Asymmetric synthesis of planar chiral (arene)tricarbonylchromium complexes via enantioselective deprotonation by conformationally constrained chiral Lithium-amide bases

Article abstract of DOI:10.1002/1522-2675(20000906)83:9<2436::AID-HLCA2436>3.0.CO;2-N

Enantioselective lithiation/electrophile addition reactions with eight chiral Li-amide bases, 1-8, and five [Cr(arene)(CO)₃] complexes, 9-13, were investigated. Restriction of conformational freedom in the chiral Li-amide base Li-1, in general, did not result in an increase in asymmetric induction. A new route to enantiomerically enriched (75-92%) planar chiral ortho-substituted benzaldehyde complexes via enantioselective lithiation of benzaldimine complexes 16 and 17 is reported. Within the (1S)-enantiomer series of o-substituted benzaldehyde complexes 18a-d, the sign of the specific rotation, [α](D)/²⁰, is found to be positive, except for the trimethylstannyl derivative 18b. This is interpreted in terms of a reversed conformation of the aldehyde group.

Full text of DOI:10.1002/1522-2675(20000906)83:9<2436::AID-HLCA2436>3.0.CO;2-N

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Helvetica Chimica Acta ± Vol. 83 (2000)
Asymmetric Synthesis of Planar Chiral (Arene)tricarbonylchromium
Complexes via Enantioselective Deprotonation by Conformationally
Constrained Chiral Lithium-Amide Bases
by Sandrine Pache, Candice Botuha, Roberto Franz, and E. Peter Kündig*
Â
Â
Â
Â
Departement de Chimie Organique, Universite de Geneve, 30 Quai Ernest Ansermet, CH-1211 Geneve 4
and Jacques Einhorn
Â
Universite Joseph Fourier, F-38041Grenoble
Dedicated to Professor Albert Eschenmoser on the occasion of his 75th birthday
Enantioselective lithiation/electrophile addition reactions with eight chiral Li-amide bases, 1 ± 8, and five
[Cr(arene)(CO)3] complexes, 9 ± 13, were investigated. Restriction of conformational freedom in the chiral Li-
amide base Li-1, in general, did not result in an increase in asymmetric induction. A new route to
enantiomerically enriched (75 ± 92%) planar chiral ortho-substituted benzaldehyde complexes via enantiose-
lective lithiation of benzaldimine complexes 16 and 17 is reported. Within the (1S)-enantiomer series of o-
substituted benzaldehyde complexes 18a ± d, the sign of the specific rotation, [a]2D0 , is found to be positive,
except for the trimethylstannyl derivative 18b. This is interpreted in terms of a reversed conformation of the
aldehyde group.
Introduction. ± Temporary complexation of an arene to the electrophilic Cr(CO)3
group results in a considerable extension of the scope of synthetically useful
transformations of arenes. [Cr(Arene)(CO)3] Complexes are accessible in high yield
and, while the metalÀarene bond is stable under a variety of reaction conditions, it is
efficiently cleaved by mild oxidation. The role of the Cr(CO)3 group is twofold: it
activates the arene and thus makes reactions possible that are not viable with the free
arene; secondly, it blocks one arene face and thus acts as an efficient stereocontrol unit.
In the complex, benzylic cations and anions are stabilized, ring substitution via
lithiation or nucleophilic addition is readily realized, and mild pathways exist for the
regio- and stereocontrolled transformation of arenes into substituted alicyclic
molecules. These characteristics have led to widerspread use of this class of compounds
in organic synthesis [1].
Complexes of ortho-disubstituted arenes with different substituents are chiral, and
reactions at the ring (e.g., nucleophilic addition) and at side-chain positions often take
place with high diastereoselectivity [2][3]. In addition, planar chiral ortho-disubsti-
tuted [Cr(arene)(CO)3] complexes are also finding use as chiral ligands in asymmetric
catalysis [4]. Stimulated by these successful applications, attention has been directed to
efficient access of enantiomerically pure and enantiomerically enriched complexes.
Resolution procedures can be applied successfully to complexes of ortho-substituted
benzaldehydes [5]. Other methods include diastereoselective complexation of chiral
arenes [3c][5c][6], diastereoselective or enantioselective nucleophilic addition/hy-

