Novel chiral diamino-oligothiophenes as valuable ligands in Pd-catalyzed allylic alkylations. On the "primary" role of "secondary" interactions in asymmetric catalysis

Article abstract of DOI:10.1002/adsc.200505109

A new class of chiral C₂-symmetrical diamino-oligothiophenes is described to be effective in catalyzing Pd-mediated asymmetric allylic alkylations in a highly enantioselective manner. The combination of experimental as well as crystallographic

Full text of DOI:10.1002/adsc.200505109

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DOI: 10.1002/adsc.200505109
Novel Chiral Diamino-Oligothiophenes as Valuable Ligands
in Pd-Catalyzed Allylic Alkylations.
On the “Primary” Role of “Secondary” Interactions in
Asymmetric Catalysis
Vincenzo Giulio Albano,a Marco Bandini,a,* Manuela Melucci,b Magda Monari,a
Fabio Piccinelli,a Simona Tommasi,a Achille Umani-Ronchia,*
a
`
Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum – Universita di Bologna, Via Selmi 2, 40126, Bologna,
Italy
Fax: (þ39)-(0)51-209-9456, e-mail: marco.bandini@unibo.it; achille.umanironchi@unibo.it
b
CNR-ISOF, via Gobetti 101, 40129 Bologna, Italy
Received: March 14, 2005; Accepted: June 13, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/
Abstract: A new class of chiral C2-symmetrical di-
amino-oligothiophenes is described to be effective
in catalyzing Pd-mediated asymmetric allylic alkyla-
tions in a highly enantioselective manner. The com-
bination of experimental as well as crystallographic
evidence revealed the key role played by sulfur-
based heteroaromatic rings in the stereodiscriminat-
ing step of the procedure. In particular, unprecedent-
ed non-covalent secondary interactions between the
inner thiophene and the metal center proved to be
essential to create the stereochemical environment
necessary in order to guarantee excellent levels of
chemical and optical yields.
tures in combination with their ready synthetic accessi-
bility render chiral thiophene derivatives potential can-
didates as ligands also in the field of asymmetric
organometallic catalysis. However, only a few examples
of their application have been reported to date.[8]
Herein, we describe our preliminary findings in the
synthesis, structural characterization and application
of a new class of C2-symmetrical diamino-oligothio-
phenes (3, DATs)[9] in the highly stereoselective Pd(0)-
catalyzed asymmetric allylic alkylation (AAA) of malo-
nates with both linear hindered and unhindered sub-
strates.[10] Moreover, the presence of an intramolecular
“secondary/weak” interaction[11] in the catalytic com-
plex between one of the thienyl rings and the palladium
center is discovered to play an active role in order to
guarantee chemo-enzymatic levels of chemical as well
as optical yields in the final product.
Keywords: asymmetric catalysis; crystal structure;
oligothiophenes; secondary interactions; palladium
complexes
A series of new chiral (R,R)-diamino-oligothiophenes
(3) bearing one (T1), two (T2) or three (T3) thienyl units
in each N-pendant, was prepared in high yields under mild
conditions following the strategy depicted in Scheme 1.
Asymmetric organic as well as organometallic catalysis
has blossomed into a mature field over the last dec-
ades.[1,2] Accordingly, the search for new chiral organic
molecules effectively acting in the control of stereo-
chemical outcomes of organic transformations is almost
a daily task for numerous synthetic chemists.
Oligothiophene molecular motifs are pivotal candi-
dates in the molecular material scenario due to their re-
markable electrical and optical properties.[3,4] Taking ad-
vantage of the extremely flexible coordination mode of
the sulfur-containing five-membered rings,[5,6] much ef-
fort has also been devoted to the preparation and char-
acterization of several metallo-oligo- and polythio-
Scheme 1. Reagents and conditions: a) MgSO4, CH2Cl2, room
temperature, 24–48 h; b) NaBH4, MeOH, room temperature
or NaBH3CN/HCl, THF/MeOH, 08C to room temperature
phene hybrid materials.[7] The unique coordinating fea- for 3c.
