A novel oxovanadium(V)-induced oxidation of organoaluminum compounds. Highly selective coupling of organic substituents on aluminum [7]

Full text of DOI:10.1021/ja973891s

5124
J. Am. Chem. Soc. 1998, 120, 5124-5125
Scheme 1
A Novel Oxovanadium(V)-Induced Oxidation of
Organoaluminum Compounds. Highly Selective
Coupling of Organic Substituents on Aluminum
Takuji Ishikawa, Akiya Ogawa, and Toshikazu Hirao*
Department of Applied Chemistry, Faculty of
Engineering, Osaka UniVersity
Yamada-oka, Suita, Osaka 565-0871, Japan
ReceiVed NoVember 13, 1997
The redox processes of organometallics are important in
synthetic transformations, especially in transition-metal-induced
reactions.1 Transmetalation and electronic interaction are able
to participate in the control of such a process.2 From this point
of view, main-group organometallics such as silicon, aluminum,
and boron have limited usage due to their restricted redox
processes. A versatile one-electron oxidation method for orga-
nosilicon compounds has been developed by using oxovanadium-
(V) compounds in a previous paper,3,4 permitting oxidative
desilylation via radical cations and radicals. Oxidative transfor-
mations of organoaluminum compounds usually lead to alcohols,5
but to the best of our knowledge, the investigation on selective
carbon-carbon bond formation of organic substituents on alu-
minum has not been undertaken with the aid of metallic oxidants.
This is partly due to the difficulty in the selection of metallic
oxidants. Herein we report a highly selective coupling of
organoaluminum compounds induced by oxovanadium(V) com-
pounds.
Table 1. The Cross-Coupling Reaction of the Ate Complex 1a
entry
1, R
2, yield, %b
1
2
3
4
5
Ph
Ph
n-C6H13
Me3Si
PhCH2OCH2
80 (76)
56c
84 (70)
77 (68)
72 (65)
a VO(OEt)Cl2, 3 equiv. The reaction conditions are shown in Scheme
1. The ate complex 1 was added to the solution of VO(OEt)Cl2 unless
otherwise stated. b Determined by 1H NMR based on 1-octyne. Isolated
yields are shown in parentheses. c VO(OEt)Cl2 was added to the solution
of the ate complex 1.
Scheme 2
The addition of 1-alkynyllithium (1.1 equiv) to the 1-octenyl-
aluminum6 at -78 °C, followed by treatment with VO(OEt)Cl2,
led to a novel cross-coupling of organic substituents on aluminum,
giving the corresponding trans-enyne 2 with excellent stereose-
lectivity (Scheme 1). Interestingly, the homo-coupled diene and
isobutylated products were not obtained at all, although the diyne
probably from excess alkynyllithium was formed as a byproduct.7
Representative results of the cross-coupling reaction are shown
in Table 1. The addition of the ate complex to the solution of
VO(OEt)Cl2 raised the yield of 2 (Table 1, entries 1 and 2).
Similar conditions can be employed with some other 1-alkynyl-
lithiums, affording the conjugate trans-enynes 2 in good yields
Scheme 3
(1) Efficiency of redox-active ligands in the transition-metal-catalyzed
oxidation reactions has been investigated: (a) Hirao, T.; Higuchi, M.; Hatano,
B.; Ikeda, I. Tetrahedron Lett. 1995, 36, 5925. (b) Higuchi, M.; Yamaguchi,
S.; Hirao, T. Synlett 1996, 1213. (c) Hirao, T.; Murakami, T.; Ohno, M.;
Ohshiro, Y. Chem. Lett. 1989, 785; 1991, 299.
(2) For recent reviews for radical generation from organic compounds by
metallic reagents, see: (a) Iqbal. J.; Bhatia. B.; Nayyar. N. K. Chem. ReV.
1994, 94, 519. (b) Dalko. P. I. Tetrahedron 1995, 51, 7579. (c) Arai, N.;
Narasaka, K. Yuki Gousei Kagaku Kyoukaishi 1996, 54, 964. (d) Hirao, T.
Chem. ReV. 1997, 97, 2707. For oxidation of d0 organometallics, see: (e)
Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett, R. J. Am.
Chem. Soc. 1987, 109, 4111. (f) Burk, M. J.; Tumas, W.; Ward, M. D.;
Wheeler, D. R. J. Am. Chem. Soc. 1990, 112, 6133.
via highly selective carbon-carbon bond formation (Table 1,
entries 3-5). Stoichiometric or excess amounts of VO(OEt)Cl2
are required for this transformation, and the absence of VO(OEt)-
Cl2 only resulted in the recovery of 1-octene and the starting
alkyne.
When the aryl-substituted aluminum 38 was used instead of
the 1-alkenylaluminum, the intramolecular coupling of the ate
complex 4 took place selectively between aryl and 1-alkynyl
groups (Scheme 2). Use of VO(OPri)2Cl with the lower oxidation
capability9 gave rise to the coupling product 5 and only trace
amount of isobutylated product despite the higher reaction
temperature (0 °C). Again, the alkynyl group is introduced to
(3) (a) Fujii, T.; Hirao, T.; Ohshiro, Y. Tetrahedron Lett. 1992, 33, 5823.
(b) Fujii, T.; Hirao, T.; Ohshiro, Y. Tetrahedron Lett. 1993, 34, 5601. (c)
Hirao, T.; Fujii, T.; Ohshiro, Y. Tetrahedron Lett. 1994, 35, 8005. (d) Hirao,
T.; Fujii, T.; Ohshiro, Y. Tetrahedron 1994, 50, 10207.
(4) For oxidative transformations of organosilicon compounds by using
other metallic oxidants, see: (a) Ito, Y.; Konoike, T.; Saegusa, T. J. Am. Chem.
