Reactions of an intramolecular frustrated Lewis pair with unsaturated substrates: Evidence for a concerted olefin addition reaction

Article abstract of DOI:10.1021/ja903511s

The intramolecular frustrated Lewis pair (mesityl)₂P-CH ₂-CH₂-B(C₆F₅)₂ was generated in situ by HB(C₆F₅)₂ hydroboration of dimesitylvinylphosphine. The comp

Full text of DOI:10.1021/ja903511s

Published on Web 08/06/2009
Reactions of an Intramolecular Frustrated Lewis Pair with
Unsaturated Substrates: Evidence for a Concerted Olefin
Addition Reaction
Cornelia M. Mo¨mming, Silke Fro¨mel, Gerald Kehr, Roland Fro¨hlich, Stefan Grimme,
and Gerhard Erker*
Organisch-Chemisches Institut der UniVersita¨t Mu¨nster, Corrensstrasse 40,
48149 Mu¨nster, Germany
Received April 30, 2009; E-mail: erker@uni-muenster.de
Abstract: The intramolecular frustrated Lewis pair (mesityl)2P-CH2-CH2-B(C6F5)2 was generated in situ
by HB(C6F5)2 hydroboration of dimesitylvinylphosphine. The compound reacts with 1-pentyne by C-H bond
cleavage. It undergoes a 1,2-addition to the carbonyl group of trans-cinnamic aldehyde to yield a zwitterionic
six-membered heterocycle by B-O and P-C bond formation. The Lewis pair regioselectively adds to the
electron-rich CdC double bond of ethyl vinyl ether, and it selectively undergoes an exo-cis-2,3-addition to
norbornene. A combined experimental/theoretical study suggests that this reaction takes place in an
asynchronous concerted fashion with the B-C bond being formed in slight preference to the P-C bond.
The addition products were characterized by X-ray crystal structure analyses.
Introduction
of which an increasing number of examples is presently
emerging in the literature.6-9
Lewis acids and Lewis bases usually undergo a neutralization
reaction when brought together. It requires special measures to
prevent this ubiquitous quenching reaction from taking place.
Attaching sufficiently bulky substituents at the respective central
atoms is a way of preserving the typical properties of a Lewis
acid in the presence of a complementary Lewis base and vice
versa. Such “antagonistic” 1,2 (or “frustrated”)3-5 Lewis pairs
can exhibit rather special chemical reactivities that result from
a probably cooperative interaction of the non-self-quenched pair.
The heterolytic splitting and activation of dihydrogen is an
important feature of a variety of P/B or N/B pairs of this type,
The addition to CdC multiple bonds is another example. This
was probably first realized by the Wittig group in Heidelberg.
They observed that 1,2-didehydrobenzene (2) formed the
zwitterionic product 3 in the presence of a mixture of, e.g., PPh3
and BPh3.1 In the same context, Tochtermann had observed that
the usual anionic butadiene polymerization reaction did not take
place when the conjugated diene was exposed to the trityl anion
initiator in the presence of the BPh3 Lewis acid; the trapping
product 6 was formed instead (see Scheme 1).2 Wittig and
Tochtermann realized the special nature of these combinations
of bulky Lewis acids and Lewis bases. Tochtermann coined the
term “antagonistisches Paar” for such Lewis pairs.2,10
(1) Wittig, G.; Benz, E. Chem. Ber. 1959, 92, 1999–2013.
(2) Tochtermann, W. Angew. Chem., Int. Ed. 1966, 5, 351–371.
Tochtermann, W. Angew. Chem. 1966, 78, 355–375.
(7) (a) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela¨, M.; Repo, T.; Rieger,
B. Angew. Chem., Int. Ed. 2008, 47, 6001–6003. Sumerin, V.; Schulz,
F.; Nieger, M.; Leskela¨, M.; Repo, T.; Rieger, B. Angew. Chem. 2008,
120, 6090–6092. (b) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem.
Commun. 2008, 1701–1703. (c) Sumerin, V.; Schulz, F.; Atsumi, M.;
Wang, C.; Nieger, M.; Leskela¨, M.; Repo, T.; Pyykko¨, P.; Rieger, B.
J. Am. Chem. Soc. 2008, 130, 14117–14119. (d) Chen, D.; Klanker-
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(8) (a) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.;
Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7428–7432. Holschu-
macher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.; Tamm, M.
Angew. Chem. 2008, 120, 7538–7542. (b) Chase, P. A.; Stephan, D. W.
Angew. Chem., Int. Ed. 2008, 47, 7433–7437. Chase, P. A.; Stephan,
D. W. Angew. Chem. 2008, 120, 7543–7547.
(3) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W. Science
2006, 314, 1124–1126.
(4) (a) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880–
1881. (b) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda,
J. D.; Wei, P.; Stephan, D. W. Dalton Trans. 2007, 3407–3414. (c)
Welch, G. C.; Holtrichter-Roessmann, T.; Stephan, D. W. Inorg. Chem.
2008, 47, 1904–1906.
(5) Reviews: (a) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535–
1539. (b) Kenward, A. L.; Piers, W. E. Angew. Chem., Int. Ed. 2008,
47, 38–41. (c) Stephan, D. W. Dalton Trans. 2009, 3129–3136.
(6) (a) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew.
Chem., Int. Ed. 2007, 46, 8050–8053. Welch, G. C.; Jurca, T.; Stephan,
D. W. Angew. Chem. 2007, 119, 8186–8199. (b) Chase, P. A.; Welch,
G. C.; Jurca, T.; Stephan, D. W. Angew. Chem., Int. Ed. 2007, 46,
9136. Welch, G. C.; Jurca, T.; Stephan, D. W. Angew. Chem. 2007,
119, 9296. (c) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. J. Am.
Chem. Soc. 2008, 130, 12632–12633. (d) Huber, D.; Kehr, G.;
Bergander, K.; Fro¨hlich, R.; Erker, G.; Tanino, S.; Ohki, Y.; Tatsumi,
K. Organometallics 2008, 27, 5279–5284. (e) Dureen, M. A.; Lough,
A.; Gilbert, T. M.; Stephan, D. W. Chem. Commun. 2008, 4303–4305.
(f) Wang, H.; Fro¨hlich, R.; Kehr, G.; Erker, G. Chem. Commun. 2008,
5966–5968. (g) Ullrich, M.; Lough, A. J. J. Am. Chem. Soc. 2009,
131, 52–53. (h) Ramos, A.; Lough, A. J.; Stephan, D. W. Chem.
Commun. 2009, 1118–1120.
(9) Theoretical studies: (a) Rokob, T. A.; Hamza, A.; Stirling, A.; Soo´s,
T.; Pa´pai, I. Angew. Chem., Int. Ed. 2008, 47, 2435–2438. Rokob,
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(10) Remarkably, this expression was only used in the German version of
Tochtermann’s Angewandte Chemie paper; he refrained from using
the equivalent expression, “antagonistic pair”, in the English version
of the paper.
9
12280 J. AM. CHEM. SOC. 2009, 131, 12280–12289
10.1021/ja903511s CCC: $40.75  2009 American Chemical Society

Evidence for a Concerted Olefin Addition Reaction
A R T I C L E S
Scheme 1
Scheme 3
Scheme 2
found that the ethylene-bridged P/B Lewis pair 12 rapidly adds
to a variety of unsaturated organic substrates, probably in a
cooperative manner to some. In this article we describe some
typical reactions and attempt a mechanistic description of this
unique addition mode based on a combination of experiment
and theory using a selected example.
Results and Discussion
Experimental Investigations: Reactions with Alkenes and
Alkynes. Stephan et al. had shown that intermolecular frustrated
Lewis pairs can undergo 1,2-addition reactions to alkynes,
yielding alkenes of type 18. However, sometimes (sp)C-H
deprotonation competes with formation of the addition product
(19, see Scheme 4).18b We have now reacted the intramolecular
frustrated Lewis pair 12, generated in situ by treatment of
Stephan et al. explored the chemistry of “frustrated” Lewis
pairs consisting of very bulky phosphines and the strong Lewis
acid B(C6F5)3.3-6 They found that some systems underwent
clean olefin addition reactions (see Scheme 2).11,12 Pa´pai et al.
analyzed the parent reaction by quantum chemical calculations
and concluded it was an antiperiplanar asynchronous concerted
addition with the B-C bond formation slightly preceding the
P-C bond formation.13
(17) See also: (a) Grobe, J.; Lu¨tke-Brochtrup, K.; Krebs, B.; La¨ge, M.;
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895. (b) Bontemps, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. J. Am.
