Photo Lewis acid generators: Photorelease of B(C6F5)3 and applications to catalysis

Article abstract of DOI:10.1039/c5dt03008k

A series of molecules capable of releasing of the strong organometallic Lewis acid B(C₆F₅)₃ upon exposure to 254 nm light have been developed. These photo Lewis acid generators (PhLAGs) can now serve as photoinitiators for several important B(C₆F₅)₃-catalyzed reactions. Herein is described the synthesis of the triphenylsulfonium and diphenyliodonium salts of carbamato- and hydridoborates, their establishment as PhLAGs, and studies aimed at defining the mechanism of borane release. Factors affecting these photolytic reactions and the application of this concept to photoinduced hydrosilylation reactions and construction of siloxane scaffolds are also discussed.

Full text of DOI:10.1039/c5dt03008k

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DOI: 10.1039/C5DT03008K
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Photo Lewis acid generators: photorelease of B(C6F5)3 and
applications to catalysis†
Received 00th January 20xx,
Accepted 00th January 20xx
Andrey Y. Khalimon,*a,b Bryan K. Shaw,a Adam J. V. Marwitz,a Warren E. Piers,*a James M.
Blackwellc and Masood Parveza
DOI: 10.1039/x0xx00000x
A series of molecules capable of releasing of the strong organometallic Lewis acid B(C6F5)3 upon exposure to 254 nm light
have been developed. These photo Lewis acid generators (PhLAGs) can now serve as photoinitiators for several important
B(C6F5)3-catalyzed reactions. Herein is described the synthesis of the triphenylsulfonium and diphenyliodonium salts of
carbamato- and hydridoborates, their establishment as PhLAGs, and studies aimed at defining the mechanism of borane
release. Factors affecting these photolytic reactions and the application of this concept to photoinduced hydrosilylation
reactions and construction of siloxane scaffolds are also discussed.
www.rsc.org/
applications, but
a
method for preparing complex,
Introduction
monodisperse silicone structures that would be difficult if not
impossible to prepare using conventional methods.18, 19 These
Tris-(pentafluorophenyl)borane, B(C6F5)3, is a widely employed
borane Lewis acid for a host of catalytic applications.1, 2 Its
appeal stems from the fact that its properties of air and
moisture tolerance3, 4 (unusual for an organoborane) and very
strong Lewis acidity2, 5 (close to that of BF3) make for a catalyst
that is both easy to use and effective. Furthermore, after it
applications rely on the “Piers-Rubinsztajn reaction”,20
a
dealkylative siloxane synthesis generally depicted in equation
1. This versatile reaction can also be used to form polymeric
structures,21, 22 or as a vector for crosslinking macromolecules
and changing the physical properties of polysiloxane
materials.23
found initial application as
a
co-catalyst for olefin
In this context, we became interested in devising a way in
which the reaction could be triggered by an external stimulus,
rather than simply upon introduction of the catalyst to the
precursor mixture. Inducing a catalytic reaction by the some
external stimulus has many benefits in materials research since
one can, in principle, control the precise timing of the onset of
a catalytic reaction. Examples of external stimuli include
heat,24 mechanical force25 or, more commonly, irradiation with
a specific wavelength of light.26-28
polymerization catalysis,6 it became widely available from
several commercial sources, increasing its attractiveness.
Organic chemists began to explore its use as a Lewis acid
catalyst for organic transformations,7 and subsequent seminal
discoveries that it is capable of mediating Si-H8 and H-H9 bond
activations that can be incorporated into hydrosilation10-13 and
hydrogenation14, 15 catalytic cycles spawned a worldwide effort
in so-called “frustrated Lewis pair” chemistry.16, 17
The ability of B(C6F5)3 to activate Si-H bonds has lead not
only to the development of a number of hydrosilation
Light-induced catalysis is of particular importance in the
area of photolithography, where photoacid generators (PAGs)
R1
30
eject protons upon irradiation,29,
which mediate the
R2
deprotection of photoresist polymers and allow for patterning
to sub-20 nm resolution. Given the importance of B(C6F5)3 and
other perfluoroaryl boranes as Lewis acid catalysts for siloxane
synthesis and modification, we became interested in devising
molecules that, when irradiated, would liberate this borane. In
other words, we sought photo Lewis acid generators, or
“PhLAGs”. While the generation of vacant coordination sites
in transition metal complexes via photodissociation of, for
example, carbonyl ligands is well known, controlled
photorelease of main group element-based Lewis acids has far
less literature precedent. Photopolymerization of epoxides
R1
Si
Si
R3
H
R2
B(C6F5)3
(1-5%)
(1)
SiR3
O
+ H3C-H
+
R3
H3C-O-SiR3
with the use of aryl diazonium salts [ArN2][EXn] (E = P, n = 6; E
32
= B, n = 4) has been demonstrated in 1970s31,
and the
reactions are believed to be initiated by Lewis acidic EXn-1
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species, which are generated along with dinitrogen and aryl interaction of Lewis acidic Li+ with the carbonyl oxygen of the
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DOI: 10.1039/C5DT03008K
halides from diazonium salts upon exposure to UV light. anion. Analogously to 1-Ph3S, treatment of 2-LiTMEDA with
However, Lewis acids generated in both these cases are air and [Ph2I][Cl] gives a diphenyliodonium derivative 1-Ph2I; however,
moisture sensitive, which raises the question regarding the the reaction is less clean and requires more rigorous
true nature of the catalytic species. Furthermore, aryl purification protocols. This significantly reduces the yield of 1-
diazonium salts are often found to be thermally unstable and Ph2I (40%) in comparison with 1-Ph3S (79%). In a similar
decompose under ambient conditions.33 Therefore, PhLAG manner, the 2,2,6,6-tetramethylpiperidine (TMP, Me4C5NH)
molecules that are thermally robust and produce a well- derivative 2-Ph3S can be prepared by the ion exchange
defined Lewis acid efficiently are of potential value in materials reaction between [Ph3S][Cl] and the corresponding lithium
synthesis.
borate [Li][(Me4C5N)CO2B(C6F5)3] (3-Li) derived from lithium
In a preliminary communication,34 we reported preparation carbamate (Me4C5N)CO2Li (4; Scheme 1B). Borate 3-Li exhibits
of the first PhLAG molecule capable of the photorelease of greater stability compared to 2-Li and does not require
B(C6F5)3 and applications of this design to photoinduced additional stabilization with TMEDA. This phenomenon can
hydrosilylation and Piers-Rubinsztajn catalysis. The compound most likely be explained by a higher basicity of the carbamato
was a triphenylsulfonium-carbamatoborate ion pair, 1-Ph3S nitrogen centre in 3-Li vs. 2-Li, which allows for additional
(Chart 1), which decomposes under irradiation with 254 nm stabilization of the lithium cation preventing its interaction
light to release one equivalent of B(C6F5)3, along with other with the carbonyl oxygen of the anion and thus, inhibiting
products that were innocuous in terms of desired hydrosilation dissociation of 3-Li to 4 and free B(C6F5)3.
catalysis.
