H2 activation and hydride transfer to olefins by Al(C 6F5)3-based frustrated lewis pairs

Article abstract of DOI:10.1002/anie.201203362

Frustrated delivery: Frustrated Lewis pairs derived from tBu₃P and Al(C₆F₅)₃ activate H₂ to give [tBu₃PH][(μ-H)(Al(C₆F₅)₃) ₂], which reacts with

Full text of DOI:10.1002/anie.201203362

Angewandte
Chemie
DOI: 10.1002/anie.201203362
Frustrated Lewis Pairs
H2 Activation and Hydride Transfer to Olefins by Al(C6F5)3-Based
Frustrated Lewis Pairs**
Gabriel Mꢀnard and Douglas W. Stephan*
The homogeneous reduction of olefins has been deemed to be
the purview of transition metal catalysis since the seminal
work of Wilkinson on Rh[1] and Ru[2] species, some 50 years
ago. Since then, numerous advances have resulted in the
evolution of such metal-based technologies to include Nobel
prize-winning work on asymmetric hydrogenations.[3] More
recent developments have produced Ru-based olefin-specific
reduction catalysts,[4] as well as remarkably active hydro-
genation catalysts based on Fe[5] and Co.[6] An alternative
strategy that is garnering attention is based on non-transition-
metal systems. As far back as the 1970s, it was reported that
strong Brønsted acids, such as HBr·AlBr3, could mediate the
alanes. A plausible mechanism is considered and is thought to
involve aluminum-activation of the olefin. This view is
supported by the characterization of the first simple inter-
molecular aluminum-olefin adduct. The implications for the
future development of an aluminum-based FLP hydrogena-
tion catalyst for unactivated olefins is considered.
Combining tBu3P and Al(C6F5)3·toluene in a 1:2 ratio in
fluorobenzene under 4 atm of H2 at 258C resulted in product
1a, which was isolated in 88% yield. The 1H NMR spectrum
of 1a showed a broad signal at 4.30 ppm and doublets at 4.09
and 0.94 ppm. The corresponding 31P{1H} NMR resonance
was seen at 60.0 ppm, but the 27Al NMR spectrum showed no
discernible resonance. The 19F{1H} NMR spectrum showed
signals at À120.5, À153.1 and À161.8 ppm. These data,
together with elemental analysis, were consistent with the
formulation of 1a as [tBu3PH][(m-H){Al(C6F5)3}2] (Scheme 1).
reduction of some aromatics to alkanes; however, changes to
[7]
À
the C C framework of the arenes were often observed.
More recently, main group systems, such as cyclic (alkyl)-
(amino)carbenes[8] or heavier group 14 alkene analogues[9]
have been shown by the research groups of Bertrand and
Power to effect H2 activation. In very recent work, the
research groups of Harder[10] and Okuda[11] have developed
Ca-based species capable of reducing the activated olefin, 1,1-
diphenylethene. Our own efforts have involved exploring
frustrated Lewis pairs (FLPs), the combination of main group
Lewis acids and bases that are sterically inhibited from
forming classical adducts. Such systems have been shown to
activate small molecules[12] and, in particular, H2.[13] The latter
finding has led to the development of boron-based hydro-
genation catalysts for the reduction of polar unsaturated
bonds, such as imines,[14] nitriles,[14a,b] aziridines,[14a,b] en-
amines,[14c] and silylenolethers.[15] Most recently, we have
demonstrated the ability of FLPs to effect the stoichiometric
reductions of anilines to cyclohexylammonium derivatives.[16]
Nonetheless, the delivery of H2 to simple, unactivated olefins
by any main group system has not been reported. In targeting
FLP systems that could effect such reductions, we[17] and
others[18] have begun to explore the use of aluminum-based
FLPs. Herein, we present an aluminum-based FLP system
capable of activating H2 that reacts stoichiometrically with the
unactivated olefins ethylene and cyclohexene, to give alkyl-
Scheme 1. Synthesis of 1–3.
In an analogous fashion, the species [(C6H2Me3)3PH][(m-H)-
{Al(C6F5)3}2] (1b) was prepared. This was subsequently
confirmed by X-ray crystallography (Figure 1).[19] While the
metric parameters of the cation are unexceptional, a single
H atom bridges the two pseudo-tetrahedral Al centers with
Al–H distances of 1.818 ꢀ and an Al-H-Al angle of 138.08.
