Synthesis and characterization of κ-2-bis-N-heterocyclic carbene rhodium(I) catalysts: Application in enantioselective arylboronic acid addition to cyclohex-2-enones

Article abstract of DOI:10.1016/j.jorganchem.2011.05.015

This manuscript describes the synthesis and characterization of new derivatives of di-N-heterocyclic carbene (diNHC) ligands derived from trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diylmethanediyl (DEAM) and trans-9,10-dihydro-9,10-ethanoanthracene-11

Full text of DOI:10.1016/j.jorganchem.2011.05.015

Journal of Organometallic Chemistry 696 (2011) 3127e3134
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of
k-2-bis-N-heterocyclic carbene rhodium(I)
catalysts: Application in enantioselective arylboronic acid addition to
cyclohex-2-enones
*
Matthew S. Jeletic, Roxy J. Lowry, Jason M. Swails, Ion Ghiviriga, Adam S. Veige
Department of Chemistry, Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville FL 32611, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
This manuscript describes the synthesis and characterization of new derivatives of di-N-heterocyclic
carbene (diNHC) ligands derived from trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diylmethanediyl
(DEAM) and trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diyl (DEA). Synthesized as the diazolium
salts, the new ligands are employed, in conjunction with known derivatives, to examine specific prop-
erties than influence the chiral induction in rhodium-catalyzed arylboronic acid addition to cyclohex-2-
enones. Three properties of the diNHC ligands are modified and include: 1) the size and composition of
the N-heterocyclic substituent (Me, iPr, Bn, CHPh2, o-MeBn, and R-CHMePh), 2) the type of N-heterocycle
(benzimidazole versus imidazole), and 3) the size of the chiral pocket (DEAM versus DEA). Results from
the catalytic studies indicate the DEAM ligand, which contains an extra CH2 group compared to DEA, is too
flexible to induce enantiomeric excess. The DEA derivatives containing an imidazole or benzimidazole
provide distinctly different results despite relatively small differences between the heterocyclic carbene
donors. An unexpected source of chiral induction is rationalized using results from catalytic data (% e.e.)
and complemented by X-ray structural data and DFT calculations.
Received 8 May 2011
Received in revised form
22 May 2011
Accepted 26 May 2011
Keywords:
Carbene
diNHC
Chiral
C2-symmetric
Rhodium
Enantioselective catalysis
Arylboronic acid
Published by Elsevier B.V.
1. Introduction
not surprising that trial and error remains the common method for
discovery.
Enantiopure di-N-heterocyclic carbene ligands (diNHCs) are
emerging as effective chiral auxiliaries in metal catalyzed asym-
metric catalysis [1e25]. However, few design principles exist to
guide chiral diNHC catalyst engineering. Representations of a chiral
metal ion environment, such as Knowles’ quadrant model [26,27]
and Trost’s version [28], serve as good approximations, especially
for asymmetric alkene hydrogenation [29] and allylic alkylation
[30,31]. These models fit well for chiral diphosphine ligands, but in
some cases they do not, resulting in some modifications to the
model [32e35]. Advances in computational techniques [36e40]
may ultimately permit catalyst designs that correctly select for
a specific product enantiomer prior to any laboratory investigation.
The problem is that small energetic differences of 1e2 kcal/mol in
the diastereo- or enantio-determining transition states lead to large
changes in % enantiomeric excess (% e.e.) [41]. Predicting or
designing ligand and catalyst features, that discriminate within
such a small energy range is difficult. After factoring in the addi-
tional variables of temperature, additives, and solvent choice, it is
Providing some guidance regarding NHC [42] ligand steric
influence on the metal ion coordination sphere is the %Vburied
model, but this model applies to monodentate ligands [43,44]. In
addition, the relative s-donation strength of NHCs, and thus the M-
NHC bond strength, is becoming clearer from both experiment and
computational results [45e53]. Despite possessing a strong M-NHC
bond, NHCs can reductively eliminate from corresponding metal-
hydrido and alkyl species to give imidazolium salts [18,54e63]. A
systematic study by Hermann et al. provides some insight into the
effects of achiral diNHC ligand bridge length, N-substituent, and
heterocycle composition on Rh(I) catalyzed hydrosilylation of
ketones [64]. Considering the limited models available for
designing effective chiral catalysts using diNHCs, it is important
that a chosen ligand platform permit rapid and facile modification.
Our previously reported DEAM (trans-9,10-dihydro-9,10-etha-
noanthracene-11,12-diylmethanediyl) and DEA (trans-9,10-dihy-
dro-9,10-ethanoanthracene-11,12-diyl) diNHC ligands provide
opportunities to easily modify steric and electron-donating
features (Fig. 1) [17,18,65e67]. In addition, the DEAM ligand
provides an opportunity to link the two NHC heterocycles through
the R groups to create chiral dicarbene cyclophane ligands [17].
