Catalytic diboration of aldehydes via insertion into the copper-boron bond

Article abstract of DOI:10.1021/ja064019z

Mesitaldehyde reacts cleanly with (IPr)CuB(pin) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene); pin = 2,3-dimethyl-2,3-butanediolate] to afford the product complex 1, the first well-defined product of carbonyl group insertion into a metal-boron

Full text of DOI:10.1021/ja064019z

Published on Web 08/04/2006
Catalytic Diboration of Aldehydes via Insertion into the Copper-Boron Bond
David S. Laitar, Emily Y. Tsui, and Joseph P. Sadighi*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
Received June 7, 2006; E-mail: jsadighi@mit.edu
Scheme 1. Syntheses of an (R-Boroxy)benzylcopper Complexa
A copper(I) boryl complex, the first of its kind to be character-
ized, reacts with carbon dioxide under remarkably mild conditions
to form carbon monoxide plus a copper(I) borate.1,2 The identity
of the initial product of CO2 insertion remains elusive, because CO
is generated fairly rapidly even at -60 °C. To explore the reactivity
of the copper-boron bond toward other substrates containing the
CdO group, we have investigated the insertion of aldehydes.
Insertion reactions of alkynes,3a-c R,â-unsaturated ketones3c and
alkenes3d into metal-boron bonds have been studied, but 1,2-
insertion products derived from carbonyl substrates had not been
described.4
A further goal of this effort was the development of new catalytic
borylation reactions using aldehyde substrates. Organoboron com-
pounds are valuable reagents in organic synthesis, because of the
a (IPr)CuB(pin) generated in situ from (IPr)CuOt-Bu and (pin)B-B(pin).
R-Borobenzyl alcohol prepared by selective hydrolysis of 2, vide infra.
synthetic versatility of the carbon-boron bond.5 An important class
of borylation reactions involves the catalytic addition of diboron
reagents6 across unsaturated organic substrates such as alkenes,7
alkynes,8 allenes,9 and R,â-unsaturated ketones.10 The 1,2-diboration
of carbon-heteroatom double bonds is rare, but has been achieved
in the case of aldimines11 and thioketones.12 The use of aldehyde
substrates has given rise to complex reactivity,13 and the isolation
of 1,2-diboration products from these reactions has not been
reported.
We now report the insertion of an aldehyde carbonyl group into
a metal-boron bond, leading to metal-carbon σ-bond formation.
Subsequent reaction with a diboron reagent in the presence of
Figure 1. (a) X-ray crystal structures of 1 (a) and 2 (b), shown as 50%
additional aldehyde results in carbon-boron bond formation and
ellipsoids. For each, one enantiomer of the racemic mixture is shown.
catalytic turnover. A closely related system catalyzes the efficient
Selected bond distances (Å) and angles (deg) for 1: Cu(1)-C(28) 1.9473-
1,2-diboration of a range of aldehydes, forming potentially useful
(19), Cu(1)-C(1) 1.8975(18), C(28)-O(1) 1.464(2), O(1)-B(1) 1.352(3),
C(1)-Cu(1)-C(28) 175.46(8), Cu(1)-C(28)-O(1) 118.74(14), Cu(1)-
C(28)-C(29) 101.94(12).
R-hydroxyalkyl anion equivalents.14
Mesitaldehyde reacts rapidly with (IPr)CuB(pin) (IPr ) 1,3-bis-
(2,6-diisopropylphenyl)imidazol-2-ylidene, pin ) pinacolate, 2,3-
dimethyl-2,3-butanediolate) in C6D6 solution, forming the single
product 1 as judged by NMR spectroscopy (Scheme 1). The 11B
NMR shift of 1 (21.8 ppm) is consistent with the presence of a
neutral boron center bound to three oxygen atoms.15 The X-ray
crystal structure of 1 confirmed the formation of Cu-C and B-O
bonds (Figure 1a). The Cu-Calkyl bond distance of 1.9473(19) Å
is similar to those in the previously reported (IPr)CuCH3 (1.913(6)
Å)16 and (IPr)CuCH(Ph)CH2B(pin) (1.948(3) Å).3d Benzene solu-
tions of 1 are stable for days at ambient temperature.
Monitoring the aldehyde insertion by 1H NMR spectroscopy, we
observed no evidence for formation of the alternative regioisomer
1′, containing a [Cu-O-C-B] linkage. An attempt to synthesize
this isomer independently17 (Scheme 1) afforded 1 as the sole
observed product complex. We cannot conclude, therefore, that the
1,2-insertion of mesitaldehyde into the Cu-B bond forms 1 directly,
because rearrangement of 1′ to 1 is apparently facile. Further studies
are in progress.
conversion observed by 1H NMR spectroscopy after 20 h at room
temperature. In the presence of both diboron reagent and mesital-
dehyde (5 equiv of each), however, 1 acts as a competent
precatalyst, with 4.5 equiv of the mesitaldehyde 1,2-diboration
product 2 formed after 24 h. The 11B NMR spectrum of 2 displays
resonances at 22.6 ppm, characteristic of a borate ester, and at 32.7
ppm, characteristic of a boronate ester.15 Complex 1 is essentially
the only IPr-containing complex observed (∼95% of ligand
1
resonances in the H NMR spectrum) during this reaction.
