Magnesium catalysis of imine hydroboration

Article abstract of DOI:10.1002/chem.201203190

The β-diketiminato magnesium alkyl complex [LMgnBu] (L=CH{CMe(NDipp)}₂, Dipp=diisopropylphenyl) is shown to be a highly effective precatalyst for the hydroboration of alkyl and aryl substituted aldimines and ketimines with pinacol borane (HBpin). Catalysis is proposed to occur through a sequence of Mg-N/B-H metathesis and rate-determining Mg-H/N=C insertion steps, a proposal strongly supported by stoichiometric studies and kinetic analysis. The reactions are observed to proceed through the intermediacy of well-defined magnesium amides, two examples of which have been isolated and structurally characterized. Mechanistic investigations suggest that the catalytic rate-determining process occurs at an isolated magnesium center and requires the presence of two molecules of the imine substrate for effective turnover. This latter observation is rationalized as a requirement for the secondary substrate molecule to displace HBpin from the coordination sphere of the catalytic magnesium center.

Full text of DOI:10.1002/chem.201203190

DOI: 10.1002/chem.201203190
Magnesium Catalysis of Imine Hydroboration
Merle Arrowsmith, Michael S. Hill,* and Gabriele Kociok-Kçhn[a]
Abstract: The b-diketiminato magnesi-
um alkyl complex [LMgnBu] (L=CH-
{CMe(NDipp)}2, Dipp=diisopropyl-
phenyl) is shown to be a highly effec-
tive precatalyst for the hydroboration
of alkyl and aryl substituted aldimines
and ketimines with pinacol borane
(HBpin). Catalysis is proposed to occur
strongly supported by stoichiometric
studies and kinetic analysis. The reac-
tions are observed to proceed through
the intermediacy of well-defined mag-
nesium amides, two examples of which
have been isolated and structurally
characterized. Mechanistic investiga-
tions suggest that the catalytic rate-de-
termining process occurs at an isolated
magnesium center and requires the
presence of two molecules of the imine
substrate for effective turnover. This
latter observation is rationalized as a
requirement for the secondary sub-
strate molecule to displace HBpin from
the coordination sphere of the catalytic
magnesium center.
G
ACHTUNGTRENNUNG
Keywords: homogeneous catalysis ·
hydroboration · imines · magnesi-
um · main group elements
ꢀ
ꢀ
through
a
sequence of Mg N/B H
ꢀ
metathesis and rate-determining Mg
H/N=C insertion steps,
a
proposal
Introduction
and co-workers have reported the use of chiral titanocenes
for the asymmetric hydrogenation and hydrosilylation of
prochiral cyclic and acyclic imines and enamines
(Scheme 1).[6] In all cases, reduction of the imine was shown
to proceed through insertion of the C=N double bond into
Imines are readily available from the condensation of car-
bonyl compounds and primary amines and are, thus, the pre-
cursors of choice for the synthesis of unsymmetrically substi-
tuted secondary amines.[1] Although the catalytic hydrogena-
tion of imines provides an effective means to effect reduc-
tion of the C=N double bond, the synthetic utility of these
reactions can be restricted by issues of selectivity and low
product yields. Similarly, the heterogeneous nature and con-
sequent poor selectivity of borohydride reducing agents,
such as NaBH4, has led to the development of a plethora of
homogenous late-transition-metal catalysts (Ru, Rh, Ir, Pd,
Pt, Au)[1f,l,m] and, more recently, Lewis basic organocata-
lysts.[1h,k] Reduction with homogenous hydride sources, such
as silanes[1d–g,j,2] or boranes,[3] to afford the corresponding hy-
drosilylation and hydroboration products, as well as trans-
fer-hydrogenation methods, commonly by using isopropanol
or formic acid as the hydrogen source,[1k,4] are also gaining
prominence over the traditional use of molecular hydro-
gen[1h,l,m,5] owing to milder reaction conditions and higher se-
lectivity, especially in the asymmetric catalytic reduction of
prochiral imines.
ꢀ
the M H bond of the catalyst, followed by s-bond metathe-
sis with molecular dihydrogen to regenerate the metal hy-
dride catalyst and liberate the amine, as illustrated in
Scheme 1.
