Carbanion mechanisms XXI. Solution acidity of triphenylsilane

Article abstract of DOI:10.1016/S0022-328X(00)00242-4

Comparative acidity measurements have been performed between triphenylsilylpotassium and a series of arylmethanes by ¹H-NMR in tetrahydrofuran: Ph₃SiK + Ar_nCH_{4 - n} ? Ph₃SiH + Ar_nCH_{3 - n}K Metalation of the arylmethane was complete in the case of Ph₃CH (pK_a 31.4) and Ph₂CH₂ (pK_a 33.4) but with (p-CH₃·C₆H₄)₂CH₂ (pK_a 35.1) ～50% metalation of the di-p-tolymethane occurred, indicating that the two acids have equal pK_a. No reaction occurred with 4-Ph·C₆H₄·CH₃ (pK_a 38.7). Thus pK_a Ph₃SiH ≈ 35.1 in THF. This represents the first solution measurement of the acidity of an organosilane.

Full text of DOI:10.1016/S0022-328X(00)00242-4

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Journal of Organometallic Chemistry 604 (2000) 208–210
Carbanion mechanisms XXI. Solution acidity of triphenylsilaneꢀ
Erwin Buncel *, T.K. Venkatachalam
Department of Chemistry, Queen’s Uni6ersity, Kingston, Ont., Canada K7L 3N6
Received 4 February 2000; received in revised form 20 April 2000; accepted 24 April 2000
Abstract
Comparative acidity measurements have been performed between triphenylsilylpotassium and a series of arylmethanes by
1H-NMR in tetrahydrofuran:
Ph3SiK+ArnCH4−n = Ph3SiH+ArnCH3−n
K
Metalation of the arylmethane was complete in the case of Ph3CH (pKa 31.4) and Ph2CH2 (pKa 33.4) but with (p-CH3·C6H4)2CH2
(pKa 35.1) ꢀ50% metalation of the di-p-tolymethane occurred, indicating that the two acids have equal pKa. No reaction
occurred with 4-Ph·C6H4·CH3 (pKa 38.7). Thus pKa Ph3SiH:35.1 in THF. This represents the first solution measurement of the
acidity of an organosilane. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Silyl anions; Triphenylsilane; Acidity; pKa in THF
investigated though a number of organosilane deriva-
tives are known to be deprotonated by potassium hy-
1. Introduction
dride [7]. It appeared to us that investigations of
The concept of carbon acidity [1] is fundamental to a
solution organosilane acidities, through deprotonation
variety of structure-reactivity relationships in organic
of the parent silanes, would be of value in allowing
chemistry. It also has practical consequences as it
direct comparison with carbon acidity and extend the
reflects the stabilities of carbanions, key intermediates
acidity scales established for carbon acids [1,3,4] to the
utilized in a multitude of synthetic transformations of
other Group 14 elements. In continuation of our stud-
organic molecules [2]. While much effort has been
ies of carbanions [8] and of other Group 14 anions [9],
devoted in quantitating both thermodynamic [3] and
kinetic [4] carbon acidities, relatively little is known
we now present the results of what to our knowledge is
the first determination of the solution acidity of an
about the acidities of hydrides of the other Group 14
organosilane derivative relative to the corresponding
elements, silicon, germanium, tin and lead. These ele-
carbon analogue.
ments have rich organic chemistry associated with
them. For example, silyl anions are utilized in a variety
of synthetic organic reactions [5]. Thus, knowledge of
the acidities of organo silicon hydrides is of practical
2. Results and discussion
utility, as in the case of carbon acids.
The classical method for determination of equi-
Some studies of organosilane acidities have, in fact,
librium carbon acidities [10] is via the reaction in Eq.
been reported but these refer almost entirely to gas
(1):
phase acidities [6]. Formation of silyl anions directly by
R1H+R2M = R1M+R2H
(1)
deprotonation in an organic solvent has rarely been
This has been used by Streitwieser to determine
relative acidities in cyclohexylamine (CHA) [3a] and in
tetrahydrofuran (THF) [3b], and by Bordwell in
dimethyl sulfoxide (DMSO) [11]. The acid R1H is al-
ꢀ For Part XX see Ref. [16].
* Corresponding author. Tel.: +1-613-5332653; fax: +1-613-
5336669.
E-mail address: buncele@chem.queensu.ca (E. Buncel).
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(00)00242-4

