New insights into the mechanism of reduction of tertiary phosphine oxides by means of phenylsilane

Article abstract of DOI:10.1002/hc.21279

The mechanism of the reduction of phosphine oxides by PhSiH₃ was established on the basis of kinetic measurements and Density Functional Theory (DFT) calculations. In particular, it has been proved that the model reaction between tri-nbutylphosphine oxide and phenylsilane occurs via a nonpolar mechanism. The data presented herein allow prediction and verification of the applicability of the new reduction reagents and conditions for industrially attractive processes.

Full text of DOI:10.1002/hc.21279

Heteroatom Chemistry
Volume 26, Number 6, 2015
New Insights into the Mechanism
of Reduction of Tertiary Phosphine Oxides
by Means of Phenylsilane
Oleg M. Demchuk,1 Radomir Jasin´ski,2 and K. Michał Pietrusiewicz1
1Department of Organic Chemistry, Maria Curie-Sklodowska University, Lublin 20614, Poland
2Institute of Organic Chemistry and Technology, Cracow University of Technology, Cracow 31155,
Poland
Received 30 March 2015; revised 16 June 2015
Ti(OiPr)4/HSi(OEt)3 [6–8], Ti(OiPr)4/PMHS [9, 10],
ABSTRACT: The mechanism of the reduction of
TMDS/Cu(OTf)2 [11], (EtO)2MeSiH/Ar2PO2H [12],
phosphine oxides by PhSiH3 was established on the
PhSiH3 [13,14], LiAlH4 [15,16], LiAlH4/MeOTf [17],
basis of kinetic measurements and Density Func-
AlH3 [18, 19], DIBAL-H [20], and Schwartz reagent
tional Theory (DFT) calculations. In particular, it has
[Cp2ZrHCL] [21]. Nevertheless, the utilization of
been proved that the model reaction between tri-n-
majority of frequently used reductants suffers from
butylphosphine oxide and phenylsilane occurs via a
their toxicity, corrosiveness, low stereoselectivity,
nonpolar mechanism. The data presented herein al-
and the necessity to use expensive additives, or un-
low prediction and verification of the applicability
favorable conditions.
of the new reduction reagents and conditions for in-
Considerably less demanding and therefore
ꢀC
dustrially attractive processes.
2015 Wiley Peri-
more commonly used is the reduction of phosphine
oxides by phenylsilanes [13]. They offer consider-
able advantages over other reducing agents [2], such
as nonaggressive behavior, simple and mild reac-
tion conditions, low toxicity, good functional group
tolerance, relatively simple purification of products,
and a predictable and highly enantioselective stereo-
chemical outcome. For these advantages, they have
been recently used as suitable reductants in the key
steps of catalytic Wittig [22–24], Appel [25], Mit-
sunobu [26–28], as well as in related reactions [29].
Thus, investigation of the precise mechanism of re-
duction of phosphine oxides by phenylsilane has an
additional practical value since it can help to develop
new reagent systems for those industrially attractive
processes.
odicals, Inc. Heteroatom Chem. 26:441–448, 2015;
View this article online at wileyonlinelibrary.com.
DOI 10.1002/hc.21279
INTRODUCTION
Phosphines constitute an important class of or-
ganic compounds with well-recognized academic
and industrial usage as ligands, organocatalysts,
extractants, etc. [1]. The main approach to phos-
phines remains reduction of phosphine oxides
by various strong reducing agents. Numerous re-
search groups have studied such reductions as
well as the applicability of different reductants.
In terms of reaction yields and selectivity, the
most promising reducing agents include silanes
[2], Si2Cl6 [3], HSiCl3/Et3N [4], HSiCl3/Ph3P [5],
Despite the potentially high value of such stud-
ies, the publications dedicated to elucidation of the
mechanism of deoxygenation of phosphine oxides
are very rare and not comprehensive. The mecha-
nism of reduction of phosphine oxides by chlorosi-
lane had been initially proposed by Horner and
Correspondence to: Oleg M. Demchuk; e-mail: oleh.demchuk@
umcs.lublin.pl.
