A stable β-silyl carbocation with allyl conjugation

Article abstract of DOI:10.1002/poc.531

Phenyl group replaced by a double bond yielding a stable β-silyl carbocation was studied. Two systems were examined in which positive charge could be stabilized by one phenyl and one double bond or two double bonds. The carbocation was found to be stable at room temperature with tetrakis(pentafluorophenyl)borate as the anion benzene as the solvents. The range of β-silyl carbocations observed under stable ion conditions was relatively circumscribed.

Full text of DOI:10.1002/poc.531

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2002; 15: 667–671
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.531
A stable b-silyl carbocation with allyl conjugation
Joseph B. Lambert,* Chunqing Liu and Timur Kouliev
Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, USA
Received 18 February 2002; revised 16 April 2002; accepted 18 April 2002
‡
ABSTRACT: The carbocation Et3SiCH2C Ph(CH
=CHPh) has been found to be stable at room temperature with
tetrakis(pentafluorophenyl)borate as the anion and benzene as the solvent. The cation constitutes the first stable b-silyl
carbocation in which positive charge is delocalized both allylically and benzylically. Copyright  2002 John Wiley &
Sons, Ltd.
KEYWORDS: allylic conjugation; benzylic conjugation; carbocations; hyperconjugation; silicon
INTRODUCTION
of silicon. Even magic acid contains nucleophiles (‘X’)
strong enough to attack silicon and lead to loss of
Early studies of the ability of silicon to stabilize positive
charge on a carbon at the b-position (R3Si—C—C )
focused on understanding the mechanism of the interac-
tion (for a review, see Ref. 1). The two major mechan-
isms of positive charge delocalization in carbocations
involve donation of p or of n electrons to the empty p
Me3Si—X and formation of CH2=CPh2, which was
‡
protonated under the reaction conditions to form
‡
CH3Ph2C as the only observable carbocation.3
In 1996, we utilized conditions developed for the
preparation of stable silylium ions to generate the first
simple, stable b-silyl carbocation.4 Addition of the
‡
orbital, for example, respectively in allylic (CH2
=
CH—
solvated silyl cation Et3Si to CH2
=CPh2 produced
‡
‡
‡
‡
CH2 $ CH2—CH
=
CH2) and oxonium (CH3O—CH2
Et3SiCH2CPh2 , which was stable at room temperature
for weeks. The stability of this ion arose because of
internal stabilization by the various substituents and
because of the use of conditions with minimal nucleo-
philicity. The anion was tetrakis(pentafluorophenyl)
borate [(C6F5)4BÀ, abbreviated here as TPFPB], and
the solvent was an arene such as toluene or benzene. Siehl
et al. later observed similar ions under magic acid
conditions by rigid exclusion of water and other
nucleophiles.5
‡
$ CH3O
=CH2) ions. A much weaker mechanism
involves resonance donation of s electrons in CH or CC
‡
‡
hyperconjugation
CH2
(as
in
H—CH2—CH2 $ H
=
CH2). Induction also plays a role, destabilizing
in the case of electron-withdrawing substituents such as
the double bond and the oxygen atom in the above allylic
and oxonium examples and stabilizing in the case of
electron-donating substituents such as trimethylsilyl.
Silicon at the b-position thus offers the twofold
advantage of stabilization both through induction as a
result of its electropositivity and through enhanced
hyperconjugation as a result of its high polarizability
and the appropriate energy of the C—Si bond. Stereo-
chemical and kinetic studies demonstrated that hyper-
conjugation is the major mechanism of the b effect of
silicon and that induction plays a minor role.2
We also were able to isolate germanium analogues.6 In
unpublished work (with Y. Zhao) we found that the
phenyl groups were critical in observing stable b-silyl
carbocations. Omission of even one phenyl ring failed to
yield isolable ions. Thus, when styrene, a-methylstyrene
or isobutene was used in place of 1,1-diphenylethene, no
stable ions were formed. Apparently the ions were
present briefly, as we were able to carry out successful
trapping experiments. The observed b-silyl carbocation
from the addition of triethylsilylium to 1,1-diphen-
Chemists also tried to observe a b-silyl carbocation
directly under stable ion conditions.3 Olah’s studies in
magic acid failed because the highly electrophilic silicon
atom suffered nucleophilic attack and elimination.
