Synthesis of isochromene and related derivatives by rhodium-catalyzed oxidative coupling of benzyl and allyl alcohols with alkynes

Article abstract of DOI:10.1021/jo201923d

The straightforward synthesis of isochromene derivatives and related cyclic ethers is achieved by the rhodium-catalyzed oxidative coupling of α,α-disubstituted benzyl and allyl alcohols with alkynes. The hydroxy groups effectively act as the key function for the regioselective C-H bond cleavage.

Full text of DOI:10.1021/jo201923d

Note
pubs.acs.org/joc
Synthesis of Isochromene and Related Derivatives by Rhodium-
Catalyzed Oxidative Coupling of Benzyl and Allyl Alcohols with
Alkynes
Keisuke Morimoto, Koji Hirano, Tetsuya Satoh,* and Masahiro Miura*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: The straightforward synthesis of isochromene derivatives and related
cyclic ethers is achieved by the rhodium-catalyzed oxidative coupling of α,α-
disubstituted benzyl and allyl alcohols with alkynes. The hydroxy groups effectively act
as the key function for the regioselective C−H bond cleavage.
cleavage by employing a copper oxidant to produce
isochromene derivatives. In addition, the reactions of related
alcohols such as a naphthalenemethanol and an allyl alcohol
have also been examined. The new findings are described
herein.
sochromene and related cyclic ether structures can be widely
Iseen in various biologically active compounds and natural
products.1 Therefore, the development of methods for their
effective construction from simple, readily available building
blocks has attracted considerable attention.2 One of the most
reliable and versatile methods for their syntheses is the
palladium-catalyzed annulation by the coupling of aryl halides
In an initial attempt, α,α-dimethylbenzyl alcohol (1a) (1.5
mmol) was treated with diphenylacetylene (2a) (0.5 mmol) in
the presence of [Cp*Rh(MeCN)3][SbF6]2 (0.02 mmol, Cp* =
pentamethylcyclopentadienyl) and Cu(OAc)2·H2O (1 mmol)
as catalyst and oxidant, respectively, in refluxing dioxane (3
mL) under N2 for 6 h. As a result, a heterocyclic product, 1,1-
dimethyl-3,4-diphenyl-1H-isochromene (3a), was formed in
82% isolated yield (entry 1 in Table 1). Under similar
conditions, the couplings of 1a with 4-methyl- (2b), methoxy-
(2c), and chloro-substituted (2d) diphenylacetylenes pro-
ceeded smoothly to form the corresponding 3,4-diarylisochro-
menes 3b−d (entries 2−4). 1-Phenylpropyne (2e) also reacted
with 1a to give a 4-methyl-3-phenylisochromene derivative 3e
predominantly, along with a minor amount of regioisomer 3e′
(entry 5). The reaction of 1a with 4-octyne (2f) gave 3,4-
dipropylisochromene 3f in a moderate yield (entry 6). We
previously reported that treatment of triphenylmethanol (1b)
with 2a in the presence of [RhCl(cod)]2, 1,2,3,4-tetraphenyl-
cyclopenta-1,3-diene, and Cu(OAc)2·H2O as catalyst, ligand,
and oxidant, respectively, in refluxing o-xylene resulted in
quantitative formation of 1,2,3,4-tetraphenylnaphthalene, ac-
companied by C−H and C−C bond cleavages.7 In contrast,
under the present milder conditions using the cationic rhodium
catalyst, isochromene formation exclusively took place in the
coupling of 1b and 2a (entry 7). In this case, no naphthalene
Scheme 1. Catalytic Synthesis Pathways for Cyclic Ethers
bearing an oxygen nucleophile with alkynes (Scheme 1, path
a).3
Obviously, a more atom- and step-economical version is the
oxidative coupling using nonhalogenated aromatic substrates
via C−H bond cleavage,4 as depicted in Scheme 1, path b.
Needless to say, compared to the disubstituted aromatic
substrates in path a, the parent monosubstituted ones in path b
are readily available.
In the course of our study of the rhodium-catalyzed oxidative
coupling of monosubstituted arenes with alkynes to form
benzannulated heterocyclic compounds,5,6 we have undertaken
the coupling of benzyl alcohols with alkynes under rhodium
catalysis. As a result, the expected reaction has been found to
proceed smoothly through hydroxy-directed C−H bond
Received: September 16, 2011
Published: October 11, 2011
© 2011 American Chemical Society
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Note
a
contrast to these tertiary alcohols, α,α-unsubstituted benzyl
alcohol showed low reactivity: only a small amount (∼15%) of
coupling product was detected by GC−MS.