The smaller ICy (1,3-dicyclohexylimidazol-2-ylidene) proved a
more effective supporting ligand for catalysis. The reaction between
mesitaldehyde and (pin)B-B(pin), using 2.3 mol % (ICy)CuOt-
Bu precatalyst, was complete after 22 h at room temperature, and
2 was isolated in 73% yield. The structure of 2 was confirmed by
X-ray crystallography (Figure 1b).
The catalytic diboration procedure developed for mesitaldehyde
works effectively for a variety of aldehyde substrates (Table 1).
Benzaldehydes substituted with electron-donating (entries 2 and 3)
and electron-withdrawing groups (entries 4-6) react in high yield,
and the [(ICy)Cu] catalyst system tolerates ortho-substitution well
Complex 1 reacts very slowly with a diboron reagent to form a
new carbon-boron bond and regenerate the copper boryl, with 10%
9
11036
J. AM. CHEM. SOC. 2006, 128, 11036-11037
10.1021/ja064019z CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Copper-Catalyzed Diboration of Aldehydes
We thank Dr. Peter Mu¨ller (MIT) for crystallographic assistance.
We are indebted to Prof. T. B. Marder (University of Durham),
Mr. H. Zhao, and Prof. Z. Lin (Hong Kong University of Science
and Technology) for graciously sharing results prior to publication,
and for helpful discussions raising the possibility of a 1′-1
rearrangement.
yield
yield
(%)a,b
entry
R
)
(%)a,b
entry
R
)
1
2
3
C6H5
88
86
94
80
81
87
91
8
9
2,4,6-Me3C6H2
n-butyl
iso-butyl
tert-butyl
cyclohexyl
3-pyridyl
2-thienyl
73
94
95
71
97
66
95
Supporting Information Available: Experimental procedures and
characterization for all products; crystallographic data (also available
as a CIF) and structure refinement details for 1 and 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
4-MeOC6H4
4-MeC6H4
4-BrC6H4
4-ClC6H4
4-CF3C6H4
2-MeOC6H4
10
11d
12
13e
14
4c
5c
6c
7
References
(1) Laitar, D. S.; Mu¨ller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005, 127,
17196-17197.
a Conditions: unless otherwise noted, 1 mol % (ICy)CuOt-Bu, 1 equiv
aldehyde, 1 equiv bis(pinacolato)diboron, 22 h. b Isolated yield, average of
two runs. c Reaction run using 2.3 mol % (ICy)CuOt-Bu. d Product was
isolated as R-hydroxyneopentyl(pinacol)boronate after column chromatog-
raphy on silica gel. e Reaction run using 10 mol % (ICy)CuOt- Bu.
(2) Copper(I) boryl complexes as likely intermediates: (a) Ito, H.; Yamanaka,
H.; Tateiwa, J.-i.; Hosomi, A. Tetrahedron Lett. 2000, 41, 6821-6825.
(b) Takahashi, K.; Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2001,
625, 47-53. (c) Ramachandran, P. V.; Pratihar, D.; Biswas, D.; Srivastava,
A.; Ram Reddy, M. V. Org. Lett. 2004, 6, 481-484. (d) Ito, H.;
Kawakami, C.; Sawamura, M. J. Am. Chem. Soc. 2005, 127, 16034-
16035.
(entries 7 and 8). Halogen substituents such as chloride and bromide
do not interfere with the reaction. Aliphatic aldehydes bearing
primary, secondary, and tertiary alkyl substituents react in excellent
yield (entries 9-12). The diboration products can generally be
isolated pure by filtration and reconcentration of their solutions in
hydrocarbons, or by recrystallization. In the case of the oily product
of pivaldehyde diboration (entry 11), an attempt at chromatographic
purification on silica gel resulted in B-O bond cleavage, and the
product was isolated as the R-hydroxyneopentylboronate ester. This
selective hydrolysis also afforded the R-borobenzyl alcohol used
in the attempted synthesis of 1′ described above.
Certain aldehydes derived from aromatic heterocycles form 1,2-
diboration products in good yield (entries 13, 14). The substrate
2-pyridinecarboxaldehyde, however, undergoes reductive coupling
to form a 1,2-dipyridyl-1,2-bis(pinacolboroxy)ethane as the major
reaction product; the reason for this change in selectivity is currently
unknown. The reaction of 4-pyridinecarboxaldehyde with (pin)B-
B(pin) results in reductive coupling even in the absence of copper
catalyst. Analogous reactions have been observed for sterically
unencumbered aldimines.11,18
In the solid state, the aldehyde diboration products are stable
for weeks when protected from air and light. No decomposition
was observed in benzene solutions, even those saturated with water,
after several days at ambient temperature. Exposure to dioxygen
results in slow oxidation, regenerating the aldehydes and forming
[(pin)B]2O. This reaction occurs more rapidly for benzylic than
for aliphatic diboration products.
