CuI-catalyzed conjugate addition of silyl boronic esters: Retracing catalytic cycles using isolated copper and boron enolate intermediates

Article abstract of DOI:10.1021/om500989w

Copper(I)-catalyzed conjugate additions of silyl boronic esters to α,β-unsaturated aldehydes, ketones, and esters are synthetically well-established reactions. For the first time central reactive intermediates as well as the boron enolates as the primary

Full text of DOI:10.1021/om500989w

Article
pubs.acs.org/Organometallics
CuI‑Catalyzed Conjugate Addition of Silyl Boronic Esters: Retracing
Catalytic Cycles Using Isolated Copper and Boron Enolate
Intermediates
Jacqueline Plotzitzka and Christian Kleeberg*
Institut fur Anorganische und Analytische Chemie, Technische Universitat Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106
̈
̈
Braunschweig, Germany
S
* Supporting Information
ABSTRACT: Copper(I)-catalyzed conjugate additions of silyl
boronic esters to α,β-unsaturated aldehydes, ketones, and esters
are synthetically well-established reactions. For the first time
central reactive intermediates as well as the boron enolates as
the primary reaction products are isolated and employed in
order to deduce catalytic cycles on an experimental basis.
Employing an NHC CuI complex as a model catalyst, it is
possible to perform efficient catalytic transformations as well as
to isolate and characterize the formed copper enolate
complexes as the key intermediates. It is shown that for this
catalytic system the nature of this enolateO- or C-enolateis
crucial for the catalytic process. For α,β-unsaturated aldehydes
and ketones the O-enolate is formed predominantly, while for
α,β-unsaturated esters the C-enolate is the major product.
Catalytic turnover is only facile for copper O-enolates, as they react efficiently with the silyl boronic ester under (re)formation of
the catalytically active Cu−Si species and a thermodynamically favored boric acid ester. Thus, the formation of copper C-enolates
is inhibiting the catalytic process, and effective turnover is possible only after solvolysis by an alcohol additive. The individual
catalytic processes were retraced by performing stepwise stoichiometric reactions monitored by in situ NMR spectroscopy.
Scheme 1. General Scheme of Copper(I)-Catalyzed
Silylation Reactions of α,β-Unsaturated Carbonyl (R = H,
Alkyl; R′ = H, Alkyl, Aryl) and Carboxyl (R = Alkoxido, R′ =
H, Alkyl, Aryl) Compounds Employing the Silyl Boronic
Ester 1
INTRODUCTION
■
Silyl boronic esters, especially pinB−SiMe2Ph (1) (pin =
OCMe2CMe2O), are well-established reagents in transition-
metal-catalyzed silylation reactions (e.g., Pt, Pd, Cu, Ni, Rh,
CuII). More recently, transition-metal-free Lewis-base-pro-
moted silylation and borylation reactions employing 1 have
also emerged.1−4 Furthermore, copper(I)-catalyzed silylation
reactions employing 1 with various organic substrates such as
α,β-unsaturated carbonyl and carboxyl compounds, aldehydes,
imines, amides, but also, for example, allyl/propargyl chlorides
have been reported in the past few years.1a,2 Using α,β-
unsaturated carbonyl and carboxyl compounds as substrates,
the products obtained, possibly after hydrolytic workup, are the
corresponding β-silyl carbonyl/carboxyl compounds, respec-
tively (Scheme 1).
A generalized catalytic cycle was proposed in agreement with
the experimental data, well-established stoichiometric cuprate
chemistry, and the apparently closely related CuI-catalyzed
diboration reactions (Scheme 2).1a,2,4−8,9a,b While diverse
catalyst systems and reaction conditions are employed in
these transformations, two general points regarding the
(pre)catalyst are to be noted: The CuI source may be a
preformed, isolated copper complex. More often, a CuI salt and
a ligand or ligand precursor, possibly under addition of an
auxiliary reagent (e.g., a base in the case of imidazolium salt in
order to furnish a (NHC)Cu species), is employed. In a simple
and yet versatile catalytic system CuCN is used in the absence
of any additional ligand.1a,2,4 The second general observation is
that catalytic amounts of an alkali metal alkoxide are required. It
has been conclusively argued that the alkoxide is necessary
(besides the possible generation of an NHC ligand, vide supra)
to form a copper alkoxido species that is crucial for an initial
B−Si bond activation step in order to generate the catalytically
Received: September 29, 2014
© XXXX American Chemical Society
A
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Scheme 2. Proposed Catalytic Cycle for the CuI-Catalyzed
and finally to study their reactivity and retrace the catalytic
cycle stepwise by stoichiometric reactions.
Conjugate Silylation of α,β-Unsaturated Carbonyl and
a
We reasoned that using a model catalyst based on the
(IDipp)CuI fragment is advantageous in order to have well-
defined, mononuclear species and to prevent the formation of
aggregates and/or polynuclear complexes as suggested by the
long-discussed aggregation behavior known from, for example,
alkyl and aryl cuprates.6 Furthermore, the (IDipp)CuI system
has already been successfully employed as a model catalyst
studying 1,2-addition reactions of 1 to aldehydes and CO2 as
well as in a number of related studies.5,9
Carboxyl Compounds with 1
RESULTS AND DISCUSSION
■
Catalytic Experiments. The exemplary α,β-unsaturated
carbonyl and carboxyl compounds 3a−f as substrates were
reacted with 1 in the presence of catalytic amounts of
(IDipp)CuOtBu (2). The substrates were selected in order to
cover carbonyl and carboxyl compounds (ketones, aldehydes,
esters) and different substitution patterns at the β-carbon atom
(Table 1).
Table 1. Conjugate Addition of 1 to α,β-Unsaturated
Substrates in the Presence of 2
a
Adopted from ref 1a.
a
a
substrate
t/h
4 (E/Z)
5
crucial Cu−Si species (Scheme 2). The effectiveness of ligand
control especially by NHC ligands has been demonstrated by
Hoveyda et al.: Using a chiral NHC-Cu complex as catalyst, the
conjugate silyl addition to α,β-unsaturated ketones and esters
results in high yields and enantioselectivity.4a
R = Et, R′ = H
R = Me, R′ = Me
cyclohexenone
3a
3b
3c
3d
3e
3f
3
87% (1:7)
67%
77%
87%
82%
68%
6−8
6
not isolated
93% (2:5)
R = H, R′ = H
R = H, R′ = Me
R = OMe, R′ = H
2
b
b
The commonly accepted catalytic process consists of three
distinct steps (Scheme 2): (1) formation of the central
copper(I) silyl complex (via σ-bond metathesis) from the
silyl boronic ester and a copper alkoxido species, initially
formed from the primary copper source and the alkali metal
alkoxide. (2) This copper(I) silyl complex reacts in an addition
reaction with the α,β-unsaturated substrate to give a β-silylated
copper enolate. (3) The reaction of this enolate with the silyl
boronic esters regenerates the copper(I) silyl complex (and,
hence, completes the catalytic cycle), and the primary product
of the catalytic process, a β-silylated boron O-enolate, is
released. Finally, hydrolytic workup of the latter results in the
formation of the finally isolated β-silylated carbonyl/carboxyl
compounds. The individual steps within the catalytic cycle are
also, as far as plausible, in agreement with computational data
on a CuI/amine-cocatalyzed enantioselective conjugate silyla-
tion of α,β-unsaturated carbonyl compounds and with
experimental studies on the 1,2-addition of 1 to aldehydes
and CO2.4g,5,10
16
0%
0%
c
4
79%
b
a
Isolated yields. E/Z ratio determined by 1H NMR spectroscopy. 1,2-
Addition product in 92% (7)/83% (8) yield (vide infra, Scheme 3). In
c
i
the presence of PrOH.
For the ketones 3a−c and the mono-β-substituted aldehyde
3d the expected β-silylated boron enolates (4a,b,d) and after
hydrolytic workup the corresponding β-silylated carbonyl
compounds (5a−d) were isolated in good yields. This
demonstrates that 2 is indeed an efficient precatalyst and that
further studies may be fruitful to gain more mechanistic insight.
Moreover, to the best of our knowledge, the β-silyl boron
enolates as the primary reaction products of a CuI-catalyzed
silyl conjugate addition reaction of a silyl boronic ester have so
far not been obtained in substance and characterized in any
detail.11
The conjugate addition reactions of 1 to 3a, 3b, and 3d were
studied by in situ H NMR spectroscopy under conditions
For a further rational development of CuI-catalyzed
conjugate addition of silyl boronic esters with α,β-unsaturated
carbonyl and carboxyl compounds a more detailed under-
standing of the species present during the individual reaction
steps and of their reactivity is highly desirable. Hence, we set
out to study the CuI-catalyzed reaction of 1 with exemplary α,β-
unsaturated aldehydes, ketones, and esters employing a
(IDipp)CuI complex (IDipp = 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene) as model catalyst. In particular we aimed to
isolate and thoroughly characterize the central intermediates
1
closely resembling the conditions used above but employing 10
mol % of (IDipp)Cu−SiMe2Ph (6) or (IDipp)Cu−OtBu (2) as
precatalysts and C6D6 as solvent. As, according to the proposed
catalytic cycle (Scheme 2) and in agreement with our studies
on related 1,2-addition chemistry, the alkoxide complex 2 is
rapidly converted to 6 upon reaction with 1, both precatalysts
eventually lead to the same catalytically active species.5
For all substrates the formation of the boron enolate as the
virtually exclusive reaction product is observed (Figure 1).