Our own research interests have centered on the catalytic
potential of similar d0 alkaline earth complexes. Recent re-
ports have shown these to be highly efficient and versatile
precatalysts for an ever-growing array of transformations, in-
cluding multiple bond heterofunctionalization, polymeriza-
tion, and dehydrocoupling reactions.[7] Mechanistic studies
have revealed that catalytic turnover generally occurs
through consecutive insertion and s-bond metathesis steps
reminiscent of lanthanide and Group 4 chemistry. Harder
and co-workers, in particular, have demonstrated the applic-
ability of a b-diketiminate-supported calcium hydride com-
plex, as well as homoleptic calcium and strontium dibenzyl
species, to the catalytic hydrogenation of activated alkenes
and the hydrosilylation of activated alkenes and ketones.[8]
In a similar manner, we have recently reported the use of
the b-diketiminato magnesium alkyl complex [LMgnBu] 1
As a counterpoint to these advances, there have been
only very limited reports of stoichiometric and catalytic
imine reduction by d0 metal hydride complexes. Buchwald
(L=CHACTHNUGTRNENUG[CMeACHTUNGTREN(NNGU NDipp)]2, Dipp=diisopropylphenyl) as an ef-
ficient catalyst for the hydroboration of aldehydes and ke-
tones,[9] as well as pyridines (Scheme 2), which may be
viewed as cyclic delocalized imine derivatives.[10] During the
course of these studies, it was also noted that cyanopyridines
[a] Dr. M. Arrowsmith, Prof. M. S. Hill, Dr. G. Kociok-Kçhn
Department of Chemistry, University of Bath
Claverton Down, Bath, BA2 7AY, (UK)
E-mail: msh27@bath.ac.uk
ꢀ
underwent double hydroboration of the nitrile C N triple
bond to form N,N-(diboryl)aminopyridines (Scheme 2B).
Following this preliminary result, we report in this paper the
magnesium-catalyzed reduction/hydroboration of imines
with pinacolborane (HBpin) under mild reaction conditions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203190. It contains full ex-
perimental details and characterization data.
2776
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 2776 – 2783

FULL PAPER
Scheme 1. Example of titanocene-catalyzed asymmetric hydrogenation of imines.[6f]
ꢁ
Scheme 2. A) Proposed mechanism for the magnesium-catalyzed hydroboration of pyridines. B) Magnesium-catalyzed double hydroboration of the C N
functionality of 3-cyanopyridine with HBpin.
Table 1. Scope of the magnesium-catalyzed hydroboration of benzalde-
hyde-derived aldimines.
Results and Discussion
Reaction scope: Although an initial NMR-scale control re-
action between HBpin and PhCH=NPh showed no sign of
hydroboration after two hours at room temperature
(Table 1, entry 1), addition of 5 mol% of compound 1 to
this reaction mixture yielded quantitative conversion to the
hydroborated amine product, PhCH2NACTHNUTRGNE(NUG Bpin)Ph (Table 1,
Entry
R
Cat.
[mol%]
t
T
[8C]
NMR yield
[%][a]
G
[h]
entry 2), identified by the characteristic 2H methylene sin-
glet at d=4.76 ppm in the 1H NMR spectrum, within less
than 10 min at room temperature. A highly exothermic
scaled up reaction in toluene with only 1 mol% precatalyst
provided complete conversion within 10 min and the amino-
borane product was isolated in near-quantitative yield
(95%) by recrystallization from cold hexanes.
Encouraged by this result, we investigated the scope of
the catalytic hydroboration of benzaldehyde-derived imine
substrates bearing a variety of alkyl and aryl substituents on
the imine nitrogen atom. Substrates with primary N-alkyl
substituents, such as benzyl, n-butyl, 2-methoxyethyl, and
allyl (Table 1, entries 5–8), underwent facile hydroboration
in essentially quantitative yield under mild reaction condi-
tions (1 (5 mol%), 258C, 0.5–2 h). It is notable that the 2-
methoxyethyl-substituted substrate (Table 1, entry 6) re-
quired a longer reaction time than the n-butyl analogue
(Table 1, entry 5), most likely due to competitive coordina-
tion to the oxophilic magnesium center through the neutral
O-donor functionality. In the case of the allyl-substituted
substrate, no hydroboration of the alkene functionality was
observed after addition of a second equivalent of HBpin
1
2
3
4
5
6
7
8
Ph
Ph
–
5
5
2
0.2
8
3 d
1
2
0.5
0.5
1 d
3 d
3 d
25
25
50
70
25
25
25
25
70
70
70
0
>99 (95)[b]
3-pyridyl
3-pyridyl
n-butyl
2-methoxyethyl
benzyl
93[c]
5[d]
5
79[e] (93:7)[f]
92
96
90
5
5
5
10
10
10
allyl
95
9
10
11
cyclohexyl
tert-butyl
2,4,6-Me3C6H2 or
2,6-iPr2C6H3
75[g]
42[g]
<2
[a] Calculated by integration versus an internal standard of tetrakis(tri-
methylsilyl)silane. [b] Isolated yield from the scaled up reaction.