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In detail, the chiral C2-symmetrical thienyldiamines tively (entries 2, 3, 5). During the preparation of the
3a–c were isolated by treating the commercially availa- manuscript, Kim and co-workers reported on the syn-
ble enantiomerically pure (R,R)-1,2-cyclohexanedi- thesis and application of 3a in the AAA of 1,3-diphen-
amine (1)[12] with the thienylaldehydes 2a–c followed yl-2-propenyl acetate under different reaction condi-
by reduction of the intermediate diimines. Best experi- tions ([Pd]¼[Pd(h3-C3H5)Cl]2), obtaining 6aa in 50%
mental conditions were found in the use of NaBH4 as a GC conversion and 88.5% ee.[15]
reductant for 3a and b (overall yield 88% and 50% re-
spectively) and HCldry/NaBH3CN[13] in the case of 3c one of the highest obtained in Pd-catalyzed AAA proc-
(yield 72%, two steps).
esses by using C2-phosphine-free chiral ligands.[16] Re-
It is worthy to note that this level of stereoinduction is
It is worthy to note that all these diamino ligands 3a–c markably high was also the enantiodiscrimination
can be obtained in relatively large amounts without any reached with 3b in the presence of the more challenging
time-consuming intermediate/final chromatographic dimethyl methylmalonate (5b) which afforded (R)-6ab
purification.
in 80% yield as a single enantiomer (ee>98% by chiral
Due to the well known affinity of functionalized thien- shift 1H NMR, entry 4, Table 1).
yls and oligothienyls for Pd(II),[14] we decided to explore
Although many papers and ligands have been report-
these compounds as chiral ligands in the Pd-catalyzed al- ed describing highly stereoselective AAA of malonates
lylic substitution of carbonate 4 as the probe reaction.
in the presence of hindered allylic compounds such as
The data collected in Table 1 reveal that the diamines 4a,b, only very few of them involved classes of chiral li-
3 are efficient ligands in the [Pd2(dba)3]·CHCl3-cata- gands that displayed poor substrates specificity.[17] Inthis
lyzed AAA of 4a,b with dimethyl malonate (5a). In par- scenario, asymmetric alkylation of the unhindered 1,3-
ticular, the desired (S)-dimethyl 1,3-diphenylallylmalon- dimethylallyl carbonate (4c) constitues a much more
ate (6aa) was isolated in variable yields and enantiomer- probing test in order to evaluate the catalytic activity
ic excess.
of chiral ligands. In the light of these considerations,
To our delight, we recorded the unique activity of A- we tested the effectiveness of our ligand of choice (3b)
T2 (3b) and A-T3 (3c), which afforded 6aa and 6ba in in AAAwith dimethylallyl carbonate (4c). After a brief
quantitative isolated yields and 99%, 95% ee, respec- survey of experimental conditions that involved palladi-
Table 1. Screening of chiral ligands in the Pd-catalyzed allylic substitution with carbonates 4.[a]
Entry
L*
[Pd]
Base
4
5
6
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
3a
3b
3c
3b
3b
3b
3b
3b
3b
3b
[Pd2dba3]·CHCl3
[Pd2dba3]·CHCl3
[Pd2dba3]·CHCl3
[Pd2dba3]·CHCl3
[Pd2dba3]·CHCl3
[Pd2dba3]·CHCl3
[Pd2dba3]·CHCl3
[Pd(p-C3H5)Cl]2
[Pd(p-C3H5)Cl]2
[Pd(p-C3H5)Cl]2
BSA/KOAc
”
”
”
”
”
NaH
NaH
Cs2CO3
Cs2CO3
4a
4a
4a
4a
4b
4c
”
”
”
”
5a
5a
5a
5b
5a
5a
”
”
”
”
6aa
6aa
6aa
6ab
6ba
6ca
6ca
6ca
6ca
6ca
65
96
99
80
99
71
45 (S)
99 (S)
99 (S)
>98[d] (R)
95
10 (S)
36 (S)
77 (S)
80 (S)
90 (S)
20
83[e]
98[f]
45[g]
10
[a]
All the reactions were carried out in anhydrous THF at room temperature, by using a 1: 1 L:Pd ratio (16 h).