The formation of this bridging hydride anion stands in
contrast to the analogous borane chemistry,[13a] although the
anionic fragment of 1a is reminiscent of the anion
[*] G. Mꢀnard, Prof. Dr. D. W. Stephan
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, Ontario, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
[**] D.W.S. gratefully acknowledges the financial support of the NSERC
of Canada and the award of a Canada Research Chair. G.M. is
grateful for the support of NSERC and Walter C. Sumner Fellow-
ships.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203362.
Figure 1. POV-Ray depiction of the anion of 1a.
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[(m-Me){Al(C6F5)3}2]À, previously reported by Chen and
Abboud[20] and the hydride-bridged species [Me2Si(Indenyl)2-
Zr(m-H)(AlR2H)2],[21] [{(C3HR2N2)AliBu2}2(m-H)],[22] and
[{(C3HMe2N2)AltBu2}2(m-H)].[23] This is consistent with our
previous results, which demonstrated that using a 1:2 ratio of
phosphine/alane was optimal for small-molecule activa-
tion.[17a,b,d] Similar to the salt [tBu3PH][HB(C6F5)3], salt 1a
shows no evidence of H2 loss when heated under vacuum.
Nonetheless, all efforts to prepare the related 1:1 species
[tBu3PH][HAl(C6F5)3] resulted only in reduced yields of the
isolated 1:2 species, 1a. This observation is consistent with
both the presumed nucleophilicity of the anion, [HAl-
(C6F5)3]À, and the Lewis acidity of Al(C6F5)3.
Exposure of 1a to an atmosphere of ethylene at 608C for
2 h results in the consumption of 1a and the formation of two
products, 2 and 3a.[19] Compound 2 was easily identified as the
previously reported salt [tBu3PH][Al(C6F5)4], based on its
spectroscopic signature.[17d] Compound 3a gave a triplet and
a broad quartet at 1.15 and 0.57 ppm, respectively, in the
1H NMR spectrum, inferring the presence of an ethyl frag-
ment. The 19F NMR signals were found to be at À120.5,
À150.9, and À160.2 ppm, similar to those previously reported
for alkylalanes of the general formula RAl(C6F5)2.[24] In this
report, compound 3a was independently generated by the
redistribution reaction of AlEt3 and B(C6F5)3 in a 3:2 ratio.[24]
The corresponding reaction of 1a with cyclohexene generates
2 and the cyclohexylalane derivative 3b. By analogy to the
previously reported alane [MeAl(C6F5)2]2,[24] the compounds
3 are thought to be dimers of the form [RAl(C6F5)2]2
(Scheme 1), although this was not unambiguously confirmed.
Several possibilities arise in considering the mechanism of
the reaction of 1 with olefins. One pathway (Scheme 2a)
could involve the redistribution of 1 to generate 2 and
HAl(C6F5)2. Subsequent reaction of the latter alane with an
olefin would afford 3. However, variable temperature NMR
studies on a salt of the anion of 1 (compound 7, see below) at
25–1208C in C6D5Br did not show any appreciable changes in
the 19F NMR spectra, suggesting that redistribution to gen-
erate HAl(C6F5)2 is unlikely.
(C6F5)3 would then be significantly activated and susceptible
to attack by the anion [HAl(C6F5)3]À, leading to the formation
of the alkylaluminate anion, [RAl(C6F5)3]À, and free
Al(C6F5)3. A subsequent redistribution reaction affords 2
and 3. This possibility is supported by the reaction of 1a with
Lewis bases, such as Et2O or PMe3, which results in the rapid
loss of H2 and the formation of the Lewis base adducts
L·Al(C6F5)3 (L = Et2O, PMe3). In this case, the donor acts to
sequester Al(C6F5)3 from 1a, generating the salt [tBu3PH]-
[HAl(C6F5)3], which is unstable with respect to H2 loss in the
absence of other electrophiles.
Support for the notion of an olefin–Al(C6F5)3 interaction
was obtained from the following reaction: cooling a solution
of Al(C6F5)3·toluene in neat cyclohexene produced crystals of
complex 4, which were isolated in 85% yield. While elemental
analysis confirmed the formulation of the crystals to be
[Al(C6F5)3·(C6H10)], the strength of the aluminum–olefin
1
interaction appears to be rather weak, as H NMR analysis
of a solution of 4 in bromobenzene showed the signature of
free cyclohexene. Nonetheless, crystallographic analysis of 4
showed that the aluminum center is pseudo tetrahedral with
an h2 coordination of the olefinic unit to the aluminum center
(Figure 2).[19] The Al–Colefin distances were found to be
=
2.471(2) and 2.540(2) ꢀ, while the C C bond is 1.340(3) ꢀ.