[c] Imine hydroboration only. [d] A second equivalent of HBpin was
added to the product of entry 3. [e] 100% imine hydroboration, 79% pyr-
idine hydroboration. [f] 1,4- versus 1,2-hydroboration of the pyridine
fragment. [g] Catalysis stops due to magnesium-promoted decomposition
of HBpin into [B2pin3] and a catalytically inactive magnesium borohy-
dride complex, as detected by 11B NMR spectroscopy.
even at elevated temperatures (Table 1, entry 8). Similar ob-
servations had already been made for the reaction of 5-
hexen-2-one with HBpin catalyzed by 1, which led to exclu-
Chem. Eur. J. 2013, 19, 2776 – 2783
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2777

M. S. Hill et al.
sive hydroboration of the ketone moiety.[10] Harsher condi-
tions (708C, 1–3 d) were required for the reduction of
imines bearing secondary (cyclohexyl) or tertiary (tert-butyl)
alkyl substituents on the imine nitrogen (Table 1, entries 9
and 10). In both cases, reactions did not reach completion
due to competing catalyst decomposition (see below).
Imines bearing even larger imine nitrogen substituents, such
as 2,4,6-Me3C6H2 or 2,6-iPr2C6H3, did not show any sign of
hydroboration with HBpin under these reaction conditions
(Table 1, entry 11). Interestingly the pyridine-substituted
substrate 3-(PhCH=N)C5H4N underwent exclusive hydrobo-
ration of the exocyclic imine residue at 508C (Table 1,
entry 3). Upon addition of a second equivalent of HBpin
and at a higher reaction temperature (708C), however, the
pyridine fragment underwent slow, albeit highly selective,
1,4-hydroboration (Table 1, entry 3: 1,4- versus 1,2-hydrobo-
ration 93:7). Single crystals of compounds 2 and 3, products
of the hydroboration and double hydroboration of PhCH=
NPh and 3-(PhCH=N)C5H4N, respectively, were isolated
from saturated solutions in hexane stored for several days at
ꢀ308C. The results of the X-ray diffraction experiments are
presented in Figure 1 and selected bond lengths and angles
and details of the analyses can be found in Tables 2 and 4
(below), respectively.
Table 2. Selected bond lengths [ꢁ] and angles [8] for aminoboranes 2
and 3.
2
3
ꢀ
ꢀ
C7 N
ꢀ
C1 N
1.470(2)
1.427(2)
C1 N1
1.4770(17)
1.4282(16)
1.504(2)
ꢀ
C8 N1
ꢀ
C8 C9
ꢀ
C9 C10
1.494(2)
ꢀ
N B
ꢀ
1.418(3)
N1 B1
1.4102(19)
1.4247(18)
111.50(11)
119.30(11)
124.00(11)
116.49(11)
110.33(12)
ꢀ
N2 B2
C8-C7-N1
C7-N-B
C1-N-B
113.97(17)
116.86(16)
126.61(17)
116.52(16)
C2-C1-N1
C1-N1-B1
C8-N1-B1
C1-N1-C8
C8-C9-C10
Figure 1. ORTEP representations of compounds 2 (top) and 3 (bottom).
Thermal ellipsoids drawn at the 20% probability level. Hydrogen atoms,
except those attached to the benzylic carbon atoms and the para-position
of the 1,4-dihydropyridine fragment of 3, are omitted for clarity.