* Corresponding author. Tel.: þ1 352 392 9844; fax: þ1 352 392 3255.
E-mail address: veige@chem.ufl.edu (A.S. Veige).
0022-328X/$ e see front matter Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2011.05.015

3128
M.S. Jeletic et al. / Journal of Organometallic Chemistry 696 (2011) 3127e3134
The 1H NMR spectrum of 3 reveals a resonance attributable to
Fine-tune sterics and
the chiral pocket
R
R
the C2 proton at 9.65 ppm (see Fig. 1 for atom notation). Other
diagnostic signals include a singlet at 2.45 ppm for the o-methyl
protons, a singlet for the bridgehead proton at 4.42 ppm, and
a third singlet for the bridge proton at 2.77 ppm. Presumably, the
dihedral angle between the bridgehead and bridge proton is w90ꢀ
because no coupling occurs, which is in agreement with the Karplus
curve [77]. A 2D HETCOR experiment permits the assignment of the
other aliphatic carbon resonances (see supporting information).
The 1H NMR spectrum of 4 confirms its identity. The spectrum
exhibits the recognizable singlet C2 proton resonance at 9.04 ppm.
The methyl and methine protons of the chiral N-alkyl fragment
resonate at 1.92 (d, J ¼ 5 Hz) and 5.78 ppm (q, J ¼ 10 Hz), respec-
tively. The bridgehead proton resonates at 4.05 ppm as a singlet
which is consistent with previously reported azolium salts [18].
Subjecting 4 to a 2D HETCOR experiment permits the assignment of
the other aliphatic carbon signals. Precluding water from the
synthesis of 4 is important because the N-alkyl group is susceptible
to hydrolysis. In the presence of a small amount of water, partial
epimerization of the N-alkyl chiral center occurs, resulting in
multiple diastereomers (confirmed by 1H NMR).
Preparation of the DEA diazolium diiodide salts (R,R)-trans-
1,10-(9,10-dihydro-9,10-ethanoanthracene-11,12-diyl)bis(3-iso-
propyl-1H-benzimidazol-3-ium) diiodide [DEA-IBI](I)2 (5) and
(azole ¼ benzimidazole) and (R,R)-trans-1,10-(9,10-dihydro-9,10-
ethanoanthracene-11,12-diyl)bis(3-isopropyl-1H-imidazol-3-ium)
diiodide [DEA-II][I]2 (6) (azole ¼ imidazole) requires treating
excess 2-iodopropane with one equivalent of the corresponding
1,10-(9,10-dihydro-9,10-ethanoanthracene-11,12-diyl)di(1H-azole)
[67] in CH3CN at 105 ꢀC (Scheme 2). The 1H and 13C{1H} NMR
spectra and combustion analysis confirm the identity and purity of
5 and 6.
N
C2
N
C5
C4
N
Fine-tune
electronic
donation
N
n = 1, 0
Modify ligand flexibility
n
chirality
chirality
Fig. 1. Parameters of optimization for the dihydroethanoanthracene diNHC backbone.
Probing the parameters depicted in Fig. 1 requires a suitable
benchmark reaction. Shi et al. report the 1,4-conjugate addition of
arylboronic acids to cyclic-enones with a chiral BINAM derived
diNHC ligand and Pd(II) as the metal [4]. The catalyst provides good
to excellent product % e.e.s ranging from 32 to 97%. Rhodium-
phosphine complexes are also common catalysts for this trans-
formation, but often require elevated temperatures [68e74]. In this
report, we present evidence indicating the identity of the
N-heterocycle significantly influences the chiral induction in
enantioselective diNHC rhodium-catalyzed arylboronic acid addi-
tion to cyclohex-2-enones. Also, adding to the library [17,18,65e67]
of DEAM and DEA ligands, we now report four new C2-symmetric
diazolium salt precursors and one isolable free chiral diNHC.
2. Results and discussion
2.1. Synthesis of diimidazolium and dibenzimidazolium ligand
precursors
The general procedure for preparing diazolium salt precursors of
the DEAM ligand involves direct substitution at the methylene
carbon. Scheme 1 is a generic synthetic methodology for the prep-
aration of C2-symmetric DEAM diazolium triflate salts [17,18,65,66].