ꢀC
2015 Wiley Periodicals, Inc.
441

442 Demchuk et al.
SCHEME 2 Reduction of 2 by means of phenylsilane (1).
TABLE 1 Representative Conditions for the Reaction be-
tween Tri-n-butylphosphine oxide (2) and Phenylsilane (1)
Ratio
(1):(2)
Time
(h)
Conv.(2)
(%)
Entry Solvent
t (°C)
SCHEME 1 Hypothetical ionic mechanism for the reaction
1
2
3
4
5
6
7
8
PhSiH3
PhSiH3
PhSiH3
PhSiH3
C6H12
DME
DMSO
CPME
80:1
80:1
80:1
80:1
20:1
20:1
20:1
20:1
4
4
17
90
5
5
5
100
75
75
50
75
75
75
75
95
39
97
99
47
10
10
98
between tertiary phosphine P-oxides and phenylsilane [13].
Balzer [4]. More detailed results of theoretical stud-
ies of the mechanism of reduction of phosphine
oxides by trichlorsilane have been presented only
recently by Krenske [30]. Based on this purely the-
oretical investigation run by means of DFT and
Møller–Plesset calculations, Krenske found that the
reaction runs through two different mechanisms
leading to products with inversion or retention of
absolute configuration at the phosphorus atom, de-
pending on the reaction conditions and reagent sys-
tems. For analogous reductions of phosphine oxides
by phenylsilane, an intuitive polar, two-step mecha-
nism involving formation of an ion-pair intermediate
(Scheme 1) was proposed by Marsi [13], but it has
not been so far verified in any way.
32
To assure reliable results, all kinetic experiments
were conducted in sealed vessels and the reaction
mixtures were always protected by argon atmo-
sphere in sealed reactors to exclude possible oxidiza-
tion and evaporation effects, which may influence
the quantitative results. The extent of substrate con-
versions was monitored by means of 31P, 1H, and
13C NMR spectroscopy of rapidly cooled samples. It
was found that in reactions where phenylsilane was
used both as the solvent and the reactant, full con-
version of 2 was observed in 4 h at 100°C and in 90
h at 50°C. Having these promising results, series of
experiments were carried out in different solvents,
with different reagent ratios, and at different tem-
peratures (Table 1). The reactions were run at tem-
peratures between 50 and 100°C, since preliminary
experiments run at room temperature (RT) took sev-
eral weeks to achieve sufficient conversion, and re-
actions run above 100°C were too short to perform
kinetic experiments with good precision.
In the detailed investigation of the reaction
course, intermolecular interactions in the elemen-
tary reaction process with respect to the recently
widely promoted [31, 32] theory of global elec-
trophilicity and nucleophilicity indexes were ana-
lyzed first. Recently, the approach based—according
to the Domingo suggestions [31]—on B3LYP/6-
31G(d) calculations has been used successfully for
the interpretation of different bimolecular reactions
[31–35]. Indices necessary for this purpose (chemi-
cal electron potentials μ, chemical hardness η, global
electrophilicity ω, as well as maximum charge that
may be potentially accepted in the electrophilic
substructure of transition complex ꢀNmax) were
In respect to the crucial importance of
phenylsilane-based reduction of phosphine oxides,
we aimed to study the mechanism of this reaction
both experimentally and theoretically, using tri-n-
butylphosphine oxide (2) and phenylsilane (1) as
model reactants.
RESULTS AND DISCUSSION
Among the number of available phosphine oxides,
tri-n-butylphosphine oxide (2) was selected as a
model reactant because of its low hygroscopicity
(experimental kinetic results not influenced by wa-
ter impurities), high solubility in numerous organic
solvents, and a moderate reduction reaction rate
observed in preliminary experiments. Besides their
applicability in the DFT calculations, cyclic phos-
phine oxides and those bearing smaller substituents
(Me, Et, Pr) at the phosphorus atom were highly
hydroscopic, while the tested aromatic derivatives
were insoluble in nonpolar solvents. It was found
that an increase in sterical hindrance at the phos-
phorus atom slowed down the reaction significantly;
therefore, the use of bulky oxides was also limited.