C7-N-C1
the benzylidene fragment caused a slight decrease in the re-
action rate compared to the parent compound, PhCH=NPh
(Table 3, entries 1 and 2). Although the polyaromatic pyren-
yl derivative underwent extremely facile and quantitative
hydroboration in less than 10 min at room temperature
(Table 3, entry 3), reduction of the more sterically demand-
ing mesityl analogue required 21 h at 708C to reach comple-
tion (Table 3, entry 4). As shown in Table 3, entry 5, secon-
dary aldimines also yielded the aminoborane product under
mild reaction conditions (508C, 3 h). Ketimines derived
from both aryl and alkyl ketones were also investigated for
hydroboration with HBpin. Although imines derived from
acetophenone and cyclohexanone underwent quantitative
reduction with only 5 mol% of 1 at room temperature
(Table 3, entries 7 and 8), the more sterically hindered sub-
strate Ph2C=NPh required higher reaction temperatures
(708C) and prolonged reaction times, and only allowed
three turnovers before complete catalyst degradation (see
Both structures unambiguously demonstrate the reduction
of the imine functionality to the corresponding aminoboro-
ꢀ
nate esters, with characteristic C N single-bond lengths [2
ꢀ
ꢀ
C7 N 1.470(2), 3 C1 N1 1.4770(17) ꢁ] and tetrahedral geo-
metries around the benzylic carbon atoms [2 C8-C7-N
113.97(17), 3 C2-C1-N1 111.50(11)8]. In the case of the
ꢀ
bis(hydroborated) compound, 3, the C C single-bond
ꢀ
ꢀ
lengths [C8 C9 1.504(2), C9 C10 1.494(2) ꢁ] and more
acute angles [C8-C9-C10 110.33(12)8] around the 4-C-posi-
tion (C9) of the dihydropyridide fragment also show clear
evidence of reduction. Compound 3 is the first crystallo-
graphically characterized example of a N-borylated 1,4-dihy-
dropyridine.
Imines synthesized from aniline and a variety of arylalde-
hyde derivatives were also readily hydroborated by using 1
(5 mol%). The presence of both activating (methoxy) and
deactivating (fluoride) substituents in the ortho-position of
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Imine Hydrobration
FULL PAPER
Table 3. Scope of the magnesium-catalyzed hydroboration for a variety
of alkyl- and aryl-substituted aldimines and ketimines.
Entry
R
R’
R’’
Cat.
AHCTUNGTRENN[GUN mol%] [h]
t
T
NMR yield
[8C] [%][a]
1
2
3
4
5
6
7
8
9
2-(OMe)C6H4 Ph
H
H
H
H
H
H
Ph
Ph
nBu
5
5
5
5
1.5 25
25
0.2 25
96
96
>99
98
2-FC6H4
1-pyrenyl
Ph
Ph
2
Figure 2. Annotated 11B NMR spectrum of the magnesium-catalyzed hy-
droboration of PhCH=NtBu with HBpin after 3 d at 708C in [D8]toluene
showing the decomposition of HBpin.
2,4,6-Me3C6H2 Ph
21
3
1d
4d
70
50
25
70
cyclohexyl
3-pyridine
Ph
nBu
5
91
[c]
Ph
Ph
Me
5[b]
–
10
5
5
32[d]
Ph
0.2 25
20 25
>99
92
product 4 (d=25.4 ppm, singlet; lit. d=22.1 ppm[11a]), and
-(CH2)5-
an
apparent
BH2
triplet
resonance
at
[a] Calculated by integration versus an internal standard of tetrakis(tri-
methylsilyl)silane. [b] Two equivalents of HBpin. [c] 100% pyridine hy-
droboration. A complex mixture of 1,2- and 1,4-dihydropyridine and par-
tial imine hydroboration products formed. [d] Catalysis stops due to mag-
nesium-promoted decomposition of HBpin into [{pinBOC(Me)2}2] and a
catalytically inactive magnesium borohydride complex, as detected by
11B NMR spectroscopy.
d=41.6 ppm (1JBꢀH ꢂ108 Hz), which could not be unambig-
uously identified. A stoichiometric reaction between 1 and
three equivalents of HBpin at 808C over a period of two
days was shown to yield the same deactivation products.
Stoichiometric studies: We have previously reported that
the reaction between 1 and one equivalent of HBpin in
C6D6 leads to the formation of (nBu)Bpin (11B NMR singlet
below). Prompted by our previous observations of the hy-
droboration of aldehydes and ketones, during which compet-
itive dehydrocoupling reactions were observed,[10] no at-
tempts were made for the hydroboration of alcohol or
amine-containing imines.