Chem. Soc. 2006, 128, 12056–12057. (c) Bontemps, S.; Gornitzka,
H.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Angew. Chem., Int. Ed.
2006, 45, 1611–1614. Bontemps, S.; Gornitzka, H.; Bouhadir, G.;
Miqueu, K.; Bourissou, D. Angew. Chem. 2006, 118, 1641–1644. (d)
Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.; Takahashi, M.;
Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem., Int. Ed. 2007,
46, 8583–8586. Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.;
Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem.
2007, 119, 8737–8740. (e) Bebbington, M. W. P.; Bontemps, S.;
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G.; Dyer, P. W.; Miqueu, K.; Bourissou, D. Inorg. Chem. 2007, 46,
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C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D.
Angew. Chem., Int. Ed. 2008, 47, 1481–1484. Bontemps, S.; Bouhadir,
G.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.;
Ozerov, O. V.; Bourissou, D. Angew. Chem. 2008, 120, 1503–1506.
(h) Sircoglou, M.; Bontemps, M.; Bouhadir, G.; Saffon, N.; Miqueu,
K.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.;
Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729–
16738. (i) Vergnaud, J.; Grellier, M.; Bouhadir, G.; Vendir, L.; Sabo-
Etienne, S.; Bourissou, D. Organometallics 2008, 27, 1140–1146. (j)
Hudnall, T. W.; Kim, Y.-M.; Bebbington, M. W. P.; Bourissou, D.;
Gabba¨ı, F. P. J. Am. Chem. Soc. 2008, 130, 10890–10891. (k) Moebs-
Sanchez, S.; Bouhadir, G.; Saffon, N.; Maron, L.; Bourissou, D. Chem.
Commun. 2008, 3435–3437. (l) Bontemps, S.; Sircoglou, M.; Bouhadir,
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We recently described the system 12,14,15 which was char-
acterized as an intramolecular “antagonistic/frustrated” Lewis
pair.14-17 It is one of the most active “metal-free” systems for
heterolytic H2 splitting. It catalyzes the hydrogenation of bulky
imines (e.g., 14) and of enamines (e.g., 16) very effectively
under mild reaction conditions (see Scheme 3).18a We have now
(11) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem., Int.
Ed. 2007, 46, 4968–4971. McCahill, J. S. J.; Welch, G. C.; Stephan,
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Commun. 2009, 2335–2337.
(13) Theoretical studies: (a) Stirling, A.; Hamza, A.; Rokob, A. T.; Pa´pai,
I. Chem. Commun. 2008, 3148–3150. (b) Guo, Y.; Li, S. Eur. J. Inorg.
Chem. 2008, 2501–2505.
(14) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fro¨hlich, R.; Grimme,
S.; Stephan, D. W. Chem. Commun. 2007, 5072–5074.
(15) Spies, P.; Kehr, G.; Bergander, K.; Wibbeling, B.; Fro¨hlich, R.; Erker,
G. Dalton Trans. 2009, 1534–1541.
(16) For topologically related systems see, e.g.: (a) Binger, P.; Ko¨ster, R.
J. Organomet. Chem. 1974, 73, 205–210. (b) Ko¨ster, R.; Hagelee,
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Arbuzov, B. A. Phosphorus Sulfur Silicon Relat. Elem. 1992, 645,
57–60. (f) Yuan, Z.; Taylor, N. J.; Sun, Y.; Marder, T. B.; Williams,
I. D.; Cheng, L.-T. J. Organomet. Chem. 1993, 449, 27–27. (g) Yuan,
Z.; Taylor, N. J.; Ramachandran, R.; Marder, T. B. Appl. Organomet.
Chem. 1996, 10, 305–316. (h) Yuan, Z.; Collings, J. J.; Taylor, N. J.;
Marder, T. B.; Jardin, C.; Halet, J.-F. J. Solid State Chem. 2000, 154,
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26, 2672–2678. (k) Hudnall, T. W.; Gabba¨ı, F. P. J. Am. Chem. Soc.
2007, 129, 11978–11986.
(18) (a) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Fro¨hlich, R.;
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543–7546. Spies, P.;
Schwendemann, S.; Lange, S.; Kehr, G.; Fro¨hlich, R.; Erker, G. Angew.
Chem. 2008, 120, 7654–7657. (b) Dureen, M. A.; Stephan, D. W.
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9
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A R T I C L E S
Mo¨mming et al.
Scheme 4
Scheme 5
Figure 1. Projection of the molecular geometry of compound 20. Selected
bond lengths (Å), angles (deg), and dihedral angles (deg): P1-C1 1.819(3),
P1-C3 1.891(3), C1-C2 1.541(4), C2-B1 1.625(4), B1-O1 1.493(3),
O1-C3 1.374(3), C3-C4 1.498(3), C4-C5 1.324(3); C1-P1-C3 95.4(1),
C11-P1-C21 108.5(1), C2-B1-O1 108.9(2), C31-B1-C41 105.7(2),
C3-C4-C5 122.1(2); P1-C1-C2-B1 66.3(3), B1-O1-C3-P1 -60.0(3),
B1-O1-C3-C4 163.4(2).
system of compound 20 contains a newly formed chirality
center. Therefore, we observe sets of 1H/13C NMR signals of a
diastereotopic pair of mesityl groups at phosphorus and 19F
NMR signals of a pair of diastereotopic C6F5 substituents at
tetracoordinated boron.20,21 The 11B NMR resonance of 20 is
found at δ -0.5, and the 31P NMR signal is at δ 16.1.
Compound 20 was characterized by an X-ray crystal structure
analysis (single crystals were obtained from benzene plus
heptane by the diffusion method). The molecule exhibits a
distorted chair conformation of the central zwitterionic six-
membered heterocycle. The former carbonyl oxygen atom is
found bonded to boron, and the former aldehyde carbonyl carbon
center is now attached to the phosphorus atom. Ring formation
has resulted in a differentiation of the individual aryl rings of
both the C6F5 pair at boron and the mesityl pair at P (cis or
trans to the trans-Ph-CHdCH- functionality at C3). The
remaining C4-C5 CdC double bond is found in a trans-
configuration (see Figure 1).
Ethyl vinyl ether was used as an electron-rich olefinic
substrate for the P/B addition reaction of the antagonistic pair
12. Mes2P-CH2-CH2-B(C6F5)2 indeed adds rather rapidly to
this substrate at ambient temperature in pentane. The reaction
is highly regioselective. We isolated the zwitterionic six-
membered heterocyclic product 21 in >80% yield (Scheme 5).
The regioselective formation of the six-membered ring product
was confirmed by an X-ray crystal structure analysis (see
below); it also followed clearly from the spectroscopic features
of the isolated product. Compound 21 shows the NMR signals
of a pair of diastereotopic mesityl substituents at phosphorus
(31P: δ +23.4) and a pair of diastereotopic C6F5 groups at boron
(11B: δ -13.3). This is due to the formation of a chirality center
Mes2PsCHdCH2 (11) with 1 equiv of HB(C6F5)2,19 with
1-pentyne, observing only the C-H cleavage pathway in this
case to yield 19b. The product was characterized spectroscopi-
cally, by C,H elemental analysis, and by X-ray diffraction (for
details see the Experimental Section and the Supporting
Information).
We chose trans-cinnamic aldehyde as an example of a
strongly electron deficient alkene substrate for the reaction with
12. This reaction has a selectivity choice: the P/B addition can
take place in a 1,2-fashion using either the CdC or the CdO
functional group, or it can employ both and add in the 1,4-
position (Scheme 5). The reaction between 12 and cinnamic
aldehyde in pentane solution at ambient temperature gave a
single reaction product, which was isolated in 70% yield. The
obtained compound (20) was identified as the product of 1,2-
addition of the bifunctional antagonistic Lewis pair to the
aldehyde moiety.
