Synthesis and reactivity of the methylene arenium form of a benzyl cation, stabilized by complexation

Article abstract of DOI:10.1021/ja067298z

Benzyl cations are unstable intermediates involved in various chemical and biological processes. Two extreme resonance forms of these cations include positive charge localization at the methylene carbon or delocalization in the ring, the latter, nonaromatic form, termed methylene arenium . The preparation of the discrete methylene arenium compound, stabilized by coordination to a metal (palladium) center, is described. It was fully characterized, including by X-ray diffraction. Reactivity patterns, resulting from charge distribution in the ring, were observed. Upon controlled release of the methylene arenium compound into solution, it demonstrates aromatic benzyl cation reactivity. Copyright

Full text of DOI:10.1021/ja067298z

Published on Web 12/08/2006
Synthesis and Reactivity of the Methylene Arenium Form of a Benzyl Cation,
Stabilized by Complexation
Elena Poverenov,† Gregory Leitus,‡ and David Milstein*,†
Department of Organic Chemistry and Unit of Chemical Research Support, The Weizmann Institute of Science,
RehoVot 76100, Israel
Received October 11, 2006; E-mail: david.milstein@weizmann.ac.il
Benzyl cations are highly reactive compounds involved as key
intermediates in various chemical and biochemical processes.1 They
can be stabilized at low temperatures under super-acidic conditions.2
The structural and electronic properties of these compounds is a
subject of fundamental importance. Experimental observations,2,3
supported by high level ab initio calculations,4 indicate that the
Figure 1. Resonance forms of a benzyl cation.
nonaromatic, “methylene arenium” resonance form (B, Figure 1)
can contribute significantly to the general energy of the benzyl
Scheme 1
cation.
However, discrete methylene arenium compounds, i.e., com-
pounds with an sp2 ipso carbon atom and ring-localized charge,
are unknown, except when cramped in a pincer-ligand framework.5,6
We report here the generation and stabilization of a simple
methylene arenium (MA) molecule, stabilized by coordination to
a metal center. The MA complex was fully characterized, including
by X-ray diffraction. Its reactivity is strongly influenced by the
arenium character of the ring. Being coordinated to a metal center
due to the carbonyl group disappears and is replaced by a signal of
C-O-CH3 at 166.08 ppm (dd, JP-C ) 7 Hz, 1 Hz). The other
ring signals are all downfield shifted with respect to anisole (129-
114 ppm) and to the related benzylic complex 4 (see later), and
via the exocyclic double bond, controlled release of the methylene
demonstrate a nonhomogeneous distribution of the positive charge.
arenium moiety into solution was possible, where it demonstrated
Notably, the ortho carbons appear at 139.88 ppm, while the meta
carbons are only slightly deshielded (129.14 ppm). Due to the
positive charge in the ring, the signal of the ipso carbon is strongly
downfield shifted relative to the parent QM complex (107.51 instead
of 78.12 ppm in 2). These data clearly indicate that the positive
charge is localized predominantly in the ring, distributed mainly at
the para and ortho positions, and almost none at the exocyclic
methylene (or metal center, see below, complex 4). For comparison,
the documented NMR data for benzyl cations (form A)2,3 show
that the methylene groups are strongly deshielded (appearing in
the range of 150-300 ppm), while the ring is only moderately
affected, indicating that the charge is mostly localized at the
methylene group.
benzyl cation reactivity.
Utilizing our method for the generation and stabilization of
quinone methides,7 the complex (tmeda)Pd(BHT-OSiMe3)Br 17
was reacted with the ligand dtpp (dtpp ) bis(di-tert-butylphosphi-
no)propane) in THF solution at -30 °C, yielding the new, stable
complex 2 of the quinone methide BHT-QM (Scheme 1).
Complex 2 was fully characterized by multinuclear NMR and
IR spectroscopies8 and by a single-crystal X-ray diffraction study.
It is similar to an analogous bis(diphenyphosphino)ethane complex,7
except for effects of the bulkiness and electron-donating properties
of the dtpp ligand (Figure 2).
Significantly, upon electrophilic attack of methyl triflate on
complex 2, an unprecedented methylene arenium compound,
Yellow X-ray quality crystals of 3 were obtained by slow
stabilized only by coordination of the exocyclic double bond, was
diffusion of pentane into its toluene solution at -20 °C. As expected
from a methylene arenium resonance structure, the X-ray structure
of 3 (Figure 2) indicates averaging of the carbon-carbon bond
obtained, complex 3 (Scheme 1), as proven by multinuclear NMR
spectroscopy and X-ray structural characterization.
Both the NMR and X-ray data strongly support sp2 hybridization
distances of the quinonoid ring in comparison with those in the
of the ipso carbon and positive charge concentration in the arenium
quinone methide complex 2. The asymmetric charge distribution
is reflected by incomplete averaging. According to the resonance
forms, most of the positive charge is expected to be localized at
the ortho- and para-carbon atoms. This is supported by recent
observations regarding the X-ray structure of the cumyl cation9 and