Helvetica Chimica Acta ± Vol. 83 (2000)
2437
dride abstraction [7], deprotonation of chirally modified complexes, followed by an
electrophilic quenching [8] and enantioselective ortho-lithiation/electrophile addition
reactions [9].
The last procedure is based on the successful and widely used directed ortho-
metalation procedure [10]. Because of the increased acidity of ring H-atoms in the
complex, deprotonation occurs readily with sec-Li-amides. A number of chiral amines
have been used successfully in these reactions, and the methodology is applicable to
substrates for which other methods fail (e.g., phenol derivatives). Moreover, it allows
introduction of a broad range of ortho-substituents, and, as the chiral information is
carried by an external reagent rather than the complex itself, it avoids the two steps
required for attachment and cleavage of a chiral auxiliary. This makes the recycling of
the chiral agent easier and has the potential to lead to a catalytic process.
The structure of the chiral base is crucial for success in these reactions. Our
knowledge of the transition state of enantioselective arene deprotonation is in its
infancy, and often even the sense of asymmetric induction cannot be predicted. This
situation parallels that found in other enantioselective deprotonation reactions [11]. In
view of this, an extension of the structural variety of reported bases and an extension of
this chemistry to arene complexes that have not received previous attention is
important. These issues are the focus of this paper. We have synthesized five-, six-, and
seven-membered enantiomerically pure cyclic amines, and they, together with other
chiral Li-amide bases, have been screened for their efficiency in regio- and in
enantioselective lithiation reactions of mono-substituted complexes with different
functionality. The bases used in this study are obtained by deprotonation of the chiral
amines 1 ± 8 with BuLi.
The Li-amide derived from 1 [12] has previously been used successfully in the
enantioselective lithiation of the anisole complex 9 [9a,b,e ± h]. The cyclic amines 2, 4,
and 6 were chosen to probe the effect of reducing conformational freedom. For 2 and 4,
the ring structure replaces the benzylic Me groups, whilst the chiral dibenzo[c,e]azepine
6 is the cyclic analogue of 1 bearing an arylÀaryl bond. In addition to the ring structure,
amines 3 and 51) incorporate tertiary benzylic centers. Finally, the non-C2-symmetric
1
)
The assignment of the (R,R) absolute configuration to (À)-5 is tentative. It is based on the analogy of CD
spectra of (‡)-(R,R)-4 and (À)-5, and the sense of asymmetric induction in products 10, 12, and 14.

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Helvetica Chimica Acta ± Vol. 83 (2000)
amines 7 and 8 were selected for their efficiency in the enantioselective deprotonation
of ketones [13 ± 15].
Enantioselective lithiation/electrophile addition was investigated with five [Cr(ar-
ene)(CO)3] complexes. Two are phenol derivatives: the anisole complex 9 and the aryl
carbamate complex 11. The other three are benzaldehyde derivatives: the acetal
complex 13 and the ꢀbenzaldimineꢁ (benzylideneamine) complexes 16 and 17. The
phenol derivatives were chosen because there are no reported efficient alternative
methods of access to this class of non-racemic ortho-substituted complexes. The
situation is different for benzaldehyde complexes, but their widespread use in synthesis
makes the search for new approaches desirable. All five complexes possess functional
groups capable of interaction with the Li-atom of the chiral base; therefore, ortho-
regioselectivity in lithiation can be expected. Complex 13 has a benzylic H-atom and
competitive deprotonation at this site is recognized as a potential problem.
Enantioselective lithiation of complexes 9, 11, and 13 has been investigated previously
[9a,b], but benzaldimine complexes 16 and 17 have not received prior attention in this
reaction sequence.
Results and Discussion. ± 1 .Chiral Amines. Chiral amines 1 ± 3 and 5 ± 7 were
prepared according to literature methods (see Exper. Part). The trans-2,6-diphenylpi-
peridine 4 was synthesized from 1,5-diphenylpentane-1,5-dione according to the same
procedure as described for the pyrrolidine 2 [16]. Amine 8 was prepared by borane
reduction of the amide obtained by reaction of enantiomerically pure sodium
methylbenzylamide with CF3COOEt.
The enantiomeric purity of the amines was confirmed prior to use either by HPLC
(for 1 and 3), or by derivatization with Mosherꢁs acid and integration of the 1H-NMR
signals assigned to benzylic protons or benzylic Me groups. In all cases, only one
enantiomer respectively one diastereoisomer was detected.
2. Enantioselective Lithiation/Electrophile Addition. The reactions were carried out
in THF at À788 with the bases derived from 1 ± 8 and trapping of the lithiated arene
complexes by in situ quenching (ISQ) with an excess of Me3SiCl. The ISQ procedure
was chosen because it was shown by Simpkins and co-workers that the lithiated anisole
complex (Li-9) racemizes rapidly by H-exchange with the starting material [9b]. Our
own experience in the reaction of the carbamate complex 11 with base Li-1 also
indicated a similar problem [9c]. Furthermore, in preliminary experiments involving
the acetal complex 13, ISQ was found to reduce competing benzylic deprotonation and
to give higher enantioselectivity.

