Copper(I) β-boroalkyls from alkene insertion: Isolation and rearrangement

Article abstract of DOI:10.1021/om060131u

The insertion of alkenes into an (NHC)copper(I) boryl affords isolable β-boroalkyl complexes in high yields; competition experiments using substituted styrenes show that electron-donating substituents slow the reaction. Although the insertion products are stable at ambient temperature, a β-hydride elimination/reinsertion sequence affords a rearranged α-boroalkyl complex on heating.

Full text of DOI:10.1021/om060131u

Organometallics 2006, 25, 2405-2408
2405
Copper(I) â-Boroalkyls from Alkene Insertion: Isolation and
Rearrangement
David S. Laitar, Emily Y. Tsui, and Joseph P. Sadighi*
Department of Chemistry, Massachusetts Institute of Technology, 2-214A, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
ReceiVed February 10, 2006
Summary: The insertion of alkenes into an (NHC)copper(I) boryl
affords isolable â-boroalkyl complexes in high yields; competi-
tion experiments using substituted styrenes show that electron-
donating substituents slow the reaction. Although the insertion
products are stable at ambient temperature, a â-hydride
elimination/reinsertion sequence affords a rearranged R-boro-
alkyl complex on heating.
insertion in some metal-catalyzed hydroboration reactions.7 The
â-boroalkyl intermediates formed through this insertion are
typically prone to â-hydride elimination,8 and the discrete
borometalation of alkenes, in contrast to that of alkynes,9 has
not been reported to date.
We recently reported the first well-characterized copper boryl
complex,10,11 which is highly reactive toward carbon dioxide,
and we were interested in examining its reactions with other
unsaturated substrates such as alkenes. Although alkyl com-
plexes of d10 metal centers undergo â-hydride elimination less
readily than those of metals with partially filled d-orbitals,
copper(I) alkyls have been shown to decompose by this route
as well as by net Cu-C bond homolysis.12,13 Because N-
heterocyclic carbene (NHC) ligands impart considerable stability
to σ-organocopper(I) complexes, we hoped that alkene insertion
into the (NHC)copper boryl complex would lead to isolable
products. Herein we report the regioselective insertion of alkenes
into the copper-boron bond, with a Hammett study of sub-
stituent electronic effects on the reactivity of vinylarenes. The
structurally characterized styrene insertion product does undergo
â-hydrogen elimination, resulting in rearrangement to an
R-boroalkyl complex, but only at elevated temperatures.
The formation of alkylboron reagents from alkenes has
generated notable interest due to the synthetic versatility of the
carbon-boron bond.1 In the catalytic addition of diboron
reagents to alkenes, which forms two carbon-boron bonds and
permits a wide range of subsequent elaboration, a key step is
the insertion of a CdC bond into a metal-boron bond.2-6 This
insertion has been implicated as competitive with metal-hydride
* To whom correspondence should be addressed. E-mail:
jsadighi@mit.edu. Fax: (617) 258-5700.
(1) See for example: (a) Organoboranes for Synthesis; Ramachandran,
P. V., Brown, H. C., Eds.; ACS Symposium Series 783; American Chemical
Society: Washington, DC, 2001. (b) Miyaura, N.; Suzuki, A. Chem. ReV.
1995, 95, 2457-2483.
(2) For diboration 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.
(3) Alkene diboration: (a) Baker, R. T.; Nguyen, P.; Marder, T. B.;
Westcott, S. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1336-1338. (b)
Iverson, C. N.; Smith M. R., III Organometallics 1997, 16, 2757-2759.