Isolated yields after flash chromatography.
Determined by chiral HPLC (6aa, 6ab) and by chiral GC for 6ca. The absolute configuration was assigned by comparison
[b]
[c]
with the known optical rotation value.
Determined by H NMR with Eu(hfc)3.
Reaction time 36 h.
Reaction time 36 h in the presence of AgSbF6 (10 mol %).
Reaction carried out at 08C, with AgSbF6 (10 mol %), 72 h.
[d]
[e]
[f]
1
[g]
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Table 2. Proving the role of thienyl rings in the Pd-catalyzed
allylic substitution with carbonate 4a.[a]
um salts, bases, solvents and additives (a collection of re-
sults is reported in Table 1, entries 6–10), we were de-
lighed to find out that, by using [Pd(p-C3H5)Cl]2 as the
palladium source, Cs2CO3 (2 equivs.) as the base and
10 mol % of AgSbF6,[17a] 3b furnished (S)-6ca in almost
quantitative yield and with high ee (80%, entry 9).
Again, to further improve the enantioselectivity in
6ca, the influence of the temperature was envisaged
by running the model reaction at 08C. Herein, beside
the observed dropin reaction rate (yield 45%), the
enantiomeric excess reached 90%, that represents one
of the highest levels of stereoinduction obtained to
date for 4c in the presence of phosphorus-free ligands
(entry 10).
Entry L*
Time [h]
6
Yield [%][b] ee [%][c]
1
2
3
3d, (A-TP) 48
3e, (A-PT) 24
3f, (A-P2) 48
6aa 99
6aa 60
6aa 30
97 (S)
50 (S)
33 (S)
[a]
All the reactions were carried out in anhydrous THF at
room temperature, by using 5 mol % of [Pd2(dba)3]·
CHCl3 as the Pd source and 1: 1 L:Pd ratio.
[b]
[c]
Isolated yields after flash chromatography.
Determined by HPLC analysis with chiral column (Chiral-
cel AD). The absolute configuration was assigned by com-
parison with the known optical rotation value.
To gain some insights into the factors governing the
activity of these oligothienyl ligands in Pd species we
synthesized and tested, in the aforementioned model al-
In an effort to learn more about the nature of these
lylic substitution, a series of properly designed chiral C2- unprecedented chiral Pd catalysts, we synthesized
diamine ligands (3d–3f, Scheme 2) in which one or both and characterized the [Pd(h3-C3H5)(3b)][BF4] and
the thienyl rings were alternatively replaced by benzene [Pd(h3-C3H5)(3f)][BF4] that were isolated as air/mois-
rings.
ture stable yellow solids (64%, 80% yields, respectively)
by reacting 3b/3f with [Pd(h3-C3H5)(m-Cl)]2 (2:1 ratio)
followed by an exchange reaction with AgBF4. The
structure of the cation [Pd(h3-C3H5)(3b)]þ was estab-
lished by X-ray diffraction and its stereogeometry is il-
lustrated in Fig. 1A.
Interestingly, the potentially multidentate ligand 3b
coordinates to Pd(II) through the nitrogen atoms form-
ing a five-membered palladacycle in an envelope con-
formation, with the nitrogen donors exhibiting opposite
configurations.[18] The bithiophene pendants are packed
in an orderly way and S(2) exhibits a significant van der
Waals interaction with palladium [3.34(1)Š].[19] It is rea-
sonable to attribute attractive character to this contact
as the estimated sum of the van der Waals radii is
3.4 Š.[20] The allyl unit is coordinated almost symmetri-
cally but some 20% of the molecules show the alterna-
tive orientation with the central carbon pointing down-
wards. As usual with the allyl ligand the asymmetric en-
vironment is not fully successful in influencing its way of
coordination.
