Rhodium-catalyzed hydroborations of allylamine and allylimines

Article abstract of DOI:10.1139/v01-177

The in situ rhodium-catalyzed addition of catecholborane (HBcat, cat = 1,2-O₂C₆H₄) and pinacolborane (HBpin, pin = 1,2-O₂C₂Me₄) to allylamine, allylimine, 2- and 4-vinylpyridines, and a thienyl imine has been examined using multinuclear NMR spectroscopy. Although reactions of allylamine (H₂NCH₂CH=CH₂) and HBcat gave complex product distributions arising from competing dehydrogenative borylation pathways, addition of HBpin to allylamine using a rhodium catalyst afforded only products arising from hydroboration (RN(Bpin)CH₂CH₂CH₂Bpin, where R = H, Bpin) and hydrogenation (RN(Bpin)CH₂CH₂CH₃). Hydroboration of allylimines (RHC=NCH₂CH=CH₂, R = Ar) with HBcat occurs initially at the more reactive imine functionality to give unsaturated borylamines (RCH₂N(Bcat)CH₂CH=CH₂). Further reaction with HBcat gives varying amounts of hydroboration products RCH₂N(Bcat)CH₂CH₂CH₂Bcat and RCH₂N(Bcat)CH₂CH(Bcat)CH₃ as well as the diboration product RCH₂N(Bcat)CH₂CH₂CH(Bcat)₂, depending on the choice of catalyst. Reactions with related unsaturated pyridine derivatives are complicated by extensive degradation, which can be avoided by coordination of the pyridine nitrogen to a Lewis acid. The first examples of metal-catalyzed hydroboration of imines using HBpin are also reported.

Full text of DOI:10.1139/v01-177

1898
Rhodium-catalyzed hydroborations of allylamine
and allylimines1
Christopher M. Vogels, Paul E. O’Connor, Trevor E. Phillips, Keith J. Watson,
Michael P. Shaver, Paul G. Hayes, and Stephen A. Westcott
Abstract: The in situ rhodium-catalyzed addition of catecholborane (HBcat, cat = 1,2-O2C6H4) and pinacolborane
(HBpin, pin = 1,2-O2C2Me4) to allylamine, allylimine, 2- and 4-vinylpyridines, and a thienyl imine has been examined
using multinuclear NMR spectroscopy. Although reactions of allylamine (H2NCH2CH=CH2) and HBcat gave complex
product distributions arising from competing dehydrogenative borylation pathways, addition of HBpin to allylamine using
a rhodium catalyst afforded only products arising from hydroboration (RN(Bpin)CH2CH2CH2Bpin, where R = H, Bpin)
and hydrogenation (RN(Bpin)CH2CH2CH3). Hydroboration of allylimines (RHC=NCH2CH=CH2, R = Ar) with HBcat
occurs initially at the more reactive imine functionality to give unsaturated borylamines (RCH2N(Bcat)CH2CH=CH2).
Further reaction with HBcat gives varying amounts of hydroboration products RCH2N(Bcat)CH2CH2CH2Bcat and
RCH2N(Bcat)CH2CH(Bcat)CH3 as well as the diboration product RCH2N(Bcat)CH2CH2CH(Bcat)2, depending on the
choice of catalyst. Reactions with related unsaturated pyridine derivatives are complicated by extensive degradation, which
can be avoided by coordination of the pyridine nitrogen to a Lewis acid. The first examples of metal-catalyzed hydroboration
of imines using HBpin are also reported.
Key words: catalysis, hydroboration, boronate esters, dehydrogenative borylation, allylimines.
Résumé : Faisant appel à la spectroscopie RMN multinucléaire, on a étudié la réaction in situ, catalysée par le rhodium,
du catécholborane (HBcat, cat= 1,2-O2C6H4) et du pinacolborane (HBpin, pin = 1,2-O2C2Me4) avec l’allylamine,
l’allylimine, les 2- et 4-vinylpyridines et une thiénylimine. Même si les réactions de l’allylamine (H2NCH2CH=CH2)
avec le HBcat conduisent à des distributions complexes de produits provenant de voies de borylation déshydrogénante
en compétition, l’addition du HBpin à l’allylamine en présence d’un catalyseur de rhodium ne conduit qu’aux produits
provenant d’une hydroboration (RN(Bpin)CH2CH2CH2Bpin dans lequel R = H ou Bpin) ou d’une hydrogénation
(RN(Bpin)CH2CH2CH3). L’hydroboration des allylimines (RHC=NCH2CH=CH2, R = Ar) à l’aide de HBcat se produit
initialement au niveau de la fonctionnalité imine qui est la plus réactive pour conduire à la formation de borylamines
(RCH2N(Bcat)CH2CH=CH2). Une réaction subséquente avec du HBcat conduit à des quantités variables de produits
d’hydroboration, RCH2N(Bcat)CH2CH2CH2Bcat et RCH2N(Bcat)CH2CH(Bcat)CH3 ainsi qu’au produit de diboration,
RCH2N(Bcat)CH2CH2CH(Bcat)2, suivant le catalyseur choisi. Les réactions avec les dérivés insaturés de la pyridine apparentés
sont compliquées par d’importantes réactions de dégradation que l’on peut éviter en procédant à une coordination de l’azote de
la pyridine à l’aide d’un acide de Lewis. On rapporte aussi les premiers exemples de réactions d’hydroboration, catalysées
par des métaux, d’imines à l’aide de HBpin.