The selective formation of 20 is evidenced by the disappear-
ance of the typical aldehyde NMR and IR features in favor of
[P]-CH-O[B] NMR signals at δ 5.74 (1H) and δ 81.3 [1JPC
) 35.1 Hz, (13C)], and we monitor the 13C NMR signals of the
remaining CdC double bond at δ 123.6 and δ 133.8 (3JPC
12.1 Hz). The zwitterionic six-membered heterocyclic ring
)
(19) (a) Parks, D. J.; von H. Spence, R. E.; Piers, W. E. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 809–811. Parks, D. J.; von H. Spence, R. E.;
Piers, W. E. Angew. Chem. 1995, 107, 895–897. (b) von, H.; Spence,
R. E.; Parks, D. J.; Piers, W. E.; McDonald, M. A.; Zaworotko, M. J.;
Rettig, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1230–1233. von
H. Spence, R. E.; Parks, D. J.; Piers, W. E.; McDonald, M. A.;
Zaworotko, M. J.; Rettig, S. J. Angew. Chem. 1995, 107, 1337–1340.
Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17,
5492–5503. von H. Spence, R. E.; Piers, W. E.; Sun, Y. E.; Parvez,
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2459–2469. Review: (e) Piers, W. E.; Chivers, T. Chem. Soc. ReV.
1997, 26, 345–354.
(20) (a) Jacobsen, H.; Berke, H.; Do¨ring, S.; Kehr, G.; Erker, G.; Fro¨hlich,
R.; Meyer, O. Organometallics 1999, 18, 1724–1735. (b) Piers, W. E.
AdV. Organomet. Chem. 2005, 52, 1–76. (c) Beringhelli, T.; Donghi,
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(21) See, for comparison: Massey, A. G.; Park, A. J. J. Organomet. Chem.
1966, 5, 218–223.
9
12282 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Evidence for a Concerted Olefin Addition Reaction
A R T I C L E S
Scheme 6
Figure 2. View of the molecular geometry of compound 21. Selected bond
lengths (Å), angles (deg), and dihedral angles (deg): P1-C1 1.817(2),
P1-C4 1.862(2), C1-C2 1.540(3), C2-B1 1.633(3), B1-C3 1.649(3),
C3-C4 1.520(3), C6-C7 1.493(5); C1-P1-C4 99.1(1), C11-P1-C21
108.0(1), C2-B1-C3 107.5(2), C31-B1-C41 104.8(2); P1-C1-C2-B1
-76.5(2), B1-C3-C4-P1 42.0(3), B1-C3-C4-O5 172.8(2).
adjacent to P; the [P]-CH(-OR)CH2[B] unit shows a 1H NMR
1
resonance at δ 5.03 and a 13C NMR signal at δ 85.1 with JPC
1
) 55.4 Hz. The H NMR features of the adjacent -CH2-[B]
hydrogens are very characteristic for a pair of diastereotopic
methylene H atoms positioned ꢀ to phosphorus in this overall
situation.14,15 They give rise to resonances at δ 1.65 and δ 2.92,
the latter of which shows typical large JPH coupling constant
of ca. 45 Hz.
in a stepwise manner; then, the addition of the boron Lewis
acid part of 12 could lead to the reactive zwitterionic intermedi-
ate 22 (see Scheme 6).22 Sextet rearrangements in the norbornyl
cation system are known to be fast.23-26 Consequently, there
would be a competition between internal capture (to form 23)
and Wagner-Meerwein rearrangement to a new zwitterion (24).
This again would face competition between internal nucleophilic
capture (to form 25) and further rearrangement. In this way, by
using the complete repertoire of sextet rearrangement combina-
tions of the norbornyl cation system [i.e., Wagner-Meerwein
rearrangement in combination with endo-2,6-H shift and exo-
2,3-H shift (Nametkin rearrangement)], it is in principle possible
to formulate the potential formation of any of the four products
depicted in Scheme 6 (and Figure 3). The isomeric zwitterionic
addition products feature the added P/B pair in the exo-2,3- (23),
2,7- (25), endo-2,3- (26), or endo-2,6-positions (27) (a complete
rearrangement scheme is provided in the Supporting Informa-
tion).
3
Compound 21 was characterized by an X-ray crystal structure
analysis (single crystals were obtained from a dichloromethane
solution at -36 °C). Compound 21 exhibits a distorted chairlike
six-membered heterocyclic core with typical bond lengths at
the heteroatoms. The corresponding endocyclic bond angles at
the formally charged heteroatoms amount to 99.1(1)° (C1-
P1-C4) and 107.5(2)° (C2-B1-C3), respectively. The
-OCH2CH3 substituent at the ring carbon atom C4 is found in
a pseudoequatorial orientation (see Figure 2). One mesityl group
at P1 is oriented cis to the -OR substituent and thus axially
(C11-C19), whereas the other is oriented trans and hence
equatorially (C21-C29). There is a similar differentiation of
the C6F5 groups at the distal boron atom (C31-C36, cis,
equatorial; C41-C46, trans, axial orientation).
We first needed to determine the relative energy of the
potential adduct isomers 23, 25-27 in order to later see whether
an addition reaction of the antagonistic P/B Lewis pair 12 to
Reaction with Norbornene: The Mechanistic Quest. Before
we describe the experimental outcome of the reaction of the
antagonistic Lewis pair with norbornene, let us briefly consider
what could happen. Let us assume the reaction would proceed
(23) (a) Meerwein, H.; Van Emster, K. Chem. Ber. 1920, 53, 1815–1829.
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1964, 86, 5680–5681. (c) Goering, H. L.; Schewene, C. B. J. Am.
Chem. Soc. 1965, 87, 3516–3518. (d) Funke, B.; Winstein, S.
Tetrahedron Lett. 1971, 19, 1477–1480. (e) Collins, C. J.; Harding,
C. E. Liebigs Ann. Chem. 1971, 745, 124–134. (f) Wenke, G.; Lenoir,
D. Tetrahedron 1979, 35, 489–498.
(22) Direct attack from the exo face is usually prevailing in norbornene
addition reactions. See, e.g.: (a) Stille, J. K.; Hughes, R. D. J. Org.
Chem. 1971, 36, 340–344. (b) Brown, H. C.; Kawakami, J. H.; Liu,
K.-T. J. Am. Chem. Soc. 1970, 92, 5536–5538. (c) Jefford, C. W.;
Hill, D. T. Tetrahedron Lett. 1969, 10, 1957–1960. (d) Bond, F. T.
J. Am. Chem. Soc. 1968, 90, 5326–5328. (e) Huisgen, R.; Knupfer,
H.; Sustmann, R.; Wallbillich, G.; Weberndoerfer, V. Chem. Ber. 1967,
100, 1580–1592. (f) Cristol, S. J.; Morrill, T. C.; Sanchez, R. A. J.
Org. Chem. 1966, 31, 2719–2725. (g) Huisgen, R.; Seidel, M.;
Wallbillich, G.; Knupfer, H. Tetrahedron 1962, 17, 3–29. (h)
Rondestvedt, C. S., Jr.; Ver Nooy, C. D. J. Am. Chem. Soc. 1955, 77,
4878–4883. (i) Ver Nooy, C. D.; Rondestvedt, C. S., Jr. J. Am. Chem.
Soc. 1955, 77, 3583–3586. (j) van Tamelen, E. E.; Shamma, M. J. Am.
Chem. Soc. 1954, 76, 2315–2317.
(26) Reviews: (a) Berson, J. A. In Molecular Rearrangements; Mayo, P.,
Ed.; Interscience: New York, 1963; Vol. 1, pp 111-231. (b) Traylor,
T. G. Acc. Chem. Res. 1969, 2, 152–160. (c) Olah, G. A. Acc. Chem.
Res. 1976, 9, 41–52. (d) Brown, H. C. In The Nonclassical Ion
Problem; Plenum Press: New York, 1977.
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12283

A R T I C L E S
Mo¨mming et al.
Scheme 7
work [e.g., C2, δ 37.6 (1H δ 3.29); C1, δ 44.0 (1JPC ) 32.9 Hz)
(1H δ 3.20)]. A 11B NMR signal appears at δ -11.8 and a 31P
NMR resonance at δ 31.2. There are 19F NMR signals for a
pair of diastereotopic C6F5 substituents at boron (p-C6F5, δ
1
-162.8/-163.8). Compound 23 shows sets of H/13C NMR
signals for a pair of diastereotopic mesityl groups at phosphorus
1
(e.g., H of p-CH3 at δ 2.02/1.94). The system is so sterically
congested that rotation of the mesityl groups around the
P-C(sp2) bonds is hindered on the NMR time scale at ambient
temperature. Consequently, we observe a total of four ortho-
mesityl-CH3 1H NMR signals at δ 2.57/1.60 and δ 2.41/1.70
(600 MHz, 298 K). The protons of the [P]-CH2-CH2-[B]
section of the zwitterionic six-membered heterocyclic subunit
of compound 23 are pairwise diastereotopic, as expected [1H,
δ 2.89/2.63 ([P]-CH2)]. One of the distal CH2 hydrogens shows
Figure 3. DFT-calculated structures and relative energies [in kcal mol-1
(B2PLYP-D/QZVP(-g,-f)//B97-D/def2-TZVP level) of the adducts 23,
25-27 (for clarity, the major parts of the Mes and C6F5 substituents are
not shown).
