Calcium-mediated hydroboration of alkenes: "trojan horse" or "true" catalysis?

Article abstract of DOI:10.1016/j.jorganchem.2011.09.025

The hydroboration of 1,1-diphenylethylene (DPE) with catecholborane (HBcat) proceeds at 100 °C. For conversion at room temperature three different organocalcium catalysts have been investigated: the calcium hydride complex [DIPPnacnacCaH·(THF)]₂ (1, DIPPnacnac = CH{(CMe)(2,6-iPr ₂C₆H₃N)}₂), Ca[2-Me ₂N-α-Me₃Si-benzyl)₂·(THF) ₂ (2) and DIPPnacnacCa(H-BBN)·(THF) (3, BBN = 9-borabicyclo[3.3.1.]nonane). Although up to 96% conversion of DPE is found, the product of the reaction is not the expected Ph₂CHCH₂Bcat but (Ph₂CHCH₂)₃B is formed instead. Organocalcium compounds catalyze the decomposition of HBcat to B ₂(cat)₃ and BH₃ (or B₂H₆) and the latter is involved in hydroboration of DPE. The calcium-catalyzed decomposition of HBcat was investigated with ¹¹B NMR and the signals were assigned to the following species: B₂(cat)₃, B(cat)₂^-, HBcat, BH₃(THF), BH₄ ^- and B₂H₇^-. A tentative mechanism for the formation of these species was proposed. The intermediate DIPPnacnacCa(BH₄)·(THF)₂ (5) was independently prepared by reaction of 1 and BH₃(Me₂S) and was structurally characterized by X-ray diffraction. Stoichiometric reaction of 1 with pinacolborane (HBpin) gave a trimeric complex [DIPPnacnacCa(H ₂Bpin)]₃ (6) which was structurally characterized by X-ray diffraction. This complex does not react with DPE, also not at elevated temperatures. The possible equilibrium between 6 and 1/HBpin is therefore fully at the side of 6. As 6 is unstable in the presence of HBpin, no further catalytic conversions have been investigated.

Full text of DOI:10.1016/j.jorganchem.2011.09.025

Journal of Organometallic Chemistry 698 (2012) 7e14
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Calcium-mediated hydroboration of alkenes: “Trojan horse” or “true” catalysis?
,
b
Sjoerd Harder a , Jan Spielmann
*
a Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, Netherlands
b Universität Duisburg-Essen, Universitätsstraße 5, 45117 Essen, Germany
a r t i c l e i n f o
a b s t r a c t
The hydroboration of 1,1-diphenylethylene (DPE) with catecholborane (HBcat) proceeds at 100 ꢀC. For
conversion at room temperature three different organocalcium catalysts have been investigated: the
calcium hydride complex [DIPPnacnacCaH$(THF)]2 (1, DIPPnacnac ¼ CH{(CMe)(2,6-iPr2C6H3N)}2), Ca[2-
Article history:
Received 25 July 2011
Received in revised form
19 September 2011
Me2N-
a
-Me3Si-benzyl)2$(THF)2 (2) and DIPPnacnacCa(H-BBN)$(THF) (3, BBN ¼ 9-borabicyclo[3.3.1.]
Accepted 28 September 2011
nonane). Although up to 96% conversion of DPE is found, the product of the reaction is not the expected
Ph2CHCH2Bcat but (Ph2CHCH2)3B is formed instead. Organocalcium compounds catalyze the decompo-
sition of HBcat to B2(cat)3 and BH3 (or B2H6) and the latter is involved in hydroboration of DPE. The
calcium-catalyzed decomposition of HBcat was investigated with 11B NMR and the signals were assigned
to the following species: B2(cat)3, B(cat)2ꢁ, HBcat, BH3(THF), BHꢁ4 and B2Hꢁ7 . A tentative mechanism for the
formation of these species was proposed. The intermediate DIPPnacnacCa(BH4)$(THF)2 (5) was inde-
pendently prepared by reaction of 1 and BH3(Me2S) and was structurally characterized by X-ray
diffraction. Stoichiometric reaction of 1 with pinacolborane (HBpin) gave a trimeric complex [DIPPnac-
nacCa(H2Bpin)]3 (6) which was structurally characterized by X-ray diffraction. This complex does not
react with DPE, also not at elevated temperatures. The possible equilibrium between 6 and 1/HBpin is
therefore fully at the side of 6. As 6 is unstable in the presence of HBpin, no further catalytic conversions
have been investigated.
Keywords:
Calcium
Hydroboration
Borate complexes
Catalysis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
[13,14]. The catalytic toolbox contains late transition metal catalysts
that recently also includes an Au complex [15]. Early group 3 and 4
Classical hydroboration [1] is an important synthetic tool for the
conversion of alkenes to boron-functionalized organics which
subsequently offer various possibilities for further transformation
into alcohols [2], amines [3] or halogenated organics [4]. Although
the hydroboration of alkenes with B2H6 generally proceeds
smoothly and under mild conditions there has been increased
interest in alkene hydroboration with heteroatom-substituted
boranes like catecholborane (HBcat; Scheme 1). As such a stabi-
lized borane is less Lewis-acidic, it shows better functional group
tolerance and is easier to store and handle. Alkene hydroboration
with HBcat, however, generally needs the assistance of a catalyst
[5,6]. Although this might seem a disadvantage, it is inherently
coupled to the crucial advantage of organometallic catalysis, i.e.
control over the course of the reaction by metal and/or ligand
choice. Transition metal-catalyzed addition of catecholborane to
carbonecarbon multiple bonds produced some remarkable chemo-
[5], regio- [7e9], diastereo- [7,9e12] and enantioselectivities
transition metal compounds have been used as catalysts already at
a preliminary stage [16e21]. Marks et al. [16] proposed that
lanthanide-catalyzed alkene hydroboration could proceed through
a succession of steps typical for organolanthanide chemistry, i.e.
alkene insertion and
s-bond metathesis (Scheme 2). The actual
catalyst in this cycle is proposed to be a highly reactive lanthanide-
O
H
H
N
N
N
N
Ca
Ca
O
1
* Corresponding author. Tel.: þ31 50 3634322; fax: þ31 50 3634296.
E-mail addresses: s.harder@rug.nl, sjoerdharder@gmx.de (S. Harder).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.09.025

8
S. Harder, J. Spielmann / Journal of Organometallic Chemistry 698 (2012) 7e14
O
B
O
O
O
H
SiMe3
Ca THF
O
Me2N
THF
H
+
N
N
Ca
H
THF Ca THF
H
NMe2
Me3Si
O
O
O
O
B
H
B
4
2
3
Scheme 1.
hydride complex. The formulation of an intermediate highly
nucleophilic lanthanide-hydride complex in the presence of
a Lewis-acidic borane seems contradictory. The borane HBcat,
however, should be regarded as a resonance-stabilized less Lewis-
acidic borane and, based on metal-ligand bond enthalpies, the
reaction of a lanthanide-alkyl complex with HBcat to give
a lanthanide-hydride and alkylBcat has been estimated to be
slightly exothermic (ꢁ3 kcal/mol) [16]. In addition, Marks et al. do
not rule out the possibility that the lanthanide-hydride catalyst is
present as a lanthanide borate species containing the (H2Bcat)ꢁ ion.
We recently introduced the well-defined calcium hydride
complex [DIPPnacnacCaH$(THF)]2 (1) [22] (DIPPnacnac ¼ CH
{(CMe)(2,6-iPr2C6H3N)}2) and could show that such a species is an
actual catalyst for alkene hydrosilylation [23] and hydrogenation
[24]. As the key steps in catalysis based on organolanthanide and
organocalcium chemistry are similar [25,26], we posed the ques-
tion whether calcium-catalyzed alkene hydroboration would be
feasible and efficient. Use of calcium compounds in catalysis has
important economical advantages (Ca is globally available and is
one of the cheapest metals) as well as environmental advantages
(Ca is biocompatible and non-poisonous). Herein we describe
preliminary investigations of alkene hydroboration with organo-
calcium catalysts and extend our earlier studies [27] on the reac-
tivity of the calcium hydride complex 1 with boranes.
Table 1
Catalytic hydroboration of DPE with HBcat.
Entry catalyst [cat]/[DPE] solvent
(%)
temperature time conversion
(ꢀC)
(hrs) (%)
1
2
3
4
5
6
e
e
1
2
1
e
e
25
100
25
50
25
72
20
72
20
72
e
e
e
96
95
90
38
43
2.5
2.5
2.5
5
e
e
benzene
benzene
3
25
192
Scheme 2.

S. Harder, J. Spielmann / Journal of Organometallic Chemistry 698 (2012) 7e14
9
Table 2
2. Results and discussion
11B NMR signals for the reaction of 1 with HBcat (THF-d8, 160 Mhz, 25 ꢀC) and their
comparison with literature data.
The calcium hydride complex 1 does not react with isolated
alkenes but shows addition to conjugated alkenes like butadienes
and styrenes. Therefore we chose 1,1-diphenylethylene (DPE) as
a model substrate. This alkene has the additional advantage to be
inert to anionic polymerization, a possible side reaction for which
organocalcium compounds have been shown to be highly effective
initiators [28]. Hydroboration experiments were run in the
temperature range of 25e100 ꢀC, either without solvent or in
benzene and with or without the use of three different catalysts
(1e3). The results are summarized in Table 1.
d
(ppm)
Multiplicity,
Possible
species
Literature values
Ref.
[31]
1JB,H
ꢁ36.5
ꢁ23.9
quint, 82.0 Hz
Ca[BH4]2
[B2H7]ꢁ
ꢁ35.8, quint 82.0 Hz,
in monoglyme
ꢁ22.7, q, in diglyme
ꢁ25.3, q 102 Hz, in
monoglyme
q, 68.2 Hz
[32]
[33]
ꢁ0.6
q, 101.6 Hz
s
BH3(THF)
[B(cat)2]ꢁ
ꢁ0.6, q 103 Hz, in THF
15.2, s, in CD2Cl2
15.1, s, in toluene-d8
14.6, s, in diglyme
29.9, d, in CDCl3
[33]
[34a]
[35]
[32]
[36]
9.2
The first two entries confirm that HBcat does not react with DPE
at room temperature. Heating to 100 ꢀC is a necessity and this gave
near to full conversion to Ph2CHCH2Bcat within 20 h. With calcium
hydride as a catalyst, however, already at room temperature 95%
conversion of DPE is observed but the reaction is slow (entry 3). Use
of catalyst 2 at the slightly raised temperature of 50 ꢀC gave a faster
conversion (entry 4). Whereas this set of experiments has been run
without solvent, use of benzene as a solvent led to slower conver-
sion (entry 5) which is likely attributed to a dilution effect. The last
experiment (entry 6) shows that also the calcium borate complex 3
catalyzes the hydroboration of DPE, albeit much slower. In all cases
oxidative work-up under basic conditions gave the expected alco-
holic product Ph2CHCH2OH.
Given these results, it seems that organocalcium compounds
can function as a catalyst for the hydroboration of DPE with HBcat.
Careful analysis of the products by 1H NMR before oxidative work-
up, however, showed that the intermediate product in question is
not the boronic ester Ph2CHCH2Bcat but the tris-alkyl borane
(Ph2CHCH2)3B. This compound was independently prepared by
reaction of excess DPE with BH3(Me2S). It is therefore likely that
organocalcium compounds catalyze the decomposition of HBcat in
BH3 (or the diborane B2H6) and further products. This is followed by
selective addition of BH3 to the 1,1-substituted alkene DPE. Such
behaviour has been pointed out earlier in Ti-catalyzed alkene
23.9
25.2
d, 185.8 Hz
s, broad
HBcat
28.8, d 193.0 Hz, in
benzene-d6
22.7, s (broad), in CD2Cl2
[this
work]
[34a]
B2(cat)3
hydroboration [17,18]. The organocalcium species is therefore not
a “true” catalyst but could be regarded a “Trojan horse”, as was
described earlier by Burgess et al. for Nb-mediated alkene hydro-
boration [29].
In order to uncover the species involved in these catalytic
reactions, we measured 11B NMR spectra of the reaction mixture in
benzene-d6 after catalysis. The very broad signal at 87.6 ppm can be
assigned to the product (Ph2CHCH2)3B (this was supported by
comparison with the independently prepared sample). The doublet
at 28.8 ppm (1JB,H ¼ 191.6 Hz) corresponds to unreacted substrate
HBcat. Minor signals at 23.2 and 9.2 have been assigned to B2(cat)3
and [B(cat)2]ꢁ (vide infra).
Earlier work from Nöth et al. showed that HBcat is also not
stable in the presence of group 4 metal hydrides like Cp2ZrH2 [30].
In order to learn more about the catalytic decomposition of HBcat
we followed the reaction of 1 with excess HBcat in THF-d8 by 11B
NMR. The reaction mixture became after a short time at room
temperature already viscous. The 11B NMR signals (Fig. 1) can be
11
Fig. 1. The B NMR spectrum (THF-d8, 160 Mhz, 25 ꢀC) for the reaction of 1 with excess HBcat. See Table 2 for assignments of signals (* indicates unknown species).

10
S. Harder, J. Spielmann / Journal of Organometallic Chemistry 698 (2012) 7e14
O
–
+
HBcat
LCa
H2B
BH3
O
HBcat
O
O
–
+
–
+
LCa
BH4
LCa
H2B
O
O
O
O
HBcat
B
H
LCaH
B
O
O
O
O
B
O
B
O
H
O
O
–
–
+
+
LCa
H3B
LCa
H3B
BH3
O
B
O
B
O
O
O
–
+
HBcat
LCa
B
O
O
O
O
O
Scheme 3.
compared to literature values for various boron species (Table 2).