Borates 1-Ph3S, 1-Ph2I, and 3-Ph3S were fully characterized
by multinuclear NMR, IR, absorption spectroscopic techniques
and X-ray diffraction analysis for triphenylsulfonium salts 1-
Chart 1
Ph3S
1
Ph3S and 3-Ph3S. The signals for the cations appear in the H
X
NMR spectra at δ 7.42, 7.65 and 7.77 ppm for 1-Ph3S, δ 7.39,
7.57 and 7.73 ppm for 1-Ph2I, and δ 7.55, 7.73 and 7.85 ppm
for 3-Ph3S. Other characteristic spectroscopic data for borates
1-Ph3S, 1-Ph2I and 3-Ph3S include the respective boron
resonances at δ -4.1, -3.8 and -5.4 ppm in the 11B NMR spectra
and the respective bands in the IR spectra at 1700, 1676 and
1653 cm-1 for the stretches of the carbonyl groups of the
anions.
N
O
tBu
O
B(C6F5)3
O
O
3-Ph3S
N
B(C6F5)3
C6F5
C6F5
tBu
X: Ph3S, 1-Ph3S; Ph2I, 1-Ph2I
X
H
B
The molecular structure of 1-Ph3S has been described34
and is consistent with its formulation as a triphenylsulfonium
carbamatoborate anion pair. Figure 1A shows the molecular
structure of the TMP derived salt 3-Ph3S with selected metrical
data. The B1-O1 distance of 1.509(3) Å in 3-Ph3S is shorter
than those typically found in the adducts between carbonyl
C6F5
X: Ph3S, 7-Ph3S
Ph2I, 7-Ph2I
Here we report the full account of this research, including
preparation of a series of next generation triphenylsulfonium
and diphenyliodonium PhLAGs (Chart 1), their applications, as
well as a detailed study of the mechanisms of B(C6F5)3
photorelease and the factors affecting these photolytic
reactions.
35
compounds and B(C6F5)3 and is close to those observed for
the carbamatoborate 1-Ph3S (1.521(3) Å)34 and previously
reported
[TMPH][(Me4C5N)CO2B(C6F5)3]
[TMPH][HCO2B(C6F5)3]
(1.546(3)
(1.496(2)
Å)36
Å).37
and
The
triphenylsulfonium cation of 3-Ph3S is pointing away from the
anion showing interaction (3.025(2) Å) with the carbonyl
oxygen of the neighbouring molecule in the unit cell (Figure
1B). However, this is likely attributed to the molecular packing
effects of 3-Ph3S.
To probe whether or not borane release from ion pairs 1-
Ph3S, 1-Ph2I and 3-Ph3S was truly induced by absorption of a
photon, we performed several control experiments. First,
exposure of 1-Ph3S, 1-Ph2I and 3-Ph3S to an atmosphere of
13CO2 under ambient conditions for several hours resulted in
no incorporation of the isotopic label into the carbamato
carbon. This indicates that they are stable to loss of CO2 under
ambient conditions. Furthermore, each ion pair is thermally
robust up to moderate temperatures as indicated by
thermogravimetric (TGA) analysis, showing no decomposition
of 1-Ph3S, 1-Ph2I and 3-Ph3S up to 185 °C, 158 °C and 214 °C,
Results and discussion
Carbamato-derived PhLAGs
The synthesis of the triphenylsulfonium carbamatoborate 1-
Ph3S34 is summarized in Scheme 1A. In a series of sequential
steps we prepared a TMEDA stabilized lithium salt of carbamic
acid, 2-LiTMEDA, derived from 3,6-di-tert-butylcarbazole
(tBu2C12H6NH). Treatment of 2-LiTMEDA with B(C6F5)3 gives a
TMEDA
stabilized
lithium
carbamatoborate
[Li(TMEDA)][(tBu2C12H6N)CO2B(C6F5)3], which can be converted
to triphenylsulfonium salt 1-Ph3S via an ion exchange reaction
with commercially available [Ph3S][Cl].34 In the absence of
TMEDA, treatment of 2-Li with B(C6F5)3 gives a lithium
carbamatoborate 1-Li in a mixture with the starting materials
suggesting an equilibrium, which is most likely induced by the
2 | J. Name., 2012, 00, 1-3
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Scheme 1 Synthesis of carbamatoborates 1-Ph3S, 1-Ph2I and 3-Ph3S.
Fig. 1 (A) X-ray structure of 3-Ph3S (thermal ellipsoids drawn to 50% probability level). Hydrogen atoms are omitted for clarity. Selected bond distances (Å): B1-O1, 1.509(3); C11-
O1, 1.328(3); C11-O2, 1.226(3); C11-N1, 1.386(3). Selected bond angles (°): B1-O1-C11, 120.96(19). (B) View showing C=O…S bonding motif in the unit cell of 3-Ph3S: S1i…O2 =
S1…O2ii, 3.025(2) Å.
respectively (Figures S1-S3 in ESI). Sulfonuim ion pairs 1-Ph3S borate is not present in the structure of 1-Ph3S34 but a weak
and 3-Ph3S were also found to be stable in refluxing CH2Cl2 and contact is seen in 3-Ph3S (Figure 1B).
THF, whereas the iodonium derivative 1-Ph2I exhibits lower
Irradiation of 1-Ph3S,38 1-Ph2I and 3-Ph3S with 254 nm light
stability in solution; slow decomposition of 1-Ph2I (approx. in either CD3CN or a mixture of CD2Cl2 and CD3CN (8.7:1,
30% by NMR) in CD2Cl2 (ca. 9.0×10-3 M) was observed respectively) (2.1×10-3 M) results in the efficient release of
overnight at room temperature in the absence of light. Based B(C6F5)3, which is effectively trapped as its acetonitrile adduct
39
on the IR spectra, which showed a significant shift of the C=O CD3CN·B(C6F5)3.3,
Qualitatively, the rate of borane
band for 1-Ph2I (1676 cm-1) compared to 1-Ph3S (1700 cm-1), photorelease from 1-Ph3S, 1-Ph2I and 3-Ph3S depends on the
we hypothesize that instability of 1-Ph2I in solution could be a solvent system used (Table S1 and Figure S6 in ESI). Thus, the
result of a stronger interaction between Ph2I+ cation and the photolysis of 1-Ph3S and 1-Ph2I ion pairs in CD2Cl2/CD3CN
carbonyl group of the anion but this could not be confirmed (8.7:1; 2.1×10-3 M) proceeds to >99% within 12 min, whereas
crystallographically.