]
3
the norbornene CdC double bond would have taken place in a
regime of thermodynamic or kinetic control. That was done by
a state-of-the-art DFT calculation (full optimization with disper-
sion-corrected B97-D functional27 and the large def2-TZVP AO
basis set;28 single-point energies for these structures using the
very accurate B2PLYP-D29 double-hybrid density functional
with the huge QZVP(-g,-f) basis set;30 in all calculations we
used TURBOMOLE;31 for technical details, see the Supporting
Information). This calculation on the fully substituted systems
23, 25-27, i.e., with the large mesityl and C6F5 substituents
attached at their backbones, gave us the structures of the four
isomers, as they are depicted in Figure 3, and their relative
energies.
the rather large JPH coupling constant that is typical for this
arrangement [1H, δ 2.21 (3JPH ) 35.5 Hz)/0.36 (CH2-[B])].
These results indicate that the product formation observed
in the reaction of 12 with norbornene to give 23 is taking place
According to these calculations, the endo-2,3-adduct (26) is
nearly isoenergetic with the exo-2,3-adduct isomer (23). The
remaining pair of isomers (25, 27) is markedly higher in energy
(see Figure 3). Formation of a mixture of the exo-2,3- (23) and
endo-2,3-products (26) would be expected upon reaction of 12
with norbornene under thermodynamic control.
This is not observed experimentally. The reaction of the
intramolecular antagonistic Lewis pair 12 with norbornene at
room temperature in pentane gave a single product (isolated in
>70% yield), which was identified as the exo-2,3-adduct 23 (see
Scheme 7 and Figures 4 and 5). The compound was character-
ized by C,H elemental analysis, by an X-ray crystal structure
analysis, and spectroscopically.
The X-ray crystal structure analysis shows that the bifunc-
tional [P]-CH2-CH2-[B] reagent has been added from the exo
side to give the exo-2,3-adduct isomer 23. The anellated
heterocyclic six-membered ring adopts a distorted half-chair
conformation.
Figure 4. View of the molecular geometry of compound 23. Selected bond
lengths (Å), angles (deg), and dihedral angles (deg): P1-C1 1.802(2),
P1-C3 1.830(2), C1-C2 1.538(3), C2-B1 1.648(3), B1-C4 1.695(3),
C3-C4 1.601(2), C4-C5 1.551(3), C3-C8 1.567(3); C1-P1-C3 102.9(1),
C11-P1-C21 105.2(1), C2-B1-C4 112.0(2), C31-B1-C41 102.4(1);
P1-C1-C2-B1 -72.3(2), P1-C3-C4-B1 8.7(2), P1-C3-C4-C5
136.7(1), C8-C3-C4-B1 -116.0(2).
The NMR spectra of compound 23 show the typical signals
of the unsymmetrically exo-2,3-disubstituted norbornene frame-
(27) Grimme, S. J. Comput. Chem. 2006, 27, 1787–1799.
(28) Scha¨fer, A.; Huber, C.; Ahlrichs, R. J. Chem. Phys. 1994, 100, 5829–
5835.
(29) Grimme, S. J. Chem. Phys. 2006, 124, 034108.
(30) Weigend, F.; Furche, F.; Ahlrichs, R. J. Chem. Phys. 2003, 119, 12753–
12762.
(31) (a) Ahlrichs, R.; Ba¨r, M.; Ha¨r, M.; Horn, H.; Ko¨lmel, C. Chem. Phys.
Lett. 1989, 162, 165–169. (b) Ahlrichs, R.; et al. TURBOMOLE,
version 6.0; Universita¨t Karlsruhe, 2009; http://www.turbomole.com.
Figure 5. Projection of the molecular structure of the core atoms of the
exo-2,3-adduct 23.
9
12284 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Evidence for a Concerted Olefin Addition Reaction
A R T I C L E S
Scheme 8
Table 1. Calculated Relative Energies and Reaction Energies of
12/12B with Norbornene (N) for Products 23, 25-27, Transition
States (TS), and the Corresponding Model Compounds without the
Substituents on Boron and Phosphorous (Denoted by Suffix B)a
Figure 6. Optimized structures of the open (gauche) and quenched
conformations of 12 and of the exo transition state 23(TS) at the B97-D/
def2-TZVP level.
reaction
B97-Da
B2PLYP-Db
No Substituents on B,P
12B + N f exo complex
12B + N f endo complex
12B + N f 23B (exo)
12B + N f 26B (endo)
12B + N f 23B(TS)
12B + N f 26B(TS)
-11.3
-6.4
-5.7
0.5
at the end of this section. Because the two different density
functional methods applied (B97-D and B2PLYP) provide very
similar results, only the higher level B2PLYP-D data are
considered here; for more details, the reader is referred to Table
1 and the Supporting Information.
-13.8(0.0)
-12.5 (1.3)
4.1
-13.5 (0.0)
-11.6 (1.9)
11.4
8.7
16.9
“Real” Systems
-16.0 (0.0)
-16.3 (-0.3c)
-12.8 (3.2)
-13.3 (3.0)
8.8
12 + N f 23 (exo)
12 + N f 26 (endo)
12 + N f 25
12 + N f 27
12 + N f 23(TS)
12 + N f 26(TS)
-19.4 (0.0)
-19.6 (-0.2c)
-16.2(3.2)
-16.8(2.6)
12.3
The open (gauche) form of 12 is found to be less stable than
a “quenched” four-membered ring (see Figure 6) by 13.0 kcal/
mol. This dissociation step is required to initiate the reaction.
We also found precomplexes of the borane fragment with
norbornene but will not discuss these further because they are
likely influenced significantly by solvation effects that are mostly
absent in our theoretical treatment. The computed reactions of
12 (closed form) with norbornene are strongly exothermic in a
range of 16-19 kcal/mol. The endo addition product (26) is
slightly (0.2 kcal/mol) lower in energy than 23(exo), while
25-27 are higher by about 2.6-3.2 kcal/mol (see Figure 3
above). The correction to enthalpy at 298 K by B97-D/SV(P)
computations of harmonic vibrational frequencies stabilizes 23
in favor of 26 by 0.5 kcal/mol. Solvent effects have been
estimated using the COSMO continuum solvation model32
employing a dielectric constant of 2 for a nonpolar alkane-type
solvent. The computed contribution is about 0.1 kcal/mol and
therefore negligible. We conclude from the results of the very
reliable B2PLYP-D method (estimated error for the relative
energy of less than (0.5 kcal/mol) that, under equilibrium
conditions, both isomers 23 and 26 should be experimentally
observable (computed equilibrium fractions between 40 and 80%
at 298 K for each when considering computational error bars).
15.3
19.4
a def2-TZVP AO basis. b def2-QZVP(-g,-f) AO basis using B97-D/
def2-TZVP optimized structures. c The correction to H298 for the relative
energy is 0.5 kcal/mol. The COSMO correction (ꢀ ) 2) is -0.1 kcal/
mol (favoring 26). a Values in parentheses are relative energies of the
isomers.
under kinetic controlsthe specific outcome is, therefore,
mechanistically relevant. The by far predominant formation of
23 under our reaction conditions can be explained either by a
stepwise reaction pathway through a reactive intermediate (22)
that undergoes internal trapping of the incipient carbenium ion
by the neighboring phosphine nucleophile (to form 23) faster
than rearrangement, or by a concerted mechanism (Scheme 8).
Although this distinction is probably not synthetically important,
it is relevant for a principal mechanistic understanding of this
general reaction type. We have therefore carried out a detailed
theoretical analysis of the reaction pathway.
Theoretical Studies. First we want to discuss basic energetic
aspects of the reactants and products (see Table 1) and the
computed transition states (TS), and then analyze the reaction
mechanism. In addition to the “real” system with the bulky C6F5/
mesityl substituents, we also considered smaller model structures
in which these large aryl substituents were replaced by hydrogen
atoms (dubbed 12B, 23B-27B in the following), which
simplified the quantum chemical computations considerably.