In conclusion, the insertion of an aldehyde CdO group into a
copper-boron bond leads to the formation of a metal-carbon
σ-bond. The product complex undergoes further reaction with a
diboron reagent in the presence of aldehyde, resulting in carbon-
boron bond formation. These key processes form the basis of a
versatile catalytic 1,2-diboration of aldehydes. The products of this
reaction are potentially useful as masked R-hydroxyalkyl anions
for organic synthesis.
(3) (a) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.;
Miyaura, N. Organometallics 1996, 15, 713-720. (b) Clark, G. R.; Irvine,
G. J.; Roper, W. R.; Wright, L. J. Organometallics 1997, 16, 5499-5505.
(c) Onozawa, S.-y.; Tanaka, M. Organometallics 2001, 20, 2956-2958.
(d) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. Organometallics 2006, 25,
2405-2408.
(4) Selected aldehyde insertions into metal-silicon bonds: (a) Si-C/Ta-O
bond formation, Arnold, J.; Tilley, T. D. J. Am. Chem. Soc. 1987, 109,
3318-3322. (b) Mn-C/Si-O bond formation, Johnson, D. L.; Gladysz,
J. A. Inorg. Chem. 1981, 20, 2508-2515.
(5) (a) Organoboranes for Syntheses; Ramachandran, P. V., Brown, H. C.,
Eds.; ACS Symposium Series 783; American Chemical Society: Wash-
ington, DC, 2001. (b) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95,
2457-2483.
(6) For reviews, see (a) Marder, T. B.; Norman, N. C. Top. Catal. 1999, 5,
63-73. (b) Ishiyama, T.; Miyaura, N. Chem. Rec. 2004, 3, 271-280.
(7) See for example (a) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott,
S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1336-1338. (b) Nguyen,
P.; Coapes, R. B.; Woodward, A. D.; Taylor, N. J.; Burke, J. M.; Howard,
J. A. K.; Marder, T. B. J. Organomet. Chem. 2002, 652, 77-85.
(8) See for example (a) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A.
J. Am. Chem. Soc. 1993, 115, 11018-11019. (b) Iverson, C. N.; Smith,
M. R., III. Organometallics 1996, 15, 5155-5165. (c) Thomas, R. L.;
Souza, F. E. S.; Marder, T. B. J. Chem. Soc., Dalton Trans. 2001, 1650-
1656 and references therein.
(9) Pelz, N. F.; Woodward, A. R.; Burks, H. E.; Sieber, J. D.; Morken, J. P.
J. Am. Chem. Soc. 2004, 126, 16328-16329.
(10) See for example Bell, N. J.; Cox, A. J.; Cameron, N. R.; Evans, J. S. O.;
Marder, T. B.; Duin, M. A. Elsevier, C. J.; Baucherel, X.; Tulloch, A. A.
D.; Tooze, R. P. Chem. Commun. 2004, 1854-1855 and references therein.
(11) Mann, G.; John, K. D.; Baker, R. T. Org. Lett. 2000, 2, 2105-2108.
(12) Carter, C. A. G.; Vogels, C. M.; Harrison, D. J.; Gagnon, M. K. J.; Norman,
D. W.; Langler, R. F.; Baker, R. T.; Westcott, S. A. Organometallics
2001, 20, 2130-2132.
(13) Carter, C. A. G.; John, K. D.; Mann, G.; Martin, R. L.; Cameron, T. M.;
Baker, R. T.; Bishop, K. L.; Broene, R. D.; Westcott, S. A. Bifunctional
Lewis Acid Reactivity of Diol-Derived Diboron Reagents. In Group 13
Chemistry/From Fundamentals to Applications; Shapiro, P. J., Atwood,
D. A., Eds.; ACS Symposium Series 822; American Chemical Society:
Washington, DC, 2002; pp 70-87.
(14) Use of R-silylalkyl-Li or -Mg reagents, followed by Si-C bond oxidation,
for this purpose: (a) Tamao, K.; Iwahara, T.; Kanatani, R.; Kumada, M.
Tetrahedron Lett. 1984, 25, 1909-1912. (b) Tamao, K.; Kanatani, R.;
Kumada, M. Tetrahedron Lett. 1984, 25, 1913-1916.
(15) Kennedy, J. D. In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New
York, 1987; pp 221-258.
(16) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004, 23,
1191-1193.
(17) (IPr)CuMe + ROH: Goj, L. A.; Blue, E. D.; Munro-Leighton, C.; Gunnoe,
T. B.; Petersen, J. L. Inorg. Chem. 2005, 44, 8647-8649.
(18) These products have been prepared previously by McMurry-type cou-
plings: Clerici, A.; Porta, O. Tetrahedron 1983, 39, 1239-1246.
Acknowledgment. We thank the NSF (Grant No. CHE-
0349204), the MIT Department of Chemistry, and Corning Inc.
for funding. The MIT UROP office has generously supported E.Y.T.
JA064019Z
9
J. AM. CHEM. SOC. VOL. 128, NO. 34, 2006 11037