Figure 2. POV-Ray depiction of 4. H atoms are omitted for clarity.
We[17d] and others[25] have previously described NMR spec-
troscopic evidence supporting the interaction of pendant
olefins to Lewis acidic aluminum[17d,25] or boron[26] centers,
and Schnçkel and co-workers have reported the tetrametallic
dimer derived from 1,4-dialumina-2,5-cyclohexadiene, which
has Al–Colefin distances of 2.355 ꢀ.[27] Compound 4 is, to the
best of our knowledge, the first crystallographically charac-
terized species derived from the interaction of a simple free
olefin with Al.
A possible alternative mechanism (Scheme 2b) involves
olefin interception by Al(C6F5)3 from 1a. The olefin–Al-
To probe the transient role of the mono-Al hydride
species [HAl(C6F5)3]À, the known salt K[HAl(C6F5)3][28] was
prepared; however, its solubility in bromobenzene proved
extremely poor and consequently the attempted cation
exchange was very sluggish. An alternative synthetic
approach involved the initial preparation of [Et4N][ClAl-
(C6F5)3] (5) in 92% yield by the combination of [Et4N]Cl and
Al(C6F5)3·toluene. Subsequent treatment of 5 with LiAlH4 at
258C followed by filtration resulted in the isolation of the
pyrophoric salt 6. The 1H NMR spectrum of 6 showed
a hydride signal at 4.79 ppm, somewhat shifted downfield
compared to 1 (4.30 ppm). The X-ray structure of 6 confirmed
Scheme 2. Possible mechanisms of the reaction of 1 with olefins.
2
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the formulation of the salt as [Et4N][HAl(C6F5)3].[19] The
metric parameters were generally unexceptional, although
Received: May 2, 2012
Published online: && &&, &&&&
À
the Al H bond lengths in the two independent molecules of
the asymmetric unit averaged 1.51(1) ꢀ, which is comparable
to that seen in K[HAl(C6F5)3] (1.59(5) ꢀ)[28] and significantly
shorter than those seen in 1. Compound 6 alone does not react
with an added olefin, even upon heating to 608C for 12 h. This
observation is consistent with the known inability of LiAlH4
to react with isolated olefins.[29] However, the addition of one
equivalent of Al(C6F5)3 to 6 in C6D5Br produced the poorly
soluble salt [Et4N][H{Al(C6F5)3}2] (7), as evidenced by 19F and
1H NMR spectroscopy. Heating 6/Al(C6F5)3, or alternatively
the isolated salt 7, under an atmosphere of ethylene at 608C
for 12 h did result in complete conversion to 3a and the salt
[Et4N][Al(C6F5)4], as evidenced by NMR spectroscopy. These
observations further support the notion that thermal/sub-
strate-induced dissociation of Al(C6F5)3 from the anion of 1a
facilitates both the activation of the olefin and the hydride
transfer. This is similar to our previously reported hydride
delivery from [HB(C6F5)3]À to the borane-activated olefin
Keywords: alkylaluminum · frustrated lewis pairs ·
.
hydride delivery · hydrogen · olefins
[1] J. A. Osborn, F. H. Jardine, J. F. Young, G. Wilkinson, J. Chem.
Soc. A 1966, 1711 – 1732.
[2] P. S. Hallman, D. Evans, J. A. Osborn, G. Wilkinson, Chem.
Commun. 1967, 305 – 306.
[3] a) R. Noyori, Angew. Chem. 2002, 114, 2108 – 2123; Angew.
Chem. Int. Ed. 2002, 41, 2008 – 2022; b) W. S. Knowles, Angew.
Chem. 2002, 114, 2096 – 2107; Angew. Chem. Int. Ed. 2002, 41,
1998 – 2007; c) N. B. Johnson, I. C. Lennon, P. H. Moran, J. A.
Ramsden, Acc. Chem. Res. 2007, 40, 1291 – 1299; d) S. J. Rose-
blade, A. Pfaltz, Acc. Chem. Res. 2007, 40, 1402 – 1411; e) C. S.
Shultz, S. W. Krska, Acc. Chem. Res. 2007, 40, 1320 – 1326.
[4] C. L. Lund, M. J. Sgro, R. Cariou, D. W. Stephan, Organo-
metallics 2012, 31, 802 – 805.
[5] S. C. Bart, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2004,
126, 13794 – 13807.
[6] S. Monfette, Z. R. Turner, S. P. Semproni, P. J. Chirik, J. Am.
Chem. Soc. 2012, 134, 4561 – 4564.
