Preorganized frustrated lewis pairs

Article abstract of DOI:10.1021/ja210214r

Geminal frustrated Lewis pairs (FLPs) are expected to exhibit increased reactivity when the donor and acceptor sites are perfectly aligned. This is shown for reactions of the nonfluorinated FLP tBu₂PCH ₂BPh₂ with H₂, CO₂, and isocyanates and supported computationally.

Full text of DOI:10.1021/ja210214r

Communication
pubs.acs.org/JACS
Preorganized Frustrated Lewis Pairs
Federica Bertini,† Volodymyr Lyaskovskyy,† Brian J. J. Timmer,† Frans J. J. de Kanter,† Martin Lutz,‡
Andreas W. Ehlers,† J. Chris Slootweg,*,† and Koop Lammertsma*,†
†Department of Chemistry and Pharmaceutical Sciences, VU University Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The
Netherlands
‡Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The
Netherlands
S
* Supporting Information
longer reaction time (25 h) and gives a much lower yield
(33%).9
ABSTRACT: Geminal frustrated Lewis pairs (FLPs) are
expected to exhibit increased reactivity when the donor
and acceptor sites are perfectly aligned. This is shown for
reactions of the nonfluorinated FLP tBu2PCH2BPh2 with
H2, CO2, and isocyanates and supported computationally.
Enhanced reactivity can be expected for preorganized FLPs
with intramolecular donor and acceptor sites,2a particularly
when the two sites are oriented in the same direction for
optimal interaction with the substrate.17 We envisioned that
geminal P/B-based FLPs might be ideally suited to activate
small molecules, possibly to the extent that strongly electron-
withdrawing substituents on B might not even be needed. We
further reasoned that intramolecular self-quenching of the
methylene-linked Lewis acid and base centers might be
adequately countered by the inherent strain associated with
the formation of the three-membered PCB ring.
ince their discovery, frustrated Lewis pairs (FLPs) have
S
gained much attention because of their ability to activate
small molecules.1 This ability arises from the synergic effect of
the donor and acceptor sites that polarizes the desired chemical
bond.2 FLPs react with molecules such as H2,1,2 CO2,3,4
alkenes,5 and alkynes,3,5b,6 thereby offering new synthetic
entries. Their ability to cleave H2 heterolytically makes them
valuable metal-free catalysts for the hydrogenation of organic
substrates,7 whereas activation of the greenhouse gas CO2 holds
potential for its conversion into products such as MeOH4b,8
and CH4.8c Mutual quenching of the Lewis acid and base sites
in the FLPs is prevented by the bulky substituents that keep the
donor and acceptor atoms sufficiently far apart, as is the case for
the associated pair tBu3P---B(C6F5)3 (1a) (Scheme 1).2a−c,9,10
To date, the only reported geminal P/B-based FLPs are 3a18
and 3b19 (Scheme 1), which were prepared by hydroboration
of substituted phosphines in a manner similar to that used to
synthesize 2.20 Both bear strongly electron-withdrawing C6F5
groups at P and at B. They react with unsaturated organic
substrates but not with H2, probably because of the reduced
Lewis basicity of the P center.