Hyperconjugation moves positive charge from carbon
‡
ylethene, Et3SiCH2CPh2 , is stabilized once by Si
hyperconjugation and twice by benzylic conjugation.
Replacement of one or both phenyl groups with methyl or
hydrogen considerably reduced the stability of the cation
and rendered it unisolable.
‡
to silicon in Olah’s system (Me3Si—CH2—CPh2 $
‡
Me3Si CH2
=
CPh2), heightening the electrophilicity
In the present investigation, we considered whether the
phenyl group can be replaced by a double bond and still
yield a stable b-silyl carbocation. The question thus is
whether allylic stabilization can serve as successfully as
*Correspondence to: J. B. Lambert, Department of Chemistry,
Northwestern University, Evanston, Illinois 60208-3113, USA.
E-mail: jlambert@northwestern.edu
Contract/grant sponsor: National Science Foundation; Contract/grant
number: CHE-0091162.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 667–671

668
J. B. LAMBERT, C.-Q. LIU AND T. KOULIEV
did benzylic stabilization. On the basis of our previous
unpublished work (with Y. Zhao), we concluded that two
such p donors are needed. We examined systems in
which positive charge could be stabilized by one phenyl
and one double bond or by two double bonds. Thus, by
analogy with the diphenyl system 1 (to which the
solvated silyl cation was added to produce the cation),
we synthesized 1,3-diphenyl-1,3-butadiene in both its E
(2) and Z (3) forms and 2,6-dimethyl-4-methylene-2,5-
heptadiene (4). Addition of the solvated silyl cation to
these species respectively gives 5 (illustrated as the Z
form) and 6. In 5, positive charge is stabilized by silicon,
phenyl and the double bond. In 6, positive charge is
stabilized by silicon and two double bonds. These
systems enable us to examine the ability of double
bonds, through allylic resonance, to supplement hyper-
conjugation from the C—Si bond and permit the isolation
of stable b-silyl carbocations.
Wittig reaction on phorone (2,6-dimethyl-2,5-heptadien-
4-one) (8).
Preparation and characterization of the cations
To produce the b-silyl carbocations, triethylsilylium
TPFPB was added to the alkenes 3 and 4. The solvated
silyl cation in turn was produced by the reaction of
triethylsilane with trityl TPFPB. These reactions were
carried out at room temperature in NMR tubes using
syringe techniques within a glove-box. The solvent was
deuterated benzene.
Reaction of the solvated silyl cation with (Z)-1,3-
diphenyl-1,3-butadiene (3) was expected to produce the
b-silyl carbocation 5, in which the positive charge is
stabilized by the silicon at the b-position, by the single
phenyl ring, and by the double bond. The 1H spectrum of
the dark red product was very simple, containing a set of
resonances in the region ꢀ 6.4–7.4 from aromatic and
alkenic resonances, two methyl and two methylene
singlets in the region ꢀ 0–1 from the ethyl group in the
product and the ethyl group in excess triethylsilylium
TPFPB, and a sharp singlet at ꢀ 3.18. This singlet is
appropriate for the methylene group between silicon and
the carbon bearing positive charge. It is shifted to high
frequency because of sp2 character conveyed by
‡
‡
hyperconjugation (Et3Si—CH2—CPhR $ Et3Si CH2
=
CPhR, in which R is the phenylvinyl group). The
general appearance of the spectrum and the resonance
position of the methylene carbon are very similar to those
‡
of the product from diphenylethene (Et3SiCH2CPh2 ), in
which the methylene carbon resonates at ꢀ 3.64.6
The 29Si spectrum contained a resonance from excess
solvated triethylsilylium TPFPB at ꢀ 91.6, and two other
peaks at ꢀ 34.2 and 28.5. The b-silyl carbocation from
diphenylethene had a single 29Si peak at ꢀ 46.4.4 The
lower frequency positions of the peaks for 5 suggest less
transfer of positive charge to silicon.