The signal for the main decomposition product is a sharp singlet at
9.2 ppm. This is assigned to the B(cat)ꢁ2 ion that in the presence of
Ca2þ likely forms the complex DIPPnacnacCaB(cat)2 or Ca
[B(cat)2]2$(THF)2 in which the Ca2þ is chelated by two B(cat)2ꢁ ions
(4). This binding mode would be in agreement with the crystal
structure of Cp2Ti[B(cat)2] that also contains a chelating B(cat)2ꢁ
ligand [21]. In contrast, the B(cat)ꢁ2 ligand binds Rh via the aromatic
ring [37]. The 1H NMR of the reaction mixture is in agreement with
the proposed structure for 4: it shows an AA0XX0 pattern for the
aromatic protons. We were not able to crystallize this major species
from solution. Further 11B signals can be assigned to residual HBcat
(doublet at 23.9 ppm), B2(cat)3 (broad signal at 25.2 ppm),
BH3(THF) (characteristic quartet at ꢁ0.6 ppm), BHꢁ4 (characteristic
quintet at ꢁ36.5 ppm) and B2H7ꢁ (broad multiplet at ꢁ23.9 ppm).
The mechanism of HBcat decomposition in the presence of an
organocalcium compound likely shows parallels with that
proposed for the phosphine promoted degradation of HBcat [34].
The tentatively proposed scheme contains all species observed in
the 11B NMR spectrum (Scheme 3).
One of the key species in this catalytic cycle is DIPPnacnac-
Ca(BH4), a compound we could prepare by reaction of 1 with
BH3(Me2S). Considering the strong Lewis-basicity and acidity of 1
and BH3, respectively, it is not likely that this compound is in
equilibrium with a calcium hydride complex and BH3 (as suggested
for the lanthanide-catalyzed cycle, Scheme 2). Indeed, 1H NMR
spectra of DIPPnacnacCa(BH4) in aromatic solvents do not show any
indication for such equilibria.
Crystals obtained from a THF solution have the composition
DIPPnacnacCa(BH4)$(THF)2 and the crystal structure could be
determined by X-ray diffraction. It crystallizes as a monomeric
complex with two nearly identical molecules in the asymmetric
unit (Fig. 2). The BHꢁ4 ion caps the Ca2þ ion via three bridging
BeH units (hydrogen atoms have been observed in the difference-
Fourier map and could be refined isotropically). A similar k
3-BH4
bonding mode is found in other known crystal structures of
calcium tetrahydroborate complexes like Ca(BH4)2$(DME) [38a],
Ca(BH4)2$(diglyme) [38b] and a series of cationic complexes [39].
The Ca/B distance of 2.600(5) Å is significantly shorter than that
in Ca(BH4)2$(DME) and Ca(BH4)2$(diglyme) (range: 2.643(5)e
2.884(5) Å) but is similar to that observed in the cationic complexes
(2.580(2)e2.610(4) Å). The coordination around the five-coordinate
Ca centre could be described as a tetragonal pyramid. Complex
DIPPnacnacCa(BH4)$(THF)2 was also fully characterized by 1H, 13
C
Fig. 2. Crystal structure of DIPPnacnacCa(BH4)$(THF)2 (5); hydrogen atoms, except
those for BH4, are not shown for clarity. The range of selected bond distances in Å
(average values for two independent molecules are given in square brackets): CaeN
2.371(3)e2.404(3) [2.399(3)], CaeO 2.410(2)e2.429(2) [2.419(2)], Ca/H 2.27(4)e
2.46(3) [2.37(3)], Ca/B 2.596(5)e2.604(5) [2.600(5)].
and 11B NMR. The 11B signal at ꢁ35.0 ppm (quint, 1JB,H ¼ 82.2 Hz)
fits well the signal observed in the decomposition studies (Fig. 1,
Table 2).

S. Harder, J. Spielmann / Journal of Organometallic Chemistry 698 (2012) 7e14
11
Fig. 3. Crystal structure of [DIPPnacnacCa(H2Bpin)]3 (6); iPr substituents and hydrogen atoms, except those on B, are not shown for clarity. Symmetry operations: X0 ¼ 1/2 þ z,
3/2 ꢁ x, 1 ꢁ y; X00 ¼ 3/2 ꢁ y, 1 ꢁ z, ꢁ1/2 þ x.
