An intramolecular: Ortho -assisted activation of the silicon-hydrogen bond in arylsilanes: An experimental and theoretical study

Article abstract of DOI:10.1039/c7dt04858k

An intramolecular activation of the Si-H bond in arylsilanes by selected ortho-assisting functional groups based on boron, carbon and phosphorus was investigated experimentally and by means of theoretical calculations. The major conclusion drawn is that the presence of a negatively charged oxygen atom in the functional group is essential for providing effective chelation to the silicon atom which in turn results in the increased hydridic character of a resulting five-coordinated species. In contrast, an intermolecular attack of hydroxide on the silicon atom in aryldimethylsilane results in the activation of the silicon-aryl bond. This increased reactivity of the Si-H bond in intramolecularly coordinated arylsilanes can be ascribed to a significant trans effect which operates in the preferred configuration. Hydrolytic cleavage of the Si-H bond results in dihydrogen elimination and the formation of various silicon heterocyclic systems such as benzosiloxaboroles, spiro-bis(siloxa)borinate, benzosilalactone and benzophosphoxasilole. In addition, intermolecular reduction of benzaldehydes with ortho-boronated arylsilane was observed whereas compounds bearing other reducible functional groups (COMe, COOEt, CN and NO₂) were inert under comparable conditions. Specifically, an intramolecular reduction of the CN group in an ortho-silylated benzonitrile derivative was observed. The mechanism of Si-H bond activation was investigated by the DFT theoretical calculations. The calculations showed that the intramolecular coordination of the silicon atom effectively prevents the cleavage of the Si-aryl bond. Furthermore, the reaction is favored in anionic systems bearing COO^-, B(OH)₃^- or CH₂O^- groups, while in the case of neutral functional groups such as PO(OEt)₂ the process is much slower.

Full text of DOI:10.1039/c7dt04858k

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Intramolecular ortho-Assisted Activation of Silicon-Hydrogen Bond in Arylsilanes: An
Experimental and Theoretical Study
Krzysztof Durka,a,* Mateusz Urban,a Maja Czub,a Marek Dąbrowski,a Patryk Tomaszewski,a
Sergiusz Lulińskia,*
aDepartment of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology,
Noakowskiego 3, 00ꢀ664 Warszawa, Poland
Abstract
Intramolecular activation of the Si–H bond in arylsilanes by selected orthoꢀassisting
functional groups based on boron, carbon and phosphorus was investigated experimentally
and by means of theoretical calculations. The major conclusion drawn is that the presence of
negatively charged oxygen atom in the functional group is essential for providing effective
chelation to the silicon atom which in turn results in the increased hydridic character of a
resulting fiveꢀcoordinated species. In contrast, an intermolecular attack of hydroxide on the
silicon atom in aryldimethylsilane results in the activation of the siliconꢀaryl bond. This
increased reactivity of Si–H bond in intramolecularly coordinated arylsilanes can be ascribed
to a significant trans effect which operates in the preferred configuration. Hydrolytic cleavage
of the Si–H bond results in dihydrogen elimination and formation of various silicon
heterocyclic systems such as benzosiloxaboroles, spiroꢀbis(siloxa)borinate, benzosilalactone
and benzophosphoxasilole. In addition, intermolecular reduction of benzaldehydes with orthoꢀ
boronated arylsilane was observed whereas compounds bearing other reducible functional
groups (COMe, COOEt, CN, NO2) were inert under comparable conditions. Specifically, an
intramolecular reduction of the CN group in an orthoꢀsilylated benzonitrile derivative was
observed. The mechanism of Si–H bond activation was investigated by the DFT theoretical
calculations. They showed that the intramolecular coordination of silicon atom effectively
prevents the cleavage of Siꢀaryl bond. Furthermore, the reaction is favored in anionic systems
ꢀ
bearing COOꢀ, B(OH)3 or CH2Oꢀ groups, while in the case of neutral functional groups such
as PO(OEt)2 the process is much slower.
Introduction
The activation of siliconꢀhydrogen bond is of recent prime interest.1 This is mainly due to the
use of hydrosilylation of unsaturated substrates as a powerful synthetic tool of a wide
practical importance with a special emphasis on largeꢀscale catalytic processes performed in
industry.2 However, the majority of developed protocols is based on the use of transition
metal catalysts for the Si–H bond activation and subsequent hydrosilylation of substrates
bearing unsaturated carbonꢀcarbon and carbonꢀheteroatom bonds such as ketones, imines and

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nitriles.3 Catalysts based on zinc, tin and aluminium have also been used.4 Furthermore,
metalꢀfree methods involving acids, bases, alkali or tetraalkylammonium fluorides were also
proposed.5 A notable example is the recently developed catalytic system, where the Si–H
bond is activated by a system comprising a strongly Lewis acidic perfluorinated phosphonium
cation [(C6F5)3PF]+,6 eventually leading to a good performance in hydrosilylation of ketones,
nitriles and imines at room temperature. Furthermore, metalꢀfree organoꢀphothocatalytic
system for hydrosilylation of alkenes was also proposed.7 Finally, in the context of our work
it is especially important to note that strong organoboron Lewis acid B(C6F5)3 acts as a highly
effective hydrosilylation catalyst as shown and mechanistically studied by Piers8 and
Oestreich,9 respectively. This was complemented by successful isolation of a borole–silane
adduct,10 which apparently can be regarded as a model intermediate. In the recent striking
example used combination of B(C6F5)3/Al(C6F5)3 catalytic system for effective reduction of
CO2 to CH4.11 In the intramolecular version of this reaction mode, the Si–H bond can be
effectively activated. In the case of orthoꢀhydrosilylꢀdimesityloboronbenzene an
intramolecular hydride ligand transfer from silicon to boron centers was observed. Recent
developments in the field of Si
Corey.1
–H bond activation were summarized by Tilley, Oestreich and
Recently, the interest in our group has focused on benzosiloxaboroles. They can be
regarded as silicon benzoxaborole congeners and therefore were considered as potential
antimicrobial agents as well as receptors of biologically relevant diols. We have found that
benzosiloxaboroles can be conveniently obtained by a facile intramolecular dehydrogenative
condensation of arylboronic acids bearing a SiHR2 group at the ortho position in the presence
of water (Scheme 1).12 In our search for novel functionalized benzosiloxaboroles we have
undertaken the synthesis of derivatives bearing formyl or cyano groups attached at various
positions of the benzene ring.13 These studies resulted in the preliminary observation that the
activation of the Si–H by the adjacent B(OH)2 group enables reduction of the CHO group
under mild aqueous conditions. The proposed initial mechanical pathway for the activation of
the Si–H bond involved the intramolecular interaction with the Lewisꢀacid boronꢀbased group
resulting in the formation of Si–H…B bridge followed by the coordination of nucleophile to
silicon centre and elimination of H2 molecule. This is in accordance with previous findings by
Kawachi on the alcoholysis of 2ꢀ(dimesitylboryl)(dimethylsilyl)benzene14 and his later
mechanistic studies on intramolecular hydride ligand transfer from hydrosilyl group to orthoꢀ
located dimesitylboryl group.15 In this contribution we present our experimental and
computational results on various mechanistic scenarios accounting for Si–H bond activation.
We went beyond the boron case to get a more general mechanistic picture. Therefore, we
studied the effect of other orthoꢀassisting groups based on carbon and phosphorus.
Gratifyingly, developed methodologies allowed us to obtain new classes of silicon
heterocycles including a unique spiroꢀbis(siloxa)borinate system and phosphoxasilole.