Soc. 1975, 97, 649. (b) Kobayashi, Y.; Taguchi, T.; Morikawa, T.; Tokuno,
E.; Sekiguchi, S. Chem. Pharm. Bull. 1980, 28, 262. (c) Baciocchi, E.; Casu,
A.; Ruzziconi, R. Synlett 1990, 679. For a review, see: ref 2c, d.
(5) (a) Eisch, J. J. In ComprehensiVe Organometallic Chemistry II; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1994;
Vol. 11, pp 277-311. (b) Zietz, J. R., Jr.; Robinson, G. C.; Lindsay, K. L. In
ComprehensiVe Organometallic Chemistry; Wilkinson, S. G., Stone, F. G.
A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; Vol. 7, pp 365-464.
(6) Zweifel, G.; Miller, J. A. Org. React. 1984, 32, 375.
(8) Arylaluminum 3 was prepared by treatment of the aryllithium, which
was derived by treatment of the bromobenzene derivative with butyllithium
(0.63 mL, 1.6 M in hexane) in ether at room temperature, with diisobutyl-
aluminum chloride in ether at room temperature. Diisobutylaluminum chloride
was prepared by treatment of triisobutylaluminum with isobutylaluminum
dichloride at 80 °C.
(9) The following reactivity order has been reported; VO(OR)Cl2-AgOTf
or Me3SiOTf > VO(OR)Cl2 > VO(OR)3 > VO(acac)2. Hirao, T.; Mori, M.;
Ohshiro, Y. Bull. Chem. Soc. Jpn. 1989, 62, 2399; J. Org. Chem. 1990, 55,
358; Chem. Lett. 1991, 783.
(7) The enyne 2 and the diyne were easily separable by recycling GPC
equipped with JAI-GEL-1H and -2H columns (Japan Analytical Industry Co.
Ltd.) using chloroform as an eluent.
S0002-7863(97)03891-2 CCC: $15.00 © 1998 American Chemical Society
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Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 20, 1998 5125
Table 2. The Cross-Coupling Reaction of the Arylaluminum 6a
of aryldiethylaluminums 6 (obtained from aryllithiums and
diethylaluminum chloride) with VO(OEt)Cl2 in ether at the room
or refluxing temperature surprisingly led to a novel ethylation
reaction (Scheme 3). The absence of VO(OEt)Cl2 again resulted
in the recovery of the arene. As exemplified in Table 2, the
procedure can be applied to a wide variety of arylaluminums
bearing an electron-donating group, indicating that intramolecular
coupling of the organic groups of organoaluminums is performed
selectively. The methylthio group was found to be intact under
the conditions, but the similar coupling was not observed in the
case of the 2-cyanophenyl derivative. The electronic nature of
the substituents in 6 appears to be an important factor to control
the reaction.10 The carbon-carbon bond formation occurred even
with the disubstituted phenyllithium (Table 2, entries 9-10). The
naphthyllithium was also converted to the ethylated product (Table
2, entry 11).
It should be noted that the coupling reaction depends on the
substituent of oxovanadium(V) compounds. Use of VO(OPri)2-
Cl decreased the yield of the ethylation product (Table 2, entry
2). Staring from diisobuthylaluminum derivative, the yield of
the expected coupling product drastically decreased with the major
formation of the biaryl (Table 2, entry 3). This result is probably
due to the steric effect, but an improved conversion to the desired
coupling product was attained by using VO(OPri)2Cl instead of
VO(OEt)Cl2 (Table 2, entry 4). Both redox and steric control
interactions are considered to operate in these transformations.
Although the intramolecular coupling mechanism is ambiguous,
coordination of oxo atom of oxovanadium(V) to organoaluminums
has been reported.11 Such an intermetallic interaction and/or
transmetalation may induce the oxidation of organoaluminum
compounds.11,12 In conclusion, these novel transformations for
the formal reductive elimination on aluminum are for the first
time achieved by oxidation of the organoaluminum ate complexes
and arylaluminums with the oxovanadium(V) compound.13 This
method is expected to be promising with regard to its synthetic
potential.
Acknowledgment. The use of the facilities of the Analytical Center,
Faculty of Engineering, Osaka University is acknowledged. This work
was partly supported by a Grant-in-Aid for Scientific Research on Priority
Areas from the Ministry of Education, Science, and Culture, Japan.
a VO(OEt)Cl2, 3 equiv. The arylaluminum 6 was added to the ether
solution of VO(OEt)Cl2. Reaction time, 2 h. b The biaryl product was
obtained in <5% yield unless otherwise stated. c VO(O-i-Pr)2Cl was
used instead of VO(OEt)Cl2. d The biaryl product, 13%. e i-Bu2AlCl
was used instead of Et2AlCl. f The biaryl product, 54%. g The biaryl
product, 22%. h The biaryl product, 26%.
Supporting Information Available: Experimental procedures and
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JA973891S
the sp2-carbon (o-anisyl group) of aluminum ligand rather than
the sp3-carbon (isobutyl group). On the other hand, the selectivity
of reaction with VO(OEt)Cl2 was lower and the alkylation product
and biaryl were formed as byproducts.
Although tri(organo)aluminum compounds are considered to
be less reactive than the corresponding ate complexes, the reaction
(10) The calculated ionization potentials of 6 based on MOPAC are
correlated to their reactivities.
(11) Feher, F. J.; Blanski, R. L. J. Am. Chem. Soc. 1992, 114, 5886.
(12) The electrochemical oxidation of diethyl(2-methoxyphenyl)aluminum
afforded a small amount of the corresponding biaryl.
(13) For an electrophile-induced 1,2-migration reaction, see: Takagi, K.;
Rousset, C. J.; Negishi, E. J. Am. Chem. Soc. 1991, 113, 1440.