Results described above show that the Ca-mediated hydro-
boration of DPE with HBcat is not “true” organometallic catalysis.
The overall catalytic decomposition, 3 HBcat / B2(cat)3 þ BH3,
generates BH3 which is the actual reactive species in the alkene
hydroboration. This “Trojan horse”-like behaviour also renders any
control of chemo-, regio- or enantioselectivity by catalyst design.
For these reasons we also investigated the reactivity of the calcium
hydride complex 1 with other boranes.
In previous work we could isolate and structurally characterize
the borate complex 3 from the reaction of 1 and 9-BBN (9-
borabicyclo[3.3.1.]nonane) [27]. This complex was simultaneously
prepared and characterized by Hill et al. [40]. Compound 3 is also
catalytically active in the hydroboration of DPE with HBcat but the
reaction is very slow (Table 1, entry 6). Complex 3 itself does not
react with DPE even under forcing conditions (benzene, 80 ꢀC).
Complex 3 likely functions as hydride transfer reagent and forms
H2Bcatꢁ which starts the catalytic degradation of HBcat. As 9-BBN
already reacts with DPE at room temperature, a Ca-controlled
catalytic system based on 4 and 9-BBN would not be feasible (at
least not at room temperature).
We directed further attention to Knochel’s pinacolborane HBpin
[41], a borane that similar to HBcat, displays reduced Lewis-acidity
and hydroboration reactivity due to partial O(p)eB(p) overlap.
Alkene hydroboration with HBpin needs the presence of a catalyst.
The borane HBpin, however, is based on a tertiary alkyl ether
instead of a phenolic ether which makes it less prone to decom-
position via ring opening reactions [41,42].
From the reaction of 1 with HBpin in benzene we could isolate
crystals of [DIPPnacnacCa(H2Bpin)]3 (6). The crystal structure
shows a trimeric aggregate with crystallographic C3-symmetry
(Fig. 3, Table 3) which is unusual in organocalcium chemistry [43].
The origin for the formation of trimeric units lies in the bonding
mode of the H2Bpinꢁ ion: instead of bonding through a BH2/Ca
interaction, the ether O atoms interact with the metal. Hitherto
there is no precedence for this coordination mode but there are few
isolated examples for R(H)Bcatꢁ/metal coordination [44]. The
star-like structure results from the bridging behaviour of the
H2Bpinꢁ ion. Although the primary bonding between the anion and
the metal cation proceeds through O/Ca2þ interaction, there is
Table 4
Crystal data for DIPPnacnacCa(BH4)$(THF)2 (5) and [DIPPnacnacCa(H2Bpin)]3 (6).
Compound
Formula
DIPPnacnacCa(BH4)$(THF)2
C37H61BCaN2O2
[DIPPnacnacCa(H2Bpin)]3
C105H165B3Ca3N6O6,
3(C6H6)
1994.43
0.2 ꢂ 0.2 ꢂ 0.2
cubic
MW
616.77
Size (mm3)
0.2 ꢂ 0.3 ꢂ 0.5
Crystal system
triclinic
Spacegroup
Pꢁ1
Paꢁ3
a (Å)
b (Å)
c (Å)
a
14.6759(3)
15.6628(3)
17.7136(4)
75.614(1)
81.328(1)
70.817(1)
3714.42(14)
4
29.2765(10)
29.2765(10)
29.2765(10)
90
b
90
90
g
V (Å3)
Z
25,093.3(15)
8
1.056
r
(g.cmꢁ3
)
1.103
0.201
m
(MoKa) (mmꢁ1
)
0.183
T (ꢀC)
ꢁ123
ꢁ70
q
(max)
Refl. total
independent
23.0
27.5
36,582, 10,295
242,723, 9612
Rint
0.089
0.098
Obsd refl.(I > 2
s(I))
5963
7253
Parameter
R1
wR2
845
481
Table 3
0.0511,
0.1416
1.11
ꢁ0.23/0.32
0.0779
0.2190
1.16
ꢁ0.46/0.60
Selected bond distances (Å) in the crystal structure of [DIPPnacnacCa(H2Bpin)]3 (6).