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Scheme 1. Formation of benzosiloxaborole via boronic groupꢀassisted activation of the Si–H
bond.
Results and Discussion
Si–H bond activation with ortho-assisting boron-based groups. We have considered two
general Si–H activation modes for the observed hydrosilylations. The first one relies on Lewis
acid properties of the boron atom. This mechanical pathway resembles the reported activation
of silicon hydrides by moderateꢀstrong Lewis acidic dimesitylboryl group located at the
vicinity of hydrosilyl group.15 The hydride transfer from silicon to boron was also used for the
preparation of some organoboron hydrides.16 Obviously, the boron atom in B(OH)2 group is
not strongly Lewis acidic. Nevertheless, taking into account previous reports on related
systems,15,17 in our previous contribution we assumed that the proximity of silicon and boron
centres may promote the interaction between them via hydride Si–H…B bridge formation.12
However, to facilitate hydride migration, initial coordination of water to the silicon atom
should be invoked and this is consistent with the negative entropy change. On the other hand,
the presence of the boronꢀbound oxygen atom provides the possibility for intramolecular
oxygenꢀsilicon coordination which, in turn, may result in the Si–H bond activation. Such
interaction was previously invoked to account for the reactivity of arylsilanes bearing
hydroxymethyl18 and formyl groups.19
We have studied the mechanism of boronicꢀgroupꢀassisted hydrosilylation using 1ꢀ
bromoꢀ2ꢀ(dimethylsilyl)ꢀ3ꢀfluorobenzene 1 as a starting material. It is readily available by the
deprotonative lithiation/silylation of 1ꢀbromoꢀ3ꢀfluorobenzene.12a Compound 1 was subjected
to bromine/lithium exchange with tBuLi/Et2O at –90 °C to give 1-Li which was then
boronated with B(OMe)3 (Scheme 2). The resulting suspension of the ate complex 2 was
warmed to the room temperature, then evaporated and redissolved in THF. 10B and 29Si
HMBC NMR analyses revealed the presence of a substantial amount of a aryltrihydroborate
salt 3 containing the Si(OMe)Me2 group (δ10B = –30.6 ppm, δ29Si = 12.2 ppm) resulting from
the substitution of all methoxy groups at the boron atom (Figure 1). The spectra showed also
the formation of large amounts of the boronic ate complex 4 (δ10B = –0.9 ppm) as well as the
neutral boronate ester 5 (δ10B = 24.9 ppm), both featuring similar δ29Si values (7.9 and 8.4
ppm). One can generally assume that the activation of the Si–H bond is due to nucleophilic
attack of the anionic methoxy group on the silicon atom. In turn, this results in moving
hydride ligands from hypervalent silicon atom to boron. Apparently, the substitution of all
methoxy group at the boron atom leading to the formation of 3 is thermodynamically
favoured. Treatment of the obtained mixture with ethereal HCl resulted in the rapid hydrogen
1
evolution and conversion of 3 and 4 to 5. The formation of hydrogen was confirmed by H
NMR spectrum of a mixture obtained by hydrolysis of 2 with D2O in THFꢀd8; in fact, a weak

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but perceptible signal of HD was observed as a triplet (J = 42 Hz) centered at 4.48 ppm. In
contrast, when 2 was quenched at –70 °C with Si(H)ClMe2, the crude boronate ester 6 bearing
the Si(H)Me2 group (δ11B = 24.0 ppm, δ29Si = –17.2 ppm) resulted. This indicates that the
neutral B(OMe)2 group does not strongly activate the Si–H bond and the absence of an
intramolecular oxygenꢀsilicon coordination is confirmed by 29Si NMR data since δ29Si values
for related Si(H)Me2ꢀsubstituted arenes are in the range of –18 ꢀ –19 ppm. A minor product of
the latter reaction is presumably the related borinic derivative 7 as evidenced by the smaller
11B NMR signal (ca. 8%) at 42.2 ppm. Naturally, quenching of arylboronate salt 2 or esters 5,
6 with aqueous acids leads to condensation of B–OH and Si–OH fragments resulting in the
formation of previously reported benzosiloxaborole structure 8.12a However, the cleavage of
Si–H bond during hydrolysis of 6 in wet THF is slow since H2 evolution was completed only
after 2ꢀ3 days. This indicates that effective activation of Si–H bond proceeds via the
coordination of oxygen atom of the anionic boronate group to the silicon centre rather than
intramolecular hydride transfer to boron atom (the latter mechanism operates for stronger
triarylborane Lewis acids).
Scheme 2. Formation and transformations of Si(H)Me2ꢀsubstituted arylboronate ate complex
2.

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Figure 1. 10B NMR spectrum of a reaction mixture obtained upon treatment of 2 with
B(OMe)3.
Further studies revealed that the reaction of 1-Li with B(OiPr)3 in the ratio of 2:1
followed by hydrolysis gave a borinateꢀtype species 9 as a sole product in good yield (ca.
70%) (Scheme 3). The Xꢀray diffraction analysis revealed that its molecular structure features
the spiro arrangement of a central boron atom linking two benzosiloxaborole systems (Figure
2). Molecules form centrosymmetric dimers due to strong and symmetrical hydrogen bonds
(O…O distance is only 2.432 Å). Notable, the acidity of 9 (pKa = 3.5 in MeOH/H2O, 1:1) is
strongly enhanced with respect to the related benzosiloxaborole (pKa = 7.2 ppm). Respective
1H and 13C NMR spectra show broad single resonance of methyl groups, which points to a
dynamic character of the molecule at room temperature. According to VT NMR studies
(Figure S2, ESI), it seems that the Hꢀbonded dimeric structure of 9 is conformationally stable
(on the NMR time scale) at low temperatures in CDCl3. On the other hand, at higher
temperatures the Si(OH)ꢀB dative bond dissociates enabling the free rotation around the Bꢀ
C(aryl) bond, which in turn allows for coordination of SiꢀOH oxygen atom from an opposite
side. Notably, a totally different behavior of 9 is observed in acetone solution. Multinuclear
NMR and mass spectral data indicate that most probably it undergoes a slow isomerization
(within several hours) to a 8ꢀmembered cyclic borinic acid 10 comprising Si–O–Si linkage.
The process is reversible as after solvent evaporation the reꢀformation of single crystals of 9
was confirmed by Xꢀray diffraction. On the other hand, we have also found that compound 10
undergoes slow degradation in acetone solution after prolonged exposure to air (> 7 d). This
behavior is typical for diarylborinic systems. A detailed discussion of observed phenomena is
given in the ESI.
F
HO
1. B(OiPr)3 (0.5 equiv),
Et2O, -80 oC
S
i
acetone
(> 5 hrs)
acetone
(>7d)
B
OH
degradation
1-Li
B
2. rt, then H3O+
solvent
evaporation
O
Si
Si
F
F
O
Si
F
10
9

