Influence of Electronic Effects on the Reactivity of Triazolylidene-Boryl Radicals: Consequences for the use of N-Heterocyclic Carbene Boranes in Organic and Polymer Synthesis

Article abstract of DOI:10.1002/chem.201500499

A small library of triazolylidene-boranes that differ only in the nature of the aryl group on the external nitrogen atom was prepared. Their reactivity as hydrogen-atom donors, as well as that of the corresponding N-heterocyclic carbene (NHC)-boryl radicals toward methyl acrylate and oxygen, was investigated by laser flash photolysis, molecular orbital calculations, and ESR spin-trapping experiments, and benchmarked relative to the already known dimethyltriazolylidene-borane. The new NHC-boranes were also used as co-initiators for the TypeI photopolymerization of acrylates. This allowed a structure-reactivity relationship with regard to the substitution pattern of the NHC to be established and the role of electronic effects in the reactivity of NHC-boryl radicals to be probed. Although their rate of addition to methyl acrylate depends on their electronegativity, the radicals are all nucleophilic and good initiators for photopolymerization reactions. Polar influence: The influence of electronic effects on the reactivity of N-heterocyclic carbene (NHC)-boryl radicals (see figure) was studied. The addition rate of the radicals to methyl acrylate depends on their electronegativity. Nevertheless, all NHC-boranes studied were good co-initiators for the TypeII photopolymerization of acrylates, which suggests that the addition rate is not the determining factor for the polymerization.

Full text of DOI:10.1002/chem.201500499

DOI: 10.1002/chem.201500499
Full Paper
&
NHC-Boranes
Influence of Electronic Effects on the Reactivity of Triazolylidene-
Boryl Radicals: Consequences for the use of N-Heterocyclic
Carbene Boranes in Organic and Polymer Synthesis
Sofia Telitel,[a] Anne-Laure Vallet,[b] Darrin M. Flanigan,[c] Bernadette Graff,[a] Fabrice Morlet-
Savary,[a] Tomislav Rovis,[c] Jacques LalevØe,*[a] and Emmanuel Lacôte*[b, d]
Abstract: A small library of triazolylidene-boranes that differ
only in the nature of the aryl group on the external nitrogen
atom was prepared. Their reactivity as hydrogen-atom
donors, as well as that of the corresponding N-heterocyclic
carbene (NHC)-boryl radicals toward methyl acrylate and
oxygen, was investigated by laser flash photolysis, molecular
orbital calculations, and ESR spin-trapping experiments, and
benchmarked relative to the already known dimethyltriazol-
ylidene-borane. The new NHC-boranes were also used as co-
initiators for the Type I photopolymerization of acrylates.
This allowed a structure–reactivity relationship with regard
to the substitution pattern of the NHC to be established and
the role of electronic effects in the reactivity of NHC-boryl
radicals to be probed. Although their rate of addition to
methyl acrylate depends on their electronegativity, the radi-
cals are all nucleophilic and good initiators for photopoly-
merization reactions.
Introduction
makes them poor H-atom donors to nucleophilic alkyl and aryl
radicals. This is why radical reactions using them as mediators
require polarity-reversal catalysis conditions.[1c]
N-Heterocyclic carbene-boranes (NHC-boranes) have emerged
as valid surrogates for the toxic tin hydrides in synthetic free-
radical chemistry[1] and as efficient co-initiators for the radical
photopolymerization of acrylates.[2] This is principally due to
their relatively low BÀH bond dissociation energies (BDEs) in
the 80–88 kcalmolÀ1 range, a consequence of the stabilization
of the boryl radicals by complexation with the NHC.[3] The
NHC-boryl radicals are highly nucleophilic and thus add rapidly
to electron-poor olefins, which is one factor explaining their
high efficiency for initiating the polymerization of acrylates.
However the NHC-boranes are also hydrides,[4] which actually
We have extensively studied the influence of sterics on the
reactivity of NHC-boranes and NHC-boryl radicals.[5] We now
report how polar effects on the NHC affect the radical reactivi-
ty of NHC-boranes, and what this means for their structural op-
timization as radical mediators and Type II photopolymeriza-
tion co-initiators. For that purpose, the library of triazolylidene-
NHCs developed by Rovis et al. was an attractive target for in-
vestigation, as one can easily prepare NHCs with strongly elec-
tron withdrawing to strongly electron donating substituents.[6]
The impact of the N-aryl substituent on NHC-catalyzed reac-
tions has been extensively documented,[6d] with some reactions
benefiting from strongly electron withdrawing groups such as
C6F5,[6e] whereas others proceed optimally when electron-rich
groups[6f] or largely neutral substituents[6g] are located at that
position.
[a] S. Telitel, Dr. B. Graff, Dr. F. Morlet-Savary, Prof. J. LalevØe
Institut de Science des MatØriaux de Mulhouse IS2M - UMR CNRS 7361
UniversitØ de Haute-Alsace, 15 rue Jean Starcky
68057 Mulhouse Cedex (France)
Fax: (+33)3-89-33-60-69
In the present work, we prepared a small library of triazolyl-
idene-boranes that differ only in the nature of the aryl group
on the external nitrogen atom (2–8, Figure 1). Their reactivity
as hydrogen-atom donors, as well as that of the corresponding
NHC-boryl radicals toward methyl acrylate and oxygen, were
investigated by laser flash photolysis (LFP), molecular orbital
calculations, and ESR spin-trapping experiments, and bench-
marked relative to the already known 1. The new NHC-boranes
were also used as co-initiators for the Type II photopolymeriza-
tion of acrylates.
E-mail: jacques.lalevee@uha.fr
[b] A.-L. Vallet, Dr. E. Lacôte
ICSN CNRS
Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex (France)
[c] D. M. Flanigan, Prof. T. Rovis
Department of Chemistry, Colorado State University
Fort Collins, CO 80523 (USA)
[d] Dr. E. Lacôte
UniversitØ de Lyon, Institut de chimie de Lyon
UMR 5265 CNRS-UniversitØ Lyon I-ESCPE Lyon
43 Bd du 11 novembre 1918, 69616 Villeurbanne Cedex (France)
Fax: (+33)4-72-43-17-95
E-mail: emmanuel.lacote@univ-lyon1.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500499.
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Table 1. Hydrogen-abstraction rate constants for reactions (1b) and (2b)
and quantum yields of NHC-boryl radicals for reaction (2b). The BDEs
were evaluated at the UB3LYP/6-31G* level of theory.
kH2(3BP)
[108 MÀ1 sÀ1
Fboryl
3BP/NHC-BH3
kH1(tBuO )
BDE (BH)
[kcalmolÀ1
]
C
NHCÀBH3
]
[108 mÀ1 sÀ1
]
1
2
3
4
5
6
7
8
3
0.9
0.9[b]
0.8
0.7
0.5
0.7
0.9
0.9
1.4[a]
1.5[b]
1.8
1.5
–
1.9
1.4
1.4
80.4[a]
80.2
78.1
79
78.6
78.1
80.2
80.5
5.5[b]
6
1.7
7
8
1.7
2.2
[a] See ref. [2d]. [b] See ref. [2f].
ing to our previously introduced procedure.[2e,f] They are listed
in Table 1.
For reaction (1b), the kH1 values were derived from the rise
C
time of NHC-BH2 (time needed for the signal to rise after the
Figure 1. Structure of the NHC-boranes used in the study.
C
pulse). tBuO is silent in LFP for l>300 nm, and thus only the
NHC-boryl radicals can be observed (see also Figure 2, inset). A
C
typical spectrum (that of NHC-boryl radical 4 ) is shown in Fig-
ure 2A.
Results and Discussion
All NHC-boranes were prepared by our standard method, that
is, from the NHC, generated in situ, and BH3·SMe2.[7] The 11B
chemical shifts of NHC-boranes 2–8 vary from À34.1 ppm for 6
to À36.7 ppm for 3 and 4. They lie in the usual chemical range
observed for other NHC-boranes (e.g., À37.6 ppm for 1), and
there is no apparent correlation between chemical shifts and
electronic effects of the aryl substituent.