Helvetica Chimica Acta ± Vol. 83 (2000)
2439
The sense of induction observed in the reactions with complex 9 correlates with the
absolute configuration of the chiral bases, i.e., the bases with (R)-configuration all gave
(1S,2R)-10 as the major product (Scheme 1), and Entries 3 and 5 in Table 1 show the
reaction to be stereospecific. In terms of the degree of induction, the best result by far
was achieved when Li-1 was employed as base, as first reported by Simpkins and co-
workers [9a]. The inductions obtained with the other bases are poor-to-moderate, and
yields also drop considerably. We note a large increase in induction upon introduction
of a second substituent in a-position to the N-atom in the pyrrolidine bases (Entry 3 vs.
Entry 2). To a much minor extent, this is also observed for the six-membered ring bases
(Entry 5 vs. Entry 4). Remarkable also is the sharp drop in induction when one of the
stereogenic centers in 1 is removed (bases derived from 7 and 8; Entries 7 and 8).
Scheme 1
Table 1. Enantioselective Lithiation/Me3SiCl Quenching with Complex 9
Entry
Base
Yield [%]a)
ee [%]b)
Productc)
1
2
3
4
5
6
7
8
Li-(‡)-(R,R)-1
Li-(‡)-(R,R)-2
Li-(‡)-(S,S)-3
Li-(‡)-(R,R)-4
Li-(‡)-(S,S)-5
Li-(À)-(R,R)-6
Li-(‡)-(R)-7
90d
69
72
67
40
43
93
81(1
90d)
0
73
9
22
14
48
(‡)-(1S)-10
rac-10
(À)-(1R)-10
(‡)-(1S)-10
(À)-(1R)-10
(‡)-(1S)-10
(‡)-(1S)-10
‡)-(1S)-10
Li-(‡)-(R3)-8
a) Yields of isolated products after flash chromatography. b) The enantiomeric excess (ee) of 10 was determined
by chiral HPLC (Chiracel OD-H column, hexane/i-PrOH). c) The absolute configuration of 10 was determined
by correlation of the sign of optical rotation with that given in the literature [9g]. d) Previous literature reports:
[9a,g]: 83% yield, 84% ee; [9e]: 95% yield, 88% ee.
Chelation of the Li-amide by the carbamate complex 11 occurs at the g-position
(carbonyl O-atom), whereas, in the anisole complex, it involved the a-position of the
side chain (Scheme 2). The sense of asymmetric induction in the lithiation step is the
same for 11 as that found for the anisole complex 9. Base Li-1 provided only low
induction (Entry 1 in Table 2). The five- and six-membered Li-amides, and the bases 7
and 8 were even less efficient. Of the series tested here, the best result was achieved
with the chiral dibenzo[c,e]azepine 6. The 62% ee achieved for (1S)-12, albeit with a
modest yield of 48% (Entry 6), is close to that obtained earlier with the base Li-19
(64% ee under ISQ conditions) [9h].

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Helvetica Chimica Acta ± Vol. 83 (2000)
Scheme 2
Table 2. Enantioselective Lithiation/Me3SiCl Quenching with Complex 11
Entry
Base
Yield [%]a)
ee [%]b)
39
Productc)
1
2
3
4
5
6
7
8
Li-(‡)-(R,R)-1
Li-(‡)-(R,R)-2
Li-(‡)-(R,R)-3
Li-(‡)-(R,R)-4
Li-(‡)-(S,S)-5
Li-(À)-(R,R)-6
Li-(‡)-(R)-7
56
510
97
65
72
48
49
88
(‡)-(1S)-12
rac-12
27
29
24
62
13
0
(‡)-(1S)-12
(‡)-(1S)-12
(À)-(1R)-12
(‡)-(1S)-12
(‡)-(1S)-12
rac-12
Li-(‡)-(R)-8
a) Yields of isolated products after flash chromatography. b) Enantiomeric excess (ee) of 12 determined by
chiral HPLC (Chiracel OD column, hexane/i-PrOH). c) Configuration of 12 determined by correlation of the
sign of optical rotation with that given in the literature [9c,h].
As anticipated, the problem with the benzaldehyde acetal complex 13 is that Li-
amides competitively deprotonate at the ring and at benzylic positions2). The data
show that the ratio of products 14 and 15 is sensitive to both the base strength and steric
effects (Scheme 3 and Table 3). Notable is the complete change in regioselectivity of
deprotonation with the two pyrrolidine bases Li-2 and Li-3. The highest ee value in
product 15 was realized with Li-1, but the formation of product mixtures renders this
approach of little synthetic value.
We have previously shown that benzaldimine complex 16 reacts with alkyllithium
reagents by 1,4-addition to give, after oxidation, ortho-substituted benzaldehydes [17].
Cr(CO)3 Complexation can be conserved if the intermediate anionic cyclohexadienyl
complex is treated with a trityl salt to effect an endo-hydride abstraction. This
procedure has been developed into an enantioselective route to Cr(CO)3 complexes of
ortho-substituted benzaldehydes (18) [7]. Enantioselective lithiation would be an
2
)
More reactive bases give higher ratios of ring vs. benzylic deprotonation. Alkyllithium reagents react
selectively at ring positions, and this has been used successfully with chiral reagents [9f].