RESULTS AND DISCUSSION
Synthesis
In the 13C spectrum, in addition to saturated reso-
nances from the ethyl carbons, resonances from the
product triphenylmethane and other aromatic resonances
in the region ꢀ 120–160 from the anion and the phenyl
groups on the cation 5, there were resonances at ꢀ 57.2,
193.7 and 235.7. The resonance at ꢀ 57.2 is from the CH2
group between silicon and the tertiary carbocation
carbon. Charge delocalization by allylic resonance places
positive charge at two positions (5 $ 9). The chemical
shift ꢀ 235.7 compares with ꢀ 225.7 for the cation from
Substrates 2–4 were selected because of their ready
accessibility. Alkene 2/3 is available from a-methylstyr-
ene in four steps.7 Previous studies of 1,3-diphenyl-1,3-
butadiene7,8 indicated that the E isomer 2 is unstable to
Diels–Alder dimerization and forms 7. Greater steric
hindrance in the Z isomer 3 rendered it stable and
isolable. Consequently, all our experiments focused on
the Z isomer 3. Substrate 4 with potential stabilization
from two double bonds was readily prepared by the
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 667–671

STABLE b-SILYL CARBOCATION WITH ALLYL CONJUGATION
669
‡
diphenylethene (Et3SiCH2CPh2 ). This resonance is from
the tertiary carbocation carbon in 5, and the higher
frequency position suggests more positive charge on this
were used. Upon stoichiometric treatment with the
‡
solvated silyl cation Et3Si TPFPBÀ either at room
temperature in C6D6 or at À50°C in CD2Cl2, 4 produced
a dark brown solution that exhibited no 13C resonances at
higher frequency than ꢀ 145 and no 29Si resonances
above ꢀ 10. Moreover, treatment of the benzene solution
with triethylsilane as a hydride source and a catalytic
amount of trityl TPFPB resulted in the loss of none of the
principal resonances, confirming that they did not derive
from a carbocation. We conclude that no b-silyl
carbocation was observed under the reaction conditions
from 4.
‡
carbon than in Et3SiCH2CPh2 . The chemical shift ꢀ
193.7 is appropriate for the secondary carbocation in 9
(from allylic delocalization as compared, for example,
‡
with ꢀ 200.2 for Ph2CH ) (for a general discussion of the
13C resonances of unsaturated carbocations, see Ref. 9).
The aryl carbons on to which positive charge is
delocalized typically are in the range9 ꢀ 140–145 and
so appear with other aromatic resonances. Note that 5 and
9 are resonance hybrids of a single carbocation.
These observations indicate that there is less deloca-
It is important to recognize that cation 6 has numerous
methyl protons that can initiate decomposition through
elimination, whereas cation 5 has no such protons. The
failure to observe the doubly allylic b-silyl cation may be
the result of this difference rather than inherent
instability. This hypothesis would have to be tested by
lization of charge to the double bond than to the second
‡
phenyl ring in Et3SiCH2CPh2 . The observation of two
29Si peaks may reflect the presence of E/Z isomers (10),
but we have no clear evidence on this issue. These
experiments were repeated in dichloromethane at
À50°C. No NMR peaks characteristic of a carbocation
were obtained. Evidently this solvent is too nucleophilic
to permit formation of the cation as a long-lived species.
The cation was trapped by reaction with triethylsilane
in the presence of a catalytic amount of trityl TPFPB.6
The peaks characteristic of carbocationic character
use of a substrate such as CH2=C(CH=CHPh)2.
