New chiral 2,2′:6′,2″-terpyridines as ligands for asymmetric catalysis: Cyclopropanation and hydrosilylation reactions

Article abstract of DOI:10.1016/S0040-4020(00)01082-6

A number of new chiral 2,2′:6′,2″-terpyridines were prepared and the corresponding rhodium and ruthenium complexes were assessed as chiral catalysts for the enantioselective hydrosilylation of acetophenone with diphenylsilane and for the cyclopropanation

Full text of DOI:10.1016/S0040-4020(00)01082-6

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 1099±1104
New chiral 2,20:60,200-terpyridines as ligands for asymmetric
catalysis: cyclopropanation and hydrosilylation reactions
Giorgio Chelucci,a,p Antonio Saba,a Davide Vignolaa and Costantino Solinasb
a
Á
Dipartimento di Chimica, Universita di Sassari, via Vienna 2, I-07100 Sassari, Italy
bDipartimento di Scienze del Farmaco, via Muroni 23/a, I-07100 Sassari, Italy
Received 14 August 2000; revised 25 October 2000; accepted 9 November 2000
AbstractÐA number of new chiral 2,20:60,200-terpyridines were prepared and the corresponding rhodium and ruthenium complexes were
assessed as chiral catalysts for the enantioselective hydrosilylation of acetophenone with diphenylsilane and for the cyclopropanation of
styrene with ethyl diazoacetate. Enantioselectivities up to 59% in the cyclopropanation and up to 13% in the hydrosilylation were obtained.
q 2001 Published by Elsevier Science Ltd.
1. Introduction
cation of chiral pyridine derivatives as ligands in metal
complexes for enantioselective catalysis,4 we have evalu-
ated the utility of the 2,20:60,200-terpyridines (trpy's) 1 and
2 (Scheme 1) as chiral controllers in some enantioselective
processes such as the Pd-catalysed allylic alkylation of
1,3-diphenylprop-2-enyl acetate with dimethyl malonate,5
the cyclopropanation of styrene with diazoacetates6 and
the hydrosilylation of acetophenone with diphenylsilane.7
Enantioselective reactions based on chiral nitrogen ligands
are currently an actively pursued research area.1 In this
context a number of bidentate-nitrogen ligands have been
demonstrated to be useful auxiliaries for metal-promoted
asymmetric reactions reaching high levels of stereocontrol.
On the other hand the use of N,N,N-terdentate ligands is
rather limited2 with the terdentate C2-symmetrical bis(oxazo-
linyl)pyridines (pybox) being the only remarkable excep-
tion.3
Our ®ndings showed that the chiral 6,6-dimethylnorpinan-2-
yl group was not able to furnish the ligand with a satis-
factory enantioselectivity. These disappointing results
were ascribed to the conformational mobility of the
Recently, pursuing our interest in the synthesis and appli-
Scheme 1. Rˆa: H; b: Me; c: n-Bu; d: i-Pr; e: Bn.
Keywords: terpyridine ligands; ruthenium and rhodium catalysts; asymmetric cyclopropanation and hydrosilylation reactions.
p Corresponding author. Tel: 139-79-229539; fax: 139-79-229559; e-mail: chelucci@ssmain.uniss.it
0040±4020/01/$ - see front matter q 2001 Published by Elsevier Science Ltd.
PII: S0040-4020(00)01082-6

1100
G. Chelucci et al. / Tetrahedron 57 (2001) 1099±1104
Scheme 2. R: a: RˆH; b: RˆMe; c: Rˆn-Bu; d: Rˆi-Pr; e: RˆBn; a: AcOH, AcONH4, re¯ux, 6 h, 63%; b: LDA, THF, 2408C, 2 h; then MeI or n-BuI or i-PrI
or BnI, 37±55%; c: RhCl3´3H2O, 85±91%.
6,6-dimethylnorpinan-2-yl group which causes a minimi-
zation of the steric interaction among the stereogenic
centres on the ligand, the substrate and the reagent in the
metal complex forming the enantioselective transition state.