Mots clés : catalyse, hydroboration, esters de l’acide boronique, borylation déshydrogénante, allylimines.
[Traduit par la Rédaction] Vogels et al. 1905
Introduction
addition of HBcat to organic substrates has become an important
and well-established technique in organic synthesis (for an
excellent review on hydroborations catalyzed by transition-metal
complexes, see ref. 2) (3, 4). These reactions can have
regio-, chemo-, or stereoselectivities, complementary, or more
remarkably, opposite to those from products obtained via the
uncatalyzed variant. Indeed, hydroborations of 5-hexene-2-
one with HBcat proceed readily at room temperature to give
exclusive formation of a borate product where the borane
has added to the more reactive carbonyl double bond (4).
However, when the reaction is carried out at 0°C in the pres-
The hydroboration of alkenes and alkynes, which constitutes
the formal addition of a B—H bond across a carbon—carbon
multiple bond, is an extremely important reaction in organic
synthesis (1). Although simple boron hydride reagents such as
borane (H3B·X, where X is a Lewis base) and 9-
borabicyclo[3.3.1]nonane react readily with alkenes at room
temperature, hydroborations with catecholborane (HBcat, cat =
1,2-O2C6H4) generally require elevated temperatures. The
discovery that transition metals can be used to catalyze the
Received 24 May 2001. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on December 10, 2001.
C.M. Vogels, P.E. O’Connor, T.E. Phillips, K.J. Watson, M.P. Shaver, P.G. Hayes, and S.A. Westcott.2 Department of
Chemistry, Mount Allison University, Sackville, NB E4L 1G8, Canada.
1Dedicated to the memory of Dr. Slayton A. Evans, Jr.
2Corresponding author (telephone: (506) 364-2351; fax: (506) 364-2313; e-mail: swestcott@mta.ca).
Can. J. Chem. 79: 1898–1905 (2001)
DOI: 10.1139/cjc-79-12-1898
© 2001 NRC Canada

Vogels et al.
1899
Scheme 1.
O
B
O
O
Uncatalyzed
O
O
+
HB
O
O
O
B
HBcat
5 mol% RhCl(PPh3)3
0oC
O
ence of
a
catalytic amount of Wilkinson’s catalyst
at 67.8 MHz. Multiplicities are reported as (s) singlet,
(d) doublet, (t) triplet, (q) quartet, (m) multiplet, (br) broad,
and (ov) overlapping. Infrared spectra were obtained using a
Mattson Genesis II FT IR spectrometer and are reported
in cm–1. Microanalyses for C, H, and N were carried out at
Desert Analytics (Tucson, Arizona).
(RhCl(PPh3)3), addition of the B—H bond occurs primarily
at the less reactive alkene moiety to give an organoboronate
ester product (Scheme 1).
Organoboronate esters (RB(OR>)2) and boronic acids
(RB(OH)2) belong to a remarkable class of compounds that
have been used extensively in organic (5–9), organometallic
(10, 11), and solid-phase synthesis (12, 13), as well as in
macrocyclic chemistry (14), molecular recognition (15),
transporting molecules across biological membranes (16),
redox switching (17), and as glucose sensors (18, 19). Interest
in these compounds also arises from their unique biological
activities (20). For instance, peptide boronic acids are among
the most potent, known inhibitors of serine proteases (21).
As a result, the synthesis of boron-containing amino acid de-
rivatives has become an area of considerable interest (22, 23).
Hydroborations of tertiary allylic amines (R2NCH2CH=CH2,
R = alkyl, aryl) with borane (H3B·X, X = SMe2, THF) are
known to give initial formation of the expected anti-
Markovnikov products, R2NCH2CH2CH2BH2 (6, 24–26).
Analogous reactions with primary and secondary amines,
however, are complicated by direct interaction with the N—H
bond to give a number of N-boryl products (27, 28). Protection
of the N—H bond is usually required in these reactions to
ensure chemoselective addition of the borane at the allyl
group (29, 30). Unfortunately, deprotection methodologies
frequently compromise the integrity of the B—C bond (31).
We, therefore, decided to investigate the in situ reactions of
allylamine (H2NCH2CH=CH2, 1) and related allylimine
derivatives with catecholborane (HBcat, cat = 1,2-O2C6H4)
and pinacolborane (HBpin, pin = 1,2-O2C2Me4) to see if we
could generate novel aminoboron compounds without using
a protecting group.