B
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1
■
□
Figure 1. Details of in situ H NMR spectra of conjugate addition reactions of 1 (1.2 equiv, ) to hex-4-en-3-one (3a), mesityloxide (3b, ), and
●
crotonaldehyde (3d) (1.0 equiv) (from top) in the presence of 10 mol % precatalyst (6 ( ) for 3a,b and 2 for 3d) at room temperature (300 MHz,
rt, C6D6). Signals of the minor isomers are denoted by a prime; only selected NOESY contacts are shown ( : pinB−O Bu; ×: 5a); denotes that
only a part of the signal is visible due to overlapping.
t
†
○
However, the reaction times to reach complete conversion are
significantly different and generally in accordance with the
preparative study (vide supra). The different rates may be
rationalized with the steric influence of the β-substituents and
the electrophilicity of the carbonyl group. Thus, for the mono-
β-substituted aldehyde 3d complete conversion is achieved
within 2 h, while for the sterically comparable ketone 3a 6 h is
required. The β-dimethyl-substituted, sterically more encum-
bered ketone 3b does not reach completion within 20 h (62%
1H NMR conversion). In comparison with other Cu-based
catalytic systems the catalyst employed here is of only moderate
activity; yet, this may render it especially suitable for the
isolation and study of catalytically relevant intermediates.1a,2
The boron O-enolates 4a and 4d are obtained as E/Z
mixtures according to 1H NMR data with ratios of 1:8 and 2:5,
respectively, resembling the ratios observed in the isolated
products (Table 1). For the enolate 4b only one isomer was
and in particular with reported values for related silyl enol
ethers.12
In addition to the signals of the silylation products 4a,b,d
signals indicative for complex 6 as well as for residual 1 are
observed in all reactions. Moreover, unreacted starting material
is observed in the reaction of 3b as well as pinB−OtBu in the
reaction of 3d. The latter results from the initial conversion of
the precatalyst 2 to the silyl complex 6 in order to enter the
catalytic cycle (vide supra).5 In the case of the reaction of 3a a
small amount (<5% relative to 4a) of 5a was identified due to
hydrolysis by adventitious moisture. The magnitude of all other
remaining signals is negligible, and their nature was not further
evaluated.
Employing the β-dimethyl-substituted aldehyde 3e under the
established conditions, the products of the 1,2-addition of 1, the
silyl alcohol derivatives 7 and 8, are obtained as major products
in excellent isolated yields (Scheme 3). The existence of a 1,2-
addition pathway is not surprising considering that the catalytic
system used is also effective for the 1,2-silylation of aryl and
alkyl aldehydes.5
1
detected by H NMR spectroscopy. The isomers were studied
1
by H−1H NOESY NMR experiments on isolated samples of
4a,b,d. For the major isomer of 4a,d no (or only very weak)
NOESY contacts were observed between the hydrogen atoms
in the positions 1 and 3, while strong contacts were observed
between the hydrogen atoms in the positions 1 and 2,
suggesting mutual Z orientation of the latter. In agreement with
these findings the opposite is observed for the minor isomer:
strong 1′−3′ (and 1′−4′) and only comparably weak 1′−2′
contacts were found, corroborating the assignment of the
predominant isomer as Z configured (Figure 1). In the case of
4b a final assignment is not possible on the basis of NOESY
NMR data, as comparable contacts 1−2 and 1−4 are observed.
Furthermore, for 4d the assignment is confirmed by the
observed mutual coupling constants for the olefinic hydrogen
1
An in situ H NMR experiment reveals that the 1,2-addition
product 7 is indeed the major product (Figure 2). However,
additional signals in the in situ 1H NMR spectrum are
suggestive of the competitive formation of the 1,4-addition
product 4e (5% relative to 7).13,14 All other signals are in
agreement with the findings discussed above.
In light of the observation of the 1,2-addition reaction for 3e
it may be speculated that the additional, very weak signals in the
range 5−6 ppm observed during the silylation reaction of 3d
may suggest the formation of traces of the 1,2-addition product
also in this case. However, finally conclusive data could not be
obtained (Figure 1, bottom).
3
3
atoms of JHH = 6.0 Hz for the major Z isomer and of JHH
=
For the α,β-unsaturated ester 3f no conjugate addition
product 4f was obtained under the conditions employed for the
carbonyl substrates, while in the presence of a stoichiometric
11.9 Hz for the minor E isomer. These data are in agreement
with the general trend observed for vicinal coupling constants
C
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Scheme 3. Reaction of 1 with 3e in the Presence of 2 Leads
to the Predominant Formation of the 1,2-Addition Product
7 (and 8)
Figure 3. Details of in situ 1H NMR spectrum of the conjugate
■
□
addition reaction of 1 ( ) to 3f ( ). Bottom: 1 (1.2 equiv), 3f (1.0
equiv), 10 mol % 2 after 8 h at room temperature. Top: 1 (1.2 equiv),
i
3f (1.0 equiv), PrOH (1.0 equiv), 10 mol % 2 after 0.5 h at room
t
○
●
temperature (300 MHz, rt, C6D6, : pinB−O Bu; : 6; ×: pinB−
OiPr; +: 9f); denotes that only a part of the signal is visible due to
†
overlapping.
and 9f may be rationalized by a slightly less (approximately 5%)
i
than equimolar amount of PrOH, leading to incomplete
alcoholysis and as a result incomplete conversion of 9f to 6
(Figure 3).
In summary, it is shown that, by preparative as well as in situ
NMR studies, the CuI-catalyzed conjugate addition of the silyl
boronic ester 1 employing a (IDipp)Cu complex as catalyst is
effective for α,β-unsaturated ketones (3a−c) and certain
aldehydes (3d). This is in general agreement with previous
reports on related systems.4 Beyond that, it is demonstrated
that the bis-β-substituted aldehyde 3e gives the 1,2-addition
product and that the ester 3f is unreactive in the absence of an
alcohol as additive.
Stoichiometric Experiments. To gain further insight into
the individual reaction steps, we aimed to retrace the catalytic
cycle by stoichiometric experiments. The first step to be studied
is the insertion of the α,β-unsaturated substrates into the Cu−
Si bond of 6 (Scheme 4).
Figure 2. Details of in situ 1H NMR spectrum of the conjugate
addition reaction of 1 (1.2 equiv, ) to 3e (1.0 equiv) in the presence
■
of 10 mol % 2 as precatalyst after 6 h at room temperature giving 7
13
t
○
●
and 4e (300 MHz, rt, C6D6, : pinB−O Bu; : 6).
i
amount of PrOH the β-silylated ester 5f was obtained (Table
1). This is unexpected insofar as the conjugate addition to 3f
was reported to proceed in the absence of alcohol, although a
similar catalytic system with a different NHC ligand was
used.4a,b The in situ 1H NMR spectrum of the silylation
reaction of 3f corroborates this and shows essentially a mixture
of the starting materials 1 and 3f after 8 h (Figure 3, bottom).
Nevertheless, the precatalyst 2 has been converted to a single
new (IDipp)Cu species along with the formation of pinB−
OtBu, indicating the initial conversion of 2 to 6. The novel
(IDipp)Cu species was later identified as the copper C-enolate
9f, the insertion product of 3f into the Cu−Si bond in 6 (vide
infra).
Scheme 4. Insertion of 3a,b,d,f into the Cu−Si Bond in 6
An in situ NMR study of the stoichiometric reaction of
3a,b,d,f with 6 revealed that single products are formed, which
eventually were assigned to the corresponding copper enolates
(Figure 4). In agreement with the results of the catalytic
reactions the rate of the reactions depends greatly on the
carbonyl compound. The conversions of aldehyde 3d and ester
3f are fast (complete conversion within 30 min), while the
ketones 3a,b react slower (complete conversion within 1.5 h in
the case of 3a, while for the β-bis-substituted ketone 3b
completion is not reached within 16 h).
In contrast, the catalytic silylation reaction does perform very
i
well in the presence of a stoichiometric amount of PrOH, as
complete conversion to 5f is observed within 30 min at room
1
temperature (Figure 3, top). Additionally, the in situ H NMR
spectrum reveals the formation of pinB−OtBu, pinB−OiPr, and
the (IDipp)Cu complexes 6 and 9f. This observation may be
rationalized assuming the alcoholysis of the insertion product 9f
resulting in the direct formation of 5f, without the intermediate
formation of a boron enolate, along with the formation of the
alkoxido complex (IDipp)Cu−OiPr. The latter then reacts with
1, yielding pinB−OiPr and regenerating 6 to complete the
catalytic cycle. The observation of both copper complexes 6
The reaction products were isolated (62−87% yield) and
identified as the copper enolates 9a,b,d,f (and also 9c) by 1D
and 2D NMR techniques as well as single-crystal X-ray
1
determinations for 9a−d. The complexes 9a,b,d exhibit H
NMR signals just below 4 ppm (Figure 4). These signals
D
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1
●
Figure 4. Details of in situ H NMR spectra of the reaction of 6 ( ) with hex-4-en-3-one (3a), mesityloxide (3b), crotonaldehyde (3d), and
15
†
□
methylcrotonate (3f) ( ) at room temperature (300 MHz, rt, C6D6). Signals of the E isomer are denoted by a prime (*: (IDipp)CuPh ); denotes
that only a part of the signal is visible due to overlapping.
Table 2. Selected Crystallographic and Geometrical Parameters for 9a, 9b, 9c(C5H12)1/2, and 9d(10)(PhMe)16
a
a
a
9a
9b
9c(C5H12)1/2
9d(10)(PhMe)
formula
C41H57CuN2OSi
monoclinic
C2/c, 8
C41H57CuN2OSi
monoclinic
P21/n, 4
C43.5H61CuN2OSi
monoclinic
C2/c, 8
C79H109BCu2N4O4Si
triclinic
cryst syst
space group, Z
wR2 (all data), Rint
temp (K)
P1, 2
̅
0.1462, 0.0513
0.1145, 0.0489
100(2)
0.1468, 0.0436
150(2)
0.1564, 0.0779
100(2)
101(3)
CCDC no.
