 h, heating was stopped. The
reaction gave monoalkynylated 5 as a major product and it was isolated
in 69% yield after distillation.
(18) Although 8 was able to handle in solution at low concentration,
it was easily converted to black-colored, structure-undefined higher
molecular weight compounds after concentration.
(19) For desilylative Sonogashira coupling, see: Nishihara, Y.; Inoue,
E.; Noyori, S.; Ogawa, D.; Okada, Y.; Iwasaki, M.; Takagi, K. Tetra-
hedron 2012, 68, 4869 and references therein.
(20) For the effect of trimethylsilylethynyl group on the fluorescence
quantum yield of pyrene, see: Maeda, H.; Maeda, T.; Mizuno, K.;
Fujimoto, K.; Shimizu, H.; Inoue, M. Chem.;Eur. J. 2006, 12, 824.
(17) Oxidative dimerization of 2a took place to afford a significant
amount of bis(triisopropylsilyl)butadiyne.
Org. Lett., Vol. XX, No. XX, XXXX
C

Table 3. One-pot Synthesis of 1,3-Dialkynylisoindoles 3a from
Isoindolines 6a
Table 4. Photophysical Properties of Alkynylated Isoindoles 3a,
5, 7, and 9 in CH2Cl2
absorption
emissiona
λmax/nm (log ε)
λmax/nm
isoindole
λab1
λab2
λem1
λem2
Φb
3aa
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
5c
376 (4.53)
375 (4.43)
377 (4.48)
382 (4.48)
374 (4.42)
382 (4.33)
375 (4.50)
393 (4.35)
389 (4.35)
353 (4.41)
375 (4.50)
395 (4.59)
374 (4.37)
401 (4.46)
397 (4.53)
449 (4.85)
414 (4.62)
413 (4.72)
475 (4.81)
396 (4.60)
395 (4.50)
396 (4.55)
402 (4.54)
394 (4.49)
400 (4.36)
394 (4.56)
409 (4.39)
409 (4.39)
407
407
410
418
422
431
407
461
449
406
406
450
411
457
453
493
445
446
528
426
427
429
435
441
0.88
0.86
0.89
0.83
0.90
0.92
0.90
0.20
0.72
0.57
0.86
0.01
0.84
0.001
0.01
0.66
0.13
0.23
0.03
temp (°C)/time (h) yield of 3a
in the second step
entry R1
R2 R3 R4
(%)b
1c
2
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
Me 6a
Et 6b
110/48
110/72
135/48
135/48
135/24
135/24
135/48
135/72
135/24
71 (3aa)
83 (3ab)
88 (3ac)
90 (3ad)
67 (3ae)
88 (3af)
55 (3ag)
79 (3ah)
86 (3ai)
426
H
3
H
i-Pr 6c
t-Bu 6d
Me 6e
Me 6f
Me 6g
4
H
7a
395 (4.57)
416 (4.66)
392 (4.40)
424 (4.43)
419 (4.65)
476 (4.89)
434 (4.57)
434 (4.71)
426
5
OMe
CF3
Me
H
7b
6
7c
427
486
486
533
472
474
7
8d
9d
Ph Me 6h
7d
7e
Ph
Ph Ph Me 6i
7f
a Pd(OAc)2 (0.015 mmol), cyclohexene (1.5 mmol), and 6 (0.30
mmol) were reacted in 1,4-dioxane (0.45 mL) at 110 °C for 14 h. 2a
(0.75 mmol) and K3PO4 (1.2 mmol) were then added to the mixture and
the resulting mixture was stirred at 110ꢀ135 °C for 24ꢀ72 h. b Isolated
yield based on 6. c 0.4 mmol scale reaction. d 0.2 mmol scale reaction.
9a
9b
9cc
a Excited at the wavelength of λab2
.
b Absolute PL quantum yield.
c Emission was excited at the wavelength of λab1
.
1,3-dialkynylisoindoles via CꢀH alkynylation of isoin-
doles. The reaction proceeded via [4 þ 2] cycloaddition
followed by a ring-opening reaction. Remarkable stability
under air and strong fluorescence were observed for some
alkynylatedisoindoles. The syntheticmethodologyand the
fundamental insights into chemicaland physical properties
shown here would be valuable in development of new
functional molecules based on alkynylisoindoles.
Scheme 2. Desilylative Conversion of 3aa
Acknowledgment. We thank Dr. Yuuya Nagata at
Kyoto Univ. for helping with X-ray crystallographic ana-
lysis. This work is supported by Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation
Directed Toward Straightforward Synthesis (No. 23105520)”
from MEXT.
shifts of both absorption maxima (449 and 476 nm) and
emission maxima (493 and 533 nm) were observed in the
spectra of phenylene-linked 7f. Strong fluorescence was
observed for 3aa-3ag, 7a, and 7c (Φ 0.83ꢀ0.92),20 whereas
7b, 7d, and 7e, which bear an arylethynyl group, showed
very weak fluorescence (Φ 0.001ꢀ0.01).
Supporting Information Available. Experimental details
and characterization data of the products. This material is
available free of charge via the Internet at http://pubs.acs.org.
In conclusion, we have established an efficient method
for synthesis of both symmetrical and unsymmetrical
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX