Controlling stereochemical outcomes of asymmetric processes by catalyst remote molecular functionalizations: Chiral diamino-oligothiophenes (DATs) as ligands in asymmetric catalysis

Article abstract of DOI:10.1002/chem.200500889

The synthesis, characterization, and structure-guided application of a new class of highly versatile chiral C₂-symmetric diamine-oligothiophene ligands in Pd-catalyzed asymmetric transformations are presented. Experimental investigations of the

Full text of DOI:10.1002/chem.200500889

FULL PAPER
Chem. Eur. J. 2006, 12, 667 – 675
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
667

DOI: 10.1002/chem.200500889
Controlling Stereochemical Outcomes of Asymmetric Processes by Catalyst
Remote Molecular Functionalizations: Chiral Diamino-oligothiophenes
(DATs) as Ligands in Asymmetric Catalysis
Vincenzo Giulio Albano,[a] Marco Bandini,*[a] Giovanna Barbarella,[b] Manuela Melucci,[b]
Magda Monari,[a] Fabio Piccinelli,[a] Simona Tommasi,[a] and Achille Umani-Ronchi*[a]
Abstract: The synthesis, characterization, and structure-guided application of a
new class of highly versatile chiral C2-symmetric diamine-oligothiophene ligands in
Pd-catalyzed asymmetric transformations are presented. Experimental investiga-
tions of the intimate role of pendant p-conjugate oligothiophenes in determining
the catalytic activity of the corresponding chiral Pd complexes are described. Their
unusual behavior opens up new routes toward the logical design of finely tuned or-
ganometallic catalysts by remote structural functionalizations.
Keywords: asymmetric catalysis ·
electronic interactions · palladium ·
thiophenes · X-ray diffraction
Introduction
tures of the coordinating atoms as well as modifications of
the ligand skeleton in the proximity of the metal center are
among the most commonly adopted strategies to control
both chemical and optical yields in asymmetric transforma-
tions. In some cases, however, these are difficult to accom-
plish, due to poor tolerance by the ligands of harsh oxida-
tive/reductive conditions and lack of structural flexibility in
the ligand skeletons; therefore, the fine-tuning of the cata-
lytic performances of the chiral species through remote
backbone functionalization would be preferable.[3] In this
context, Gilheany and Buntꢁs results are noteworthy: they
reported elegantly on the possibility of modulating the per-
formances of Cr–salen and Pd–PHOX (asymmetric epoxida-
tion of alkenes, and asymmetric allylic alkylation, respec-
tively), simply by choosing the substitution pattern of the
arene rings in the chiral ligands appropriately.[4]
While the design of new stereodiscriminating systems has
frequently dealt with biological stereoselective transforma-
tions (that is, enzyme-based cascade sequences),[5,6] itis in-
teresting that the interchange of know-how between organo-
metallic asymmetric catalysis and hybrid organic–inorganic
materials disciplines[7] remains poorly explored.
Theoretically, the p-conjugate system of oligoaryl com-
pounds, which is fundamental for several modern electro-op-
tical devices, guarantees an ideal pattern throughout the
structural backbone and the coordinated metal.[8] In this
field, oligothiophene molecular motifs are of pivotal impor-
tance,[9] because of their ready synthetic accessibility and un-
usual coordinating features that allow transition metals to
Asymmetric catalysis has blossomed over the last six deca-
des.[1] Tremendous progress has been made toward the de-
velopment of new catalytic protocols for the facile synthesis
of ever more challenging stereochemically defined, enantio-
pure compounds (EPCs) in pharmacology, agrochemicals,
and materials science.[2] In this context, the search for new,
effective, chiral organic molecules to control the stereo-
chemical outcomes of organic transformations is a long-
sought goal of numerous synthetic chemists. Generally, the
overall performances of chiral organometallic catalysts are
the result of reciprocal and synergic influences between the
catalaphoric (metal center) and chiraphoric (ligand) units.
Among these, the nature of coordinating atoms present in
the ligand framework affects the catalytic activity of the
metal center significantly. Variations in the electronic fea-
[a] Prof. Dr. V. G. Albano, Dr. M. Bandini, Prof. Dr. M. Monari,
Dr. F. Piccinelli, Dr. S. Tommasi, Prof. Dr. A. Umani-Ronchi
Dipartimento di Chimica “G. Ciamician”
Università di Bologna
Via Selmi 2, 40126 Bologna (Italy)
Fax : (+39)512-099-456
E-mail: marco.bandini@unibo.it
achille.umanironchi@.unibo.it
[b] Dr. G. Barbarella, Dr. M. Melucci
CNR-ISOF
Via Gobetti 101, 40129 Bologna (Italy)
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Asymmetric Catalysis
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couple conveniently to the organic backbone with conse-
quent modulation of their overall electro-optical proper-
ties.[10] Of particular relevance is the affinity of functional-
ized thienyls and oligothienyls for PdII salts; this has been
widely exploited for the synthesis of inner-sphere architec-
tural-type hybrid organic–inorganic materials.[11] All of these
considerations make chiral oligothiophene structural motifs
potentially useful also for the design of fine-tunable stereo-
selective organometallic catalysts (Figure 1).
Scheme 1. Synthesis of DATs 3a, b: i) (R,R)-1 (1.0 equiv), 2 (2.0 equiv),
MgSO4 (10.0 equiv), CH2Cl2, RT, 32 h; ii) NaBH4 (5.0 equiv), MeOH, RT,
16 h.
in late-transition metal chemistry, namely asymmetric allylic
alkylation. Although extensive literature already addresses
this transformation with aromatic hindered substrates
(4a,b),[12,15] the search for broadly effective chiral ligands on
more challenging aliphatic unhindered allylic substrates is
still ongoing.
With model malonates 5a and b (Table 1), excellentlevels
of chemical (ꢀ99%) as well as optical (ee up to 99%)
yields were obtained with 3b in the presence of aromatic
Figure 1. Chiral oligothiophene catalysts for asymmetric catalysis inspired
by materials science.
Table 1. DAT ligands in the Pd-catalyzed allylic substitution with carbon-
ate 4: proving the role of the thienyl rings.[a]
We have reported recently the synthesis, chemo-optical
characterization, and catalytic properties of a new family of
chiral ligands containing oligothienyl systems (diamino-oli-
gothiophenes, DATs) that proved effective in the stereose-
lective Pd-catalyzed asymmetric allylic alkylation (AAA)[12]
of allylcarbonates.[13]
The presence of a sizable intramolecular interaction be-
tween the apparently spectator ancillary inner thienyl rings
through the sulfur atom and the metal center suggests inter-
esting guidelines for the design, prediction, and fine-tuning
of the catalytic activity of chiral organometallic species
simply by conveniently engineering the electronic features
of the thienyl side arm; we call this approach molecular
remote control (MRC).
We do not intend to add this class of ligands to the al-
ready rich collection of valuable auxiliaries for this transfor-
mation. On the contrary, we wish to highlight and to prove
the remarkable effect of their intramolecular “weak/secon-
dary” interactions on the stereochemical outcome of asym-
metric catalytic processes.
[c]
Entry
L
Conditions[b]
ProductYield [%]
ee [%][d]
1
2
3
4
5
6
3a
3b
3b
3b
3b
3b
A
A
A
A
B
6aa
6aa
6ab
6ba
6ca
6ca
65
45 (S)
99 (S)
>98
95 (R)
80 (S)
90 (S)[e]
96
80
99
98
40
B
[a] All the reactions were carried out under a nitrogen atmosphere at RT
(16 h). [b] Conditions: A) [Pd2(dba)3]·CHCl3, BSA, KOAc; B) [Pd-
(allyl)Cl]2/Cs2CO3/AgSbF6. [c] After flash chromatography. [d] Deter-
1
mined by chiral HPLC for 6aa and 6ba, chiral GC for 6ca, and H NMR
with [Eu(hfc)3] for 6ab. The absolute configuration was assigned by com-
parison with the known optical rotation value. [e] Reaction time 4 d at
08C.
Results and Discussion
carbonates 4a and 4b. Remarkably high also was the enan-
tiodiscrimination achieved with 3b in the presence of the di-
methyl methylmalonate 5b, which afforded (R)-6ab in 80%
yield as a single enantiomer (ee>98% by chiral shift
1H NMR, entry 3, Table 1). Then we tested the effectiveness
of our ligand of choice, 3b, with the more challenging dime-
thylallyl carbonate 4c. After a brief survey of experimental
conditions, both yield and enantiomeric excess in (S)-6ca
could be increased, to 98 and 80%, respectively, using [Pd-
(h3-C3H5)Cl]2/AgSbF6/3b (0.05:0.1:0.1, entry 5) as the cata-
lytic system. Again, to improve the enantioselectivity in 6ca
In the course of our continuing efforts to develop new chiral
organometallic catalysts for enantiodiscriminating transfor-
mations, we reported recently on the synthesis of a family of
chiral C2-symmetric multidentate diamino-oligothiophenes
(DATs, 3a,b).[13,14] DATs were readily obtainable through re-
ductive amination of commercially available (R,R)-1,2-cy-
clohexanediamine (1) by the corresponding thienyl (T) alde-
hydes 2a,b (Scheme 1). Then DATs 3a,b were successfully
employed in a bench-test reaction for new chiral auxiliaries
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A. Umani-Ronchi, M. Bandini etal.
further, the influence of the temperature was investigated.
In particular, when the model reaction was run at 08C, al-
though the reaction rate dropped (yield 40%, entry 6), the
enantiomeric excess reached 90%.
Crystallographic evidence on [Pd(h3-C3H5)(3b)]+ provid-
ed insight into the role of thiophene rings in the stereo-
chemical outcome of the process. In particular, one inner
thiophene was found to interact attractively with the transi-
tion-metal center [S2···Pd 3.34(1) Š].[13] On the basis of the
experimental findings previously reported,[13] we hypothe-
sized the presence of such a hemilabile contact[16] in solution
also, dynamically bringing the oligothienyl side arm closer
to the catalaphoric unit with increased overall stereodiffer-
entiation. It is noteworthy that AAA processes have already
proven to be strongly affected in solution, even by “weak/
secondary” intramolecular contacts between side arms of
the ligand and metal center.[17]
We thought that the well-known ability of oligothiophenes
to shuttle charges across their organic backbone would offer
the opportunity of fine-tuning the electronic (coordinating)
properties of the inner sulfur atom and the catalytic activity
of the corresponding Pd complex, simply by modifying the
ligand skeleton even in positions remote from the metal-
bonding heteroatoms (that is, by molecular remote control,
MRC).[18]
Figure 2. a) UV/Vis spectra for 3b, c–f recorded in CH2Cl2. b) Conver-
sion profiles in the Pd-catalyzed AAA between 4a and 5a for variously
substituted DATs.
In this context, the easily accomplishable electronic alter-
ation of oligothienyls, by grafting electron-releasing or -with-
drawing substituents, would make MRC a potentially useful
and versatile tool for fine-tuning of the catalyst for several
chiral organometallic promoted reactions.
To properly control and assess the role of the electronic
interactions in modulating the catalytic activity of the metal
center, we synthesized a series of 5’’-functionalized, enantio-
merically pure T2 ligands (3c–f) in moderate to good overall
yields (31–55%), starting from the corresponding aldehydes
2c–f (Scheme 2).
To verify the overall changes of electronic distribution in
these ligands, we analyzed and compared UV/Vis spectra of
ligands 3b–f (Figure 2a). Here, the p–p bands in the ligand
exhibited a red-shift absorption that increased progressively
along with the electron-donating group (EDG) strength (labs
(CH2Cl2): 3c 411(271) nm, 3b 312 nm, 3d 317 nm, 3e
326 nm, 3 f 371 nm) due to the well-known ability of elec-
tron-releasing groups to increase the HOMO level of p-con-
jugate systems. Of particular relevance is the maximum ab-
sorbance at411 nm shown by T NO2 (3c) that can be associ-
2
ated with a charge-transfer absorption band.[19] In addition
to this absorbance (typical for molecules that behave as
push–pull systems),[20] 3c showed also a weak band at
271 nm that may be correlated with the intense bands re-
corded for 3b,d–f spanning 312–371 nm.
The end-capped ligands (R,R)-3c–f were then tested
under the optimized conditions in the model reaction (4a +
5a). The results are summarized in Table 2: apart from 3c,
containing the nitro group (ee 60%, entry 1), all the ligands
furnished 6aa with comparable high enantioselectivity (97–
99%). However, markedly different reaction rate profiles
were observed over the range of ligands (Figure 2b). Here, a
good correlation between reaction rate and substitution pat-
tern of the bithienyl pendants was observed, with a rate in-
creasing progressively with the electron-donating character
of the thiophene substituent (NR>OMe>Me>H>NO2).
In particular, the initial reaction rate with T2NR (3 f, R =
-(CH2)4-), bearing the strongest EDG, was estimated to be
approximately 1.6, 2.1, 2.6, and 3.9 times that of T2MeO
(3e), T2Me (3d), T2H (3b), and T2NO2 (3c), respectively.
These results are fully in agreement with Zheng and co-
workersꢁ recent study on the dependence of AAA outcomes
on the electronic features of the P,N ligand.[21]
This distinctive behavior was even more evident for the
less sterically hindered carbonate 4c (Table 2). Indeed,
when DATs bearing ever-increasing electron-donating end-
caps were used, both yield and enantiomeric excess of 6ca
increased, with yields in the range 20–50% and ee values of
68–92%.[22] Besides the marked increase in chemical yield
Scheme 2. i) (R,R)-1 (1.0 equiv),
2 (2.0 equiv), MgSO4 (10.0 equiv),
CH2Cl2, RT, 48 h; ii) [H]2c: NaBH3CN (3.0 equiv), HCldry in Et2O
(2.0 equiv), THF, 08C!RT, 16 h; [H]2d–f: NaBH4 (5.0 equiv), MeOH,
RT, 16 h.
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Table 2. Application of functionalized DATs 3 in AAA.
Entry
3
6aa
Yield [%][a]
6ca
Yield [%][a]
ee [%][b]
ee [%][c]
1
2
3
4
5
3c
3b
3e
3 f
3g
75
96
98
98
98
60 (S)
99 (S)
99 (S)
99 (S)
98 (S)
20
40
42
68 (S)
90 (S)
91 (S)
92 (S)
–
50
[d]
–
[a] After flash chromatography Reaction conditions: (6aa) [Pd2-
(dba3)]·CHCl3, BSA, KOAc; (6ca) [Pd(allyl)Cl]2/AgSbF6/Cs2CO3].
[b] Determined by chiral HPLC. The absolute configuration was assigned
by comparison with the known optical rotation value. [c] Determined by
chiral GC. The absolute configuration was assigned by comparison with
the known optical rotation value. [d] See ref. [22].
Figure 3. Molecular structure of the cation [Pd(h3-C3H5)(3d)]+ as the
BF4 salt, determined by X-ray diffraction. Selected bond lengths (Š):
À
À
À
À
À
Pd1 C27 2.109(9) Pd1 C28 2.10(1), Pd1 C29 2.144(7), Pd1 N1 2.117(5),
À
À
À
Pd1 N2 2.144(4), C27 C28 1.35(2), C28 C29 1.35(2), Pd···S2 3.293(2).
for 6ca, our attention was also attracted by the narrow
trend in the relative ee values, which can be rationalized in
terms of the peculiar electronic as well as steric properties
of these organometallic catalysts. Here, the presence of elec-
tron-releasing groups on the pendant oligothienyls, by favor-
ing the attractive character of the thienyl–metal interaction,
should shorten the S–Pd contact, with consequent higher
steric congestion in the proximity of the metal center. To
support this assumption, 3d was treated with [Pd(h3-
C3H5)Cl]2/AgBF4 to give [Pd(h3-C3H5)(3d)]BF4 in a straight-
forward manner (95% yield).