1019948
1019947
1019946
1019950
1.849(2)
1.815(2)
174.78(9)
1.293(4)
1.331(6)
1.529(6)
CNHC−Cu1 (Å)
Cu1−O1 (Å)
C1−Cu1−O1 (deg)
O1−C1 (Å)
C1−C2 (Å)
1.861(2)
1.855(2)
1.865(3)
1.819(1)
1.799(2)
1.823(2)
174.35(7)
1.342(2)
172.48(8)
1.298(3)
179.4(1)
1.345(3)
1.350(3)
1.347(4)
1.323(4)
C2−C3 (Å)
1.503(3)
1.508(3)
1.512(4)
C1−C7 (Å)
1.513(3)
1.507(4)
1.509(6)
C1−C2−C3 (deg)
τ (deg)
125.4(2)
128.9(2)
124.8(3)
124.6(4)
b
−5.5(3)
−8.9(5)
179.0(3)
−173.0(4)
a
b
Values for the major of disordered parts only. Torsion angle along O1,C1,C2,C3.
correspond to carbon atoms with chemical shifts of 96−103
enolates differ only slightly in energy, while for esters the
copper C-enolate is significantly thermodynamically favored.
Moreover, in this study it is also shown that for copper O-
enolates the metathesis reaction with a diboron compound is
associated only with a small energy barrier, while for copper C-
enolates the energy barrier is substantial and, hence, the
reaction pathway is ineffective. Efficient catalytic conversion is
predicted to be possible only after alcoholysis and formation of
a Cu alkoxide intermediate.7
Only single isomers of the complexes 9a,b are observed
during the in situ NMR study and in the isolated compounds;
unfortunately, the nature of the isomers could not be
established unambiguously by means of NMR spectroscopy.
For 9d, however, a mixture of the E and Z isomers in a 1:2 ratio
is observed on the basis of 1H NMR data (Figure 4). This is in
agreement with a 2:5 E/Z ratio found for the boron enolate 4d.
The E and Z isomers are assigned on the basis of the coupling
1
ppm and J(C,H) coupling constants of 150−155 Hz
1
(determined from H−13C HMBC spectra), indicating an sp2-
character of the carbon atom. Therefore, these signals were
assigned to the 2-CH groups of the O-enolate isomer of 9a,b,d
1
(Figure 4). In contrast, for 9f no H/13C NMR signals in the
respective ranges are observed and the signal for the 2-CH
group is detected at 2.35 ppm (13C: 37.6 ppm). Thus, a C-
enolate structure was assigned to 9f (Figure 4). In accordance
with these assignments a 13C NMR signal at 179.4 ppm
indicative for a carboxyl group is observed for 9f, while for
9a,b,d no signals indicative for carbonyl groups are observed.
Here, the signals of the O-bound carbon atoms are detected at
151−162 ppm.
A computational study on the closely related CuI-catalyzed
diborylation reaction generally agrees with this observation: for
aldehydes (and presumably ketones) the copper O- and C-
E
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Figure 5. Molecular structures of 9a−d. For clarity only selected hydrogen atoms and only the ipso-carbon atoms of the arene groups are shown;
disorder present in the structures of 9a,b,d is omitted.16 Thermal ellipsoids are drawn at the 50% probability level; an adopted numbering scheme
was used.
constants between the olefinic protons (5.6 Hz (Z) and 11.1
Hz (E), respectively); this is in accordance with findings for the
boron O-enolate 4d.12 Intriguingly the E and Z isomers of 9d
are in a slow equilibrium, as concluded from the observation of
sharp 1H NMR signals as well as signals indicative for chemical
(vide supra)the Z/E isomers equilibrate in solution, no
conclusions on the isomer present in solution can be drawn
from the solid-state structures.
Having established the copper enolates as central inter-
mediates (Scheme 2), a last step is required in order to close
the catalytic cycle and to retrace it by stoichiometric reactions.
In this last step a copper enolate reacts with 1 to form the
respective boron enolate, the primary product of the catalytic
process (vide supra), and to regenerate 6 (Scheme 5).
15,16
1
exchange in a H−1H NOESY spectrum of pure 9d.
Single crystals suitable for X-ray structure determination of
the enolates 9a−d were obtained by crystallization from
solutions in PhMe/n-pentane at low temperatures (Table 2),
although for 9d the crystals obtained were repeatedly of low
quality.16 A single crystal of higher quality containing 9d was
obtained by slow evaporation of a solution in PhMe/n-pentane,
but it contains, apart from one molecule of 9d, one molecule of
PhMe and one molecule of (IDipp)Cu−OBpin (10) in the
asymmetric unit (Table 2).15 While the geometrical data of
both molecular structures of 9d are comparable, the latter
structure determination is of significantly better quality and is
used exclusively for further discussion.16 The molecular
structures of 9a,d show both enantiomers of the enolate
moieties mutually disordered; for 9b a structurally related
disorder of two conformers is observed (Figure 5).17
For 9a−d the distances O1−C1 are in between a single and a
double bond, the distances C1−C2 indicate clearly a CC
double bond, and the C1−C2−C3 angles are above 120° and
suggestive for an sp2-hybridized C2 carbon atom.18 The
distances C1−C7 and C2−C3 are in the range expected for a
C(sp2)−C(sp3) single bond.18 Moreover, the geometry of the
CNHC−Cu1−O1 moieties is similar to those found in alkoxido
complexes of the type (NHC)Cu−OR (Cu1−O1: 1.80−1.84
Å, CNHC−C1: 1.86−1.87 Å, CNHC−Cu1−O1: 173−
179°).9d,e,19a All four crystal structures consist of a single Z/E
isomer. For 9a,b the solid-state structures represent the Z
isomer, while for 9c,d the E isomer is found as indicated by the
O1−C1−C2−C3 torsion angles. However, asat least for 9d
Scheme 5. Reaction of the Copper Enolates 9a,b,d with 1 to
the Boron O-Enolates 4a,b,d and 6
The reaction of 9a,b,d with an excess of 1 in C6D6 was
studied by in situ 1H NMR spectroscopy (Scheme 5, Figure 6).
For all three copper enolates virtually clean and quantitative
conversions to the expected products 4a,b,d and 6 were
observed within 2 h at rt. Again, for 4a and 4d a mixture of the
E and Z isomers is observed with ratios of 1:8 (4a) and 1:3
(4d), in reasonable agreement with the values obtained from
the catalytic reactions (vide supra). It is worth mentioning that
those values are not necessarily directly correlated to the E/Z
ratio observed for the enolates 9a,d asat least for 9dthe E/
Z isomers of the copper enolates are interconvertible.
On performing the reaction of the copper C-enolate 9f with
an equimolar amount of 1, virtually no formation of 5f is
observed within 2 h. Nevertheless, clean and complete
conversion to 5f, 6, and pinB−OiPr is observed upon addition
i
of PrOH within a few minutes (Figure 7). This demonstrates
F
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1
■
●
Figure 6. Details of in situ H NMR spectra of the reaction of 9a,b,d with excess 1 ( ) giving 6 ( ) and 4a,b,d at room temperature after 2 h (300
MHz, rt, C6D6). Signals of the E isomers of 4a,d are denoted by a prime (×: 5a); † denotes that only a part of the signal is visible due to overlapping.
that the copper C-enolate 9f does not efficiently react with 1
(assuming that the traces of 5f observed are due to adventitious
moisture). Hence, under the catalytic conditions employed here
and in agreement with a computational study on the related
diboration reaction, efficient conversion is not possible in the
absence of an alcohol, as the catalytic cycle rests at the stage of
the copper C-enolate.7 In accordance with this and excluding
possible side reactions, further NMR experiments show that the
Cu−Si complex 6 does not efficiently react with iPrOH and are
in agreement with the formation of 5f and (IDipp)CuOiPr
16
i
upon reaction of 9f with PrOH.
CONCLUSION
■
Six exemplary α,β-unsaturated carbonyl/carboxyl compounds
(3a−f) are efficiently β-silylated by the silyl boronic ester
pinB−SiMe2Ph (1) in the presence of catalytic amounts of
(IDipp)Cu−SiMe2Ph (6). Depending on the steric and
electronic properties of the substrates, the reaction gives
Figure 7. Details of the in situ 1H NMR spectrum of the reaction of 9f
i
■
with 1 ( ) in the absence and presence of PrOH (300 MHz, rt, C6D6,
i
i
□
●
× : pinB−OPr; : PrOH; : 6).
a
Scheme 6. Generalized Catalytic Cycles for the CuI-Catalyzed Conjugate Silyl Addition to Carbonyl/Carboxyl Compounds
a
Isolated species are denoted by their respective identifiers (species in parentheses suggested by NMR data only).
G
dx.doi.org/10.1021/om500989w | Organometallics XXXX, XXX, XXX−XXX

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different products and/or pursues different reaction pathways.
Using the (IDipp)Cu-based catalyst as a model system, the
different pathways were studied in detail, isolating central
intermediates, retracing the different catalytic cycles stepwise by
stoichiometric experiments, and monitoring the individual steps
by in situ NMR spectroscopy. In conclusion three different, yet
closely related, catalytic cycles for the Cu-catalyzed silylation of
carbonyl/carboxyl compounds can be deduced (Scheme 6).
In agreement with previously proposed catalytic cycles and
our findings on the silylation of alkyl/aryl aldehydes and CO2,
the Cu-silyl complex (IDipp)Cu−SiMe2Ph (6) is initially
formed as a catalytically active species.5 This complex reacts
efficiently with the α,β-unsaturated substrates by insertion into
the Cu−Si bond (9a−d,f). The following insertion products are
observed (Scheme 6): (a) for α,β-unsaturated ketones the 1,4-
addition product, a copper O-enolate, is formed; (b) for α,β-
unsaturated aldehydes, depending on the β-substitution pattern,
either the 1,4-addition product as for ketones (one β-
substituent) or the 1,2-addition product (two β-substituents)
is predominantly formed; (c) for α,β-unsaturated esters the 3,4-
addition product, a copper C-enolate, is formed. While the
isolated 1,4- and 3,4-insertion products alone allow no
conclusions on the mechanism of the insertion reaction, the
findings agree with a computational study on the closely related
borylation of α,β-unsaturated substrates. Here generally a 3,4-
addition of a Cu−boryl species onto the α,β-unsaturated
compound is suggested. This primary 3,4-insertion product
then rearranges for aldehydes (and presumably ketones) in a
subsequent reaction step via a keto-to-enol isomerization to the
thermodynamically favored 1,4-addition product (copper O-
enolate).7
For the conditions and the model catalyst employed
conversion of the insertion product to the primary catalytic
reaction products 4a−d accompanied by regeneration of the
Cu−Si species 6 and hence completion of the catalytic cycle
upon reaction with the silyl boronic ester 1 is effective only for
the copper O-enolates 9a−d. It is ineffective for the copper C-
enolate 9f, and, hence, the ester 3f is not reactive under the
catalytic conditions described. However, in the presence of a
stoichiometric amount of iPrOH alcoholysis of the C-enolate 9f
to the β-silylated ester 5f and a Cu-alkoxido complex is facile,
and the latter regenerates efficiently 6 upon reaction with 1.
Hence in the latter reaction the β-silylated ester (5f) is obtained
as the primary reaction product, while for aldehydes and
ketones the β-silylated carbonyls (5a−d) are obtained only
after hydrolytic workup.
EXPERIMENTAL SECTION
■
General Considerations. pinB−SiMe2Ph (1), (IDipp)CuOtBu
(2), and (IDipp)CuSiMe2Ph (6) were prepared according to literature
procedures.5b,19 All other compounds were commercially available and
were used as received; their purity and identity was checked by
appropriate methods. All α,β-unsaturated compounds 3a−f were
commercially available andif applicablepredominantly (>95%)
the E isomer. All solvents were dried using a solvent purification
systems, deoxygenated using the freeze−pump−thaw method, and
stored under purified nitrogen. All manipulations were performed
using standard Schlenk techniques under an atmosphere of purified
nitrogen or in a nitrogen-filled glovebox. Air-sensitive samples were
measured in NMR tubes equipped with screw caps. Chemical shifts
(δ) are given in ppm, using the residual resonance signal of the
1
solvents (C6D6: 99.5% deuteration, H NMR: 7.16 ppm, 13C NMR:
128.06 ppm).20 11B chemical shifts are reported relative to external
BF3·Et2O. 11B NMR spectra were processed applying a back linear
prediction in order to suppress the broad background signal due to the
borosilicate glass of the NMR tube and a Lorentz-type window
function (LB = 10 Hz); the spectra were carefully evaluated to ensure
that no genuinely broad signals of the sample were suppressed. If
necessary 2D NMR techniques were employed to establish
connectivity and to assign the individual signals (1H−1H NOESY (1
s mixing time), 1H−1H COSY, 1H−13C HSQC, and 1H−13C HMBC).
Complicated coupling patterns were analyzed with the aid of
simulations. The same numbering scheme as in Figures 1−4 and 6/
7 is used in the experimental part. Melting points were determined in
flame-sealed capillaries under nitrogen and are not corrected.
Elemental analyses were performed at the Institut fur Anorganische
and Analytische Chemie of the Technische Universitat Carolo-
̈
̈
Wilhelmina zu Braunschweig. GC/MS measurements were performed
in positive EI mode (70 eV, 60−700 m/z) with the following
conditions: injection temperature 250 °C; interface temperature 280
°C; temperature program: start temperature 50 °C for 3 min, heating
rate 12 °C min−1, end temperature 300 °C for 8 min; column type: 5%
phenyl-arylene/95% dimethylpolysiloxane, 30 m × 0.25 mm, 0.25 μm
film thickness; He carrier gas (1.5 mL min−1). For HRMS
measurements a time-of-flight mass spectrometer operating in EI
mode (70 eV) coupled to a gas chromatograph was used. Infrared (IR)
spectra were recorded using an ATR unit. Analytical thin-layer
chromatography (TLC) was performed on precoated silica gel
polyester sheets; flash column chromatography (⦶ 3 cm × 20 cm)
on silica gel 60.
Preparation of 4a,b,d and 7 (General Procedure). In a
nitrogen-filled glovebox 2 (18 mg, 34 μmol, 3 mol %) and 1 (300 mg,
1.14 mmol, 1.0 equiv) were mixed in dry toluene (5 mL). The α,β-
unsaturated substrate (1.14 mmol, 1.0 equiv) was added, and the
mixture stirred at room temperature. The progress of the reaction was
monitored by GC/MS analysis of an aliquot of the reaction mixture
(0.05 mL) after dilution with Et2O (2 mL) and filtration over Celite.
After complete consumption of 1 the solvent and excess α,β-
unsaturated carbonyl substrate were evaporated at room temperature
under vacuum (10−3 mbar), and the residue was extracted with n-
pentane (2 × 2 mL) and filtered over Celite, in order to remove
insoluble NHC-Cu complexes, to give a clear solution. Evaporation of
the solvent and possibly residual α,β-unsaturated carbonyl substrate at
room temperature under vacuum (10−3 mbar) yielded the reaction
products.
An exception to this general scheme is the bis-β-substituted
aldehyde 3e. Here, presumably due to steric reasons, the 1,2-
addition product is predominantly formed, and only a minor
amount of the 1,4-addition product is observed. Still, catalytic
conversion is effective but leading to α-silyl alcohols, as
reported for alkyl and aryl aldehydes.5a
While experimental investigations are inevitably exemplary
and strictly valid only for the particular system studied, the
findings discussed may be carefully generalized, as they agree
well with a computational study on the related conjugate
addition reactions of tetraalkoxidodiboron reagents.7 So our
findings may contribute to the understanding and further
development of CuI-catalyzed conjugate silyl additions and
other related transformations.
pinB-O-C6H10-SiMe2Ph (4a): hex-4-en-3-one (3a) (112 mg, 1.14
mmol); 3 h; pale yellow, sticky oil (357 mg, 0.99 mmol, 87%). Despite
all efforts the contamination of 4a with a small amount (<5 mol %) of
the hydrolysis product 5a was unavoidable according to 1H NMR
spectroscopy, likely due to adventitious moisture. 1H NMR (600 MHz,
C6D6, rt): δ (Z isomer) 7.55−7.53 (m, 2 H, o-CHPh), 7.22−7.19 (m, 3
3
4
H, m,p-CHPh), 4.49 (dt, 1 H, JHH = 10.9 Hz, JHH = 1.0 Hz, 2-CH),
2.38 (dq, 3JHH = 10.9 Hz, 3JHH = 7.3 Hz, 1 H, 3-CH), 2.28 (qt, 3JHH
=
4
3
7.5 Hz, JHH = 1.0 Hz, 2 H, 1-CH2), 1.10 (d, JHH = 7.3 Hz, 3 H, 4-
CH3), 1.06 (bs, 12 H, 6-CH3), 1.03 (t, 3JHH = 7.5 Hz, 3 H, 1a-CH3), 2
× 0.33 (s, 3 H, 5-CH3); δ (E isomer) 7.58−7.56 (m, 2 H, o-CHPh),
H
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Organometallics
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7.25−7.21 (m, 3 H, m,p-CHPh), 4.98 (d, 1 H, 3JHH = 11.6 Hz, 2′-CH),
2.19−2.13 (m, 1 H, 1′-CH2), 2.10−2.05 (m, 1 H, 1′-CH2), 1.71 (dq,
3JHH = 11.6 Hz, 3JHH = 7.3 Hz, 1 H, 3′-CH), 1.01 (d, 3JHH = 7.3 Hz, 3
H, 4′-CH3), 1.06 (s, 12 H, 6′-CH3), 1.03−1.00 (m, 3 H, 1a′-CH3),
0.17 (s, 3 H, 5-CH3), 0.16 (s, 3 H, 5-CH3). The signals of 1, 1a, 1′, 1a′,
and m,p-CHPh overlap with signals of 4a or 5a. 11B{1H} NMR (96
MHz, C6D6, rt): δ 22.0 (pinB-O). 13C{1H} NMR (150 MHz, C6D6,rt):
δ (Z isomer) 149.5 (C-O), 138.3 (ipso-CPh), 134.5 (o-CHPh), 129.1
(m/p-CHPh), 128.8 (m/p-CHPh), 112.2 (2-CH), 82.9 (C(CH3)2), 28.4
(1-CH2), 19.2 (3-CH), 24.7 (C(CH3)2), 24.6 (C(CH3)2), 15.5 (4-
CH3), 11.1 (1a-CH3), −3.8 (5-CH3), −5.2 (5-CH3); δ (E isomer)
149.3 (C-O), 138.1 (ipso-CPh), 134.7 (o-CHPh), 113.6 (2′-CH), 23.8
(1′-CH2), 20.7 (3′-CH), 16.2 (4′-CH3), −4.6 (5′-CH3), −5.1 (5′-
CH3). The m/p-CHPh, 1a′-CH3, C(CH3)2, and C(CH3)2 signals could
not be assigned unambiguously and are not given. MS (GC/MS, EI):
360 [M]+, 331 [M − C2H5]+, 135 [SiMe2Ph]+. Anal. Calcd for
C20H33BO3Si: C, 66.66; H, 9.23. Found: C, 66.54; H, 8.98.
pinB-O-C6H10-SiMe2Ph (4b): mesityl oxide (3b) (112 mg, 1.14
mmol); 6 h; pale yellow oil (275 mg, 0.76 mmol, 67%). 1H NMR (600
MHz, C6D6, rt): δ 7.57−7.54 (m, 2 H, CHPh), 7.25−7.20 (m, 3 H,
CHPh), 4.37 (q, JHH = 1.0 Hz, 1 H, 2-CH), 1.94 (d, JHH = 1.0 Hz, 3 H,
1-CH3), 1.29 (s, 6 H, 4-CH3), 1.05 (s, 12 H, 6-CH3), 0.42 (s, 6 H, 5−
CH3). 11B{1H} NMR (96 MHz, C6D6, rt): δ 21.8 (pinB-O). 13C{1H}
NMR (150 MHz, C6D6, rt): δ 144.6 (C-O), 138.1 (ipso-CPh), 135.4
(o/m-CHPh), 129.1 (p-CHPh), 128.1 (o/m-CHPh), 117.3 (2-CH), 82.9
(C(CH3)2), 25.0 (4-CH3), 24.6 (C(CH3)2), 24.4 (3-C), 22.6 (1-CH3),
−5.2 (5-CH3). MS (GC/MS, EI): 360 [M]+, 345 [M − CH3]+, 135
[SiMe2Ph]+. Anal. Calcd for C20H33BO3Si: C, 66.66; H, 9.23. Found:
C, 66.53; H, 9.07.
Preparation of 5a−d,f and 8 (General Procedure). In a
nitrogen-filled glovebox 2 (12 mg, 23 μmol, 6 mol %) and 1 (100 mg,
0.38 mmol, 1.0 equiv) were mixed in dry toluene (2 mL). The α,β-
unsaturated substrate (0.38 mmol, 1.0 equiv) was added, and the
mixture stirred at room temperature. The progress of the reaction was
monitored by GC/MS analysis of an aliquot of the reaction mixture
(0.05 mL) after dilution with Et2O (2 mL) and filtration over Celite.
After complete conversion the solvent was evaporated and the residue
purified on air by flash column chromatography using the indicated
eluents. Compounds 5a−d,f were reported earlier, and the
spectroscopic data match the literature.
(C2H5)C(O)CH2CH(SiMe2Ph)CH3 (5a): hex-4-en-3-one (3a) (37
mg, 0.38 mmol); 3 h; n-pentane/EtOAc (8:1); colorless oil (69 mg,
0.29 mmol, 77%). 1H NMR (300 MHz, CDCl3, rt): δ 7.52−7.46 (m, 2
H, CHPh), 7.39−7.32 (m, 3 H, CHPh), 2.45−2.23 (m, 3 H, OC-
3
3
CH2), 2.17 (dd, JHH = 10.8 Hz, JHH = 16.2 Hz, 1 H, OC-CH2),
3
1.60 (bs, 1 H, OH), 1.58−1.46 (m, 1 H, Si-CH), 1.00 (t, JHH = 7.3
Hz, 3 H, CH2-CH3), 0.91 (d, 3JHH = 7.3 Hz, 3 H, CH-CH3), 0.27 (s, 6
H, Si(CH3)2).4d 1H NMR (300 MHz, C6D6, rt): δ 7.45−7.39 (m, 2 H,
CHPh), 7.24−7.19 (m, 3 H, CHPh), 2.1 (dd, 3JHH = 4.0 Hz, 3JHH = 16.4
Hz, 1 H, OC-CH2), 1.98−1.72 (m, 3 H, OC-CH2), 1.68−1.54
(m, 1 H, Si-CH), 0.91 (d, 3JHH = 7.3 Hz, 3 H, CH-CH3), 0.89 (t, 3JHH
= 7.3 Hz, 3 H, CH2-CH3), 2 × 0.17 (s, 6 H, Si(CH3)2). MS (GC/MS,
EI): 234 [M]+, 219 [M − CH3]+, 205 [M − C2H5]+, 135 [SiMe2Ph]+.
H3CC(O)CH2CH(SiMe2Ph)(CH3)2 (5b): mesityl oxide (3b) (37.4
mg, 0.38 mmol); 8 h; n-pentane/Et2O (5:1); colorless oil (77 mg, 0.33
1
μmol, 87%). H NMR (300 MHz, CDCl3, rt): δ 7.52−7.47 (m, 2 H,
CHPh), 7.38−7.32 (m, 3 H, CHPh), 2.29 (s, 2 H, CH2), 2.04 (s, 3 H,
CH3), 1.04 (s, 6 H, CH3), 0.32 (s, 6 H, Si(CH3)2).21a MS (GC/MS,
EI): 234 [M]+, 219 [M − CH3]+, 135 [SiMe2Ph]+.
pinB-O-C4H6-SiMe2Ph (4d): crotonaldehyde (3d) (80 mg, 1.14
mmol); 2 h; colorless, highly viscous oil (350 mg, 1.06 mmol, 93%).
1H NMR (600 MHz, C6D6, rt): δ (Z isomer) 7.51−7.49 (m, 2 H, o-
CHPh), 7.24−7.18 (m, 3 H, m,p-CHPh (overlapping with signal of E
OC6H9SiMe2Ph (5c): cyclohexenone (3c) (37 mg, 0.38 mmol); 6
1
h; n-pentane/Et2O (8:1); colorless oil (73 mg, 0.31 μmol, 82%). H
NMR (300 MHz, CDCl3, rt): δ 7.50−7.47 (m, 2 H, CHPh), 7.40−7.34
(m, 3 H, CHPh), 2.42−2.04 (m, 5 H, CH/CH2), 1.87−1.60 (m, 2 H,
CH/CH2), 1.50−1.18 (m, 2 H, CH/CH2), 2 × 0.31 (s, 6 H,
Si(CH3)2).21b MS (GC/MS, EI): 232 [M]+, 217 [M − CH3]+, 156,
135 [SiMe2Ph]+.21b
3
4
isomer)), 6.74 (dd, JHH = 6.0 Hz, JHH = 1.0 Hz, 1 H, 1-CH), 4.44
(dd, 3JHH = 10.8 Hz, 3JHH = 6.0 Hz, 1 H, 2-CH), 2.66 (dqd, 3JHH = 10.8
Hz, 3JHH = 7.3 Hz, 4JHH = 1.0 Hz, 1H, 3-CH), 1.02 (d, JHH = 7.4 Hz, 3
H, 4-CH3), 1.00 (s, 12 H, 6-CH3), 0.31 (s, 3 H, 5-CH3), 0.30 (s, 3 H,
HC(O)CH2CH(SiMe2Ph)CH3 (5d): crotonaldehyde (3d) (27 mg,
5-CH3); δ (E isomer) 7.44−7.42 (m, 2 H, o-CHPh), 7.24−7.18 (m, 3
0.38 mmol); 2 h; n-hexane/EtOAc (5:1); yellow oil (53 mg, 0.26
3
H, m,p-CHPh (overlapping with signal of Z isomer)), 6.72 (dd, JHH
=
=
=
1
mmol, 68%). H NMR (300 MHz, CDCl3, rt): δ 9.67 (dd, 4JHH = 3.3
11.9 Hz, 4JHH = 1.3 Hz, 1 H, 1′-CH), 5.51 (dd, 3JHH = 11.9 Hz, 3JHH
4
Hz, JHH = 1.2 Hz, 1 H, OCH), 7.52−7.46 (m, 2 H, CHPh), 7.40−
3
4
8.9 Hz, 1 H, 2′-CH), 1.57 (dqd, JHH = 8.9 Hz, 3JHH = 7.2 Hz, JHH
7.33 (m, 3 H, CHPh), 2.43 (ddd, 2JHH = 16.2 Hz, 3JHH = 3.6 Hz, 4JHH
=
2
3
1.3 Hz, 1 H, 3′-CH), 1.00 (s, 12 H, 6-CH3), 0.97 (d, JHH = 7.2 Hz, 3
H, 4′-CH3), 0.19 (s, 3 H, 5′-CH3), 0.18 (s, 3 H, 5′-CH3). 11B{1H}
NMR (96 MHz, C6D6, rt): δ 22.0 (pinB-O). 13C{1H} NMR (150
MHz, C6D6,rt): δ (Z isomer) 138.1 (ipso-C), 135.8(1-CH), 134.5 (o-
CHPh), 129.1 (m/p-CHPh), 128.0 (m/p-CHPh), 115.6 (2-CH), 83.1
(C(CH3)2), 24.7 (C(CH3)2), 18.4 (3-CH), 15.3 (4-CH), −4.2 (5-
CH3), −5.4 (5-CH3); δ (E isomer) 137.8 (ipso-C), 137.6 (1′-CH),
134.4 (o-CHPh), 129.2 (m/p-CHPh), 128.0 (m/p-CHPh), 116.0 (2′-
CH), 83.1 (C(CH3)2), 24.6 (C(CH3)2), 20.8 (3′-CH), 14.6 (4′-CH),
−4.6 (5-CH3), −5.5 (5-CH3). MS (GC/MS, EI): 332 [M]+, 317 [M −
CH3]+, 135 [SiMe2Ph]+. Anal. Calcd for C18H29BO3Si: C, 65.06; H,
8.80. Found: C, 65.16; H, 8.51.
1.2 Hz, 1 H, OC-CH2), 2.15 (ddd, JHH = 16.2 Hz, JHH = 10.9 Hz,
3
3
3JHH = 3.3 Hz, 1 H, OC-CH2), 1.51 (dqd, JHH = 10.9 Hz JHH = 7.3
Hz, 3JHH = 3.6 Hz, 1 H, Si-CH), 0.98 (d, 3JHH = 7.3 Hz, 3 H, CH3), 2
× 0.30 (d, 6 H, Si(CH3)2).4g,21a MS (GC/MS, EI): 191 [M − CH3]+,
135 [SiMe2Ph]+.