Preparation of Rh(acac)(coe)2
Rh(acac)(coe)2 was prepared by modification of an estab-
lished procedure (39). Tl(acac) (0.42 g, 1.40 mmol) was
dissolved in 10 mL of THF and added dropwise to a solution
of [RhCl(coe)2]2 (0.50 g, 0.70 mmol) in 10 mL of THF. The
reaction was allowed to stir for 18 h then stored for 48 h
at –25°C. A greenish precipitate was filtered whereupon
subsequent removal of the THF under vacuum afforded an
orange solid. IR (Nujol): 2904, 1581, 1520, 1464, 1377,
1
1317, 1269, 1020, 897, 766, 735, 604, 550, 519. H NMR
(in C6D6) d: 5.03 (s, 1H), 2.51 (br, 4H), 2.41 (br, 8H), 1.68
(br s, 12H), 1.55 (br, 4H), 1.40 (br, 6H). 13C NMR d: 185.0,
98.8 (CH), 78.0 (d, JC-Rh = 12 Hz, -CH=CH-), 30.3 (CH2),
27.9 (CH2), 27.0 (CH2), 26.8 (CH3).
Addition of catecholborane (HBcat) to allylamine (1)
In a typical reaction, 2 equiv of HBcat in 0.5 mL of C6D6
were added dropwise to a 0.5 mL C6D6 solution of
allylamine. The mixture was allowed to stir for 1 h and then
analyzed by multinuclear NMR spectroscopy. Attempts to
control the selectivity of these reactions always gave minor
amounts of 2, which precipitated out of solution after ca.
1 h. 2: IR (Nujol): 3200, 2940, 2868, 2443, 1600, 1464,
1
1377, 1350, 1232, 1099, 1057, 1083, 912, 808, 739, 700. H
NMR (in d6-acetone) d: 8.00 (br, cat), 7.09 (br ov m, cat),
6.48 (br, cat), 6.05 (br ov m, 1H, CH=CH2), 5.40 (br ov m,
2H, CH=CH2), 3.88 (br, 2H, -CH2CH=CH2). 11B NMR d: 23
1
(br, NBcat), 13 (s, N J HBcat). 3: H NMR (in C6D6) d:
Experimental
7.01 (d of d, J = 6, 3 Hz, 4H, cat), 6.73 (d of d, J = 6, 3 Hz,
4H, cat), 5.82 (ov d of d of t, J = 17, 10, 7 Hz, 1H, -CH=CH2),
5.16 (d of d, J = 17, 2 Hz, 1H, -CH=CHH), 4.94 (d of d, J = 10,
2 Hz, 1H, -CH=CHH), 3.92 (d, J = 7 Hz, 2H, -CH2CH=CH2).
11B NMR d: 26 (br, NBcat). 13C NMR d: 148.6, 136.3, 122.3,
115.1, 112.1, 46.3.
All reagents and solvents used were obtained from Aldrich
Chemicals. Complexes RhCl(PPh3)3 (32), [RhCl(coe)2]2 (coe =
cis-cyclooctene, (33)), and [RhCl(cod)]2 (cod
cyclooctadiene, (34)) were prepared as described elsewhere.
Imines were prepared by well-established procedures (35–38).
NMR spectra were recorded on a JEOL JNM-GSX270 FT
= cis-
1
Catalyzed hydroborations of allylamine (1) with HBcat
In a typical reaction, 5 equiv of HBcat in 0.5 mL of C6D6
were added to a 0.5 mL C6D6 solution of allylamine and
1 mol% RhCl(PPh3)3. The mixture was allowed to stir for 1 h
and then analyzed by multinuclear NMR spectroscopy. 4: IR
NMR spectrometer. H NMR chemical shifts are reported in
ppm and referenced to residual protons in deuterated solvent
at 270.1 MHz. 11B NMR chemical shifts are referenced to
external F3B·OEt2 at 86.6 MHz. 13C NMR chemical shifts
are referenced to solvent carbon resonances as internal standards
© 2001 NRC Canada

1900
Can. J. Chem. Vol. 79, 2001
(Nujol): 3060, 2962, 2935, 2873, 1610, 1511, 1467, 1344,
1232, 1182, 1130, 1083, 1007, 941, 866, 806, 733, 680, 498.
1H NMR (in d6-acetone) d: 7.60 (br ov m, min), 7.23 (br s,
cat), 7.13 (br s, cat), 6.57 (br s, cat), 4.1–3.9 (ov m, min),
3.80 (t, J = 8 Hz, -NCH2-), 3.58 (t, J = 8 Hz, -NCH2-), 2.09
(ov m, -CH2CH2CH2-), 1.72 (ov t of t, J = 8 Hz, -CH2CH2CH2-),
1.34 (t, J = 8 Hz, -CH2B), 0.88 (t, J = 8 Hz, -CH2B). 11B
NMR d: 35 (br, CBcat), 22 (br, NBcat), 13 (s, N J HBcat).
5: 1H NMR (in C6D6) d: 7.06–6.68 (ov m, 12H, cat), 3.46 (t, J =
8 Hz, 2H, -NCH2-), 1.90 (ov t of t, J = 8 Hz, 2H, -CH2CH2CH2-),
1.07 (t, J = 8 Hz, 2H, -CH2B). 11B NMR d: 34 (br, CBcat),
1.11 (s, 12H, pin), 0.75 (t, J = 8 Hz, 3H, -CH3). 11B NMR d:
25 (br, NBpin). 15: 1H NMR d: 3.39 (t, J = 8 Hz, 2H, -NCH2-),
1.74 (ov t of q, J = 8 Hz, 2H, -CH2CH2CH3), 1.05 (s, 24H,
pin), 0.93 (t, J = 8 Hz, 3H, -CH3). 11B NMR d: 25 (br,
NBpin).