An
interaction
between
the less efficient borane release (58% and 46% conversion for 1-
triphenylsulfonium cation and the carbonyl moiety in the Ph3S and 3-Ph3S, respectively) observed in pure CD3CN under
the same conditions. Furthermore, we have previously
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demonstrated for 1-Ph3S that the generation of CD3CN·B(C6F5)3
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A slightly different pathway for the release of B(C6F5)3 is
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only proceeds when the light source is on; turning the lamp off proposed for the ion pair 3-Ph3S on theDbOaIs: i1s0.o10f3t9h/Ce5fDoTll0o3w00in8Kg
stops the photorelease of borane immediately.34 Similar observations. First, photolysis of 3-Ph3S in CD2Cl2 in the
behaviour was also observed for 3-Ph3S and control absence of acetonitrile does not give B(C6F5)3. Instead, NMR
experiments with 1-Ph3S and 3-Ph3S showed no borane analysis revealed the formation of a new borate product with a
photorelease within several days under ambient light. In 11B NMR signal at δ -3.6 ppm, characteristic of a four-
contrast, exposure of a solution of 1-Ph2I in CD2Cl2/CD3CN coordinate borate species. In the 1H NMR spectrum, this
(8.7:1) to ambient light leads to a slow (≈ 40% in 18 h) product shows a broad downfield NH resonance at δ 6.93 ppm,
formation of CD3CN·B(C6F5)3 indicating the increased light coupled in the 1H-15N HMQC NMR spectrum with the 15N NMR
sensitivity of diphenyliodonium cation vs. triphenylsulfonium signal at δ 85.0 ppm with 1J(N-H) ≈ 70 Hz (found from 1H-15N JC
cation.40 In the absence of acetonitrile as trapping reagent, HMQC NMR) (Figures S8 and S9 in ESI). Taking into account the
photolysis of CD2Cl2 solutions of 1-Ph3S and 1-Ph2I gives higher basicity of the amide moiety in 3-Ph3S (pKa of the
B(C6F5)3; however, the reactions were found to be less clean conjugated acid of TMP is 11.1)44 vs. 1-Ph3S, these
due to slow photodecomposition of borane, shown in separate spectroscopic features are consistent with the formation of a
experiments.
zwitterionic species [(Me4C5NH)CO2B(C6F5)3] (5, Scheme 3),
produced via protonation of the nitrogen atom of the anion in
3-Ph3S. Although compound 5 formally represents the product
of CO2 activation by the “frustrated Lewis pair” (FLP)
combination of 2,2,6,6-tetramethylpiperidine and B(C6F5)3,45
our previous studies on the reactivity of this FLP with carbon
dioxide revealed the reversible formation of an ammonium
carbamatoborate [TMPH][(Me4C5N)CO2B(C6F5)3] (3-TMPH)37
instead of 5. Borate 3-TMPH can be also viewed as a result of
deprotonation of 5 with tetramethylpiperidine, and, in fact,
treatment of 5 with one equivalent of TMP immediately gives
3-TMPH (Scheme 3). We have previously demonstrated that
formation of 3-TMPH from B(C6F5)3, TMP and CO2 is reversible
as evidenced by incorporation of 13C label into 3-TMPH upon
exposure to 13CO2.37 Based on the X-ray analysis of 3-TMPH,
which shows a hydrogen bond between the ammonium cation
and the carbamato carbonyl oxygen,37 we hypothesized that
the reverse process is aided by protonation of the carbamato
carbonyl oxygen. In contrast, zwitterion 5, formed in the
absence of excess base, was found to be stable under ambient
X
OH
O
hν, 254 nm
t
t
Carb
Bu2
Carb
Bu2
CD3CN or
CD2Cl2/CD3CN
O
B(C6F5)3
O
B(C6F5)3
1-H
- Ph3S+ or Ph2I+
photoproducts
(see refs. 29, 30)
X: Ph3S, 1-Ph3S; Ph2I, 1-Ph2I
B(C6F5)3
CD3CN
OH
O
t
tBu2CarbH
Carb
Bu2
- CO2
CD3CN•B(C6F5)3
2-H
tBu2Carb = 3,6-di-tert-butylcarbazolide (tBu2C12H6N-)
Scheme 2 Suggested mechanism for photorelease of B(C6F5)3 from 1-Ph3S.
We have previously suggested the mechanism of borane
photorelease from 1-Ph3S.34 An analogous mechanism can be
envisioned for the ion par 1-Ph2I with diphenyliodonium cation
being the source of protons30 instead of triphenylsulfonium
cation29 (Scheme 2). Taking into account the negligible basicity
of the carbazolide nitrogen centre due to the conjugation of its
lone pair with the aromatic system (the pKa of the conjugated
acid of carbazole is -4.9),41 we proposed that the reaction
starts with the photoinduced protonation of the carbonyl
group of the anion, which results in the formation of a
zwitterionic intermediate 1-H. The latter species
spontaneously ejects borane and the carbamic acid 2-H, which
undergoes decomposition to 3,6-di-tert-butylcarbazole and
CO2.42 This is supported by the observation of resonances at δ
153.4 ppm (assigned to 2-H) and 126.2 ppm (assigned to 13CO2)
in the 13C NMR spectrum of the photoproducts of irradiated
solutions of 1-Ph3S selectively labelled with 13C in the
carbamate carbon position (Figure S7, ESI). Furthermore, GS-
MS analysis of the reaction mixture after photolysis of 1-Ph3S
in CH2Cl2 revealed the presence of 3,6-di-tert-butylcarbazole
(51%)43 along with a mixture of Ph3S+ photoproducts:29 o-, m-
and p-(phenylthio)biphenyls (11.3%), Ph2S (28.8%) and
biphenyl (2.2%).
conditions and shows no exchange with either 13CO2 or a
47
different borane, B(p-H-C6F4)3.46,
However, addition of a
base, in the form of acetonitrile, to a solution of 5 in CD2Cl2
immediately leads to the release of CO2 and formation of
CH3CN·B(C6F5)3 and free TMP. This observation is consistent
with the facile formation of CD3CN·B(C6F5)3 upon photolysis of
CD3CN solutions of 3-Ph3S (2.1·10-3 mol/L) and suggests that
the reaction starts with the protonation of the nitrogen centre
of the anion (Scheme 3), followed by CD3CN-assisted proton
transfer to the carbonyl oxygen (as illustrated in 6) and
formation of the zwitterion 3-H. The latter salt spontaneously
ejects B(C6F5)3 and the carbamic acid 4-H, which decomposed
to TMP and CO2. Thus, PhLAGs of the type represented by
compounds 1 and 3 release borane via slightly different paths
depending on the basicity of the nitrogen center in the
carbamato unit in the anion.
