Monomeric (dialkylamino)boranes: A new and efficient boron source in palladium catalyzed C-B bond formation with aryl halides

Article abstract of DOI:10.1039/b306874a

Thermal decomposition of hindered amine-borane adducts leads in high yields to monomeric (dialkylamino)boranes R₁R₂N-BH ₂ (R₁ and R₂ = alkyl) that are new and efficient boron-sources in the Pd⁰

Full text of DOI:10.1039/b306874a

Monomeric (dialkylamino)boranes: a new and efficient boron source in
palladium catalyzed C–B bond formation with aryl halides
Lisenn Euzenat, David Horhant, Yann Ribourdouille, Christophe Duriez, Gilles Alcaraz* and Michel
Vaultier
S. E. S. O. UMR-CNRS 6510, Université de Rennes1-Beaulieu, F-35042, Rennes, France.
E-mail: gilles.alcaraz@univ-rennes1.fr; Fax: 33 22323 6955; Tel: 33 22323 5672
Received (in Cambridge, UK) 17th June 2003, Accepted 17th July 2003
First published as an Advance Article on the web 31st July 2003
Thermal decomposition of hindered amine-borane adducts
leads in high yields to monomeric (dialkylamino)boranes
R1R2N–BH2 (R1 and R2 = alkyl) that are new and efficient
boron-sources in the Pd0 catalyzed borylation reaction
affording monomeric aryl(dialkylamino)boranes R1R2N–
BHR3 (R3 = aryl).
vs. 51% with the overcrowded 2c; satisfactory conversion being
obtained with 2d and 2b.
It was also of interest to investigate the scope of the reactivity
of aminoboranes 2 toward different haloarenes. Selected results
with 2a and representative aryl halides 3 are listed in Table 1.
2a displays an H : B ratio of 2 as compared to 1 in
pinacolborane. Nevertheless, hydrogenolysis of the carbon–
halogen bond was barely observed with aryliodides and limited
to some extent with arylbromides except in the case of
bromonaphthalene (Table 1-entry 11). Best results were
obtained with electron-rich aryl halides compare to electron
poor haloarenes (Table 1-entries 12, 13).
We have also tried to get some insights into the mechanism of
catalyzed borylation reactions7,8 with organoboranes. In Ma-
suda borylation,9 authors had inconclusive evidences for a
mechanism involving an aryl-PdII-X intermediate resulting
from the oxidative addition of 3 to the [(Ph3P)2Pd0] active
intermediate. However, they did not exclude the formation of
aryl–Pd–B(OR)2 via a s-bond metathetic pathway between the
aryl halide and a H–Pd–B(OR)2 intermediate resulting from a
first addition of the organoborane to palladium. In our case with
2a, two experimental features are different from Masuda’s
observations. Starting from iodobenzene and 2a in the presence
Aminoboranes (R1R2N–BR3R4) are well known in material
science as precursors of BN-based ceramics.1 Among them and
since discovery of the hydroboration reaction in 1956, (amino)-
dihydridoboranes (R1R2N–BH2) were scarcely studied as useful
tools for organic synthesis probably because of their lack of
reactivity as hydroborating agents.2
One would expect (amino)dihydridoboranes to offer a
moderate synthetic attraction and the usual borane reductive
properties via hydride transfer. Interestingly, their potential is
greatly broadened with the present description of their unprece-
dented boron transfer ability in palladium-catalyzed Csp2–B
bond formation.
(Amino)dihydroboranes readily form mixtures of cyclic and
linear oligomers3 that often prevent any purification. Provided
that the nitrogen atom is hindered enough, they can be obtained
in their monomeric form by tedious reduction of corresponding
(amino)dihaloboranes4 or via the under-used thermally-induced
dehydrogenation5 of chosen secondary amine-borane adducts.
We then revisited this thermal decomposition considering the
incidence of steric hindrance on the physical properties of the
final product in order to define the best monomeric candidates,
including a chiral version6 for further synthetic purposes.
Monomeric aminoboranes 2a–f were easily isolated in more
than 80% yield in a multigram scale as distillable and non-
pyrophoric liquids (Scheme 1). 11B NMR spectroscopy re-
vealed in all cases a deshielded triplet in the range of 35–36 ppm
1
(122 < JBH < 137 Hz) which is in agreement with the
expected monomeric aminoboranes 2 structures and excluded
ipso facto the cyclic patterns.3
Scheme 2 Catalyzed borylation of 4-iodoanisole with (dialkylamino)bor-
anes 2.
The potential of (dialkylamino)boranes 2 was then evaluated
through their ability to react in a palladium-catalysed coupling
process, in dioxane at 80 °C for 15 h in the presence of
4-iodoanisole (3a-I), triethylamine as the base and a catalytic
amount (5 mol%) of (Ph3P)2PdCl2. Interestingly, (dialkylami-
no)boranes 2a–d led to the corresponding aryl(dialkylamino)-
monohydridoborane 4a–7a (Scheme 2) in good to high yields.
The outcome of the borylation of 4-iodoanisole (3a(I)) appeared
to be dependent on the steric bulk of the amino moiety in 2.
Borylation was performed in 99% yield with less hindered 2a
Table 1 Borylation of representative aryl halides 3 with 2a
Entry
3
Yield (%)a
1
2
3
4
5
6
7
8
9
10
11
12
13
3b-I: C6H5–I
3b-Br: C6H5–Br
4b
4b
4c
4c
4d
4d
4e
4e
4f
4g
4g
4h
4i
99%
79%
85%
80%
91%
73%
94%
50%
99%
80%b
20%c
70%
34%d
3c-I: 4-Me–C6H4–I
3c-Br: 4-Me–C6H4–Br
3d-I: 3-Me–C6H4–I
3d-Br: 3-Me–C6H4–Br
3e-I: 2-Me–C6H4–I
3e-Br: 2-Me–C6H4–Br
3f-I: 4-Me2N–C6H4–I
3g-I: 1-C10H7–I
3g-Br: 1-C10H7–Br
3h-Br: 4-F–C6H4–Br
3i-Br: 4-NC–C6H4–Br
a Isolated yields. 20%. b 60%. c And 40%. d Hydrogenolysis.
Scheme 1 Thermal decomposition of secondary amine-borane adducts 1.
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This journal is © The Royal Society of Chemistry 2003