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Scheme 3. The synthesis of spiroꢀbis(siloxa)borinate derivative 9 and its subsequent
transformation to siloxane 10.
Figure 2. Crystal structure of 9 showing the formation of a dimer with two strong and
symmetrical hydrogenꢀbond O…H…O interactions.
Si–H bond activation with ortho-assisting carbon- and phosphorus-based groups. The
discussed above mechanism for the activation of Si–H bond via O…Si coordination should
also operate for other orthoꢀassisting groups. Thus, we have carboxylated a suspension of
aryllithium 1-Li in Et2O with gaseous CO2 at –90 °C. The mixture was evaporated to dryness
under reduced pressure to leave a white solid 11 well soluble in THF. The 29Si HMBC
spectrum of the solution in C6D6 showed a resonance at –26.0 ppm which points to some
additional shielding of silicon atom in SiHMe2 group due to interaction with the carboxylate
oxygen atom. Respective 1H NMR spectrum shows broadened resonances of protons from the
Si(H)Me2 group as well as two aromatic protons located ortho and meta to the carboxylate
group. This points to a fluxional behaviour of obtained compound, which can be due to the
lability of dative oxygenꢀsilicon interaction. However, it should be stressed that activation of
Si–H bond is strong as the addition of water results in vigorous H2 evolution giving rise to
benzosilalactone 12 (δ29Si = 20.5 ppm, Scheme 4). Formation of 5ꢀmembered silactone ring
was also confirmed by Xꢀray diffraction analysis (Figure 3). In another experiment, 1-Li was
quenched with DMF. Hydrolysis of the adduct afforded 1,3ꢀbis(2ꢀfluoroꢀ6ꢀformylphenyl)ꢀ
1,1,3,3ꢀtetramethyldisiloxane 14 (δ29Si = –1.36 ppm, Scheme 4) resulting from facile
cleavage of the Si–H bond. This suggests that activation of the Si–H bond is achieved through
the aminoalkoxide interaction with silicon atom in the intermediate 13. Hydrolysis leads to
cleavage of the Si–H bond followed by subsequent condensation of SiOH fragments to the
siloxane bridge (with concomitant liberation of pendant formyl groups).
In the next step we have studied the effect of a phosphorusꢀbased functional group.
The reaction of 1-Li with (EtO)2P(O)Cl and subsequent hydrolysis resulted in diethyl aryl
1
phosphonate 15 obtained in the initial step as a crude material and identified by H and 31P
NMR spectroscopy. This indicates that the phosphonate ester group does not activate the Si–H

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bond significantly. However, in the presence of water 15 undergoes slow transformation
(within several days) to a novel benzophosphoxasilole heterocycle 16 with concomitant H2
evolution. We suppose that hydrolysis of one of POEt groups is a rateꢀlimiting step en route to
16. It liberates the anionic Oꢀethyl phosphonate function which is able to activate the Si–H
bond effectively via generation of a hypervalent silicon species. Formation of 16 was clearly
established by 1H NMR spectrum showing signals of two nonꢀequivalent methyl groups at Si
centre. 29Si NMR chemical shift (δ29Si =16.85 ppm) also points to a formation of a 5ꢀ
membered ring involving the ArSi(CH3)2O moiety.12,13
O
O
OLi
H2O
- H2
CO2
O
Si
Si
Me2
H
F
F
12
11
NMe2
F
Me2
CHO
Si
H2O
- H2
DMF
OLi
O
1-Li
Si
Si
OHC
Me2
H
F
F
14
13
EtO
O
P(O)(OEt)2
P
H2O
-EtOH, -H2
O
(EtO)2P(O)Cl
Si
Si(H)Me2
F
F
15
16
Scheme 4. Si–H bond activation with selected orthoꢀassisting carbonꢀ and phosphorusꢀbased
groups.
Figure 3. Crystal structure of benzosilalactone 12.
Interestingly, trapping of 1-Li with PhCHO gave rise to two siliconꢀcontaining
compounds with concomitant formation of benzyl alcohol (Scheme 5). One can assume that
benzosilole 17 is produced via a mechanism similar to that operative for 12. The formation of

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diarylsilane 18 is rather unexpected and can be rationalized in terms of an attack of aryl
carbanion on silicon atom in mixed aryllithium/alkoxide aggregate, which can persist during
benzaldehyde quench.20 Presumably, Si–H bond activation in an intermediate fiveꢀ
coordinated silicon species results in hydride transfer to benzaldehyde. The crude reaction
mixture contained 17 and 18 in the ratio of 1:2. Both products were separated by column
chromatography. The structure of 18 was confirmed by HRMS analysis and multinuclear
1
NMR spectroscopy. Specifically, HMBC H,29Si NMR spectra showed two signals at –9.59
and –20.52 ppm which can be assigned to Ar2SiMe2 and ArSi(H)Me2 moieties, respectively.
Scheme 5. Reaction of 1-Li with benzaldehyde.
The observation of the formation of benzyl alcohol is in line with our recent studies on
synthesis of formylꢀsubstituted benzosiloxaboroles which showed that reduction of the CHO
group can also occur under aqueous acidic conditions required for initial hydrolysis of
corresponding acetal precursors.13 Consequently, the reducing potential of 2 towards various
functionalized benzaldehydes was investigated. In all cases, reduction of the formyl group
occured resulting in the formation of mixtures containing respective benzyl alcohols and
benzosiloxaborole 8 as a byꢀproduct (Scheme 6, for details, see Supporting Information,
Table S1). The reaction is chemoselective as other unsaturated functionalities such as CN,
COMe, COOEt and NO2 were inert. One can assume that the hydride transfer occurs directly
from the activated hypervalent silicon atom in 2 to the formyl group. This is supported by the
fact that the reagent 11 bearing carboxylate group has proved to be also effective reducing
agent towards the formyl group of 4ꢀcyanobenzaldehyde.
OH
CH2OH
CHO
B
2, then H3O+
O
+
Si
Me2
R
R
F
8
Scheme 6. Reduction of benzaldehydes with the boronic ate complex 2 (details are given in
the Supporting Information, Table S1).
Further insight into possible activation modes of the Si–H bond in the context of
hydrosilylation reaction was provided by the attempted synthesis of cyanoꢀsubstituted
benzosiloxaborole 20 from 3ꢀbromoꢀ2ꢀdimethylsilylbenzonitrile 19. In our recent study two
examples of related cyanoꢀsubstituted benzosiloxaboroles were obtained in good yields.13 To