(Scheme 2).
Heteroatom Chemistry DOI 10.1002/hc

New Insights into the Mechanism of Reduction of Tertiary Phosphine Oxides by Means of Phenylsilane 443
TABLE 2 Global Electronic Properties for Phenylsilane (1)
and Tri-n-butylphosphine oxide (2)
for a number of organophosphorus compounds, in-
cluding phosphines and their oxides [37] and, addi-
tionally, using a new generation M062x functional
[38]. For the calculations of the solvent effect on
the reaction paths, the polarizable continuum model
(PCM) was used.
Compound
μ (a.u)
η (a.u)
ω (eV)
1
2
–0.1347
–0.1108
0.2343
0.2018
1.05
0.83
The results of B3LYP/6-31g(d) (PCM) sim-
ulations prove that the reaction between tri-n-
butylphosphine oxide (2) and phenylsilane (1) is
a multistage process. In particular, two transition
complexes (TS-1 and TS-2) were localized between
the minimum linked with existence of individual
substrates (1 + 2) and the minimum linked with
existence of products (3 + 4) separated by the inter-
mediate minimum (I) and a minimum correspond-
ing to the prereaction complex (PC) (Tables 3 and 4).
The shape of the enthalpic reaction profile is shown
in Fig. 1, and the visualisations of the critical struc-
tures are shown in Fig. 2.
Interactions between the reagent molecules in
the first stage lead to the formation of PC. This in-
volves the enthalpy of the reaction system reduced
by 1.7 kcal/mol. The complex has no properties of a
charge transfer one. Distances between the reaction
centers in the complex are still much larger than the
range typical of respective transition state bonds.
calculated as a function of Frontier Molecular
Orbitals (FMO) energies (Table 2).
The analysis of chemical electron potentials
proved that the electron transfer in the reaction
should occur from oxide 2 (μ = –0.1108 a.u.)
to phenylsilane 1 (μ = –0.1347 a.u.). The maxi-
mum charge potentially accepted by the phenylsi-
lane molecule within the transition complex (ꢀNmax
)
is almost 0.6 eV. Both reaction components are con-
sidered moderate electrophiles [31], where phenyl-
silane global electrophilicity is slightly higher. The
difference between global electrophilicity values for
the substrates is not large (ꢀω = 0.22 eV). There-
fore, the reaction should be considered a nonpolar
process.
The character of the interactions in the first re-
action stage is evaluated based on electronic prop-
erties. Its mechanism, however, cannot be analyzed
in this way. Therefore, to describe precisely the na-
ture of critical structures on the substrate conversion
pathway toward the products, we simulated the re-
action pathway based on the DFT calculations data.
The calculations were made in several basis sets us-
ing the B3LYP functional [36], successfully verified
Further
movement
of
the
reaction
system along the reaction coordinate lead to
the formation of intermediate TS-1. This involves
an increase in the enthalpy of the reaction system
by 28.5 kcal/mol. This is the limiting step for
the whole process. With the increased enthalpy,
TABLE 3 Eyring Parameters of the Reaction between Phenylsilane (1) and Tri-n-butylphosphine oxide (2) in Solution according
to DFT (PCM) Calculations
Functional
B3LYP
M062x
Basis set
Transition
ꢀH
ꢀG
ꢀS
ꢀH
ꢀG
ꢀS
6-31G(d)
1 + 2 → PC
1+2→TS-1
1+2→I
−1.7
28.5
14.0
18.2
−2.5
−0.1
31.0
15.3
17.1
−3.1
7.55
43.06
26.70
30.33
−2.59
7.57
45.64
27.61
27.92
−2.87
–31.1
–48.9
–42.6
–40.9
0.2
–25.8
–49.0
–41.4
–36.3
–0.8
–4.0
18.7
2.6
12.0
–5.7
–3.0
20.3
2.9
10.7
−6.4
−3.1
20.2
2.6
3.55
30.95
15.72
25.36
–6.40
4.35
33.34
18.14
22.59
−7.11
4.96
−25.2
−41.0
−43.9
−44.7
2.4
−24.6
−43.6
−51.0
−40.0
2.4
−27.2
−43.1
−50.9
−37.6
2.3
1+2→TS-2
1+2→3+4
1+2→ PC
1+2→TS-1
1+2→I
6-31+G(d)
1+2→TS-2
1+2→3+4
1+2→ PC
1+2→TS-1
1+2→I
6-31+G(d,p)
30.8
15.0
16.3
–6.0
45.23
27.33
27.77
−5.75
−48.4
−41.4
−38.4
−0.9
33.05
17.75
16.03
–9.90
1+2→TS-2
4.8
−9.2
1+2→3+4
ꢀH and ꢀG values are expressed in kcal/mol; ꢀS values are expressed in cal/molꢀK.
