Carbon-carbon bond cleavage of diynes through the hydroamination with transition metal catalysts

Article abstract of DOI:10.1021/ja034105o

The C-C bond cleavage of terminal and internal diynes takes place readily in the presence of catalytic amounts of Ru₃(CO)₁₂ or Pd(NO₃)₂ and of 2-aminophenol, giving the corresponding benzoxazoles and ketones in good to high yields. There are two different modes of the bond cleavage: (a) an alkyne C-C triple bond is cleaved, and (b) the C-C single bond between the two alkyne groups is cleaved. Copyright

Full text of DOI:10.1021/ja034105o

Published on Web 05/10/2003
Carbon-Carbon Bond Cleavage of Diynes through the Hydroamination with
Transition Metal Catalysts
Tomohiro Shimada and Yoshinori Yamamoto*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Sendai 980-8578, Japan
Received January 9, 2003; E-mail: yoshi@yamamoto1.chem.tohoku.ac.jp
Table 1. Ru-Catalyzed Carbon-Carbon Bond Cleavage of
Terminal Diynes 1 with 2-Aminophenol 2aa
Althogh a number of researches on C-C bond cleavage have
been carried out,1 only a few examples on the cleavage of alkynes
are known: C-C bond cleavage of alkynes ligated to metal
complexes2 and oxidative cleavage.3 Furthermore, most of these
reactions do not proceed in a catalytic manner, and the use of a
stoichiometric amount of reagents is needed.4 Recently, Jun et al.
reported catalytic C-C triple bond cleavage through the rhodium-
catalyzed hydroiminoacylation.5 During our investigation on pal-
ladium-catalyzed hydroamination of alkynes using o-aminophenol,6
we found that the C-C bond cleavage of diynes 1 with o-
aminophenols 2 is catalyzed efficiently by Ru and Pd catalysts to
give the corresponding 2-substituted benzoxazoles 3 and 4 (eq 1).
1
2
b
entry
R
R
1
reaction time, h
yield, %
3:4
3:1
4:1
7:1
1
2
3
4
5
6
7
nHex
nDec
H
H
H
H
H
H
H
1a
1b
1c
1d
1e
1f
20
36
20
20
20
20
20
98
74c
81
81
77
70
58
Cyclohexyl
tBu
>30:1
Ph(CH2)2
TIPSO(CH2)4
Cl(CH2)3
3:1
3:1
1:1
1g
a Reaction conditions: 0.75 mmol 2-aminophenol 2a, 0.50 mmol diyne,
1 mol % Ru3(CO)12, 3 mol % NH4PF6, methanol (10 M), 80 °C. b Combined
isolated yields based on 2-aminophenol. c 2-Dodecanone was obtained in
11% yield. TIPS ) tris(triisopropyl)silyl
Table 2. Ru-Catalyzed Carbon-Carbon Bond Cleavage of
1,3-Decadiyne 1a with Substituted Aminophenols 2a
3
b
entry
R
2
reaction time, h
yield, %
3:4
1
2
3
4
5
6c
7
4-NO2
4-Cl
2b
2c
2d
2e
2f
20
20
20
48
20
20
20
81
97
95
70
68
87
79
3:1
3:1
3:1
3:1
3:1
3:1
4:1
4-Me
4-OMe
5-NO2
5-Me
3-Me
2g
2h
The results are summarized in Table 1. The reaction of
1,3-decadiyne 1a (1.0 equiv) with o-aminophenol 2a (R3 ) H, 1.5
equiv) in the presence of a catalytic amount of Ru3(CO)12 and
NH4PF6, followed by hydrolysis of the products, gave the benz-
oxazoles 3a and 4a, almost quantitatively, in a ratio of 3:1 (entry
1). Apparently, 3a was obtained through the C-C triple bond
cleavage as shown in path a, and 4a was obtained through path b.
Since R2 ) H, acetone 3a′ must be produced in the same yield as
3a and 2-octanone 4a′ in the same yield as 4a. Although detection
of acetone was not attempted due to its volatile characteristics, the
formation of 2-octanone was detected (see also entry 2, vide infra).
