Olefin-borane "van der Waals Complexes": Intermediates in frustrated Lewis pair addition reactions

Article abstract of DOI:10.1021/ja205598k

The nature of the borane-olefin interactions that take place prior to frustrated Lewis pair addition reactions has been probed employing a Lewis acidic borane tethered to a vinyl group through an alkyl chain. ¹H{¹⁹F} HOESY spectral d

Full text of DOI:10.1021/ja205598k

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pubs.acs.org/JACS
OlefinÀBorane “van der Waals Complexes”: Intermediates in
Frustrated Lewis Pair Addition Reactions
Xiaoxi Zhao and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S Supporting Information
b
has been overlooked due in part to rapidity of products forma-
ABSTRACT: The nature of the boraneÀolefin interactions
that take place prior to frustrated Lewis pair addition
reactions has been probed employing a Lewis acidic borane
tion. Identification of such intermediates may also provide
mechanistic insights into allyl group abstraction reactions by
boron-based Lewis acids.9 In this communication, we probe the
nature of the intramolecular interaction of olefinic and B(C6F5)2
fragments, and provide the first NMR spectroscopic evidence
of a weakly interacted olefinÀborane van der Waals complex.
Such systems are formed en route to zwitterionic FLP addition
products.
1
tethered to a vinyl group through an alkyl chain. H{19F}
HOESY spectral data obtained at À50 °C demonstrated the
spatial proximity of the boryl and vinyl groups and compu-
tational data support the initial formation of a van der Waals
boraneÀolefin complex. Such species serve as intermediates
undergoing facile addition reactions with phosphine bases
to afford cyclic zwitterionic products.
In approaching the question of the mechanism of FLP-olefin
addition reactions, we proposed to explore a tethered olefinÀ
borane species to minimize entropic destabilization of complex
formation by chelate effect. It is noted that a related strategy has
been employed by the research groups of Casey10 and Jordan11 in
probing group III and IV d0 metalÀolefin interactions important
for olefin polymerization. We began by using DFT calculations to
assess the feasibility of this idea. To this end, computations were
performed at the M06 level of theory12 (6-311++G(d,p) for B, C
on B, olefinic C and H; 6-31G(d) for all other atoms), for two
conformers of B(C6F5)2(CH2CH2CH2CHdCH2). The “open”
form (1a) with the least steric congestion and the “closed” form
(1b) in which the alkenyl group is oriented toward the B center
were both found to be energy minima, with the “closed” form
being lower in free energy by 3.3 kcal/mol (with thermal energy
corrections at STP) (Figure 1). In contrast with the classical view
of a coordination complex bearing a dative bond, 1b does not
show any sign of a chemical bond between B and the vinyl group.
This includes minimum elongation of the CdC bond and no
pyramidalization of the B center. Atomic charges and molecular
orbital considerations also do not indicate notable charge transfer
or pÀπ orbital overlap. Nonetheless, the BÀCvinyl distances
(3.04 and 3.26 Å) are within the sum of the van der Waals radii of
B and C (3.62 Å),13 consistent with the existence of a weakly
interacted boraneÀolefin van der Waals complex, held in close
proximity by noncovalent interactions. The use of the popular
density functional B3LYP also displayed an energetic preference for
the “closed” form, but to a lesser extent (ΔG1bÀ1a = À1.9 kcal/mol).
This is consistent with the well-documented underestimation of
medium-range exchange-correlation energies, such as van der
Waals forces, by the B3LYP method.12 Single point energy
calculations at the B3LYP/def2-TZVP level of theory on the
optimized structures gave a ΔE1bÀ1a of 1.4 kcal/mol, while such
calculations employing Zhao and Truhlar’s M06-2X (improved
performance for noncovalent interactions)14 or Grimme’s B97-D
(inclusion of empirical dispersion correction)15 method with the
ctivation and transformation of small molecules had long
A
been considered the domain of transition metal compounds.