The synthesis of the diazolium salts (S,S)-trans-1,10-(9,10-dihydro-
9,10-ethanoanthracene-11,12-diyldimethanediyl)bis(3-(2-methylphenyl-
1-benzyl)-1H-benzimidazol-3-ium)bis(triflouromethansulfonate)
[DEAM-o-MBBI][OTf]2 (3) and (S,S,R,R)-trans-1,10-(9,10-dihydro-
9,10-ethanoanthracene-11,12-diyldimethanediyl)bis(3-(R-1-phe-
nylethane)-1H-imidazol-3-ium) [DEAM-PEI][OTf]2 (4) involves
treating two equivalents of 1-benzyl(2-methylphenyl)-benz-
imidazole (1) or R-1-(1-phenylethyl)-1H-imidazole (2) [75] with
an equivalent of (S,S)-trans-9,10-dihydro-9,10-ethanoanthracene-
11,12-diyldimethanediyl bis(trifluoromethanesulfonate) [76] in
refluxing DME. After 1 h the reactions are complete. In a nitrogen-
filled glovebox, evaporation of the solvent in vacuo provides both
salts 3 and 4 as off-white, crunchy, hygroscopic solids in 93 and 80%
yield, respectively. A combination of 1D and 2D-NMR spectroscopic
techniques and combustion analysis confirms the purity and iden-
tity of 3 and 4 (see supporting information).
2I
N
N
N
N
5
=
=
3-4 eq.
N
N
N
N
iodopropane
H
H
CH3CN
6
105 o
C
Scheme 2. General synthesis of DEA azolium salts 5 (azole ¼ benzimidazole) and 6
(azole ¼ imidazole).
The 1H NMR spectrum of 5 in CDCl3 indicates the compound is
C2-symmetric and reveals a diagnostic C2 proton resonance at
9.31 ppm. This is 0.55 ppm downfield of the same proton resonance
in 6. Changing the N-heterocycle composition at positions C4 and
C5 changes the electron-donation properties at the C2 carbon of the
corresponding carbene [51e53]. The 0.55 ppm shift in location of
the C2 proton resonance between 5 and 6 may reflect this differ-
ence; the more electron-withdrawing benzimidazole moiety
deshields the C2 proton.
The 1H NMR spectrum of 6 has similar features to 5 including
a downfield resonance for the C2 proton at 8.76 ppm. Characteristic
resonances in the 13C{1H} NMR spectrum include one at 138.8 ppm
for the C2 carbon, two at 22.2 and 22.0 ppm for the isopropyl-methyl
carbons and one at 52.5 ppm for the isopropyl methine carbon.
N
2OTf
R
N
R
N
N
1
OTf
TfO
N
N
DME
2.2. Synthesis of the free di-N-heterocyclic carbene 7
N
It is useful to have access to the corresponding free dicarbene
derivative of these ligands for the purpose of in situ catalyst
generation. The synthesis of the free diNHC (þ/ꢁ) trans-1,10-(9,10-
dihydro-9,10-ethanoanthracene-11,12-diyl)bis(3-isopropylbenzimida-
zolidine-2-ylidene) [DEA-IBY] (7) requires treating one equivalent of 5
N
3:
benzimidazole, R = o-MeBn
2
4
:imidazole ,R = R-CHMePh
Scheme 1. General synthetic scheme for DEAM salts 3 (benzimidazole, R ¼ o-MeBn)
and 4 (imidazole, R ¼ R-CHMePh).

M.S. Jeletic et al. / Journal of Organometallic Chemistry 696 (2011) 3127e3134
3129
with 2.1 equivalents of KN(Si(CH3)3)2 in THF (60% yield as a white
powder; Scheme 3). Multinuclear NMR spectroscopy and combustion
analysis confirm the identity of 7. The 1H NMR spectrum displays
diagnostic isopropyl proton resonances for the methyl and methine
groups at 1.20/1.27 ppm (J ¼ 6 Hz) and 4.19 ppm (J ¼ 6 Hz), respec-
tively. The bridgehead proton signal appears as a singlet at 4.68 ppm.
The bridge proton resonance in 7 shifts downfield into the aromatic
proton region obscuring its exact location (between 6.92 and
7.16 ppm). This is 4 ppm more downfield than the bridge proton
signal in the previously reported isopropyl DEAM free diNHCs [65].
The downfield shift for 7 may be a consequence of directly attaching
the N-heterocycle to the ethanoanthracene, whereas in the DEAM
version a CH2 spacer exists. The 13C{1H} NMR spectrum of 7 confirms
that it is indeed a free dicarbene and not a tetraaminoethylene
(enetetramine) [78,79], which would be the result of carbene
dimerization. The carbene carbon resonance appears downfield at
222.5 ppm [80,81], instead of upfield between 120 and 140 ppm,
typical for enetetramines of DEA and DEAM [65,67].
procedures (8-Me [65], 8-iPr [18], 8-diPh [18], 10-Me [67], 11
[67,82], and 13 [66]), as well as four new species 8-o-MeBn, 9-R-
CHMePh, 10-iPr, and 12. Complexes 8-o-MeBn and 9-R-CHMePh
are synthesized in a one-pot method via combination of a metal
precursor and in situ generation of the diNHC. For example, treating
one equivalent of the precursor azolium salt 3 with 2.1 eq. of
KN(Si(Me)3)2 and 1 eq. of the metal precursor [Rh(NBD)2]BF4 (where
NBD ¼ norbornadiene) at ꢁ35 ꢀC, provides the monometallic
complexes 8-o-MeBn according to Scheme 4. A similar procedure
provides complex 9-R-CHMePh.
2I
N
N
Scheme 4. Synthesis of the rhodium complexes 8-o-MeBn and 9-R-CHMePh.
N
N
N
N
)
2.1 eq KN(SiMe3 2
N
N
THF
1D and 2D-NMR techniques provide evidence for the formation
of complexes 8-o-MeBn and 9-R-CHMePh. Similar to previously
reported complexes with the ethanoanthracene backbone, the
NMR spectra of 8-o-MeBn and 9-R-CHMePh exhibit an elaborate
set of resonances indicating a lowering of the symmetry of the
ligand upon coordination from C2 to C1. Presented here is the NMR
data for 8-o-MeBn. Not discussed specifically here because the
data is similar to 8-o-MeBn, the NMR spectral data for 9-R-
CHMePh are located in the supplementary information. The NBD
5
7
Scheme 3. Synthesis of the free diNHC 7.
2.3. Synthesis and characterization of rhodium complexes
Chart 1 depicts complexes employed in the catalytic studies
(vide infra) that were prepared according to known literature
Ph
Ph
OTf
OTf
R
R
R
R
I
N
N
N
N
N
N
Rh
Rh
N
N
N
N
Rh
N
N
i
i
9-R-CHMePh
8-R R = Pr, diPh,o-MeBn, Me
10-R R = Me, Pr
Ph
Ph
OTf
Me
Rh
Me
N
I
N
N
N
Cl
N
N
Cl
N
Rh
Rh
N
N
N
Rh
N
N
11
12
13
Chart 1. Complexes employed in catalytic arylboronic acid addition to cyclohex-2-enones.

3130
M.S. Jeletic et al. / Journal of Organometallic Chemistry 696 (2011) 3127e3134
I
2I
N
N
N
2.1 eq. KN(Si(Me)3)2
0.5 eq. [Rh(COD)Cl]2
N
Rh
N
N
N
N
THF, -35 oC
6
10-iPr
Scheme 5. Synthesis of 10-iPr.
methylene bridge protons (RhCHCHCH2), and the DEAM bridge
and bridgehead proton resonances provide unique signatures for
complex 8-o-MeBn. The NBD bridge protons resonate as two
overlapping doublet of doublets at 1.25 and 1.34 ppm (J ¼ 5 Hz).
The diNHC bridgehead protons resonate as broad singlets at 4.42
and 4.91 ppm and the diNHC bridge protons resonate at 2.19 and
5.49 ppm.
doublets (J ¼ 53 Hz) at 178.0 and 177.8 ppm, consistent with other
rhodium imidazole-NHCs [14,66,67]. The COD olefinic carbons also
resonate as doublets at 92.4, 88.6, 87.5, and 85.4 ppm with coupling
constants that range between 5 and 9 Hz.
The bimetallic complex 12 is synthesized by treating 0.5 eq. of
[Rh(COD)Cl]2 with an equivalent of free diNHC 7 in THF at ꢁ35 ꢀC
(Scheme 6). After combining the reagents and stirring the solution
for 40 h while slowly warming to ambient temperature, 12 forms
and precipitates upon addition of hexanes. Golden-yellow crystals
will grow from slow diffusion of hexanes into a saturated chloro-
form solution providing 12 in 89% yield. The original intention was
to synthesize the corresponding mononuclear complex; however,
the combination of the constrained backbone, by virtue of
removing the methylene spacer, and the larger N-isopropyl group,
provides the dinuclear complex. Methods involving varying the
temperature, metal precursor (including mononuclear [Rh(NBD)2]
BF4), reagent equivalents, and whether the diNHC is generated in
situ or isolated, all result in the dinuclear product. Multinuclear
NMR techniques and combustion analysis confirm the identity and
purity of 12.
The 13C{1H} spectrum for 8-o-MeBn indicates the diNHC is
bound to the rhodium ion. The carbene carbon resonances appear
as doublets (J ¼ 57 Hz) at 194.2 and 194.7 ppm. The coupling
constants observed are consistent with other Rh-NHC complexes
[1,18,65e67,83e88]. The olefinic carbons of the NBD ligand reso-
nate as doublets, but the coupling with rhodium is smaller (7.5 Hz),
and appear at 67.6, 74.2, 76.2, 79.6 ppm. Other diagnostic reso-
nances include the NBD bridge carbon at 67.9 ppm and the o-
methyl carbons at 19.3 and 19.4 ppm.
Complex 10-iPr forms in one-pot by treating an equivalent of
the isopropyl diiodide salt precursor 6 with 0.5 eq. of [Rh(COD)Cl]2
and 2.1 equivalents of KN(Si(Me)3)2 at ꢁ35 ꢀC (Scheme 5). Purifi-
cation by recrystallizing from 3:1 dry THF:CH2Cl2 provides
analytically pure 10-iPr in 92% yield. The complex is very hygro-
scopic. Atmospheric water causes 10-iPr to become oily, and
subsequent isolation as a powder is not feasible. The 1H NMR
spectrum of 10-iPr indicates it is C1-symmetric. Four sets of iso-
propyl-methyl proton resonances appear as doublets at 1.51/1.38/
1.28/1.27 ppm (J ¼ 6 Hz) and the corresponding methines resonate
at 5.31 and 4.39 ppm (J ¼ 6 Hz). Another characteristic signal is the
ethanoanthracene bridge protons, which resonate as a doublet of
doublets at 3.78 and 8.31 ppm (J ¼ 10 Hz, J ¼ 1 Hz). The unexpected
splitting is a result of an ABA'B' spin system containing virtual
coupling [89]. The unusual downfield shift for an aliphatic bridge
proton to 8.31 ppm is a consequence of its close proximity to the
rhodium center. The analogous methyl derivative 10-Me [67] also
exhibits an extremely downfield signal at 8.23 ppm.
The 1H and 13C{1H} NMR spectra exhibit signals consistent with
a C2-symmetric complex. For the isopropyl methine and methyl
protons, the 1H NMR spectrum reveals a septet at 6.79 ppm and two
doublets at 1.79 and 1.77 ppm (J ¼ 6 Hz), respectively. This contrasts
with the C1 symmetric species 10-iPr, which exhibits two methine
and four methyl resonances. In addition, only a single carbene
carbon resonates at 197.6 ppm as a doublet (J ¼ 50 Hz) in the 13C
{1H} NMR spectrum. One of the COD methylene protons resonates
unusually upfield at ꢁ0.38 ppm. The COD olefinic carbons resonate
in two distinct groups as doublets. One set appears at 100.3 and
99.4 ppm (J ¼ 5 Hz) and the second set appears at 69.2 and
67.8 ppm (J ¼ 15 Hz). The trans influence of the chloride versus the
NHC accounts for the large disparity (c.a. 30 ppm) between the two
groups of carbon olefin resonances. A 2D COSY experiment
confirms all the other assignments in the 1D spectrum (see
supplementary information).
The 13C{1H} NMR spectrum corroborates the C1 symmetry of
10-iPr. In particular, four isopropyl methyl resonances appear at
22.5, 23.6, 23.9, and 25.4 ppm and the corresponding methines
resonate at 54.9 and 52.1 ppm. The carbene carbons resonate as
The formation of the dinuclear species 12, instead of the
mononuclear derivative akin to 11-Me [67], 10-Me [67], and 10-iPr,
N
N
Rh
N
Cl
Cl
Rh
N
N
N
0.5 eq. [Rh(COD)Cl]2
THF, -35 oC
N
N
7
12
Scheme 6. Synthesis of dinuclear 12.

M.S. Jeletic et al. / Journal of Organometallic Chemistry 696 (2011) 3127e3134
3131
Table 2
indicates there is a steric limit. Combined with the larger benz-
imidazole N-heterocycle and an isopropyl group, the DEA ligand is
too constrained to chelate. The fact an imidazole isopropyl version
forms the mononuclear derivative 10-iPr foreshadows a key steric
factor in enantioselective catalytic induction (vide infra).
Catalytic 1,4-conjugate addition of phenylboronic acid to cyclohex-2-enone with
catalyst 8e13.
Entry
Catalyst
Time (h)
Yielda
% e.e.b
1
2
3
4
5
6
7
8
(S,S) 13
(R,R) 11
(R,R) 10-Me
(R,R) 10-iPr
(S,S) 8-Me
18
24
23
24
14
14
14
14
14
24
24
24
98
56
19
26
98
95
96
95
98
40
70
41
0
78 (R)
5 (R)
7 (R)
0
0
0
5 (R)
0
82 (S)
20 (S)
15 (R)
3. Enantioselective catalysis results
(S,S) 8-o-MeBn
3.1. 1,4-Conjugate addition of arylboronic acids to cyclohex-2-
enone
(S,S) 8-iPr
(S,S,R,R) 9-R-CHMePh
(S,S) 8-diPh
(S,S) 11
9
10c,d
11c
12c
The enantioselective catalytic activity of complexes 8e11 and 13
were examined via 1,4-conjugate addition of arylboronic acids to
cyclohex-2-enone. Table 1 outlines optimization experiments for
the catalytic reaction using catalyst 11 and phenylboronic acid. A
solvent mixture of 10/1 dioxane:methanol at 80 ꢀC with 33 mol% of
KOH and 1.5 mol% catalyst over 24 h affords the best results (entry
9, Table 1). While other rhodium catalysts achieve high yields and
enantioselectivities for this reaction at room temperature, this
system requires temperatures above 70 ꢀC because the removal of
the strongly bound COD or NBD ligand is slow at lower tempera-
tures. Potassium hydroxide provides the best results for both
conversion and enantioselectivity. Altering the solvent combina-
tions also influences the % e.e. and product yield, although the
effect is minor. The presence of water reduces the % e.e. to near
zero, indicating water has a negative effect during catalytic turn-
over. Replacing methanol with anhydrous methanol led to
a moderate decrease in yield and % e.e. (entries 12 and 13, Table 1).