In fact, it is indicated in the literature that effective chiral
controllers are those ligands in which the substituents at the
asymmetric centres are forced to be directed toward the
metal ion on its complex formation.8
ligands 5b±e in moderate yields (37±55%). The reactions
of alkylation afforded single diastereomers in which the new
substituents replace the hydrogen atoms, situated on the two
tetrahydroquinoline rings of 5a, in the trans-position to the
substituted bridge. Finally, Rh(trpy)Cl3 complexes 8a±e
were prepared in high yield by heating a methanolic solution
of 5a±e with RhCl3´3H2O (85±91%) under re¯ux. The
enantiomeric excess of 5a was not determined. (2)-Pino-
carvone 7 was prepared from (1R)-(1)-a-pinene having an
enantiomeric excess (ee) .80%, however, and as no race-
mization process occurs during this synthesis, an enantio-
meric purity .80% can be attributed to this compound and
to its derivatives.
Since, 2,20-bipyridines 39 and 1,10-phenanthrolines 4,10
incorporating the 6,6-dimethylnorpinane framework in the
form of a cycloalkeno-condensed substituent have found
useful application in asymmetric catalysis,7 we became
interested in obtaining an analogue class of trpy's.
2.2. Rhodium-catalysed asymmetric hydrosilylation
In this paper we report the preparation and application of a
number of new chiral trpy's of type 511 in the rhodium-
catalysed hydrosilylation of acetophenone with diphenyl-
silane12 and in the ruthenium- and rhodium-catalysed cyclo-
propanation of styrene with ethyl diazoacetate.13
Firstly we compared the results obtained with ligands 1 and
2 for the asymmetric hydrosilylation of acetophenone using
rhodium(I)±trpy catalysts prepared in situ from [Rh-
(cod)Cl]2 (codˆ1,5-cyclooctadiene) and the ligand with a
molar ratio of rhodium to ligand of 1:1. The degree of
hydrosilylation (conversion of acetophenone) and the rela-
tive amounts of the silyl enol ether 11 and of the silylalkyl
2. Results and discussion
1
2.1. Synthesis of the ligands
ether 10 were determined by H NMR17 while the ee was
determined by GC on a chiral column on the carbinol 12
obtained after acid hydrolysis (Scheme 3).
The trpy 5a was readily accessible by reaction of (2)-pino-
carvone 714 with 2,6-bis(pyridinioacetyl)pyridine iodide 615
following the Krohnke methodology16 (63% yield) (Scheme
To compare the results among all ligands, the reactions were
carried out for 24 h at room temperature. After this time
moderate levels of conversion with all ligands were
obtained (45±59%). Only trpy's 5a,b gave, although in
È
2). Then the red solution of lithiated 5a, obtained by treat-
ment with lithium diisopropylamine (LDA) at 2408C for
2 h, was quenched with the proper alkyl iodide to give
Scheme 3. a: [Rh(cod)Cl]2, ligand, H2SiPh2; b: p-TsOH, CH3OH.

G. Chelucci et al. / Tetrahedron 57 (2001) 1099±1104
1101
Scheme 4.
low yield (29 and 18%, respectively), the desired silylalkyl
ethers 10. The resultant carbinols, however, did not show
signi®cant enantioexcesses (7 and 13%, respectively).
Surprisingly, the other trpy's 5c±e afforded the silyl enol
ether 11 as the sole product. It is plain that increasing the
steric hindrance of the substituent on the tetrahydroquino-
line ring favours the formation of 11 to the detriment of 10.
to our surprise, the resultant carbinols showed very low
enantioexcesses (2±6%).
2.3. Ruthenium(II)- and rhodium(III)-catalysed
asymmetric cyclopropanation
It has been shown by Nishiyama et al. that particular ruth-
enium(II)±pybox complexes are capable of providing high
ee's in the enantioselective cyclopropanation of ole®ns with
diazoesters. Interestingly, these authors found that similar
results can be also obtained using catalysts prepared in situ
from the procatalyst [RuCl2(p-cymene)]2 (p-cymeneˆ
p-isopropyltoluene) and an excess of pybox.19
The particularly disappointing results obtained with Rh(I)
catalysts prompted us to examine the ability of the Rh(III)
complexes of these ligands. This research was inspired by
the works of Nishiyama et al. who showed that trivalent
rhodium complexes, derived from terdentate C2-symmetri-
cal bis(oxazolinyl)pyridines (pybox) and RhCl3 with the aid
of AgBF4, are effective chiral catalysts for the hydrosilyl-
ation of ketones.18
On this basis, the reaction of styrene with ethyl diazoacetate
to give the trans- and cis-cyclopropanes 13 and 14 (Scheme
4) was chosen as the model for the evaluation of the
ef®ciency of trpy's 5a±e in the ruthenium(II)-catalysed
asymmetric cyclopropanation of ole®ns. The reactions
were carried out at room temperature for 24 h by slow
addition (2 h) of ethyl diazoacetate to a solution of styrene
in methylene chloride containing the ruthenium(II)±
ligand adduct. This was prepared in situ by adding two
equivalents of ligands 5a±e to the [RuCl2(p-cymene)]2
The Rh(trpy)Cl3 complexes 8a±e were assessed in the
presence of AgBF4, in the hydrosilylation of acetophenone
with diphenylsilane. The reaction was carried out at room
temperature and stopped after 24 h.