However, comparison of the results for these two systems also
provides insight into the effect of the substituents, which seems
to be not entirely clear also in the related hydrogen activation
reactions by such frustrated Lewis pairs. This is briefly discussed
The computed reaction barrier for the exo attack is about 7
kcal/mol lower than for the corresponding endo reaction.22 The
structures of both TS look qualitatively similar except for a
slightly higher asymmetry regarding B-C and P-C bond
formation (see below) in the endo case. The B2PLYP-D exo
forward barrier is computed to be 12.3 kcal/mol, which seems
to be a reasonable value for a fast chemical process that occurs
at ambient temperature. From benchmark studies of B2PLYP
(32) Klamt, A.; Schu¨u¨rmann, G. J. Chem. Soc., Perkin Trans. 1993, 2,
799–805.
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J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12285

A R T I C L E S
Mo¨mming et al.
Figure 7. Optimized structures of the exo transition state 23B(TS) and of the product 23B at the B97-D/def2-TZVP level with interatomic distances in Å
(Wiberg bond order in brackets).
more stable than the endo form (by a similar amount of about
6-7 kcal/mol), which reflects its “early” character as expected
for an exothermic reaction. We also tried to find zwitterionic
species like 22 (see Schemes 6 and 8). For the model compound,
we initially stabilized 22B by rotation around the PC-CB bond
to a trans conformation in order to avoid self-quenching of the
formal zwitterion pair. However, in the unconstrained geometry
optimization, such an assumed intermediate 22B was converted
to the already described borane-group-double-bond complex.
Whether this result is due to neglect of the solvent or points to
a nonexistence of species 22 is beyond the capabilities of the
present quantum chemical procedures.
Figure 8. View of the localized Kohn-Sham MOs (LMO) at the transition-
state structure 23B(TS) corresponding to the P(lone-pair)-C(π*) (left) and
the B(p)-C(π) (right) interactions.
In summary, according to the theoretical results, the addition
of 12 to the double bond of norbornene occurs via an initial
P-B “bond” dissociation followed by a concerted but slightly
asynchronous bimolecular reaction induced by B(p)-π and
P(lone pair)-π* frontier orbital interactions. The reaction is
strongly exergonic (about -19 kcal/mol), with an energy barrier
of 12.3 kcal/mol and a corresponding rather “early” transition
state. The bulky Mes/C6F5 substituents do not interfere sterically
with the norbornene (also not in the TS) but rather stabilize the
product side by several kcal/mol due to noncovalent interac-
tions.34 Of the four possible stereoisomers of the product, the
exo-2,3 and endo-2,3 forms are most stable (by about 3 kcal/
mol). When including corrections to H298, the exo form 23
becomes lower in enthalpy than the endo isomer by 0.3 kcal/
mol. Considering the error in the computations, however, the
two isomers (23, 26) should be regarded as isoenthalpic and
therefore should both be detectable under experimental equi-
librium conditions (which is not observed). Thus, in agreement
with the results of computations of the endo and exo transition
states, we conclude that the reaction occurs under kinetic control
in the experiment.
and related functionals,33 it is known that such barriers should
be accurate to (1-2 kcal/mol.
Analysis of the electronic structure of the transition state has
been performed in detail for the model compounds only. The
localized (frontier) Kohn-Sham molecular orbitals formed by
B(p)-π and P(lone pair)-π* interactions are shown in Figure
8. It is clearly seen that the two bonds are formed in a
synchronous but asymmetric manner. As also indicated by the
bond orders (and bond lengths), the B-C bond is slightly ahead
of the P-C bond. The corresponding bond orders for 23(TS)
are reduced to 0.37 and 0.21, respectively, for B-C and P-C,
i.e., to about 50-60% of the values for 23B(TS).
For the substituent effects, a rather mixed picture emerges.
In contrast to 12, the open (gauche) form of 12B is found to be
almost as stable as the quenched conformer (energy difference
of 0.6 kcal/mol at the B97-D/def2-TZVP level). On the other
hand, the overall reaction energies for 12 and 12B are not very
different, indicating a rather small substituent effect. The same
holds for the relative energies of 23(B), 25-27(B) and also the
barriers. For the transition-state structures (see Figures 6 and
7) we note only an elongation of the forming B-C and P-C
bonds in 23(TS) compared to 23B(TS) (by about 0.36 and 0.23
Å, respectively), probably due to steric reasons. The increase
of the former CdC bond length to about 1.40 Å in 23(TS) is
almost identical with and without substituents (1.43 Å in
23B(TS)), indicating similar progress along the reaction coor-
dinate. The complexation energies of the borane group at the
double bond are also given in Table 1 for the model system
(for the substituted systems, the corresponding complexes, due
to steric reasons, are more of van der Waals type and therefore
not discussed here). Similarly to the TS, the exo complex is
Conclusions
The bifunctional frustrated Mes2P-CH2-CH2-B(C6F5)2
Lewis pair 12 undergoes 1,2-addition reactions with a variety
of unsaturated organic substrates. These reactions feature distinct
(34) (a) El-azizi, Y.; Schmitzer, A.; Collins, S. K. Angew. Chem. 2006,
118, 982-987; Angew. Chem., Int. Ed. 2006, 45, 968-973 and
references cited therein. (b) Cockroft, S. L.; Hunter, C. A.; Lawson,
K. R.; Perkins, J.; Urch, C. J. J. Am. Chem. Soc. 2005, 127, 8594–
8595. (c) Williams, J. H. Acc. Chem. Res. 1993, 26, 593–598. See
also: (d) Martin, E.; Spendley, C.; Mountford, A. J.; Coles, S. J.;
Horton, P. N.; Hughes, D. L.; Hursthouse, M. B.; Lancaster, S. J.
Organometallics 2008, 27, 1436–1446. (e) Blackwell, J. M.; Piers,
W. E.; Parvez, M.; McDonald, R. Organometallics 2002, 21, 1400–
1407. (f) Parks, D. J.; Piers, W. E.; Parvez, M.; Atencio, R.; Zaworotko,
M. J. Organometallics 1998, 17, 1369–1377.
(33) (a) Schwabe, T.; Grimme, S. Acc. Chem. Res. 2008, 41, 569–579. (b)
Karton, A.; Tarnopolsky, A.; Lamere, J. F.; Schatz, G. C.; Martin,
J. M. L. J. Phys. Chem. A 2008, 112, 12868–12886.
9
12286 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Evidence for a Concerted Olefin Addition Reaction
A R T I C L E S
glassware (or in a glovebox) under an atmosphere of argon. Solvents
were dried according to Grubbs38 or were distilled from appropriate
drying agents and stored under an argon atmosphere. The following
NMR instruments were used for physical characterization of the
compounds: Bruker ARX 300 spectrometer (19F, 282.4 MHz; 11B,
96.3 MHz; 31P, 121.5 MHz), Varian Inova 500 (1H, 499.9 MHz;
13C, 125.7 MHz; 19F, 470.3 MHz; 11B, 160.4 MHz; 31P, 202.3 MHz),
and Varian UnityPlus 600 (1H, 599.9 MHz; 13C, 150.8 MHz; 19F,
chemo- and regioselectivities. Compound 12 adds to the
aldehyde CdO functionality of trans-cinnamic aldehyde and
leaves the CdC bond untouched. The 1,2-addition to the
-CHdO functional group takes place regioselectively with
B-O and P-C bond formation. Also the 1,2-addition of 12 to
the polar enol ether CdC double bond is regioselective. The
boron atom adds to the dCH2 terminus. The observed regiose-
lective reactions of 12 with both the carbonyl CdO group and
the enol ether RO-CHdCH2 moiety are as would be expected
for a stepwise reaction pathway involving the respective
stabilized internal ion pairs. This regioselective outcome would,
however, probably also be expected from a concerted addition
pathway with the weakly interaction P/B system of 12 acting
as a cooperative pair.35
We have obtained evidence from our combined experimental
and theoretical study that the 1,2-addition of 12 to the nor-
bornene CdC double bond is a concerted reaction, although it
is probably markedly asynchronous, with the B-C bond formed
to a larger extent in the transition state (23(TS)) than the P-C
bond. The cis addition mode, of course, is enforced in our
example by the short -CH2-CH2- bridge connecting the
antagonistic P/B Lewis pair in the compound 12.