However, in recent years, there has been an increasing number of
reports on nontransition metal systems that show reactivity
traditionally characteristic of transition metal species.1 Such
systems offer considerable advantages in future applications from
both economic and environmental viewpoints. One approach to
achieve transition metal-free chemical transformations is the use
of sterically encumbered pairs of Lewis acids and bases, or
“frustrated Lewis pairs” (FLPs).2 Besides mediating catalytic
reduction of a range of polar substrates using H2,3 FLPs have
also been shown to undergo intriguing addition reactions to a
variety of substrates including olefins.4 Unlike many transition
metal-based systems where d-orbitals are accessible and back-
bonding stabilizes η2Àolefin complexes, a combination of a Lewis
acidic borane with an olefin does not show a detectable coordination
complex. Thus, the nominally termolecular reaction of an FLP
addition to an olefin raises interesting and important mechanistic
questions. Computational studies by Papai and co-workers exam-
ined the reaction of tBu3P and B(C6F5)3 with ethylene.5 These
authors proposed an “encounter complex” in which steric conges-
tion limits the PÀB approach, affording a shallow energy minimum
at PÀB distance of 4.2 Å. It is this intermediate that is proposed to
subsequently react with ethylene. In contrast, calculations of Guo
and Li showed that the three-component reaction may be
initiated by the formation of a weak association complex of
B(C6F5)3 with ethylene.6 There is a paucity of direct experi-
mental evidence that might illuminate the mechanism of action
of FLPs, although an earlier argon matrix isolation study did
describe IR data that suggested the formation of BF3Àethylene
and BF3Àpropylene van der Waals complexes at 93À125 K.7
BoraneÀolefin π-complexes are also believed to take a part in the
noncatalyzed hydroboration reaction,8 one of the most useful
organic transformations. However, the nature of the intermediates
Received: June 16, 2011
Published: July 19, 2011
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2011 American Chemical Society
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Figure 2. Partial 1H{19F} HOESY spectra of (top) 1 and (bottom) 2 (X-
axis, 1H spectrum; Y-axis, 19F spectrum; 32 equally spaced contour levels).
Figure 1. Optimized structures of 1a and 1b with pertinent distances
and NPA charges.
Scheme 1. Equilibria Governing the Formation of 1a and 1b
same AO basis set gave ΔE1bÀ1a = À5.0 and À4.6 kcal/mol,
respectively. This is an indication of van der Waals interactions that
afford stabilization to 1b. Despite the small free-energy difference of
3.3 kcal/mol, simple calculations show that the equilibrium should
lie heavily toward 1b (K298 = 2.6 Â 102; K223 = 1.7 Â 103),
encouraging us to attempt to examine such van der Waals complex
spectroscopically. It is noted that, using the IEF-PCM solvent model
for dichloromethane, no significant deviation in the geometries and
energies was identified (ΔG1bÀ1a = À3.0 kcal/mol).
Figure 3. Syntheses and POV-ray depictions of 3À5.
stays in the positive NOE regime to avoid overestimation of
enhancements.21 It is noteworthy that no significant change in
1H, 19F, and 11B NMR chemical shifts was observed at À50 °C.
Interestingly, these cross-peaks were all but lost at 25 °C,
consistent with the small dissociation energy of the van der
Waals complex. Similarly, no NOE cross-peaks for olefinic protons
and ortho-F atoms were observed upon addition of 1 equiv of
acetonitrile to the sample. This was attributed to the facile forma-
tion of the stable boraneÀnitrile adduct. Such NOE cross-peaks
were also not observed for saturated analogue B(C6F5)2(CH2)4-
(CH3) (2), generated in situ from hydroboration of 1-pentene
with [HB(C6F5)2]n, demonstrating that the presence of a vinyl
group is essential to stabilizing the “closed” form. (Figure 2)
Collectively, these data provide spectroscopic evidence of close
proximity of the olefinic fragment in 1 to the B center, and thus, the
van der Waals olefinÀborane complex, or the “closed” form 1b.
As isolation of 1 was not possible, it was reacted with
phosphine bases in attempt to form stable adducts. Addition of
The boryl-alkyl-alkene species (1) was generated in a NMR
sample in CD2Cl2 by hydroboration of 1,4-pentadiene with 1 equiv
of [HB(C6F5)2]n, commonly known as Piers borane.16 Isolation of
the target species B(C6F5)2(CH2CH2CH2CHdCH2) (1) was not
possible due to the facility of retrohydroboration. Nonetheless,
species 1 was indeed detected as the major species existing in
equilibrium with the starting materials as well as some of the double
hydroboration product (CH2)((CH2)2B(C6F5)2)2 as assessed by
1H and 19F NMR spectra (Scheme 1).
First described by Yu and Levy17 and by Rinaldi18 indepen-
dently in 1983, the 2D heteronuclear NOE (HOESY) technique
has provided valuable information in structural and conforma-
tional analysis of organic and biologically relevant molecules19
and detection of ion pairing.20 The 1H{19F} HOESY spectrum
obtained on a sample of 1 in CD2Cl2 at À50 °C showed signi-
ficant cross-peaks between each of the three olefinic protons,
especially those on the terminal carbon, and the ortho-F atoms of
C6F5 groups, inferring that the olefinic protons undergo a
significant amount of cross-relaxation with the ortho-F nuclei.