This suggests a small amount of water in the system is beneficial
and that a fine balance between water-mediated catalyst activation
and degradation exists. These results are in agreement with the
findings of Hayashi [71,72].
(S,S) 10-Me
(R,R) 10-iPr
a
Isolated yield.
b
c
Enantiomer in parenthesis determined by HPLC.
4:1 dioxane:MeOH.
3 mol% 11, T ¼ 76 ꢀC, 33 mol% KOH, 1.5 eq. o-tolyboronic acid.
d
extra chiral centers on (S,S,R,R)-9 provide an added benefit during
catalysis when compared to the results of the similar catalysts 8-R
and 13. Regardless, it appears the DEAM ligand is too flexible [18]
for this reaction and the conditions, most notably the elevated
temperature (80 ꢀC). Interestingly, the yield drastically decreases
when using the more rigid catalysts 10-R and 11, but significantly
increases when using catalysts 8-R and 9-R-CHMePh. Though
flexibility diminishes % e.e., it permits better turnover.
An important finding is the more rigid DEA [67] catalysts (10-R,
11) provide higher % e.e. than the DEAM catalysts. Notably, even
amongst the DEA versions (benzimidazole (11) versus imidazole
(10-R)) a large disparity between the % e.e. exists. This is surprising
since the only difference between 10-Me and 11 is the possible
electron-donating capacity of the NHC and the additional arene
unit for 11. An example of the disparity appears in entries 2 versus 3
and 4. Complex 11 achieves 78% e.e. (entry 2), however, complexes
10-Me and 10-iPr achieve only 5 and 7% e.e. (entries 3 and 4),
respectively. In addition, entries 10e12 (Table 2) display the results
when the boronic acid is the larger o-tolyl derivative. All the cata-
lysts demonstrate a slight increase in the % e.e., attributable to the
larger substrate. However, complex 11 achieves 82% e.e., whereas
complex 10-Me provides only 20% e.e (Table 2; entry 11). Changing
the size of the N-alkyl substituent in 10-R from Me to the larger iPr
has little effect on the % e.e., in fact it decreases by 5% (Table 2; entry
12), implying the sterics at the N-alkyl group play only a minor role
in enantio-discrimination. The only clear steric difference between
11 and 10-R is the presence of the arene moiety on the benzimid-
azole heterocycle. How can this seemingly small difference have
such a dramatic effect on the % e.e.?
The activities of catalysts 8e11 and 13 were tested under
optimal conditions (entry 9, Table 1) for 1,4-conjugate addition of
arylboronic acids to cyclohex-2-enone (Table 2). These studies
reveal that the DEAM ligands are not capable of transferring
chirality at the high temperatures required for catalytic turnover
(entries 1, 5e9, Table 2). However, as an exception (S,S,R,R)-9 did
provide low chiral induction (5%), comparable to the DEA ligands
with imidazole NHCs (entries 3 and 4, Table 2). This implies that the
Table 1
Optimization of catalytic 1,4-conjugate addition of phenylboronic acid to cyclohex-
2-enone with catalyst 11-Me.
Entry Solvent
T ꢀ
C
Time (h) Base
Yielda % e.e.b
3.2. DFT geometry calculation of 10-Me and 11
1
Dioxane/MeOH (1:1)
50
75
80
80
80
80
80
80
80
80
80
80
17
24
24
17
17
17
24
24
24
24
24
24
24
KOH
KOH
KOH
none
pyridine
K2CO3
KOH
KOH
KOH
KOH
KOH
KOH
KOH
0
33
55
0
0
70
69
0
2
3
4
5
6
7
Dioxane/MeOH (1:1)
Dioxane/MeOH (1:1)
Dioxane/MeOH (1:1)
Dioxane/MeOH (1:1)
Dioxane/MeOH (1:1)
THF/MeOH (1:1)
The enantioselectivities of the closely related catalysts 10-R and
11 differ drastically under the same reaction conditions despite
only altering the azole moiety. Changing the groups at C4 and C5 of
the NHC typically only alters the electron-donation of the NHC to
the metal center, and not the topology around the metal center.
Located on the periphery of the catalyst, the azole moiety should
not directly influence the chiral environment at the metal ion.
Though the electron-donating capacity of the NHCs may contribute
to the different % e.e., a steric argument seems more rational. Thus,
a valuable comparison would be to examine the differences
between the solid-state structures between 10-Me and 11.