Is this actually a new type of a concerted reaction? A simple
topological analysis reveals that each of the reactive centers in
the antagonistic Lewis pair 12 is involved with one specific
orbital, namely the empty p-orbital at boron and the orbital
containing the electron pair at phosphorus. Therefore, our 1,2-
addition reaction of the antagonistic P/B Lewis pair of 12 to
norbornene bears some resemblance to the class of “cheletropic”
reactions in Woodward-Hoffmann chemistry,36 except that the
pair of doubly occupied/empty orbitals in the typical cheletropic
examples37 is located at the same single atom. Therefore, one
might term examples such as the concerted 1,2-addition of an
antagonistic Lewis pair, where these two essential orbitals are
located at different atoms, as “two-site cheletropic reactions”.
Analysis of further examples of these and topologically related
reactions that are beginning to increasingly appear in the
literature will show how important and how widely spread this
new reaction type might be.
1
564.4 MHz; 11B, 192.4 MHz; 31P, 242.7 MHz). H NMR and 13C
NMR, chemical shift δ is given relative to TMS and referenced to
the solvent signal; 19F NMR, chemical shift δ is given relative to
CFCl3 (external reference); 11B NMR, chemical shift δ is given
relative to BF3 ·Et2O (external reference); 31P NMR, chemical shift
δ is given relative to H3PO4 (85% in D2O) (external reference).
NMR assignments are supported by additional 2D NMR experi-
ments. Elemental analyses were performed on a Elementar Vario
El III instrument. IR spectra were recorded on a Varian 3100 FT-
IR spectrometer (Excalibur Series).
X-ray Crystal Structure Analyses. Data sets were collected
with a Nonius KappaCCD diffractometer. Programs used: data
collection, COLLECT (Nonius B.V., 1998); data reduction, Denzo-
SMN (Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276,
307-326); absorption correction, Denzo (Otwinowski, Z.; Borek,
D.; Majewski, W.; Minor, W. Acta Crystallogr. 2003, A59,
228-234); structure solution, SHELXS-97 (Sheldrick, G. M. Acta
Crystallogr. 1990, A46, 467-473); structure refinement, SHELXL-
97 (Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112-122);
graphics, XP (BrukerAXS, 2000).
Materials. Dimesitylvinylphosphine14 and bis(pentafluorophe-
nyl)borane19 were prepared according to literature procedures.
Norbornene was used without further purification; cinnamaldehyde
was distilled under reduced pressure prior to use. 1-Pentyne was
distilled using vacuum transfer. Ethyl vinyl ether was distilled from
sodium using vacuum transfer.
Synthesis of Compound 19. Dimesitylvinylphosphine (100 mg,
0.34 mmol) and HB(C6F5)2 (117 mg, 0.34 mmol) were dissolved
in pentane (6 mL) and stirred for 15 min. 1-Pentyne (37 µL, 0.37
mmol) was added at room temperature, and after 1 h a white-yellow
solid began to precipitate. After the reaction mixture had been stirred
for 4 days, the solid was isolated via cannula filtration and washed
twice with pentane (2.5 mL). Removal of all volatiles in vacuo
yielded 19 (167 mg, 69%) as a white powder. Crystals suitable for
X-ray crystal structure analysis were grown by slow diffusion of
pentane into a solution of 19 in benzene. Anal. Calcd for
C37H34BF10P: C, 62.55; H, 4.82. Found: C, 62.10; H, 4.40. 1H NMR
Experimental Section
1
3
General Procedures. All syntheses involving air- and moisture-
sensitive compounds were carried out using standard Schlenk-type
(600 MHz, 298 K, C6D6): δ ) 6.49 (1H, dt, JPH ) 475 Hz, JHH
4
) 7.7 Hz, P-H), 6.35 (4H, d, JPH ) 4.0 Hz, m-Mes), 2.93 (2H,
m, PCH2), 2.34 (2H, t, JHH ) 7.1 Hz, tCH2), 1.91 (12 H, s,
3
o-CH3Mes), 1.83 (6H, s, p-CH3Mes), 1.64 (2H, sext, JHH ) 7.1 Hz,
3
(35) We note an increasing number of papers concerning, e.g., “cooperative
catalysis” and related features currently emerging in the literature. See,
e.g.: (a) Park, Y. J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res. 2008,
41, 222–234. (b) Xu, X.; Zhou, J.; Yang, L.; Hu, W. Chem. Commun.
2008, 6564–6566. (c) Hu, W.; Xu, X.; Zhou, J.; Liu, W.-J.; Huang,
H.; Hu, J.; Yang, L.; Gong, L.-Z. J. Am. Chem. Soc. 2008, 130, 7782–
7783. (d) Sorimachi, K.; Terada, M. J. Am. Chem. Soc. 2008, 130,
14452–14453, and references cited therein.
B
3
CH2), 1.52 (2H, br, CH2 ), 1.10 (3H, t, JHH ) 7.1 Hz, CH3).
13C{1H} NMR (126 MHz, 298 K, C6D6): δ ) 148.9 (dm, JFC
)
)
1
4
2
238 Hz, C6F5), 145.0 (d, JPC ) 2.8 Hz, p-Mes), 142.8 (d, JPC
9.5 Hz, o-Mes), 138.7 (dm, 1JFC ) 246 Hz, C6F5), 137.4 (dm, 1JFC
3
) 249 Hz, C6F5), 131.5 (d, JPC ) 10.8 Hz, m-Mes), 129.0 (br,
i-C6F5), 113.6 (d, 1JPC ) 75.9 Hz, i-Mes), 98.0 (br, BCt), 94.8 (br,
1
P
tC), 25.3 (d, JPC ) 35.4 Hz, CH2), 23.9 (CH2), 23.1 (tCH2),
(36) Woodward, R. B.; Hoffmann, R. Angew. Chem., Int. Ed. 1969, 8, 781–
853. Woodward, R. B.; Hoffmann, R. Angew. Chem. 1969, 81, 797–
869.
3
B
21.6 (d, JPC ) 6.6 Hz, o-CH3Mes), 20.8 (p-CH3Mes), 20.5 (CH2 ),
13.9 (CH3). 11B{1H} NMR (192 MHz, 298 K, C6D6): δ ) -17.4
(ν1/2 ) 60 Hz). 31P NMR (243 MHz, 298 K, C6D6): δ ) -5.3 (1JPH
) 475 Hz). 31P{1H} NMR (243 MHz, 298 K, C6D6): δ ) -5.3
(ν1/2 ) 48 Hz). 19F{1H} NMR (564 MHz, 298 K, C6D6): δ )
-132.1 (4F, o-C6F5), -162.7 (2F, p-C6F5), -165.8 (4F, m-C6F5).
IR (KBr): V˜ [cm-1] ) 2359 (w), 2341 (w).
(37) (a) Baldwin, J. E. Can. J. Chem. 1966, 44, 2051–2056. (b) Hoffmann,
R.; Hayes, D. M.; Zeiss, C. A. J. Phys. Chem. 1971, 75, 340–344. (c)
Sonobe, B. I.; Fletcher, T. R.; Rosenfeld, R. N. J. Am. Chem. Soc.
1984, 106, 4352–4356. (d) Sua´rez, D.; Iglesias, E.; Sordo, T. L.; Sordo,
J. A. J. Phys. Org. Chem. 1996, 9, 17–20. (e) Unruh, G. R.; Birney,
D. M. J. Am. Chem. Soc. 2003, 125, 8529–8533. (f) Rodr´ıguez-Otero,
J.; Cabaleiro-Lago, E. M.; Hermida-Ramo´n, J. M.; Pen˜a-Gallego, A.
J. Org. Chem. 2003, 68, 8823–8830. (g) Lai, C.-H.; Li, E. Y.; Chen,
K.-Y.; Chow, T. J.; Chou, P.-T. J. Chem. Theory Comput. 2006, 2,
1078–1084. Reviews: (h) Moss, R. A. Acc. Chem. Res. 1980, 13, 58–
64. (i) Kla¨rner, F. G.; Wurche, W. J. Prakt. Chem. 2000, 342, 609–
636. (j) Vogel, P.; Sordo, J. A. Curr. Org. Chem. 2006, 10, 2007–
2036. (k) Vogel, P.; Turks, M.; Bouchez, L.; Markovic´, D.; Varela-
X-ray Crystal Structure Analysis of 19. Formula C37H34BF10-
P·1.5C6H6, M ) 827.58, colorless crystal 0.25 × 0.10 × 0.10 mm,
a ) 17.2244(4), b ) 12.5179(4), and c ) 21.0243(7) Å, ꢀ )
110.039(1)°, V ) 4258.7(2) Å3, Fcalc ) 1.291 g cm-3, µ ) 1.227
(38) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
´
Alvarez, A.; Sordo, J. A. Acc. Chem. Res. 2007, 40, 931–942.