A plausible mechanism for the coupling of 1 with 2 is
illustrated in Scheme 2, in which neutral ligands are omitted.
Table 1. Coupling of Alcohols 1 with Alkynes 2
Scheme 2. Plausible Mechanism for the Reaction of 1 with 2
Coordination of the alcoholic oxygen of 1 to Cp*Rh(III)X2
gives a rhodium(III) alcoholate A. Subsequent ortho-rhodation
to form a rhodacycle intermediate B, alkyne insertion, and
reductive elimination occur to produce an isochromene 3. The
resulting Cp*Rh(I) species may be oxidized by a copper(II)
salt to regenerate Cp*Rh(III)X2.
Besides aromatic alcohols, α,α-dimethylallyl alcohol (5) also
underwent the oxidative coupling. Thus, the reaction of 5 (1.5
mmol) with 2a (0.5 mmol) in the presence of [Cp*Rh-
(MeCN)3][SbF6]2 (0.02 mmol) and Cu(OAc)2·H2O (1 mmol)
in refluxing dioxane for 6 h proceeded smoothly, accompanied
by vinylic C−H bond cleavage to produce the corresponding
2H-pyran derivative 6 in 60% yield (Scheme 3). However,
Scheme 3. Coupling of Alcohol 5 with 2a
treatment of γ-substituted allyl alcohols such as cinnamyl
alcohol and (E)-2-methylpent-3-en-2-ol with 2a under similar
conditions gave no or trace amount of coupling products.
In summary, we have demonstrated that the oxidative
coupling of α,α-disubstituted benzyl alcohols with alkynes can
be performed in the presence of a rhodium catalyst and a
copper salt oxidant to afford the corresponding isochromene
derivatives. The reaction system has also been found to be
applicable to the 2H-pyran synthesis from an allyl alcohol.
a
Reaction conditions: 1 (1.5 mmol), 2 (0.5 mmol), [Cp*Rh-
(MeCN)3][SbF6]2 (0.02 mmol), Cu(OAc)2·H2O (1 mmol), in
b
refluxing dioxane (3 mL) under N2 for 6 h. 4 (0.12 mmol) was
also formed.
formation could be detected. para-Substituted benzyl alcohols
1c and 1d also underwent the coupling with 2a to form 3h and
3i, respectively (entries 8 and 9). In the former case,
homocoupling8 of relatively electron-rich 1c took place
competitively to form a considerable amount (0.12 mmol) of
4 in addition to 3h. The reaction using α,α-dimethyl-2-
naphthalenemethanol (1e) in place of benzyl alcohols occurred
in a similar manner to give a tricyclic product 3j (entry 10). In
EXPERIMENTAL SECTION
■
General Procedures. 1H and 13C NMR spectra were recorded at
400 and 100 MHz for CDCl3 solutions. MS data were obtained by EI.
GC analysis was carried out using a silicon OV-17 column (i.d. 2.6 mm
× 1.5 m). GC−MS analysis was carried out using a CBP-1 capillary
column (i.d. 0.25 mm × 25 m). The structures of all products listed
below were unambiguously determined by H and 13C NMR with the
1
aid of NOE, COSY, HMQC, and HMBC experiments.
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Note
Alcohols 1c−e,9 diarylacetylenes 2b−d,10 and [Cp*Rh(MeCN)3]-
1,1,6-Trimethyl-3,4-diphenyl-1H-isochromene (3h). A solid:
mp 81−82 °C; H NMR (400 MHz, CDCl3) δ 1.77 (s, 6H), 2.21 (s,
6e
1
[SbF6]2 were prepared according to published procedures. Other
3H), 6.70 (s, 1H), 7.01 (d, J = 8.0 Hz, 1H), 7.09−7.13 (m, 4H), 7.22−
7.24 (m, 4H), 7.27−7.36 (m, 3H); 13C NMR (100 MHz, CDCl3) δ
21.3, 27.2, 77.6, 115.6, 122.2, 124.1, 126.8, 127.4, 127.5, 127.7, 128.5,
128.8, 131.7, 131.8, 133.6, 136.1, 136.8, 137.1, 148.2; HRMS m/z
calcd for C24H22O (M+) 326.1671, found 326.1668.