Scheme 2. Reagents and conditions: a) aldehyde, MgSO4,
CH2Cl2, room temperature, 24–48 h; b) NaBH4, MeOH,
08C to room temperature. Overall yield: 3d: 40%; 3e: 39%;
3f: 60%.
The determining role of the intramolecular S(2)...Pd
From the data collected in Table 2 some conclusions interaction in the enantiodiscriminating stepof the reac-
can be drawn. i) The crucial participatory role of the tion can be appreciated by inspection of the molecular
thienyl rings in the enantiodiscriminating stepof the
structure of the poorly effective [Pd(h3-C3H5)(3f)][BF4]
process was unambiguously proved by employing A- complex(Fig. 1B). Here, although asimilar overall coor-
P2 as the chiral promoter. In this case in fact, 6aa was iso- dination geometry is found, the phenyl-Pd interaction is
lated in poor yield (30%) and low enantioselectivity not of special significance [Pd···C(13) 3.55(1)Š and
(33%, entry 2). ii) The pivotal role of the closest thienyl Pd···H(13) 3.20(2)Š].[21] Even more telling is the super-
units (inner thiophenes) to the stereogenic centers of the imposition diagram of these structures containing 3b
ligand was demonstrated by comparative stereoselec- and 3f (Fig. 2). It is evident that the 1,4-diphenyl unit
tive allylic substitutions of 5 in the presence of A-TP is less effective in producing beneficial steric hindrance
(yield: 99%, ee: 97%) and A-PT (yield: 60%, ee: 50%). than the bent bithienyl counterpart when properly ori-
iii) The second (outer) rings (thienyl, 3b or phenyl, 3d) ented by the sulfur-palladium interaction. Therefore,
exerted a positive influence on the stereochemical out- the weak Pd···S interaction is crucial in generating a
come of the process [A-T1, (3a) ee: 45%; A-T2, (3b) more stringent chiral environment around the metal
ee: 99%; A-TP (3d) ee: 97%].
center.[22] Moreover, since the catalytic efficiency of li-
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Figure 1. A) X-ray molecular structure of [Pd(h3-C3H5)(3b)]þ as [BF4]ꢀ salt. The dominant orientation (80%) of the allyl li-
ꢀ
ꢀ
ꢀ
ꢀ
gand is shown. Selected bond lengths (Š): Pd C(25) 2.103(6), Pd C(26) 2.126(7), Pd C(27) 2.112(6), Pd N(1) 2.111(4),
ꢀ
Pd N(2) 2.143(3), C(25)–C(26) 1.402(12), C(26)–C(27) 1.323(14). Configurations of the stereogenic centers: N(1)(R),
N(2)(S), C(1)(R), C(2)(R), conformation of the chelate ring l. B) X-ray molecular structure of [Pd(h3-C3H5)(3f)]þ as
[BF4]ꢀ salt. The dominant orientation (60%) of the allyl ligand is shown. Selected bond lengths (Š): Pd(1)–C(33) 2.066(6),
Pd(1)–C(34) 1.999(9), Pd(1)–C(35) 2.066(5), Pd(1)–N(1) 2.086(4), Pd(1)–N(2) 2.081(4), C(33)–C(34) 1.283(19), C(34)–
C(35) 1.306(16). Configurations of the stereogenic centers: N(1)(R), N(2)(S), C(1)(R), C(2)(R), conformation of the chelate
ring l.
gands 3b and 3c on aromatic carbonates are almost
Finally, to prove that the crystallographic information
equal, we can conclude that the third thienyl ring in 3c, was consistent with the species present in solution, we
bonded with 2,2’ connectivity, has no significant role in synthesized the [Pd(h3-1,3-Ph2-C3H3)(3b)][BF4] and
the enantioselectivity of the reaction.
the isolated complex was tested in the model reaction.