Heteroatom Chemistry DOI 10.1002/hc

444 Demchuk et al.
TABLE 4 Key Geometrical Parameters of Critical Structures PC, TS-1, I, and TS-2 in Solution according to B3LYP/6-31g(d)
(PCM) Calculations
˚
Interatomic Distances r (A)
Degree of Bond Development
Structure
Si–H
H–P
P–O
O–Si
H–O
H–P
Si–O
O–H
PC
TS-1
I
1.488
1.708
2.894
3.255
4.622
1.934
1.411
1.412
1.515
1.617
1.936
2.767
3.730
1.786
1.628
1.600
3.439
2.167
2.252
1.853
0.629
0.903
TS-2
0.087
where rTS
is the distance between the reaction
X – Y
centers X and Y at the transition structure and rP
X – Y
is the same distance at the corresponding product.
The complex proved to be nonsymmetrical. The
˚
Si–O (r = 1.786 A, l = 0.903) bond forms much more
rapidly. The Si–H bond is broken when the new σ-
bonds form. Further development of the reaction
system lead to the formation of intermediate I. Con-
trary to what Marsi [13] suggested, this is not an ion
pair. In the intermediate, the Si–O bond is already
˚
formed (r = 1.628 A). The O–P distance is also typi-
cal of the formed chemical bond. It should be noted
at this point that the presence of a similar phospho-
rane intermediate has been suggested by Mislow and
co-workers [3] in the case of reduction of optically
active acyclic phosphine oxides by means of hex-
achlorodisilane. Intermediate I is unstable, rapidly
converted further through the transition state TS-
2. TS-2 formation involves a relatively lower en-
ergy expense than for TS-1. It is a three-membered
complex. An O–H bond forms within its structure.
The degree of advancement of the bond in TS-2
does not exceed 10%, however. P–O and H–P bonds
are simultaneously broken. Similar transformation
has been previously studied both experimentally and
theoretically [39]. Further movement of the reaction
system leads to the products (3 + 4).To conclude,
the reaction mechanism is shown in Scheme 3.
DFT calculation data at other theory levels
(Tables 3 and 4) provide a similar profile of the re-
action. In particular, energy profiles are very similar
and the geometry parameters of critical structures
are practically identical as for the B3LYP/6-31G(d)
calculations. This changes only qualitative descrip-
tion of critical points in the enthalpic profiles. All
attempts to find ionic intermediates as well as hypo-
thetical one-step reaction paths (including calcula-
tions in more advanced 6–31+G(d) and 6–31+G(d,p)
basis sets) were not successful.