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our surprise, a mixture contains mainly formylꢀsubstituted compound 21 resulted from the
reduction of cyano group and subsequent hydrolysis of imine (Scheme 7). Formation of 21
can be reasonably explained by means of a cooperative mechanism21 involving coordination
of oxygen atom from the boronate group to the silicon centre resulting in a concomitant
intramolecular hydride transfer to the electrophilic nitrile carbon atom.
Scheme 7. Reduction of the CN group via boronicꢀgroup assisted intramolecular
hydrosilylation.
Computational studies on plausible mechanistic pathways.
In order to gain a deeper insight into mechanism of observed Si–H bond activation we
have performed a series of DFT (M06ꢀ2X22/6ꢀ31+G(d)23) theoretical calculations in
Guassian09.24 Our intention was to (i) compare the reactivity of Si–H and Si–C bonds in the
fiveꢀcoordinated silicon intermediates resulted from the interꢀ or intramolecular reaction with
model base, (ii) evaluate the kinetics of Si–H bond activation effected by studied functional
groups attached at the ortho position, (iii) compare two considered activation modes of boronꢀ
containing group (via Si–H…B bridge formation or oxygenꢀsilicon coordination). To simplify
calculations, we have used unsubstituted phenyldimethylsilane PhSi(H)Me2 and analogous
ꢀ
compounds bearing anionic (CH2Oꢀ, CH(NMe2))Oꢀ, COOꢀ, B(OH)3 ) or neutral (B(OH)2,
P(O)(OEt)2) functional groups attached at the ortho position.
In this work we have clearly demonstrated that the presence of anionic or neutral
functional group bearing oxygen atom facilitates activation of Si–H bond in the neighboring
Si(H)Me2 group. However, this process may compete with the dissociation of siliconꢀaryl
bond. In a more recognized mechanistic pathway, the reaction of silanes with mild to strong
ꢀ
bases such as OHꢀ, CH3Oꢀ, NH2 involves formation of fiveꢀcoordinated silicon ateꢀcomplex
followed by elimination of allyl or aryl substituents.18,25 Indeed, the reaction of 1 with NaOH
(in aqueous or THF solutions) generates phenyl anion which is immediately quenched with
water or trapped with benzaldehyde under in situ conditions (Scheme 8). The latter reaction
was observed previously for related arylsilanes using TBAF as a base.26
Scheme 8. Testing the reactivity of arylsilane toward OHꢀ.

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To compare the reactivity of Si–H and Si–C bonds we have first optimized the
geometry of model hypervalent silicon complexes resulted from the interꢀ (A) and
intramolecular (C) reaction of arylsilane with base (Scheme 9, Table 1). As the stability of
such hypervalent adducts are affected by electronic features of the substituents we
supplemented these studies with corresponding perfluorinated analogues (B and D). To
provide more realistic estimation of dissociation energies the anionic charge of the used
molecular species were neutralized by the introduction of lithium cation. Furthermore,
calculations were performed using conductor polarizable continuum model with THF as a
o
solvent and the temperature set to 25 C. According to our estimations the nucleophilic
addition of MeOꢀ(Li+) to silicon gives small reaction barrier of 18 kJmolꢀ1 (A) and 6 kJmolꢀ1
(B). In an intramolecular version of this reaction, coordination of oxygen atom to silicon
centre is exothermic and equals to –22 kJmolꢀ1 (C and D). Subsequent decomposition
pathways of fiveꢀcoordinated silicon ate complexes are different for interꢀ and intramolecular
systems. Nevertheless, in all studied cases cleavage of Si–C(Me) bond is strongly disfavored
(ꢁG = 70ꢀ90 kJmolꢀ1). Comparison of intermolecular reaction modes (A and B) shows that the
cleavage of Si–C(Ar) bond is energetically more advantageous than dissociation of Si–H
bond, which is in accordance with our experimental observations. In turn, the elimination of
the hydride anion is more favored in the case of intramolecular oxygen coordination for C.
This can be partially ascribed to trans configuration of oxygen with respect to a hydride
ligand. However, fluorination of aromatic ring promotes desilylation, i.e., Si–C(Ar) bond
cleavage. Thus, for D both processes can operate to a similar extent.
Scheme 9. Possible reaction pathways for the dissociation of fiveꢀcoordinated silicon ate
complexes resulting from the interꢀ or intramolecular reaction of arylsilane with base.
Table 1. Gibbs free energies (kJmolꢀ1) for the dissociation of fiveꢀcoordinated silicon ate
complexes.
MeꢀLi+ PhꢀLi+ HꢀLi+
(A) [PhSi(H)(OMe)Me2] ꢀLi+
(B) [C6F5Si(H)(OMe)Me2] ꢀLi+
(C) [Ph(oꢀCH2O)Si(H)Me2] ꢀLi+
(D) [C6F4(oꢀCH2O)Si(H)Me2]ꢀLi+
75
72
80
90
–12
–34
68
23
12
15
21
19