(c) Ishiyama, T.; Yamamoto, M.; Miyaura, N. Chem. Commun. 1997, 689-
690. (d) Dai, C.; Robins, E. G.; Scott, A. J.; Clegg, W.; Yufit, D. S.; Howard,
J. A. K.; Marder, T. B. Chem. Commun. 1998, 1983-1984. (e) Marder, T.
B.; Norman, N. C.; Rice, C. R. Tetrahedron Lett. 1998, 39, 155-158. (f)
Ishiyama, T.; Momota, S.; Miyaura, N. Synlett 1999, 1790-1792. (g) Mann,
G.; John, K. D.; Baker, R. T. Org. Lett. 2000, 2, 2105-2108. (h) 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. (i) Morgan,
J. B.; Miller, S. P.; Morken, J. P. J. Am. Chem. Soc. 2003, 125, 8702-
8703. (j) Ram´ırez, J.; Corbera´n, R.; Sanau´, M.; Peris, E.; Fernandez, E.
Chem. Commun. 2005, 3056-3058. (k) Trudeau, S.; Morgan, J. B.; Shrestha,
M.; Morken, J. P. J. Org. Chem. 2005, 70, 9538-9544.
The structurally characterized copper(I) boryl complex (IPr)-
CuB(pin)10 (IPr ) 1,3-bis(2′,6′-diisopropylphenyl)imidazol-2-
ylidene), pin ) pinacolate: 2,3-dimethyl-2,3-butanediolate)
reacts rapidly and cleanly with styrene (Scheme 1) to form a
single product as judged by 1H NMR spectroscopy. Protonolysis
of the styrene insertion product 1 with ethanol produces
(7) Westcott, S. A.; Marder, T. B.; Baker, R. T. Organometallics 1993,
12, 975-979, and references therein.
(8) Baker, R. T.; Calabrese, J. C.; Westcott, S. A.; Nguyen, P.; Marder,
T. B. J. Am. Chem. Soc. 1993, 115, 4367-4368.
(4) Allene diboration: (a) Ishiyama, T.; Kitano, T.; Miyaura, N.
Tetrahedron Lett. 1998, 39, 2357-2360. (b) Yang, F.-Y.; Cheng, C.-H. J.
Am. Chem. Soc. 2001, 123, 761-762. (c) Pelz, N. F.; Woodward, A. R.;
Burks, H. E.; Sieber, J. D.; Morken, J. P. J. Am. Chem. Soc. 2004, 126,
16328-16329.
(9) Clark, G. R.; Irvine, G. J.; Roper, W. R.; Wright, L. J. Organome-
tallics 1997, 16, 5499-5505. (b) Onozawa, S.-y.; Tanaka, M. Organome-
tallics 2001, 20, 2956-2958. (c) Sagawa, T.; Asano, Y.; Ozawa, F.
Organometallics 2002, 21, 5879-5886.
(10) Laitar, D. S.; Mu¨ller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005,
127, 17196-17197.
(5) R,â-Unsaturated ketone diboration: (a) Lawson, Y. G.; Lesley, M.
J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R. Chem. Commun. 1997,
2051-2052. (b) Takahashi, K.; Ishiyama, T.; Miyaura, N. Chem. Lett. 2000,
982-983. (c) Ali, H. A.; Goldberg, I.; Srebnik, M. Organometallics 2001,
20, 3962-3965. (d) Takahashi, K.; Ishiyama, T.; Miyaura, N. J. Organomet.
Chem. 2001, 625, 47-53. (e) Kabalka, G. W.; Das, B. C.; Das, S.
Tetrahedron Lett. 2002, 43, 2323-2325. (f) 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.
(6) Alkyne diboration: (a) Ishiyama, T.; Matsuda, N.; Miyaura, N.;
Suzuki, A. J. Am. Chem. Soc. 1993, 115, 11018-11019. (b) Ishiyama, T.;
Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.; Miyaura, N. Organome-
tallics 1996, 15, 713-720. (c) Lesley, G.; Nguyen, P.; Taylor, N. J.; Marder,
T. B.; Scott, A. J.; Clegg, W.; Norman, N. C. Organometallics 1996, 15,
5137-5154. (d) Iverson, C. N.;. Smith, M. R., III. Organometallics 1996,
15, 5155-5165. (e) Thomas, R. L.; Souza, F. E. S.; Marder, T. B. J. Chem.
