C-H activation of isobutylene using frustrated lewis pairs: Aluminum and boron σ-allyl complexes

Article abstract of DOI:10.1002/anie.201200328

Activation frustration: The frustrated Lewis pairs derived from the Lewis base, tBu₃P, and the Lewis acids, Al(C₆F₅) ₃ and B(C₆F₅)₃, react with isobutylene to give complexes 1 an

Full text of DOI:10.1002/anie.201200328

Angewandte
Chemie
DOI: 10.1002/anie.201200328
À
C H Activation
À
C H Activation of Isobutylene Using Frustrated Lewis Pairs:
Aluminum and Boron s-Allyl Complexes**
Gabriel Mꢀnard and Douglas W. Stephan*
Although the activation of small molecules has been the
purview of transition-metal chemistry for the past half-
century, parallels between the reactivity of transition-metal
and main-group compounds have become increasingly prev-
alent in recent years.[1] Among the approaches that involve
main-group compounds, frustrated Lewis pairs (FLPs), that is,
the combination of Lewis acids and bases that are sterically
inhibited from forming classical adducts, have drawn partic-
ular attention. While FLPs initially garnered much attention
for their ability to heterolytically activate H2,[2] they have also
been shown to activate small molecules such as CO2,[3] N2O,[4]
NO,[5] alkenes, alkynes, among others.[1c] The majority of the
published work on FLPs exploits boron-based Lewis acids,
particularly B(C6F5)3.[1c] More recently, we[3b,d,6] and
others,[3i,7] have begun to explore the use of aluminum in
such chemistry. Whereas boron/phosphine (B/P) and alumi-
num/phosphine (Al/P) FLPs are capable of CO2 capture, only
the adducts derived from the Al/P FLPs have been shown to
undergo further reactions, resulting in CO2 reduction.[3b,d]
Allyl groups are important fragments in organic syn-
thesis;[8] however, the installation of such groups using
unactivated olefin compounds usually requires the use of
harsh bases, such as KOtBu/nBuLi.[9] In addition, the use of
activated olefins,[10] such as allyl halides,[11] ethylene ketals,[12]
allyl alkoxides,[13] and trimethylsilyl olefin derivatives,[14] have
been used to install allyl substituents.[8c,9,15] While such
reagents can be employed for the allylation of carbonyl
functionalities,[11,16] the allylation of olefins has only been
achieved in a limited number of cases.[10a,17] To this end, we
have explored the reactivity of boron- and aluminum-based
tube at 258C, led to an exothermic reaction with the
generation of a 1:1 mixture of a new product and tBu3P, as
indictated by the 31P NMR resonances at 60 and 62 ppm,
respectively. Repetition of the reaction with a 1:2 ratio of
tBu3P/Al(C6F5)3 led to the formation of a single product. This
species 1 was isolated from a scaled-up reaction by trituration
of the reaction mixture with hexanes (Scheme 1). The 31P{1H}
and 31P NMR spectra revealed a resonance at 60 ppm with
a strong P–H coupling constant of 426 Hz, which is consistent
with the value of a one-bond coupling and thus the formation
of the tBu3PH+ ion. The 19F{1H} NMR spectrum exhibited
only three resonances, thus suggesting that the Al(C6F5)3
fragments exist in equivalent environments. The 1H NMR
spectrum exhibited a broad singlet at 3.91 ppm and a sharp
singlet at 2.10 ppm, with a relative integration of four and
three, respectively. When the 1H NMR spectrum was acquired
at low temperature (À308C), the resonance at 3.91 ppm split
into two broad singlets.[18] Collectively, these data imply the
formation of an anionic compound containing a bridging allyl
moiety between two equivalent Al(C6F5)3 fragments together
with the corresponding phosphonium cation and thus the
formulation of 1 as [tBu3PH][{(C6F5)3Al}2{CH2C(CH3)CH2}].