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12287

A R T I C L E S
Mo¨mming et al.
mm-1, empirical absorption correction (0.749 e T e 0.887), Z )
4, monoclinic, space group P21/c (No. 14), λ) 1.54178 Å, T )
223(2) K, ω and ꢁ scans, 31 506 reflections collected (h,k,l), [(sin
θ)/λ] ) 0.60 Å-1, 7483 independent (Rint ) 0.055) and 5991
observed reflections [I g 2σ(I)], 541 refined parameters, R ) 0.056
wR2 ) 0.154, max. (min.) residual electron density 0.32 (-0.32) e
Å-3, disorder in the group C5-C7 refined with split positions,
hydrogen atom at phosphorus from difference Fourier map, others
calculated and refined as riding atoms.
Synthesis of Compound 20. Dimesitylvinylphosphine (100 mg,
0.34 mmol) and HB(C6F5)2 (117 mg, 0.34 mmol) were dissolved
in pentane (10 mL) and stirred for 15 min. Upon addition of
cinnamaldehyde (42 µL, 0.34 mmol) at room temperature, a white
precipitate formed immediately. After the reaction mixture was
stirred overnight, the precipitate was isolated via cannula filtration.
The residue was then washed twice with pentane (2 mL), and all
volatiles were removed in vacuo to yield 20 (184 mg, 70%). Crystals
suitable for X-ray crystal structure analysis were obtained by gas
diffusion of heptane into a solution of 20 in benzene at room
temperature. Anal. Calcd for C41H34BF10OP: C, 63.58; H, 4.42.
Found: C, 63.12; H, 4.54. 1H NMR (600 MHz, 298 K, CD2Cl2): δ
) 7.29 (2H, m, m-Ph), 7.23 (3H, m, p-, o-Ph), 6.98 (2H, br s,
m-MesA), 6.94 (2H, br s, m-MesB), 6.93 (1H, ddd, 3JHH ) 15.5 Hz,
2.15 (6H, s, o-CH3MesB), 1.88 (6H, s, o-CH3MesA), 1.88 (2 × 3H, s,
p-CH3MesA,B), 1.96 (dm, 3JPH ) 39.9 Hz), 1.27 (m) (each 1H, BCH2),
2
3
3
1.65 (1H, ddd, JHH ) 13.5 Hz, JHH ) 12.4 Hz, JPH ) 8.7 Hz,
OCH2 ), 0.68 (3H, t, JHH ) 7.1 Hz, CH3). 13C{1H} NMR (125
MHz, 298 K, C6D6): δ ) 149.3, 148.9 (each dm, each 1JFC ) 237
Hz, each 2 × C6F5), 143.1, 142.9 (each d, 4JPC ) 3.0 Hz, p-MesA,B),
142.2 (d, 2JPC ) 8.8 Hz, o-MesB), 141.9 (d, 2JPC ) 8.8 Hz, o-MesA),
B
3
1
138.7 (dm, 1JFC ) 246 Hz, 2 × C6F5), 137.6 (dm, JFC ) 248 Hz,
3
3
4 × C6F5), 132.2 (d, JPC ) 10.9 Hz, m-MesA), 131.6 (d, JPC
)
10.9 Hz, m-MesB), 129.0 (br, i-C6F5), 122.3 (d, JPC ) 69.9 Hz,
1
i-MesB), 121.8 (d, JPC ) 69.9 Hz, i-MesA), 85.1 (d, JPC ) 55.4
1
1
Hz, CH), 66.3 (d, 3JPC ) 11.0 Hz, CH2Et), 28.3 (d, 1JPC ) 39.0 Hz,
PCH2), 27.6 (br, CH2 ), 23.9 (d, JPC ) 4.2 Hz, o-CH3MesB), 23.8
O
B
3
3
5
(d, JPC ) 3.2 Hz, o-CH3MesA), 20.7, 20.6 (each d, JPC ) 1.5
Hz, p-CH3MesA,B), 16.4 (br, BCH2), 14.8 (CH3), 11B{1H} NMR (160
MHz, 298 K, C6D6) δ ) -13.3 (ν1/2 ) 80 Hz). 31P{1H} NMR
(202 MHz, 298 K, C6D6): δ ) 23.4 (ν1/2 ) 6 Hz). 19F{1H} NMR
(470 MHz, 298 K, C6D6): δ ) -133.5 (2F, o-), -162.2 (1F, p-),
-165.1 (2F, m-) (C6F5A), -133.7 (br, 2F, o-), -161.1 (1F, p-),
B
-164.5 (2F, m-) (C6F5 ).
X-ray Crystal Structure Analysis of 21. Formula C36H34BF10OP,
M ) 714.41, colorless crystal 0.40 × 0.30 × 0.25 mm, a )
33.6177(10), b ) 12.8660(4), and c ) 16.5773(5) Å, ꢀ )
111.635(2)°, V ) 6665.0(4) Å3, Fcalc ) 1.424 g cm-3, µ) 1.496
mm-1, empirical absorption correction (0.586 e T e 0.706), Z )
8, monoclinic, space group C2/c (No. 15), λ ) 1.54178 Å, T )
223(2) K, ω and ꢁ scans, 22 950 reflections collected (h,k,l), [(sin
θ)/λ] ) 0.60 Å-1, 5877 independent (Rint ) 0.042) and 5364
observed reflections [I g 2σ(I)], 449 refined parameters, R ) 0.048,
wR2 ) 0.135, max. (min.) residual electron density 0.36 (-0.30) e
Å-3, hydrogen atoms calculated and refined as riding atoms.
Synthesis of Compound 23. To a solution of dimesitylvi-
nylphosphine (100 mg, 0.34 mmol) and HB(C6F5)2 (117 mg, 0.34
mmol) in pentane (10 mL) was added norbornene (621 mg, 6.6
mmol). The reaction mixture was stirred for 8 days at room
temperature, and during that time a white solid precipitated. The
precipitate was isolated via cannula filtration, washed with pentane
(10 mL), and dried in vacuo to yield 23 (173 mg, 72%). Crystals
suitable for X-ray crystal structure analysis were obtained by slow
diffusion of pentane into a solution of 23 in benzene. Anal. Calcd
4
3
4JPH ) 6.2 Hz, JHH )1.9 Hz, dCHPh), 6.11 (1H, dt, JHH ) 15.5
3
3
2
Hz, JPH ≈ JHH ) 4.3 Hz, dCH), 5.74 (1H, ddd, JPH ) 6.2 Hz,
4JHH ) 1.9 Hz, 3JHH ) 4.3 Hz, HCO), 3.31, 3.04 (each 1H, each br
P
m, CH2), 2.36 (6H, s, o-CH3MesB), 2.32 (3H, s, p-CH3MesA), 2.28
(3H, s, p-CH3MesB), 2.23 (6H, s, o-CH3MesA), 1.67, 1.02 (each 1H,
each br m, BCH2). 13C{1H} NMR (151 MHz, 298 K, CD2Cl2): δ )
144.3 (p-MesA), 143.7 (p-MesB), 142.8 (o-MesB), 142.5 (o-MesA),
136.7 (d, 4JPC ) 3.8 Hz, i-Ph), 133.8 (d, 3JPC ) 12.1 Hz, dCHPh),
132.7 (br d, JPC ) 7.7 Hz, m-MesA), 131.5 (br d, JPC ) 7.7 Hz,
3
3
m-MesB), 129.0 (m-Ph), 128.3 (p-Ph), 126.9 (o-Ph), 123.6 (dCH),
122.8 (d, JPC ) 60.5 Hz, i-MesB), 119.6 (d, JPC ) 60.5 Hz,
1
1
i-MesA), 81.3 (d, JPC ) 35.1 Hz, HCO), 28.4 (d, JPC ) 42.1 Hz,
PCH2), 24.5 (o-CH3MesA), 23.6 (o-CH3MesB), 21.1 (p-CH3MesA,B), 13.9
(br, BCH2), n.o. (C6F5). 11B{1H} NMR (192 MHz, 298 K, CD2Cl2):
δ ) -0.5 (ν1/2 ) 150 Hz). 31P{1H} NMR (243 MHz, 298 K,
CD2Cl2): δ ) 16.1 (ν1/2 ) 12 Hz). 19F{1H} NMR (564 MHz, 298
K, CD2Cl2): δ ) -134.6 (2F, o-), -162.5 (1F, p-), -166.4 (2F,
m-) (C6F5A), -134.8 (2F, o-), -161.4 (1F, p-), -165.6 (2F, m-)
1
1
1
for C39H36BF10P: C, 63.60; H, 4.93. Found: C, 63.91; H, 5.42. H
B
(C6F5 ).