Soc., Dalton Trans. 2001, 1650-1656, and references therein.
(11) Copper(I) boryl complexes have been inferred in other systems: Ito,
H.; Kawakami, C.; Sawamura, M. J. Am. Chem. Soc. 2005, 127, 16034-
16035. See also ref 5d.
(12) Wada, K.; Tamura, M.; Kochi, J. J. Am. Chem. Soc. 1970, 92, 6656-
6658. (b) Miyashita, A.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1977, 50,
1102-1108. (c) Miyashita, A.; Yamamoto, T.; Yamamoto, A. Bull. Chem.
Soc. Jpn. 1977, 50, 1109-1117. (d) Stein, T.; Lang, H. J. Organomet. Chem.
2002, 664, 142-149. (e) Goj, L. A.; Blue, E. D.; Munro-Leighton, C.;
Gunnoe, T. B.; Petersen, J. L. Inorg. Chem. 2005, 44, 8647-8649.
(13) Some alkyl cuprates display increased stability relative to neutral
copper(I) alkyls; see for example: (a) Lipshutz, B. H.; Parker, D.;
Kozlowski, J. A.; Miller, R. D. J. Org. Chem. 1983, 48, 3334-3336. (b)
Bertz, S. H.; Dabbagh, G. J. Org. Chem. 1984, 49, 1119-1122. (c) Mu¨ller,
A.; Neumu¨ller, B.; Dehnicke, K. Angew. Chem., Int. Ed. Engl. 1997, 36,
2350-2352. (d) Boche, G.; Bosold, F.; Marsch, M.; Harms, K. Angew.
Chem. Int. Ed. 1998, 37, 1684-1686.
10.1021/om060131u CCC: $33.50 © 2006 American Chemical Society
Publication on Web 04/14/2006

2406 Organometallics, Vol. 25, No. 10, 2006
Communications
Scheme 1
Table 1. Insertion of Alkenes into (IPr)CuB(pin)a
Figure 1. Solid-state structure of 1‚(0.5 C5H12) shown as 50%
ellipsoids. For clarity, hydrogen atoms, disorder, and solvent have
been omitted. Select bond lengths (Å) and angles (deg): Cu(1)-
C(28) 1.948(3), Cu(1)-C(1) 1.898(4), C(28)-C(29) 1.526(5),
C(29)-B(1) 1.579(6), B(1)-O(1) 1.353(6), B(1)-O(2) 1.359(5),
C(1)-Cu(1)-C(28) 175.07(16), Cu(1)-C(28)-C(29) 114.9(3), Cu-
(1)-C(28)-C(30) 104.5(2).
To examine the role of electronic effects, competitive insertion
experiments were carried out using para-substituted styrenes.16
A benzene solution containing styrene (2.0 equiv) and 4-XC6H4-
CHdCH2 (X ) NMe2, OMe, Me, F; 2.0 equiv) was rapidly
added to a solution of (IPr)CuB(pin). The relative product ratios,
a (IPr)CuB(pin) was generated in situ from (IPr)CuOtBu and (pin)BB(pin)
(1 equiv). Unless noted otherwise, insertions were carried out in n-pentane
solvent at room temp for 20 min, using 1.1 equiv of alkene. b Carried out
under 1 atm of C2H4. c Reaction time was 15 h. d 2 equiv of cis-stilbene
used; yield refers to both isomers. e Indicated stereochemistry is relative.
1
assessed by H NMR spectroscopy, indicated that electron-
donating substituents slow the reaction: styrene reacts ca. 60
times more rapidly than 4-(dimethylamino)styrene. The reactions
were complete within minutes after mixing, and the product
ratios did not change over several hours. Competition experi-
ments using more electron-poor styrenes (p-Cl, p-CF3) gave
qualitatively similar results; however, these substrates undergo
observable side-reactions17 and are not included in the study.