The NMR data support an allyl geometry that is likely
m2-h1:h1, although the m2-h3:h3 geometry could not be dis-
missed (Scheme 1). Single crystal X-ray crystallography
confirmed the former binding mode (Figure 1).[18] Whereas
À
FLPs in the C H activation of isobutylene as a route to s-allyl
anion salts. The nature of these products is established and
contrasted herein. Whereas allyl borates form in a reversible
reaction, the corresponding allyl aluminate undergoes sub-
sequent ethylene insertion, thus providing a rare example of
the allylation of an unactivated olefin.
Scheme 1. Synthesis of 1 and 2.
the metric parameters of the cation are unexceptional, those
of the anion are rather interesting. The planar allyl fragment
is linked to the two aluminum centers through sigma
interactions with the methylene units, thus giving rise to
The addition of 1 atm of isobutylene to a 1:1 solution of
tBu3P and Al(C6F5)3 in C6D5Br, contained in a J-Young NMR
À
Al C distances of 2.080(4) ꢀ and 2.094(4) ꢀ. The two
Al(C6F5)3 moieties are oriented in a transoid disposition,
[*] 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
À
which minimizes the steric congestion. The C C distances
between these terminal carbon atoms and the central carbon
atom of the allyl fragment are 1.415(6) ꢀ and 1.411(6) ꢀ, with
a corresponding CAl-C-CAl’ angle of 121.8(4)8.
Whereas a number of recent reports have described early
main-group allyl metal compounds,[9,19] as well as allyl
aluminum compounds,[11c,20] the compound described above
is, to the best of our knowledge, the first example of an
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
[**] D.W.S. gratefully acknowledges the financial support of NSERC of
Canada, the award of a Canada Research Chair. G.M. is grateful for
the support of an NSERC and a Walter C. Sumner Fellowship. We
thank Dr. Timothy Burrow and Dr. Darcy Burns for their advice and
support with the NMR spectrometers.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200328.
À
isolable allyl species that is derived from facile C H
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Figure 1. POV-ray depiction of the anion of 1. H atoms are omitted for
clarity.
Figure 2. POV-ray depiction of 2. H atoms are omitted for clarity.
À
activation. It should be noted that C H activation of alkynes
Interestingly, when a solution of 2 (E = B) in either CD2Cl2
or C6D5Br in a J-Young NMR tube was monitored by NMR
spectroscopy, the formation of approximately one equivalent
of free tBu3P and free isobutylene, as well as significant
by FLPs has been previously reported.[3i,6,21] Furthermore,
compound 1 is the first example of a compound having
a bridging allyl moiety between two aluminum centers. It is
noteworthy that the formation of 1 appears to be reversible:
slow regeneration of the initial FLP mixture with concurrent
release of isobutylene was observed upon prolonged heating
of 1 at 608C under vacuum.
1
broadening of the 19F, 11B, and allylic-based H NMR reso-
nances was observed, thus suggesting that the loss of
isobutylene is facile. In the presence of excess borane, an
allyl bis(borate) analogue of 1 was not observed spectroscopi-
cally. Indeed, all efforts to generate and isolate an allyl
bis(borate) species through variation of the stoichiometry
were unsuccessful. The apparent preference of boron to form
an allyl monoborate anion, which incorporates a terminal
olefinic unit, is consistent with the weak interactions of
B(C6F5)3 with olefins[22] and the weaker Lewis acidity of
B(C6F5)3 compared with Al(C6F5)3.[23]
The reaction of 1 and 2 (E = B) with ethylene was probed.
In the case of 2 (E = B), its treatment with ethylene resulted
only in the release of isobutylene with concomitant formation
of the known ethylene addition product tBu3PCH2CH2B-
(C6F5)3.[24] The addition of more borane did not lead to the
formation of different products. In marked contrast, the
treatment of the bridging allyl species 1 with ethylene resulted
in a slow reaction at ambient temperature. When the temper-
ature was raised to 608C, the reaction was complete in
6 hours. NMR analysis of the reaction mixture revealed the
formation of three products, 3, 4, and 5 in addition to the
generation of a small amount of Al(C6F5)3 (see Scheme 2).