this result represents one of the highest levels of stereoin-
duction obtained so far with unhindered aliphatic acyclic
substrate 4c.[25]
To gain more insight into this unusual behavior we
needed to shed light on the reaction mechanism. For this
purpose, the pre-catalytic species [Pd(h3-1,3-Ph2C3H3)-
(3b)]BF4 was also synthesized in quantitative yield (95%)
as an air-stable complex, starting from 3b and [Pd(h3-1,3-
Ph2C3H3)Cl]2. In t hesyn,syn isomer molecule (Figure 4), the
central carbon of the allyl moiety points upward (pseudo-
exo type) and no disorder was detected. We attempted to
identify the actual species present in solution by liquid
1H NMR analysis of the [Pd(h3-1,3-Ph2C3H3)(3b)]BF4 over a
range of temperatures and with different solvents. In the
The overall conformation of the [Pd(h3-C3H5)(3d)]+
cation is similar to that of [Pd(h3-C3H5)(3b)]+, exhibiting
opposite absolute configurations of the coordinated nitrogen
atoms [(S)-N1; (R)-N2] and l conformation of the five-
membered palladacycle (Figure 3). It is noteworthy that the
contact between S2 and Pd is significantly shorter than that
in [Pd(h3-C3H5)(3b)]+ [(3.293(2) Š against3.340(1) Š)],
fully supporting our hypothesis regarding the influence of
remote substituents on the catalyst activity.
The remarkable connections between chemical outcomes
and electronic features of the ligand were further highlight-
ed by synthesizing the diamino
1
collected H NMR spectra (RT, CD2Cl2) coalescence of the
signals of the allyl protons (anti) was observed. However, at
À208C the spectrum showed two sets of signals that have
been assigned to the syn,syn-pseudo-exo and syn,syn-
pseudo-endo isomers (1.6:1) by the allyl J(H,H) coupling
constants.[26] When the temperature was still lower (À508C)
compound 3g, in which the
inner thienyl rings were re-
placed by the strongly electron-
donating 3,4-ethylenedioxythio-
phene (EDOT, Scheme 3).[23]
Our choice was dictated by the
distinctive effects of EDOT
units embedded in oligoaryl
systems. These structural motifs
generally lead to pronounced
flattening of p-conjugate sys-
tems, with consequent superior
electron-releasing properties of
the hetero-substituted thienyl
rings.[24] Interestingly, under op-
timal reaction conditions, 3g al-
Scheme 3. Synthesis of AEDOT-T (3g) and its application in the AAA between 4c and 5a: i) EDOT
(1 equiv), nBuLi (2.5m, 1 equiv), À788C!08C (over 20 min) ! À788C, then DMF (1.9 equiv), RT, 1 h;
ii) EDOT-CHO (1 equiv), N-bromosuccinimide (1 equiv), CH2Cl2/AcOH 1:1, 08C, 2 h (in the dark) 95%. 2g:
[PdCl2(PPh3)2] (3 mol%), Na2CO3 (5 equiv), DME/EtOH 1:1, 508C, 16 h. 3g: i) (R,R)-1 (1.0 equiv), 2g
lowed (S)-6ca to be isolated
with a remarkably higher yield
(73%) and excellent ee (94%).
To the best of our knowledge, (2.0 equiv), MgSO4 (10.0 equiv), toluene, 808C, 48 h; ii) NaBH4 (5.0 equiv), MeOH, RT, 16 h, 45% (two steps).
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A. Umani-Ronchi, M. Bandini etal.
Scheme 4. Rationale of the S absolute configuration obtained on the
basis of the preferential rotation (PR) model.
ment almost parallel to the 1,3-diphenylallyl unit (Figure 4).
If nucleophilic attack is postulated to occur on C1, the coun-
À
terclockwise rotation will operate in order to orient the C
C double bond properly with respect to the palladium
center. Contrarily, a clockwise rotation should be considered
for a hypothetical nucleophilic attack on C3. While strong
steric interaction of the pendant thiophene groups with the
allyl unit will prevent the clockwise movement, nucleophilic
attack on C1, producing a less sterically hindered rotation of
the Pd-coordinated product, will result as the favored ste-
reochemical pathway.
Figure 4. Molecular structure of the cation [Pd(h3-1,3-Ph2C3H3)(3b)]+ as
À
the BF4 salt, determined by X-ray diffraction. Selected bond lengths
À
À
À
À
(Š): N1 Pd 2.14(1), N2 Pd 2.15(1), Pd C25 2.21(1), Pd C26 2.11(2),
À
Pd C27 2.19(1), Pd···S1 3.443(5).
a shiftof ht e pseudo- exo–endo equilibrium toward the ther-
modynamic pseudo-exo species was recorded (2.8:1). As ex-
pected, the solvent drastically affected the Pd–allyl species
exchange. Yet attempts to reveal the presence of minor iso-
mers in the reaction solvent ([D8]THF) failed. In fact, over
the temperature range studied (25!À908C), only one setof
NMR signals was observed. This finding can be ascribed to
the presence in solution of a single isomer (pseudo-exo) or
to a rapid exchange between the two isomeric Pd complexes
on the NMR time-scale.
The close contact between the thienyl sulfur and the pal-
ladium center is still present in the [Pd(h3-1,3-Ph2C3H3)-
(3b)]+ crystal structure [(S2···Pd 3.44(1) Š)], with the bi-
thiophene ancillary arm occupying a pseudo-axial position.
The thienyl-containing pendant that does not interact inti-
mately with Pd adopts an axial orientation in order to mini-
mize the steric congestion with the more encumbering
phenyl groups. In contrast to the crystal structures in
Figure 3, both nitrogen atoms of the ligand adopt an S con-
figuration. The molecular geometry shows that the palladi-
um is almost symmetrically bonded to the allyl termini:
On the basis of these considerations, the observed overall
increase in reaction rate with higher electron density should
be rationalized in terms of electronic [Pd0-k] stabilization.
