Electrophilic activation of Lewis base complexes of borane with trityl tetrakis(pentafluorophenyl)borate

Article abstract of DOI:10.1021/om070228w

Borenium ions do not accumulate under the conditions of hydride abstraction from Lewis base borane complexes (L·BH₃) using trityl cation because subsequent rapid reaction with L·BH₃ occurs to form B - H - B bonds. The hydride-bridged

Full text of DOI:10.1021/om070228w

Organometallics 2007, 26, 3079-3081
3079
Electrophilic Activation of Lewis Base Complexes of Borane with
Trityl Tetrakis(pentafluorophenyl)borate
Timothy S. De Vries and Edwin Vedejs*
Department of Chemistry, UniVersity of Michigan, 930 North UniVersity AVenue,
Ann Arbor, Michigan 48109-1055
ReceiVed March 9, 2007
Summary: Borenium ions do not accumulate under the condi-
tions of hydride abstraction from Lewis base borane complexes
(L‚BH3) using trityl cation because subsequent rapid reaction
with L‚BH3 occurs to form B-H-B bonds. The hydride-bridged
cations are sufficiently stabilized to resist abstraction of the
remaining hydride by excess trityl cation; howeVer, reVersible
cleaVage of the 3c2e bond does take place to release borenium
ion equiValents, as eVidenced by interaction with weak nucleo-
philes.
The borenium ion 3 is isoelectronic with benzyl cation and
should benefit from significant π delocalization. We therefore
attempted to observe 39 using NMR methods. While the 11B
NMR spectrum of 1 activated by Ph3C+-B(C6F5)4 (TrTPFPB,
5)10 in CD2Cl2 (room temperature) has a major peak (among
several) at δ 44 ppm, well within the range where trisubstituted
borenium ions have been reported,4 the signal is not coupled to
protons and cannot be due to 3 or to the solvent adduct (4, Nuc
+
) CD2Cl2). We have assigned this signal as PyBCl2 (6) on
the basis of 11B chemical shift comparisons and a pyridine
Trityl cation has been used extensively as a potent hydride
acceptor to generate reactive cationic species from neutral
hydride donors.1 In an early example, Benjamin et al. reported
the reaction of Ph3C+-BF4 with pyridine borane (1) in the
+ 11
quench to form the known Py2BCl2
.
None of the expected
Ph3CH singlet was found in the 1H NMR spectrum after 1 h at
room temperature, apparently due to decomposition, although
shorter reaction times did afford Ph3CH.
Cleaner reactions were observed between 5 and amine boranes
(1:1 molar ratio) at -78 °C in CD2Cl2. This procedure gave
little decomposition, and the best spectra were acquired from
activation of Et3N‚BH3 (7). Samples were warmed to -20 °C
1
for H and 11B NMR analysis, conditions that minimize line
broadening, especially for the 11B signals.12 Surprisingly, a 1H
NMR assay indicated complete conversion of 7 but only ca.
50% conversion of trityl cation, as evidenced by a 1:1 ratio of
Ph3CH to unreacted Ph3C+. A highly shielded peak appeared
at δ(1H) -2.6 ppm that was integrated for 1H relative to Ph3-
CH. By 11B NMR, signals were observed for -B(C6F5)4 (sharp
singlet at -17 ppm) and for a new broad peak at -3 ppm.
Warming the sample to room temperature resolved coupling to
two protons for this peak but did not result in greater conversion
of trityl cation prior to quenching with methanol and did not
produce signals in the trivalent boron region. Qualitatively
similar results were obtained when 1, 7, or other Lewis base
borane complexes were treated with 50 mol % TrTPFPB (Table
1), although 1 still produced 2 and other contaminants along
presence of pyridine to give Py2BH2+ (2, a four-coordinate boron
cation, bis(pyridine)boronium according to the conventional
nomenclature),2 as well as Ph3CH.3 A three-coordinate boron
cation, the (pyridine)borenium ion 3, was later proposed as an
intermediate,4 but no attempts to detect 3 or other primary
borenium ions have been reported. Little is known regarding
such species, although the trivalent boron cation should be
highly electrophilic at boron,5 perhaps sufficiently so for
applications in borylation,6 hydroboration,7 and hydrodefluori-
nation chemistry triggered by interaction with weakly nucleo-
philic n or π electrons, as in 4.8
* To whom correspondence should be addressed. E-mail: edved@
umich.edu.
(6) For leading references see: (a) Varela, J. A.; Pen˜a, D.; Goldfuss, B.;
Denisenko, D.; Kulhanek, J.; Polborn, K.; Knochel, P. Chem. Eur. J. 2004,
10, 4252. (b) Zhou, Q. J.; Worm, K.; Dolle, R. E. J. Org. Chem. 2004, 69,
5147.
