Synthesis of 2-(lutidinyl)organoboranes and their reactivities against dihydrogen and pinacol borane

Article abstract of DOI:10.1039/c5dt03815d

Two 2,4,6-tris(trifluoromethyl)phenyl-substituted 2-(lutidinyl)organoboranes (5a and 5b) were prepared. These complexes can function as intramolecular vicinal B/N frustrated Lewis pairs to heterolytically activate dihydrogen. When these complexes were treated with HBpin, two different reaction pathways took place. Whereas the reaction between 5a and HBpin affords a formal ligand-redistribution product, the reaction of 5b with HBpin leads to a dearomative dehydroborylation product.

Full text of DOI:10.1039/c5dt03815d

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Synthesis of 2-(lutidinyl)organoboranes and their
reactivities against dihydrogen and pinacol
borane†
Cite this: DOI: 10.1039/c5dt03815d
Junhao Zheng, Yue-Jian Lin and Huadong Wang*
Two 2,4,6-tris(trifluoromethyl)phenyl-substituted 2-(lutidinyl)organoboranes (5a and 5b) were prepared.
These complexes can function as intramolecular vicinal B/N frustrated Lewis pairs to heterolytically acti-
vate dihydrogen. When these complexes were treated with HBpin, two different reaction pathways took
place. Whereas the reaction between 5a and HBpin affords a formal ligand-redistribution product, the
reaction of 5b with HBpin leads to a dearomative dehydroborylation product.
Received 30th September 2015,
Accepted 15th October 2015
DOI: 10.1039/c5dt03815d
www.rsc.org/dalton
In recent years, the chemistry of “frustrated Lewis pairs” (FLPs) as pyridine can only provide limited steric protection for the
has received substantial attention.1,2 The unquenched Lewis Lewis base center, pyridine-based intramolecular FLPs often
acid and base centers in FLPs can cooperate in a synergistic bear strong Lewis acid–base interactions. For example, very
manner to activate small molecules, such as H2, CO2 and recently Mitzel et al. reported the synthesis of a series of intra-
alkenes. Intramolecular FLPs, exemplified by Erker’s vicinal molecular boron–pyridine Lewis pairs (3 and 4) (Scheme 1).8
P/B FLPs (1)3,4 and Repo’s “molecular tweezers” (2),5 can sub- Complexe 3 contains strong B–N coordination and complex 4
stantially enhance the synergistic effects, thus leading to some shows no B–N interaction due to the large substitution at the
remarkable reactivities (Scheme 1). Besides the phosphine and 2-position of pyridine. Although H/D-scrambling was observed
amine moieties, pyridine moieties were also introduced as the when complex 4 was treated with a H2/D2 mixture, none of
Lewis base components for intramolecular FLPs.6–8 However, these complexes showed reactivity against H2, CO2 and THF.
We recently showed that the organoborane substituted with
bulky ArF (ArF = 2,4,6-tris(trifluoromethyl)phenyl) ligands,
ArF2BMe, does not form a Lewis adduct with pyridine.9 We
assumed that incorporation of the ArF2B moiety into the intra-
molecular pyridine–boron systems could lead to the formation
of vicinal N/B FLPs without strong B–N interactions. Such
systems could show typical FLP reactivity against small mole-
cules. Herein, we present the preparation of the ArF-substi-
tuted 2-(lutidinyl)organoboranes 5a and 5b and the study of
their reactivities against H2 and HBpin.
Results and discussion
Treating ((4-methyl-2-pyridyl)methyl)lithium with ArF2BF can
afford the target complex 5a in 71% yield after recrystallization
(Scheme 2). The 11B NMR spectrum of 5a displays a singlet at
3.57 ppm, within the range for a tetracoordinated boron,
Scheme 1
suggesting interactions between the boryl and pyridinyl moi-
eties. A possible intermolecular B–N interaction in the solution
of 5a was excluded as addition of the strong base DMAP to the
solution of 5a does not lead to any coordination of DMAP to
Department of Chemistry, Fudan University, Handan Rd, 220, Shanghai, 200433,
China. E-mail: huadongwang@fudan.edu.cn
†Electronic supplementary information (ESI) available. CCDC 1426309–11,
5a. Single-crystal X-ray analysis revealed that the structure of
5a contains a constrained four-membered ring (Fig. 1). The
1426313 and 1426315–17. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c5dt03815d
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Scheme 2
Fig. 2 Molecular structure of 5b (thermal ellipsoids are shown with
30% probability). Selected bond lengths [Å] and angles [°]: N(1)–B(1)
1.661(2), N(1)–C(1) 1.353(2), C(6)–C(1) 1.487(2), C(6)–B(1) 1.678(2), N(1)–
C(1)–C(6) 100.56(13), N(1)–B(1)–C(6) 81.79(11), C(1)–N(1)–B(1) 91.34(12),
C(1)–C(6)–B(1) 86.19(12).
Fig. 1 Molecular structure of 5a (thermal ellipsoids are shown with 30%
probability). Selected bond lengths [Å] and angles [°]: N(1)–B(1) 1.643(3),
N(1)–C(2) 1.346(3), C(2)–C(1) 1.490(3), C(1)–B(1) 1.671(3), N(1)–C(2)–C(1)
99.75(17), N(1)–B(1)–C(1) 81.81(14), C(2)–N(1)–B(1) 92.15(16), C(2)–C(1)–
B(1) 86.13(16).
Scheme 3
5b is close to that determined by single crystal analysis. The
B–N distance in the calculated structure of 5b is 1.640 Å. In
the B/N “open” isomer 5b′, the B–N distance is increased to
2.781 Å and it is calculated to be 8.6 kcal mol−1 less stable
than 5b. Similar values were also reported for Erker’s vicinal
P/B FLPs.4a On the other hand, the B/C “open” isomer 5b″ is
13.5 kcal mol−1 less stable than 5b, thus suggesting that com-
plexes 5a and 5b are likely to function as B/N instead of B/C
FLPs.
distance between B and N atoms is 1.643 Å, shorter than that
observed in the 2,6-lutidine–B(C6F5)3 adduct (1.661 Å), a inter-
molecular FLP which readily reacts with H2 under ambient
conditions.10 The distance between the boron atom and the
methylene carbon is 1.671 Å, comparable to the other two B–C
bonds (1.657 and 1.671 Å). To increase the steric bulk around
the N atom, we synthesized (6-methyl-2-pyridyl)methyl-substi-
tuted organoborane 5b with a similar strategy and it can be
isolated in 63% yield (Scheme 2). The 11B NMR spectrum of 5b
reveals a singlet at 19.78 ppm which is in a lower field com-
pared to 5a, indicating weakened pyridine–borane coordi-
nation. This is corroborated by structure analysis (Fig. 2). In
the solid state of 5b, the B–N distance of 1.661 Å is substan-
tially longer than that of 5a, suggesting a weakened B–N dative
bond due to larger steric congestion. The distance between the
boron atom and the methylene carbon of 5b is 1.678 Å, com-
parable to that observed in 5a.
Treatment of complex 5a with H2 (ca. 4 bar) in hexane at
25 °C or 80 °C led to no reaction. When the pressure of H2 was
increased to 60 bar, complex 5a was quantitatively converted to
the 2,4-lutidine–HBArF adduct 6a, which can be subsequently
2
isolated in 89% yield after workup (Scheme 4). Although we
As both complexes 5a and 5b contain a highly strained
4-membered ring, there are two ways to open this 4-membered
ring, which can lead to a B/N or B/C FLP. To examine which
way is favoured, we carried out DFT (M06-2X) calculations11 to
optimize the structure of 5b, its B/N “open” isomer 5b′ and
B/C “open” isomer 5b″ (Scheme 3). The optimized geometry of
Scheme 4
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Scheme 6
Complex 9a was fully characterized by NMR spectroscopy,
elemental analysis and single crystal X-ray analysis. We con-
sider that the reaction of HBpin and 5b could first produce
intermediate 10a. Then the highly-electrophilic borenium
moiety of 10a attacks the methylene group to afford 2-(boryl-
methyl)-4-methylpyridine and HBArF2, which subsequently
Scheme 5
were not able to observe any intermediate during the for- bind together to form 9a.
mation of 6a, we assume that complex 5a activates H2 to afford
When 1 equiv. of HBpin was added to the C6D6 solution of
a zwitterionic complex 7a, which then undergoes a proton 5b, surprisingly, we noticed slow formation of a new complex
transfer reaction to generate the protodeboronation product 11b with the concomitant formation of HBArF and 2,6-
2
6a. In the solid state of 6a, we observed a short B–N bond lutidine. When the reaction temperature was increased to
(1.617 Å), which could be the cause of the lack of reactivity of 60 °C, complete consumption of 5b was observed. Meanwhile,
6a against H2 even under harsh conditions.
only 0.5 equiv. of HBpin was consumed. The molar ratio of the
When complex 5b was exposed to H2 (ca. 4 bar) in C6D6 at formed HBArF (both free and coordinated) and 11b is around
2
80 °C, quantitative conversion to an equilibrium mixture of 1 : 1. In a larger scale carried out in hexane solution, we were
HBArF and 2,6-lutidine was observed (Scheme 5). The equili- able to isolate 11b as an orange solid in 68% yield (Scheme 7).
2
brium constant between free HBArF2/2,6-lutidine and its The 11B NMR spectrum of 11b displays two singlets at 22.6
adduct 6b in C6D6 is 56 at 298 K as determined by 1H NMR and 44.5 ppm, indicating that both the ArF2B and Bpin moi-
analysis. In a larger scale reaction carried out in hexane, 6b eties are likely three-coordinated. In the 1H NMR spectra of
can be obtained in 81% yield. The increased reactivity of 5b 11b, no methylene signal was observed. Instead we found a
compared to 5a is possibly due to the larger steric demand singlet at 5.58 ppm which was assigned to a methine frag-
around the nitrogen atom in 5b. The molecular structure of 6b ment. Furthermore, the signals from the pyridine ring are
was determined and the B–N distance (1.643 Å) is longer than upfield shifted to 6.11, 5.98 and 5.20 ppm, suggesting that the
the one in 6a. At higher pressure of H2 (60 bar), complex 5b pyridine ring has lost its aromaticity. The molecular structure
was completely transformed to piperidinium dihydridoborate of 11b was determined by single-crystal X-ray analysis (Fig. 3).
salt 8b after stirring at 80 °C for 24 hours (Scheme 5). We Complex 11b contains a dearomatized pyridine ring. The dis-
believe that the original formed adduct 6b undergoes further
H2 activation to yield the hydrogenation product 8b. Similar
B(C6F5)3 mediated hydrogenation of pyridine derivatives was
also observed by the groups of Stephan12 and Du.13 It is note-
worthy that the related intermolecular FLPs comprised of
ArF2BMe and 2,4-lutidine or 2,6-lutidine showed no reactivity
against H2 (60 bar) at 80 °C, highlighting the advantage of
vicinal N/B pairs in H2 activation.14
Besides cleavage of the H–H bond, B–H bond activation by
FLPs has also attracted some attention. The groups of
Stephan15 and Crudden16 reported that HBcat or HBpin can be
activated by phosphine/borane or amine/borane pairs, which
has been successfully applied in borenium catalyzed imine
hydroboration. Recently our group discovered that the FLP
comprised of ArF2BMe and pyridine can effectively activate
HBpin which led to ArF2BMe catalyzed pyridine 1,4-hydrobora-
tion.9 We are interested to know if a similar reaction mode can
be observed for intramolecular B/N Lewis pairs 5a,b. Addition
of HBpin to the solution of 5a in hexane resulted in the for-
mation of 9a, which can be obtained in 90% yield (Scheme 6).