Here we address the potential of simple, nonfluorinated
geminal P/B-based FLPs to activate small molecules by
comparing the reactivities of the methylene-bridged tBu2P-
phosphinoboranes with pentafluorophenyl (4a) and phenyl
substituents (4b) on B (Scheme 2). The enhanced reactivity
Scheme 1. Frustrated Lewis Pairs 1a,b, 2b, and 3a,b
Scheme 2. Preorganized FLPs 4a,b and Rearrangement of 4a
to 6a
Although weakly bonded Lewis pairs can be used,11 the energy
needed to separate the Lewis acid and base components
reduces their reactivity. For example, whereas the strained four-
membered ring 2a can dissociate into the reactive open gauche
form 2b (Scheme 1),11a,12,13 the five- and six-membered rings
Mes2P(CH2)nB(C6F5)2 (n = 3, 4) possess much stronger P−B
bonds, rendering them unreactive.14
was recently demonstrated by the intramolecular activation of
transient perfluorinated 4a to form the undesired five-
membered heterocycle 6a.21 Our M06-2X/6-31+G(d,p)
calculations22 on the full system confirmed that 4a undergoes
intramolecular substitution via σ-complex 5a [ΔE⧧ = 20.1 kcal
mol−1, ΔE = 10.4 kcal mol−1; zero-point energy (ZPE)-
corrected] and a facile [1,3]-F shift to the thermodynamically
While many Lewis bases1,15,16 have been employed in FLP
chemistry, the Lewis acid component is generally restricted to
borane moieties bearing highly electron-withdrawing penta-
fluorophenyl substituents to enhance the acidity of the acceptor
site. This effect is illustrated by the intermolecular FLP 1a,
which reacts smoothly with H2 (rt, 13 h; 90% yield) while the
nonfluorinated analogue tBu3P---BPh3 (1b) requires a much
Received: November 8, 2011
Published: December 13, 2011
© 2011 American Chemical Society
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Communication
favored 6a (ΔΔE = −65.2 kcal mol−1; ΔE = −54.8 kcal mol−1;
Scheme 2).
mol−1; Figure 3) to the phosphonium borate product 7b (ΔE =
−1.6 kcal mol−1). The computed covalent bond orders (bo's)
Interestingly, the “quenched” ring-closed FLP 4a′ with a P−B
distance of 2.04 Å is slightly more stable (ΔE = −1.5 kcal
mol−1) than the ring-opened isomer 4a (P1−B1 = 2.89 Å),23
but only the open form exists for the nonfluorinated derivative
4b (P1−B1 = 2.69 Å), indicating reduced Lewis acidity of the
acceptor site (Figure 1). Preorganized 4b with Lewis base and
Figure 1. M06-2X/6-31+G(d,p) geometries of 4a′ and 4b; H atoms
have been omitted for clarity. Selected bond lengths (Å) and angles
(deg) for 4a′: P1−B1, 2.04; P1−C1, 1.83; B1−C1, 1.59. For 4b: P1−
B1, 2.69; P1−C1, 1.90; B1−C1, 1.57; P1−C1−B1, 101.3.
Figure 3. Relative M06-2X/6-31+G(d,p) energies (kcal mol−1) for the
reaction of 4b with H2 to give 7b; the transition state for the H2 uptake
by 4a is also shown. Computed bond orders for TS7a and TS7b are
given in italics. Selected bond lengths (Å) and angles (deg) for 7b-
vdW: B1−H31, 3.61; B1−H32, 3.64; P1−C1−B1, 115.6; B1−H31−
H32, 86.9; B1−H32−H31, 81.3. For TS7b: P1−H31, 1.70; H31−
H32, 1.01; B1−H32, 1.40; P1−C1−B1, 104.3; P1−H31−H32, 135.5;
B1−H32−H31, 107.2. For 7b: P1−H31, 1.40; B1−H32, 1.23; P1−
C1−B1, 108.2; H31−P1−C1−B1, −13.7; H32−B1−C1−P1, −47.1.
acid sites perfectly oriented for small-molecule activation (see
the HOMO and LUMO in Figure 2) became our target.
for TS7b show that heterolytic H2 splitting occurs in a
concerted manner, with the formation of the B−H bond (bo =
0.55) proceeding slightly earlier than that of the P−H bond (bo
= 0.50) (Figure 3).2a The significant barrier for the nearly
thermoneutral H2 splitting is consistent with our experimental
findings. Apparently, pentafluorophenyl substituents may not
be needed for FLPs to function. Their presence may even be
detrimental, as illustrated in Scheme 2. Our calculations showed
that heterolytic uptake of H2 by 4a to give 7a is far more
exothermic (ΔE = −14.8 kcal mol−1) and has only a modestly
lower activation energy (TS7a, ΔE⧧ = 16.6 kcal mol−1; Figure
3), indicating that pentafluorophenyl substituents facilitate H−
H bond cleavage while limiting the reversibility of this process.