(7) Clay, J. M.; Vedejs, E. J. Am. Chem. Soc. 2005, 127, 5766.
(8) For leading references see: (a) Vela, J.; Smith, J. M.; Yu, Y.; Ketterer,
N. A.; Flaschenriem, C. J.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem.
Soc. 2005, 127, 7857. (b) Scott, V. J.; C¸ elenligil-C¸ etin, R.; Ozerov, O. V.
J. Am. Chem. Soc. 2005, 127, 2852.
(1) (a) Corey, J. Y.; West, R. J. Am. Chem. Soc. 1963, 85, 2430. For
reviews see: (b) Lambert, J. B.; Zhao, Y.; Zhang, S. M. J. Phys. Org.
Chem. 2001, 14, 370. (c) Reed, C. A. Acc. Chem. Res. 1998, 31, 325. (d)
Beck, W.; Su¨nkel, K. Chem. ReV. 1988, 88, 1405. For leading references
to recent work see: (e) Panisch, R.; Bolte, M.; Mu¨ller, T. J. Am. Chem.
Soc. 2006, 128, 9676. (f) Cheng, T.-Y.; Szalda, D. J.; Zhang, J.; Bullock,
R. M. Inorg. Chem. 2006, 45, 4712.
(2) For reviews of cationic boron compounds see: (a) Piers, W. E.;
Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed. 2005, 44, 5016. (b)
Ko¨lle, P.; No¨th, H. Chem. ReV. 1985, 85, 399.
(9) This highly electrophilic species may exist as the solvent-coordinated
cation (4, Nuc ) solvent), which would technically be a boronium ion. To
avoid confusion, this distinction in nomenclature and structure is left unspec-
ified, and 4 will be considered equivalent to the free borenium ion 3.
(10) (a) Vedejs, E.; Nguyen, T.; Powell, D. R.; Schrimpf, M. R. Chem.
Commun. 1996, 2721. (b) Chien, J. C. W.; Tsai, W.-M.; Rausch, M. D. J.
Am. Chem. Soc. 1991, 113, 8570.
(3) Benjamin, L. E.; Carvalho, D. A.; Stafiej, S. F.; Takacs, E. A. Inorg.
Chem. 1970, 9, 1844.
(4) Ryschkewitsch, G. E.; Miller, V. R. J. Am. Chem. Soc. 1973, 95,
2836.
(5) Despite formal charge assignment in 3 to nitrogen, computational
studies of this cation show significant positive charge character at the boron
atom, supporting its electrophilic character and its description as a borenium
ion: Schneider, W. F.; Narula, C. K.; No¨th, H.; Bursten, B. E. Inorg. Chem.
1991, 30, 3919.
(11) The 11B NMR signal for 4-Me-C5H4N‚BCl2+-Al2Cl7 is reported at
+47 ppm by: Ryschkewitsch, G. E.; Wiggins, J. W. J. Am. Chem. Soc.
1970, 92, 1790. See the Supporting Information for quenching studies.
(12) Beall, H.; Bushweller, C. H.; Dewkett, W. J.; Grace, M. J. Am.
Chem. Soc. 1970, 92, 3484.
10.1021/om070228w CCC: $37.00 © 2007 American Chemical Society
Publication on Web 05/22/2007

3080 Organometallics, Vol. 26, No. 13, 2007
Communications
Table 1. 1H and 11B NMR Data for Activated Borane
Complexesa
bridged cationic M-H-M species1d,18 with trityl cation, a
process which typically results in full conversion to the M+
cation (M ) transition metal).19 The Si-H-Si 3c2e bond of
15 also undergoes further activation upon treatment with
TrTPFPB.1e,20 The only reported case of intermolecular stabi-
lization of a silylium cation with an external Si-H bond (16)
requires a large excess of silane, suggesting an equilibrium
between 16 and solvent- and anion-coordinated species.20 In
contrast, 8-13 were formed in the absence of excess borane
complex L‚BH3, and the chemical shifts were not affected by
the presence of unreacted L‚BH3. However, the highly electro-
philic cations 8-13 could not be isolated and were only
observed in solution.
Given the structural and electronic analogies to silylium cation
chemistry,1 we were interested to learn whether 8 might interact
with iPr3SiH as a potential 3c2e hydride donor. No NMR
evidence for the unsymmetrical structure 17 was obtained.
a In CD2Cl2 at -20 °C. In all cases X- ) TPFPB. b Bridging hydride
signal, in ppm. c Contaminated with 2 and unidentified pyridinium impuri-
ties.
with 10. In each example, conversion to a dominant product
1
having a high-field H NMR signal (δ 0.5 to -3.7 ppm) was
observed.