Scheme 7
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functionalization,18 such dearomative dehydroborylation, to
the best of our knowledge, has not been reported before. This
observation could potentially provide a new method for the
dearomatization of pyridine derivatives. Efforts to elucidate
the mechanism of this reaction are currently underway.
Experimental
Synthesis of complex 5a
A solution of ((4-methyl-2-pyridyl)methyl)lithium (0.569 g,
5.04 mmol) in Et2O (15 mL) was added to the solution of
ArF2BF (2.69 g, 4.55 mmol) in Et2O (20 mL) at −70 °C. The
reaction mixture was warmed up to room temperature and
stirred for 16 h. Then the volatile was removed under vacuum.
The resulting oil was extracted with hexane (55 mL). The
hexane solution was concentrated to ∼3 mL and stored at
−40 °C, affording a brown solid which was filtered and dried
under vacuum to give complex 5a (2.20 g, 71%). X-ray quality
crystals were grown from hexane at −20 °C. 1H NMR (400 MHz,
C6D6): δ [ppm] = 8.01 (s, 4H, C6H2), 7.66 (d, 1H, 3JH–H = 5.8 Hz,
Fig. 3 Molecular structure of 11b (thermal ellipsoids are shown with
30% probability).
tances of C2–C3 (1.401 Å) and C4–C5 (1.424 Å) are close to the
value of a single bond. On the other hand, the bond lengths
for C1–C2 (1.355 Å) and C3–C4 (1.354 Å) are in the range of a
standard double bond. Interestingly, the exo-ring double bond
C5–C6 (1.409 Å) is substantially longer than a typical double
bond. This might be caused by electron charge transfer from
the double bond to the adjacent boron atom. Indeed, we
noticed that the bond of C6–B2 (1.463 Å) is significantly
shorter than a typical C(sp2)–B bond. The ArF2B moiety is anti
to the nitrogen atom of the lutidine moiety and the C5–C6–B2
angle is deviated from the ideal 120° due to the steric conges-
tion. Right now the mechanism of formation of 11b still
remains elusive. We tentatively suggest that the initial coordi-
nation of 5b to HBpin and a following intermolecular hydride
abstraction by another 5b could result in the formation of a
borenium species 12b. With a more acidic methylene moiety,17
12b can be deprotonated to form 11b with the concomitant
formation of 7b, which can go through rearrangement to
afford HBArF2 and 2,6-lutidine.
3
H6(py)), 6.42(s, 1H, H3(py)), 6.26 (d, 1H, JH–H = 5.8 Hz,
H5(py)), 2.79 (s, 2H, CH2), 1.64 (s, 3H, CH3). 19F NMR
(376 MHz, C6D6): δ [ppm] = −55.34 (s, 12F, ortho-CF3-C6H2),
−63.33 (s, 6F, para-CF3-C6H2). 13C NMR (125 MHz, C6D6):
δ [ppm] = 169.39 (C2(py)), 154.65 (C4(py)), 142.20 (C6(py)),
2
2
135.95 (q, JC–F = 34 Hz, ortho-C6H2), 129.50 (q, JC–F = 34 Hz,
1
para-C6H2), 127.15 (meta-C6H2), 124.46 (q, JC–F = 275 Hz,
ortho-CF3), 124.11 (C3(py)), 123.72 (q, JC–F = 275 Hz, para-
1
CF3), 122.72 (C5(py)), 27.21 (br s, CH2), 21.35 (CH3), ipso-C6H2
not observed. 11B NMR (160 MHz, C6D6): δ [ppm] = 3.57(s).
Element analysis: calcd for C25H12BF18N: C 44.21, H 1.78,
N 2.06%; found C 44.36, H 1.91, N 2.04%.
Synthesis of complex 5b
A solution of ((6-methyl-2-pyridyl)methyl)lithium (0.391 g,
3.46 mmol) in Et2O (20 mL) was added to the solution of
ArF2BF (1.70 g, 2.87 mmol) in Et2O (15 mL) at −70 °C. The
reaction mixture was stirred at room temperature for 12 h.
Then the volatile was removed under vacuum. The resulting oil
Conclusions
In summary, we have prepared and characterized two 2-(lutidi- was extracted with hexane (40 mL). The hexane solution was
nyl)organoboranes 5a and 5b. Although interaction between concentrated to ∼5 mL and stored at −40 °C, affording an
the nitrogen of the lutidine fragment and the boron atom was orange solid which was filtered and dried under vacuum to
observed in both solution and solid state, these complexes can give complex 5b (1.22 g, 63%). X-ray quality crystals were
function as intramolecular FLPs to heterolytically activate di- grown from hexane at −20 °C. 1H NMR (400 MHz, C6D6):
3
hydrogen. Their reactivity against H2 is strongly dependent on δ [ppm] = 8.04 (s, 4H, meta-C6H2), 6.89 (t, 1H, JH–H = 7.8 Hz,
3
the steric bulk around the nitrogen atom. For less congested H4(py)), 6.47 (d, 1H, JH–H = 7.8 Hz, H3(py)), 6.20 (d, 1H,
5a, harsh conditions (60 bar of H2, 80 °C) are required for 3JH–H = 7.8 Hz, H5(py)), 2.84 (s, 2H, CH2), 1.78 (s, 3H, CH3). 19
F
efficient H2 activation. For more congested 5b, H2 can be acti- NMR (376 MHz, C6D6): δ [ppm] = −54.67 (s, 12F, ortho-CF3-
vated under much milder conditions (4 bar of H2, 80 °C). C6H2), −63.39 (s, 6F, para-CF3-C6H2). 13C NMR (125 MHz,
When complexes 5a and 5b were treated with HBpin, two C6D6): δ [ppm] = 167.19 (C2(py)), 156.04 (C6(py)), 151.74 (br,
2
different reaction pathways took place. Whereas the reaction ipso-C6H2), 140.20 (C4(py)), 135.76 (q, JC–F = 34 Hz, ortho-
between 5a and HBpin affords a formal ligand-redistribution C6H2), 130.29 (q, 2JC–F = 34 Hz, para-C6H2), 127.29 (meta-C6H2),
1
1
product 9a, the reaction of 5b with HBpin leads to a dearoma- 124.37 (q, JC–F = 275 Hz, ortho-CF3), 123.52 (q, JC–F = 275 Hz,
tive dehydroborylation product 11b. Although dehydroboryl- para-CF3), 122.52 (C3(py)), 120.89 (C5(py)), 32.17 (br, CH2),
ation has been very well studied as
a
way of C–H 20.14 (CH3). 11B NMR (160 MHz, C6D6): δ [ppm] = 19.78(s).
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Element analysis: calcd for C25H12BF18N: C 44.21, H 1.78, N 7.95 (s, 4H, meta-C6H2), 4.27 (br s, 2H, NH2), 3.32 (m, 2H,
1
2.06%; found C 44.40, H 1.98, N 1.98%.
CH2), 2.82 (q, 2H, JB–H = 78 Hz, BH2), 2.00 (m, 2H, CH2CH),
1.92 (m, 1H, CH), 1.59 (m, 1H, CH), 1.36 (m, 2H, CH2CH), 1.30
(d, 6H, JH–H = 6.4 Hz, CH3). 19F NMR (376 MHz, CD2Cl2):
Synthesis of complex 6a
A solution of complex 5a (186 mg, 0.275 mmol) in hexane δ [ppm] = −57.83 (s, 12F, ortho-CF3-C6H2), −63.01 (s, 6F, para-
(2 mL) was placed in a Parr autoclave and subjected to 60 atm CF3-C6H2). 13C NMR (125 MHz, CD2Cl2): δ [ppm] = 164.85 (br,
2
H2. After stirring at 80 °C for 18 h, the reaction mixture was ipso-C6H2), 136.53 (q, JC–F = 34 Hz, ortho-C6H2), 126.21 (q,
taken out of the autoclave and dried under vacuum. The 2JC–F = 34 Hz, para-C6H2), 125.50 (meta-C6H2), 125.45 (q, 1JC–F
residue was redissolved in hexane (1 mL) and stored at −20 °C, 275 Hz, ortho-CF3), 124.50 (q, JC–F = 275 Hz, para-CF3), 56.57
=
1
affording a brown solid which was filtered and dried under (CH2CH2CH2), 31.52 (CH2CH2CH2), 22.60 (CHCH3), 19.64
1
vacuum to give complex 6a (162 mg, 89%). X-ray quality crys- (CHCH3). 11B NMR (160 MHz, CD2Cl2) δ = −21.37 (t, JB–H
=
tals were grown from CH2Cl2 at −20 °C. 1H NMR (400 MHz, 83 Hz). Element analysis: calcd for C25H22BF18N: C 43.57,
C6D6): δ [ppm] = 8.20 (br s, 4H, meta-C6H2), 7.47 (d, 1H, H 3.22, N 2.03%; found C 43.57, H 3.30, N 2.09%.
3JH–H = 6.2 Hz, H6(py)), 6.20 (s, 1H, H3(py)), 5.99 (d, 1H, JH–H
= 6.2 Hz, H5(py)), 5.24 (br s, 1H, BH), 2.11 (s, 3H, C2(py)-CH3),
3
Synthesis of complex 9a
1.49 (s, 3H, C4(py)-CH3). 19F NMR (376 MHz, C6D6): δ [ppm] = Complex 5a (154 mg, 0.228 mmol) was mixed with HBpin
−55.52 (br s, 3F, ortho-CF3-C6H2), −56.58 (br s, 3F, ortho-CF3- (32 mg, 0.25 mmol) in hexane (2 mL). A white precipitation
C6H2), −58.30, (br s, 3F, ortho-CF3-C6H2), −58.74 (br s, 3F, appeared and the reaction mixture was stirred at room tem-
ortho-CF3-C6H2), −63.09 (s, 6F, para-CF3-C6H2). 13C NMR perature for 4 h. The resulting slurry was dried under vacuum,
(125 MHz, C6D6): δ [ppm] = 159.34 (C6(py)), 155.61 (br, ipso- affording complex 9a (166 mg, 90%) as a white solid. X-ray
C6H2), 154.08 (C2(py)), 148.77 (C4(py)), 136.87 (br, ortho- quality crystals were grown from hexane at −20 °C. 1H NMR
2
C6H2), 129.54 (q, JC–F = 34 Hz, para-C6H2), 128.19 (C3(py)), (400 MHz, C6D6): δ [ppm] = 8.22 (br s, 4H, meta-C6H2), 7.47 (d,
1
3
126.88 (meta-C6H2), 124.52 (q, JC–F = 275 Hz, ortho-CF3), 1H, JH–H = 6.1 Hz, H6(py)), 6.87 (s, 1H, H3(py)), 6.00 (d, 1H,
123.82 (q, JC–F = 275 Hz, para-CF3), 122.72 (C5(py)), 22.66 3JH–H = 6.1 Hz, H5(py)), 5.27 (br s, 1H, BH), 2.68 (br s, 1H,
1
(C2(py)-CH3), 20.42 (C4(py)-CH3). 11B NMR (160 MHz, C6D6): CH2), 2.29 (br s, 1H, CH2), 1.53 (s, 3H, CH3), 1.06 (s, 12H, CH3).