The reactivity of 4b toward H2 encouraged us to examine the
capture of CO2, which is unprecedented for nonfluorinated
borane-based FLPs. Bubbling CO2 through a solution of 4b in
toluene under ambient conditions for 30 s quantitatively
formed 8b, which was isolated as a colorless solid in 89% yield
Figure 2. HOMO (left) and LUMO (right) of 4b.
Phosphinoborane 4b was readily obtained as a colorless oil in
94% isolated yield by a salt metathesis reaction between readily
accessible tBu2PCH2Li24 and chlorodiphenylborane.25 The
broad 11B{1H} NMR resonance at 72.3 ppm is characteristic
of a tricoordinated B center and supports the FLP nature of 4b
(δ31P = 39.4 ppm). Next, we examined its reactivity with H2,
CO2, and isocyanates and studied the full systems computa-
tionally using ZPE-corrected M06-2X/6-31+G(d,p) energies.22
Bubbling H2 through a solution of 4b in toluene under
ambient conditions for 5 h afforded the H2 adduct 7b as a
colorless solid in 28% isolated yield [δ31P = 53.0 ppm, 1J(P,H)
= 460.4 Hz; δ11B{1H} = −9.0 ppm; Scheme 3). This process
1
(δ31P = 48.0 ppm; δ13C{1H} (CO2) = 167.8 ppm, J(C,P) =
Scheme 3. Reactions of 4b with H2, CO2, and tBuNCO
89.7 Hz; δ11B{1H} = 5.0 ppm; Scheme 3). CO2 fixation by 4b
was confirmed by the X-ray structure of 8b (Figure 4), which
displays an almost planar five-membered ring with P−C and
B−O distances of 1.8690(12) and 1.5645(15) Å, respectively,
and CO/C−O lengths of 1.2098(15)/1.2938(14) Å, which
are comparable to those observed for 2-CO2.4a The short
endocyclic C22−O1 bond suggests double-bond character, as
in all previously reported FLP−CO2 adducts [i.e., 1.2988(15) Å
for 1-CO2; 1.284(4) Å for 2-CO2],4a underlining the
contribution of resonance structure 8B (Scheme 3).
The calculations revealed that the generation of 8b (ΔE =
−20.3 kcal mol−1) starts with the formation of a van der Waals
complex (8b-vdW, ΔE = −4.1 kcal mol−1; Figure 5) with an
almost linear CO2 fragment (177.4°) and long P−C (3.34 Å)
and B−O (3.81 Å) distances. In contrast to the reaction with
H2, the computed bo's show that CO2 uptake occurs via facile
nucleophilic attack on the CO2 C atom by the P atom (TS8b,
reflects a modest increase in reactivity relative to 1b.26 The
computations revealed that H2 is delivered to the FLP cavity
through van der Waals complexes [see the Supporting
Information (SI)]. One of these results from the side-on
interaction of H2 with the B atom (7b-vdW, ΔE = −0.7 kcal
mol−1), which leads via transition state TS7b (ΔE = 22.6 kcal
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Journal of the American Chemical Society
Communication
1.8341(11) and 1.5514(15) Å, respectively, and a short
endocyclic C22−O1 bond [1.3198(13) Å] (Figure 4). Thus,
the substrate substituents also influence the product formation.