the methylene arenium moiety incorporated in PCP-type ligand
ring. The 31P NMR spectrum of 3 shows two doublets at 52.91
and 39.05 ppm (JP-P ) 20 Hz) for the nonequivalent phosphine
groups. The methoxy group gives rise to singlets at 4.22 and
62.89 ppm in the 1H and 13C{1H} NMR spectra, respectively. It is
noteworthy that the exocyclic methylene group remains almost the
1
same as in complex 2. In the H NMR spectrum it gives rise to a
structures,5 the only crystallographic examples of related com-
signal which is only slightly downfield shifted (by 0.07 ppm), while
pounds. Our structure is in agreement with this, the charge being
in the 13C{1H} NMR spectrum its signal is slightly shifted upfield
localized mostly in the para position: C22-C23 (1.387 Å) is
to 45.89 (d, JP-C ) 34 Hz) vs 47.67 (d, JP-C ) 33 Hz) in complex
shorter than the rest of the distances in the ring, while C23-C24
2. On the other hand, the positive charge has a strong effect on the
(1.423 Å) is the longest bond. The tert-butyl substituents in the
arenium ring, as evidenced by 13C NMR. As expected, the signal
ring preclude C21-C22 from being the longest bond, which is what
is expected in an unsubstituted ring. The C23-C24 bond was
initially much longer than other ring distances (1.488 Å in parent
† Department of Organic Chemistry.
‡ Unit of Chemical Research Support.
9
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J. AM. CHEM. SOC. 2006, 128, 16450-16451
10.1021/ja067298z CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
mild conditions, using a ligand substitution process. Thus, upon
addition of diphenylacetylene (dpa) to a CD2Cl2 solution of complex
3a containing 99 equiv of water at room temperature, the methylene
arenium moiety dissociated from the metal center and reacted with
water, giving 3,5-di-tert-butyl-4-hydroxybenzyl alcohol, as expected
from benzyl cation reactivity (Scheme 2).
Slow rearrangement of 3 to give the benzylic complex 4 takes
place at room temperature (Scheme 2). 4 was fully characterized
by multinuclear NMR spectroscopy. The signals due to the ring
carbon atoms in the 13C{1H} NMR spectrum clearly indicate
conversion to aromaticity. For example, the para and ipso carbons
in the methylene arenium ring, which significantly deviate from
aromaticity in 3, return to the normal range of a metal coordinated
benzyl ligand in 13C{1H} NMR : para C-O-CH3 (147.98 vs
166.08 ppm in 3) and ipso carbon (125.45 vs 107.51 ppm in 3).
To summarize, a methylene arenium cation, an extreme resonance
form of a benzyl cation, was generated, stabilized by coordination
of the exocyclic double bond, and fully characterized in solution
and in the solid state. It exhibits reactivity patterns resulting from
the positively charged ring, such as facile ether C-O bond
hydrolysis to the corresponding alcohol, and deprotonation to form
a p-quinone methide complex. Controlled release of the methylene
arenium moiety was achieved by associative substitution, forming
and trapping the benzyl cation. This approach can potentially be
utilized for the delivery and controlled generation of benzyl cations.
Figure 2. ORTEP views of complexes 3 (left) and 2 (right) at 50%
probability. Hydrogen atoms are omitted for clarity.
Scheme 2
complex 2), whereas the C24-C25 bond (symmetric to C23-24)
is the second longest distance in both 3 (1.415 Å) and 2 (1.461 Å).
Similarly to 2, the quinonoid ring is not symmetric in complex 3.
Notably, the C-C bond lengths of the ring are longer than average
bond lengths in aromatic compounds,9 as expected for a methylene
arenium structure.
Acknowledgment. This research was supported by the Minerva
Foundation, Munich, Germany, and by the Helen and Martin
Kimmel Center for Molecular Design. D.M. is the Israel Matz
Professor. We thank Dr. Arkadi Vigalok for fruitful discussions.
The Pd-coordinated C20-C21 (1.459 Å) bond is elongated as
compared with the corresponding bond in 2 (1.439 Å) as a result
of more back-bonding from the Pd center to the electron-deficient
exocyclic bond. This value is in the range of other bond distances
of alkenes coordinated to Pd(0).5,10 Interestingly, the Pd-C21
distance is also longer than the corresponding bond in 2 (2.360 vs
2.274 Å), probably as a result of diminished π-donation from the
electron-deficient methylene arenium group to the Pd-C bond. This
value is comparable with values reported for palladium complexes
of nonsymmetric olefins bearing an electron-withdrawing group.11
As a result of strong back-bonding,12 the exocyclic methylene is
bent out of the pseudo-aromatic ring plane by 6.1° (compared with
2.5° in the parent complex). No interaction between the metal and
the pseudo-aromatic ring is indicated. Pd-C22 (3.150 Å) and Pd-
C26 (2.884 Å) distances exclude the possibility of a π-benzylic
interaction. As expected, the C24-O bond distance of 1.384 Å no
longer indicates a double bond. For comparison, C-O bond length
of anisole in the gas phase is 1.357 Å.13
Supporting Information Available: Crystallographic data of 2 and
3 (CIF) and preparation and characterization of 2, 3, 4, and 5. This
material is available free of charge via the Internet at http://pubs.acs.org.
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arenium moiety makes possible its controlled release under very
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