CaꢁN1
CaꢁN2
Ca/O1
2.367(3)
2.372(3)
2.313(3)
Ca/O200
Ca/H1
Ca/B
2.299(3)
2.60(4)
2.830(5)
Ca/ B00
BꢁO1
BꢁO2
3.259(5)
1.489(6)
1.459(6)
GOF
min/max residual
e-density (e.Åꢁ3
)

12
S. Harder, J. Spielmann / Journal of Organometallic Chemistry 698 (2012) 7e14
a tendency for BH/Ca bonding (H1/Ca). The bridging H2Bpinꢁ ion
is rotated to one of the Ca2þ ions which is demonstrated by unequal
BeOeCa angles (B1eO1eCa 93.7(1)ꢀ; B1eO2eCa0 118.5(1)ꢀ). This
brings H1 in close vicinity of one of the Ca2þ ions. Although the
H1/Ca distance of 2.60(1) Å is significantly longer than that of
2.45(1) Å in DIPPnacnacCa(BH4)$(THF)2, it is well within the sum of
the van der waals radii for B and H (3.33 Å). All other BH/Ca
distances are well over 3 Å. The bonding interaction between H1
can be clearly seen in the space-filling model (Fig. 3b). The hole in
the middle of the ring is too small for interaction with metal
cations, however, ring enlargement could produce crown ether-like
species for entrapment of cations through BH2/metal interactions.
Another feature is the extreme side-on coordination of the DIPP-
nacnac ligand: the C(H)eC(Me)eNeCa torsion angles are 45.1(1)ꢀ
and 51.6(1)ꢀ.
addition, the less sensitive pinacolborane HBpin was investigated in
reactivity studies. Reaction of 1 with HBpin gave a trimeric orga-
nocalcium complex [DIPPnacnacCa(H2Bpin)]3 (6) which in benzene
solution is invariably stable also at higher temperatures (60e80 ꢀC).
However, in the presence of excess of HBpin the complex and the
borane decompose already at room temperature and therefore this
system is not useful for “true” catalytic conversion. Complex 6 does
not react with DPE even at higher temperatures (80 ꢀC). This
demonstrates that there is no equilibrium between
6 and
1 þ HBpin. It is possible that more Lewis-acidic metals like the
lanthanide 3þ cations may give an equilibrium between borate and
hydride. Therefore, also more Lewis-acidic organomagnesium
might be productive in alkene hydroboration catalysis. We will
address this possibility in future investigations.
Trimeric [DIPPnacnacCa(H2Bpin)]3 (6) dissolves well in aromatic
solvents and was characterized by 1H, 13C and 11B NMR. It likely
retains its trimeric structure in this solvent. The 1H NMR spectrum
shows two singlet signals for the Me groups of H2Bpin and four
signals for the iPr groups of DIPPnacnac. The 13C NMR spectrum
shows two CH3 signals and two quaternary signals for the carbon
atoms in H2Bpin. If one assumes fast exchange between ring-
puckering conformations in H2Bpin, these data are in agreement
with the crystal structure.
NMR investigations on 6 do not show any sign of this species
being in equilibrium with 1 and HBpin. A benzene solution of 6
in benzene-d6 is even stable for several days at 60 ꢀC. As addition
of DPE and heating to 80 ꢀC for several days did not result in
reaction of the hydride with the C]C bond, it can be ruled out
that 6 is in equilibrium with small amounts of 1 and HBpin (1
reacts smoothly with DPE at 60 ꢀC [24]). Reaction of 1 with
HBpin seems therefore irreversible. The high stability of calcium
borate complexes has been reported earlier by Hanusa et al.:
attempted synthesis of a calcium hydride complex by decom-
position of (Me3Si)3CpCa(HBEt3) into (Me3Si)3CpCaH and BEt3
failed [45].
Failure of 6 and DPE to react, inherently means that the borate
anion H2Bpinꢁ does not react directly with DPE. However,
a benzene solution of complex 6 is unstable in the presence of
HBpin. Therefore, any crystals obtained by reaction of 1 with HBpin
must be isolated immediately. Prolonged crystallization times lead
to decomposition of 6 and HBpin. Although we did not analyze this
decomposition reaction in detail, it is likely that similar ring
opening mechanisms apply as for HBcat (Scheme 3). For these
reasons catalytic conversions with HBpin were not further
considered.
4. Experimental
4.1. General procedures
All experiments were carried out under argon using dry solvents
and Schlenk techniques. The following complexes have been
prepared according to literature procedures: 1 [22], 2 [28a] and 3
[27]. Crystals were measured on a Siemens Smart diffractometer
with APEXII area detector system. The structures were solved by
Direct Methods (SHELXS-97) and refined with SHELXL-97 [46].
All geometry calculations and graphics were performed with PLA-
TON [47].