of 5 mol% of authentically isolated [PhPd(I)(PPh3)2], phenyl-
(diisopropylamino)borane 4b was isolated in 85% yield in the
same reaction conditions. Moreover, starting from 2a and
4-fluorobromobenzene (Table 1-entry 12), coupling product 4h
was contaminated by about 10% of 4b (PhBHNiPr2) resulting
from aryl exchanges of [4-F–C6H4–Pd(Br)(PPh3)2] giving
[PhPd(Br)(PPh3)(PPh2(4-F–C6H4))].10 Accordingly, with (dii-
sopropylamino)borane 2a as the boron-donor, the likely
oxidative addition step of 3 is clearly confirmed. Also of interest
is the impact of electronic effects in the organic electrophiles for
the outcome of the borylation reaction.
Notes and references
† Selected spectroscopic data: for 2a: 1H NMR (300 MHz, CDCl3): d 1.17
(d, CH3), 3.38 and 3.41 (hept, CH). 13C{1H} NMR (75 MHz, CDCl3): d
25.1 (s, CH3), 52.3 (s, CH). 11B NMR (96 MHz, CDCl3): 35.0 (d, JBH
1
126.2, BH). 14N NMR (21.7 MHz, CDCl3) 2209 (s, br, NB). For 5a 1H
NMR (300 MHz, CDCl3): d 1.21 and 1.34 (d, CH3), 3.42 and 4.34 (hept,
CH), 3.87 (s, OCH3), 6.96 and 7.47 (d, CH aryl). 13C{1H} NMR (75 MHz,
CDCl3): d 22.6 and 27.4 (s, CH3 iPr), 44.9 and 49.4 (s, CH Pr), 55.2 (s,
i
OCH3), 113.5 and 135.5 (s, CH aryl), 159.8 (s, CIV–OMe). 11B NMR (96
MHz, CDCl3): 37.6 (d, 1JBH 90, BH). HRMS [M+·] calcd. for C13H22NOB:
m/z 219.1794. Found 219.1794 (0 ppm).
It is known that the oxidative addition is favoured with
electron poor aryl halides. This fact takes much importance in
the Suzuki–Miyaura coupling in which oxidative addition is the
rate-determining step of the catalytic cycle.11 In our case with
aminoborane 2a, the results suggest that rate-limitation in this
catalytic process could be attributed to a slow transmetalation
that allow the phosphine-bound aryl side coupling to take place
in enhanced ratio. However, the transmetalation step is a
debatable question. To rationalize this point, Masuda et al.
proposed an intriguing triethylamine/pinacol- or catecholborane
interaction mode. Lewis acid–base adduct is the most common
interaction mode that one would normally expect between an
amine and a BIII-containing organoborane.12 Authors however
suggested, by analogy with trichlorosilane reactivity, this
interaction would lead to an ammonium/boride ion pair of type
Et3NH+.2B(OR)2 with the boride as the active transmetalating
anion.13 With pinacol- or catecholborane acting as a proton
provider able to protonate triethylamine into triethylammon-
ium, this process is difficult to conceive outside the coordina-
tion sphere of a transition-metal.14 In order to check the
proposed hypothesis, various amounts of triethylamine were
added to 2a and pinacolborane respectively. Monitoring the
reactions by variable temperature 11B NMR and 14N NMR
spectroscopy did not allow us to detected either ionic nor Lewis
acid–base interaction between the reagents even with triethyla-
mine as the solvent.15 Aminoborane 2a and (Ph3P)4Pd in
stoichiometric amounts did neither afford any detectable entity
exhibiting the H–Pd–B sequence. From PhPd(I)(PPh3)2 and a
two fold excess of aminoborane 2a, triethylamine and heat
appeared both necessary to give 4b. These results are not in
favour of Masuda’s hypothesis even if they do not completely
clarify the role of triethylamine in the post oxidative-addition
mechanism. The involvement of a PdIV-transient species
resulting of the oxidative addition of the borane to the X-PdII-
aryl intermediate would be an elegant way out but without extra
evidences the mechanism keeps open.
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In conclusion, monomeric aminoboranes 2 are easily and
cleanly obtained from secondary amine-borane adducts 1 in a
multigram scale. They are excellent and new boron transferring
agents that tolerate a wide range of aryl halides 3. Experimental
evidence now clearly confirm a mechanism involving aryl-Pd-
X species via the oxidative addition of 3 to the [(Ph3P)2Pd]
active catalyst in a first non rate-determining elementary step.
The role of the amine in the transmetallation step is not
understood and the mechanism is currently under investiga-
tion.†
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