J. Am. Chem. Soc. 1998, 120, 5327-5328
5327
commonly used chelating Lewis acid, TiCl4 (1 equiv) in toluene
at -78 °C for 10 min, and subsequent chelation-induced reduction
with Bu3SnH (1.1 equiv) gave rise to R-methoxy alcohol 3
predominantly accompanied by 4 in a ratio of 7:1. Under similar
reaction conditions, reduction of 1 and 2 (1:1 ratio) with (C6F5)3B
(1 equiv),8,9 which is originally identified as a nonchelating Lewis
acid, afforded R-methoxy alcohol 3 as a major product (ratio of
3/4 ) >20:<1). A similar tendency in selectivity between 1 and
2 was also observed with Me3Al. These results imply the
preferable formation of chelating pentacoordinate E (MX3 )
Hypercoordination of Boron and Aluminum:
Synthetic Utility as Chelating Lewis Acids
Takashi Ooi, Daisuke Uraguchi, Naoko Kagoshima, and
Keiji Maruoka*
Department of Chemistry, Graduate School of Science
Hokkaido UniVersity, Sapporo 060-0810, Japan
ReceiVed October 23, 1997
(C6F5)3B or Me3Al) rather than a tetracoordinate F (MX3
)
Boron and aluminum compounds have been widely utilized in
both organic and inorganic syntheses.1 Many of the reaction
characteristics common to both these elements depend on the
availability of the empty p orbital that makes these compounds
electrophilic or Lewis acidic.2 Accordingly, trivalent B and Al
compounds A (BX3 and AlX3) readily react with a variety of
neutral or negatively charged Lewis bases (L) to form the
corresponding tetracoordinate complexes B. Several restricted
examples of neutral pentacoordinate, trigonal-bipyramidal com-
plexes of type C (M ) B, Al), where ligands L occupy two axial
positions, have recently been isolated and characterized.3,4
However, the majority still involve the tricoordination/tetraco-
ordination chemistry of boron and aluminum, and hence trivalent
B and Al compounds A have long been regarded as nonchelating
Lewis acids.2,5 Little attention has been given to the existence
of another pentacoordinate, chelate-type complex D,6 and its
nature remains elusive despite its potential importance from
mechanistic as well as synthetic points of view. Here we wish
to report such a pentacoordinate complex D is observable with
commercially available (C6F5)3B and Me3Al in simple reaction
systems, and should find considerable utility in organic synthesis.
(C6F5)3B or Me3Al).5,6
Moreover, (C6F5)3B-promoted reduction of simple R-substituted
ketone 5a (X ) CH2) with Bu3SnH gave a mixture of diastere-
omeric alcohols 6, whereas chelation-controlled reduction of
R-methoxy-R-methyl ketone 5b (X ) O) with (C6F5)3B/Bu3SnH
afforded single diastereomer 7 exclusively.5
Since boron and aluminum have high affinity to oxygen, as
evident from the bond strengths in several diatomic molecules
of metal-oxygen (B-O ) 808.8 kJ/mol, Al-O ) 511 kJ/mol),7
we chose R-methoxy ketone 1 and its deoxy analogue 2 as model
substrates for chelation-induced selective reduction with Bu3SnH
in the presence of several Lewis acids. We assumed that the
chelate formation of substrate 1 with Lewis acids is generally a
favorable process, and therefore accelerates the rate of reduction
by the effective activation of carbonyl moiety compared to the
nonchelation case.5 Indeed, initial treatment of an equimolar
mixture of R-methoxy ketone 1 and its deoxy analogue 2 with a
A discrimination experiment between o- and p-methoxyphen-
ylcarbonyl compounds, 8 and 9, was carried out in a manner
similar to that described above. Again, chelation-induced selec-
tive reduction of o-methoxyisobutyrophenone 8 (R ) i-Pr) was
observed to furnish o-methoxyphenyl carbinol 10 (R ) i-Pr; X
) H) preferentially with (C6F5)3B and Me3Al. The (C6F5)3B-
and Me3Al-promoted discriminative allylation of an equimolar
mixture of o- and p-anisaldehyde, 8 (R ) H) and 9 (R ) H),
with allyltributyltin afforded o-methoxy homoallylic alcohol 10
(R ) H; X ) CH2CHdCH2) predominantly.
(1) Negishi, E. Organometallics in Organic Synthesis; John Wiley &
Sons: New York, 1980.
(2) Santelli, M.; Pons, J.-M. Lewis Acids and SelectiVity in Organic
Synthesis; CRC Press: Boca Raton, 1995.
(3) Lee, D. Y.; Maetin, J. C. J. Am. Chem. Soc. 1984, 106, 5745.
(4) (a) Heitsch, C. W.; Nordman, C. E.; Parry, P. W. Inorg. Chem. 1963,
2, 508. (b) Palenick, G. Acta Crystallogr. 1964, 17, 1573. (c) Beattie, I. R.;
Ozin, G. A. J. Chem. Soc. A 1968, 2373. (d) Bennett, F. R.; Elms, F. M.;
Gardiner, M. G.; Koutsantonis, G. A.; Raston, C. L.; Roberts, N. K.
Organometallics 1992, 11, 1457. (e) Muller, G.; Lachmann, J.; Rufinska, A.
Organometallics 1992, 11, 2970. (f) Fryzuk, M. D.; Giesbrecht, G. R.;
Olovsson, G.; Rettig, S. J. Organometallics 1996, 15, 4832.
(5) Reetz, M. T. Angew. Chem., Int. Ed. Engl. 1984, 23, 556. Detailed
kinetic experiments show that steric effects are completely overriden in the
case of R-alkoxy and amino ketones, see: Reetz, M. T.; Maus, S. Tetrahedron
1987, 43, 101.
(6) Ooi, T.; Kagoshima, N.; Maruoka, K. J. Am. Chem. Soc. 1997, 119,
5754. See also: Heller, D. P.; Goldberg, D. R.; Wulff, W. D. J. Am. Chem.
Soc. 1997, 119, 10551. Murakata, M.; Jono, T.; Mizuno, Y.; Hoshino, O. J.
Am. Chem. Soc. 1997, 119, 11713. Arai, T.; Sasai, H.; Yamaguchi, K.;
Shibasaki, M. J. Am. Chem. Soc. 1998, 120, 441.
S0002-7863(97)03682-2 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/13/1998
Ooi