1
at d=37.5 ppm) and [{LMgH}2] 5, as identified by H NMR
spectroscopy, in equilibrium with a magnesium borohydride
complex of the [HACTHNUTRGNEUNG
(nBu)Bpin]ꢀ ion (broad 11B NMR feature
at d=ꢀ1.3 ppm). Subsequent reaction with one equivalent
of PhCH=NPh instantly afforded a bright-orange solution,
the color of which slowly faded over a period of three hours
at room temperature. As shown by the stack-plot displayed
Catalyst decomposition: Hydroboration reactions requiring
temperatures higher than 608C for reaction times exceeding
1 day invariably produced poor yields due to competing cat-
alyst deactivation (Table 1, entries 9 and 10; Table 3,
entry 6). Redistributive degradation of HBpin has been re-
ported to be quite facile, particularly in the presence of
bases such as triphenylphosphine to form to form Ph3P·BH3
1
in Figure 3, the H NMR spectrum of the resulting colorless
solution showed complete disappearance of the aldimine
and MgH singlets (d=8.12 and 4.00 ppm, respectively) and
quantitative conversion to amido complex 6, as displayed in
Scheme 3. Addition of a further equivalent of HBpin to this
solution at room temperature instantly led to a broadening
and tris(pinacolato)diboron,
4 (commonly described as
1
B2pin3).[11a] Crystals of B2pin3 were isolated from several re-
action mixtures and identified
and a downfield shift of the methylene H NMR singlet to
by comparison of their X-ray
crystallographic unit cell pa-
rameters
with
literature
data.[11b] The 11B NMR spec-
trum recorded for the hydrobo-
ration reaction of PhCH=NtBu
after 3 d at 708C displayed five
distinct resonances (Figure 2).
These can be attributed to the
catalyst activation byproduct
nBuBpin (d=37.4 ppm, sin-
glet), the substrate HBpin (d=
31.4 ppm, doublet), the amino-
borane
product
PhCH2N-
ACHTUNGTRENNUNG
1
glet), the B2pin3 decomposition Figure 3. Annotated H NMR stack plot of the stoichiometric hydroboration of PhCH=NPh with 1.
Chem. Eur. J. 2013, 19, 2776 – 2783
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2779

M. S. Hill et al.
Scheme 3. Stoichiometric step-by-step hydroboration of PhCH=NPh by complex 1.
d=4.75 ppm, attributed to the hydroborated product
PhCH2NACHTUNGTRENNUNG
(Bpin)Ph and a corresponding 13C{1H} NMR peak
1
at d=52.9 ppm. This H NMR data, however, showed that
the magnesium hydride species 5 had only been partially re-
generated (ꢂ10%), as seen by the reappearance of a small
MgH resonance at d=4.00 ppm (Figure 3). Complex split-
1
ting patterns in the iPrCH region of the H NMR spectrum
(d=2.76–3.54 ppm), as well as a further singlet at d=
2.87 ppm (ꢂ35%), suggested the formation of an additional
magnesium hydride species potentially stabilized by a neu-
tral PhCH2N
this was accompanied by the appearance of a new singlet at
d=28.4 ppm assigned to the aminoborane PhCH2N(Bpin)Ph
ACHTUNGTRENNUNG
(Bpin)Ph adduct. In the 11B NMR spectrum,
AHCTUNGTRENNUNG
and a broader resonance further upfield at approximately
d=1.6 ppm, tentatively attributed to a [{(PhCH2)PhN}-
ACHTUNGTRENNUNG
(HBpin)]ꢀ aminoborohydride ion loosely bound to another
b-diketiminate magnesium species.
Although no crystals of 6 suitable for X-ray structural
analysis could be isolated, the analogous reaction with (1-
pyrenyl)CH=NPh in toluene at room temperature led to the
isolation of yellow single crystals of the heteroleptic magne-
sium amide complex LMgNPhACHTNUTGRENNUG{CH2ACHTUNGTERN(NUGN 1-pyrene)} 7, resulting
ꢀ
from the insertion of the imine into the Mg H bond of the
magnesium hydride complex formed by s-bond metathesis
with HBpin. The results of this experiment are displayed in
Figure 4 and details of the analysis and selected bond
lengths and angles are presented in Tables 4 and 5. The
1H NMR spectrum displayed a characteristic 2H singlet at
d=4.46 ppm attributed to the new methylene protons,
which could be correlated with a 13C{1H} NMR peak at d=
50.2 ppm. A single-crystal X-ray structural analysis showed
7 to be a monomeric compound with a three-coordinate pyr-
amidal magnesium center [N1-Mg-N2 96.55(7), N2-Mg-N3
122.99(7), N3-Mg-N1 125.96(7)8]. The magnesium atom lies
0.54 ꢁ above the plane of the b-diketiminate framework
Figure 4. ORTEP representations of compounds 7 (top) and 8 (bottom).