Hydrogen-atom abstraction from NHC-boranes: access to
NHC-boryl radicals
The NHC-boryl radicals 1–8 were generated by hydrogen ab-
straction from the corresponding NHC-boranes[2f] in two differ-
ent ways: 1) by a tert-butoxyl radical [Eq. (1b)] generated by
UV photocleavage of di-tert-butyl peroxide [Eq. (1a)]; 2) by hy-
drogen-atom transfer to the triplet state of benzophenone
[3BP, Eq. (2b)], obtained on photoexcitation of benzophenone
[Eq. (2a)].
C
Figure 2. A) Time-resolved UV/Vis spectrum of 4 in di-tert-butylperoxide/ace-
tonitrile for t=200 ns; [4]=0.05m. Inset: the kinetics at 360 nm. B) Kinetics
3
at 525 nm of BP in acetonitrile for different [4].
The kH2 values for reaction (2b) were determined from the
quenching of 3BP, the signal of which is easily observed at
525 nm (Figure 2B). In the absence of any NHC-BH3, the ob-
served signal disappearance corresponds to the regular decay
C
tBuOOtBu þ hn ! 2 tBuO
ð1aÞ
ð1bÞ
ð2aÞ
ð2bÞ
3
3
of BP (Figure 2B). When an NHC-borane is present, BP under-
C
C
tBuO þ NHC-BH3 ! tBuOH þ NHC-BH2
ðkH1
Þ
C
goes a fast H-atom exchange to deliver ketyl radical BPH . The
3
C
signal for BP is thus rapidly quenched and BPH shows a resid-
3
BP þ hn ! BP
C
ual absorption (Figure 2B). Because BPH is formed together
with the NHC-boryl radical by H-atom transfer, the quantum
3
C
C
BP þ NHC-BH3 ! BPH þ NHC-BH2
ðkH2
Þ
C
yield for the formation of BPH is equal to that of the NHC-
boryl radical in all cases. It can be extracted from the optical
[2e,f]
C
The decay of the UV/Vis signal of the NHC-boryl radicals was
monitored by nanosecond LFP with a Q-switched nanosecond
Nd/YAG laser (lexc =355 nm, 9 ns pulses, energy reduced to
10 mJ). The hydrogen-abstraction rate constants kH1 and kH2 for
reactions (1b) and (2b), respectively, were determined accord-
density of BPH .
ESR spin-trapping experiments confirmed that the observed
species are the desired NHC-boryl radicals (Figure 3). In the
presence of phenyl N-tert-butyl nitrone (PBN), NHC-boranes 3,
4, and 8 led to the respective nitroxyl radical adducts, which
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Figure 3. Experimental (a) and simulated (b) ESR spectra in spin-trapping experiments for the PBN adducts of the NHC-boryl radicals derived from A) 3 in di-
tert-butyl peroxide, B) 4 in di-tert-butyl peroxide/tert-butylbenzene, and C) 8 in di-tert-butyl peroxide/tert-butylbenzene (the ESR spectral simulation includes
10B).
were characterized. The signals exhibit the following hyperfine
coupling constants (HFCs): aN =15.1 G, aB =4.0 G, aH =2.5 G for
C
C
3 ; aN =15.1 G, aB =4.1 G, aH =2.7 G for 4 ; aN =15.1 G, aB =
C
3.8 G, aH =2.6 G for 8 . These values are in the range of the
known HFCs for boryl radicals.
All rate constants measured for hydrogen abstraction were
very high (>108 mÀ1 sÀ1) for both the tert-butoxyl radical and
3BP (Table 1, entries 1–8). However, whereas the kH1 values
were almost constant, some slight variations were observed in
kH2 [(1.7–8)”108 mÀ1 À1], and the NHC-boranes featuring elec-
s
tron-donating groups were the slowest electron donors (with
one exception; compare Table 1, entries 7 and 8 to entries 2–
6). The quantum yields for H exchange are higher for the elec-
tron-rich NHC-boranes 7 and 8 and slightly lower for the elec-
tron-poor 3–6.