A plot of the relative ratios of the insertion products against
σp gave a moderate fit (R2 ) 0.92) with F ) +1.9 ( 0.4 (Figure
2).18 If alkene insertion into the copper-boron bond is rate-
determining, the F value is consistent with the buildup of
negative charge on the incipiently copper-bound carbon. The
carbocupration of enones and alkynes by dialkylcuprate(I)
reagents has been described in terms of significant electron
donation from the electron-rich copper center to the substrate,19
followed by rate-determining insertion; both substrate binding
and insertion are facilitated by the π-acidity of the substrate.20
Here, the relative rates for the borocupration of styrenes suggest
that the substrate likewise behaves essentially as an electrophile.
2-phenethyl(pinacol)boronate as the only boron-containing
product, corroborating its assignment as an R-phenyl-â-boroethyl
complex.
The results of insertion reactions using styrenes and other
alkene substrates are given in Table 1. A number of para-
substituted styrenes (entry 1) react efficiently, forming a single
regioisomer in each case. Although alkyl-substituted alkenes
such as 1-hexene and cyclopentene react very slowly with (IPr)-
CuB(pin), ethylene itself undergoes insertion in high yield (entry
2). Both trans- and cis-stilbene (entries 3, 4) show high
selectivity for syn addition, although the insertion of cis-stilbene
leads to a detectable degree of isomerization (∼5% as judged
by 1H NMR spectroscopy) to form the anti product, suggesting
that a radical pathway may be involved to some extent. It is
worth noting that an internal alkyne, 2-butyne, also inserts
readily, affording the corresponding cis-2-borovinylcopper
complex in an isolated yield of 90%.
Single crystals of 1 were grown by the diffusion of pentane
vapor into an ether solution at -40 °C. Analysis by X-ray
diffraction revealed a nearly linear two-coordinate structure and
confirmed the regiochemistry of the styrene insertion (Figure
1).14 Although the complex (IPr)CuEt was recently isolated and
characterized spectroscopically,12e structurally characterized
copper alkyl complexes possessing â-hydrogens are rare.12d,13c,d
The Cu(1)-Calkyl bond distance in 1 (1.948(3) Å) is similar to
that of (IPr)CuCH3 (1.913(6) Å).15
(15) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P. Organo-
metallics 2004, 23, 1191-1193.
(16) For related studies on migratory insertion reactions of p-substituted
styrenes, see for example: (a) Halpern, J.; Okamoto, T. Inorg. Chim. Acta
1984, 89, L53-L54. (b) Doherty, N. M.; Bercaw, J. E. J. Am. Chem. Soc.
1985, 107, 2670-2682. (c) Burger, B. J.; Santarsiero, B. D.; Trimmer, M.
S.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 3134-3146. (d) Rix, F. C.;
Brookhart, M.; White, P. S. J. Am. Chem. Soc. 1996, 118, 2436-2448.
(17) The byproducts appear to arise from further reaction between the
copper alkyl products and the remaining electron-poor olefins.
(18) σp values taken from: Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV.
1991, 91, 165-195.
(14) Crystal data for 1‚0.5C5H12: C43.50H62BCuN2O2, monoclinic, space
group P2(1)/n, a ) 20.3264(8) Å, b ) 9.6981(4) Å, c ) 21.9595(8) Å, â
) 106.996(2)°, V ) 4139.8(3) Å3, Z ) 4, Fcalcd ) 1.154 g/cm3, µ ) 0.563
mm-1, λ(Mo KR) ) 0.71073 Å, T ) 100(2) K. A total of 46 339 reflections
were collected in the θ range 1.94-25.02°, of which 7312 were unique
(Rint ) 0.0826). A semiempirical absorption correction was applied based
on equivalent reflections. Least-squares refinement on 544 parameters
converged normally with R1(I > 2σ(I)) ) 0.0657, wR2(all data) ) 0.1449,
GOF ) 1.133.
(19) The dominant importance of metal-to-alkene electron donation has
been demonstrated both for the binding of substituted styrenes by a d10
Pd(0) center and for the associative substitution of Pd(0)-bound styrenes:
Popp, B. V.; Thorman, J. L.; Morales, C. M.; Landis, C. R.; Stahl, S. S. J.