Thermal ellipsoids drawn at the 20% probability level. Isopropyl methyl
groups and hydrogen atoms, except those attached to the benzylic carbon
atoms, are omitted for clarity.
ꢀ
ꢀ
and all three Mg N bonds are of similar lengths [Mg N1
2.0310(17), Mg-N2 2.0250(17), Mg-N3 1.9738(18) ꢁ]. The
tively occupied by the planar pyrenyl fragment, which forms
an angle of 818 with the plane of the ligand backbone, thus
preventing dimerization. The N3-C37-C38 angle of
109.09(16)8 clearly shows the sp3 character of C37, as does
ꢀ
magnesium–amide bond length, Mg N3, is also comparable
to that reported for other monomeric b-diketiminato mag-
ꢀ
ꢀ
nesium amide complexes [LMg N
ACHTUNGRTEN(NUGN iPr)2 1.938(2), LMg N-
(SiMe3)2 1.961(2) ꢁ].[12] The fourth coordination site is effec-
the N3 C37 bond length of 1.463(3) ꢁ, characteristic of a
ꢀ
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Chem. Eur. J. 2013, 19, 2776 – 2783

Imine Hydrobration
FULL PAPER
Table 4. Details of the X-ray diffraction analyses for compounds 2, 3, 7, and 8.[a]
The iPrCH resonances (d=
2.90–3.18 ppm) were split into
four inequivalent septets due to
restricted rotation about the
2[b]
3
7
8
molecular formula
C19H24BNO2
309.20
C24H36B2N2O4
438.17
C52H57MgN3
748.32
C48H57MgN3
700.28
triclinic
formula weight [gmolꢀ1
]
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
monoclinic
P21
11.7827(6)
6.0575(3)
12.4906(6)
90
101.786(3)
90
872.71(7)
2
monoclinic
P21/c
12.2092(2)
9.3494(2)
22.3278(3)
90
101.2777(12)
90
2499.48(8)
4
triclinic
P1
ꢀ
Mg N bond caused by the
¯
¯
P1
large N-(diphenylmethyl)anilide
ligand. Upon addition of a fur-
ther equivalent of HBpin, the
methine 1H NMR singlet reso-
nance was observed to broaden
and was shifted downfield to
12.3469(4)
12.3919(5)
16.0597(6)
76.723(2)
82.705(2)
61.650(2)
2104.20(13)
2
12.3741(4)
12.4465(3)
15.0774(3)
97.9476(16)
91.9122(16)
117.2601(10)
2032.03(9)
2
a [8]
b [8]
g [8]
V [ꢁ3]
Z
d=6.18 ppm
whereas
the
1 [gcmꢀ3
]
1.177
0.074
1.164
0.077
1.181
0.081
1.145
0.080
11B NMR spectrum displayed
two new resonances: a singlet
at d=27.3 ppm corresponding
to the aminoborane product
m [mmꢀ1
]
q range [8]
4.08 to 27.50
0.0473, 0.0838
0.1113, 0.0997
16598/3971/
0.0984
4.04 to 27.50
0.0479, 0.1068
0.0679, 0.1187
36758/5695/
0.0660
4.61 to 27.52
0.0575, 0.1242
0.1288, 0.1522
41137/9542/
0.0976
3.85 to 27.47
0.0496, 0.1129
0.0875, 0.1284
38039/9218/
0.0685
R1,[c] wR2 [I>2s(I)][d]
R1, wR2 (all data)
Ph2CHN
(Bpin)Ph (ꢂ65%) and
measured/independent reflns/
Rint
a broad resonance at approxi-
mately d=1 ppm tentatively at-
tributed to a [{(Ph2CH)PhN}-
[a] CCDC-900215 (7), CCDC-900216 (2), CCDC-900217 (3), and CCDC-900218 (8) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. [b] Absolute structure parameter ꢀ1.1(13).
AHCTUNGTRENNUNG
(HBpin)]ꢀ aminoborohydride
2
2
[c] R1 =S j jFo j-jFc j j/S jFo j. [d] wR2 ={S[w(Fo - Fc2)2]/S[w
(Fo )2]}1/2
ACHTUNGTERNNUNG .
ion loosely bound to a b-diket-
AHCTUNGTERGiNNUN minate magnesium complex.