CONCLUSIONS
A tricoordinate, tertiary carbocation has three opportu-
1
disappeared: H at ꢀ 3.18, 13C at ꢀ 235.7 and 193.7 and
nities for stabilization by its substituents. Thus three p
29Si at ꢀ 34.2 and 28.5. The spectra were consistent with
the expected product, 11 (stereochemistry indetermi-
nate), including in particular the benzylic proton at ꢀ
2.67. The mass spectrum confirmed the structure.
donors, as in the triphenylmethyl carbocation, Ph3C
‡
(trityl or tritylium), provide the strongest stabilization. In
the known4–6 b-silyl carbocation Et3SiCH2CPh2 , C—Si
‡
hyperconjugation (sp conjugation) has replaced benzylic
conjugation of one of the phenyl groups of trityl.
Nonetheless, the cation is sufficiently stable to exist for
long periods of time at room temperature under our
conditions4,6 or at À100°C under magic acid conditions.5
We found in this study that replacement of a second
phenyl group on trityl with phenylvinyl still permits
observation of a cation that is stable at room temperature.
This cation, 5, is stabilized by C—Si hyperconjugation,
benzylic conjugation and allylic conjugation. Replace-
ment of both the second and the third phenyl rings of
trityl by dimethylallyl groups, however, does not permit
the isolation of a stable carbocation (6) at room
temperature or at À50°C under the current conditions.
This cation would have been stabilized by C—Si
hyperconjugation and twice by allylic conjugation.
Substrate
4
(2,6-dimethyl-4-methylene-2,5-hepta-
diene) was prepared in order that double allylic
stabilization can replace double benzylic stabilization in
‡
Et3SiCH2CPh2 . Procedures identical with those used for
1,1-diphenylethene and (Z)-1,3-diphenyl-1,3-butadiene
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 667–671

670
J. B. LAMBERT, C.-Q. LIU AND T. KOULIEV
It is evident from these observations that allylic
was concentrated by rotary evaporation, and the orange
residue was allowed to stand at room temperature for 72 h
to complete dimerization of the unwanted trans product,
2. The desired cis product, 3, was purified by chroma-
tography on a silica gel column with pentane as eluent.
The dimer of 2 remained on the column to give pure 3: 1H
conjugation is not so effective as benzylic conjugation
in stabilizing a carbocation. When benzylic stabilization
is replaced entirely by allylic stabilization, no cation is
observed. We even used the most favorable substitution
mode of the allylic group (dimethylallyl) in 6, in which
allylic delocalization moves charge to a tertiary position
NMR (CDCl3), ꢀ 5.27 (d, J = 1.5 Hz, 1H, CH2
(d, J = 1.5 Hz, 1H, CH2 ), 6.37 (d, J = 12 Hz 1H, CH
6.63 (d, J = 12 Hz, 1H, CH ), 7.1–7.5 (m, 10H,
=
), 5.54
‡
‡
(R'R@C —CH
=
CMe2 $ R'R@C
=
CH—CMe2 in which
=
=
),
R' is Me2C CH and R@ is Et3SiCH2). This structure,
=
=
however, may be susceptible to elimination. It may be
noteworthy that in the case of cation 5, allylic
delocalization moves charge to a benzylic position, from
aromatic); 13C NMR (CDCl3), ꢀ 115.8, 126.5, 127.0,
127.8, 128.0, 128.4, 128.9, 130.4, 131.8, 136.8, 139.3,
144.8. MS 51, 77, 91, 115, 128, 167, 178, 191, 206
(molecular ion); UV (cyclohexane), lmax 250 nm (" =
16 200).
‡
which charge may be further delocalized (R'R@C —
‡
CH=
CHPh $ R'R@C
=
CH— CHPh $ R'R@C
=C—
‡
CH=
Ph , in which R' is Ph and R@ is Et3SiCH2). This
additional delocalization may provide an increment of
stabilization that is critical.