HO−HC(SiMe2Ph)CHC(CH3)2 (8): 3-methylcrotonaldehyde (3e) (32
mg, 0.38 mmol); 16 h; n-hexane/Et2O (5:1); colorless oil (70 mg, 0.32
1
mmol, 84%). H NMR (300 MHz, C6D6, rt): δ 7.63−7.56 (m, 2 H,
3
4
CHPh), 7.25−7.18 (m, 3 H, CHPh), 5.29 (dsept., JHH = 10.1 Hz, JHH
3
= 1.4 Hz, 1 H, OC(H)-C(H)), 4.25 (d, JHH = 10.1 Hz, 1 H,
OC(H)-C(H)), 1.57−1.55 (bs, 3 H, CH3), 1.38 (bs, 1H, OH), 1.29
4
(bd, JHH = 1.4 Hz, 3 H, CH3), 0.35 (s, 3 H, Si(CH3)2), 0.32 (s, 3 H,
Si(CH3)2). 13C{1H} NMR (75 MHz, C6D6, rt): δ 137.3 (ipso-CPh),
134.7 (CHPh), 131.2 (C(CH3)2), 129.4 (CHPh), 128.0 (CHPh),
126.8 (OC(H)-C(H)), 64.1 (OC(H)-C(H)), 25.9 (C(CH3)2),
pinB-O-C5H8-SiMe2Ph (7): 3-methylcrotonaldehyde (3e) (96 mg,
1.14 mmol); 16 h; from the n-pentane extract after filtration over
Celite colorless crystals separated that were dried in vacuo (365 mg,
1
1.05 mmol, 92%). H NMR (300 MHz, C6D6, rt): δ 7.65−7.61 (m, 2
18.3 (C(CH3)2), −5.3 (Si(CH3)2), −5.5 (Si(CH3)2). IR (liquid): ν
̃
H, o−CHPh), 7.22−7.17 (m, 3 H, m,p-CHPh), 5.52 (d of v sept., 3JHH
=
3382, 3069, 2963, 2911, 2856, 1662, 1427, 1375, 1247, 1113, 971, 831,
812, 776, 734, 698, 642. MS (GC/MS, EI): 220 [M]+, 205 [M −
CH3]+, 135 [SiMe2Ph]+. HRMS (GC/MS, EI): calcd for C13H20OSi
220.1283, found 220.1267.
10.1 Hz, J = 1.3 Hz, 1 H, 2-CH), 5.13 (d, 3JHH = 10.1 Hz, 1 H 1-CH),
1.55−1.54 (m, 3 H, 3/4-CH3), 1.44 (d, 3JHH = 1.3 Hz, 3 H, 3/4-CH3),
1.02 (s, 6 H, 6-CH3), 1.01 (s, 6 H, 6-CH3), 0.40 (s, 3 H, 5-CH3), 0.35
(s, 3 H, 5-CH3). 11B{1H} NMR (96 MHz, C6D6, rt): δ 22.7 (pinB-O).
13C{1H} NMR (75 MHz, C6D6, rt): δ 137.0 (ipso-C), 134.8 (CHPh),
131.6 (C(CH3)2), 129.4 (CHPh), 127.9 (CHPh), 125.2 (2-CH), 82.3
(C(CH3)2), 67.7 (1-CH), 25.9 (3/4-CH3), 24.8 (C(CH3)2), 24.7
(C(CH3)2), 18.5 (3/4-CH3), −5.3 (5-CH3), −5.6 (5-CH3). Mp: 57−
60 °C. MS (GC/MS, EI): 346 [M]+, 331 [M − CH3]+, 135
[SiMe2Ph]+. Anal. Calcd for C19H31BO3Si: C, 65.89; H, 9.02. Found:
C, 65.57; H, 9.04.
OC(OMe)CH2CH(SiMe2Ph)CH3 (5f). The reaction was conducted
i
as described for 5a−d and 8, but additionally PrOH (29 μL, 23 mg,
0.38 mmol) was added after combining 3f, 1, and 2: methylcrotonate
(3f) (38 mg, 0.38 mmol); 4 h; n-hexane/Et2O (8:1); colorless oil (71
1
mg, 0.30 mmol, 79%). H NMR (300 MHz, CDCl3, rt): δ 7.52−7.46
(m, 2 H, CHPh), 7.39−7.33 (m, 3 H, CHPh), 3.61 (s, 3 H, OCH3), 2.39
3
3
(dd, JHH = 15.2 Hz, 3JHH = 4.2 Hz, 1 H, CH2), 2.06 (dd, JHH = 15.2
Hz, 3JHH = 11.2 Hz, 1 H, CH2), 1.44 (dqd, JHH = 11.2 Hz 3JHH = 7.6
3
I
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Organometallics
Article
Hz, 3JHH = 4.2 Hz, 1 H, CH), 0.97 (d, 3JHH = 7.6 Hz, 3 H, CH3), 0.28
(s, 6 H, Si(CH3)2).21c 1H NMR (300 MHz, C6D6, rt): δ 7.41−7.35 (m,
2 H, CHPh), 7.20−7.15 (m, 3 H, CHPh), 3.32 (s, 3 H, OCH3), 2.33
13C{1H} NMR (150 MHz, C6D6, rt): δ 182.8 (CCarbene), 161.0 (O-C
CH), 2 × 145.7 (ipso-CDipp), 140.7 (ipso-CPh), 135.1 (ipso-NCDipp),
134.6 (o-CHPh), 130.7 (p-CHDipp), 128.5 (m/p-CHPh), 127.8 (m/p-
CHPh), 124.3 (m-CHDipp), 122.6 (NCH), 90.4 (O−CCH), 35.1
(CH-Si), 29.0 (CH(CH3)2), 25.7 (CH2), 25.5 (CH2), 25.1 (CH-
(CH3)2), 24.5 (CH2), 2 × 23.8 (CH(CH3)2), −3.7 (Si(CH3)2), −4.3
(Si(CH3)2). Mp: 86−92 °C (dec). Anal. Calcd for C41H55CuN2OSi:
C, 72.04; H, 8.11; N, 4.10. Found: C, 72.17; H, 8.36; N, 4.02.
(IDipp)Cu-O-C4H6-SiMe2Ph (9d): crotonaldehyde (3d) (7 mg, 100
μmol, 1.2 equiv); 2 h; colorless microcrystals (41 mg, 62 μmol, 73%).
1H NMR (600 MHz, C6D6, rt): (Z isomer) δ 7.72−7.69 (m, 2 H, o-
3
3
(dd, JHH = 15.3 Hz, 3JHH = 4.2 Hz, 1 H, CH2), 2.02 (dd, JHH = 15.3
Hz, 3JHH = 10.8 Hz, 1 H, CH2), 1.53 (dqd, 3JHH = 10.8 Hz, 3JHH = 7.3
Hz, 3JHH = 4.2 Hz, 1 H, CH), 0.97 (d, 3JHH = 7.3 Hz, 3 H, CH3), 0.13
(s, 3 H, Si(CH3)2), 0.12 (s, 3 H, Si(CH3)2). MS (GC/MS, EI): 236
[M]+, 221 [M − CH3]+, 205 [M − OCH3]+, 159 [M − Ph]+, 135
[SiMe2Ph]+.21c
Preparation of 9a-d,f (General Procedure). In a nitrogen-filled
glovebox 6 (50 mg, 85 μmol, 1 equiv) was dissolved in toluene (2
mL), and a small excess of the α,β-unsaturated carbonyl compound
3a−d,f was added. After the given reaction time at room temperature
the mixture was layered with n-pentane (approximately 2 mL) and
stored at −40 °C. After separation of a (crystalline) solid the
supernatant solution was decanted and the residue washed with n-
pentane (approximately 2 × 1 mL) and dried in vacuo.