Scaleup of the 1:1:1 reaction
between 1, HBpin, and Ph2C=
NPh in toluene yielded a crop
of large colorless crystals of 8
after storage for one week at
room temperature. X-ray struc-
tural analysis confirmed the
monomeric nature of the com-
plex. The results of this experi-
ment are displayed in Figure 4
and details of the analysis and
selected bond lengths and
Table 5. Selected equivalent bond lengths [ꢁ] and angles [8] for magnesium amides 7 and 8.
7
8
7
8
ꢀ
Mg N1
2.0310(17)
2.0250(17)
1.9738(18)
1.377(3)
1.463(3)
1.519(3)
2.0245(13)
2.0396(13)
1.9896(13)
1.3787(19)
1.4672(19)
1.518(2)
N1-Mg-N2
N2-Mg-N3
N3-Mg-N1
96.55(7)
94.92(5)
ꢀ
Mg N2
122.99(7)
125.96(7)
126.90(14)
116.69(13)
116.40(16)
109.09(16)
121.73(6)
129.55(6)
126.35(10)
117.28(9)
115.91(12)
107.52(12)
ꢀ
Mg N3
[a]
[a]
C31/30-N3-Mg[a]
C37/36-N3-Mg[a]
C31/30-N3-C37/36[a]
N3-C37/36-C38/37[a]
ꢀ
N3 C31/30
ꢀ
N3 C37/36
[a]
ꢀ
C37/36 C38/37
[a] Atom numbering for 7/8, respectively.
ꢀ
N C single bond. In contrast, a stoichiometric NMR-scale
reaction of 1 with one equivalent of HBpin and the more
sterically hindered substrate PhCH=NtBu] at room tempera-
angles can be found in Tables 4 and 5, respectively. One of
the rings of the diphenylmethanide fragment effectively
blocks the fourth coordination site. The magnesium center
ꢀ
ꢀ
ture showed no sign of insertion of the imine into the Mg
is, thus, three-coordinate and pyramidal with Mg N bond
ꢀ
H bond of intermediate 5. Even after heating this reaction
mixture for one day at 708C, only 25% of the imine had
been consumed, as determined by 1H NMR spectroscopy.
Addition of a further equivalent of HBpin, however, led to
rapid consumption of the magnesium amide intermediate
and formation of the hydroborated product PhCH2N-
lengths comparable to those in compound
7
[Mg N1
ꢀ
ꢀ
2.0245(13), Mg N2 2.0396(13), Mg N3 1.9896(13) ꢁ]. As in
compound 7, the angles around the new benzylic center,
C36, are characteristic of an sp3 carbon atom [N3-C36-C37
107.52(12),
N3-C36-C43
115.44(13),
C37-C36-C43
ꢀ
110.83(12)8] and the C36 N3 distance of 1.4672(19) ꢁ is in-
dicative of the reduction of the imine substrate to the amido
ligand by the intermediate magnesium hydride species 5.
ACHTUNGTRENNUNG(Bpin)tBu, suggesting that imine insertion is the rate-deter-
mining step during catalytic turnover and is strongly influ-
enced by the imine nitrogen substituent.
Similarly, addition of one equivalent of HBpin followed
by the benzophenone-derived imine Ph2C=NPh to a solution
of 1 in C6D6 led to quantitative formation of the amido com-
Kinetic studies: Based on these observations a kinetic study
of the catalytic hydroboration of CyCH=NnBu and PhCH=
NCH2Ph with 1 in C6D6 was carried out by H NMR spec-
1
plex [LMgNPh
(CHPh2)] 8, albeit after heating for several
troscopy to provide further mechanistic insight into these
catalytic reactions. Solutions for NMR spectroscopy were
prepared in a glovebox, immediately cooled to 08C, and
then transferred to the NMR spectrometer. Conversions
were followed by integration of the substrate methine and
1
hours at 508C. H NMR data showed complete disappear-
ance of the MgH singlet at d=4.00 ppm and the appearance
of a new singlet at d=5.27 ppm, attributed to the methine
proton of the diphenylmethyl fragment resulting from the
insertion of the imine into the magnesium–hydride bond.