6-Chloro-1,1-dimethyl-3,4-diphenyl-1H-isochromene (3i). A
solid: mp 105−106 °C; 1H NMR (400 MHz, CDCl3) δ 1.77 (s, 6H),
6.85 (d, J = 2.3 Hz, 1H), 7.10−7.16 (m, 5H), 7.20−7.24 (m, 4H),
7.28−7.37 (m, 3H); 13C NMR (100 MHz, CDCl3) δ 22.0, 77.6, 114.8,
123.3, 123.7, 126.5, 127.2, 127.5, 128.0, 128.7, 128.8, 131.6, 133.3,
134.0, 134.5, 135.6, 136.3, 149.4; HRMS m/z calcd for C23H19ClO
(M+) 346.1124, found 346.1128.
1,1-Dimethyl-3,4-diphenyl-1H-benzo[g]isochromene (3j). A
solid: mp 153−154 °C; 1H NMR (400 MHz, CDCl3) δ 1.88 (s, 6H),
7.13−7.15 (m, 3H), 7.26−7.40 (m, 10H), 7.60 (d, J = 6.9 Hz, 1H),
7.63 (s, 1H), 7.76 (d, J = 6.9 Hz, 1H); 13C NMR (100 MHz, CDCl3)
δ 27.5, 77.9, 115.8, 120.9, 121.9, 125.4, 125.9, 127.0, 127.4, 127.5,
127.8, 127.8, 128.6, 128.8, 130.5, 131.8, 132.6, 132.9, 136.1, 136.5,
137.2, 148.8; HRMS m/z calcd for C27H22O (M+) 362.1671, found
362.1661.
starting materials and reagents were commercially available.
General Procedure for Coupling of Alcohols with Alkynes.
To a 20 mL two-necked flask with a reflux condenser, a balloon, and a
rubber cup were added alcohol 1 or 5 (1.5 mmol), alkyne 2 (0.5
mmol), [Cp*Rh(MeCN)3][SbF6]2 (0.02 mmol, 17 mg), Cu-
(OAc)2·H2O (1 mmol, 200 mg), 1-methylnaphthalene (ca. 50 mg)
as internal standard, and dioxane (3 mL). Then, the resulting mixture
was stirred under nitrogen at 110 °C (bath temperature) for 6 h. After
cooling, the reaction mixture was extracted with ethyl acetate (100
mL) and ethylenediamine (2 mL). The organic layer was washed by
water (100 mL, three times) and dried over Na2SO4. After evaporation
of the solvents under vacuum, product 3 or 6 was isolated by column
chromatography on silica gel using hexane/ethyl acetate (97:3, v/v) as
eluant.
1,1-Dimethyl-3,4-diphenyl-1H-isochromene (3a). A solid: mp
1
90−91 °C; H NMR (400 MHz, CDCl3) δ 1.78 (s, 6H), 6.89 (d, J =
8.0 Hz, 1H), 7.10−7.14 (m, 4H), 7.19 (d, J = 5.1 Hz, 2H), 7.22−7.35
(m, 7H); 13C NMR (100 MHz, CDCl3) δ 27.1, 77.7, 115.7, 122.2,
123.6, 126.8, 126.9, 127.2, 127.4, 127.7, 128.5, 128.8, 131.7, 132.0,
136.0, 136.3, 137.0, 148.1; HRMS m/z calcd for C23H20O (M+)
312.1514, found 312.1523.
4-Menthyl-2,4-bis(4-methylphenyl)pent-1-ene (4).8d An oil:
1H NMR (400 MHz, CDCl3) δ 1.18 (s, 6H), 2.30 (s, 3H), 2.32 (s,
3H), 2.78 (s, 2H), 4.73 (s, 1H), 5.12 (s, 1H), 7.05 (d, J = 6.4 Hz, 4H),
7.18 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 20.8, 21.1, 28.7, 38.4,
49.5, 116.1, 125.8, 126.4, 128.5, 128.7, 134.8, 136.5, 140.6, 146.5,
146.7; HRMS m/z calcd for C20H24 (M+) 264.1878, found 264.1879.
2,2-Dimethyl-5,6-diphenyl-2H-pyran (6). A solid: mp 77−78
°C; 1H NMR (400 MHz, CDCl3) δ 1.74 (s, 3H), 1.89 (s, 3H), 5.88 (d,
J = 11.9 Hz, 1H), 7.06 (d, J = 11.9 Hz, 1H), 7.19−7.30 (m, 3H), 7.34−
7.42 (m, 4H), 7.51 (t, J = 7.3 Hz, 1H), 7.95 (d, J = 6.0 Hz, 2H); 13C
NMR (100 MHz, CDCl3) δ 18.4, 26.5, 121.6, 126.0, 126.8, 127.5,
128.6, 128.7, 129.8, 133.4, 136.9, 137.7, 137.8, 141.4, 198.9; HRMS m/
z calcd for C19H18O (M+) 262.1358, found 262.1347.