CHPh), 7.26−7.21 (m, 3 H, m,p-CHPh), 7.19 (dd, 3JHH = 5.6 Hz, 4JHH
=
3
0.9 Hz, 1 H, 1-CH), 7.19 (t, JHH = 7.8 Hz, 2 H, p-CHDipp), 7.04 (d,
3
3JHH = 7.8 Hz, 4 H, m-CHDipp), 6.23 (s, 2 H, NCH), 3.91 (dd, JHH
=
=
10.0 Hz, 3JHH = 5.6 Hz, 1 H, 2-CH), 2.74 (dqd, 3JHH = 10.0 Hz, 3JHH
7.5 Hz, 4JHH = 0.9 Hz, 1 H, 3-CH), 2 × 2.53 (sept, 3JHH = 6.8 Hz, 2 H,
CH(CH3)2), 1.35 (d, 3JHH = 6.8 Hz, 6 H, CH(CH3)2), 1.34 (d, 3JHH
=
6.8 Hz, 6 H, CH(CH3)2), 1.09 (d, 3JHH = 7.5 Hz, 3 H, 4-CH), 1.05 (d,
3JHH = 7.8 Hz, 12 H, CH(CH3)2), 0.40 (s, 6 H, Si(CH3)2); (E isomer)
δ 7.63−7.60 (m, 2 H, o-CHPh), 7.26−7.21 (m, 3 H, m,p-CHPh), 7.17 (1
H, 1′-CH, overlapping with signals of Z isomer), 6.23 (s, 2 H, NCH,
(IDipp)Cu-O-C6H10-SiMe2Ph (9a): hex-4-en-3-one (3a) (9 mg, 92
μmol, 1.1 equiv); 3 h; light brown microcrystals (64 mg, 74 μmol,
87%). 1H NMR (600 MHz, C6D6, rt): δ 7.74−7.70 (m, 2 H, o-CHPh),
7.28−7.22 (m, 3 H, m,p-CHPh), 7.19 (t, JHH = 7.8 Hz, 2 H, p-CHDipp),
7.04 (d, JHH = 7.8 Hz, 4 H, m-CHDipp), 6.25 (s, 2 H, NCH), 3.97 (d,
JHH = 9.5 Hz, 1 H, 2-CH), 2.72 (dq, 3JHH = 9.5 Hz, 3JHH = 7.5 Hz, 2 H,
overlapping with signal of Z isomer), 3.98 (dd, 3JHH = 11.1 Hz, 3JHH
=
=
9.4 Hz, 1 H, 2′-CH), 1.74 (dqd, JHH = 9.4 Hz, JHH = 7.6 Hz,4JHH
3
3
3
3
3-CH), 2.54 (sept., JHH = 6.9 Hz, 4 H, CH(CH3)2), 1.96 (q, JHH
=
0.6 Hz, 1 H, 3′-CH), 1.13 (d, 3JHH = 7.6 Hz, 3 H, 4′-CH), 0.33 (s, 3 H,
Si(CH3)2), 0.31 (s, 3 H, Si(CH3)2), the signals of the Dipp moiety
were not independently assigned due to overlapping with the signals of
the Z isomer. 13C{1H} NMR (150 MHz, C6D6, rt): (Z isomer) δ 182.6
(CCarbene), 151.4(O-CCH), 145.7 (ipso-CDipp), 142.0 (ipso-CPh),
135.0 (ipso-NCDipp), 134.9 (o-CHPh), 130.8 (p-CHDipp), 128.3 (m/p-
CHPh), 127.6 (m/p-CHPh), 124.4 (br, m-CHDipp), 122.6 (NCH), 101.4
(2-CH), 29.0 (CH(CH3)2), 25.0 (CH(CH3)2), 24.9 (CH(CH3)2), 23.8
(CH(CH3)2), 23.7 (CH(CH3)2), 16.8 (4-CH3), 16.5 (3-CH), −3.1
(Si(CH3)2), −4.5 (Si(CH3)2); (E isomer) δ 182.7 (CCarbene), 154.7 (1′-
CH), 141.8 (ipso-CPh), 134.7 (o-CHPh), 128.5 (m/p-CHPh), 127.8 (m/
p-CHPh), 100.1 (2′-CH), 20.1 (3′-CH), 18.0 (4′-CH3), −2.9
(Si(CH3)2), −5.2 (Si(CH3)2), the signals of the Dipp moiety were
not independently assigned due to overlapping with the signals of the
Z isomer. Mp: 94−99 °C. Anal. Calcd for C39H53CuN2OSi: C, 71.24;
H, 8.13; N, 4.26. Found: C, 70.35; H, 8.07; N, 4.21. Repeated
elemental analysis for different, independently prepared samples failed
to give more satisfactory results; all samples gave consistent values but
were low in carbon. It may be speculated that this is caused by SiC
and/or carbide/carbonate formation.22
15.0 Hz, 3JHH = 7.5 Hz, 1 H, 1-CH2), 1.93 (dq, 3JHH = 16.0 Hz, 1 H, 1-
CH2), 2 × 1.36 (d, JHH = 6.9 Hz, 6 H, CH(CH3)2), 1.14 (d, JHH = 7.6
Hz, 3 H, 4-CH3), 1.06 (bd, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.02 (t,
3JHH = 7.5 Hz, 3 H, 1a-CH3), 0.41 (s, 3 H, Si(CH3)2), 0.39 (s, 3 H,
Si(CH3)2). 13C{1H} NMR (150 MHz, C6D6, rt): δ 182.8 (CCarbene),
162.1 (O-CCH), 145.7 (ipso-CDipp), 142.1 (ipso-CPh), 135.2 (ipso-
NCDipp), 134.8 (o-CHPh), 130.7 (p-CHDipp), 128.2 (m/p-CHPh), 127.6
(m/p-CHPh), 124.4 (m-CHDipp) 122.6 (NCH), 95.5 (2-CH), 35.5 (1-
CH2), 29.0 (CH(CH3)2), 24.9 (CH(CH3)2), 23.9 (CH(CH3)2), 17.8
(3-CH), 16.8 (4-CH3), 13.6 (1a-CH3), −2.7 (Si(CH3)2), −4.8
(Si(CH3)2). Mp: 110−114 °C. Anal. Calcd for C41H57CuN2OSi: C,
71.83; H, 8.38; N, 4.09. Found: C, 69.56; H, 8.23; N, 4.17. Repeated
elemental analysis for different, independently prepared samples failed
to give more satisfactory results; all samples gave consistent values but
were low in carbon. It may be speculated that this is caused by SiC
and/or carbide/carbonate formation.22
(IDipp)Cu-O-C6H10-SiMe2Ph (9b): mesityl oxide (3b) (9 mg, 92
μmol, 1.1 equiv); 20 h; off-white microcrystals (39 mg, 57 μmol, 67%).
1H NMR (600 MHz, C6D6, rt): δ 7.78−7.74 (m, 2 H, o-CHPh), 7.26−
7.18 (m, 3 H, m,p-CHPh), 7.20 (t, 3JHH = 7.8 Hz, 2 H, p-CHDipp), 7.05
(d, JHH = 7.8 Hz, 4 H, m-CHDipp), 6.26 (s, 2 H, NCH), 3.87 (bs, 1 H,
(MeO)OC-((IDipp)Cu)-C4H5-SiMe2Ph (9f): methyl crotonate (3f)
(10 mg, 100 μmol, 1.2 equiv); 48 h; colorless solid (46 mg, 67 μmol,
79%). 1H NMR (600 MHz, C6D6, rt): δ 7.59−7.55 (m, 2 H, o-CHPh),
7.17−7.12 (m, 3 H, m,p-CHPh), 7.19 (t, 3JHH = 7.7 Hz, 2 H, p-CHDipp),
7.07 (dd, 3JHH = 7.7 Hz, 3JHH = 1.3 Hz, 2 H, m-CHDipp), 7.06 (dd, 3JHH
= 7.7 Hz, 3JHH = 1.3 Hz, 2 H, m-CHDipp), 6.30 (s, 2 H, NCH), 3.24 (s,
3 H, 1-CH3), 2.55 (m, 3JHH = 7.0 Hz, 2 H, CH(CH3)2), 2.55 (m, 3JHH
= 6.9 Hz, 2 H, CH(CH3)2), 2.35 (bd, 3JHH = 7.9 Hz, 1 H, 2-CH), 2.05
(dq, 1 H, 3JHH = 7.9 Hz, 3JHH = 6.8 Hz, 1 H, 3-CH), 1.38 (d, 3JHH = 6.9
3
2-CH), 2.53 (sept., JHH = 6.9 Hz, 4 H, CH(CH3)2), 1.59 (s, 3 H, 1-
CH), 1.36 (d, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.36 (s, 6 H, 4-CH3),
3
1.06 (d, JHH = 6.9 Hz, 12 H, CH(CH3)2), 0.52 (s, 3 H, Si(CH3)2).
13C{1H} NMR (150 MHz, C6D6, rt): δ 182.7 (CCarbene), 157.0 (O-C
CH), 145.7 (ipso-CDipp), 142.6 (ipso-CPh), 135.2 (o-CHPh), 135.1 (ipso-
NCDipp), 130.7 (p-CHDipp), 127.9 (m/p-CHPh), 127.3 (m/p-CHPh),
124.3 (m-CHDipp), 122.6 (NCH), 102.6 (2-CH), 30.4 (1-CH2), 29.0
(CH(CH3)2), 26.8 (4-CH3), 24.9 (CH(CH3)2), 23.9 (CH(CH3)2),
22.6 (3-CH), −3.0 (Si(CH3)2). Mp: 128−134 °C. Anal. Calcd for
C41H57CuN2OSi: C, 71.83; H, 8.38; N, 4.09. Found: C, 70.70; H, 8.29;
N, 4.22. Repeated elemental analysis for different, independently
prepared samples failed to give more satisfactory results; all samples
gave consistent values but were low in carbon. It may be speculated
that this is caused by SiC and/or carbide/carbonate formation.22
(IDipp)Cu-O-C6H8-SiMe2Ph (9c): cyclohexenone (3c) (9 mg, 94
μmol, 1.1 equiv); 1 h; crystallization at −78 °C, colorless microcrystals
3
Hz, 6 H, CH(CH3)2), 1.38 (d, JHH = 6.9 Hz, 6 H, CH(CH3)2), 1.07
(d, 3JHH = 7.0 Hz, 12 H, CH(CH3)2), 0.88 (bd, 3JHH = 6.8 Hz, 1 H, 4-
CH), 0.28 (s, 3 H, Si(CH3)2), 0.25 (bs, 3 H, Si(CH3)2). 13C{1H}
NMR (150 MHz, C6D6, rt): δ 183.8 (CCarbene), 179.4 (O-CO),
145.8 (ipso-CDipp), 141.4 (ipso-CPh), 135.4 (ipso-NCDipp), 134.5 (o-
CHPh), 130.5 (p-CHDipp), 128.2 (m/p-CHPh), 127.6 (m/p-CHPh), 2 ×
124.2 (br, m-CHDipp), 122.5 (NCH), 49.2 (1-CH), 37.6 (br, 2-CH),
29.0 (CH(CH3)2), 24.9 (CH(CH3)2), 24.8 (CH(CH3)2), 24.0
(CH(CH3)2), 23.9 (CH(CH3)2), 21.1 (4-CH), 19.8 (br, 3-CH3),
−2.7 (br, Si(CH3)2), −5.2 (br, Si(CH3)2). Mp: dec >100 °C. Anal.