product methylene peaks versus an internal tetrakis(triACHTUNGTRENNUNGmeth-
Chem. Eur. J. 2013, 19, 2776 – 2783
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2781

M. S. Hill et al.
A
behavior to the dimeric nature of [(Ae{NACTHUNGTREUNN(G SiMe3)2}2)2] (Ae=
strates consistently displayed second-order kinetics up to a
period of three half-lives. PhCH=NCH2Ph in the presence
of ten molar equivalents of HBpin ([HBpin]t ꢂ[HBpin]0)
also provided a second-order dependence on [imine] as
Mg, Ca, Sr) in solution. In contrast, the current observation
of a first -order dependence on catalyst concentration is in
line with our reports of molecular catalysis employing mag-
nesium and calcium precatalysts supported by the same b-di-
ketiminate ligand.[12a] We, thus, deduce that the rate-deter-
mining process during catalytic turnover for this magnesi-
um-catalyzed imine hydroboration is mediated by an isolat-
ed Group 2 center. Although interpretation of kinetic order
for the assignment of molecularity to individual reaction
steps of multi-step reactions should be applied with cau-
tion,[15] an inverse dependence of reaction rate on HBpin
concentration is consistent with the noted effect of reversi-
ble borane coordination/de-coordination and consequent
catalyst inhibition (see above). Similarly, the second-order
variation of reaction rate with [imine] implicates two mole-
cules of imine substrate during the rate-determining process
and we, thus, suggest that imine pre-coordination and rate-
shown by the linear 1/ACHTNUGRTENUNG[imine] versus time plot in Figure 5.
Figure 5. Comparison of room temperature hydroboration rates of
ꢀ
determining Mg H insertion is facilitated by the ability of a
~
^
PhCH=NCH2Ph (0.8m in C6D6) with one ( ) and ten ( ) equivalents of
HBpin (5 mol% of 1).
secondary substrate molecule to displace HBpin from the
coordination sphere of the catalytic magnesium center.
The reaction is, thus, second order in [imine] and zero order
in [HBpin]. Moreover, in parallel room-temperature experi-
ments by using the same catalyst/imine stock solution
(0.04m of 1, 0.8m of PhCH=NCH2Ph in C6D6) the presence
of ten equivalents of HBpin induced a more than 300-fold
decrease in reaction rate, consistent with HBpin acting as a
very strong inhibitor by coordination to the magnesium
center, either as a neutral donor or as the [H2Bpin]ꢀ dihy-
droborate ion, thus impeding imine pre-coordination and in-
sertion. In contrast experiments by using the same catalyst/
HBpin stock solution (0.02m of 1, 0.4m of HBpin in C6D6)
led to 94–97% conversion of HBpin after 30 min at room
temperature, showing no imine inhibition at higher initial
concentration.
Conclusion
A b-diketiminato magnesium complex is reported as an effi-
cient and easily accessed precatalyst for the hydroboration
of a broad range of imine substrates under mild conditions.
The reactions occur through magnesium hydride and amide
intermediates, which are formed by consecutive Mg N/B H
and C=N/Mg H metathesis and insertion steps, and mecha-
nistic studies suggest that turnover is strongly dependent
upon the basicity and steric demands of the imine sub-
strates.
ꢀ
ꢀ
ꢀ
The order in catalyst concentration for the hydroboration
of CyCH=NnBu was determined by monitoring of reactions
performed at 508C with a constant substrate concentration
(0.8m) and by using five different precatalyst concentrations
(Figure S1 in the Supporting Information). Over a range of
0.8 to 55.8 mm, the reaction was observed to be first order in
[catalyst], which is consistent with the monomeric nature of
the insertion intermediate (Figure S1 in the Supporting In-
formation).
Acknowledgements
We thank the EPSRC (UK) for funding.
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In accordance with observations on the relative rates of
stoichiometric imine insertion versus HBpin s-bond me
ACHTUNGTNERtNUNG ath-
ACHTUNGTRENNUNG
ration reactions may thus be written:
2
2
½catꢃ½imineꢃ
½catꢃ½imineꢃ
rate ¼ ꢀk
¼ ꢀk0
kinhib½HBpinꢃ0
½HBpinꢃ0
We have previously observed a second-order dependence
upon precatalyst concentration in intra-[13] and intermolecu-
lar[14] hydroamination reactions catalyzed by homoleptic
bis(trimethylsilyl)amide derivatives and have ascribed this
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Chem. Eur. J. 2013, 19, 2776 – 2783

Imine Hydrobration
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Received: September 7, 2012
Published online: January 10, 2013
Chem. Eur. J. 2013, 19, 2776 – 2783
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2783