Catalyzed hydroborations of allylimines (16a–16d) with
HBcat
In a typical reaction, 3 equiv of catecholborane in 0.5 mL
of C6D6 were added to a 0.5 mL C6D6 solution of allylimine
16c in the presence of 1 mol% RhCl(PPh3)3. The mixture
was allowed to stir for 1 h and then analyzed by
1
26 (br, NBcat). 6: H NMR d: 7.06–6.68 (ov m, 12H, cat),
1
3.76 (2nd order d of d, J = 14, 8 Hz, 1H, -NCHH-), 3.64
(2nd order d of d, J = 14, 8 Hz, 1H, -NCHH-), 1.87 (ov t of
q, J = 8 Hz, 1H, -CH(Bcat)CH3), 1.15 (d, J = 8 Hz, 3H, -CH3.
multinuclear NMR spectroscopy. 17c: H NMR (in C6D6) d:
7.01–6.65 (ov m, 11H, cat and Ar), 4.30 (s, 2H, -NCH2Ar),
3.09 (t, J = 8 Hz, 2H, -NCH2-), 1.69 (ov t of t, J = 8 Hz, 2H,
-CH2CH2CH2-), 0.99 (t, J = 8 Hz, 2H, -CH2B). 11B NMR d:
1
11B NMR d: 34 (br, CBcat), 26 (br, NBcat). 8: H NMR d:
1
7.06–6.68 (ov m, 16H, cat), 3.62 (t, J = 8 Hz, 2H, -NCH2-),
2.48 (ov d of t, J = 8 Hz, 2H, -CH2CH2CH-), 1.70 (t, J =
8 Hz, 1H, -CH(Bcat)2). 11B NMR d: 34 (br, CBcat), 26 (br,
35 (br, CBcat), 26 (br, NBcat). 18c: H NMR d: 7.01–6.65
(ov m, 11H, cat and Ar), 4.31 (s, 2H, -NCH2Ar), 3.46 (2nd
order d of d, J = 14, 8 Hz, 1H, -NCHH-), 3.31 (2nd order d
of d, J = 14, 8 Hz, 1H, -NCHH-), 1.68 (ov t of q, J = 8 Hz,
1H, -CH(Bcat)CH3), 1.07 (d, J = 8 Hz, 3H, -CH3). 11B
NMR d: 35 (br, CBcat), 26 (br, NBcat). 19c: 1H NMR d: 7.01–
6.65 (ov m, 15H, cat and Ar), 4.30 (s, 2H, -NCH2Ar),
3.25 (t, J = 8 Hz, 2H, -NCH2-), 2.18 (ov d of t, J = 8 Hz,
2H, -CH2CH2CH-), 1.63 (t, J = 8 Hz, 1H, -CH(Bcat)2). 11B
1
NBcat). 9: H NMR d: 7.17–6.64 (ov m, 8H, cat), 3.34 (t, J =
8 Hz, 2H, -NCH2-), 1.57 (ov t of q, J = 8 Hz, 2H, -CH2CH2CH3),
0.81 (t, J = 8 Hz, 3H, -CH3).
Catalyzed hydroborations of H2NCH2CHϵCH with HBcat
In a typical reaction, 5 equiv of HBcat in 0.5 mL of C6D6
were added to a 0.5 mL C6D6 solution of propargyl amine
and 1 mol% RhCl(PPh3)3. The mixture was allowed to stir
for 1 h and then analyzed by multinuclear NMR spectroscopy.
Products were similar to those observed for reactions with 1.
1
NMR d: 35 (br, CBcat), 26 (br, NBcat). 20c: H NMR d:
7.01–6.65 (ov m, 7H, cat and Ar), 4.30 (s, 2H, -NCH2Ar),
3.00 (t, J = 8 Hz, 2H, -NCH2-), 1.38 (ov t of q, J = 8 Hz,
2H, -CH2CH2CH3), 0.71 (t, J = 8Hz, 3H, -CH3). 11B NMR d:
26 (br, NBcat).
Addition of pinacolborane (HBpin) to allylamine (1)
In a typical reaction, 2 equiv of HBpin in 0.5 mL of C6D6
were added dropwise to a 0.5 mL C6D6 solution of allylamine.