With complexes 8a±e high levels of conversion (65±95%)
and satisfactory yields (55±71%) were obtained but, much
a
Table 1. Enantioselective cyclopropanation of styrene with ethyldiazoacetate using [RuCl2(p-cymene)]2
Ligand
Yieldb (%) 13114
trans:cis ratioc 13:14
% eec
Con®gurationd
14
13
2
0
0
0
14
4
0
2
0
13
5a
5b
5c
5d
5e
88
83
77
89
81
65:35
65:35
62:38
64:36
36:64
1R,2R
±,±
±,±
±,±
±,±
lR,2S
±,±
1R,2S
±,±
lS,2R
0
4
a A solution of the ligand (0.1 mmol), [RuCl2(p-cymene)]2 (0.025 mmol) in CH2Cl2 (2 ml) was stirred under argon atmosphere for 30 min. After the addition of
styrene (12.5 mmol) the solution of ethyl diazoacetate (2.5 mmol) in CH2Cl2 (2.5 ml) was added dropwise over a period of 2 h and then stirred for 24 h.
b Isolated yield, based on the diazoacetate, for the mixture of trans and cis cyclopropanes.
c Determined by GC analysis on a chiral column.
d Assignment according to Ref. 20.
Table 2. Enantioselective cyclopropanation of styrene with ethyldiazoacetate and Rh(III) complexesa
Complex
Yieldb (%) 13114
trans:cis ratioc 13:14
% eec
Con®gurationd
14
13
14
13
8a
8b
8c
8d
8e
75
68
64
57
53
46:54
34:66
46:54
71:29
57:43
7
0
lR,2R
1S,2S
lS,2S
1S,2S
1S,2S
±,±
49
37
39
35
59
45
33
25
lR,2S
lR,2S
1R,2S
1R,2S
a To a solution of Rh(trpy)Cl3 complex (0.05 mol) in THF (2 ml) was added AgOTf (0.1 mmol) under argon atmosphere. After 30 min stirring, styrene
(12.5 mmol) was added and then a solution of ethyl diazoacetate (2.5 mmol) in THF (2.5 ml) was added dropwise over a period of 2 h and then stirred for
24 h.
b Isolated yield, based on the diazoacetate, for the mixture of trans and cis cyclopropanes.
c Determined by GC analysis on a chiral column.
d Assignment according to Ref. 20.

1102
G. Chelucci et al. / Tetrahedron 57 (2001) 1099±1104
complex (molar ratio of ruthenium/ligandˆ112). The
results obtained in these runs are summarized in Table 1.
Trpy's 5a±e provided effective Ru(II) catalysts but negli-
gible enantioselectivities.
[Rh(cod)Cl]2, [RuCl2(p-cymene)]2, RhCl3´3H2O, ethyl
diazoacetate were purchased from Aldrich. 2,6-Bis(pyri-
dinioacetyl)pyridine iodide 6 was obtained from 2,6-di-
acetylpyridine (Aldrich) following the Ortoleva±King
procedure.15 (2)-Pinocarvone 7 was prepared from (1R)-
(1)-a-pinene (801% ee, Acros).14
Subsequently, the behaviour of these derivatives in the
Rh(III)-catalysed cyclopropanation reaction of styrene was
investigated.
3.1.1. (5S,7S)-2,6-Bis(6,6-dimethyl-5,6,7,8-tetrahydro-
5,7-methanoquinolin-2-yl)pyridine (5a). A solution of
2,6-bis(pyridinioacetyl)pyridine iodide (12 g, 21 mmol),
(2)-pinocarvone (6.3 g, 42 mmol), ammonium acetate
(32.3 g, 0.42 mol) in glacial acetic acid (120 ml) was heated
at 120±1258C for 4 h under nitrogen. Then, most of the
acetic acid was removed under reduced pressure and the
residue taken up with H2O (600 ml) and extracted with
CH2Cl2 (2£150 ml). The organic phase was washed with a
5% NaOH solution and then extracted with a 10% HCl
solution. The acid solution was alkalinized with a 10%
NaOH solution and extracted with CH2Cl2 (2£150 ml).
The organic phase was dried on anhydrous Na2SO4, the
solvent was evaporated and the residue was recrystallized
from dichloromethane±diethylether to give pure 5a as a
The reactions were carried out at room temperature for 24 h
by slow addition of ethyl diazoacetate (2 h) to a solution of
styrene in THF containing the active catalyst which was
prepared by the addition of two equivalents of silver tri¯ate
to the Rh(tryp)Cl3 complex in THF, a method analogous to
that used by Nishiyama et al. to generate pybox±rhodium
catalysts used for the asymmetric hydrosilylation of
ketones.18
The cyclopropanes recovered from the reactions were
obtained in moderate yield as a mixture of trans and cis
isomers in a ratio which varies appreciably with the nature
of the substituents present on the tetrahydroquinoline rings
(Table 2). Enantioselectivities were also moderate with the
best result being obtained with the trpy 5b (59%).
25
white solid: 5.57 g (63%); mp 230±2328C; ‰aŠ 178 (c
D
0.8, CHCl3); H NMR (CDCl3) d 0.70 (s, 6H), 1.33 (d,
1
2H, Jˆ9.3 Hz), 1.43 (s, 6H), 2.41 (m, 2H), 2.72 (m, 2H),
2.83 (t, 2H, Jˆ5.7 Hz), 3.21 (d, 4H, Jˆ2.7 Hz), 7.37 (d, 2H,
Jˆ7.8 Hz), 7.90 (t, 1H, Jˆ7.8 Hz), 8.29 (d, 2H, Jˆ7.8 Hz),
8.37 (d, 2H, Jˆ7.8 Hz). Anal. calcd for C29H31N3: C, 82.62;
H, 7.41; N, 9.97. Found: C, 82.67; H, 7.55; N, 9.92.
As a general trend, the substitution of two hydrogen atoms
onto the basic structure 5a displayed a positive effect on the
enantioselectivity of the reaction leading to an increase of
the ee's for both trans and cis isomers 13 and 14. However,
the increase of the steric bulk of the alkyl substituent from
methyl to benzyl led to the reduction of the ee for both
cyclopropanes. Notably, the presence of substituents in the
backbone of 5b±e caused a chiral switch of the con®gu-
ration of trans cyclopropanes with respect to that obtained
from the unsubstituted ligands 5a. This fact indicates that
the stereochemistry of the reaction is basically dictated by
the new stereocentres generated by alkylation and that these
stereocentres have a mismatching stereotopic relation with
those pre-existing on the bridge.
3.1.2.
(5S,7S,8R)-2,6-Bis(6,6,8-trimethyl-5,6,7,8-tetra-
hydro-5,7-methanoquinolin-2-yl)pyridine (5b). A solu-
tion of the trpy 5a (1.05 g, 2.5 mmol) in anhydrous THF
(5 ml) was added at 2788C to a solution of lithium diiso-
propylamine (5 mmol) in anhydrous THF (25 ml). The
resulting solution was stirred at 2408C for 2 h and then a
solution of methyl iodide (0.71 g, 5 mmol) in THF (4 ml)
was added dropwise at 2408C. After 30 min at 2408C, the
solution was allowed to reach room temperature slowly and
then treated with H2O. The organic phase was separated and
the aqueous phase extracted with diethylether (3£50 ml).
The organic phase was dried on anhydrous Na2SO4, the
solvent evaporated and the residue puri®ed by chroma-
tography on neutral aluminium oxide (petroleum ether/
Taken overall, the obtained results point out that the
2,20:60,200-terpyridine ligands used throughout this work
are poorly suitable chiral controllers in the Rh-catalysed
hydrosilylation of acetophenone, while they seem to deserve
attention for their possible applications in the ®eld of
Rh(III)-catalysed cyclopropanation. Further studies on this
subject are currently in progress in our laboratory.
ethyl acetateˆ20/1) to give pure 5b as a white solid:
25
D
0.62 g (55%); mp 163±1658C; ‰aŠ
215.4 (c 0.5,
1
CHCl3); H NMR (CDCl3) d 0.69 (s, 6H), 1.35 (d, 2H, Jˆ
9.6 Hz), 1.44 (s, 6H), 1.49 (d, 6H, Jˆ6.9 Hz), 2.19 (m, 2H),
2.59 (m, 2H), 2.82 (t, 2H, Jˆ5.4 Hz), 3.27 (m, 2H), 7.34 (d,
2H, Jˆ7.8 Hz), 7.90 (t, 1H, Jˆ7.8 Hz), 8.30 (d, 2H, Jˆ
7.5 Hz), 8.45 (d, 2H, Jˆ7.5 Hz). Anal. calcd for
C31H35N3: C, 82.81; H, 7.85; N, 9.35. Found: C, 82.67; H,
7.97; N, 9.44.
3. Experimental
3.1. General
3.1.2.
(5S,7S,8R)-2,6-Bis(6,6-dimethyl-8-butyl-5,6,7,8-
Boiling points are uncorrected. Melting points were deter-
mined on a Buchi 510 capillary apparatus and are uncor-
rected. The 1H NMR (300 MHz) spectra were obtained with
a Varian VXR-300 spectrometer. Optical rotations were
measured with a Perkin-Elmer 241 polarimeter in a 1 dm
tube. Elemental analyses were performed on a Perkin-Elmer
240 B analyser. Gas chromatographic analyses were
performed with a HP 5900 chromatograph using He
(60 kPa) as the carrier gas.
tetrahydro-5,7-methanoquinolin-2-yl)pyridine (5c). Com-
pound 5c was obtained as a white solid following the pro-
cedure described for the preparation of 5b using butyl
25
iodide: 0.64 g (48%); mp 70±758C; ‰aŠ 219.6 (c 0.2,
D
CHCl3); H NMR (CDCl3) d 0.60±1.60 (m, 18H), 0.67 (s,
1
6H), 1.34 (d, 2H, Jˆ9.9 Hz), 1.45 (s, 6H), 2.37 (m, 2H), 2.56
(m, 2H), 2.81 (t, 2H, Jˆ5.7 Hz), 3.06 (m, 2H), 7.33 (d, 2H,
Jˆ7.8 Hz), 7.92 (t, 1H, Jˆ7.8 Hz), 8.30 (d, 2H, Jˆ 7.8 Hz),

G. Chelucci et al. / Tetrahedron 57 (2001) 1099±1104
1103
8.45 (d, 2H, Jˆ7.8 Hz). Anal. calcd for C37H47N3: C, 83.24;
3.1.8. Rh(trpy 5d)Cl3 (8d). Compound 8b was obtained as
an orange solid following the procedure described for the
preparation of 8a: 0.850 g (91%); IR (KBr) nCyN 1610 cm21
H, 8.88; N, 7.88. Found: C, 83.35; H, 8.78; N, 7.85.
1
3.1.3. (5S,7S,8R)-2,6-Bis(6,6-dimethyl-8-methylethyl-5,6,
7,8-tetrahydro-5,7-methanoquinolin-2-yl)pyridine (5d).