C
C
Figure 4. Calculated SOMOs for 2 (left) and 3 (right) at the UB3LYP/6-31G*
The two reactions differ in the nature of the H-abstracting
level of theory for two different views (B is indicated by an arrow).
partner. Unlike tBuO , 3BP is a more conjugated species that
C
can develop stacking interactions with the NHC-boranes,
which affect the exchange for reaction (2b). Steric hindrance is
another factor that could explain the different behaviors of the
C
BÀH BDE. We thus calculated the SOMOs of 2 (N-phenyl) and
C
3 (N-pentafluorophenyl). The two orbitals are highly similar
C
C
initiators. The H transfer to tBuO induces an alignment be-
(Figure 4). The spin density on boron is slightly lower for 3
C
tween the three atoms (O, H, and B). Hence, the influence of
(0.511 versus 0.502 for 2 at the UB3LYP/6-31G* level of
theory). This trend is in agreement with the Rablen model, in
which spin delocalization influences the BÀH BDE.[3b] The more
spin is delocalized on the Lewis base (owing to the electron-
withdrawing substituents on the NHC), the lower the BDE. On
the contrary, the system is not modified by the presence of
the donating groups: the two nitrogen atoms already contrib-
ute to the overlap with the boryl radical, and the additional
donation from the substituents is negligible.
3
the tert-butyl substituent is minimal. In contrast, BP is a conju-
gated biradical that requires a different geometry for H trans-
fer, which might generate more steric strain. It is interesting to
consider the structurally different imidazolydinene-borane
DiMe-Imd-BH3,[2e] which transfers its hydrogen atom faster to
8
3BP (9.6”108 mÀ1 sÀ1) than to tBuO (2.7”10 mÀ1 sÀ1). DiMe-Imd-
C
BH3 is a faster hydrogen donor than 1, likely because of the
electronic differences of the carbenes: imidazolydinene-car-
benes are more electron rich than triazolydinenes,[8] so that
the BÀH bonds have greater hydridic character and are more
suitable to react with electrophilic radicals.
Reactivity of NHC-boryl radicals towards methyl acrylate
and O2
We next calculated the BÀH BDEs using the hybrid function-
al B3LYP.[9] They were determined from reactants and products
fully optimized at the B3LYP/6-31G* level of theory. Compared
to reference compounds 1 and 2, the BDEs were similar for
NHC-boranes 7,8 and slightly lower for NHC-boranes 3–6 (78–
79 kcalmolÀ1, Table 1).
Because of their relevance to the photopolymerization of acryl-
ates in the presence of air, the rate constants for the addition
of NHC-boryl radicals to methyl acrylate (Scheme 1a) and cou-
pling with dioxygen (Scheme 1b) were measured (Figure 5).
C
C
For that purpose, we selected 3 and 4 as representative elec-
C
Although caution must be exercised because of the difficult
modeling of the BÀH BDEs of NHC-boranes[3a,10] and the small
differences within the range of error (Æ2.6 kcalmolÀ1), it ap-
pears that electron-withdrawing groups tend to diminish the
tron-poor NHC-boryl radicals and 8 as representative electron-
C
C
rich NHC-boryl radical. The known values for 1 and 2 were
also added for comparison. The rates of decay of the signals of
the NHC-boryl radicals were measured for different quencher
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This property of NHC-boryl radicals is at the root of the high
oxygen tolerance of radical photopolymerizations initiated by
NHC-boranes. In contrast, the addition rate to methyl acrylate
ranged from 0.8”106 to 17”106 mÀ1 sÀ1 (compare the rates for
C
C
1 and 3 ; Table 2, entries 1 and 3). The more electron with-
drawing the substituents on the NHC-borane, the less rapid
C
C
the addition to methyl acrylate (compare 3 and 4 ; Table 2, en-
C
C
C
tries 3 and 4). Radical 8 adds faster than both 3 and 4 , but
C
C
significantly slower than 1 and 2 (Table 2, entry 5). On the
whole, though, all the NHC-boryl radicals considered add rap-
idly to the electron-poor acrylate, and are thus nucleophilic
species. For comparison, the fastest imidazolydinene-boryl radi-
Scheme 1. Reactions of NHC-boryl radicals: addition to methyl acrylate (a)
and coupling with dioxygen (b).
cal adds to methyl acrylate with a rate of 3.8”107 mÀ1 À1, close
s
[2e]
C
to that of 1 .