As sulfur atoms have a well-known tendency to interact
strongly with soft metals,[30] we may speculate that the ob-
served thiophene–metal interaction could contribute to sta-
bilization of the Pd0 intermediate complexes. In fact, the in-
creased electron population of the sulfur atoms caused by
introducing EDGs will contribute to the overall stabilization
of the entire complex as well as that of the reaction transi-
tion state. In support of this last statement, secondary inter-
actions between coordinating functional groups and Pd0 spe-
cies (the proximity effect) have been reported recently to be
crucial in the transition state stabilization of Pd-catalyzed
cross-coupling reactions.[31]
Conclusion
À
À
[Pd C25 2.21(1) Š, Pd C27 2.19(1) Š], with consequently
similar electronic surroundings for C25 and C27. This evi-
dence tends to exclude the operation of an early transition
state[27] during the nucleophilic attack and, as already dis-
cussed elsewhere, the preferential rotation (PR) model
should be applied in order to justify the overall stereoinduc-
tion of the chiral Pd catalyst.[28] Therefore, a rationale for
the absolute stereochemistry emerges from comparison of
the energies of the corresponding [Pd0-k] species represent-
ed schematically in Scheme 4,[29] in which the differently ori-
ented pendant thiophene groups are indicated by black and
gray spots. In particular, while the bithiophene interacting
with the Pd (black spot) points above the organic frame-
work, the second pendant (gray spot) has a spatial arrange-
In this work we have described the synthesis, characteriza-
tion, and application in AAA of a new class of chiral oligo-
thienyl ligands. The high efficiency of intramolecular deloc-
alization of their p-electrons calls for possible fine-tuning of
the catalytic properties through “remote control” groups. In
particular, the unusual trends in the chemical and stereo-
chemical outcomes recorded for the probe reaction suggest
the presence in solution also of the key attractive intramo-
lecular contact recorded in the solid state.
In view of the simplicity of the ligand–catalyst assembly,
the employment of the present “molecular remote control”
model for the development of new catalytic stereoselective
transformations appears to be promising and is currently
under investigation.
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Asymmetric Catalysis
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(d, J = 14.8 Hz, 2H), 2.37–2.40 (m, 2H), 1.50–1.82 (br, 4H), 1.10–1.35
(br, 4H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d = 24.7, 31.5, 45.3,
60.7, 121.7, 125.5, 126.3 (2C), 129.7, 133.5, 145.7, 149.3; IR (Nujol): n˜ =
3286(w), 3104(w), 2926(s), 2849(s), 2351(w), 1604(w), 1515(m), 1493(m),
Experimental Section
General methods: All reactions were performed in a dry nitrogen atmos-
phere using standard Schlenk techniques. Anhydrous solvents and re-
agents were purchased from Fluka and Aldrich, and used as received. We
1465(s), 1327(s), 1249(m), 1094(s), 795(s) cmÀ1; UV/Vis (CH2Cl2): labs
=
271, 411 nm; elemental analysis calcd (%) for C24H24N4O4S4 (560.73): C
54.41, H 4.31, N 9.99; found: C 54.45, H 4.33, N 10.00.
recorded 1H NMR and 13C NMR spectra on
a Gemini Varian 200
(200 MHz) or Gemini Inova 300 (300 MHz) spectrometer. GC-MS spec-
tra were recorded with a Hewlett-Packard 5971 instrument. LC-electro-
spray ionization mass spectra were obtained with an Agilent Technolo-
gies MSD1100 mass spectrometer. IR analysis was performed with an
FT-IR Nicolet 205, and HPLC with a Hewlett-Packard HP1100 equipped
with a chiral column (Chiralcel OD/AD). Analytical GC was performed
on a Hewlett-Packard HP6890 gas chromatograph using a chiral Mega-
dex-5 (25 m) column. Melting points were determined with a Büchi 150
melting point unit and are not corrected. The UV/Vis spectrophotometer
was a Perkin-Elmer model 554.
Compound 3d–f: NaBH4 (5 mmol) was added in portions to a cooled so-
lution (08C) of diimine (1 mmol) in MeOH (5 mL), and the mixture was
stirred at RT until the diimine had disappeared completely (checked by
1H NMR). Then water (10 mL) was added, MeOH was evaporated, and
the product was extracted with CH2Cl2 (3”4 mL). Evaporation of the
volatiles afforded crude 3, which was purified by consecutive washings or
flash chromatography.
(R,R)-T2Me (3d): Purified by flash chromatography: pale yellow solid;
Rf = 0.30 (CH2Cl2/MeOH 99:1); yield 45% (two steps); m.p. 77–788C;
[a]D
=
+8.1 (c = 0.42 in CHCl3); 1H NMR (200 MHz, CDCl3, 258C,
Synthesis of 2c–g
TMS): d = 6.90 (t, J = 3.6 Hz, 4H), 6.75 (d, J = 3.6 Hz, 2H), 6.61 (d, J
= 3.6 Hz, 2H), 4.08 (d, J = 14.2 Hz, 2H), 3.86 (d, J = 14.2 Hz, 2 Hz),
2.47 (s, 6H), 3.32–2.37 (m, 2H), 1.55–1.88 (m, 4H), 1.12–1.38 (m, 4H);
13C NMR (75 MHz, CDCl3, 258C, TMS): d = 143.1, 138.6, 136.7, 135.4,
Compound 2c: Prepared by Suzuki coupling ([PdCl2(PPh3)2]/Na2CO3(aq)
/
dimethoxyethane/EtOH/508C, 95% yield) between commercially avail-
able 5-nitro-2-bromothiophene and 5-formyl-2-thiopheneboronic acid.