The above data are consistent with the initial formation of
transient borenium ion intermediates that undergo rapid capture
by the B-H bond of unreacted substrate to form the symmetrical
cations 8-13. The key event leading to 8 corresponds to overlap
between a nucleophilic B-H σ orbital of 7 with the empty p-
orbital of [Et3N‚BH2]+ (14a) or the equivalent displacement of
dichloromethane (DCM) from the solvent adduct [Et3N‚BH2‚Cl2-
CD2]+ (14b). Coordination of B-H bonds into electrophilic
However, when 8-d5 was generated from 7-d3 followed by
exposure to iPr3SiH at room temperature in CD2Cl2 (1 h), H/D
1
exchange was observed in 8 as well as iPr3SiH by H NMR,
2H NMR, and MS assay. According to these results, 8-d5
dissociates reversibly to release a small amount of the borenium
ion 14-d2. Reversible formation of a 3c2e bond with iPr3SiH
leads to 18, and equilibration with 19 provides the pathway for
H/D exchange. The symmetrical cation 8 therefore functions
as a source of the highly electrophilic borenium species 14. We
regard these data as the strongest evidence so far that mono-
substituted borenium ions such as 14a (or the equivalent DCM
adduct 14b) are viable intermediates.
With a clear picture of the activated species, the unexpected
decomposition of Ph3CH was studied more closely. Prior
literature implicitly assumes that Ph3CH is inert to the potent
electrophiles produced by hydride abstraction using TrTPFPB,
in contrast to our findings at room temperature. A reductive
quench (Bu4NBH4) of the solution obtained from activation of
7 with TrTPFPB (CH2Cl2, 1 h, room temperature) gave Ph2CH2
as the major byproduct, along with a complex mixture of
hydrocarbons. Analysis by GC/MS revealed benzene, toluene,
MeC6H4CH2Ph (20), and MeC6H4CHPh2 (21). The use of
CD2Cl2 as solvent increased the masses of toluene, 20, and 21
by 2 amu, indicating solvent incorporation. This likely occurs
from 14b-d2 by Cl-C heterolysis, Friedel-Crafts alkylation of
centers to form 3-center, 2-electron (3c2e) bonds is well
established,13 but cationic species with the B-H-B structural
motif have not been reported previously. The upfield 1H NMR
signals for 8-13 are in the range of those for B-H-B bonds
of neutral structures, including diborane,14 as well as the B2H7
-
anion15a and cyclic derivatives.15b The hydride-bridged structure
8 is also consistent with the -3 ppm 11B NMR chemical
shift.15,16 The “dimeric” structure of 8 explains the stoichiometry,
with 50 mol % of the TrTPFPB required for reaction with 7.
In the absence of nucleophiles, excess TrTPFPB (beyond 50
mol %) did not react with the hydride-bridged products 8-13
and did not produce signals that could be assigned to borenium
ions.17 This is in contrast to the reaction of singly hydride
(13) (a) Merle, N.; Koicok-Ko¨hn, G.; Mahon, M. F.; Frost, C. G.;
Ruggerio, G. D.; Weller, A. S.; Willis, M. C. Dalton Trans. 2004, 3883.
(b) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.; Uruichi,
M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704. (c) Parry, R. W.;
Kodama, G. Coord. Chem. ReV. 1993, 128, 245. (d) Marks, T. J.; Kolb, J.
R. Chem. ReV. 1977, 77, 263.
(14) Kern, C. W.; Lipscomb, W. N. J. Chem. Phys. 1962, 37, 275.
(15) (a) Hertz, R. K.; Johnson, H. D., II; Shore, S. G. Inorg. Chem. 1973,
12, 1875. (b) Katz, H. E. J. Am. Chem. Soc. 1985, 107, 1420.
(16) No¨th, H.; Wrackmeyer, B. Nuclear Magnetic Resonance Spectros-
copy of Boron Compounds; Springer-Verlag: New York, 1978.
(17) Treatment of DMAP‚BH3 with 1 equiv of TrTPFPB led to a new
11B NMR signal at +20 ppm (doublet). The structure responsible for this
signal remains unclear, but the chemical shift is not consistent with a
borenium ion.
(18) Venanzi, L. M. Coord. Chem. ReV. 1982, 43, 251.
(19) A cationic species with a Mo-H-Mo bond was reported to be
unreactive toward excess trityl cation: Voges, M. H.; Bullock, R. M. Dalton
Trans. 2002, 759.
(20) Hoffmann, S. P.; Kato, T.; Tham, F. S.; Reed, C. A. Chem. Commun.
2006, 767.