To help with the interpretation of the results we also calcu-
lated the Mulliken electronegativity c of the five NHC-boryl
radicals (cM =1= (IP+EA), Table 2, column 2). Both the ionization
2
potential (IP) and electron affinity (EA) were calculated at the
UB3LYP/6-31+G* level of theory by optimization of the rele-
vant radicals and ions.
C
Radical 1 had the lowest c value (2.70). Introduction of
a phenyl substituent led to an increase in the electronegativity
to 2.85. The addition of halogen substituents further increased
C
C
the c value (to 2.98 for 4 and 3.13 for 3 ). In contrast, the addi-
tion of three methoxyl substituents to the phenyl ring did not
C
C
affect the c value of the radical (2.81 for 8 ). DiMe-Imd-BH2 has
a calculated c value of 2.72.[1c] Interestingly, the Mulliken elec-
tronegativity appears to be strongly related to the rate con-
stant for addition to methyl acrylate: the lower the electro-
negativity, the faster the addition to methyl acrylate (Figure 6).
C
Figure 5. Kinetics at 380 nm for 8 in di-tert-butylperoxide/acetonitrile (75/
25%); [8]=0.032m for different [methyl acrylate] from 0 to 0.11m; Inset:
C
zoom of the rise time of 8 in the absence of methyl acrylate.
Figure 6. Relationship between the rate constants for addition to acrylate
kadd and the calculated electronegativities of the NHC-boryl radicals (see
values and compound structures in Table 2 and Figure 1).
Table 2. Rate constants characterizing the reactivity of NHC-boryls to-
wards methylacrylate and oxygen. See text for the calculation of the Mul-
liken electronegativity c.
Entry
Radical
c [eV]
kadd(methylacrylate)
kadd(O2)
[108 mÀ1 sÀ1
]
[106 mÀ1 sÀ1
]
We believe that the pattern observed in the triazolydinene
family is a direct result of electronic effects. Increasingly less
donating substituents at nitrogen lead to more electron back-
donation from the boron atom to the NHC in the NHC-boryl
radicals. The latter are thus less prone to cede an electron,
which is reflected by a higher c value. Accordingly, they are
also less prone to add to an electron-poor olefin. The case of
C
C
C
C
C
1
2
3
4
5
1
2
3
4
8
2.70
2.85[a]
3.13
2.98
2.81
17[2d]
6[2f]
0.8
2.5
3.8
>6[a]
>2[b]
>5
>5
>5
C
C
concentrations (see Figure 5 for the addition of 8 to methyl
acrylate), and these data were processed in the usual way to
give the elementary rate constants listed in Table 2.[2e,f]
8 is more problematic at first sight, since the aryl group has
three strongly electron donating substituents. However, as for
the other NHC-boryl radicals, in fact the trimethoxyphenyl
C
All radicals react extremely fast with oxygen, and the reac-
tions are not affected by the substitution pattern of the NHCs.
group in 8 adopts an out-of-plane conformation (Figure 7).
The strongly donating character of the methoxyl groups is
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Figure 8. Radical photopolymerization profiles for different BP/co-initiator
couples (1/1 wt% w/w in TMPTA as laminate). a) Without co-initiator, b) BP/
3, c) BP/4, d) BP/7, e) BP/1, f) BP/6, and g) BP/8.
chromatic light source, and the evolution of the double-bond
content was followed by real-time FTIR spectroscopy
(ꢀ1630 cmÀ1) at room temperature.[2f] The benchmark photo-
polymerization was run in the presence of BP alone (Fig-
ure 8a). It led to a final conversion of about 45% with an in-
hibition time of 24 s. In contrast, when the NHC-boranes were
present, no such inhibition was observed and the conversion
was generally better (in the 50–55% range, Figure 8b–g). The
differences between the different NHC-boranes were not large,
and the polymerizations were fast in all cases. However, the in-
itial rates of the systems vary in the order Figure 8cꢀd<g<
bꢀf<e. After a short lag time, the rate for Figure 8 f was simi-
lar to that of Figure 8b.