The mixture was allowed to stir for 1 h and then analyzed by
Catalyzed hydroborations of vinylpyridines-BF3 with HBcat
In a typical reaction, 1.1 equiv of BF3·OMe2 were added
to a 0.5 mL solution of the vinylpyridine in C6D6. The resulting
solution was allowed to stir for 2 h whereupon 1 mol% of
Rh(acac)(coe)2/dppb (dppb = 1,4-bis(diphenylphosphino)butane)
in 0.25 mL of C6D6 was added to the mixture, followed by
the addition of a 0.25 mL C6D6 solution of 1.2 equiv of
HBcat. The mixture was allowed to stir for 1 h and then
analyzed by multinuclear NMR spectroscopy. 22: 1H (in
C6D6) d: 8.55 (d, J = 5 Hz, 1H), 7.04–6.78 (ov m, 6H), 6.50
(app t, J = 5 Hz, 1H), 4.34 (q, J = 8 Hz, 1H, -CH(Bcat)-),
1.36 (d, J = 8 Hz, 3H, -CH3). 11B NMR d: 34 (br, CBcat), 1
(s, NBF3). 13C NMR d: 162.2, 148.5, 143.7, 142.5, 125.4,
123.0, 122.4, 112.7, 23.8 (br, CB), 14.9. 24: 1H NMR d: 8.28
(d, J = 5 Hz, 2H), 7.07 (d of d, J = 3, 1 Hz, 2H), 6.88 (d of
d, J = 3, 1 Hz, 2H), 6.66 (d, J = 5 Hz, 2H), 2.39 (q, J =
8 Hz, 1H, -CH(Bcat)-), 1.14 (d, J = 8Hz, 3H, -CH3). 11B
NMR d: 34 (br, CBcat), 1 (s, NBF3). 13C NMR d: 161.2,
148.8, 143.3, 125.5, 124.1, 113.2, 25.6 (br, CB), 15.0.
1
multinuclear NMR spectroscopy. 10: H NMR (in C6D6) d:
5.73 (ov d of d of t, J = 16, 8, 8 Hz, 1H, -CH2CH=CH2),
5.10 (d of d, J = 16, 1 Hz, 1H, -CH2CH=CHH), 4.92 (d of d,
J = 8, 1 Hz, 1H, -CH2CH=CHH), 3.53–3.50 (br d, J = 8 Hz,
2H, -CH2CH=CH2), 2.21 (br s, 1H, NH), 1.10 (s, 12H, pin).
13C NMR d: 139.4, 112.7, 81.7, 43.7, 24.5. 11B NMR d: 24
(br, NBpin).
Catalyzed hydroborations of allylamine (1) with HBpin
In a typical reaction, 3 equiv of HBpin in 0.5 mL of C6D6
were added to a 0.5 mL C6D6 solution of allylamine and
RhCl(PPh3)3 (1 mol%). The reaction mixture was allowed to
stir for 12 h and then analyzed by multinuclear NMR
1
spectroscopy. 11: H NMR (in C6D6) d: 6.50 (ov d of d of t, J =
18, 10, 8 Hz, 1H, -CH2CH=CH2), 4.50 (d of d, J = 18, 2 Hz, 1H,
-CH2CH=CHH), 4.37 (d of d, J = 10, 2 Hz, 1H, -CH2CH=CHH),
3.90 (d, J = 8 Hz, 2H, -CH2CH=CH2), 1.10 (s, 24H, pin).
1
11B NMR d: 26 (br, NBcat). 12: H NMR (in C6D6) d: 3.00
Catalyzed hydroborations of vinylpyridines-BF3 with HBpin
In a typical reaction, 1.1 equiv of BF3·OMe2 were added
to a 0.5 mL solution of the vinylpyridine in C6D6. The resulting
solution was allowed to stir for 2 h whereupon RhCl(PPh)3
(1 mol%) in 0.25 mL of C6D6 was added to the mixture,
followed by the addition of a 0.25 mL C6D6 solution of
1.2 equiv of HBpin. The mixture was allowed to stir for 12 h
and then analyzed by multinuclear NMR spectroscopy. Selected
(ov d of t, J = 8 Hz, 2H, -NCH2-), 2.27 (br, 1H, NH), 1.60
(ov t of t, J = 8 Hz, 2H, -CH2CH2CH2-), 1.07–0.99 (ov m,
26H, -CH2B and pin). 11B NMR d: 33 (br, CBpin), 25 (br,
NBpin). 13: 1H NMR d: 3.50 (t, J = 8 Hz, 2H, -NCH2-), 2.04
(ov t of t, J = 8 Hz, 2H, -CH2CH2CH2-), 1.26–1.03 (ov m,
38H, -CH2B and pin). 11B NMR d: 33 (br, CBpin), 25 (br,
NBpin). 14: 1H NMR d: 2.84 (ov d of t, J = 8 Hz, 2H, -NCH2-),
2.18 (br, 1H, NH), 1.26 (ov t of q, J = 8 Hz, 2H, -CH2CH2CH3),
1
spectroscopic data for reactions with 21: H NMR (in C6D6)
© 2001 NRC Canada

Vogels et al.
1901
Scheme 2.