Compound 5d was obtained as a white solid following the
procedure described for the preparation of 5b using iso-
(m); H NMR (CDCl3) d 0.48 (d, 6H, Jˆ6.9 Hz), 0.86 (s,
6H), 1.07 (d, 6H, Jˆ6.9 Hz), 1.42 (s, 6H), 1.64 (d, 2H, Jˆ
8.7 Hz), 2.42±2.53 (m, 4H), 2.78 (t, 2H, Jˆ5.4 Hz), 3.76±
3.88 (m, 2H), 5.25±5.41 (m, 2H), 7.45 (d, 2H, Jˆ7.8 Hz),
7.83 (d, 2H, Jˆ6.8 Hz), 8.04 (d, 2H, Jˆ6.9 Hz), 8.10±8.16
(m, 1H). Anal. calcd C35H43Cl3N3Rh: C, 58.79; H, 6.06; N,
5.88. Found: C, 58.74; H, 6.11; N, 5.91.
25
propyl iodide: 0.47 g (37%); mp 175±1808C; ‰aŠ 213.2
D
(c 0.4, CHCl3); H NMR (CDCl3) d 0.66 (s, 6H), 0.89 (d,
1
6H, Jˆ6.9 Hz), 1.20±1.70 (m, 2H), 1.26 (d, 6H, Jˆ6.9 Hz),
1.41 (d, 2H), 1.44 (s, 6H), 2.40 (m, 2H), 2.60 (m, 2H), 2.79
(t, 2H, Jˆ5.7 Hz), 2.93 (m, 2H), 7.35 (d, 2H, Jˆ7.8 Hz),
7.90 (t, 1H, Jˆ7.8 Hz), 8.33 (d, 2H, Jˆ7.8 Hz), 8.44 (d, 2H,
Jˆ7.8 Hz). Anal. calcd for C35H43N3: C, 83.11; H, 8.58; N,
8.31. Found: C, 83.08; H, 8.64; N, 8.40.
3.1.9. Rh(trpy 5e)Cl3 (8e). Compound 8e was obtained as
an orange solid following the procedure described for the
preparation of 8a: 0.869 g (90%); IR (KBr) nCyN 1610 cm21
1
(m); H NMR (CDCl3) d 0.84 (s, 6H), 1.58 (d, 2H, Jˆ
13.2 Hz), 1.98±2.07 (m, 2H), 2.38 (t, 4H, Jˆ12.3 Hz),
2.79±2.83 (m, 2H), 4.64 (dd, 2H, Jˆ13.2, 4.1 Hz), 6.14±
6.20 (m, 2H), 7.17 (d, 2H, Jˆ7.5 Hz), 7.26±7.35 (m, 4H),
7.47 (d, 2H, Jˆ7.5 Hz), 7.60 (d, 4H, Jˆ7.2 Hz), 7.85 (d, 2H,
Jˆ6.9 Hz), 7.94±8.06 (m, 2H), 8.09±8.12 (m, 1H). Anal.
calcd for C43H43C13N3Rh: C, 63.68; H, 5.34; N, 5.18.
Found: C, 63.79; H, 5.29; N, 5.22.
3.1.4. (5S,7S,8R)-2,6-Bis(6,6-dimethyl-8-methylphenyl-5,
6,7,8-tetrahydro-5,7-methanoquinolin-2-yl)pyridine (5e).
Compound 5e was obtained as a white solid following the
procedure described for the preparation of 5b using benzyl
25
iodide: 0.78 g (52%); mp 207±2098C; ‰aŠ 1158 (c 0.6,
D
CHCl3); H NMR (CDCl3) d 0.64 (s, 6H), 1.36 (s, 6H),
1
1.46 (d, 2H, Jˆ9.6 Hz), 2.14 (m, 2H), 2.59 (m, 2H), 2.78
(m, 4H), 3.41 (m, 2H), 3.89 (m, 2H), 7.34 (m, 5H), 7.39 (d,
2H, Jˆ7.8 Hz), 7.93 (t, 1H, Jˆ7.8 Hz), 8.37 (d, 2H, Jˆ
7.8 Hz), 8.49 (d, 2H, Jˆ7.8 Hz). Anal. calcd for
C43H43N3: C, 85.81; H, 7.21; N, 6.99. Found: C, 85.95; H,
7.14; N, 6.92.
3.1.10. Asymmetric hydrosilylation of acetophenone
using [Rh(cod)Cl]2: typical procedure. The catalyst was
prepared by dissolving under argon the precursor
[Rh(cod)Cl]2 (10 mg, 0.02 mol), the ligand (0.2 mmol)
and acetophenone (1 ml, 8.5 mmol) in CCl4 (2 ml). After
30 min stirring, the mixture was cooled at 08C and
diphenylsilane (1.6 ml, 8.6 mmol) was added. The solution
was slowly warmed up to room temperature and then stirred
for 24 h. A sample was then taken (0.2 ml), diluted with
3.1.5. Rh(trpy 5a)Cl3 (8a). A mixture of appropriate ter-
pyridine (1.19 mmol) and RhCl3´3H2O (313 g, 1.19 mmol)
in methanol (9 ml) was heated under re¯ux for 4.5 h. After
cooling the precipitate was ®ltered off, and recrystallized
from dichloromethane±ethyl ether. Finally the crystals
were washed with ethyl ether and dried under vacuo to
give pure 8a as an orange solid: 0.676 g (90%); IR (KBr)
1
CDCl3 (0.4 ml) and a H NMR was recorded to determine
the amount of silylenol ether (11/10111), the degree of
hydrosilylation (conversion of acetophenone, 10111/
9110111) and the chemical yield of silylalkyl ether (10/
9110111).14 The following integrals were used for the
analysis: dˆ5.70 ppm (s, SiZH, silylenol ether, 11), dˆ
5.40 ppm (s, SiZH, silylalkyl ether, 10) and dˆ2.50 ppm
(s, CH3, acetophenone, 9).
n
CyN 1600 cm21 (m); 1H NMR (CDCl3) d 0.68 (s, 6H), 1.28
(d, 2H, Jˆ9.9 Hz), 1.38 (s, 6H), 2.47 (m, 2H), 2.61 (dd, 2H,
Jˆ9.9, 5.7 Hz), 2.82 (t, 2H, Jˆ5.7 Hz), 4.46 (m, 4H), 7.44
(d, 2H, Jˆ7.8 Hz), 7.83 (d, 2H, Jˆ7.8 Hz), 8.03 (s, 3H).