Purified by flash chromatography (c-hex/AcOEt7:3 !6:4): yellow-orange
solid; m.p. 139–1418C; 1H NMR (200 MHz, CDCl3, 258C, TMS): d =
9.94 (s, 1H), 7.90 (d, J = 4.4 Hz, 1H), 7.75 (d, J = 4.0 Hz, 1H), 7.43 (d,
J = 4.0 Hz, 1H), 7.28 (d, J = 4.4 Hz, 1H); 13C NMR (50 MHz, CDCl3,
258C, TMS): d = 182.5, 151.9, 147.0, 146.1, 144.4, 136.8, 129.5, 126.8,
126.8, 125.2, 123.2, 122.4, 60.1, 45.4, 31.2, 24.9, 15.3; IR (Nujol): n˜
=
3280(w), 3000(w), 2921(s), 2853(s), 1695(w), 1462(m), 1376(m), 1262(w),
1097(m), 1022(m), 789(m) cmÀ1; UV/Vis (CH2Cl2): labs = 317 nm; ele-
mental analysis calcd (%) for C26H30N2S4 (498.79): C 62.61, H 6.06, N
5.62; found: C 62.58, H 6.01, N 5.60.
124.7; IR (Nujol): n˜
= 2926(s), 2853(m), 1732(s), 1660(m), 1461(s),
1335(w), 1209(m) cmÀ1; UV/Vis (MeOH): labs = 340 nm; MS (70 eV):
m/z (%): 239 (100), 209 (10), 149 (50), 121 (20).[32]
(R,R)-T2MeO (3e): Used without further purification; pale beige solid;
yield 49% (two steps); m.p. 76–778C; [a]D
= +15.0 (c = 1.22 in
CHCl3); 1H NMR (200 MHz, CDCl3, 258C, TMS): d = 6.78–6.83 (m,
4H), 6.72 (d, J = 3.6 Hz, 2H), 6.07 (d, J = 4.2 Hz, 2H), 4.09 (d, J =
14.2 Hz, 2H), 3.90 (s, 6H), 3.85 (d, J = 14.2 Hz, 2H), 2.33–2.39 (m, 2H),
1.65–1.88 (br, 4H), 1.20–1.56 (br, 4H); 13C NMR (50 MHz, CDCl3, 258C,
TMS): d = 165.1, 142.4, 136.7, 124.8, 124.2, 121.6, 120.8, 104.3, 60.3, 60.2,
Compound 2d: Prepared by Suzuki coupling ([Pd(PPh3)4]/K2CO3/tolu-
ene/MeOH/reflux) of commercially available 5-methyl-2-thiophenebor-
onic acid and 5-bromothiophenecarboxaldehyde.[32]
Compound 2e: Prepared by Stille coupling ([Pd(PPh3)4]/THF/reflux,
85% yield) of 5-methoxy-2-tributyltinthiophene[33] and 5-bromothiophe-
necarboxaldehyde.[34]
45.7, 31.5, 24.9; IR (Nujol): n˜
= 3291(w), 2921(s), 2846(s), 2362(w),
1572(w), 1540(m), 1508(m), 1456(s), 1386(m), 1196(w), 1062(w),
793(w) cmÀ1; UV/Vis (CH2Cl2): labs = 326 nm; elemental analysis calcd
(%) for C26H30N2O2S4 (530.79): C 58.83, H 5.70, N 5.28; found: C 58.79,
H 5.65, N 5.26.
Compound 2 f: Prepared by Stille coupling ([Pd(PPh3)4]/toluene/reflux,
70% yield) of 5-pyrrolidine-2-tributyltinthiophene and 5-bromothiophe-
necarboxaldehyde.
Compound 2g: Prepared by Suzuki coupling ([PdCl2(PPh3)2]/Na2CO3/
DME/EtOH/508C, 70% yield). Purified by flash chromatography: white
Compound 3 f, 3g: The imine precursors were synthesized as described
in the typical procedure by reaction in toluene at 808C (24 h).
solid; Rf
=
0.33 (c-hex/AcOEt8:2); m.p. 135–137 8C; 1H NMR
(R,R)-T2NR (3 f): Purified by flash chromatography: pale brown solid;
Rf = 0.31 (CH2Cl2/MeOH 98.5:1.5); yield 31% (two steps); m.p. 80–
(200 MHz, CDCl3, 258C, TMS) d = 9.92 (s, 1H), 7.44–7.46 (m, 1H), 7.39
(d, J = 1.6 Hz, 1H), 7.09 (t, J = 4.2 Hz, 1H), 4.44 (s, 4H); 13C NMR
828C; [a]D
258C, TMS): d = 6.83 (dd, J = 2.4, 4.2 Hz, 2H), 6.72–6.84 (m, 4H), 5.61
(dd, J 2.1, 4.2 Hz, 2H), 4.09 (d, J 14.1 Hz, 2H), 3.84 (d, J =
=
+56.6 (c = 0.5 in CHCl3); 1H NMR (300 MHz, CDCl3,
(50 MHz, CDCl3, 258C, TMS): d
= 179.3, 148.6, 136.8, 133.3, 127.5,
126.8, 125.8, 114.7, 97.1, 65.2, 64.7; IR (Nujol): n˜ = 3002(m), 1648(s),
1.520(m), 1488(m), 1463(s), 1373(m), 1271(m) cmÀ1; UV/Vis (MeOH):
labs = 370 nm; MS (70 eV): m/z (%): 252 (68), 156 (18), 127 (100), 111
(33), 69 (55).