Bcat
Hydroboration
catB
catB
N(Bcat)2
N(Bcat)2
+
5
6
NH2
2HBcat
-2H2
hydrogenation
N(Bcat)2
N(Bcat)2
Dehydrogenative
borylation
HBcat
catB
N(Bcat)2
hydroboration
1
1 mol% Rh
3
Bcat
HBcat
7
8
HBcat
NHR
N(Bcat)2
HBcat
NHR
Hydrogenation
HBcat
1 mol% Rh
9
catB
4
2
R = H, Bcat
d: 3.90 (br q, J = 8 Hz, 1H, -CH(Bpin)-), 3.07 (q, J = 8 Hz,
2H, -CH2CH3), 1.28 (d, J = 8 Hz, 3H, -CH(Bpin)CH3), 1.00
(s, Bpin), 0.94 (t, J = 8 Hz, 3H, -CH2CH3). 11B NMR d: 33
(br, CBpin), 23, 21, 1 (s, NBF3). Selected spectroscopic data
for reactions with 23: 1H NMR d: 2.72 (d, J = 8 Hz), 2.30 (t,
J = 8 Hz, -CH2CH2B), 2.14 (q, J = 8 Hz, 1H, CH(Bpin)-),
1.95 (q, J = 8 Hz, 2H, -CH2CH3), 1.17 (br s), 1.06 (d, J =
8 Hz, 3H, -CH(Bpin)CH3), 0.98 (s, Bpin), 0.66 (t, J = 8 Hz,
3H, -CH2CH3). 11B NMR d: 33 (br, CBpin), 21, 1 (s, NBF3).
Addition of excess HBcat is required in all of these catalyzed
reactions to ensure 100% conversion of the starting alkene.
While hydroboration of intermediate 2 gave minor amounts
of HBcat·HNRCH2CH2CH2Bcat (4, R = H, Bcat), reactions
with 3 gave surprisingly complex product distributions,
regardless of the catalyst system used to affect this
transformation (Scheme 2). Although significant amounts of
the hydroboration products 5 and 6 were formed in these
reactions, products (such as 8) derived from a competing
dehydrogenative borylation pathway were also observed (by
NMR spectroscopy). We propose that these unique products
originate from the transient alkenylboronate ester 7, which
presumably results from insertion of the activated alkene into
the Rh—B bond (3, 41). Subsequent b-hydride elimination
would afford the alkenylboronate ester with concomitant
formation of dihydrogen (2, 41). It is possible that
“hydroboration product” 5 also arises from hydrogenation of
alkenylboronate ester 7. Hydride elimination appears to be
specific as products arising from abstraction of a methylene
hydrogen a to the activated amine group are not observed.
Alkenylboronate ester 7 can also add another equiv of HBcat
to give 8. Interestingly, we have found that compound 8 can
also be generated as the major product in analogous
hydroborations of H2NCH2CϵCH. As with most catalyzed
hydroboration reactions, a small amount of hydrogenation
product (9) is almost always observed.
The formation of these products is somewhat unusual as
catalyzed hydroborations of simple alkenes, such as 1-octene,
proceed smoothly to give predominant formation of the expected
organoboronate ester product (2, 4). In this study, replacement
of the two N—H bonds with N-Bcat groups in 3 appears to
be ineffective in deactivating the amine group and catalyzed
reactions proceed to give complex product distributions. The
formation of multiple boronated compounds has recently
been an area of considerable interest (42–44).
Metal-catalyzed hydroborations of unsaturated C—C
bonds using HBpin have been reported previously (45–47).3
In a further attempt to control selectivity we decided to examine
hydroborations using HBpin (Scheme 3). Unlike reactions
with HBcat, which is a stronger Lewis acid, no adduct formation
was observed. Addition of excess HBpin gave the monoboryl
amine HN(Bpin)CH2CH2=CH2 (10), where a second equiv
of the boronate ester failed to add to the amine N—H bond
even at elevated temperatures. Remarkably, we have found
Catalyzed hydroborations of aldimine 25 with
pinacolborane (HBpin)
To a 0.5 mL C6D6 solution of 25 and 1 mol% RhCl(PPh3)3
was added 1.2 equiv of HBpin in 0.5 mL of C6D6. The reaction
was heated at reflux for 2 h and then analyzed by multinuclear
NMR spectroscopy. 26: 1H NMR (in C6D6) d: 6.88–6.73
(ov m, 3H, Ar), 4.33 (s, 2H, -NCH2Ar), 3.01 (t, J = 8 Hz,
2H, -NCH2-), 1.42 (ov t of q, J = 8 Hz, 2H, -CH2CH2CH3),
1.15 (s, 12H, pin), 0.78 (t, J = 8 Hz, 3H, -CH3). 13C NMR d:
145.8, 126.4, 124.8, 124.1, 82.2, 46.8, 44.4, 24.5, 22.0, 11.1.
11B NMR d: 24 (br, NBpin).
Results and discussion
As with reactions using borane, we have found that the in
situ addition of HBcat to allylamine (1) resulted in the initial
formation of a mixture of products. Although minor amounts
of Lewis acid–base adducts HBcat·HNRCH2CH=CH2 (2, R = H,
Bcat) are formed (40),
a competing reaction gave
N(Bcat)2CH2CH=CH2 (3) as the major boron-containing
product in solution. No adduct formation is observed with 3,
however, as coordination of two electron-withdrawing boryl
groups has either significantly reduced the nucleophilic nature
of the amine or increased the steric hindrance around the
nitrogen atom. Although HBcat eventually adds to the
activated allyl group in these aminoboryl species, reactions
take several days. However, we have found that certain rhodium
complexes can be used to catalyze this addition. Rhodium-
catalyzed hydroborations are believed to arise via oxidative
addition of the B—H bond of the boronate ester at the metal
centre, followed by coordination and subsequent insertion of
the alkene into the Rh—H or Rh—B bond (41). Reductive
elimination affords the desired organoboronate ester product.