Based on the integration of the 19F{1H} NMR spectrum of the
reaction mixture, compound 5 and Al(C6F5)3 accounted for
approximately 20% of the available aluminum, with the
remainder being associated with the major products 3 and 4.
This mixture was separated by careful workup.[18] The NMR
spectra of 3 indicated the presence of the [tBu3PH]+ ion; in
addition, the 19F{1H} NMR spectrum exhibited three multip-
lets at À122.1, À157.4, and À163.5 ppm and the 27Al NMR
spectrum exhibited a broad singlet at 116 ppm, thus indicating
the presence of a symmetric aluminate species. These data,
together with elemental analysis, allowed us to identify
compound 3 as being the salt [tBu3PH][Al(C6F5)4].
The analogous reaction employing B(C6F5)3 was per-
formed in fluorobenzene solution. Thus, a 1:2 mixture of
tBu3P and B(C6F5)3 was exposed to an atmosphere of
isobutylene for 1 hour. The addition of pentane to the
solution resulted in the isolation of product 2 (E = B) in
modest yield (< 50%) by precipitation and then filtration.
31P{1H} NMR analysis of a solution of 2 (E = B) in CD2Cl2
revealed the formation of the expected tBu3PH+ cation.
Furthermore, the 11B{1H} NMR spectrum exhibited a sharp
singlet at À13 ppm and the 19F{1H} spectrum exhibited
resonances at À131.6, À164.3, and À167.2 ppm, all of which
are associated with C6F5 rings on a four-coordinate boron
center. However, the 1H NMR spectrum exhibited four
resonances in addition to the resonances at 5.02 and
1.59 ppm for the phosphonium cation: two mutually coupled
doublets each integrating for one proton were found in the
olefinic region at 4.01 and 3.83 ppm, and two singlets were
found in the aliphatic region at 2.20 and 1.45 ppm, which
integrated to two and three protons, respectively. These data
were consistent with an asymmetric allyl fragment and
suggested the presence of an allyl borate anion. Indeed, X-
ray crystallography confirmed the structure of 2 (E = B) as
being [tBu3PH][B(C6F5)3{CH2C(CH3) CH2}] (Figure 2).[18]
The geometry of the boron center is tetrahedral with the
B C bond distance for the allyl fragment being 1.665(2) ꢀ.
The B-C-C angle was found to be 116.01(8)8. The central
carbon atom of the allyl fragment is bonded to a boron-bound
methylene, a terminal methylene, and a terminal methyl
=
À
À
group with C C bond distances of 1.507(2) ꢀ, 1.337(2) ꢀ, and
1.501(2) ꢀ, respectively.
The sensitive species 2 (E = B) was synthesized in higher
yield (83%) when the stoichiometry of the reagents, tBu3P
and B(C6F5)3, was reduced to 1:1. The facile synthesis of 2
(E = B) at ambient temperature from commercially available
reagents stands in marked contrast to the multistep syntheses
that are often used to access known allyl borates.[11a,b]
Detailed 1D and 2D NMR analysis of 4 indicated that it
was derived from the incorporation of one equivalent of
ethylene into the aluminum allyl moiety of 1. This species also
exhibited 19F NMR resonances at À122.0, À152.1, and
À160.8 ppm but no signal was observed in the 27Al NMR
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spectrum because of line broadening. These data allow us to
=
propose CH2 C(CH3)CH2CH2CH2Al(C6F5)2 as being the
structure of 4, which is presumably derived from the insertion
of ethylene into the aluminum–allyl bond, reminiscent of the
insertions into aluminum–alkyl bonds that were first reported
by Ziegler.[25] Inspection of the 13C{1H} NMR spectrum
indicated that the fully substituted carbon atom of the olefinic
residue resonated at 188.5 ppm, which is significantly down-
field of the corresponding resonance of the unbound alkene
analogue,
2-methyl-1-pentene
(147 ppm).