NMR (600 MHz, 298 K, C6D6/CD2Cl2): δ ) 6.65 (1H, br, m-MesA),
6.55 (1H, br, m′-MesA), 6.55 (1H, br, m-MesB), 6.48 (1H, br, m′-
MesB), 3.29 (1H, m, 2-H), 3.20 (1H, m, 3-H), 2.89, 2.63 (each 1H,
X-ray Crystal Structure Analysis of 20. Formula C41H34BF10OP,
M ) 774.46, colorless crystal 0.20 × 0.15 × 0.10 mm, a )
9.4496(3), b ) 30.8893(11), and c ) 12.4289(4) Å, ꢀ ) 98.642(2)°,
V ) 3586.7(2) Å3, Fcalc ) 1.434 g cm-3, µ) 1.440 mm-1, empirical
absorption correction (0.762 e T e 0.869), Z ) 4, monoclinic,
space group P21/c (No. 14), λ ) 1.54178 Å, T ) 223(2) K, ω and
P
each m, CH2), 2.57 (3H, s, o-CH3MesB), 2.41 (3H, s, o-CH3MesA),
3
2.21 (1H, dm, JPH ) 35.5 Hz, BCH2), 2.02 (3H, s, p-CH3MesA),
3
2.00 (1H, br, 1-H), 1.94 (3H, s, p-CH3MesB), 1.79 (1H, dd, JHH
)
12.0 Hz, 3.5 Hz, 4-H), 1.70 (3H, s, o′-CH3MesA), 1.60 (3H, s, o′-
ꢁ scans, 29 245 reflections collected (h,k,l), [(sin θ)/λ] ) 0.60 Å-1
,
CH3MesB), 1.42 (2H, m, 6-H), 1.23, 1.11 (each 1H, each m, 5-H),
6310 independent (Rint ) 0.061) and 4839 observed reflections [I
g 2σ(I)], 493 refined parameters, R ) 0.050, wR2 ) 0.127, max.
(min.) residual electron density 0.20 (-0.25) e Å-3, hydrogen atoms
calculated and refined as riding atoms.
1.19, 0.72 (each 1H, each m, 7-H), 0.36 (1H, m, CH2). 13C{1H}
B
1
NMR (151 MHz, 298 K, C6D6/CD2Cl2): δ ) 149.2 (dm, JFC
)
1
238 Hz, 2 × C6F5), 148.5 (dm, JFC ) 234 Hz, 2 × C6F5), 143.9
(d, 2JPC ) 8.3 Hz, o-MesB), 143.2 (d, 4JPC ) 2.8 Hz, p-MesA), 142.9
(d, 4JPC ) 2.8 Hz, p-MesB), 142.7 (d, 2JPC ) 8.2 Hz, o-MesA), 141.3
Preparation of Compound 21. Dimesitylvinylphosphine (100
mg, 0.34 mmol) and HB(C6F5)2 (117 mg, 0.34 mmol) were
dissolved in pentane (6 mL) and stirred for 15 min. Upon addition
of ethyl vinyl ether (36 µL, 0.37 mmol) at room temperature, a
white solid began to precipitate. After the reaction mixture was
stirred for 24 h, the solid was isolated via cannula filtration and
washed twice with pentane (2 mL). Removal of all volatiles in
vacuo yielded 21 (204 mg, 84%). Crystals suitable for X-ray crystal
structure analysis were grown by slow concentration of a solution
of 21 in dichloromethane at -36 °C. Anal. Calcd for C36H34BF10P:
(d, JPC ) 8.2 Hz, o′-MesA), 141.1 (d, JPC ) 9.1 Hz, o′-MesB),
2
2
1
1
138.1 (dm, JFC ) 263 Hz, 2 × C6F5), 137.2 (dm, JFC) 251 Hz,
3
3
4 × C6F5), 132.7 (d, JPC ) 10.6 Hz, m′-MesB), 132.4 (d, JPC
)
10.2 Hz, m′-MesA), 132.0 (d, JPC ) 10.9 Hz, m-MesA), 131.5 (d,
3JPC ) 11.0 Hz, m-MesB), 123.0 (d, 1JPC ) 72.2 Hz, i-MesA), 122.3
3
(d, JPC ) 75.9 Hz, i-MesB), 44.0 (d, JPC ) 32.9 Hz, C3), 43.7
1
1
(C1), 42.8 (C4), 38.3 (C7), 37.6 (br, C2), 33.9 (d, 3JPC ) 14.9 Hz,
1
P
C5), 32.4 (C6), 25.9 (d, JPC ) 41.5 Hz, CH2), 24.8 (o-CH3MesB),
3
3
24.6 (d, JPC ) 2.4 Hz, o-CH3MesA), 22.3 (d, JPC ) 5.2 Hz, o′-
1
C, 60.52; H, 4.80. Found: C, 60.23; H, 4.71. H NMR (500 MHz,
3
5
CH3MesA), 22.2 (d, JPC ) 5.1 Hz, o′-CH3MesB), 20.9 (d, JPC ) 1.4
Hz, p-CH3MesA), 20.6 (d, 5JPC ) 1.3 Hz, p-CH3MesB), 14.8 (br, BCH2),
n.o. (i-C6F5). 11B{1H} NMR (96 MHz, 298 K, C6D6/CD2Cl2): δ )
-11.8 (ν1/2 ) 70 Hz). 31P{1H} NMR (122 MHz, 298 K, C6D6/
CD2Cl2): δ ) 31.2 (ν1/2 ) 15 Hz). 19F{1H} NMR (282 MHz, 298
298 K, C6D6): δ ) 6.46 (2H, d, 4JPH ) 4.0 Hz, m-MesB), 6.39 (2H,
4
3
2
d, JPH ) 3.7 Hz, m-MesA), 5.03 (1H, dt, JHH ) 12.4 Hz, JPH
≈
3JHH ) 4.4 Hz, CH), 3.52, 2.74 (each 1H, each dq, 4JPH ) 9.0 Hz,
3JHH ) 7.1 Hz, CH2Et), 2.92 (1H, ddd, 3JPH ) 44.6 Hz, 2JHH ) 13.5
Hz, JHH ) 4.4 Hz, OCH2 ), 2.47, 2.34 (each 1H, each m, CH2),
3
B
P
9
12288 J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009

Evidence for a Concerted Olefin Addition Reaction
A R T I C L E S
K, C6D6/ CD2Cl2): δ ) -132.1 (br, 4F, o-C6F5),-162.8, -163.8
(each 1F, p-C6F5), -165.9, -166.5 (each 2F, m-C6F5).
electron density 0.31 (-0.30) e Å-3, hydrogen atoms calculated
and refined as riding atoms.
X-ray Crystal Structure Analysis of 23. Formula C39H36BF10-
P·1.5C6H6, M ) 853.62, colorless crystal 0.30 × 0.20 × 0.10 mm,
a ) 12.1477(5), b ) 12.2155(5), and c ) 15.1806(6) Å, R )
74.541(2), ꢀ ) 72.625(2), and γ ) 82.110(2)°, V ) 2068.01(15)
Å3, Fcalc ) 1.371 g cm-3, µ ) 1.281 mm-1, empirical absorption
Acknowledgment. Financial support from the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank the
BASF for a gift of solvents.
Supporting Information Available: Experimental details and
information about the quantum chemical calculations and the
X-ray crystal structure analyses. This material is available free
of charge via the Internet at http://pubs.acs.org.
j
correction (0.700 e T e 0.883), Z ) 2, triclinic, space group P1
(No. 2), λ) 1.54178 Å, T ) 223(2) K, ω and ꢁ scans, 30 905
reflections collected (h,k,l), [(sin θ)/λ] ) 0.60 Å-1, 7290 indepen-
dent (Rint ) 0.049) and 6649 observed reflections [I g 2σ(I)], 547
refined parameters, R ) 0.049, wR2 ) 0.133, max. (min.) residual
JA903511S
9
J. AM. CHEM. SOC. VOL. 131, NO. 34, 2009 12289