(Figure 2) It is noted that the mixing time was kept relatively
short (400 ms) to minimize contribution from spin diffusion, and
the temperature was maintained high enough so that the system
1,4-pentadiene to HB(C6F5)2 PtBu3 in toluene gave a colorless
3
mixture that upon standing at 25 °C for 2 days gave a crystalline
product 3 in 85% yield (Figure 3). Resonances attributable to the
vinyl group of 1 were not observed in the 1H NMR spectrum of 3,
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while new 11B and 31P{1H} resonances appeared at À12.8 and
50.1 ppm, respectively. 19F signals corresponding to two inequi-
valent C6F5 groups suggest the formation of a substituted cyclic
borate. Crystallography confirmed compound 3 to be the six-
membered borate ring with a phosphonium substituent
(C6F5)2B(CH2CH2CH2CH(PtBu3)CH2) rather than the sim-
ple Lewis-acidÀbase adduct. It is noteworthy that the P adds to
the substituted carbon of the olefin fragment, affording a
substituent that adopts an equatorial position on the chair
confirmation of the six-membered ring. The metric parameters
of this zwitterion are unexceptional. In a similar fashion, the
significantly less sterically encumbered and less nucleophilic PPh3
was used to give the analogous species (C6F5)2B(CH2CH2CH2CH-
(PPh3)CH2) (4) in 90% yield.
In a related reaction, a mixture of [HB(C6F5)2]n and tBu3P in
toluene was exposed to an atmosphere of 1,3-butadiene and
heated at 60 °C for 18.5 h in a closed vessel, during which time a
microcrystalline solid 5 precipitated. NMR spectroscopic and
crystallographic studies affirmed the nature of 5 as the zwitterion
(C6F5)2B(CH2CH2CH(PtBu3)CH2), which was isolated in
62% yield. This cyclic borate product is analogous to 3 and 4
with a phosphonium fragment also adopting a pseudoequatorial
position on the puckered five-membered borate-ring.
would like to dedicate this paper to Professor Gerhard Erker on the
occasion of his 65th birthday.
’ REFERENCES
(1) (a) Power, P. P. Organometallics 2007, 26, 4362–4372.
(b) Nagendran, S.; Roesky, H. W. Organometallics 2008, 27, 457–492.
(c) Martin, D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2011,
2, 389–399. (d) Power, P. P. Nature 2010, 463, 171–177.
(2) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46–76.
(3) (a) Chen, D.; Wang, Y.; Klankermayer, J. Angew. Chem., Int. Ed.
2010, 49, 9475–9478. (b) Wang, H.; Frohlich, R.; Kehr, G.; Erker, G.
Chem. Commun. 2008, 5966–5968. (c) Chase, P. A.; Jurca, T.; Stephan,
D. W. Chem. Commun. 2008, 1701–1703. (d) Chase, P. A.; Welch, G. C.;
Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46, 8050–8053.
(e) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fr€ohlich, R.; Erker,
G. Angew. Chem., Int. Ed. 2008, 47, 7543–7546. (f) Geier, S. J.; Chase,
P. A.; Stephan, D. W. Chem. Commun. 2010, 46, 4884–4886. (g) Sumerin,
V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.; Leskel€o, M.; Repo, T.;
Pyykk€o, P.; Rieger, B. J. Am. Chem. Soc. 2008, 130, 14117–14119.
(4) (a) Sortais, J.-B.; Voss, T.; Kehr, G.; Frohlich, R.; Erker, G. Chem.
Commun. 2009, 7417–7418. (b) Voss, T.; Chen, C.; Kehr, G.; Nauha, E.;
Erker, G.; Stephan, D. W. Chem.ÀEur, J. 2010, 16, 3005–3008. (c) McCahill,
J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., Int. Ed. 2007,
46, 4968–4971.