In particular, the air-stable orange solid (95% yield)
showed a less fluxional 1H NMR spectrum (room tem-
perature, CD3CN) with respect to [Pd(h3-C3H5)(3b)]
[BF4], in which the presence of a single conformer, prob-
ably controlled by the phenyl/thienyl sterical constrains,
was observed. Attempts to prepare crystals of the
[Pd(h3-1,3-Ph2-C3H3)(3b)][BF4] have not been success-
ful so far, however, the isolated cationic tetrafluorobo-
rate 1,3-diphenylallyl-Pd species confirmed its authen-
ticity as the catalyst in the AAA of 4 with 5a, giving
rise to (S)-6aa in 75% yield and 99% ee.
In conclusion, in this communication we have present-
ed our preliminary findings in the synthesis, crystallo-
graphic characterization and application in asymmetric
catalysis of a new series of chiral thiophene-based
oligomers. The unprecedented coordination features re-
vealed by DATs candidate such a type of chiral motif as
easily tunable ligands for asymmetric metallo-catalyzed
transformations. Moreover, studies addressed towards
the investigation of chirooptical/electrochemical prop-
Figure 2. Superimposition of the [Pd(h3-C3H5)(3b)]þ (solid
lines) and [Pd(h3-C3H5)(3f)]þ (dashed lines) complexes.
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16.183(2) Š, b¼97.694(3)8, V¼3085.2(5) Š3, T¼293(2) K,
space group C2, Z¼4, m¼0.917 mmꢀ1, 18613 reflections col-
lected 7035 reflections I>2s (I) (R1 ¼0.0531 and wR2 ¼
0.1207), CCDC 258610.
erties of these systems both in liquid and solid phases are
currently under way and will be published in due course.
Crystal data of [Pd(h3-C3H5)(3f)][BF4]: C35H39BF4N2Pd;
M¼680.89, monoclinic, a¼8.278(3), b¼17.853(6), c¼
10.509(3) Š, b¼111.635(7)8, V¼1443.7(8)Š3, T¼293(2) K,
space group P21, Z¼2, m¼0.698 mmꢀ1, 15548 reflections col-
lected 5891 reflections I>2s (I) (R1 ¼0.0514 and wR2 ¼
0.1297), CCDC 258611.
Experimental Section
Representative Procedure for Pd-Catalyzed AAA (in
situ Procedure)
Crystallographic data (excluding structure factors) for the
structures reported in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supplementa-
ry publication no. CCDC 258610 for [Pd(h3-C3H5)(3b)][BF4]
and no. CCDC 258611 for [Pd(h3-C3H5)(3f)][BF4]. Copies of
the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: int.
codeþ44(1223)336–033; E-mail: deposit@ccdc.cam.ac.uk].
A 25-mL two-necked flask was charged, under a nitrogen at-
mosphere, with [Pd2(dba)3]·CHCl3 (2.6 mg, 2.5ꢁ10ꢀ3
mmol), diamine 3b (2.4 mg, 5.0ꢁ10ꢀ3 mmol) and 1.5 mL of an-
hydrous THF. The mixture was stirred at room temperature for
5 min (the colour of the solution gradually turned from purple
to brown) then 4a (14 mg, 0.05 mmol), dimethyl malonate
(29 mL, 0.25 mmol), BSA (12 mL, 0.05 mmol) and a catalytic
amount of KOAc were added sequentially. The reaction mix-
ture was stirred overnight at room temperature and, after
24 h, was judged complete by TLC. The reaction was then
quenched with a saturated solution of NaHCO3 (3 mL), the
two phases separated and the aqueous phase was extracted
with AcOEt (3ꢁ5 mL). Finally, the organic layers were col-
lected, dried with Na2SO4 and then concentrated under re-
duced pressure. The desired product (S)-6aa was isolated as a
yellow oil after flash chromatography (c-hex/Et2O, 9/1); yield:
16 mg (96%). The ee of the product (99%) was determined by
chiral HPLC (Chiralcel AD: IPA:n-hex, 10:90, 1.0 mL/min
flow, 214 nm, RtR: 9.3 min.; RtS: 12.6 min.). [a]D: ꢀ9.0 (c 0.7,
CHCl3), lit. (R)-6aa [a]D: þ19.2 (c 1.3, CHCl3).[23]
Acknowledgements
This work was supported by M. I. U. R. (Rome), National proj-
ect “Stereoselezione in Sintesi Organica: Metodologie and Ap-
plicazioni”, FIRB Project (Progettazione, preparazione e valu-
tazione biologica e farmacologica di nuove molecole organiche
quali potenziali farmaci innovativi) and from University of Bo-
logna (funds for selected research topics).