E. Buncel, T.K. Venkatachalam / Journal of Organometallic Chemistry 604 (2000) 208–210
209
lowed to equilibrate with R2M, where M is an alkali
metal; when R1H is a much weaker acid than R2H the
position of the equilibrium is far to the left, and con-
versely the position is far to the right when the inverse
acidity order obtains, but when they are comparable
then the relative acidities are given directly by the ratio
[R1H]/R2H] which is determined usually by spectro-
scopic techniques such as NMR or UV–vis. We had
previously used this method to evaluate by spectropho-
tometric techniques the equilibrium acidity of H2 rela-
tive to a series of arylmethanes of known acidities in
THF solvent [12], and similarly for NH3 and CH3NH2
[13].
the two acids have equal pKa. No reaction was ob-
served in the case of p-phenyltoluene (pKa 38.7). It is
noted that these pKa values refer to ion pair acidity,
following Streitwieser [3a].
The finding that Ph3SiH is a weaker acid than Ph3CH
is in accord with previous NMR [15] and UV–vis [16]
evidence that charge delocalization over the phenyl
groups in Ph3Si− does not occur to significant extent,
i.e. structures such as 1b contribute to negligible extent:
We have thus evaluated the acidity of triphenylsilane,
Ph3SiH, relative to the series of arylmethanes ArnCH4−
n (n=1–3) of known pKa values, in THF solvent, by
means of the general reaction,
Ph3SiK+ArnCH4−n ? Ph3SiH+ArnCH3−n
K
(2)
The present result on comparison of the pKa of
Ph3SiH (35.1) with that of Ph3CH (31.4) represents the
first quantitative measure of the effect of lack of reso-
nance stabilisation in the Ph3Si− anion on a fundamen-
tal thermodynamic property [17]. We plan to extend the
studies to other organosilanes as well as hydrides of the
other Group 14 elements.
starting with triphenylmethane, Ph3CH, and going
down the series of arylmethanes with decreasing acidi-
ties until equilibration would occur. Ph3SiK was pre-
pared by cleavage of Ph3SiSiPh3 with potassium metal
in THF and was allowed to react with equimolar
Ph3CH. The position of equilibrium was monitored by
1
1H-NMR; the H-NMR characteristics of Ph3SiK and
Ph3CK have been reported previously [14,15].
The results, given in Table 1, show that Ph3SiH is a
weaker acid than Ph3CH (pKa 31.4) since the metala-
tion of the arylmethane proceded to completion. Simi-
larly, it was found that Ph3SiH is also a weaker acid
than Ph2CH2 (pKa 33.4). However, when the reaction
was performed with di-p-tolymethane (pKa 35.1) the
metalation went half way to completion indicating that
3. Experimental
Hexaphenyldisilane (Ph3SiSiPh3) was prepared by the
reaction of Ph3SiCl with sodium metal using a proce-
dure modified from that of Gilman [18]. A 100 ml
3-necked flask fitted with a reflux condenser, drying
tube and nitrogen inlet was charged with 30 ml dry
xylene (distilled over Na wire), 5 g Ph3SiCl, and 0.7 g
finely cut sodium. The reaction mixture was refluxed
for 3 h during which time a deep violet coloured
solution formed. After 6 h additional refluxing the
contents were allowed to cool and the white precipitate
was filtered. Traces of unreacted sodium were allowed
to react with 95% EtOH and the solution was re-filtered
to remove more product. The precipitate was washed
with 95% EtOH, then with Et2O, recrystallized from
xylene and dried in vacuo, yielding 2.1 g Ph3SiSiPh3,
m.p. 358–360°C, lit. [18] 362°C.
Triphenylsilylpotassium was prepared by reaction of
Ph3SiSiPh3 with potassium metal flakes under dry THF
in an ultrasonic bath at 0°C under an argon atmo-
sphere [15]. The solutions were kept in septum-sealed
reaction flasks in the ultrasonic bath for 3–4 h for
reaction to be completed. The solution of Ph3SiK in
THF (0.2 M) was transferred by means of a syringe
into argon-flushed NMR tubes containing one equiva-
lent of the arylmethane and dry cyclohexane as an
internal standard. Spectra were taken intermittently on
Table 1
Results on extent of metalation of arylmethanes in the equilibration
with Ph3SiK in THF solvent from 1H-NMR experiments a
Reaction
Time
Temperature Metalation
(°C)
(%) b
Ph3SiK+Ph3CH
Ph3SiK+Ph3CH
Ph3SiK+Ph3CH
Ph3SiK+Ph2CH2
Ph3SiK+Ph2CH2
Ph3SiK+Ph2CH2
30 min −40
45 min −40
90
95
95
10
40
95
0
24 h
25
45 min −40
45 min 25
24 h
25
Ph3SiK+(p-MeC6H4)2CH2 45 min −40
Ph3SiK+(p-MeC6H4)2CH2 24 h
Ph3SiK+(p-MeC6H4)2CH2 48 h
25
25
50
60
0
0
0
Ph3SiK+p-PhC6H4CH3
Ph3SiK+p-PhC6H4CH3
Ph3SiK+p-PhC6H4CH3
30 min −40
45 min −40
24 h
25
a Cyclohexane internal reference (l1.43); 0.2 M Ph3SiK and aryl-
methane; spectra taken on AM-400 Bruker spectrometer.
b Estimated from relative integrals of SiꢀH and ArCꢀH peaks with
uncertainty of 910%.

210
E. Buncel, T.K. Venkatachalam / Journal of Organometallic Chemistry 604 (2000) 208–210
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