1,1-Dimethyl-3,4-bis(4-methylphenyl)-1H-isochromene
1
(3b). An oil: H NMR (400 MHz, CDCl3) δ 1.79 (s, 6H), 2.28 (s,
3H), 2.40 (s, 3H), 6.91 (d, J = 7.6 Hz, 1H), 6.97 (d, J = 8.3 Hz, 2H),
7.12−7.21 (m, 9H); 13C NMR (100 MHz, CDCl3) δ 21.2, 21.3, 27.0,
77.5, 115.04, 122.1, 123.5, 126.6, 127.1, 128.2, 128.6, 129.3, 131.5,
132.3, 133.3, 134.1, 136.3, 136.4, 137.5, 148.0; HRMS m/z calcd for
C25H24O (M+) 340.1827, found 340.1824.
3,4-Bis(4-methoxyphenyl)-1,1-dimethyl-1H-isochromene
1
(3c). An oil: H NMR (400 MHz, CDCl3) δ 1.76 (s, 6H), 3.73 (s,
3H), 3.82 (s, 3H), 6.67 (d, J = 9.1 Hz, 2H), 6.90−6.91 (m, 3H), 7.10−
7.23 (m, 7H); 13C NMR (100 MHz, CDCl3) δ 27.0, 55.1, 55.2, 77.4,
112.9, 114.1 (overlapped), 122.1, 123.4, 126.5, 127.1, 128.6, 129.4,
130.1, 132.5, 132.7, 136.2, 147.8, 158.4, 158.9; HRMS m/z calcd for
C25H24O3 (M+) 372.1725, found 372.1727.
ASSOCIATED CONTENT
■
S
3,4-Bis(4-chlorophenyl)-1,1-dimethyl-1H-isochromene
* Supporting Information
1
1
Copies of H and 13C NMR spectra of products. This material
(3d). A solid: mp 115 °C; H NMR (400 MHz, CDCl3) δ 1.78 (s,
6H), 6.84 (d, J = 7.6 Hz, 1H), 7.13−7.24 (m, 9H), 7.34 (d, J = 8.3 Hz,
2H); 13C NMR (100 MHz, CDCl3) δ 27.1, 78.0, 114.8, 122.3, 123.4,
127.3, 127.4, 127.9, 129.0, 130.0, 131.3, 132.9, 133.1, 133.8, 134.2,
135.3, 136.2, 147.4; HRMS m/z calcd for C23H18Cl2O (M+) 380.0735,
found 380.0732.
1,1,4-Trimethyl-3-phenyl-1H-isochromene (3e) and 1,1,3-
Trimethyl-4-phenyl-1H-isochromene (3e′). An oil: 1H NMR
(400 MHz, CDCl3) δ 1.67 (s, 6H, 3e′), 1.69 (s, 6H, 3e), 1.79 (s,
3H, 3e′), 2.13 (s, 3H, 3e), 6.60 (d, J = 7.3 Hz, 1H, 3e′), 7.03−7.08 (m,
1H, 3e′), 7.11 (d, J = 3.6 Hz, 2H, 3e′), 7.15 (d, J = 6.5 Hz, 1H, 3e),
7.20−7.42 (m, 6H, 3e and 5H, 3e′), 7.50−7.53 (m, 2H, 3e); 13C NMR
(100 MHz, CDCl3) δ 13.9, 18.5, 26.8, 27.1, 77.0, 106.8, 113.9, 121.8,
122.0, 122.1, 122.7, 126.0, 126.6, 126.8, 127.1, 127.3, 127.8, 128.2,
128.4, 129.4, 131.1, 131.9, 132.6, 135.6, 135.9, 137.1, 137.5, 147.6,
148.6; HRMS m/z calcd for C18H18O (M+) 250.1358, found 250.1353
and 250.1356.
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: satoh@chem.eng.osaka-u.ac.jp, miura@chem.eng.
osaka-u.ac.jp.
■
ACKNOWLEDGMENTS
This work was partly supported by Grants-in-Aid from MEXT
and JSPS, Japan.
■
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