References and Notes
(R)-6ab was obtained by an analogous procedure starting
from 0.05 mmol of 4a and with use of 5b as the malonate.
Through purification (flash chromatography, c-hex/Et2O, 8/2)
of the reaction crude, 6ab was isolated as a yellow oil in 99%
yield (18 mg) and ee>98%. The enantiomeric excess of the
product was determined by 1H NMR chiral shift with Eu(hfc)3
(MeO groups). [a]D: þ44.0 (c 0.4, CHCl3), lit. (R)-6ab [a]D:
þ60.1 (c 1.2, CH2Cl2).[24]
[1] E. N. Jacobsen, A. Pfaltz, H. Yamamoto (Eds.), Compre-
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[2] B. M. Trost, Proc. Nat. Acad. Sci. 2004, 101, 5348–5353.
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[4] T. A. Skotheim, R. L. Elsenbaumer, J. R. Reynolds,
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[5] T. B. Rauchfuss, The Coordination Chemistry of Thio-
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(4b), the reaction was carried out with the above described con-
ditions for 24 h. After work-up, 6ba was isolated by flash chro-
matography (c-hex/Et2O, 9/1) as a pale yellow oil in 99% yield
and 95% ee. [a]D: ꢀ2.5 (c 1.02, CHCl3), lit. (ꢀ)-6ba [a]D: ꢀ2.8
(c 1.12, CHCl3), 93% ee.[25]
(S)-6ca was obtained by an analogous procedure starting
from 0.15 mmol of 4c and with use of 5a as the malonate. The
reaction was carried out at 08C for 72 h. AgSbF6 (10 mol %)
was added during the catalyst preparation and Cs2CO3
(98 mg, 0.30 mmol) proved to be the base of choice. Through
purification (flash chromatography, c-hex/AcOEt, 95/5) of
the reaction crude 6ca was isolated as a yellow oil in 45% yield
and 90% ee. The enantiomeric excess of the product was deter-
mined by chiral GC analysis (method: 508C for 6 min, rampof
18C/min to 1808C for 15 min, RtS: 43.8 min.; RtR: 44.0 min.).
[a]D: ꢀ12.0 (c 0.7, CHCl3), lit. (S)-6cb [a]D: ꢀ28.0 (c 1.4,
CHCl3).[26]
[6] R. J. Angelici, Organometallics 2001, 20, 1259–1275.
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[10] a) B. M. Trost, D. L. Van Vraken, Chem. Rev. 1996, 96,
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X-Ray Crystallographic Study
Crystal data of [Pd(h3-C3H5)(3b)][BF4]: C27H31BF4N2PdS4;
M¼704.99, monoclinic, a¼19.760(2), b¼9.7360(9), c¼
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`
´
[11] Non-covalent “secondary/weak” interactions are of pivo-
tal importance in many areas of organic as well as orga-
nometallic chemistry, biology and material science: see
a) G. R. Desiraju, Angew. Chem. Int. Ed. Engl. 1995,
34, 2311–2327; b) J. C. Ma, J. C. Dougherty, Chem. Rev.
1997, 97, 1303–1324.
125, 11792–11793; b) O. Pamies, M. Dieguez, C. Claver,
J. Am. Chem. Soc. 2005, 127, 197.
[18] Interestingly, the coordination mode displayed by 3b in
the solid state is markedly in contrast with the specula-
tive N,S-Pd coordination previously proposed for N,N’-
dimethyl-3a (see ref.[15]).
[12] Enantiomerically pure 1,2-cyclohexandiamine (1) is
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