Anal. calcd for C29H31Cl3N3Rh: C, 55.21; H, 4.95; N,
6.66. Found: C, 55.15; H, 4.89; N, 6.78.
The mixture was diluted with methanol (10 ml) and treated
with a few crystals of p-TsOH. After 30 min stirring at room
temperature the solvent was evaporated and the residue
was distilled in a kugelrohr apparatus at 1308C/2 Torr.
The enantiomeric excess (ee) was determined by GC analy-
sis on a diethyl-t-butylsilyl b-cyclodextrin column operated
at 608C for 5 min, then programmed at 38C min21 to 1508C.
Retention times: 23.75 min [(R)-1-phenylethanol] and
24.20 min [(S)-1-phenylethanol].
3.1.6. Rh(trpy 5b)Cl3 (8b). Compound 8b was obtained as
an orange solid following the procedure described for the
preparation of 8a: 0.666 g (85%); IR (KBr) nCyN 1610 cm21
(m); 1H NMR (CDCl3) d 0.84 (s, 6H), 1.43 (s, 6H), 1.61 (d,
6H, Jˆ6.6 Hz), 1.62±1.68 (m, 2H), 2.21±2.27 (m, 2H),
2.56±2.44 (m, 2H), 2.84 (t, 2H, Jˆ6.0 Hz), 5.61±5.71 (m,
2H), 7.44 (d, 2H, Jˆ6.9 Hz), 7.5 (d, 2H, Jˆ7.8 Hz), 8.00±
8.15 (m, 3H). Anal. calcd for C31H35Cl3N3Rh: C, 56.51; H,
5.35; N, 6.38. Found: C, 56.55; H, 5.30; N, 6.42.
3.1.11. Asymmetric hydrosilylation of acetophenone
using Rh(trpy)Cl3 complexes: typical procedure. A
mixture of Rh(trpy)Cl3 complex (0.08 mol), ligand
(0.32 mmol), AgBF4 (31 mg, 0.16 mmol) and acetophenone
(0.93 ml, 8.0 mmol) was stirred at 258C for 1 h. Diphenyl-
silane (2.37 ml, 12.8 mmol) was added at 08C and then the
mixture was slowly warmed to 258C and stirred for 24 h.
After this time the mixture was worked up as described
above.
3.1.7. Rh(trpy 5c)Cl3 (8c). Compound 8c was obtained as
an orange solid following the procedure described for the
preparation of 8a: 0.789 g (89%); IR (KBr) nCyN 1610 cm21
1
(m); H NMR (CDCl3) d 0.84 (s, 6H), 0.87 (t, 6H, Jˆ
6.9 Hz), 1.10±1.28 (m, 8H), 1.46 (s, 6H), 1.61±1.79 (m,
4H), 2.42±2.51 (m, 4H), 2.81 (t, 2H, Jˆ4.7 Hz), 2.94±
3.06 (m, 2H), 5.57±5.63 (m, 2H), 7.39 (d, 2H, Jˆ7.8 Hz),
7.75 (d, 2H, Jˆ7.8 Hz), 7.96 (d, 2H, Jˆ7.8 Hz), 7.97±8.03
(m, 1H). Anal. calcd for C37H47Cl3N3Rh: C, 59.81; H, 6.38;
N, 5.65. Found: C, 59.72; H, 6.43; N, 5.72.