FIGURE 1 Enthalpy profile of the reaction between phenyl-
silane (1) and tri-n-butylphosphine oxide (2) according to cal-
culations at selected DFT (PCM) levels.
entropy of the reaction system is reduced by
48.9 cal/molꢀK. This suggests that TS-1 is
highly rigid. Indeed, a detailed geometry analy-
sis proved the structure to be a four-membered
complex, at the first view similar to that postulated
by Marsi [13]. However, its nature is different from
that suggested by the authors. In particular, two new
σ-bonds form within TS-1. They are silicon–oxygen
and hydrogen–phosphorus bonds. The degrees of
their advancement were estimated based on the
relationship (Eq. (1)):
Next, we performed similar calculations for the
reaction in the gas phase. It was found (Table 5) that
the reaction mechanism is practically identical as in
the presence of a dielectric medium. It was further
TS
X−Y
r
− rXP−Y
lX−Y = 1 −
(1)
rXP−Y
Heteroatom Chemistry DOI 10.1002/hc

New Insights into the Mechanism of Reduction of Tertiary Phosphine Oxides by Means of Phenylsilane 445
FIGURE 2 Critical structures of the reaction between phenylsilane (1) and tri-n-butylphosphine P-oxide (2) according to
B3LYP/6-31G(d) (PCM) calculations.
confirmed that the reaction between 1 and 2 has a
nonionic character.
To determine which theory level is better in de-
scribing the energetic aspects of reaction course,
we decided to establish activation enthalpy and en-
tropy values under experimental conditions. There-
fore, measurements of rate constants at three tem-
peratures were performed (Table 6) and the Eyring
relationship was determined (Eq. (2)) [40].
log k/T = 10.319 + ꢀS=/4.576 − ꢀH=/4.576T
(
)
(2)
SCHEME 3 Mechanism of the reaction between phenylsi-
lane and tertiary phosphine P-oxides according to the DFT
calculations.
The resulting activation enthalpy was
17.02 kcal/mol, while the activation entropy
TABLE 5 Eyring Parameters of the Reaction between Phenylsilane (1) and Tri-n-butylphosphine oxide (2) in the Gas Phase
according to DFT Calculations
B3LYP/6–31G(d)
B3LYP/6–31+G(d)
B3LYP/6–31+G(d,p)
M062x/6–31g(d)
Transition
ꢀH
ꢀS
ꢀH
ꢀS
ꢀH
ꢀS
ꢀH
ꢀS
1+2→ PC
1+2→TS-1
1+2→I
–2.7
27.6
11.6
19.7
–3.8
−30.0
−48.6
−41.6
−40.5
0.6
–1.6
29.5
12.5
18.6
−4.9
−26.8
−48.6
−42.0
−37.3
−0.8
−1.6
29.4
12.2
17.7
−7.9
−26.9
−48.6
−42.0
−37.8
−1.0
−9.7
18.8
−0.2
14.0
−6.4
−30.4
−34.6
−47.5
−33.2
6.5
1+2→TS-2
1+2→3+4
ꢀH values are expressed in kcal/mol; ꢀS values are expressed in cal/molꢀK.
Heteroatom Chemistry DOI 10.1002/hc

446 Demchuk et al.
TABLE 6 Experimental Measured Rate Constants of the Re-
action between Phenylsilane (1) and Tri-n-butylphosphine ox-
ide (2)
of the reaction leads to conversion of the inter-
mediate to phenylsilanol and tri-n-butylphosphine
through a three-membered transition complex. All
attempts to find an alternative mechanism for tri-n-
butylphosphine reduction were—irrespective of the
theory level—not successful. The results obtained in
our investigations allow prediction and verification
(by DFT calculation according to the proposed al-
gorithm) of the applicability of the new reduction
reagents and conditions for industrially attractive
processes.
Entry
t (°C)
k × 10−6 (1/molꢀs)
1
2
3
50
75
100
1.16
8.47
46.81
was 33.2 cal/molꢀK. The values satisfactorily
correlate with the M062x/6-31g(d) (PCM) calcula-
tions. The relatively low activation enthalpy value
confirms that the energy changes in the reacting
system caused by breaking the existing σ-bonds are
compensated by changes related to the formation
of new bonds. Furthermore, the high negative
activation entropy confirms that TS-1 is highly
ordered.