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In the next step we have evaluated the kinetics of orthoꢀassisted activation of Si–H
bond via different anionic or neutral functional groups. The Si–H bond cleavage involves
hydride ligand transfer from the fiveꢀcoordinated silicon centre to the electrophile (water).
Thus, sixꢀmembered cyclic transition state structures have been proposed for observed
reactions. The calculations showed that the activation of Si–H bond in simple
aryldimethylsilane with water requires a large amount of energy (166 kJmolꢀ1), therefore the
process is not observed. Coordination of oxygen atom from a neighbouring functional group
to the silicon centre leads to weakening of the Si–H bond and considerably decreases
ꢀ
activation barrier (Table 2). In the case of anionic B(OH)3 , CH(NMe2)Oꢀ (Figure 4) groups
the activation energy ꢁEa equals to 68 kJmolꢀ1 and 88 kJmolꢀ1, respectively, and it drops to
only 8 kJmolꢀ1 for carboxylate group, while for CH2Oꢀ group, the coordination of oxygen
atom leads to the stable silicon ate complex (–17 kJmolꢀ1). Not surprisingly, for neutral
functional groups the kinetics are significantly less favorable (P(O)(OEt)2: ꢁEa = 99 kJmolꢀ1,
B(OH)2: ꢁEa = 120 kJmolꢀ1). In all studied cases the Si–H bonds are noticeably elongated in
TS (1.58ꢀ1.82 Å) with respect to starting compounds (1.49 Å). The previously considered
reaction pathway involving the formation of Si–H…B bridge required the coordination of
water molecule to the silicon centre and is kinetically unfavorable as the reaction barrier is
122 kJmolꢀ1, which is by 54 kJmolꢀ1 higher than the activation through coordination of the
ꢀ
B(OH)3 oxygen atom to silicon. Nevertheless, it is noticeable that both reaction pathways
may compete in neutral boronic compounds such as 6.
Table 2. Energy barriers for the Si–H bond activation processes in the reaction of 2ꢀR
substituted aryldimethylsilane with water or benzaldehyde (values in brackets).
a
b
R (neutral)
H
B(OH)2
120
B(OH)2
122
P(O)(OEt)2
99
ꢁE / kJmolꢀ1 166 (182)
ꢀ
R (anionic)
ꢁE / kJmolꢀ1 68 (63)
B(OH)3
CH2Oꢀ c
COOꢀ
8
CH(NMe2)Oꢀ
–17
88
avia coordination of B–O(H)…Si bond, via formation of Si–H…B bridge, located as
b
c
minimum on potential energy surface.

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Figure 4. The reaction pathway for activation of Si–H bond by boronate group in the presence
of water. Reaction pathways for other tested systems are presented in Supporting Information.
Considering the hydrosilylation of benzaldehyde, the reduction with boronated
arylsilane is again kinetically favored over the reduction with simple PhSi(H)Me2 (63 kJmolꢀ1
vs. 182 kJmolꢀ1). Furthermore, the obtained energy barriers are slightly lower when compared
to corresponding processes with water, however, the latter process may still occur to some
extent. In the specific case of cyanoꢀsubstituted compound 19, reduction apparently involves
an intramolecular hydride transfer from fiveꢀcoordinated silicon center to the adjacent
electrophilic carbon atom. According to DFT calculations the reaction barrier is 82 kJmolꢀ1,
which is by 14 kJmolꢀ1 higher than dehydrogenation with water. However, since the reduction
of CN group is additionally favored (due to its intramolecular character), both reactions can
be observed experimentally.
Conclusions
In our previous work we proposed that the activation of the Si–H bond by adjacent B(OH)2
can be explained assuming the transition state featuring Si–H…B bridge in accordance with a
mechanism observed for other (and generally stronger) boron Lewis acids. However, a closer
inspection revealed that the process is driven by a coordination of an oxygen atom from the
ꢀ
B(OH)2, or more probable – the anionic B(OH)3 group (ꢁE = 63 kJmolꢀ1), to the silicon atom.
This results in a facile release of a hydridic ligand which in the absence of an external
ꢀ
electrophile is transferred to the boron atom resulting in formation of the ArBH3 anion as one
of final products. The activation of two Si–H bonds in a respective diarylborinic system gives
rise to an unprecedented spiroꢀbis(siloxa)borinate derivative which is a relatively strong
Brønsted acid and shows dynamic behavior in solution. The activating effect of other related
functional group bearing an anionic or neutral oxygen atom such as COOꢀ and P(O)(OEt)2
was also observed. The products of these reactions were respective benzoxasiloles including
heterocycles featuring Si–O–C(O) or Si–O–P linkage. The activation effect was also observed
in the case of CH(NMe2)Oꢀ and CH(Ph)Oꢀ groups, however, acyclic compounds 14 and 18,

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were obtained as main products. The activated Si–H bond is also able to reduce various
benzaldehydes. Finally, theoretical calculations indicate that the intramolecular coordination
of silicon atom effectively prevents the cleavage of Siꢀaryl bond. In contrast, this process is
favored in the case of an intermolecular reaction when arylsilane of the type ArSi(H)Me2 is
attacked by an external oxygen nucleophile such as MeOꢀ. Finally, calculated reaction barriers
for tested arylsilanes strongly depend on charge and nucleophilicity of orthoꢀassisting
functional group.
Experimental Section
General comments. Solvents used for reactions were dried by refluxing with
sodium/benzophenone and distilled under argon. Starting materials including various benzene
derivatives, Si(H)ClMe2, B(OMe)3 were used as received without further purification. NMR
1
11
13
spectra were recorded on 300 MHz (Bruker AvanceꢀIII, nucleus: H, B, C, 19F), 400 MHz
13
19
1
(Agilent, nucleus: 1H, C, F) and 500 MHz (Agilent, nucleus: H, 10B, 29Si) spectrometers.
In the 13C NMR spectra the resonances of boronꢀbound carbon atoms were not observed in
most cases because of their broadening by a quadrupolar boron nucleus. H, 13C and 29Si
1
NMR chemical shifts are given relative to TMS. 10B, 11B and 19F NMR chemical shifts are
given relative to BF3ꢂEt2O and CFCl3, respectively.
Synthesis:
Spiro-bis(siloxa)borinate 9. A solution of 1ꢀbromoꢀ3ꢀfluoroꢀ2ꢀdimethylsilylbenzene 1 (2.33
g, 10 mmol) in Et2O (10 mL) was added dropwise to a solution of tꢀBuLi (1.7 M in pentane,
12 mL, 20 mmol) in Et2O (30 mL) at –90 °C. A mixture was stirred for 20 min at –90 °C and
the obtained white suspension of 1-Li was treated with B(OiPr)3 (1.15 mL, 5 mmol). The
mixture was allowed to warm slowly to the ambient temperature with stirring and quenched
with 1 M aq. H2SO4. Hydrogen evolution was observed immediately. The aqueous phase was
separated followed by the extraction with Et2O (2 × 20 mL). The extracts were added to the
organic phase, which was dried with anhydrous MgSO4, filtered and concentrated under
reduced pressure. Solvents were removed to leave a white waxy residue which was stirred
overnight with hexane (10 mL). The resulting suspension was filtered and the crude product
was crystallized from CHCl3/hexane (1:1) to give 8 as colorless crystals, m.p. 173ꢀ175 °C.
1
Yield: 1.2 g (69%). H NMR (400 MHz, CDCl3) δ 7.33ꢀ7.23 (m, 2H), 6.94–6.79 (m, 4H),
0.29 (broad, 12H) ppm. 1H NMR (300 MHz, Acetoneꢀd6) δ 7.32 (ddd, J = 8.1, 7.2, 5.7 Hz,
2H), 6.96ꢀ6.87 (m, 4H), 0.48 (s, 12H) ppm. 1H NMR (300 MHz, D2O/DMSO/K2CO3) δ 7.07
13
(ddd, J = 8.0, 7.1, 5.8 Hz, 2H), 6.77–6.61 (m, 4H), 0.36 (s, 6H), 0.32 (s, 6H) ppm. C NMR
(101 MHz, CDCl3) δ 165.05 (d, J = 244.3 Hz), 160.95, 132.57 (d, J = 6.3 Hz), 125.44 (d, J =
2.7 Hz), 124.51 (d, J = 28.7 Hz), 112.53 (d, J = 23.7 Hz), 0.38 (broad) ppm. 11B NMR (96
MHz, CDCl3) δ 12.6 ppm. 11B NMR (96 MHz, Acetoneꢀd6) δ 17.6 ppm. 11B NMR (96 MHz,
D2O/DMSO/K2CO3) δ 8.4 ppm. 19F NMR (376 MHz, CDCl3) δ –104.16 (ddd, J = 8.0, 5.7, 2.5
Hz) ppm. 19F NMR (282 MHz, Acetoneꢀd6) δ –105.96 (ddd, J = 8.1, 5.7, 2.6 Hz) ppm. 29Si
NMR (99 MHz, CDCl3) δ 24.40 ppm. 29Si NMR (99 MHz, D2O/DMSO/K2CO3) δ 12.90 (d, J
= 2.3 Hz) ppm. HRMS (EI): calcd. for C16H19BF2O2Si2 [MꢀCH3]+ 333.0750 found 333.0758.