3 C.M. Crudden and A. Chen. Unpublished results.
© 2001 NRC Canada

1902
Can. J. Chem. Vol. 79, 2001
Scheme 3.
that a rhodium catalyst could be used to facilitate the addition
of another equiv of HBpin to give N(Bpin)2CH2CH2=CH2
(11). This observation provides the first example of a metal-
catalyzed hydroboration of N—H bonds.
aminopropylboronate esters has been increased in these reac-
tions, products arising from competing Markovnikov
hydroboration, dehydrogenative borylation, and hydrogenation
are all still observed to some extent (Table 1).
Compound 11 was observed in only minor amounts (<5%
by H NMR spectroscopy) as a competing hydroboration
Reactions with 16c were carried out with 3 equiv of HBcat
to ensure complete conversion of the allyl group. Although
RhCl(PPh3)3 (entry 1) gave 71% of the anti-Markovnikov
product 17c, use of [RhCl(coe)2]2/4PPh3 (coe = cis-cyclooctene,
entry 7) as a catalyst precursor gave 88% of this desired
1
reaction occurred at the alkene moiety to afford a mixture of
HN(Bpin)CH2CH2CH2Bpin (12) and N(Bpin)2CH2CH2CH2Bpin
(13). Not surprisingly, significant amounts of hydrogenation
products RN(Bpin)CH2CH2CH3 (where R = H or Bpin, 14
and 15, respectively) were also observed in these reactions.
1
product (using H NMR spectroscopy). Significant amounts
(23%) of the hydroboration product 18c were observed in
reactions using [RhCl(coe)2]2 as a catalyst precursor (entry
6). This product arises from a Markovnikov addition of the
borane to the allyl moiety. Remarkably, the diboronated ester
19c was a major product (38%) in reactions using
Rh(acac)(coe)2/dppm (dppm = 1,1-bis(diphenylphosphino)meth-
ane) (entry 10). Catalyst precursors of this type are known to
generate the active zwitterionic catalyst Rh(dppm)(ꢀ6-catBcat)
(52, 53). It is interesting to note that no significant change in
product distributions is observed when reactions were conducted
using an excess of HBcat (entry 12) or when hydroborations
were carried out in CDCl3 (entry 11). Although only minor
differences in product distributions were observed when 16a
was used as the substrate (cf. entries 1 and 13), reactions
with 16b gave significant amounts of hydrogenation product
20b (entry 14). Hydrogenation products are also observed in
other reactions where extensive borane and (or) catalyst
decomposition occurs (entries 6 and 8; (29)).
1
The H NMR data for the propyl group in 13 is similar to
that observed for 12, except that chemical shifts are moved
to a lower field due to addition of another electron-withdrawing
Bpin group. For instance, the middle CH2 resonance in 12 is
observed as an overlapping triplet of triplets at 1.60 ppm, yet
for 13 this peak is found at 2.04 ppm. The 11B NMR of
RN(Bpin)CH2CH2CH2Bpin (where R = H or Bpin) shows a
peak at 25 ppm corresponding to the B—N bond and a reso-
nance at 33 ppm for the new B—C bond. No appreciable
intramolecular interactions are observed in these molecules
as the boron atoms appear to be three-coordinate (40, 48).
This result is consistent with previous NMR data on related
NH2CH2CH2CH2B(OH)2·HCl which shows a peak in the 11
B
NMR at 32.6 ppm (49). Interestingly, the analogous neutral
compound (NH2CH2CH2CH2B(OH)2) displays a resonance
at 8 ppm suggesting intramolecular adduct formation between
the nitrogen and the boron atom (50).
To avoid complications arising from addition of the boronate
esters at the amine N—H bond, we decided to investigate
hydroborations of allylimine derivatives 16a–d. Addition of
1 equiv of catecholborane to allylimines 16a–c proceeds
cleanly to give the corresponding borylamines, where the
electron-deficient boron group has added to the nitrogen
atom of the imine double bond (51). A rhodium catalyst can
once again be used to facilitate the addition of a second
equiv of HBcat to the alkene moiety. While selectivity to 3-
Hydroborations of the pyridine derivative 16d gave rise to
a number of different boron-containing products, also arising
from the degradation of HBcat (2). Since no degradation was
observed with the thiophene imine 16c using HBcat, it is
plausible that the harder nitrogen atom in the pyridine ring is
responsible for this unwanted degradation pathway. To test
this hypothesis, we decided to investigate the analogous
hydroborations with 2- and 4-vinylpyridine (21 and 23,
respectively). Catalyzed hydroborations of styrene proceed
© 2001 NRC Canada

Vogels et al.
1903
Table 1. Hydroboration of allylimines with HBcat.