2,6-Dimethyl-4-methylene-2,5-heptadiene (4).7,10 Ad-
dition of 39 ml (0.07 mol) of a 1.8 ml solution of
phenyllithium in 70:30 cyclohexane–diethyl ether to a
stirred solution of 28.6 g (0.08 mol) of methyltriphenyl-
phosphonium bromide in 400 ml of dry diethyl ether
under N2 afforded a solution of the desired Wittig
reagent. The solution was stirred for 0.5 h, and 5.5 g
(0.04 mol) of phorone in 60 ml of dry diethyl ether was
added. The mixture was stirred an additional 0.5 h, H2O
(8 ml) was added, stirring was continued for 0.5 h and
MgSO4 was added. The mixture was filtered and the
filtrate was washed with saturated aqueous NaCl solution
(3 Â 100 ml), dried (MgSO4) and filtered. The ether was
removed by rotary evaporation and the residue was
fractionally distilled at reduced pressure to give 4.2 g of
The range of b-silyl carbocations that may be observed
under stable ion conditions (low nucleophilic anion,
hydrocarbon or halocarbon solvent, temperature À50°C
or higher) is therefore relatively circumscribed. Stabili-
zation in addition to that from the b-silyl group always is
necessary, and further stabilization by allylic conjugation
alone does not suffice.
EXPERIMENTAL
(Z) 1,3-Diphenyl-1,3-butadiene (3).7 A solution of
47.2 g (50 ml, 0.4 mol) of a-methylstyrene in 55 ml of
CCl4 was mixed with 78.3 g (0.44 mol) of N-bromosuc-
cinimide and stirred at 95–100°C for 10 h. The solution
was cooled, filtered from the precipitated succimide and
washed with CCl4. The solvent was removed by
distillation at reduced pressure (house vacuum), and the
residue was distilled [90–95°C/0.45 Torr (1 Torr =
133.3 Pa)] to give 60.7 g (77%) of a mixture of
brominated alkenes containing the desired a-(bromo-
methyl)styrene and also unwanted (E/Z)-2-bromo-1-
methylstyrene. A solution of 20.8 g (0.11 mol) of the
distillate in 100 ml of benzene was mixed with 17.3 g
(0.07 mol) of Ph3P. The resulting phosphonium salt
crystallized out in 48 h, and the unreacted and undesired
unsaturated bromides remained in solution. The crude,
white product was isolated by filtration and washed
(3 Â 100 ml) with benzene. Recrystallization from di-
chloromethane–diethyl ether produced 27.6 g (91%) of
colorless crystals of the desired phosphonium salt. To
21.4 g (46.6 mmol) of the phosphonium salt in 400 ml of
dry tetrahydrofuran cooled to À25 to À50°C under N2
was added 18.48 ml (46.2 mmol, 2.5 M in hexane) of
butyllithium. The solution of the Wittig reagent was
stirred for 30 min and 4.94 g (46.6 mmol) of benzalde-
hyde were added. The solution was stirred for 1.5 h at
room temperature and concentrated by rotary evapora-
tion. Pentane was added to precipitate triphenylphos-
phine oxide, which was removed by filtration. The filtrate
1
410 (0.03 mol, 77%, 78–79°C, 10–20 Torr): H NMR
(CDCl3), ꢀ 1.77 (s, 12H), 4.94 (s, 2H), 5.77 (s, 2H); 13
C
NMR (CDCl3), ꢀ 19.6, 26.9, 116.2, 126.7, 134.6, 143.6;
UV (cyclohexane), lmax 218 nm (" = 15 500). Anal.
Calcd for C10H16: C, 88.16; H, 11.84. Found: C, 87.98; H,
11.89%.