Am. Chem. Soc. 2004, 126, 14832-14842.
(20) Nakamura, E.; Mori, S. Angew. Chem., Int. Ed. 2000, 39, 3750-
3771, and references therein.

Communications
Organometallics, Vol. 25, No. 10, 2006 2407
Figure 2. Hammett plot of the relative rates of insertion of
4-substituted styrenes into (IPr)CuB(pin) at room temperature in
C6H6.
Scheme 2a
Figure 3. Solid-state structure of 2a‚(0.5C7H8) shown as 50%
ellipsoids. For clarity, hydrogen atoms, disorder, and solvent have
been omitted. Select bond lengths (Å) and angles (deg): Cu(1)-
C(28) 1.959(3), Cu(1)-C(1) 1.895(3), Cu(1)-B(1) 2.608(3),
C(28)-C(29) 1.536(5), C(28)-B(1) 1.520(5), B(1)-O(1) 1.393-
(5), B(1)-O(2) 1.371(4), C(1)-Cu(1)-C(28) 169.51(13), Cu(1)-
C(28)-C(29) 106.9(2), Cu(1)-C(28)-B(1) 96.3(2).
(pinacol)boronate. This observation is consistent with the
generation of a copper-alkene complex, in which the alkene is
substitutionally labile, as an intermediate in the rearrangement.
Heating a benzene solution of 2a in the presence of 2-(p-tolyl)-
vinyl(pinacol)boronate (1.5 equiv) for 20 h likewise leads to a
mixture of 2a and 2b,24 indicating that the R-boroalkyl
complexes can also undergo â-hydride elimination and hinting
at the participation of a similar intermediate.
An X-ray diffraction study was performed on single crystals
grown by vapor diffusion of pentane into a toluene solution of
2a (Figure 3).25 Other metal R-boroalkyls show varying degrees
of metal-boron orbital interaction.26 The somewhat acute Cu-
(1)-C(28)-B(1) angle of 96.3(2)° might indicate an attractive
copper-boron interaction; however, the long Cu(1)-B(1) bond
distance of 2.608(3) Å (the only measured Cu-B σ-bond is
2.002(3) Å) and the trigonal planar geometry about boron imply
that any such interaction is weak in this case. In solution, no
boron-copper interaction is observed for 2a on the basis of its
11B NMR spectrum: The chemical shift observed for 2a (33.4
ppm) is typical of neutral, three-coordinate boron27 and differs
only slightly from that of 1 (34.7 ppm).
a (a) C6H6, 70 °C, 24 h; 54%. (b) [(IPr)CuH]2 generated in situ from
(IPr)CuOtBu and (EtO)3SiH; n-pentane, room temp, 1 h; 91%. (c) C6D6,
70 °C, 24 h; mixture of 2b and 2a observed.
In contrast, the small and negative F values determined for
hydroboration reactions suggest the buildup of some positive
charge in the substrate during insertion.21,22 A more detailed
mechanistic discussion must await further investigation.
Although 1 is stable for prolonged periods at ambient
temperature, heating in benzene solution at 70 °C for 20 h
resulted in the formation of a new complex, along with some
1
deposition of elemental copper (Scheme 2). Analysis by H
NMR spectroscopy indicated the formation of the rearranged
alkyl complex (IPr)CuCH[B(pin)]CH2Ph (2a). This rearrange-
ment presumably occurs via â-hydride elimination followed by
reinsertion of the resulting olefin into a Cu-H bond. In a
separate experiment, trans-2-phenylvinyl(pinacol)boronate, which
should be formed by â-hydride elimination from 1, reacted
cleanly and rapidly with the previously characterized23 copper
hydride [(IPr)CuH]2 to form 2a. The rapidity of this hydro-
metalation suggests that â-hydride elimination is the slow step
in the rearrangement of 1 to 2a. The thermal rearrangement of
1 was also conducted in the presence of trans-2-(p-tolyl)vinyl-
(pinacol)boronate (1.5 equiv). The resulting 1H NMR spectrum
indicated the formation of both 2a and (IPr)CuCH[B(pin)]CH2-
(p-tolyl) (2b) and the presence of free trans-2-phenylvinyl-
In conclusion, alkenes insert cleanly and regioselectively into
(IPr)CuB(pin) to give isolable â-boroalkyl complexes. A Ham-
mett study using 4-substituted styrenes showed that electron-
releasing substituents slow the reaction markedly. At elevated
(24) Under the reaction conditions, some decomposition occurs with
deposition of metallic copper and formation of uncharacterized byproducts.