Bcat
Bcat
catB
catB
catB
catB
N
N
Bcat
N
N
N
Catalyst / n HBcat
+
+
Bcat
+
R
H
R
R
R
R
16a: R = 4-MeOC6H4
16b: R = 4-O2NC6H4
16c: R = 2-SC4H3
16d: R = 2-NC5H4
18a – d
20a – d
17a–d
19a – d
Entry
Allylimine
Catalyst system
RhCl(PPh3)3
RhCl(PPh3)3/10PPh3
[Rh(cod)Cl]2/dppb/AgBF4
Rh(H)(CO)(PPh3)3
Rh(H)(PPh3)4
[RhCl(coe)2]2
[RhCl(coe)2]2/4PPh3
Rh(acac)(coe)2
Rh(acac)(coe)2/2PPh3
Rh(acac)(coe)2/dppm
Rh(acac)(coe)2/dppm
RhCl(PPh3)3
RhCl(PPh3)3
RhCl(PPh3)3
Solvent
HBcat (n)
17a–17d
18a–18d
19a–19d
20a–20d
1
2
3
4
5
6
7
8
9
10
11
12
13
14
16c
16c
16c
16c
16c
16c
16c
16c
16c
16c
16c
16c
16a
16b
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
CDCl3
C6D6
C6D6
C6D6
3
3
3
3
3
3
3
3
3
3
3
6
3
3
71
78
86
80
77
63
88
51
79
37
59
75
88
51
7
8
5
20
13
6
10
5
2
1
3
3
4
12
2
21
1
5
17
1
7
14
23
2
16
18
20
9
5
6
11
2
8
12
2
38
15
19
5
1
37
1
1
Note: All reactions were conducted with 1 mol% catalyst at room temperature. Yields were calculated using H NMR data.
Scheme 4.
Scheme 5.
Bpin
N
N
Bcat
catB
1 mol% Rh /∆
H
HBpin
+
1 mol% Rh
+
+
S
S
N
N
N
25
26
+
HBcat
catB
N
spectroscopy) using Rh(acac)(coe)2/dppb (dppb
= 1,4-
1) F3B.OMe2
4-Vinylpyridine, 23
bis(diphenylphosphino)butane) as catalyst precursor
a
2) 1 mol% Rh
N
(Scheme 4) and HBcat. Reactions with HBpin gave a mixture
of hydroboration products along with a significant amount of
hydrogenation. Unfortunately, attempts to protect the nitrogen
group with BF3 in the hydroboration of allylamines 1 and
16d led to polymerization of the activated alkene moiety.
We then decided to examine the rhodium-catalyzed hydro-
boration of allylimine 16c using HBpin. Although no reac-
tion was observed at room temperature, complex product
distributions arising from addition at the allyl group and the
imine functionality were observed when reactions were carried
out at elevated temperatures (60°C). Reduction of the imine
was observed by disappearance of the aldimine hydrogen at
7.89 ppm with concomitant appearance of a benzylic hydro-
gen at 4.36 ppm. A peak at 24 ppm in the 11B NMR spec-
trum arises from the newly formed N—Bpin bond. This
result is somewhat surprising as previous attempts to hydro-
borate imines using HBpin proved unsuccessful (58). To
confirm that the imine was being reduced in these reactions,
we decided to investigate hydroborations with saturated
aldimine 25 (Scheme 5). Indeed, these reactions proceeded
BF3
24
smoothly to give selective formation of either the
Markovnikov or anti-Markovnikov product, depending upon
the metal catalyst employed (2). Not surprisingly, we have
found that catalyzed reactions of the vinylpyridines with
HBcat gave a mixture of hydroboration products, along with
a significant amount of ethylpyridine. Although a number of
metal complexes were examined as catalysts for this reaction, all
gave mixtures of products. Oxidation of the resulting
hydroborated mixtures with NaOH–H2O2 gave 4-ethylpyridine
as the only isolable organic product (54). Similar results are
also seen in reactions with HBpin. Coordination of a strong
Lewis acid, such as BF3 (55–57), to the vinylpyridines
effectively eliminated these degradation pathways and the
Marknovnikov hydroboration products (22 and 24, respec-
tively) could be obtained in high yields (>95% by NMR
© 2001 NRC Canada

1904
Can. J. Chem. Vol. 79, 2001
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We have found that the in situ addition of HBcat to allylamine
using a number of rhodium catalysts gave products derived
from competing hydroboration and dehydrogenative borylation
pathways.4 The use of HBpin effectively eliminated the
dehydrogenative borylation reaction and a novel rhodium-
catalyzed hydroboration of N—H bonds was observed to
give N(Bpin)2CH2CH2CH2Bpin along with various amounts
of hydrogenation product. Hydroboration of allylimines with
HBcat occurs initially at the more reactive imine functionality
to give unsaturated borylamines. Further reaction gives the
corresponding anti-Markovnikov and Markovnikov hydro-
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pyridine derivatives are complicated by extensive degradation,
which can be avoided by coordination of the pyridine nitrogen
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Further work will examine other catalyst systems to fine-
tune product selectivities in these reactions.
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Acknowledgements
Thanks are gratefully extended to the Natural Sciences
and Engineering Research Council of Canada (NSERC) and
Mount Allison University for financial support and Johnson
Matthey Ltd. for the generous gift of rhodium chloride. We
also wish to thank Dan Durant (MAU), Roger Smith (MAU),
and John Marcone (DuPont Co.) for their expert technical
assistance, Dr. R. Tom Baker (Los Alamos) and Dr. Cathleen
Crudden (UNB) for very valuable discussions, and anonymous
reviewers for helpful comments.
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