δ [ppm] = −7.01(s). Element analysis: calcd for C25H14BF18N: 19F NMR (376 MHz, C6D6): δ [ppm] = −55.10 (br s, 3F, ortho-
C 44.08, H 2.07, N 2.06%; found C 44.04, H 2.45, N 1.79%.
CF3-C6H2), −56.42 (br s, 3F, ortho-CF3-C6H2), −57.80, (br s, 3F,
ortho-CF3-C6H2), −58.12 (br s, 3F, ortho-CF3-C6H2), −63.10 (s,
6F, para-CF3-C6H2). 13C NMR (125 MHz, C6D6): δ [ppm] =
Synthesis of complex 6b
A solution of complex 5b (220 mg, 0.324 mmol) in hexane 161.56 (C2(py)), 155.74 (br, ipso-C6H2), 153.13 (C4(py)), 148.13
(3 mL) was degassed twice and backfilled with 4 atm H2. After (C6(py)), 137.93 (q, 2JC–F = 34 Hz, ortho-C6H2), 129.44 (q, 2JC–F
stirring at 80 °C for 20 h, the reaction mixture was concen- 34 Hz, para-C6H2), 128.55 (C3(py)), 127.00 (meta-C6H2), 124.71
=
1
1
trated to ∼1 mL and stored at −40 °C overnight, affording a (q, JC–F = 275 Hz, ortho-CF3), 123.89 (q, JC–F = 275 Hz, para-
white solid which was filtered and dried under vacuum to give CF3), 121.97 (C5(py)), 84.26 (C(CH3)2), 24.81(C(CH3)2), 21.88
pure complex 6b (176 mg, 81%). X-ray quality crystals were (br s, CH2), 20.49 (CH3). 11B NMR (160 MHz, C6D6): δ [ppm] =
grown from hexane at −20 °C. 1H NMR (400 MHz, C6D6): 32.16 (s, Bpin), −6.50 (s, BArF). Element analysis: calcd for
δ [ppm] = 8.26(s, 1H, meta-C6H2), 8.17(s, 2H, meta-C6H2), 8.07 C31H25B2F18NO2: C 46.13, H 3.12, N 1.74%; found C 46.44,
(s, 1H, meta-C6H2), 6.66 (t, 1H, 3JH–H = 7.5 Hz, H4(py)), 6.24 (d, H 3.45, N 1.67%.
3
3
1H, JH–H = 7.5 Hz, H3(py)), 6.13 (d, 1H, JH–H = 8 Hz, H5(py)),
5.27 (br s, 1H, BH), 2.08 (s, 3H, CH3), 1.61 (s, 3H, CH3).
Synthesis of complex 11b
19F NMR (376 MHz, C6D6): δ [ppm] = −55.60 (q, JH–F = 12 Hz, Complex 5b (358 mg, 0.530 mmol) was mixed with HBpin
3F, ortho-CF3-C6H2), −56.91 (q, 3F, JH–F = 12 Hz, ortho-CF3- (38 mg, 0.30 mmol) in hexane (5 mL). After stirring at 60 °C
C6H2), −58.30 (s, 3F, ortho-CF3-C6H2), −58.95 (s, 3F, ortho-CF3- for 10 h, the reaction mixture was dried under vacuum. The
C6H2), −63.02 (s, 3F, para-CF3-C6H2), −63.09 (s, 3F, para-CF3- residue was sublimated at 80 °C under vacuum to remove
C6H2). 11B NMR (160 MHz, C6D6): δ [ppm] = −6.33(s). Element ArF2BH. The remaining solid was dissolved in hexane (1 mL)
analysis: calcd for C25H14BF18N: C 44.08, H 2.07, N 2.06%; and stored at −20 °C overnight, affording an orange solid
found C 44.19, H 2.13, N 1.89%.