We have previously demonstrated the efficacy of 1-Ph3S as
a PhLAG in photoinduced catalytic hydrosilylation of carbonyl
compounds, silation of alcohols, silanols and ethers as well as
siloxane forming Piers-Rubinsztajn reactions,34 all well
established transformations catalysed by B(C6F5)3. In the
absence of the UV light, no reactions were observed upon
addition of 1 mol % of 1-Ph3S to substrate/hydrosilane
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Ph3S
H
N
H
- CO2
H
hν, 254 nm
TMP
N
N
O
O
B(C6F5)3
+
2
N
H
CD2Cl2
+ CO2
N
O
O
O
B(C6F5)3
- Ph3S+
photoproducts
(see ref. 29)
B(C6F5)3
O
3-Ph3S
5
B(C6F5)3
3-TMPH
δ−
NCCH3
CD3CN
H
O
H+ transfer
N
CD3CN
CD3CN•B(C6F5)3
B(C6F5)3
O
6
B(C6F5)3
N
- CO2
OH
N
NCO2H
H
O
B(C6F5)3
4-H
TMP
3-H
Scheme 3 Generation of 5 from 3-Ph3S under 254 nm light and suggested mechanism for photorelease of B(C6F5)3 from 3-Ph3S in the presence of acetonitrile.
mixtures; however, irradiation of these mixtures at 254 nm for B(C6F5)3. We hypothesized that the hydridoborates
15 min results in the initiation of the catalytic reactions which [X][HB(C6F5)3] (X = Ph3S, 7-Ph3S; Ph2I, 7-Ph2I; Chart 1) would be
were complete in time consistent with the reactions catalysed ideal targets because photolysis of these salts should result in
by B(C6F5)3 itself. In contrast, instability of the iodonium liberation of more inert by-products than the basic amines of
derivative 1-Ph2I in solution under ambient conditions hamper the first generation PhLAGs described above.
its applications as a latent catalyst and this compound was not
Compounds 7-Ph3S and 7-Ph2I were prepared in good yield
as effective in catalytic trials. For 3-Ph3S, however, despite its via ion exchange reactions between the potassium salt
stability under ambient conditions and its demonstrated ability [K][HB(C6F5)3] (7-K)48 and either [Ph3S][Cl] or [Ph2I][Cl] (Scheme
to release B(C6F5)3 upon irradiation, no catalytic hydrosilylation 4). Both hydridoborates 7-Ph3S and 7-Ph2I are air and moisture
and Piers-Rubinsztajn reactions were observed even upon stable, thermally robust solids (Tdecomp = 245 °C and 130 °C,
longer exposure to UV light. We believe the lack of catalytic respectively, as determined by TGA; Figures S4 and S5 in ESI)
reactivity observed for 3-Ph3S vs. 1-Ph3S is associated with the and have been characterized by multinuclear NMR, IR and UV
difference in the mechanism of borane release and, in absorption spectroscopy and X-ray diffraction analysis for 7-
particular, with the formation of the zwitterionic intermediate Ph2I.
5 and the necessity for base mediated conversion of this salt
The 1H NMR spectra of 7-Ph3S and 7-Ph2I show the
into B(C6F5)3 and the carbamic acid 4-H (see Scheme 3). expected peaks for the Ph3S+ cation (δ 7.85, 7.74 and 7.66
However, attempts to elucidate the species that form from 3- ppm) and Ph2I+ cation (δ 8.07, 7.72 and 7.54 ppm),
Ph3S upon exposure to 254 nm light under catalytic conditions respectively, in addition to a broad quartet centered at δ 3.61
were unsuccessful and so the reasons for a lack of efficacy for ppm for the hydride signal. A complementary doublet is
this PhLAG remain ill defined.
located in the 11B NMR spectra of 7-Ph3S and 7-Ph2I pairs at δ -
25.3 ppm and δ -25.1 ppm, respectively, for the B-H moiety
1
Hydridoborate-derived PhLAGs
with J(B-H) ≈ 90 Hz. Moreover, the chemical shift difference
(∆m,p) between the meta and para F atoms of the C6F5 unit in
each species is consistent with a four coordinate borate as
Although all three ion pairs 1-Ph3S, 1-Ph2I and 3-Ph3S are
capable of photorelease of B(C6F5)3, the instability of the
iodonium salt 1-Ph2I and a lack of catalytic reactivity of 3-Ph3S
hamper application of these two borates as latent catalysts. It
appears that the basicity of the nitrogen centre and the
presence of the carbonyl group in the carbamato substituent
play the crucial role in the stability and reactivity of
carbamatoborate PhLAGs. Therefore, we sought PhLAG
molecules with a borane protecting group was simpler and less
prone to chemistry that complicated the smooth release of
C6F5
C6F5
C6F5
C6F5
[X]+[Cl]-
K
H
B
X
H
B
CH2Cl2
- KCl
C6F5
7-K
C6F5
X: Ph3S, 7-Ph3S
Ph2I, 7-Ph2I
Scheme 4 Synthesis of hydridoborates 7-Ph3S and 7-Ph2I.
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DOI: 10.1039/C5DT03008K
Fig. 2 (A) X-ray structure of 7-Ph2I (thermal ellipsoids drawn to 50% probability level). Hydrogen atoms except B-H hydride are omitted for clarity. Selected bond distances (Å): B1-
H1, 1.24(4); B1-C1, 1.632(5); B1-C7, 1.629(5); B1-C13, 1.636(5); I1-C19, 2.100(4); I1-C25, 2.102(4). Selected bond angles (°):C1-B1-H1, 103.3(19); C1-B1-C7, 112.8(3); C1-B1-C13,
107.9(3); C19-I1-C25, 96.29(14). (B) View showing B-H…I interactions (I1…H1i = H1…I1ii, 2.57(4) Å) and a π-π contact between the benzene ring and C6F5 group (centroid-centroid
distance = 3.605(6) Å) in the unit cell of 7-Ph2I.
expected.11 The structure of 7-Ph2I was confirmed by X-ray M) at 254 nm and monitoring the amount of borane release
crystallography (Figure 2). The B1-H1 bond distance of 1.24(4) through formation of CD3CN·B(C6F5)33, 39 by NMR spectroscopy
Å is within the range of [HB(C6F5)3]- for other reported (Table S1 and Figure S6 in ESI). The diphenyliodonium salt 7-
structurally characterized hydridoborate ion pairs.49-51 Ph2I was found to be the most efficient in releasing the borane
Although the iodonium ion is pointing away from the with essentially full conversion to the acetonitrile adduct after
hydridoborate unit, a look at the crystal packing shows the 12 min of irradiation, whereas 7-Ph3S shows 81% conversion
iodonium centre is oriented towards another borate anion in to CD3CN·B(C6F5)3 during the same exposure time. Similar to
the unit cell with non-bonding I•••H distance of 2.57(4) Å. A the carbamatoborate 1-Ph2I, ion pair 7-Ph2I shows increased
short π-π contact (3.605(6) Å) was also observed between one sensitivity to the ambient light when compared to its
of the phenyl substituents of iodonium ion and C6F5 group of triphenylsulfonium analogue, showing complete conversion to
the anion of another molecule (Figure 2).
CD3CN·B(C6F5)3 after 96 h in CD3CN, whereas only 14% adduct
The ability of 7-Ph3S and 7-Ph2I to behave as PhLAGs was
demonstrated by irradiation of acetonitrile solutions (2.1×10-3
Table 1 Photoinduced catalytic hydrosilylationa and Piers-Rubinsztajn reactionsb with hydridoborates 7-Ph3S and 7-Ph2I.