=
fragment of [(C6F5)2B(CH2)4CH CH2] and [(C6F5)2BOC-
[30]
=
(CF3)2(CH2)CH CH2].
In the final step of the pathway, a redistribution reaction is
proposed to give 2 and 3. To model this, the salt [Et4N][EtAl-
(C6F5)3] (8) was prepared in 79% yield by treating the
precursor 5 with EtMgBr. The solid state structure of 8 was
confirmed crystallographically.[19] The 1H NMR signals arising
from the Al–Et fragment in 8 are observed at 1.40 and
0.75 ppm, considerably shifted downfield relative to those in 3
(1.15, 0.57 ppm). Similarly, the 19F resonances attributable to
the para and meta-F atoms are shifted to À159 and À164 ppm,
respectively, from À151 and À160 ppm for the corresponding
fragments in 3. Reaction of 8 with one equivalent of
Al(C6F5)3·toluene in C6D5Br leads to rapid redistribution,
thus affording 3a and the salt [Et4N][Al(C6F5)4]. This
observation stands in contrast to the analogous reaction of
the anion [MeAl(C6F5)3]À, where the bridging methyl ana-
logue
[7] J. Wristers, J. Am. Chem. Soc. 1975, 97, 4312 – 4316.
[8] G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller, G.
Bertrand, Science 2007, 316, 439 – 441.
[9] a) Y. Peng, B. D. Ellis, X. Wang, P. P. Power, J. Am. Chem. Soc.
2008, 130, 12268 – 12269; b) G. H. Spikes, J. C. Fettinger, P. P.
Power, J. Am. Chem. Soc. 2005, 127, 12232 – 12233.
[10] J. Spielmann, F. Buch, S. Harder, Angew. Chem. 2008, 120, 9576 –
9580; Angew. Chem. Int. Ed. 2008, 47, 9434 – 9438.
[11] P. Jochmann, J. P. Davin, T. P. Spaniol, L. Maron, J. Okuda,
Angew. Chem. 2012, 124, 4528 – 4531; Angew. Chem. Int. Ed.
2012, 51, 4452 – 4455.
[12] D. W. Stephan, G. Erker, Angew. Chem. 2010, 122, 50 – 81;
Angew. Chem. Int. Ed. 2010, 49, 46 – 76.
[13] a) G. C. Welch, D. W. Stephan, J. Am. Chem. Soc. 2007, 129,
1880 – 1881; b) G. C. Welch, R. R. S. Juan, J. D. Masuda, D. W.
Stephan, Science 2006, 314, 1124 – 1126; c) M. Ullrich, A. J.
Lough, D. W. Stephan, J. Am. Chem. Soc. 2009, 131, 52 – 53.
[14] a) P. A. Chase, T. Jurca, D. W. Stephan, Chem. Commun. 2008,
1701 – 1703; b) P. A. Chase, G. C. Welch, T. Jurca, D. W. Stephan,
Angew. Chem. 2007, 119, 8196 – 8199; Angew. Chem. Int. Ed.
2007, 46, 8050 – 8053; c) P. Spies, S. Schwendemann, S. Lange, G.
Kehr, R. Frohlich, G. Erker, Angew. Chem. 2008, 120, 7654 –
7657; Angew. Chem. Int. Ed. 2008, 47, 7543 – 7546.
[(m-Me){Al(C6F5)3}2]À[20] is isolated. We recently reported
a similar redistribution following ethylene addition to the
allyl-bridged species [tBu3PH][{(C6F5)3Al}2{(CH2)2CMe}]
=
affording [tBu3PH][Al(C6F5)4] and [CH2 C(Me)(CH2)3Al-
(C6F5)2].[17d] It appears that such redistribution reactions are
driven by steric congestion, as smaller substituents, such as
hydride, methyl, or fluoride, do not redistribute.[20,31]
[15] H. D. Wang, R. Frohlich, G. Kehr, G. Erker, Chem. Commun.
2008, 5966 – 5968.
[16] T. Mahdi, Z. M. Heiden, S. Grimme, D. W. Stephan, J. Am.
Chem. Soc. 2012, 134, 4088 – 4091.
In conclusion, we have presented the facile activation of
H2 using Al-based FLPs and demonstrated subsequent
hydride transfer to unactivated olefins. These reactions are
thought to involve Al–olefin activation, nucleophilic attack by
the anion [HAl(C6F5)3]À, and subsequent redistribution to
aluminate and alane. This proposed route is supported by the
isolation of the first Al–olefin complex, as well as by parallel
reactivity studies. It is interesting to note that, if redistribution
could be inhibited, the protic cation derived from FLP
activation of H2 could react with the transient alkylaluminate
to provide an entry to a catalytic cycle for hydrogenation.