=
=
14.1 Hz, 2H), 3.26–3.29 (m, 8H), 2.32–2.35 (m, 2H), 2.18–2.20 (m, 4H),
2.02–2.12 (m, 8H), 1.72–1.75 (2H), 1.21–1.25 (m, 2H); 13C NMR
(75 MHz, CDCl3, 258C, TMS): d = 154.9, 141.2, 138.3, 125.3 (2C), 123.9,
(1R,2R)-N,N’-Bis(thienylmethyl)cyclohexane-1,2-diamines (3)
120.7, 120.0, 100.5, 66.1, 60.4, 51.0, 45.8, 26.0, 25.2; IR (neat): n˜
3283(w), 3058(w), 2936(m), 2851(m), 1558(w), 1535(s), 1505(s), 1481(s),
1456(m), 1352(m), 1054(m), 739(m) cmÀ1
UV/Vis (CH2Cl2): labs
371 nm; elemental analysis calcd (%) for C32H40N4S4 (608.21): C 63.12, H
6.62, N 9.20; found: C 63.17, H 6.57, N, 9.19.
=
Typical procedure: The desired aldehyde 2 (2 mmol) and (1R,2R)-diami-
nocyclohexane 1 (1 mmol) were dissolved in dry CH2Cl2 (10 mL) under
an N2 atmosphere in a two-necked flask (50 mL). MgSO4 (5 mmol) was
added and the mixture was stirred at RT for 24–48 h (checked by
1H NMR). The solution was filtered on Celite and the solvent was re-
moved under vacuum. The crude diimine was diluted with cyclohexane
and stirred overnight. The purified insoluble product was collected by fil-
tration.
;
=
(R,R)-EDOT-T (3g): Purified by flash chromatography: white solid; Rf
= 0.32 (CH2Cl2/MeOH 9:1); yield 45% (two steps); m.p. 50–528C; [a]D
1
= +40.0 (c = 0.38 in CHCl3); H NMR (300 MHz, CDCl3, 258C, TMS):
d = 7.13–7.16 (m, 2H), 6.95–6.98 (m, 2H), 4.27–4.29 (m, 4H), 4.19–4.22
(m, 4H), 3.95 (d, J = 15.2 Hz, 2H), 3.73 (d, J = 15.2 Hz, 2H), 2.28–2.31
(m, 2H), 2.15 (pseudod, J = 12.9 Hz, 2H), 2.00 (br, 4H), 1.72–1.75 (m,
2H); 13C NMR (75 MHz, CDCl3, 258C, TMS): d = 138.2, 137.1, 135.0,
127.0, 123.2, 122.4, 114.5, 109.7, 65.0, 64.6, 60.2, 41.2, 31.3, 24.9; IR
(R,R)-T2NO2 (3c): HCl (2 mmol, 1m in Et2O) was added to a cooled so-
lution (08C) of diimine (1 mmol) in THF (5 mL). The resulting orange
slurry was stirred for 10 min, then NaBH3CN (3 mmol) was added. The
clear solution was allowed to warm at RT and stirred overnight. Then
water (10 mL) was added, MeOH was evaporated, and the product was
extracted with CH2Cl2 (3”4 mL). Evaporation of the volatiles afforded
crude 3c, which was purified by flash chromatography: deep brown solid;
Rf = 0.33 (CH2Cl2/MeOH 99:1); yield 55% (two steps); m.p. 119–1218C;
[a]D = À9.3 (c = 0.95 in CHCl3); 1H NMR (200 MHz, CDCl3, 258C,
TMS): d = 7.78 (d, J = 5.6 Hz, 2H), 7.21 (d, J = 3.6 Hz, 2H), 7.00 (d, J
= 3.6 Hz, 2H), 6.35 (d, J = 3.4 Hz, 2H), 4.15 (d, J = 14.8 Hz, 2H), 3.94
(neat):
n = 3424(s), 2924(s), 2846(m), 1643(m), 1534(m), 1436(s),
1362(m), 1088(s) cmÀ1; LC-ESI-MS: m/z: 587 [M+H]+; elemental analy-
sis calcd (%) for C28H30N2O4S4 (586.11): C 57.31, H 5.15, N 4.77; found:
C 57.28, H 5.12, N 4.76.
Pd-catalyzed AAA (4c/5a): A two-necked flask (25 mL) was charged,
under
a
nitrogen atmosphere, with [Pd(allyl)Cl]2 (2.7 mg, 7.5”
10À3 mmol), diamine 3g (8.8 mg, 0.015 mmol), and anhydrous THF
Chem. Eur. J. 2006, 12, 667 – 675
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
673

A. Umani-Ronchi, M. Bandini etal.
(500 mL). While the mixture was being stirred at room temperature for
15 min, a white solid was formed. The subsequent addition of AgSbF6
(0.015 mmol, 5 mg) dissolved the white solid and the solution turned
brown. After a few minutesꢁ stirring (with formation of solid AgCl), 4c
(24 mL, 0.15 mmol) was added, the reaction flask was cooled at 08C and
dimethyl malonate (0.75 mmol, 86 mL) and Cs2CO3 (0.3 mmol, 99 mg)
were added sequentially. The reaction was stirred for four days, then
quenched with a saturated solution of NaHCO3 (3 mL). The two phases
separated and the aqueous phase was extracted with AcOEt (3”5 mL).
The organic layers were collected, dried over Na2SO4, then concentrated.