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Silalactone (12). The suspension of 1-Li was obtained as described above. It was cooled to –
100 °C with vigorous stirring and carboxylated by saturation with dried gaseous CO2. The
resulting white suspension was warmed slowly to the room temperature and was quenched
with 2 M aq. H2SO4 to reach the pH = 1ꢀ2. The aqueous phase was separated followed by the
extraction with Et2O (2 × 20 mL). The extracts were added to the organic phase, which was
dried with anhydrous MgSO4 and concentrated under reduced pressure. A viscous oily residue
was dissolved in hexane (5 mL) and cooling to –30 °C gave the product 12 as colorless
1
crystals, m.p. 64ꢀ66 °C. Yield: 0.40 g (83%). H NMR (400 MHz, CDCl3) δ 7.89 (ddd, J =
7.5, 1.4, 0.7 Hz, 1H), 7.63 (ddd, J = 8.1, 7.5, 5.4 Hz, 1H), 7.30 (ddd, J = 8.1, 6.8, 0.7 Hz, 1H),
0.64 (s, 6H) ppm. 13C NMR (101 MHz, CDCl3) δ 166.84 (d, J = 1.5 Hz), 164.45 (d, J = 247.6
Hz), 139.36 (d, J = 10.0 Hz), 134.18 (d, J = 6.9 Hz), 126.76 (d, J = 35.1 Hz), 123.72 (d, J =
3.3 Hz), 119.93 (d, J = 24.0 Hz), –1.52 ppm. 19F NMR (376 MHz, CDCl3) δ –101.80 ꢀ –
101.88 (m) ppm. 29Si NMR (99 MHz, CDCl3) δ 20.59 ppm. HRMS (EI): calcd. for
C9H9FO2Si [M]+ 196.0356; found 196.0352.
1,1’-Bis(2-fluoro-6-formylphenyl)tetramethyldisiloxane (14)
1H NMR (400 MHz, CDCl3) δ 10.31 (s, 1H), 7.72 (d, J = 7.5 Hz, 1H), 7.50 (td, J = 7.8, 5.4
Hz, 2H), 7.19 (t, J = 8.7 Hz, 1H), 0.50 (d, J = 2.6 Hz, 8H) ppm. 13C NMR (75 MHz, CDCl3) δ
192.54 (d, J = 2.7 Hz), 167.07 (d, J = 244.4 Hz), 143.59 (d, J = 8.7 Hz), 131.78 (d, J = 9.3
Hz), 126.81 (d, J = 26.9 Hz), 126.03 (d, J = 2.6 Hz), 120.69 (d, J = 28.6 Hz), 3.43 (d, J = 4.3
Hz) ppm. 29Si{1H} NMR (99 MHz, CDCl3) δ –1.36 ppm. HRMS (EI): calcd. for
C18H20F2O3Si2 [MꢀCH3]+ 363.0684 found 363.0695.
4-Fluoro-(P-ethoxy)(P-oxo)benzophosphoxasilole (16)
To a suspension of 1-Li (5 mmol), P(O)(OEt)2Cl (0.75 mL, 5 mmol) dissolved in 5 mL Et2O
was added dropwise at –95 °C. The resulting solution was warmed to 0 °C and hydrolyzed
with 1.5 M H2SO4 to reach pH 2. Then the resulting mixture was warmed to 30 °C and stirred
for 24 h. The aqueous phase was separated followed by the extraction with Et2O (2 × 10 mL).
The combined organic phases were washed with 0.01 M NaOH and brine. The organic phase
was dried with anhydrous MgSO4 and concentrated under reduced pressure giving the final
1
product as viscous oil (1.10 g, 84%). H NMR (400 MHz, CDCl3) δ 7.76ꢀ7.40 (m, 2H, Ar),
7.20 (t, J = 7.6 Hz, 1H, Ar), 4.10ꢀ4.00 (m, 2H, OEt), 1.33 (t, J = 7.1 Hz, 3H, OEt), 0.61 (s,
3H, SiMe), 0.58 (s, 3H, SiMe) ppm. 13C NMR (101 MHz, CDCl3) δ 164.70 (dd, J = 247.3,
25.3 Hz), 138.98 (dd, J = 181.6, 8.7 Hz), 134.26 (dd, J = 17.1, 6.9 Hz), 127.84 (dd, J = 34.6,
28.5 Hz), 124.83 (dd, J = 13.3, 3.4 Hz), 118.45 (dd, J = 23.5, 3.1 Hz), 62.79 (d, J = 6.2 Hz),
16.40 (d, J = 6.3 Hz), –0.52 (d, J = 2.8 Hz), –0.68 (d, J = 2.8 Hz) ppm. 19F NMR (376 MHz,
CDCl3) δ –101.95 ꢀ –102.17 (m) ppm.31P{1H} NMR (162 MHz, CDCl3) δ 20.80 (d, J = 4.5
Hz) ppm. 29Si{1H} NMR (99 MHz, CDCl3) δ 16.85 (dd, J = 7.8, 4.0 Hz) ppm. HRMS (EI):
calcd. for C10H14FO3PS [M]+ 260.0434, found 260.0430.
1,1-Dimethyl-7-fluoro-3-phenylbenzoxasilole
fluorophenyl][2-(phenylhydroxymethyl)-6-fluorophenyl]dimethylsilane
(17)
and
[2-(dimethylsilyl)-3-
(18). The
suspension of 1-Li (10 mmol scale) was obtained as described above. It was treated with a
solution of PhCHO (2.12 g, 20 mmol) in Et2O (10 mL) at –90 °C. A resulting white
suspension was warmed slowly to 0 °C and quenched with 2 M aq. H2SO4 to reach the pH =