DFT calculations of the reaction profile confirmed 1b as an
intermediate en route to 3 (see Supporting Information). It is
noteworthy that Erker, Grimme, and co-workers22 have de-
scribed the addition of a linked P/B system to norbornene,
providing experimental and computational evidence for an asyn-
chronously concerted process. These authors concluded that
BÀC bond formation occurs to a larger extent in the transition
state, although the evidence for BÀolefin or PÀolefin interac-
tions was necessarily indirect. In the present study, using a
tethering approach, we have detected an interaction of a Lewis
acidic B center with a pendant alkenyl group by 2D heteronuclear
NOE techniques. This provides a compelling piece of evidence
that the addition reaction of a sterically encumbered Lewis pair to
a pendant vinyl group first forms a boraneÀolefin van der Waals
complex. Subsequent nucleophilic attack by a Lewis base was
used to afford the novel zwitterionic cyclic borate species 3À5.
While studies continue to target further insights into the
mechanistic details of FLP reactions, the utility of this reactivity
as a tool in developing new synthetic strategies is also ongoing.
(5) Stirling, A.; Hamza, A.; Rokob, T. A.; Papai, I. Chem. Commun.
2008, 3148–3150.
(6) Guo, Y.; Li, S. Eur. J. Inorg. Chem. 2008, 2008, 2501–2505.
(7) Herrebout, W. A.; van der Veken, B. J. J. Am. Chem. Soc. 1997,
119, 10446–10454.
(8) Jones, P. R. J. Org. Chem. 1972, 37, 1886–1889.
(9) Blackwell, J. M.; Piers, W. E.; McDonald, R. J. Am. Chem. Soc.
2002, 124, 1295–1306.
(10) (a) Casey, C. P.; Hallenbeck, S. L.; Wright, J. M.; Landis, C. R.
J. Am. Chem. Soc. 1997, 119, 9680–9690. (b) Casey, C. P.; Klein, J. F.;
Fagan, M. A. J. Am. Chem. Soc. 2000, 122, 4320–4330. (c) Casey, C. P.;
Carpenetti, D. W.; Sakurai, H. J. Am. Chem. Soc. 1999, 121, 9483–9484.
(d) Casey, C. P.; Hallenbeck, S. L.; Pollock, D. W.; Landis, C. R. J. Am.
Chem. Soc. 1995, 117, 9770–9771.
(11) (a) Carpentier, J.-F.; Wu, Z.; Lee, C. W.; Str€omberg, S.;
Christopher, J. N.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122,
7750–7767. (b) Wu, Z.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc.
1995, 117, 5867–5868. (c) Carpentier, J.-F.; Maryin, V. P.; Luci, J.;
Jordan, R. F. J. Am. Chem. Soc. 2001, 123, 898–909.
(12) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157–167.
(13) Mantina, M.; Chamberlin, A. C.; Valero, R.; Cramer, C. J.;
Truhlar, D. G. J. Phys. Chem. A 2009, 113, 5806–5812.
(14) Zhao, Y.; Truhlar, D. Theor. Chem. Acc. 2008, 120, 215–241.
(15) Grimme, S. J. Comput. Chem. 2006, 27, 1787–1799.
(16) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492–5503.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental, computational,
b
and crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
(17) Yu, C.; Levy, G. C. J. Am. Chem. Soc. 1983, 105, 6994–6996.
(18) Rinaldi, P. L. J. Am. Chem. Soc. 1983, 105, 5167–5168.
(19) (a) Ampt, K. A. M.; Aspers, R. L. E. G.; Jaeger, M.; Geutjes, P. E.
T. J.; Honing, M.; Wijmenga, S. S. Magn. Reson. Chem. 2011,
49, 221–230. (b) Li, D.; Sun, C.; Williard, P. G. J. Am. Chem. Soc.
2008, 130, 11726–11736.
’ AUTHOR INFORMATION
Corresponding Author
dstephan@chem.utoronto.ca
(20) Pregosin, P. S.; Kumar, P. G. A.; Fernꢀandez, I. Chem. Rev. 2005,
105, 2977–2998.
(21) Neuhaus, D.; Williamson, M. P. The Nuclear Overhauser Effect in
Structural and Conformational Analysis; Wiley-VCH: New York, 2000.
(22) M€omming, C. M.; Fr€omel, S.; Kehr, G.; Fr€ohlich, R.; Grimme,
S.; Erker, G. J. Am. Chem. Soc. 2009, 131, 12280–12289.
’ ACKNOWLEDGMENT
D.W.S. gratefully acknowledges the financial support of
NSERC of Canada, the award of a Canada Research Chair and a
Killam Research Fellowship. X.Z. is grateful for the support of an
NSERC-CGS Scholarship. The authors thank Dr. Edwin Otten
and Dr. Zach Heiden for fruitful discussions. The peer-reviewers of
the manuscript are also thanked for insightful suggestions. We
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