which was filtered and dried under vacuum to give pure
complex 11b (146 mg, 68%). X-ray quality crystals were grown
from a mixture of hexane and toluene at −20 °C. 1H NMR
Synthesis of complex 8b
A solution of complex 5b (232 mg, 0.343 mmol) in hexane (400 MHz, C6D6): δ [ppm] = 8.35(s, 1H, meta-C6H2), 8.17(s, 2H,
(3 mL) placed in a Parr autoclave was subjected to 60 atm H2. meta-C6H2), 7.98(s, 1H, meta-C6H2), 6.11 (d, 1H, 3JH–H = 6.9 Hz,
3
The reaction was stirred at 80 °C for 24 h. The resulting CHCCH), 5.98 (t, 1H, JH–H = 6.9 Hz, CHCHCH), 5.58 (s, 1H,
3
hexane solution was concentrated to ∼2 mL, affording a white BCH), 5.20 (d, 1H, JH–H = 6.9 Hz, CHCCH3), 1.58 (s, 3H, CH3),
solid which was filtered and dried under vacuum to give pure 0.86 (s, 12H, C(CH3)2). 19F NMR (376 MHz, C6D6): δ [ppm] =
complex 8b (163 mg, 71%). X-ray quality crystals were grown −50.23 (br s, 3F, ortho-CF3-C6H2), −53.69 (br s, 3F, ortho-CF3-
from CH2Cl2 at −20 °C. 1H NMR (400 MHz, CD2Cl2): δ [ppm] = C6H2), −56.08, (br s, 3F, ortho-CF3-C6H2), −59.24 (br s, 3F,
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ortho-CF3-C6H2), −62.98 (s, 3F, para-CF3-C6H2), −63.20 (s, 3F,
para-CF3-C6H2). 13C NMR (125 MHz, C6D6): δ [ppm] = 164.17
(CHCCH), 152.89 (br, ipso-C6H2), 149.70 (br, ipso-C6H2), 146.77
L.-M. Elmer, C. Rosorius, C. G. Daniliuc, S. Grimme,
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M. Leskelä, T. Repo, P. Pyykkö and B. Rieger, J. Am. Chem.
Soc., 2008, 130, 14117; (b) K. Chernichenko, M. Nieger,
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M. Leskelä and T. Repo, Chem. – Eur. J., 2013, 19, 10412;
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M. Leskelä and T. Repo, Nat. Chem., 2013, 5, 718;
(e) M. Lindqvist, K. Borre, K. Axenov, B. Kótai, M. Nieger,
M. Leskelä, I. Pápai and T. Repo, J. Am. Chem. Soc., 2015,
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2
(CHCCH3), 138.79 (q, JC–F = 34 Hz, ortho-C6H2), 137.65
2
(CHCHCH), 135.19 (q, JC–F = 34 Hz, ortho-C6H2), 130.73 (q,
2
2JC–F = 34 Hz, ortho-C6H2), 129.94 (q, JC–F = 34 Hz, para-C6H2),
1
125.37 (meta-C6H2), 125.28 (q, JC–F = 275 Hz, ortho-CF3),
1
123.91 (q, JC–F = 275 Hz, para-CF3), 122.42 (CHCCH), 112.20
(CHCCH3), 110.21 (BCH), 86.49 (C(CH3)2), 24.27 (C(CH3)2),
19.91 (CH3). 11B NMR (160 MHz, C6D6): δ [ppm] = 26.59 (s,
Bpin), 44.53 (s, BArF). Element analysis: calcd for
C31H23B2F18NO2: C 46.25, H 2.88, N 1.74%; found C 46.13, H
2.96, N 1.51%.
Acknowledgements
Financial support from the National Nature Science Foun-
dation of China (21372048), Shanghai Science and Technology
Committee (Shanghai Rising-Star Program 13QA1400500) and
Fudan University is gratefully acknowledged.
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