However, in our hands, only complex 11 forms single crystals, see
reference 67 for details.
0
0
30
73
82
86
56
70
98
74
46
77
5
82
78
61
54
43
8
Dioxane/H2O (1:1)
9
Dioxane/MeOH (10:1)
Dioxane/MeOH (4:1)
Dioxane/MeOH (2:1)
Dioxane/MeOH (10:1)
10
11
12c
13c
Dioxane/MeOHd (10:1) 80
a
Isolated yield.
Determined by HPLC.
3 mol% catalyst.
Anhydrous MeOH.
b
c
d

3132
M.S. Jeletic et al. / Journal of Organometallic Chemistry 696 (2011) 3127e3134
Table 3
Comparison of bond lengths (Å) and angles (ꢀ) between 11 and 11', and 10-Me’.
11
11'
10-Me’
Rh-C31
Distances
Rh-C31
Rh-C23
N2-C24
N4-C32
N2-C23
N1-C23
N4-C31
N3-C31
2.051(5)
2.033(4)
1.461(5)
1.467(5)
1.375(5)
1.365(5)
1.347(5)
1.370(5)
Rh-C31
Rh-C23
N2-C24
N4-C32
N2-C23
N1-C23
N4-C31
N3-C31
2.052
2.068
1.468
1.466
1.384
1.395
1.372
1.391
2.053
2.078
1.472
1.47
1.385
1.396
1.375
1.386
Rh-C23
N2-C24
N4-C32
N2-C23
N1-C23
N4-C31
N3-C31
Angles
C31-Rh-C23 84.14(17) C31-Rh-C23 85.3 C31-Rh-C23 87.1
N1-C23-N2 104.7(4) N1-C23-N2 105.2 N1-C23-N2 103.6
N3-C31-N4 105.9(4) N3-C31-N4 106.0 N3-C31-N4 104.2
N1-C15-C16- 68.8(5) N1-C15-C16- 70.36 N1-C15-C16- 80.64
Torsion
angle
N3
N3
N3
Distances
C4-C36
Rh-H16
3.831
2.306
C4-C36
Rh-H16
4.238
2.298
C4-C36
Rh-H16
4.946
2.394
Not able to form single crystals of 10-Me, ground-state geom-
etry optimization calculations using the B3LYP hybrid density-
functional were performed on both catalysts [90]. All calculations
use the LANL2DZ [91e94] basis set for all atoms, which utilizes
effective core potentials (ECPs) for treating the core rhodium
electrons. This methodology yields reliable geometries for similar
rhodium-containing complexes in previous studies [95]. All calcu-
lations are performed using the Gaussian 03 program suite [96].
The gas-phase geometry optimization, using the atomic coor-
dinates from the X-ray structural data for 11 as a starting point,
produces the calculated structure 11’. Table 3 lists selected bond
lengths and angles for the calculated 11’ and crystallographically
determined structure 11. The minimum root mean squared devia-
tion (RMSD) of all atoms between 11 and 11’ is 0.168 Å. The similar
metric parameters between 11 and 11’ indicate the calculation
method is appropriate for these complexes. Using the atomic
coordinates of 11 again, but replacing the arene ring carbons (C18
through C21 and C26 through C29) of the benzimidazole with
protons, permits the geometry optimization of 10-Me’ (Table 3).
Fig. 2 shows the gas-phase geometry calculation of 11’ and 10-
Me’. Visual inspection reveals little variation between the two
structures. Allowing a more meaningful inspection, Fig. 3 presents
the two structures by overlapping the ethanoanthracene fragment.
Cleary, from Fig. 3 the ethanoanthracene fragments perfectly
overlap between 10-Me’ and 11’. More importantly, comparison of
atoms C1-C14 between 10-Me’ and 11’ reveal an RMSD of only
Fig. 3. Overlay of geometry optimized structures 11' and 10-Me’.
0.0485 Å. In contrast, the remaining atoms in common between 10-
Me’ and 11’ provide an RMSD of 0.7870 Å, indicating significant
deviation.
One significant difference between the two complexes is the
location of the metal ion in relation to the chiral centers. The
smaller imidazole ring permits the rhodium center to shift up and
away from the ethanoanthracene fragment. The distance from the
Rh ion to the bridge proton H16 differs by 0.1 Å in the two calcu-
lated structures. Comparison of the distance from the bottom of the
COD ring to the aryl ring directly below shows more than a 0.5 Å
increase in the size of the chiral pocket for 10-Me’ relative to 11’.
Some error in the calculated parameters is expected; however,
there must be a difference in the location of the Rh ion relative to
H16. In the 1H NMR spectrum of 11 and 10-Me, the H16 proton
resonates at 9.26 ppm and 8.23 ppm respectively [67], a 1.03 ppm
shift, reflecting a significant difference in proximity. The shift of the
rhodium center away from the backbone in 10-Me presumably
reduces the definition of the chiral pocket, and the % e.e drops.