To address the selectivity of the reaction of 4b with
tBuNCO, we calculated the relative energies of E-9bCO and the
feasible but unobserved Z-9bCO and 9bCN (Scheme 4). Steric
Scheme 4. Possible Reaction Products of 4b + tBuNCO with
Their Connectivities and Calculated Energies (kcal mol−1)
Figure 4. Molecular structures of 8b and E-9bCO. Ellipsoids are set at
50% probability; H atoms and disordered solvent molecules have been
omitted for clarity. Selected bond lengths (Å) and angles (deg) for 8b:
P1−C22, 1.8690(12); C22−O1, 1.2938(14); C22−O2, 1.2098(15);
O1−B1, 1.5645(15); P1−C1, 1.7791(13); C1−B1, 1.6893(18); O2−
C22−O1, 125.85(11); P1−C1−B1, 105.22(8); P1−C22−O1,
110.04(8); C22−O1−B1, 120.40(9). For E-9bCO: P1−C22,
1.8341(11); C22−O1, 1.3198(13); O1−B1, 1.5514(15); P1−C1,
1.7812(12); B1−C1, 1.6719(16); C22−N1, 1.2708(15); P1−C22−
N1, 120.01(8); P1−C22−O1, 110.08(8); N1−C22−O1, 129.84(10);
C22−O1−B1, 119.64(9); C22−P1−C1, 97.93(5); P1−C1−B1,
105.32(8); C1−B1−O1, 106.79(9).
repulsion between the tert-butyl groups on N and P appears to
underlie the lower exothermicity of Z-9bCO (ΔE = −5.4 kcal
mol−1) relative to the experimentally obtained E-9bCO (ΔE =
−29.5 kcal mol−1). The analogous but sterically less hindered
Ph-substituted adducts show a less pronounced E/Z difference
(ΔΔE = 10.0 kcal mol−1; see the SI). Whereas the calculations
gave an energetic preference of 2.4 kcal mol−1 for CN adduct
1
9bCN over E-9bCO, no changes in the 31P{1H} and H NMR
spectra were observed upon heating a solution of E-9bCO in
C6D6 for 1 h at 100 °C, indicating the irreversibility of the
isocyanate capture by 4b.
To unravel the reaction mechanism, we computationally
compared (a) nucleophilic attack of the P atom on the
electrophilic isocyanate C atom to generate ylide 10PC, (b)
formation of the Lewis adduct 11BN by coordination of
tBuNCO’s N atom, and (c) formation of B−O Lewis adduct
11BO by coordination of tBuNCO’s O atom (Scheme 4).
Phosphine−isocyanate adducts like 10PC have been proposed
as intermediates in the phosphine-catalyzed oligomerization of
isocyanides,28 but no such ylides have been isolated to date.
Our computations showed that the formation of 10PC with the
isocyanate unit orthogonal to the PCB plane is endothermic by
0.5 kcal mol−1. Rotation of the isocyanate along the P−C axis
results in the barrierless formation of E-9bCO (clockwise) and
9bCN (counterclockwise) by B−O and B−N bond formation,
respectively. As both pathways are equally facile, this
mechanism cannot account for the observed exclusive
formation of the less stable E-9bCO. N-Coordination of
tBuNCO to give Lewis adduct 11BN is even more endothermic
(2.6 kcal mol−1) despite the favorable energetics for ring
closure to give 9bCN (ΔΔE⧧ = 0.6 kcal mol−1, ΔΔE = −34.5
kcal mol−1). In contrast, O-coordination of the isocyanate to
give Lewis adduct 11BO is exothermic (ΔE = −3.2 kcal mol−1).
Subsequent ring closure by P−C bond formation gives E-9bCO
(ΔΔE⧧ = 7.4 kcal mol−1, ΔΔE = −26.3 kcal mol−1; Scheme 4).
Consequently, we conclude that the formation of E-9bCO is
kinetically controlled with 11BO as the key intermediate.
In summary, this work underscores the potential of geminal
methylene-bridged phosphinoboranes as preorganized FLPs
Figure 5. Relative M06-2X/6-31+G(d,p) energies (kcal mol−1) for the
reaction of 4b with CO2. Selected bond lengths (Å) and angles (deg)
for 8b-vdW: P1−C22, 3.34; B1−O1, 3.81; C22−O1, 1.16; C22−O2,
1.16; P1−C22−O1, 95.7; B1−O1−C22, 107.8; P1−C1−B1, 100.9;
O1−C22−O1, 177.4. For TS8b: P1−C22, 2.40; C22−O1, 1.19; C22−
O2, 1.19; B1−O1, 2.66; P1−C1−B1, 113.6; P1−C22−O1, 99.6; B1−
O1−C22, 114.4; O1−C22−O2, 153.0. For 8b: P1−C22, 188.14;
C22−O1, 1.29; C22−O2, 1.21; B1−O1, 1.55; P1−C1−B1, 104.1;
O1−C22−O2, 128.5; P1−C22−O1, 109.0; B1−O1−C22, 121.5.