Communications
Organometallics, Vol. 26, No. 13, 2007 3081
Ph3CH at an ipso carbon, fragmentation to Ph2CH+, and
reductive trapping by hydride to give Ph2CH2.21 Similar events
+
involving DCM explain the formation of PyBCl2 (5) from 1
via 10 and the solvent adduct 4 (Nuc ) CH2Cl2).
We attribute the reactivity of 8 with the weak nucleophiles
Ph3CH and DCM as well as iPr3SiH to the presence of a small
amount of borenium ion 14 in equilibrium with 8. To gain
further insight, the compatibility of 8 with various counterions
was explored. No B-H-B-bonded structures were detected
when 7 was reacted with excess TrBF4,2,22 although Ph3CH
(>95%) was formed along with Et3N‚BF3.23,24 Furthermore,
treatment of 7 with the strong acids HOTf (22a) and HNTf2
(22b) as hydride acceptors formed tetravalent adducts 23a,b as
dominant products. Preformed 8 with X ) TPFPB also gave
23a,b (as well as 7) when the corresponding -OTf and -NTf2
salts 24a,b were added, thereby confirming cleavage of the
B-H-B bond by these anions. On the other hand, 8 was only
partly converted to 23c (ca. 1:1.1 8:23c) upon addition of 24c
or on treatment of 7 with 0.5 equiv of the strong, bulky carbon
acid HC(C6F5)Tf2 (22c)25,26 at room temperature. In this case,
unreacted 7 competes with the weakly nucleophilic anion
-C(C6F5)Tf2 for coordination into the unoccupied orbital of
borenium ion 14a.26
To summarize, borenium ions do not accumulate under the
conditions of hydride abstraction from Lewis base borane
complexes (L‚BH3) due to subsequent rapid reaction with L‚
BH3 to form B-H-B bonds. However, reversible cleavage of
the 3c2e bond releases borenium ion equivalents, as evidenced
by the interaction with weak nucleophiles. Hydride-bridged
cations such as 8 are sufficiently stabilized to resist abstraction
of the remaining hydride by excess trityl cation. The isotopic
exchange between 8-d5 and iPrSiH suggests that borenium ions
such as 14a may resemble silylium cations in terms of
electrophilicity. Considering the solvent-assisted decomposition
of Ph3CH reported here, the use of TrTPFPB for generation of
other reactive electrophiles may warrant closer scrutiny, espe-
cially in cases where decomposition of cationic products has
been noted.27
(21) Electrophilic substitution with E+ ) CD2Cl+ affords i. After
fragmentation to PhCD2Cl (ii; E ) CD2Cl) and Ph2CH+, reductive
quenching by hydride capture forms toluene-d2 (observed by GC/MS) and
Ph2CH2. When the reaction solvent was switched to benzene or toluene,
slow formation of Ph2CH2 was still observed, indicating the involvement
of other sources of E+ in these solvents. To test the possibility that 8 or 14
can act as E+, 7 was treated with TrTPFPB in the electron-rich arene
p-xylene as solvent. Oxidative workup after 20 h at room temperature gave
the expected 2,5-dimethylphenol, albeit in <5% yield. This experiment raises
the possibility that formation of i with E ) BH2‚NEt3 may also contribute
to C-C bond cleavage. For an example of the reaction of chloromethyl
cation with hexamethylbenzene, see: Davlieva, M. G.; Lindeman, S. V.;
Neretin, I. S.; Kochi, J. K. J. Org. Chem. 2005, 70, 4013.
Acknowledgment. This work was supported by the National
Institutes of Health (Grant No. GM067146). We also thank Prof.
M. Barybin for kindly providing a sample of 22c.
Supporting Information Available: Text and figures giving
experimental procedures and characterization data. This material
is available free of charge via the Internet at at http://pubs.acs.org.
OM070228W
(25) Ishihara, K.; Hasegawa, A.; Yamamoto, H. Angew. Chem., Int. Ed.
2001, 40, 4077.
(26) Et3NH+ was a major product from the reaction of 7 + 22c and a
minor product from 8 + 24c if the salt was dried prior to use; see the
Supporting Information.
(27) (a) Okuda, J.; Musikabhumma, K.; Sinnema, P.-J. Isr. J. Chem. 2002,
42, 383. (b) Song, X.; Thornton-Pett, M.; Bochmann, M. Organometallics
1998, 17, 1004.
(22) For the reaction of diarylcarbenium fluoroborates with amine
boranes, see: Funke, M.-A.; Mayr, H. Chem. Eur. J. 1997, 3, 1214.
(23) Formation of Et3N‚BF3 is evidence that 8 or 14 can extract fluoride
from -BF4.
(24) Heitsch, C. W. Inorg. Chem. 1965, 4, 1019.