3.1.12. Asymmetric cyclopropanation of styrene using
Rh(trpy)Cl3 complexes: typical procedure. To a solution

1104
G. Chelucci et al. / Tetrahedron 57 (2001) 1099±1104
4. (a) Chelucci, G.; Saba, A.; Sanna, G.; Soccolini, F. Tetrahedron:
Asymmetry 2000, 11, 3427. (b) Chelucci, G.; Pinna, G. A.; Saba,
A.; Sanna, G. J. Mol. Catal. A 2000, 159, 423. (c) Chelucci, C.;
Craba, S.; Saba, A.; Soccolini, F. J. Mol. Catal. A, 2000, 164,
179.
of Rh(trpy)Cl3 complex (0.05 mol), in THF (2 ml) was
added AgOTf (25.7 mg, 0.1 mmol) under argon atmos-
phere. After 30 min stirring, styrene (1.43 ml, 12.5 mmol)
was added and then a solution of ethyl diazoacetate
(0.263 ml, 2.5 mmol) in THF (2.5 ml) was added dropwise
over a period of 2 h. The mixture was stirred for 24 h and
then the solvent and excess ole®n were removed under
vacuum. The residue was puri®ed by column chromato-
graphy on silica gel (hexane/ethyl acetateˆ15/l) to afford
a mixture of ethyl trans- and cis-2-phenylcyclopropane-1-
carboxylates as a colourless oil. The trans/cis ratio and the
ee were determined by GC analysis on a diethyl-t-butylsilyl
b-cyclodextrin capillary column 25 m£0.25 mm operated
at 608C for 5 min, then programmed at 38C min21 to
1608C. Retention times: 33.2 (1S,2S) and 33.5 (1R,2R)
min for trans-13; retention times: 31.4 (1S,2R) and 31.8
(1R,2S) min for cis-14. The results are reported in Table 2.
5. Chelucci, G.; Caria, V.; Saba, A. J. Mol. Catal. A. 1998, 130,
51.
6. Chelucci, G.; Cabras, M. A.; Saba, A. J. Mol. Catal. A 1995,
95, L7.
7. Chelucci, G.; Gladiali, S.; Sanna, M. G.; Brunner, H.
Tetrahedron: Asymmetry 2000, 11, 3419.
8. (a) Leutenegger, U.; Umbricht, G.; Fahrni, C.; von Matt, P.;
Phaltz, A. Tetrahedron 1992, 48, 2143. (b) Lowenthal, R. E.;
Abiko, A.; Mamasume, S. Tetrahedron Lett. 1991, 32, 7373.
9. Chelucci, G.; Pinna, G. A.; Saba, A. Tetrahedron: Asymmetry
1998, 9, 531.
10. Chelucci, G.; Pinna, G. A.; Saba, A. Tetrahedron: Asymmetry
1998, 9, 2575.
3.1.13. Asymmetric cyclopropanation of styrene using
[RuCl2(p-cymene)]2: typical procedure. A solution of
the ligand (0.1 mmol), [RuCl2(p-cymene)]2 (15.3 mg,
0.025 mmol) in CH2Cl2 (2 ml) was stirred under argon
atmosphere for 30 min. After the addition of styrene
(1.43 ml, 12.5 mmol), a solution of ethyl diazoacetate
(0.263 ml, 2.5 mmol) in CH2Cl2 (2.5 ml) was added drop-
wise over a 2 h period. After 2 h stirring at room tempera-
ture, the mixture was worked up as described above. The
results are reported in Table 1.
11. Ziegler, M.; Monney, V.; Stoeckli-Evans, H.; Von Zelewsky,
A.; Sasaki, I.; Dupic, G.; Daran, J.-C.; Balavoine, G. G. A.
J. Chem. Soc., Dalton Trans. 1999, 667 After our research in
this ®eld had begun Von Zelewsky et al. reported the synthesis
of the terpyridine 5a and of the corresponding Rh(III)
complex.
12. For a review on asymmetric hydrosilylation, see: Brunner, H.;
Nishiyama, H.; Itoh, K. In Catalytic Asymmetric Synthesis,
Ojima, I., Ed.; VCH: New York, 1993; pp 303.
13. For recent reviews on asymmetric cyclopropanation, see:
(a) Doyle, M. P.; Protonova, M. N. Tetrahedron 1998, 54,
7919. (b) Singh, V. K.; DattaGupta, A.; Sekar, G. Synthesis
1997, 137.
Acknowledgements
Thanks are due to Mr Mauro Mucedda for experimental
assistance. Financial support by M.U.R.S.T. and by Regione
Autonoma Sardegna is gratefully acknowledged.
14. Mihelich, E. D.; Eickhoff, D. J. J. Org. Chem. 1983, 48, 4135.
È
15. (a) Krohnke, F. Angew. Chem., Int. Ed. Engl. 1963, 2, 386 and
references therein. (b) Sasaki, I.; Daran, J.-C.; Balavoine,
G. G. A. Synthesis 1999, 815.
È
16. Krohnke, F. Synthesis 1976, 1.
References
È
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