#
Substrate/Silane
PhLAG
Irradiation time, min
Conversion, % c
Products
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
PhMeC(O) / Et3SiH
7-Ph3S
7-Ph2I
7-Ph3S
7-Ph2I
7-Ph3S
7-Ph2I
7-Ph3S
7-Ph2I
7-Ph3S
7-Ph2I
7-Ph3S
7-Ph2I
7-Ph3S
7-Ph2I
7-Ph3S
7-Ph2I
15
10
15
10
10
5
>99
>99
90
>99
>99
>99
>99
>99
98e
PhMeCH(OSiEt3)
PhHC(O) / Et3SiH
Cyclohex-2-enone / Et3SiH
PhOH / Et3SiH
PhCH2(OSiEt3)
5
5
PhOSiEt3
Et2O / Et3SiH
10d
10d
15f
15f
15
15
15
15
Et3SiOEt (86%), (Et3Si)2O (12%)
Et3SiOEt (91%), (Et3Si)2O (9%)
Me3Si(OSiEt3)
>99e
>99
>99
>99
>99
>99
>99
Me3SiOH / Et3SiH
Me3SiOEt / PMHSg
Si(OMe)4 / TMCTSh
(MeHSiO)x(MeSi(OSiMe3)Oy
tetra(trimethoxysiloxy)TMCTS
a Conditions: Csubstrate = 0.25 M, 1.0 mol % of PhLAG, substrate : Et3SiH = 1:1 unless stated otherwise, CD2Cl2 (solvent), 254 nm. b Conditions: Csubstrate = 0.25 M, 1.0 mol %
of PhLAG, CD2Cl2 (solvent), substrate : hydrosilane = 1:1 unless stated otherwise, 254 nm. c Conversion of organic substrate (determined by 1H NMR). d After irradiation,
the sample was left for 2 h at RT in dark. e Conversion and ratio of products determined by GC-MS. f After irradiation, the mixture was left for 2 h at RT in dark. g PMHS
– poly(methylhydrosiloxane). h TMCTS - 2,4,6,8-tetramethylcyclotetrasiloxane, Si(OMe)4 : TMCTS = 4:1.
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT03008K
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ARTICLE
formation was observed for 7-Ph3S under these conditions. To solutions of d1-7-Ph3S and d1-7-Ph2I (deuterated in the
further investigate the potential of the hydridoborate species hydridoborate anion) were exposed to 254 nm light and
7-Ph3S and 7-Ph2I as PhLAGs, each was tested as a latent cleanly formed HD (δ 4.57 ppm, 1JH-D = 42.6 Hz). The detection
catalyst in the photoinduced hydrosilylation of carbonyl of
the
intramolecular
recombination
products
compounds, silation of alcohols, silanols and ethers and Piers- (phenylthio)biphenyl and iodobiphenyl in the GC-MS (Scheme
Rubinsztajn reactions. Over the entire substrate scope (Table 5) is further evidence that the main proton source is the cation
1; Tables S2 and S3 in ESI) both compounds displayed high itself and not the solvent. Also observed in these reactions
product yields, consistent with the thermal B(C6F5)3-catalyzed were significant quantities of benzene and d1-benzene (δ 7.39
reactions with the only difference being that shorter exposure ppm in methylene chloride53), indicating that the phenyl cation
times can be utilized with the diphenyliodonium salt 7-Ph2I can also liberate borane by abstraction of the hydride
owing to its increased photo-sensitivity. These data indicate (deuteride) from the counteranion.
that hydridoborates 7 release borane with high efficiency;
Photolysis of PhLAGs 7 in acetonitrile, which was used to
generally, shorter irradiation times are required for these quantify the efficiency of borane release through formation of
PhLAGs compared to the first generation PhLAG 1-Ph3S CD3CN·B(C6F5)3, was also examined. In the case of the
described above.34
triphenylsulfonium compound, the path to photoliberation of
Clearly, ion pairs 7 efficiently release B(C6F5)3 upon borane was apparently similar to that observed in methylene
irradiation and we were interested in determining the chloride. Here, H2 (δ 4.57 ppm) was the main product, along
dominant pathway for borane release in these PhLAG systems. with a similar distribution of diarylthioether products as
The photodegradation of the Ph3S+ and Ph2I+ cations has been determined by GC-MS analysis. Photolysis of 7-Ph2I, however,
studied in some detail and most of the products we observe did not produce any detectable H2. Instead, in addition to PhI
can be rationalized on the basis of these earlier studies.29, 30 and iodobiphenyl (detected by NMR spectroscopy and GCMS),
Essentially, excitation of the [Ph3S]+ and [Ph2I]+ leads to either a major product with a distinct singlet resonance at δ 3.42 ppm
1
homolytic or heterolytic cleavage of an S-C or I-C bond, in the H NMR spectrum was observed along with a minor
producing Ph• or Ph+, respectively (Scheme 5).52 These highly
amount of benzene (δ 7.37 ppm in acetonitrile53). Both of
these products were confirmed to originate from the
hydridoborate by repeating the reaction with [Ph2I][DB(C6F5)3]
(d1-7-Ph2I) in CH3CN and observing resonances at δ 3.42 and
7.40 ppm in the 2H NMR spectrum.54
While the origin of benzene is relatively clear, the identity
and origin of the product giving rise to the signal at 3.42 ppm
was shown to be a result of reaction of photoproduced phenyl
X
- Phn-1
E
Phn-1E
E
Phn-2
Ph
Ph
Ph
(solvent)
+
+
heterolytic
X
X
254 nm
proton
H
X
*
solvent
PhnE
PhnE
homolytic
excited
state
Ph•
n = 3, E = S
n = 2, E = I
- Ph•
X
E
Phn-2
(solvent)
Phn-1
E
HB(C6F5)3
254 nm
Scheme 5 Proton generation from photolysis of PhnE cations
HB(C6F5)3
Ph2I
HB(C6F5)3
C6H5
CH3CN
CH3CN
PhN
C
CH3
- PhI
reactive species then rapidly react with the sulphur or iodine
containing products of bond cleavage or solvent in reactions
that eject protons. When the counter anion is [HB(C6F5)3]-, the
consequence of this sequence would presumably be
protonation of the anion, loss of H2 and liberation of B(C6F5)3.
An analysis of the other photoproducts reveals that the path to
borane liberation is somewhat more complex than this and
dependant on the nature of the cation and the solvent in
which the photochemistry is conducted.