EXPERIMENTAL
Materials and Equipment
The reactions were carried out under dry argon at-
mosphere. Unless noted otherwise, materials were
obtained from commercial suppliers and used as re-
ceived. Thin layer chromatography was performed
on Merck silica gel 60 coated aluminum plates. NMR
spectra were recorded on a Bruker Advance III spec-
Finally, it has to be noted that despite the fact of
the different reactivity of chlorosilane and phenyl-
silane, our comprehensive (theoretical and exper-
imental) results correlate well with the previous,
only theoretical study presented by Krenske [30].
Therefore, this may suggest a general character of
the described transformation and that the proposed
mechanism can be extrapolated to reactions involv-
ing different classes of silicon-based reductants such
as several phosphine oxides. This is confirmed ad-
ditionally by our calculations of the reaction paths
for reductions of other tertiary phosphine oxides by
means of several arylsilanes. All of these reactions
proceed via an identical, two-step mechanism, as in
the case of tri-n-butylphosphine oxide reduction. It
should also be mentioned that 1-phenylphospholane
oxide is converted into 1-phenylphospholane via an
activation barrier, which is a few kcal/mol lower
than in the case of similar reduction involving tri-
n-butylphosphine oxide. This observation correlated
well with the recent results from the Krenske group
[41]. Our experiments also show that a similar ki-
netic effect stimulates electron withdrawing groups
in the phenyl ring of silane [22].
1
trometer (500 MHz at H NMR). All commercially
available reagents and solvents were purchased form
Sigma-Aldrich and used without further purifica-
tion. Tetramethylsilane, deuterium oxide (in a sealed
capillary), or phosphoric acid (75% in a sealed capil-
lary) were used as internal standards. All NMR spec-
tra were reported in delta (δ) units, parts per million
(ppm) downfield from the internal standard. Cou-
pling constants (J) are reported in hertz (Hz).
Reduction of Tri-n-butylphosphine oxide—
Typical Procedure
Tri-n-butylphosphine oxide (448 mg, 2.22 mmol)
and a Polytetrafluoroethylene (PTFE)-coated mag-
netic stir bar were placed in a 10-mL flask; 2 mL of
benzene-d6 was added and the solution was cooled
down to 5°C. Next, the air was replaced with argon
in a usual vacuum degasification sequence. The solu-
tion was warmed up to RT and phenylsilane (285 mg,
333 mkL, 2.64 mmol) was added. The flask was
sealed with a glass stopper secured by a metal clam
and placed in a 100°C hot oil bath on a hot plate mag-
netic stirrer for 6 h. After that time, the reaction mix-
ture was cooled down to RT and a sample was care-
fully taken to record NMR spectra, which indicate
the full conversion of the substrate. The pure prod-
uct was isolated by distillation under reduced pres-
sure to yield 348 mg (72%) of tri-n-butylphosphine.
The 1H and 31P NMR spectra are consistent with the
literature [42, 43], bp 36–38°C/0.2 mmHg (lit.: [44]
38–40°C, 0.2 mmHg).
CONCLUSIONS
The theoretical and experimental studies provide
new insights into the mechanism of reactions be-
tween tertiary phosphine oxides and silanes. In par-
ticular, it has been proved that the model reaction
between tri-n-butylphosphine oxide and phenylsi-
lane occurs via a nonionic mechanism. The first
stage involves the formation of a PC which, subse-
quently, is converted to the intermediate through
the first transition complex. Further development
31P NMR 202 MHz, CDCl : δ = −32 ppm;
(
)
3
Heteroatom Chemistry DOI 10.1002/hc

New Insights into the Mechanism of Reduction of Tertiary Phosphine Oxides by Means of Phenylsilane 447
spheres, was used. DFT (PCM) calculations were per-
formed for vacuum/isolated molecules and for the
1H NMR 500 MHz, CDCl : δ = 0.8 − 1.0 m, 9H ,
(
)
(
)
3
simulated presence of toluene, which is parameter-
ized by the Gaussian package, and the polarity is
relatively close to the polarity of phenylsilane.