4. Conclusions
This investigation probes factors that govern enantioselectivity
in Rh(I)-diNHC catalyzed arylboronic acid addition to cyclohex-2-
Fig. 2. Calculated equilibrium geometries of 11' and 10-Me’.

M.S. Jeletic et al. / Journal of Organometallic Chemistry 696 (2011) 3127e3134
3133
enones. Changing the size of the N-alkyl group has little effect on
enantioselectivity. Appending an additional chiral center to the N-
alkyl group, as in 9-R-CHMePh, did provide a meager 5% e.e.
Instead, the linker arm length between the chiral backbone and
NHC units (DEA vs. DEAM) clearly plays a central role in the
enantio-discrimination of the products. DEAM ligands provide
minimal to no enantioselectivity, whereas DEA ligands provide
15e82% e.e., even at elevated temperatures (76e80 ꢀC). A simplistic
explanation is the DEAM ligand is too flexible. A similar distinction
between linker arm length within diNHCs and enantioselectivity
already exists. Shi et al. report several successful examples of
enantioselective catalysis based on the BINAM-diNHC ligand
architecture [1e13]. Yet interestingly, the groups of RajanBabu [97]
and Trudell [98], who reported the first chiral metal-diNHC using
BINAM in 2000, have yet to report successful enantioselective
catalysis. The difference between these two ligands is the linker
arm length. RajanBabu and Trudells’ version contains one addi-
tional methylene linker group [99].
The steric influence of the benzimidazole is significant; even the
smallest modification, in this case an arene ring (proton), can have
a dramatic effect on the product % e.e. The activation barrier
difference for the enantio-determining transition state between,
benzimidazole 11 and imidazole 10-Me, must only be w 1 kcal/mol
to give the 73% e.e. improvement.
Finally, not all diNHCs with longer linker arm lengths will
provide poor enantioselectivity. In fact, a recent report demon-
strates that a diNHC with a longer linker arm length than DEAM is
able to achieve excellent % e.e [14].. At this stage in development,
one design element is clear; a constrained [17,67] rigid ligand
framework for the DEAM/DEA ligand sets will likely provide the
best % e.e. Modification of the N-heterocycle, instead of the N-
substituent can however, provide the sometimes-necessary subtle,
additional discrimination required to achieve exclusive product
enantioselectivity.
Acknowledgements
The interesting finding within our work is the identity of the
azole ring has a significant effect on enantioselectivity. Using the
DEA ligand framework, the benzimidazole provides high % e.e.,
whereas the imidazole provides low % e.e. The electron-donating
capacity of the NHC (benzimidazole versus imidazole) is not the
reason for the increased % e.e. Several IR studies support that
electronically, benzimidazole and imidazole NHCs, are practically
identical [51e53]. For instance, there is only a 3 cmꢁ1 difference for
the carbonyl stretching vibration between 1,3-dimethyl-imidazol-
2-ylidene and 1,3-dimethyl-benzimidazol-2-ylidene (NHC)Ni(CO)3
complexes [52]. The similar electron-donating capacity of benz-
imidazole and imidazole NHCs suggests a steric argument likely
predominates.
ASV is a Camille and Henry Dreyfus New Faculty awardee and an
Alfred P. Sloan fellow. K.A.A. thanks the NSF (CHE-0821346) and UF
for funding the purchase of X-ray equipment. Research quantities of
DEA and DEAM ligands are available for purchase from Strem
Chemical Inc. Computational resources were provided by the
University of Florida High-Performance Computing Center.
Appendix. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2011.05.015.
Computational data reveals the arene ring of the benzimidazole
imparts an additional, albeit subtle, steric component to the chiral
environment at the metal ion. More specifically, one of the protons
(Fig. 4) on the benzimidazole ring sterically clashes with the arene
ring of the ethanoanthracene backbone. Consequently, the metal ion
is forced down intothe chiral pocket and closer tothe chiral atoms of
the ethanoanthracene bridge. Experimental evidence for this
observation comes in the form of a 1.03 ppm shift downfield for the
bridge proton resonance for 11 versus 10-Me. This is not a simple
ring current effect from the additional arene on the benzimidiazole
because that proton resonates in the identical position for both
ligands in the non-metallated imidazolium salt ligand precursors.
The result is an improvement of 73% e.e between the catalysts
(Table 2; entries 2 and 3). Additional support for the benzimidazole
steric influence is that only the dinuclear isopropyl derivative 12 is
isolable, whereas using the smaller imidazole heterocycle, the
chelated mononuclear 10-iPr forms in 92% yield. Confirming that
a larger N-substituent alone does not impart a significant steric
component to the chiral environment, the H16 proton for the larger
10-iPr again appears upfield from 11 at 8.31 ppm, and catalyst 10-iPr
provides only 15% e.e. (Table 2; entry 12).
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