We examined a number of reaction conditions and finally found
that the highest chemical yield and highest ratio of 3a/4a were
obtained under the conditions similar to those of Wakatsuki’s
hydroamination.7 We tested various kinds of d8 and d10 transition
metals that exhibit catalytic activities for hydroamination.8 When
AuCl3 and CuCl were used instead of Ru3(CO)12, a 3:1 mixture of
3a and 4a was obtained almost quantitatively. However, the use of
PdCl2, PtCl2, and CuI afforded the products in 45-64% yields
although the product ratio was again 3:1. The reaction of 1,3-
tetradecadiyne 1b gave a 4:1 mixture of 3b and 4b in 74% yield
along with 2- dodecanone 4b′ (11% yield) (entry 2). Cyclohexyl-
substituted diyne 1c also gave a mixture of benzoxazoles 3c and
4c in the ratio of 7:1 in 81% yield (entry 3). The sterically bulkier
diyne 1c gave higher ratio of 3:4. Interestingly, sterically more
hindered benzoxazoles 3a-c, in comparison with 4a-c, were
obtained as the major product. The reaction of 1d having tBu group
as R1 substituent gave almost exclusively 3d in 81% yield (entry
a Reaction conditions: 0.75 mmol 2-aminophenol 2, 0.50 mmol diyne,
1 mol % Ru3(CO)12, 3 mol % NH4PF6, methanol (10 M), 80 °C. b Combined
isolated yields based on 2-aminophenols. c Concentration 5 M, due to the
insolubility of 2g.
4). When the phenyl- and siloxy-substituted diynes 1e and 1f were
employed, the cleavage reaction also proceeded in good yields
(entries 5 and 6). Even the chlorine-substituted diyne 1g gave the
C-C bond cleavage products 3g and 4g in 58% yield (entry 7).
Next, we investigated the C-C bond cleavage with substituted
aminophenols (Table 2). When a proton at the C-4 position was
substituted with an electron-withdrawing NO2 group (2b), the yield
of the products became lower than that of 2a (entry 1 of Table 2
vs entry 1 of Table 1). The chloro- and methyl-substituted
aminophenols 2c and 2d gave results similar to those for 2a. The
reaction of 4-methoxy-aminophenol 2e, having a strong electron-
donating group, was very sluggish (entry 4). Thus, both a strong
EWG and EDG at the C-4 position retarded the C-C bond
cleavage. This trend was also observed when the 5-substituted
aminophenols 2f and 2g were employed (entries 5 and 6).9 The
sterically hindered aminophenol, 3-methyl-aminophenol 2h, gave
the products in lower yield, but the selectivity for 3 was slightly
higher than for the others (entry 7).
The C-C cleavage reaction of internal diynes proceeded
smoothly with palladium(II) catalysts. The catalyst Pd(NO3)2 gave
the best result among palladium catalysts.10 Other catalysts such
as Ru and Au did not work at all in the case of internal diynes.
Among various solvents, nBuOH was the most efficient.11 The
9
6646
J. AM. CHEM. SOC. 2003, 125, 6646-6647
10.1021/ja034105o CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 3. Pd-Catalyzed Carbon-Carbon Bond Cleavage of
Internal Diynes 1 with 2-Aminophenol 2aa
1
2
b
entry
R
R
1
reaction time, h
yield, %
3:4
1
2
nBu
Ph
nBu
Ph
1o
1p
4
24
97
89
1:1
1:3
a Reaction conditions: 0.75 mmol 2-aminophenol 2a, 0.50 mmol diyne,
6 mol % Pd(NO3)2, n-butanol (10 M), 120 °C. b Combined isolated yields
based on 2-aminophenol.
Catalytic C-C bond cleavage procedures are not popular in
organic chemistry, and no method employing diynes is known. The
present method supplies a new pair of scissors for C-C bond
cleavage.
Scheme 1. A Proposed Mechanism for the C-C Bond Cleavage
of Diynes
Supporting Information Available: Spectroscopic and analytical
data of synthesized compounds and information on procedures (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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2001, 42, 464. (c) Mitsudo, T.-A.; Kondo, T. Synlett 2001, 309. (d) Werner,
H.; Bleuel, E. Angew. Chem., Int. Ed. 2001, 40, 145. (e) Rybtchinski, B.;
Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (f) Jennings, P. W.;
Johnson, L. L. Chem. ReV. 1994, 94, 2241. (g) Crabtree, R. H. Chem.
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Bonds and Organic Synthesis; Murai, S. Ed.; Springer: Berlin, 1999; pp
97-129.
(2) (a) Morris, M. J. In Metal Clusters in Chemistry; Braunstein, P., Oro, L.
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235. (b) Chin, C. S.; Chong, D. C.; Maeng, B.; Ryu, J.; Kim, H.; Kim,
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Y.; Morris, M. J.; Spey, S. E. J. Chem. Soc., Dalton Trans. 2002, 2907.
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E. B. Organometallics 2002, 21, 2724. (e) Hayashi, N.; Ho, D. M.; Pascal,
R. A., Jr. Tetrahedron Lett. 2000, 41, 4261. (f) Cairns, G. A.; Carr, N.;
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M.; Pu, L. J. Am. Chem. Soc. 1990, 112, 9013. (h) Degani, Y.; Willner,
I. J. Chem. Soc., Chem. Commun. 1985, 648. (i) Sullivan, B. P.; Smythe,
R. S.; Kober, E. M.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 4701.
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Y. Bull. Chem. Soc. Jpn. 1983, 56, 1133.