(25) Crystal data for 2a‚0.5C7H8: C44.50H60BCuN2O2, monoclinic, space
group P2(1)/n, a ) 12.5994(8) Å, b ) 19.7548(15) Å, c ) 17.9349(14) Å,
â ) 110.025(2)°, V ) 4194.1(5) Å3, Z ) 4, Fcalcd ) 1.155 g/cm3, µ )
0.557 mm-1, λ(Mo KR) ) 0.71073 Å, T ) 100(2) K. A total of 75 308
reflections were collected in the θ range 1.59-26.37°, of which 8592 were
unique (Rint ) 0.0409). A semiempirical absorption correction was applied
based on equivalent reflections. Least-squares refinement on 586 parameters
converged normally with R1(I > 2σ(I)) ) 0.0671, wR2(all data) ) 0.1672,
GOF ) 1.029.
(26) Scollard, J. D.; McConville, D. H.; Rettig, S. J. Organometallics
1997, 16, 1810-1812. (b) Zhang, S.; Piers, W. E.; Gao, X.; Parvez, M. J.
Am. Chem. Soc. 2000, 122, 5499-5509. (c) Cook, K. S.; Piers, W. E.;
Woo, T. K.; McDonald, R. Organometallics 2001, 20, 3927-3937.
(27) Kennedy, J. D. In Multinuclear NMR; Mason, J., Ed.; Plenum
Press: New York, 1987; pp 221-258.
(21) Vishwakarma, L. C.; Fry, A. J. Org. Chem. 1980, 45, 5306-5308.
(b) Garner, C. M.; Chiang, S.; Nething, M.; Monestel, R. Tetrahedron Lett.
2002, 43, 8339-8342.
(22) We thank a reviewer for calling our attention to ref 21a, which plots
relative rates against σ+. This reviewer astutely points out the somewhat
better correlation (R2 ) 0.95) obtained when our data are plotted against
σ+, which would give F ) +1.0 ( 0.2. The use of σ+ in our study, however,
would imply direct resonance between the aryl group and a significant
positive charge on the carbon forming a bond to boron. While we cannot
rule out such a contribution, we find it difficult to reconcile with the overall
trend in rates, in which electron-rich styrenes react more slowly.
(23) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004,
23, 3369-3371.

2408 Organometallics, Vol. 25, No. 10, 2006
Communications
temperatures, the styrene insertion product rearranges via
â-hydrogen elimination and reinsertion to give an (R-boroalkyl)-
copper(I) complex.
crystallographic assistance. We thank our reviewers and Profes-
sor John E. Bercaw (Caltech) for insightful comments.
Supporting Information Available: Text and figures giving
experimental procedures, spectroscopic data, and structure refine-
ment details; tables giving crystal structure data, atomic coordinates,
bond lengths and angles, and anisotropic displacement parameters
for 1 and 2a. Data for the structures are also available as a CIF
file. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank the National Science Founda-
tion (CAREER CHE-0349204), Corning Inc., and the MIT
Department of Chemistry for funding. E.Y.T. received support
from the MIT Undergraduate Research Opportunities Program.
Dr. David Bray (MIT DCIF) provided helpful assistance in
NMR spectroscopy. We are grateful to Dr. Peter Mu¨ller for
OM060131U