7-Ph2I
B(C6F5)3
CH3
N
C
Ph
H
7-Ph I,
254 nm
2
HB(C6F5)3
CH3
H
H
N
CH2CH3
N
C
When photolysis (254 nm) of 7-Ph3S and 7-Ph2I was carried
out in CD2Cl2 a resonance consistent with H2 production was
observed in the H NMR spectra for both PhLAGs, with no
Ph
Ph
H
B(C6F5)3
1
Scheme 6 Proposed path to secondary products in the photodegradation of PhLAG 7-
Ph2I
detectable amount of HD. This indicates that, under these
conditions, the proton that liberates borane (formed cleanly)
stems from the PhnE cation; this was confirmed when CD2Cl2
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cation and the acetonitrile solvent.55 Analysis using 13C APT The 13C label was introduced to the carbamato substituent
View Article Online
NMR and 1H-13C HSQC NMR experiments showed that the using CO2 (see SI for details). H NMRD(O4I:0100.1M03H9z/;C5CDDT20C3l020; 8δK,
resonance at 3.42 ppm could be assigned as a CH2 group. GC- ppm): 8.20 (d, 3JH-H = 8.8 Hz, 2H, Carb); 7.95 (s, 2H, Carb); 7.73
MS analysis of this product in the photolysis of 7-Ph2I in CH3CN (d, 3JH-H = 7.6 Hz, 4H, o-H, IPh2); 7.57 (t, 3JH-H = 7.6 Hz, 2H, p-H,
indicated it has an m/z of 121, which increases to 124 when IPh2); 7.39 (m, 6H, m-H of IPh2 (4H) and 2H from Carb); 1.42 (s,
13
1
changing the solvent to CD3CN. Furthermore, photolysis of d1- 18 H, 2 tBu). 1H NMR (400 MHz; CD3CN; δ, ppm): 8.07 (d, 3JH-H
=
7-Ph2I in CH3CN gives the same product with m/z of 123 8.8 Hz, 2H, Carb); 8.11 (d, 4JH-H = 2.0 Hz, 2H, Carb); 8.06 (d, 3JH-H
-
3
suggesting that two [DB(C6F5)3] units are involved in the = 7.6 Hz, 4H, o-H, IPh2); 7.72 (t, JH-H = 7.6 Hz, 2H, p-H, IPh2);
3
3
reaction. Based on comparison with an authentic sample, we 7.54 (t, JH-H = 7.6 Hz, 4H, m-H, IPh2); 7.45 (dd, JH-H = 8.8 Hz,
assign this product as N-ethylaniline, and suggest it is formed 4JH-H = 2.0 Hz, 2H, Carb); 1.42 (s, 18 H, 2 Bu). 13C{1H} NMR
t
t
via the path shown in Scheme 6. This proposal has some (100.6 MHz; CD2Cl2; δ, ppm): 32.2 (s, CH3 of Bu); 35.2 (s, Cq,
precedence in previously observed reactivity of phenyl cations tBu); 116.2 (s, CH, Carb); 116.3 (s, CH, Carb); 125.1 (s, CH,
with acetonitrile56 and is consistent with greater tendency of Carb); 125.69 (s, Cq, Carb); 125.72 (s, Cq, Carb); 133.7 (s, CH,
the [Ph2I]+ cation to release phenyl cations, compared to IPh2); 134.3 (s, CH, IPh2); 135.3 (s, CH, IPh2); 138.0 (s, Cq, Carb);
[Ph3S]+.30 Although it is plausible that adduct formation 145.7 (s, Cq, IPh2) 153.6 (s, Cq, CO2, found for 13C enriched
between this by-product and B(C6F5)3 could happen, we do not analogue); carbon signals of C6F5 were not observed due to
observe this. It is known that N-alkyl anilines form very weak broadening. 11B NMR (128.4 MHz, CD2Cl2; δ, ppm): -3.8 (br s).
adducts with B(C6F5)357 and so acetonitrile easily displaces this 19F{1H} NMR (376.5 MHz; CD2Cl2; δ, ppm): –136.3 (app d, 3JF-F
base.
=
18.8 Hz, 6F, o-F); –163.5 (t, 3JF-F = 18.8 Hz, 3F, p-F); –168.2 (br s,
6F, m-F). 19F{1H} NMR (376.5 MHz; CD3CN; δ, ppm): –137.1
(app d, 3JF-F = 18.8 Hz, 6F, o-F); –164.7 (t, 3JF-F = 18.8 Hz, 3F, p-
F); –169.7 (br s, 6F, m-F). UV-vis (CH2Cl2; 1.1·10-5 M; λ, nm (ε,
M-1·cm-1)): 234 (88.04×103); 291 (20.11×103); 310 (7.58×103);
323 (6.88×103). IR (nujol, selected bands): 1676 cm-1 (C=O).
ESI-MS: {C39H24BF15NO2]- 834.2 m/z. TGA (5.8 mg; N2 (60
mL/min); T = 23 °C – 450 °C (2.0 °C/min)): Tdecomp = 158 °C.
Conclusions
We have developed two families of photo Lewis acid
generators (PhLAGs) based on tryphenylsulfonium and
diphenyliodonium carbamato- and hydridoborate ion pairs
designed to efficiently release the strong Lewis acid, B(C6F5)3,
upon exposure to UV light. While the carbamatoborate
compounds release borane efficiently, the amine functions in
both the borate ions and the photo byproducts can interfere
with the catalytic function of the release borane in B(C6F5)3
catalyzed hydrosilation and silation reactions. The
hydridoborates 7, however, are more well-behaved since their
photoproducts in CH2Cl2 solution are largely innocuous (H2,
benzene) or very weakly Lewis basic (thioethers or aryl
iodides). Compounds 7 are also significantly easier to
synthesize and can be prepared on a gram scale as air stable
materials. Their use as photoinitiators for several reactions,
some of which are highly relevant to siloxane materials
synthesis, has been demonstrated. Given the prominence of
B(C6F5)3 in modern transition metal free catalysis, applications
of these PhLAGs to a variety of reactions are possible.
Preparation of [Li][(Me5C5N)CO2B(C6F5)3] (3-Li)
A solution of B(C6F5)3 (667 mg, 1.3 mmol) in CH2Cl2 (30 mL) was
added in one portion at room temperature to a solid
(Me4C5N)CO2Li (4) (249 mg, 1.3 mmol). The resulting solution
was stirred at room temperature for 1 h. All volatiles were
pumped off to leave a beige residue, which was washed with
hexanes (20 mL) and dried in vacuum. The product appears as
a beige powder. Yield: 838 mg, 92%. 1H NMR (400 MHz; CD2Cl2;
7
δ, ppm): 1.63 (m, 6H); 1.33 (s, 12 H). Li NMR (155.5 MHz;
CD2Cl2; δ, ppm): -0.6 (br s). 13C{1H} NMR (100.6 MHz; CD2Cl2; δ,
ppm): 16.1 (s); 30.2 (s); 40.8 (s); 55.6 (s); 138.0 (br s); 138.9 (br
s); 148.5 (br s); 149.3 (br s); ipso-carbon of C6F5 and carbonyl
carbon were not observed due to broadening. 11B NMR (128.4
MHz, CD2Cl2; δ, ppm): –4.9 (br s). 19F{1H} NMR (376.5 MHz;
CD2Cl2; δ, ppm): –135.1 (br s, 6F, o-F, B(C6F5)3); –159.3 (br s, 3F,
p-F, B(C6F5)3); –164.0 (br s, 6F, m-F, B(C6F5)3). IR (nujol, selected
bands): 1647 cm-1 (C=O). UV-vis (CH2Cl2; 1.7·10-3 M; λ, nm (ε,
M-1·cm-1)): 232 (2280); 260 (831); 301 (112).
Experimental section
General experimental methods are described in the ESI. The
preparation, properties and reactivity of 1-Ph3S were reported
previously.34 (Me4C5N)CO2Li (4)58 and [K][HB(C6F5)3] (7-K)48
were synthesized according to literature procedures.