Similarly
deshielded carbon nuclei resonances have been observed for
vinyl aluminum species.[26] The terminal olefinic carbon atom
was observed as a quintet at 105.5 ppm; in 1H- and 19F-
decoupled 13C NMR spectra this signal is a singlet, thus
implying that the carbon nucleus was coupling to four
equivalent fluorine atoms.[18] Collectively, these data suggest
that the terminal carbon atom is weakly bound to the
aluminum center, thus resulting in a significantly polarized
olefin double bond. However, attempts to obtain crystals
suitable for X-ray diffraction failed, therefore the structure of
4 was probed through DFT computations performed at the
M06-2X/6-311 ++ G(d,p) level[27] (Figure 3). The computed
Figure 4. POV-ray depiction of 5. H atoms are omitted for clarity.
The formation of 3–5 is consistent with competing
reactions involving components from the equilibrium govern-
ing the formation of 1 (Scheme 2). Thus, thermal loss of
isobutylene from 1 generates free phosphine and alane, which
Scheme 2. Reaction pathways of 1 with ethylene affording 3–5.
can react with ethylene to give 5 with residual Al(C6F5)3.
Alternatively, the direct reaction of ethylene with 1 results in
À
insertion of the olefin moiety into the Al C bond of the allyl
À
Figure 3. Computed geometry of 4 showing Al C distances and
Muliken charges on olefinic carbon atoms.
aluminum fragment and subsequent aryl group redistribution
to afford the salt 3 and the alane 4. The separation of the salts
3 and 5 from the alanes 4 and Al(C6F5)3 was readily achieved
based on their relative solubilities, and 3 was subsequently
separated from 5 by careful precipitation. The separation of 4
from Al(C6F5)3 required an alternative strategy. Based on the
integration of the 19F NMR spectrum, an equimolar amount
of tBu3P, relative to the Al(C6F5)3, was added; subsequent
exposure of the mixture to ethylene, allowed the reaction of
residual Al(C6F5)3 and tBu3P to occur to give 5, thus allowing
the extraction of 4 into hexanes.[18]
geometry shows a slight pyramidalization of aluminum (the
sum of angles being 352.98) with the olefinic fragment being
oriented near the aluminum center and with Al C distances
À
of 2.39 ꢀ and 2.67 ꢀ for the terminal and substituted carbon
atoms, respectively. Mulliken charges at the terminal and
substituted carbon atoms were found to be À1.149 and 0.689,
respectively, consistent with a highly polarized olefin. This
result is also consistent with 13C NMR data[28] and the analysis
of the molecular orbitals, which reveals an interaction of the
terminal carbon atom with the aluminum center. Further-
more, interactions of the Lewis acidic center with olefin
fragments have been observed in closely related boron and
aluminum species.[22,29]
The minor product 5 was independently synthesized by
the reaction of tBu3P, Al(C6F5)3, and ethylene at 258C, and
exhibited a 31P resonance at 45.1 ppm and a broad singlet at
132 ppm in the 27Al NMR spectrum. These data, taken
together with the 19F and 1H NMR data, were consistent with
tBu3PCH2CH2Al(C6F5)3 as being the structure of 5, for which
a boron analogue is known[24]. The structure of 5 was
confirmed by X-ray crystallography (Figure 4).[18]
It is interesting to note that P/B FLP combinations have
previously shown two divergent pathways in their reaction
with alkynes, thus affording either a zwitterionic material
derived from P/B FLP addition to the alkyne or deprotona-
tion of the alkyne affording an alkynyl borate salt.[3e,6] In
contrast, only addition product zwitterions have been
reported for the reactions of FLPs with ethylene, propene,
hexene,[24] and norborene.[30] Presumably in the present cases,
the use of the basic phosphine tBu3P and the disubstituted
olefin, isobutylene, disfavors such addition reactions because
of steric hindrance, thus affording boron- and aluminum-
based s-allyl anion salts instead. These species are the first
À
examples of products derived from FLP-based C H activa-
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À
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Received: January 12, 2012
Published online: March 23, 2012
À
Keywords: allylation · aluminum · boron · C H activation ·
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