4.2. General procedure for catalytic hydroboration of DPE with
HBcat
Catecholborane (HBcat) and pinacolborane (HBpin) were puri-
fied by distillation prior to use. 1,1-Diphenylethylene (DPE) was
dried by overnight heating to 60 ꢀC over finely powdered CaH2
and subsequently isolated by centrifugation. NMR spectra were
recorded on a Bruker DPX300 (300 MHz) spectrometer. The indi-
cated amount of an organocalcium catalyst (Table 1) was added to
a solution of HBcat (360 mg, 3.00 mmol) in DPE (360 mg,
2.00 mmol) which previously had been brought to the indicated
temperature. The conversion was followed by taking samples and
analyzing these with 1H NMR. For experiments in benzene solution,
3.0 ml of benzene was added prior to catalyst addition.
Reactions without organocalcium catalyst (entry 2) gave
Ph2CHCH2Bcat. 1H NMR (300 MHz, benzene-d6 þ THF-d8, 25 ꢀC):
d
¼ 1.95 (d, 3JH,H ¼ 8.1 Hz, 2H, CH2), 4.52 (t, 3JH,H ¼ 8.1 Hz, 1H, CH),
6.70 (m, 2H, cat), 6.93 (m, 2H, cat), 6.98 (m, 2H, aryl), 7.07 (m, 2H,
aryl), 7.20 (m, 2H, aryl); 11B NMR (160 MHz, benzene-d6 þ THF-d8,
3. Conclusion
25 ꢀC):
d
¼ 35.1 ppm (br).
Reactions in the presence of an organocalcium catalyst gave
Several organocalcium compounds, the hydride 1, the benzyl
complex 2 and the borate 3, are catalytically active in the hydro-
boration of DPE with HBcat. The products, however, are not
the expected boronic ester Ph2CHCH2OBcat but the borane
(Ph2CHCH2)3B. Therefore, Ca-catalyzed alkene hydroboration
should not be regarded as “true” catalysis but as a “Trojan horse”-
type of reaction [29]: i.e. the organocalcium compounds catalyze
the decomposition of HBcat to mainly B2(cat)3 and BH3 (or B2H6)
and the latter is the actual reagent. NMR studies on the reaction
solutions and stoichiometric reactions between 1 and HBcat indeed
indicate the formation of several new species including B(cat)ꢁ2 ,
B2(cat)3, BH3(THF), BH4ꢁ and B2Hꢁ7. The organometallic complex
with the BH4 ion as a ligand was prepared via a different route and
DIPPnacnacCa(BH4)$(THF)2 (5) could be fully characterized.
Hydroboration according to this protocol does not have the
advantage of chemo-, regio- or enantiocontrol by catalyst design. In
(Ph2CHCH2)3B. 1H NMR (300 MHz, benzene-d6, 25 ꢀC):
d
¼ 1.97 (d,
3JH,H ¼ 7.9 Hz, 2H, CH2), 4.28 (t, 3JH,H ¼ 7.9 Hz, 2H, CH), 7.02 (m, 4H,
aryl), 7.10 (m, 2H, aryl), 7.02 (m, 4H, aryl); 11B NMR (160 MHz,
benzene-d6, 25 ꢀC):
d
¼ 87.8 ppm (br).
4.3. Stoichiometric reaction of [(DIPPnacnac)CaH$(THF)]2 (1) with
HBcat
A
J. Young NMR tube was charged with [(DIPPnacnac)
CaH$(THF)]2 (30 mg, 0.028 mmol) and 0.6 ml of THF-d8. After
addition of HBcat (68 mg, 0.56 mmol) the solution became
viscous. The reaction was followed by 1H and 11B NMR spectros-
copy. Major species: 1H NMR (300 MHz, THF-d8, 25 ꢀC):
d
¼ 6.22
(m, AA0XX0 spin system, 2H), 6.38 (m, AA0XX0 spin system, 2H),
6.65 (m, AA0XX0 spin system, 2H), 6.71 (m, AA0XX0 spin system,
2H). 11B NMR (160 MHz, THF-d8, 25 ꢀC):
d
¼ 9.2 ppm (sharp). The

S. Harder, J. Spielmann / Journal of Organometallic Chemistry 698 (2012) 7e14
13
signal for the major species was assigned to the B(cat)ꢁ2 ion.
Asymmetry in the 1H NMR spectrum is caused by coordination to
Ca2þ (like in 4). See Table 2, Fig. 1 and the discussion for the
assignment of other 11B signals.
Details for the refinement of [DIPPnacnacCa(H2Bpin)]3 (6): The
H2Bpinꢁ ligand shows strong disorder due to ring-puckering in the
five-membered ring. This was solved with an appropriate disorder
model but affects the quality of the structure. All hydrogen atoms,
except those on B, have been placed on calculated positions and
were refined in a riding mode. The hydrogen atoms on B have been
found in the Difference-Fourier map and were refined isotropically.