The desired product, (S)-6ca, was isolated as a yellow oil in 73% yield
(22 mg) after flash chromatography (c-hex/AcOEt95:5). The ee of the
product (94%) was determined by GC with a chiral column (flow rate
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[16] Phenomena of hemilability are frequently invoked in the rationali-
zation of numerous organometallic catalyses. For a recent review re-
garding P/P(O) ligands, see: V. V. Grushin, Chem. Rev. 2003, 103,
1629–1662.
15 mLminÀ1; 508C for 6 min, ramp @ 18CminÀ1 to 1808C); tR(S)
=
42.7 min; tR(R) = 43.0 min; [a]D = À15.0 (c = 0.34 in CHCl3), lit. (S)-
6ca [a]D = À27.9 (c = 1.1 in CHCl3).[36]
[Pd(h3-C3H5)(3d)]BF4: In a Schlenk flask, flamed and flushed with nitro-
gen, 3d (30 mg, 0.06 mmol) was dissolved in a THF/CH2Cl2 mixture (6:1,
3.5 mL). Then [Pd(h3-C3H5)(m-Cl)]2 (11 mg, 0.03 mmol) was added and
the mixture was stirred at room temperature for 1 h. A sample of AgBF4
(12 mg, 0.06 mmol) was added and the mixture was stirred in the dark
for 24 h and then filtered off. The solvent was removed under reduced
pressure. The productwas dried under vacuum. The residue was washed
with Et2O (5 mL) and the desired Pd complex was recovered as a yellow
solid by filtration (42 mg, 87%). 1H NMR (CD3CN, 300 MHz, 258C,
TMS) appeared fluxional. IR (KBr): n˜ = 3250, 3100, 2928, 2859, 1610,
1461, 830, 802, 707 cmÀ1. Crystals for structure determination were ob-
tained by diffusion of n-hexane into a CH2Cl2 solution of [Pd(h3-C3H5)-
(3d)]BF4. XRD details: Bruker APEX II CCD diffractometer (MoKa ra-
diation, l 0.71073 Š), empirical absorption correction, anisotropic full-
matrix least-squares on F2. Results: C29H35BF4N2PdS4, Mr
monoclinic, space group C2, 19.549(6), 9.652(2),
18.331(4) Š, 92.146(5)8, 4, 1x
3456.3(15) Š3,
1.409 MgmÀ3, m = 0.821 mmÀ1, F(000) = 1496, T = 296(2) K, qmax
= 733.04,
a
V
=
b
=
c
=
=
=
b
=
=
Z =
28.688, 13138 reflections collected, 6919 with I>2s(I). Final R1 = 0.0627,
wR2 = 0.1829, GOF = 1.031, absolute structure parameter = 0.07(5).
[Pd(h3-1,3-Ph2C3H3)(3b)]BF4: See ref. [13]. Crystals for X-ray diffraction
were obtained by slow diffusion of n-hexane into an acetone solution of
[Pd(h3-1,3-Ph2-C3H3)(3b)]BF4. XRD details: Bruker AXS CCD diffrac-
tometer (MoKa radiation, l 0.71073 Š). Results: C45H51BF4N2O2PdS4·
2(C3H6O), Mr = 973.33, orthorhombic, P212121, a = 10.1154(15), b =
13.758(2),
c
=
33.294(5) Š,
V
=
4633.4(12) Š3,
Z
=
4, 1x
=
1.395 MgmÀ3, m = 0.635 mmÀ1, F(000) = 2008, T = 293(2) K, qmax
=
24.998, 39545 reflections collected, 4425 with I>2s(I). Final R1 = 0.0957,
wR2 = 0.2215, GOF = 1.081, absolute structure parameter = 0.07(9).
Crystallographic data: CCDC 273523 and 273524 contain the supplemen-
tary crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data request/cif/
[17] Intramolecular Pd0–HO interactions were recently reported on sta-
bilizing Pd–BOX catalysts for AAA: K. Hallman, A. Frolander, T.
Wondimagegn, M. Svensson, C. Moberg, Proc. Natl. Acad. Sci. USA
2004, 101, 5400–5404.
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Acknowledgements
[19] J. Casado, T. M. Pappenfus, L. L. Miller, K. R. Mann, E. Ortí, P. M.
Viruela, R. Pou-AmØrigo, V. Hernµndez, J. T. L. Navarrete, J. Am.
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This work was supported by M.I.U.R. (Rome), National Project “Stereo-
selezione in Sintesi Organica: Metodologie and Applicazioni”, FIRB
Project “Progettazione, preparazione e valutazione biologica e farmaco-
logica di nuove molecole organiche quali potenziali farmaci innovativi,”
and by the University of Bologna. We are also indebted to “Graphilandia
Media & Communication” (Taranto, Italy) for the striking artwork for
the frontispiece. PRIN project “Nuove strategie per il controllo delle rea-
zioni: interazione di frammenti molecolari con siti metallici in specie non
convenzionali”. FIRB project: Synthesis of novel organic materials and
supramolecular architectures for high efficiency optoelectronics and pho-
tonic systems.
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[22] Unfortunately, the use of the pyrrolidine-based ligand 3 f in the
AAA with 4c was ruled out by formation, during the course of the
reaction, of a dark brown, highly insoluble solid that removed the
chiral Pd catalyst from the solution. Although a detailed investiga-
tion has not been performed so far, this peculiar behavior of 3 f
674
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Chem. Eur. J. 2006, 12, 667 – 675

Asymmetric Catalysis
FULL PAPER
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Received: July 26, 2005
Published online: November 11, 2005
Chem. Eur. J. 2006, 12, 667 – 675
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
675