Modification of the aluminum-based FLPs to produce
catalysts for the reduction of unactivated olefins is under
intense study in our laboratory.
[17] a) G. Mꢁnard, D. W. Stephan, J. Am. Chem. Soc. 2010, 132,
1796 – 1797; b) G. Mꢁnard, D. W. Stephan, Angew. Chem. 2011,
123, 8546 – 8549; Angew. Chem. Int. Ed. 2011, 50, 8396 – 8399;
c) M. A. Dureen, D. W. Stephan, J. Am. Chem. Soc. 2009, 131,
8396 – 8397; d) G. Mꢁnard, D. W. Stephan, Angew. Chem. 2012,
124, 4485 – 4488; Angew. Chem. Int. Ed. 2012, 51, 4409 – 4412.
[18] a) Y. Zhang, G. M. Miyake, E. Y. X. Chen, Angew. Chem. 2010,
122, 10356 – 10360; Angew. Chem. Int. Ed. 2010, 49, 10158 –
10162; b) J. Boudreau, M.-A. Courtemanche, F.-G. Fontaine,
Chem. Commun. 2011, 47, 11131 – 11133; c) C. Appelt, H.
Westenberg, F. Bertini, A. W. Ehlers, J. C. Slootweg, K. Lam-
mertsma, W. Uhl, Angew. Chem. 2011, 123, 4011 – 4014; Angew.
Chem. Int. Ed. 2011, 50, 3925 – 3928.
[19] CCDC 889612 – 889615 (1a, 4, 6, and 8) contain the supplemen-
tary crystallographic data for this paper. These data can be
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.
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Communications
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[20] E. Y. X. Chen, K. A. Abboud, Organometallics 2000, 19, 5541 –
5543.
[21] S. M. Baldwin, J. E. Bercaw, L. M. Henling, M. W. Day, H. H.
Brintzinger, J. Am. Chem. Soc. 2011, 133, 1805 – 1813.
[22] Z. Yu, J. M. Wittbrodt, A. Xia, M. J. Heeg, H. B. Schlegel, C. H.
Winter, Organometallics 2001, 20, 4301 – 4303.
[23] W. Uhl, A. Vogelpohl, Z. Naturforsch. B 2010, 65, 687 – 694.
[24] J. Klosin, G. R. Roof, E. Y. X. Chen, K. A. Abboud, Organo-
metallics 2000, 19, 4684 – 4686.
[25] T. W. Dolzine, J. P. Oliver, J. Am. Chem. Soc. 1974, 96, 1737 –
1740.
[26] X. Zhao, D. W. Stephan, J. Am. Chem. Soc. 2011, 133, 12448 –
12450.
[27] H. Schnçckel, M. Leimkꢂhler, R. Lotz, R. Mattes, Angew. Chem.
1986, 98, 929 – 930; Angew. Chem. Int. Ed. Engl. 1986, 25, 921 –
922.
[28] Y. Hu, L. O. Gustafson, H. Zhu, E. Y. X. Chen, J. Polym. Sci.
Part A 2011, 49, 2008 – 2017.
[29] a) E. C. Ashby, J. J. Lin, J. Org. Chem. 1978, 43, 2567 – 2572; b) F.
Sato, S. Sato, H. Kodama, M. Sato, J. Organomet. Chem. 1977,
142, 71 – 79; c) P. W. Chum, S. E. Wilson, Tetrahedron Lett. 1976,
17, 15 – 16.
[30] X. Zhao, D. W. Stephan, Chem. Sci. 2012, 3, 2123 – 2132
[31] M.-C. Chen, J. A. S. Roberts, T. J. Marks, Organometallics 2004,
23, 932 – 935.
4
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Frustrated Lewis Pairs
G. Mꢁnard,
D. W. Stephan*
&&&& — &&&&
H2 Activation and Hydride Transfer to
Olefins by Al(C6F5)3-Based Frustrated
Lewis Pairs
Frustrated delivery: Frustrated Lewis
pairs derived from tBu3P and Al(C6F5)3
activate H2 to give [tBu3PH][(m-H)(Al-
(C6F5)3)2], which reacts with unactivated
olefins to give RAl(C6F5)2 (R=Et or Cy)
and [tBu3PH][Al(C6F5)4]. The proposed
mechanism involves olefin activation by
aluminum, which is supported by the
isolation of the cyclohexene complex [Al-
(C6F5)3·(C6H10)].
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