1-Phenyl-1-(b-phenylvinyl)-2-(triethylsilyl)ethylium tet-
rakis(penta¯uorophenyl)borate (5). In a nitrogen-filled
glove-box, 160 mg (0.17 mmol) of triphenylmethyl
tetrakis(pentafluorophenyl)borate (TPFPB) (Asahi Glass)
and 0.7 ml of dry benzene-d6 were placed in a valved
5 mm NMR tube. Addition of 25 mg (0.22 mmol) of
triethylsilane followed by vertical shaking of the tube
with the valve closed produced two layers. The lower,
light brown layer contained the solvated silyl cation:
triethylsilylium TPFPB. The colorless upper layer,
containing the byproduct triphenylmethane and excess
triethylsilane, was removed with a syringe. Addition of
45 mg (0.22 mmol) of 3 yielded a dark brown solution of
the carbocation: 1H NMR (C6D6), ꢀ 0.18 (s, 2H,
SiCH2CH3), 0.58 (s, 3H, CH3), 3.18 (2H, SiCH2C),
6.4–7.4 (m, 12H, aromatic and alkenic); 13C NMR
(C6D6), ꢀ 4.8, 8.1, 57.2, 120–150 (aromatic carbons),
193.7, 235.7; 29Si NMR (C6D6), ꢀ 28.5, 34.2 [all spectra
also contained peaks from excess triethylsilylium tetra-
kis(pentafluorophenyl)borate].
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 667–671

STABLE b-SILYL CARBOCATION WITH ALLYL CONJUGATION
671
Product studies. Triphenylmethyl TPFPB (20 mg,
0.02 mmol) was placed in a valved 5 mm NMR tube,
which was pumped for 2 h under high vacuum and placed
in a nitrogen-filled glove-box. The tube was opened and
charged with 0.27 g (2.3 mmol) of triethylsilane, 0.21 g
(1.0 mmol) of 3 and 1 ml of C6D6. The tube was shaken
vertically for a few minutes and two layers appeared. The
colorless top layer was removed and concentrated to give
0.2 ml of a colorless oil, which by GC contained several
components. The 13C and 29Si NMR spectra lacked the
peaks characteristic of carbocations. The 29Si spectrum
contained a single peak at ꢀ 7.4 in addition to the peak
from excess Et3SiH at ꢀ À 0.3. MS of the principal GC
peak showed the molecular ion at m/z 322, corresponding
to the structure 1,3-diphenyl-4-(triethylsilyl)butane (11).
The next highest peak was at m/z 206, from
(Grant No. CHE-0091162) for support of this work and
Asahi Glass Company for a gift of triphenylmethyl
tetrakis(pentafluorophenyl)borate.
REFERENCES
1. Lambert JB. Tetrahedron 1990; 46: 2677.
2. Lambert JB, Zhao Y, Emblidge RW, Salvador LA, Liu X, So J-H,
Chelius EC. Acc. Chem. Res. 1999; 32: 183.
3. Olah GA, Berrier AL, Field LD, Prakash GKS. J. Am. Chem. Soc.
1982; 104: 1349.
4. Lambert JB, Zhao Y. J. Am. Chem. Soc. 1996; 118: 7867.
5. Siehl H-U, Mu¨ller B, Malkina O. In Organosilicon Chemistry III:
From Molecules to Materials, Auner N, Weis J (eds). Wiley-VCH:
Weinheim, 1998; 25–40.
6. Lambert JB, Zhao Y, Wu H. J. Org. Chem. 1999; 64: 2729.
7. Mulzer J, Bru¨ntrup G, Ku¨hl U, Hartz G. Chem. Ber. 1982; 115:
3453.
8. Mulzer J, Ku¨hl U, Bru¨ntrup G. Tetrahedron Lett. 1978; 2953;
Mulzer J, Ku¨hl U, Huttner G, Bru¨ntrup G. Chem. Ber. 1988;
121: 2231.
9. Stothers JB. Carbon-13 NMR Spectroscopy. Academic Press: New
York, 1972; 217–224.
‡
(M À Et3Si) .
Acknowledgements
The authors thank the National Science Foundation
10. Sorenson, TS. Can. J. Chem. 1964; 42: 2781.
Copyright  2002 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2002; 15: 667–671