ΔΔE⧧ = 6.9 kcal mol−1; P−C bo = 0.34, B−O bo = 0.06;
Figure 5) followed by ring closure.4a The CO2 adduct 8b was
thermally stable at 100 °C under vacuum (5 × 10−2 mbar) and
showed no release of CO2. This reaction underscores the fact
that nonfluorinated FLPs can capture small molecules.
Finally, we examined the reaction of 4b with isocyanates. To
date, three CO adducts for the reaction of phenyl isocyanate
with FLPs have been reported,27 whereas with the electron-
poor FLPs 3a and 3b, CN adducts were obtained.18,19 For
the reaction of PhNCO with 4b, we observed two 31P NMR
resonances (47.2 and 58.5 ppm) in a 1:1 ratio that we attribute
to the CO and CN adducts. Reaction of 4b with tBuNCO
instead yielded only the CO adduct, which was isolated as a
colorless solid in 76% yield (δ31P = 50.5 ppm; δ13C{1H}
1
(tBuNCO) = 155.3 ppm, J(C,P) = 115.1 Hz; δ11B{1H} = 6.3
ppm; Scheme 3). Single-crystal X-ray analysis revealed the
formation of E-9bCO having P−C and B−O bond lengths of
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(8) (a) Ashley, A. E.; Thompson, A. L.; O'Hare, D. Angew. Chem.
2009, 121, 10023; Angew. Chem., Int. Ed. 2009, 48, 9839. (b) Dureen,
M. A.; Stephan, D. W. J. Am. Chem. Soc. 2010, 132, 13559.
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and shows that remarkable reactivity can be achieved without
the need for strongly electron-withdrawing substituents on B.
ASSOCIATED CONTENT
■
S
* Supporting Information
(9) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
Experimental details, Cartesian coordinates and energies of all
stationary points, and crystallographic data (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(10) Kim, H. W.; Rhee, Y. M. Chem.Eur. J. 2009, 15, 13348.
(11) (a) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Frohlich, R.;
̈
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072. (b) Dureen,
M. A.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 8396. (c) Welch,
G. C.; Prieto, R.; Dureen, M. A.; Lough, A. J.; Labeodan, O. A.;
AUTHOR INFORMATION
■
Holtrichter-Rossmann, T.; Stephan, D. W. Dalton Trans. 2009, 1559.
̈
̈
Corresponding Author
j.c.slootweg@vu.nl; k.lammertsma@vu.nl
(d) Jiang, C.; Blacque, O.; Berke, H. Organometallics 2011, 30, 2117.
(e) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.
(12) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme, S.;
̈
̈
Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 6643.
ACKNOWLEDGMENTS
■
(13) (a) Momming, C. M.; Kehr, G.; Wibbeling, B.; Frohlich, R.;
̈
̈
This work was supported by the Council for Chemical Sciences
of The Netherlands Organization for Scientific Research
(NWO/CW) and benefitted from interactions within the
European PhoSciNet (COST Action CM0802). We thank J. W.
H. Peeters for measuring the high-resolution mass spectra.
Erker, G. Dalton Trans. 2010, 39, 7556 and refs 9, 13, and 15−18
cited therein. (b) Spies, P.; Kehr, G.; Bergander, K.; Wibbeling, B.;
Frohlich, R.; Erker, G. Dalton Trans. 2009, 1534.
̈
(14) Spies, P.; Frohlich, R.; Kehr, G.; Erker, G.; Grimme, S. Chem.
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Eur. J. 2008, 14, 333.
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