C
Figure 7. SOMO for 8 (UB3LYP/6-31G*).
thus largely canceled, because the aryl ring is not fully conju-
C
C
gated, and 8 behaves as a less electron rich version of 1
owing to the ÀI effects of the oxygen atoms.
C
C
The fact that 1 and DiMe-Imd-BH2 have roughly the same
electronegativity despite the electronic difference of the NHC
shows that one must remain highly cautious in building a struc-
ture–electronegativity correlation over structurally different
radicals. We will thus only consider the triazolydinene family
for the polymerization part of this article.
The inhibition observed in Figure 8a is due to the residual
oxygen in the sandwiched formulation. Dioxygen is an inhibi-
tor, and the polymerization only starts when it has been con-
sumed.[11] This inhibition is suppressed in the presence of the
NHC-borane co-initiators, and better polymerization profiles
were obtained (higher polymerization rates and final conver-
sions). We have shown that the suppression of the inhibition
period is due to 1) the highly efficient formation of NHC-boryl
NHC-boranes as co-initiators for the Type II photopolymeri-
zation of acrylates
We carried out the radical photopolymerization of trimethylol-
propane triacrylate (TMPTA) using a two-component photoini-
tiating system (benzophenone (BP)/NHC-BH3; Scheme 2 and
Figure 8). The film polymerizations were carried under in lami-
nated conditions, in which the formulations (monomer and
photoinitiating system) were sandwiched between two poly-
propylene films. The 20 mm-thick laminated films were deposit-
ed on a BaF2 pellet and irradiated with a 150 W Xe–Hg poly-
3
radicals by H transfer to BP (Table 1); 2) their high reactivity
towards electron-deficient acrylates (Table 2); 3) fast quenching
of oxygen by NHC-boryl radicals; and 4) fast reaction of the
electrophilic NHC-boraperoxyl adduct radicals with a hydrogen
atom from another, nucleophilic BÀH bond, which converts
NHC-boraperoxyl radicals back to polymerization initiating
boryl radicals.[2e]
Interestingly, the efficiency of the NHC-borane co-initiators is
not correlated to their addition rate to the monomer, since 3
performs very well despite being the slowest to add to methyl
acrylate. On the other hand, the better co-initiators seem to be
3
those with the higher H-transfer rates to BP (see 1, 3, 6, and
8). It thus appears that the rate-limiting event for the co-initia-
tor efficiency is not the addition to the monomer, which is
rapid even for the slower NHC-boranes, but the H transfer to
3BP. The latter reaction is bimolecular and its rate depends on
the stacking/steric interactions that develop between the two
reacting partners.
Scheme 2. NHC-borane as co-initiator in the photopolymerization of TMPTA.
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Conclusion
We have studied the influence of polar effects on the reactivity
of NHC-boryl radicals derived from triazolylidene-carbenes. For
the compounds studied, polar effects have a weak effect on
the BH BDE and the direct H-atom transfer rate to oxygen-cen-
tered radicals. In contrast, we observed a correlation between
the addition rate of the NHC-boryl radicals to methyl acrylate
and their calculated electronegativity. The bimolecular interac-
3
tion BP/NHC-borane was also influenced by the NHC-borane
[5] a) J. C. Walton, M. Makhlouf Brahmi, L. Fensterbank, E. Lacôte, M. Malac-
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132, 2350; b) J. C. Walton, M. Makhlouf Brahmi, J. Monot, L. Fensterbank,
M. Malacria, D. P. Curran, E. Lacôte, J. Am. Chem. Soc. 2011, 133, 10312.
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structure, presumably because of steric/stacking interactions.
Overall, the present work shows that NHC-boranes featuring
an improved interaction with the excited state of the initiator
should be better Type II photopolymerization co-initiators.
Acknowledgements
This work was supported by the French Agence Nationale de
Recherche (grant ANR-11-BS07-008, NHCX), UHA, ENSCMu, the
Institut Universitaire de France (IUF), CNRS, and NIGMS
(GM72586).
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Keywords: boranes
·
carbenes
·
radical reactions
·
polymerization · radicals
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Received: February 6, 2015
Published online on August 3, 2015
Chem. Eur. J. 2015, 21, 13772 – 13777
www.chemeurj.org
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