Preparation of [Ph3S][(Me5C5N)CO2B(C6F5)3] (3-Ph3S)
CH2Cl2 (50 mL) was added via vacuum transfer to a mixture of
solid [Ph3S][Cl] (143.6 mg, 0.481 mmol) and 3-Li (338 mg, 0.481
mmol). The reaction mixture was stirred at room temperature
for 1 h. During this time, formation of white precipitate was
observed. The mixture was filtered through Celite. All volatiles
were pumped off to leave a yellowish foamy residue which
was washed with cold (-30 °C) hexanes (20 mL) and dried in
vacuum. The product appears as a beige powder. Yield: 277
mg, 60%. 1H NMR (400 MHz; CD2Cl2; δ, ppm): 7.85 (t, 3JH-H = 7.6
Preparation of [Ph2I][(tBu2C12H6N)CO2B(C6F5)3] (1-Ph2I)
1-Ph2I was prepared analogously to 1-Ph3S from
[Li(TMEDA)][(tBu2C12H6N)CO2B(C6F5)3] (508 mg, 0.531 mmol)
and [Ph2I][Cl] (168 mg, 0.531 mmol). The product was
additionally recrystallized from CH2Cl2 : hexanes (approx. 1:1)
at -30 °C. Yield: 237 mg, 40%. Borate 1-Ph2I selectively labeled
with 13C in the carbonyl position was prepared analogously.
8 | J. Name., 2012, 00, 1-3
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3
Hz, 3H, p-H, SPh3); 7.73 (t, JH-H = 7.6 Hz, 6H, m-H, SPh3); 7.55 13C{1H} NMR (100.6 MHz; CD2Cl2; δ, ppm):Vie1w6A.r6ticle(Osn,line-
(d, 3JH-H = 7.6 Hz, 6H, o-H, SPh3); 1.53 (br m, 6H); 1.32 (s, 12 H). CH2CH2C(CH3)2N-); 28.1 (s, -CH2CH2C(DCOHI3:)120N.1-0)3;9/3C65D.0T03(0s0, 8K-
3
1H NMR (400 MHz; CD3CN; δ, ppm): 7.84 (t, JH-H = 7.6 Hz, 3H, CH2CH2C(CH3)2N-); 59.1 (s, -CH2CH2C(CH3)2N-), 136.1, 138.8,
p-H, SPh3); 7.74 (t, 3JH-H = 7.6 Hz, 6H, m-H, SPh3); 7.67 (d, 3JH-H
=
147.2, 149.6 (all s, C6F5); all attempts to observe carbonyl
7.6 Hz, 6H, o-H, SPh3); 1.54 (br m, 6H); 1.32 (s, 12 H). 13C{1H} carbon were unsuccessful. All attempts to obtain 5 in
NMR (100.6 MHz; CD2Cl2; δ, ppm):16.9 (s); 31.3 (s); 42.6 (s); analytically pure form for elemental analysis were
ipso-carbon of TMP is obscured by CD2Cl2 signal; 124.0 (s); unsuccessful.
131.2 (s); 132.5 (s); 135.9 (s); 169.4 (s); carbon signals of C6F5
were not observed due to broadening. 13C{1H} NMR (100.6
Preparation of [Ph3S][HB(C6F5)3] (7-Ph3S)
MHz; CD3CN; δ, ppm):17.4 (s); 30.8 (s); 43.1 (s); 56.3 (s); 125.9
To a stirring suspension of triphenylsufonium chloride (0.250 g,
0.837 mmol) in 10 mL of CH3CN was added [K][HB(C6F5)3] (7-K)
(0.462 g, 0.837 mmol) in several portions and the reaction
mixture was allowed to stir at room temperature for 2 h. Over
(s); 132.5 (s); 133.0 (s); 136.2 (s); carbon signals of C6F5 and
carbonyl carbon were not observed. 11B NMR (128.4 MHz,
CD2Cl2; δ, ppm): -5.4 (br s). 19F{1H} NMR (376.5 MHz; CD2Cl2; δ,
ppm): –133.5 (br s, 6F, o-F, B(C6F5)3); –164.2 (t, 3JF-F = 18.8 Hz,
this time a cloudy white solution was formed. Filtration of the
3F, p-F, B(C6F5)3); –168.0 (br s, 6F, m-F, B(C6F5)3). IR (nujol,
solution through an Acrodisc followed by evaporation of all
selected bands) 1653 cm–1 (C=O). UV-vis (CH2Cl2; 1.1·10-5 M; λ,
volatiles gave a white, tacky solid. The tacky residue was
nm (ε, M-1·cm-1)): 228 (23656); 247 (16112); 276 (2926). ESI-
washed/sonicated with hexane (10 mL) and dried under
MS: [C28H18BF15NO2]- 696.1 m/z. TGA (6.600 mg; N2 (60
vacuum to give a white powder. Further purification was
mL/min); T = 25 °C – 450 °C (2.0 °C/min)): T(decomp) = 214 °C.
achieved by dissolving the product in a minimal volume of hot
Elem. Anal. (C46H33BF15NO2S, M.W.: 959.6): calcd. C(57.57),
CH2Cl2 and allowing the product to precipitate at -20 ºC. Yield:
86%, 560 mg. Deuterium labeled salt d1-7-Ph3S was prepared
H(3.47), N(1.46); found C(57.58), H(3.56), N(1.31). Single
1
crystals of 3-Ph3S suitable for X-ray diffraction analysis were
grown by slow vaporization of hexanes into a toluene solution
at -30 °C.
analogously from [K][DB(C6F5)3]. H NMR (400 MHz; CD3CN; δ,
3
3
ppm): 7.85 (t, JH-H = 7.5 Hz, 3H, p-H, Ph3S); 7.74 (t, JH-H = 8.2
3
Hz, 6H, m-H, Ph3S); 7.66 (d, JH-H = 7.4 Hz, 6H, o-H, Ph3S); 3.61
(br q, 1JB-H = 91 Hz, 1H, B-H). 19F{1H} NMR (376.5 MHz; CD3CN;
3
Photogeneration of [(Me4C5NH)CO2B(C6F5)3] (5) from 3-Ph3S
δ, ppm): -136.5 (d, JF-F = 18.8 Hz, 6F, o-F, B(C6F5)3); -166.9 (t,
3-Ph3S (15.0 mg, 0.0156 mmol) was dissolved in 3.0 mL of 3JF-F = 19.5 Hz, 3F, p-F, B(C6F5)3); -170.2 (m, 6F, m-F, B(C6F5)3).