The asymmetric unit contains two cocrystallized benzene mole-
cules. One is relatively ordered and was refined with large aniso-
tropic displacement parameters, the other is completely disordered
and was treated with the SQUEEZE procedure (hole: 250 Å3, 59
electrons) incorporated in PLATON [47].
4.4. Synthesis of DIPPnacnacCa(BH4)$(THF)2 (5)
[(DIPPnacnac)CaH$(THF)]2 (1, 212 mg; 0.40 mmol) were dis-
solved in 1.3 ml of THF. To this solution was slowly added
BH3(Me2S) (33 mg, 0.43 mmol). Slow cooling of the resulting
solution to 8 ꢀC gave the crystalline product in the form of large
colourless blocks. Yield: 148 mg, 0.24 mmol, 60%. Anal. (%): Calcd.
for C37H61BCaN2O2 (616.78): C, 72.05; H, 9.97. Found: C, 71.71; H
9.71. 1H NMR (300 MHz, benzene-d6, 25 ꢀC):
d
¼ ꢁ0.14 (q,
Acknowledgement
3
1JB,H ¼ 82.2 Hz, 4H, BH4), 1.23 (d, JH,H ¼ 6.8 Hz, 12H, iPr), 1.25 (d,
3JH,H ¼ 6.8 Hz,12H, iPr),1.33 (m, 8H, THF),1.68 (s, 6H, Me backbone),
We are grateful to the Deutsche Forschungsgemeinschaft for
financing this project and acknowledge Prof. R. Boese and D. Bläser
for collection of the X-ray data.
3
3.24 (sept, JH,H ¼ 6.8 Hz, 4H, iPr), 3.53 (m, 8H, THF), 4.80 (s, 1H,
backbone), 7.13 (m, 6H, aryl); 11B NMR (160 MHz, benzene-d6,
25 ꢀC):
d8, 25 ꢀC):
d
¼ ꢁ33.3 (quint, 1JB,H ¼ 82.2 Hz); 11B NMR (160 MHz, THF-
d
¼ ꢁ35.0 (quint, 1JB,H ¼ 82.2 Hz); 13C NMR (300 MHz,
Appendix A. Supplementary material
benzene-d6, 25 ꢀC):
d
¼ 24.8 (iPr), 25.0 (iPr), 25.1 (Me backbone),
25.5 (iPr), 28.4 (THF), 69.0 (THF), 94.5 (backbone),123.8 (aryl),124.5
Crystallographic data (excluding structure factors) for the
structure of DIPPnacnacCa(BH4)$(THF)2 (5) (CCDC 836486) and
[DIPPnacnacCa(H2Bpin)]3 (6) (CCDC 836485) can be obtained free
of charge from the Cambridge Crystallographic Data Centre: CCDC,
12 Union Road, Cambridge, CB2 1EZ, UK (Fax: þ44-1223-336033; e-
mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.ck).
(aryl), 142.1 (aryl), 146.9 (aryl), 165.8 (backbone).
4.5. Synthesis of [DIPPnacnacCa(H2Bpin)]3 (6)
A solution of [(DIPPnacnac)CaH$(THF)]2 (1, 200 mg; 0.38 mmol)
and excess of HBpin (191 mg; 1.49 mmol) in 3.0 ml of benzene was
stirred for 40 min at room temperature and then cooled to 8 ꢀC.
After circa 30 min crystals of [(DIPPnacnac)Ca(H2Bpin)]3 in the
form of colourless blocks appeared. These were immediately iso-
lated from the cloudy mother liquor. Prolonged crystallization
times led to decomposition of the product. Yield: 120 mg,
0.18 mmol, 48%. Anal. (%): Calcd. for C35H55BCaN2O2$(C6H6)
(664.45): C, 74.07; H, 9.25. Found: C, 74.52; H 8.92. 1H NMR
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(300 MHz, benzene-d6, 25 ꢀC):
6H, Me H2Bpin), 1.19 (d, JH,H
d
¼ 0.89 (s, 6H, Me H2Bpin), 0.90 (s,
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(160 MHz, benzene-d6, 25 ꢀC):
d
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d
¼ 25.6 (iPr), 25.8 (iPr), 25.8 (iPr),
26.0 (iPr), 26.7 (Me backbone), 29.5 (iPr), 29.8 (H2Bpin), 29.9
(H2Bpin), 81.5 (H2Bpin), 82.5 (H2Bpin), 95.0 (backbone), 125.1
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148.9 (aryl), 166.9 (backbone).
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14
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