(4) Alkyne metathesis proceeds in a catalytic manner, but in our opinion, the
metathesis is categorized as an exchange reaction of alkynes, and not as
the C-C bond cleavage reaction mentioned in this paper. For review see;
Bunz, U. H. F.; Kloppenburg, L. Angew. Chem., Int. Ed. 1999, 38, 478.
See also: (a) Rosenthal, U.; Arndt, P.; Baumann, W.; Burlakov, V. V.;
Spannenberg, A. J. Organomet. Chem. 2003, 670, 84. (b) Chisholm, M.
H.; Davidson, E. R.; Quinlan, K. B. J. Am. Chem. Soc. 2002, 124, 15351.
(5) Jun, C.-H.; Lee, H.; Moon, C. W.; Hong, H.-S. J. Am. Chem. Soc. 2001,
123, 8600.
(6) Shimada, T.; Yamamoto, Y. J. Am. Chem. Soc. 2002, 124, 12670.
(7) The Ru-catalyzed hydroamination of terminal alkynes, see: Tokunaga,
M.; Eckert, M.; Wakatsuki, Y. Angew. Chem., Int. Ed. 1999, 38, 3222.
(8) Various kinds of catalysts are investigated for intramolecular hydro-
amination of alkynes, see: Mu¨ller, T. E.; Pleier, A.-K. J. Chem. Soc.,
Dalton Trans. 1999, 583.
(9) The precise reason for this electronic effect of the substituents is not clear,
but a smilar effect was also observed in the Pd-catalyzed hydroamination
of monoalkynes using o-aminophenols (unpublished results).
(10) The yields of the reaction of 5,7-dodecadiyne 1o with 2a in the presence
of palladium catalysts (6 mol %) and NH4PF6 (12 mol %) are as follows;
Pd(NO3)2 (97%), PdCl2(PPh3)2 (76%), Na2PdCl4 (69%) and PdCl2 (65%).
(11) Higher reaction temperature was needed (120 °C); therefore, nBuOH was
used instead of MeOH.
(12) The steric congestion at the quaternary carbon center of the oxazole ring
is not so influential on the relative stability of 9 and 10 since the substituent
R2CH2 or R1 takes an orthogonal direction to the oxazole plane.
reaction of 5,7-dodecadiyne 1o (0.50 mmol) with 2a (0.75 mmol)
in the presence of Pd(NO3)2 gave a 1:1 mixture of 3o and 4o in
97% yield (entry 1, Table 3). Here again, 2-heptanone 3o′ and
2-hexanone 4o′ were obtained along with 3o and 4o. Very similarly,
the reaction of diphenylbutadiyne 1p produced a 1:3 mixture of
3p and 4p in 89% yield together with 1-phenyl-2-pentanone 3p′
and acetophenone 4p′ (entry 2, Table 3).
The catalytic hydroamination of one of the alkyne groups of the
diynes 1 to give the enamines 5 is the key step of the C-C bond
cleavage (Scheme 1).6-8 Tautomerization of 5 gives the corre-
sponding R,â-unsaturated imines 6. The conjugate addition of
aminophenol 2a to 6 provides the â-aminoimines 7 and their
tautomers 8. The intramolecular cyclization of the iminophenol
groups of 7 and 8 gives the ketals 9 and 10, respectively. The C-C
bond cleavage through the retro-Mannich-type reaction produces
the benzoxazoles 4 and 3. The selectivity of products is determined
by the relative size of R1 and R2CH2. When sterically hindered
terminal diynes are used, the relative size of R1 is larger than that
of R2CH2. Then, formation of 10 becomes more favorable than
that of 9; with the bulkier R1, steric repulsion between R1 and Ar
of the sp2 plane of 9 becomes stronger than that between R2CH2
and Ar of 10.12 In the case of internal diyne 1p, the size of R1 is
smaller than that of R2CH2, and 4p is selectively obtained.
To obtain supportive data for this mechanism, 2.2 equiv of aniline
were treated with 1a under the reaction conditions of the C-C bond
cleavage. As expected, the â-aminoimine 11 was obtained in 62%
yield (eq 2). Next, the ketone 12 and aminophenol 2a were reacted
in MeOH at 80 °C, giving the expected oxazole products 13 and
14 in 57% yield in a ratio of 3:1 (eq 3). These results strongly
support the proposed mechanism shown in Scheme 1.
JA034105O
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