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1ꢀ2. The aqueous phase was separated followed by the extraction with Et2O (2 × 20 mL). The
extracts were added to the organic phase, which was dried with anhydrous MgSO4 and
concentrated under reduced pressure. According to 1H NMR, the proportion of 17:18 is 1:2.
A viscous oily residue was subjected to column chromatography (silica gel, eluent
hexane/EtOAc, 20:1) to give 17 as a white solid and 18 as a colorless viscous oil. The fraction
of benzyl alcohol was also isolated.
Compound 17: m.p. 56ꢀ57 °C, 0.43 g. 1H NMR (300 MHz, CDCl3) δ 7.49ꢀ7.17 (m, 6H), 6.99ꢀ
6.89 (m, 1H), 6.88ꢀ6.80 (m, 1H), 6.18 (s, 1H, CH), 0.62 (s, 3H, Me), 0.54 (s, 3H, Me) ppm.
19F NMR (282 MHz, CDCl3) δ –102.19 ꢀ –102.24 (m) ppm. 13C NMR (75 MHz, CDCl3) δ
164.9 (d, J = 244.1 Hz), 155.6 (d, J = 10.2 Hz), 143.2 , 132.7 (d, J = 7.1 Hz), 128.6, 128.0,
127.1, 121.7 (d, J = 36.8 Hz), 119.8 (d, J = 3.2 Hz), 113.1 (d, J = 24.2 Hz), 84.0 , 1.2 , 0.4
ppm. 29Si NMR (99 MHz, CDCl3) δ 27.24 ppm. HRMS (EI): calcd. for C15H15FOSi [M]+
258.0876; found 258.0869.
1
Compound 18: 0.76 g. H NMR (500 MHz, CDCl3) δ 7.52 (dt, J = 7.3, 1.2 Hz, 1H), 7.41
(ddd, J = 8.2, 7.3, 6.0 Hz, 1H), 7.35ꢀ7.18 (m, 4H), 7.10ꢀ7.01 (m, 3H), 6.98 (dd, J = 7.8, 1.0
Hz, 1H), 6.93 (ddd, J = 9.5, 8.1, 1.0 Hz, 1H), 5.87 (d, J = 3.1 Hz, 1H, CH), 4.60ꢀ4.50 (m, 1H,
SiH), 1.66 (d, J = 3.7 Hz, 1H, OH), 0.77 (d, J = 3.5 Hz, 3H, Ar2SiMe2), 0.70 (d, J = 2.2 Hz,
3H, Ar2SiMe2), 0.15 (ddd, J = 11.6, 3.8, 1.9 Hz, 6H, Si(H)Me2) ppm. 13C NMR (75 MHz,
CDCl3) δ 168.42 (d, J = 241.4 Hz), 166.94 (d, J = 242.0 Hz), 151.22 (d, J = 8.4 Hz), 149.52
(dd, J = 6.7, 1.5 Hz), 143.20 , 131.75 (d, J = 9.7 Hz), 131.51 (d, J = 8.2 Hz), 130.10 (d, J =
2.5 Hz), 129.69 (d, J = 26.8 Hz), 128.19 , 127.26 , 126.50 , 124.74 (d, J = 26.0 Hz), 124.47 (d,
J = 2.7 Hz), 115.90 (d, J = 26.3 Hz), 114.54 (d, J = 27.6 Hz), 73.30 (d, J = 2.1 Hz), 2.55 (d, J
= 5.3 Hz), 2.51 (d, J = 5.5 Hz) –3.71 (dd, J = 9.1, 3.5 Hz) ppm. 19F NMR (282 MHz, CDCl3)
δ –93.29 ꢀ –95.73 (m), –97.17 ꢀ –98.27 (m) ppm. 29Si HMBC NMR (99.3 MHz, CDCl3) δ –
9.59, –20.52 ppm. HRMS (ESI): calcd. for C23H26F2OSi2 [M+Na]+ 435.1388; found
435.1377.
3-Bromo-2-(dimethylsilyl)benzonitrile (17): A solution of 3ꢀbromobenzonitrile (18.2 g, 0.1
mol) in THF (80 mL) was added at –100 °C to a stirred solution of LDA, freshly prepared
from diisopropylamine (10.5 g, 0.105 mol) and nBuLi (10 M, 10 mL, 0.1 mol) in THF (150
mL). After ca. 30 min stirring at ca. –95 °C chlorodimethylsilane (10.0 g, 0.106 mol) was
added slowly. The mixture was stirred for 30 min at –85 °C and then allowed to warm to the
room temperature. Solvents were removed under reduced pressure and the residue was
redissolved in hexane (100 mL). The mixture was filtered under argon through a Celite pad to
remove LiCl byproduct. The filtrate was concentrated and the residue was subjected to a
fractional distillation under reduced pressure. The product was obtained as a colorless liquid,
b.p. 84ꢀ87 °
C (1 Tr). Yield 20.7 g (86%). 1H NMR (400 MHz, CDCl3) δ 7.74 (dd, J = 8.1, 1.1
Hz, 1H), 7.65 (dd, J = 7.7, 1.1 Hz, 1H), 7.30 (t, J = 7.9 Hz, 1H), 5.03 (sp, J = 3.9 Hz, 1H),
0.55 (d, J = 3.9 Hz, 6H) ppm. 13C NMR (100.6 MHz, CDCl3) δ 142.51, 136.94, 132.90,
131.38, 130.91, 120.39, 118.34, –3.44 ppm. Anal. Calcd for C9H10BrNSi (240.17): C, 45.01;
H, 4.20. Found: C, 44.95; H, 4.54.
7-Cyano-1,3-dihydro-3-hydroxy-1,1-dimethyl-1,2,3-benzosiloxaborole (18) and 7-formyl-
1,3-dihydro-3-hydroxy-1,1-dimethyl-1,2,3-benzosiloxaborole (19): Compound 17 (2.40 g,