The reactivity indexes were estimated according
to the equations and theoretical level recommended
by Parr and Yang [45] and Domingo and co-authors
[31]. In particular, the electronic chemical potentials
(μ) and chemical hardness (η) of reactants 1 and 2
were evaluated in terms of one-electron energies of
FMO:
1.3 − 1.5 m, 18H ;
(
)
13C dept NMR 125 MHz, CDCl : δ = 13.8, 24.5
(
)
(
)
3
d, J = 11.1 Hz , 26.9 d, J = 10.5Hz ,
(
)
(
)
28.1 d, J = 12.3 Hz .
(
)
Kinetic Measurements
The starting reaction mixtures were prepared
by introducing weighed quantities of tri-n-
butylphosphine oxide and phenylsilane into an arg-
onated NMR tube. The initial amount of tri-n-
butylphosphine oxide was 10.6 mg (0.05 mmol),
whereas phenylsilane was used in an 80-fold molar
excess (500 μL, 4.0 mmol), when the reaction was
run without solvent addition, or in a 20-fold molar
excess (120 μL), when the reaction was run in a so-
lution in 380 μL of the chosen solvent. The NMR
tube was sealed and placed in a preheated oil bath at
constant temperatures. The reaction mixtures were
analyzed by NMR using TMS (1H NMR) and cap-
illaries with H3PO4 in D2O (31P NMR) as internal
standards. The conversion of phosphine oxide was
determined from 31P NMR spectroscopy in 30-min
periods (the NMR tube was rapidly chilled before
each measurement). We also checked whether the
ratio of Bu3PO/Bu3P remained constant during the
measurements. To this end, the measured sample
was left at RT for 16 h and the 31P NMR spectrum
was recorded again—the data analysis revealed no
reaction progress between the two measurements.
The pseudo–first-order rate constants kꢂ were fol-
lowed by measuring the area of the NMR peak of
Bu3P (–32 ppm) and Bu3PO (43 ppm). The kinetic
runs were carried out at all temperatures with more
than 80% completion of the reactions. The second-
order rate constants k were obtained from the quo-
tient of pseudo–first-order rate constants kꢂ and the
initial concentration of phenylsilane according to
the standard procedure [40].
μ = EHOMO+ELUMO /2; η = ELUMO − EHOMO
The values of μ and η were then used for calcula-
tion of the global electrophilicity (ω) according to
the formula:
(
)
ω = μ2/2η
The maximum amount of electronic charge that the
electrophile system may accept was calculated by the
following formula [46]:
ꢀNmax = − μ/η
(
)
Optimizations of the stable structures were per-
formed with the Berny algorithm, whereas the tran-
sition states were calculated using the QST2 proce-
dure followed by the TS method. Stationary points
were characterized by frequency calculations. All re-
actants, products, and local minima had positive
Hessian matrices. All transition states showed only
one negative eigenvalue in their diagonalized Hes-
sian matrices, and their associated eigenvectors were
confirmed to correspond to the motion along the re-
action coordinate under consideration. IRC calcu-
lations were performed to connect previously com-
puted transition structures with suitable minima.
ACKNOWLEDGMENTS
The generous allocation of computing time by the re-
gional computer center “Cyfronet” in Cracow (Grant
No.: MNiSW/Zeus_lokalnie/PK/009/2013) and sup-
port from the Polish National Science Centre (NCN
grants number 2012/05/B/ST5/00362) as well as kind
aid of Prof. D. Gryko (ICHO PAN, Warsaw) are grate-
fully acknowledged.
Quantumchemical Calculations
The quantum-chemical calculations were performed
on a SGI-Altix-3700 computer in the Cracow Com-
puting Center “CYFRONET.” Hybrid B3LYP as well
as M062x functionals and 6–31G(d), 6–31+G(d), 6–
31+G(d,p) basis sets included within GAUSSIAN
2009 software [36] were applied. For the calculations
of the solvent effect on the reaction paths, a PCM, in
which a cavity is created via a series of overlapping
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