Calcd for C40H55CuN2O2Si: C, 69.88; H, 8.06; N, 4.07. Found: C,
69.72; H, 8.02; N, 4.21.
1
(36 mg, 53 μmol, 62%). H NMR (600 MHz, C6D6, rt): δ 7.64−7.60
3
(m, 2 H, o-CHPh), 7.25−7.21 (m, 3 H, m,p-CHPh), 7.20 (t, JHH = 7.7
Hz, 2 H, p-CHDipp), 7.04 (d, JHH = 7.7 Hz, 4 H, m-CHDipp), 6.23 (s, 2
H, NCH), 4.56 (bs, 1 H, O−CCH), 2 × 2.53 (sept., 3JHH = 6.9 Hz,
2 H, CH(CH3)2), 2.05−1.97 (m, 2 H, CH-Si and CH2), 1.94−1.89
(m, 1 H, CH2), 1.79−1.69 (m, 2 H, CH2), 1.66−1.56 (m, 1 H, CH2),
In Situ NMR Experiments: Catalytic Conjugate Silyl Addition
to 3a,b,d,e,f (Figure 1−3). In a nitrogen-filled glovebox a screw cap
NMR tube was charged with 3a,b,d,e or f (57 μmol, 1.0 equiv), 1 (18
mg, 68 μmol, 1.2 equiv), and 2 (3.3 mg, 5.7 μmol, 10 mol %) or 6 (3.0
3
1.49−1.43 (m, 1 H, CH2), 1.34 (d, JHH = 6.9 Hz, 6 H, CH(CH3)2),
1
1.33 (d, 3JHH = 7.0 Hz, 6 H, CH(CH3)2), 2 × 1.06 (d, 3JHH = 6.9 Hz, 6
H, CH(CH3)2), 0.32 (s, 3 H, Si(CH3)2), 0.31 (s, 3 H, Si(CH3)2).
mg, 5.7 μmol, 10 mol %) and C6D6 (0.7 mL). H NMR spectra were
recorded after the given time intervals at rt: 3a: 5.6 mg, 6 h. 3b: 5.6, 20
J
dx.doi.org/10.1021/om500989w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
h. 3d: 4.0 mg, 2 h. 3e: 4.8 mg, 6 h. 3f: 5.7 mg, 8 h; a separate, identical
experiment was performed under addition of PrOH (approximately
3.4 mg, 57 μmol, 1.0 equiv, 0.5 h).
(4) (a) Lee, K.-s.; Hoveyda, A. H. J. Am. Chem. Soc. 2010, 132,
2898−2900. (b) Lee, K.-s.; Wu, H.; Haeffner, F.; Hoveyda, A. H.
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i
In Situ NMR Experiments: Formation of 9a,b,d,f (Figure 4).
In a nitrogen-filled glovebox a screw cap NMR tube was charged with
6 (15 mg, 26 μmol, 1.0 equiv), 3a,b,d or f (26 μmol, 1.0 equiv), and
1
C6D6 (0.7 mL). H NMR spectra were recorded after the given time
intervals at rt. 3a: 2.8 mg, 1.5 h. 3b: 2.8 mg, 16 h. 3d: 2.0 mg, 0.5 h. 3f:
2.8 mg, 0.5 h.
́
4462−4465. (g) Ibrahem, I.; Santoro, S.; Himo, F.; Cordova, A. Adv.
Synth. Catal. 2011, 353, 245−252. (h) Delvos, L. B.; Vyas, D. J.;
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Angew. Chem., Int. Ed. 2014, 53, 4159−4163.
In Situ NMR Experiments: Formation of 4a,b,d/5f and 6 from
9a,b,d,f and 1 (Figure 6,7). In a nitrogen-filled glovebox a screw cap
NMR tube was charged with 9a,b,d or f (15 mg, 1 equiv), 1 (7 mg, 27
μmol, 1.2 equiv), and C6D6 (0.7 mL). 1H NMR spectra were recorded
after 2 h at rt. 9a: 22 μmol. 9b: 22 μmol. 9d: 23 μmol. 9f: 22 μmol;
i
after 2 h a 1H NMR spectrum was recorded, PrOH (approximately 2
1
mg, 3 μmol, 1.5 equiv) was added, and another H NMR spectrum
(5) (a) Kleeberg, C.; Feldmann, E.; Hartmann, E.; Vyas, D. J.;
Oestreich, M. Chem.Eur. J. 2011, 17, 13538−13543. (b) Kleeberg,
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(7) Dang, L.; Lin, Z.; Marder, T. B. Organometallics 2008, 27, 4443−
4454.
was recorded.
ASSOCIATED CONTENT
■
S
* Supporting Information
The SI available contains additional NMR spectroscopic and
crystallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org. Crystallographic data
(excluding structure factors) for the structures reported in
this paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication nos. CCDC
1019946−1019951. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [fax: +44(1223) 336-033; e-mail: deposit@ccdc.
cam.ac.uk].
(8) For selected examples and an overview, see: (a) Dang, L.; Lin, Z.;
Marder, T. B. Chem. Commun. 2009, 3987−3995. (b) Takahashi, K.;
Ishiyama, T.; Miyaura, N. J. Organomet. Chem. 2001, 625, 47−53.
(c) Takahashi, K.; Ishiyama, T.; Miyaura, N. Chem. Lett. 2000, 29,
982−983. (d) O’Brien, J. M.; Lee, K.-s.; Hoveyda, A. H. J. Am. Chem.
Soc. 2010, 132, 10630−10633.
(9) See for selected examples: (a) Laitar, D. S.; Muller, P.; Sadighi, J.
̈
P. J. Am. Chem. Soc. 2005, 127, 17196−17197. (b) Laitar, D. S.; Tsui,
E. Y.; Sadighi, J. P. J. Am. Chem. Soc. 2006, 128, 11036−11037.
(c) Vergote, T.; Nahra, F.; Merschaert, A.; Riant, O.; Peeters, D.;
Leyssens, T. Organometallics 2014, 33, 1953−1963. (d) Goj, L. A.;
Blue, E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.; Pierpont, A.
W.; Petersen, J. L.; Boyle, P. D. Inorg. Chem. 2006, 45, 9032−9045.
(e) Goj, L. A.; Blue, E. D.; Munro-Leighton, C.; Gunnoe, T. B.;
Petersen, J. L. Inorg. Chem. 2005, 44, 8647−8649.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: ch.kleeberg@tu-braunscheweig.de.
Notes
The authors declare no competing financial interest.
(10) (a) Tan, G.; Blom, B.; Gallego, D.; Irran, E.; Driess, M. Chem.
Eur. J. 2014, 20, 9400−9408. (b) Ariafard, A.; Brookes, N. J.; Stranger,
R.; Yates, B. F. Organometallics 2011, 30, 1340−1349. (c) Ohishi, T.;
Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2008, 47, 5792−5795.
(11) Nonetheless, for the Pt-catalyzed β-borylation of carbonyl/
carboxyl compounds the intermediate O- and C-enolates have been
studied by NMR spectroscopy as well as computationally: (a) Lawson,
Y. G.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice, C. R.
Chem. Commun. 1997, 2051−2052. (b) Bell, N. J.; Cox, A. J.;
Cameron, N. R.; Evans, J. S. O.; Marder, T. B.; Duin, M. A.; Elsevier,
C. J.; Baucherel, X.; Tulloch, A. A. D.; Tooze, R. P. Chem. Commun.
2004, 1854−1855. (c) Liu, B.; Go, M.; Dang, L.; Zhao, H.; Marder, T.
B.; Lin, Z. Organometallics 2012, 31, 3410−3425.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support by the Deutsche
Forschungsgemeinschaft (DFG). C.K. thanks the Fonds der
Chemischen Industrie for a Liebig-Stipendium. The authors
wish to thank Oliver Haslett (RISE program, DAAD) for help
with the laboratory work.
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1
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3
the coupling constant of JHH = 12.1 Hz for 1′−2′ suggests the E
isomer to be predominant. 1H NMR (400 MHz, C6D6, rt): δ (E
isomer) 6.61 (d, 3JHH = 12.1 Hz, 1 H, 1-CH), 5.57 (d, 3JHH = 12.1 Hz,
1 H, 2-CH), 0.94 (s, 6 H, 3′,4′-CH3), 0.21 (s, 6 H, 5-CH3). Moreover,
two more signals (not visible in Figure 2) at 6.66 and 4.29 ppm (d, JHH
= 6.9 Hz) are suggestive for the additional presence of the Z isomer
(approximately 0.5% of 7).
K
dx.doi.org/10.1021/om500989w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(14) The molecular structure of 7 has also been established by an X-
ray structure analysis; see SI for details.
(15) Crystallization of 9d proved especially challenging as repeatedly
crystals of (IDipp)CuPh were obtained from samples containing 9d
and that were originally free of (IDipp)CuPh. Moreover, formation of
(IDipp)CuPh was also observed in C6D6 solution after prolonged
periods of time. The intriguing observations regarding 9d, its
decomposition to (IDipp)CuPh, the formation of 10, and the
equilibrium of the E/Z isomersmay it be related or notare
subjects of ongoing studies.16 For the spectroscopic data of (IDipp)
CuPh see ref 9d.
(16) See SI for details.
(17) In all cases but 9d the refinement of suitable split atom models
was successful. Similarity restraints had to be employed and a common
ADP was refined for each disordered atom pair. The refined
occupancies of the main component were 9a: 0.950(1), 9b:
0.811(1), and 9d(10)PhMe: 0.656(3). Only the geometrical data of
the major component are discussed.16
(18) Wiberg, N. Holleman-Wiberg, Lehrbuch der Anorganischen
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L
dx.doi.org/10.1021/om500989w | Organometallics XXXX, XXX, XXX−XXX