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0.01mol) in 5 mL of THF was slowly added to a stirred solution of tꢀBuLi (1.7 M, 12.0 ml,
o
0.02 mol) in THF/Et2O (15/15 mL) at –75 C. After 1 h of stirring a solution of B(OEt)3 (3.4
mL, 0.02 mol) in 5 mL of THF was added dropwise. After 0.5 h of stirring the solution was
o
warmed to –15 C and hydrolyzed with 1.5 M H2SO4 to pH = 3ꢀ4. The water phase was
separated followed by extraction with Et2O (20 mL). Organic phases were combined, dried
with anhydrous MgSO4 and concentrated under reduced pressure. The precipitate was
o
dissolved in hexane, then water was added and the mixture was heated to 60 C. The hexane
o
phase was separated and cooled to –20 C, which resulted in product crystallization. It was
filtered and dried under reduced pressure to give the mixture of 18 and 19 in the ratio of 1:4.
1
Due to a similar character of obtained compounds their attempted separation failed. H NMR
(300 MHz, CDCl3): 17: δ 8.03 (dd, J = 7.4, 1.0 Hz, 1H, Ph), 7.77 (dd, J = 7.7, 1.0 Hz, 1H,
Ph), 7.58 (t, 1H, J = 7.6, Ph), 5.62 (s, 1H, OH), 0.58 (s, 6H, SiMe2); 18: 10.06 (s, 1H, CHO),
8.09 (dd, J = 7.3, 1.0 Hz, 1H, Ph), 7.94 (dd, J = 7.5, 1.1 Hz, 1H, Ph), 7.69 (t, J = 7.4 Hz, 1H,
Ph), 5.71 (s, 1H, OH), 0.51 (s, 6H, SiMe2) ppm.
Testing the reactivity of 1 with NaOH. (a) Reaction in water. A solution of compound 1
(0.23 g, 1mmol) in 3 mL of THF was added to the 10 mL of 5% NaOHaq. The reaction was
o
stirred at 40 C for 1h. The evolution of hydrogen gas was observed. The reaction was
extracted with Et2O (5 mL) and organic phase was concentrated in vacuo. The GCꢀMS
analysis showed complete desilylation of 1 to give 1ꢀbromoꢀ3ꢀfluorobenzene; (b) Reaction in
THF. A solution of 1 (0.23 g, 1 mmol) was mixed with benzaldehyde (0.106 g, 1 mmol) in
THF and a saturated solution of NaOH (0.040 g, 1 mmol) in THF was added. The reaction
o
was stirred at 40 C for 1 h. The organic phase was washed with brine and concentrated in
vacuo. The GCꢀMS analysis show the formation of (2ꢀbromoꢀ6ꢀfluorophenyl)(phenyl)ꢀ
methanol (88% conversion with respect to 1) along with 1ꢀbromoꢀ3ꢀfluorobenzene (12% with
respect to 1), benzyl alcohol (6% conversion with respect to PhCHO), and benzoic acid (6%
conversion with respect to PhCHO), two latter products presumably resulting from
Cannizzaro reaction of PhCHO.
Structural measurement and refinement details. The single crystals of 9 and 12 were
measured at 100 K on SuperNova diffractometer equipped with Atlas detector (CuꢀKα
radiation, λ = 1.54184 Å). Data reduction and analysis were carried out with the CrysAlisPro
program.27 All structures were solved by direct methods using SHELXSꢀ9728 and refined
using SHELXLꢀ2014.29 All nonꢀhydrogen atoms were refined anisotropically.
Crystallographic Information Files (CIFs) have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publications no. 1581785 (9) and 1581784
(12).
Crystal data for 9: C16H19BF2O2Si2, Mr = 348.30 a.u.; monoclinic; C 2/c; a = 20.3719 (4) Å, b
= 8.5430 (2) Å, c = 20.6238 (5) Å, β = 90.992 (2)°, V = 3588.77 (14) Å3; dcalc = 1.289 gꢂcm−3;
ꢁ = 0.221 mm−1; Z = 8; F(000) = 1456; number of collected / unique reflection (Rint = 2.25%)
(min/max)
= 24087 / 6060, R[F] / wR[F] (I ≥ 3σ(I)) = 3.45 % / 10.15%, ∆ϱres
eꢂÅ−3.
= −0.51/ +0.47
Crystal data for 12: C9H9FO2Si, Mr = 196.25 a.u.; monoclinic; C 2/m; a = 17.2067 (7) Å, b =
7.1378 (3) Å, c = 7.5388 (3) Å, β = 100.265 (4)°, V = 911.08 (7) Å3; dcalc = 1.431 gꢂcm−3; ꢁ =

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0.235 mm−1; Z = 4; F(000) = 408; number of collected / unique reflection (Rint = 3.41%) =
10042 / 1797, R[F] / wR[F] (I ≥ 3σ(I)) = 3.61 % / 11.16%, ∆ϱres(min/max) = −0.33/ +0.58 eꢂÅ−3.
Computational methods. All geometry optimizations and frequency calculations were
carried out with the GAUSSIAN0924 suite of programs and M06ꢀ2X22 functional with 6ꢀ
31+G(d)23 was applied to calculate the optimal geometries. The minima were confirmed by
vibrational frequency calculations [M06ꢀ2X/6ꢀ31+G(d)] within harmonic approximation (no
imaginary frequencies). To optimize the structures of transition states, a synchronous transitꢀ
guided quasiꢀNewton approach (QST3) was applied. In this method, three input structures are
needed: one corresponds to reactants, one to products and one is a guess of a transition state.
To verify the structures of the transition states, frequency calculations were carried out at the
same level of theory. One imaginary frequency was found in all cases except of CH2Oꢀ
functional group, where corresponding state bearing fiveꢀcoordinated silicon centre was
located as minimum on potential energy surface. As all reductions processes proceed in THF,
the computations were performed using reaction field calculation with the integral equation
formalism model [SCRF(CPCM, solvent = THF)].30 The geometry of optimized structures
and transition states are provided in Supporting Information. Gibbs free energies were
obtained from the frequency calculations with the temperature set to 298 K.
Acknowledgment
This work was supported by Warsaw University of Technology. K. Durka thanks the
Foundation for Polish Science for financial support within the START program. The Xꢀray
measurements were undertaken in the Crystallographic Unit of the Physical Chemistry
Laboratory at the Chemistry Department of the University of Warsaw. Authors thank Prof.
Krzysztof Woźniak for providing the access to this laboratory. We would like to thank Jacek
Olędzki from Institute of Biochemistry and Biophysics Polish Academy of Sciences for
performing MS analysis and Marcin Wilczek from Faculty of Chemistry, University of
Warsaw, for performing gaseous 1H NMR experiment. Authors thank the Interdisciplinary
Centre for Mathematical and Computational Modelling in Warsaw (G33ꢀ14) for providing
computational facilities.
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