CH2Cl2 and the resulting solution was transferred into a quartz 11B NMR (128.4 MHz; CD3CN; δ, ppm): -25.3 (br d, 1JB-H = 91 Hz,
test tube which was sealed with a septum under argon B-H). 13C{1H} NMR (150.9 MHz; CD3CN; δ, ppm): 149.1 (dm, 1JC-F
1
1
atmosphere. The tube was exposed to 254 nm light for 2 = 244.3 Hz); 138.8 (dm, JC-F = 242.7 Hz); 137.4 (dm, JC-F
=
hours. All volatiles were pumped off to give yellow oily 237.1 Hz); 135.7, 132.5, 132.1, 125.5. IR (nujol, selected bands)
material, which was dried in vacuum and washed/sonicated 2344 cm–1 (B-H). UV-vis (CH2Cl2; 1.1·10-5 M; λ, nm (ε, M-1·cm-
with 5.0 mL of pentane. After pentane was decanted the 1)): 227 (36660). TGA (8.978 mg; N2 (60 mL/min); T = 25 °C –
residue was dried in high vacuum for a day to yield yellow- 450 °C (2.0 °C/min)): Tdecomp = 245 °C. Elem. Anal. (C36H16BF15S;
brown foamy material of [(Me4C5NH)CO2B(C6F5)3] (5), which M.W.: 776.4): calcd. C(55.69), H(2.08); found C(55.12), H(2.05).
was dissolved in CD2Cl2 for NMR studies. Subsequent addition
Preparation of [Ph2I][HB(C6F5)3] (7-Ph2I)
of 100.0 μL of CD3CN to a solution of 5 in CD2Cl2 resulted in
immediate disappearance of the starting material and
formation of an adduct CD3CN·B(C6F5)3. In the absence of
acetonitrile (or any similar base) 5 is stable under ambient
conditions (room temperature and ambient light) as well as
under UV light. An analogous addition of TMP (5.0 μL, 0.03
mmol) to a CD2Cl2 solution of 5 leads to immediate formation
of 3-TMPH. Borate 5 does not show exchange with 13CO2 or
B(p-H-C6F4)3 under ambient conditions in the absence of a
base. 5 can be also generated on NMR scale via stoichiometric
treatment of either 3-Li or 3-Ph3S with HOTf in CD2Cl2;
however, these reactions were found to be less clean than
photolysis of 3-Ph3S in CD2Cl2.
7-Ph2I was prepared analogously to 7-Ph3S from 7-K (225 mg,
0.408 mmol) using [Ph2I][Cl] (129 mg, 0.408 mmol) in CH2Cl
instead of [Ph3S][Cl] in CH3CN. Yield: 300 mg, 90%. X-ray
quality crystals were obtained by dissolving the solid in a 50:50
mixture of CH2Cl2/hexane and cooling the solution to -20 °C.
Deuterium labeled salt d1-7-Ph2I was prepared analogously
1
from [K][DB(C6F5)3]. H NMR (400 MHz; CD3CN; δ, ppm): 8.07
3
4
3
(dd, JH-H = 8.4 Hz, JH-H = 0.9 Hz, 4H, o-H, Ph2I); 7.72 (t, JH-H
=
3
7.5 Hz, 2H, p-H, Ph2I); 7.54 (t, JH-H = 7.9 Hz, 4H, m-H, Ph2I);
3.61 (br q, JB-H = 93 Hz, 1H, B-H). 19F{1H} NMR (376.5 MHz;
1
3
CD3CN; δ, ppm): -136.5 (d, JF-F = 19.3 Hz, 6F, o-F, B(C6F5)3); -
3
166.8 (t, JF-F = 19.5 Hz, 3F, p-F, B(C6F5)3); -170.2 (m, 6F, m-F,
1
B(C6F5)3). 11B NMR (128.4 MHz; CD3CN; δ, ppm): -25.1 (br d, 1JB-
5: H NMR (400 MHz; CD2Cl2; δ, ppm): 1.49 (s, 12H, 4CH3,
= 93 Hz, B-H). 13C{1H} NMR (150.9 MHz; CD3CN; δ, ppm):
TMP); 1.67 (br m, 4H, 2CH2, TMP); 1.77 (br m, 2H, CH2, TMP);
H
1
1
6.93 (br s, 1H, NH; JN-H ≈ 70 Hz, found by H-15N HMQC, see
Figure S9 in ESI). 1.53 (br m, 6H); 1.32 (s, 12 H). 11B NMR (128.4
MHz, CD2Cl2; δ, ppm): -3.6 (br s). 19F{1H} NMR (376.5 MHz;
CD2Cl2; δ, ppm): –136.0 (br s, 6F, o-F, B(C6F5)3); –160.4 (br s 3F,
149.1 (dm, 1JC-F = 229.7 Hz); 139.0 (dm, 1JC-F = 248.7 Hz); 137.4
1
(dm, JC-F = 243.9 Hz); 136.3, 134.1, 133.5, 114.7. IR (nujol,
selected bands) 2285 cm–1 (B-H). UV-vis (CH2Cl2; 1.1·10-5 M; λ,
nm (ε, M-1·cm-1)): 226 (28636). TGA (8.978 mg; N2 (60 mL/min);
T = 25 °C – 450 °C (2.0 °C/min)): Tdecomp = 130 °C. Elem. Anal.
3
p-F, B(C6F5)3); –165.6 (t, JF-F = 18.8 Hz 6F, m-F, B(C6F5)3).
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Page 10 of 11
ARTICLE
Journal Name
(C30H11BF15I; M.W.: 794.6): calcd. C (45.37), H (1.40); found C 254 nm light for 15-45 min and after that lVeiefwt AartticlerOonolimne
DOI: 10.1039/C5DT03008K
(45.40), H (1.47).
temperature for 10 min – 48 hours. Yields of products were
determined by NMR spectroscopy and in the case of
monomeric substrates were confirmed by GS-MS analysis.
Photolysis of 1-Ph2I: trapping B(C6F5)3
CH3CN (65.3 μL, 1.25 mmol) was added to a solution of 1-Ph2I
(1.4 mg, 1.25·10-3 mmol) in 534.7 μL of CD2Cl2 in a quartz NMR
tube ([1-Ph2I]total = 2.1·10-3 mol/L). No reaction was observed
in the absence of UV light within 15 min at room temperature.
The sample was exposed to 254 nm light and the reaction was
followed by NMR spectroscopy for 12 min. During this time
>99% conversion of 1-Ph2I to CH3CN·B(C6F5)3 (Table S1 and
Figure S6 in ESI) was observed by 1H and 19F NMR.
Acknowledgements
Funding for this work was provided by Intel Corp. and NSERC
of Canada via CRD Grant to W.E.P. W.E.P. also thanks the
Canada Research Chair secretariat for a Tier I CRC (2013–
2020).
Notes and references
Photolysis of 3-Ph3S: trapping B(C6F5)3
0.7 mL of a solution of 3-Ph3S (2.1·10-3 mol/L) and Si(SiMe3)4
(1.6 mg, 0.0048 mmol) in CD3CN was placed into a quartz NMR
tube and the sample was sealed with the septum under argon
atmosphere. The mixture was left at room temperature and
ambient light for 1.5 hours showing no reaction by NMR
spectroscopy. The mixture was exposed to 254 nm light and
the reaction was monitored by NMR spectroscopy every 4 min
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1
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10 | J. Name., 2012, 00, 1-3
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52. Scheme 5 depicts the major processes involved in photolysis of
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reactions involving, for example, coupling of phenyl radicals or
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solvent, are operative but, for clarity, not depicted in this
Scheme.
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