Helvetica Chimica Acta ± Vol. 83 (2000)
2441
Scheme 3
Table 3. Enantioselective Lithiation/Me3SiCl Quenching with Complex 13
Entry
Base
Product ratio [%]a)
14 15
48 52
ee [%]b)
Configurationc)
1
2
3
4
5
6
7
8
Li-(‡)-(R,R)-1
Li-(‡)-(R,R)-2
Li-(À)-(S,S)-3
Li-(‡)-(R,R)-4
Li-(À)-(R,R)-5
Li-(À)-(R,R)-6
Li-(‡)-(R)-7
81(
±
51
5
4
11
±
‡)-(1S)-14
±
d
0
100
30
57
57
0
1
)
00
00
0e)
70
43
43
( À)-(1R)-14
(‡)-(1S)-14
(‡)-(1S)-14
(‡)-(1S)-14
(‡)-(1S)-14
±
f
1
)
Li-(‡)-(R)-8
±
±g)
±
a) Product ratio 14/15 determined by HPLC. b) Enantiomeric excess (ee) of 14 determined by chiral HPLC
(Chiracel OD column, hexane/i-PrOH). c) Configuration of 14 determined by correlation of the sign of optical
d
rotation with reported literature data [9c]. ) Ratio 13/15 1 : 1 by 1H-NMR of the crude mixture, 46% isolated
f
yield of 15. e) 40% of conversion. ) Ratio 13/15 2 : 1determined by 1H-NMR of the crude mixture. g) Only 13
1
was detected in H-NMR of the crude mixture.
attractive alternative and complementary pathway since the ortho-substituent would be
introduced as an electrophile rather than as a nucleophile. We show here that this can
be successfully realized. Enantioselective deprotonation of the benzaldimine complexes
with several bases led to poor conversion, and we suspect that some of the Li-amides
undergo 1,2-nucleophilic addition to the imine function. The base Li-1 deprotonates
complexes 16 and 17 with high enantioselectivity, however, and these results are shown
in Scheme 4 and Table 4. Mild hydrolysis affords enantiomerically enriched arylalde-
hyde complexes. Enantiomer ratios are between 87:13 (for 18c) and 96 :4 (for 18a).
Scheme 4

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Helvetica Chimica Acta ± Vol. 83 (2000)
Table 4. Enantioselective Lithiation/External Quenching (EQ) with Complexes 16 and 17
Entry
Starting
complex
Reaction
conditions
Electrophile
Yield [%]
ee [%]a)
Productb)
1
2
3
4
5
6
7
8
9
16
16
16
16
16
17
17
17
17
ISQ
EQ
EQ
EQ
EQ
ISQ
EQ
EQ
EQ
Me3SiCl
Me3SiCl
Me3SnCl
MeI
ClCO2Me
Me3SiCl
Me3SiCl
Me3SnCl
ClCO2Me
84
82
67
62
66
76
67
65
68
84
78
78
72
74
88
92
89
90
(‡)-(1S)-18a
(‡)-(1S)-18a
(À)-(1S)-18b
(‡)-(1S)-18c
(‡)-(1S)-18d
(‡)-(1S)-18a
(‡)-(1S)-18a
(À)-(1S)-18b
(‡)-(1S)-18d
a) Determined by chiral HPLC for 18a, 18b, and 18d; and with chiral SFC (Chiracel OD-H column, 10%
MeOH) for 18c. The formation of the chiral aminal with (R,R)-cyclohexane-1,2-diamine and (‡)-18c confirmed
both the configuration and the enantioselectivity [5c]. b) Absolute configuration assigned based on CD
spectroscopy.
An aspect that surprised us initially was the finding that the enantiomerically
enriched complex (1S)-18b (E ˆ SnMe3) has the opposite sign of rotation of polarized
light when compared to (1S)-18a (E ˆ SiMe3) (Scheme 5). An interpretation of
opposite sense of chirality appears unlikely since this would require that the
intermediate aryl-lithium complex rapidly undergoes equilibration, and that the
reaction with the electrophiles Me3SiCl and Me3SnCl, under the influence of the chiral
amine, leads to opposite dynamic resolution. The finding of very similar induction in
both in situ quenching (ISQ) and external quenching (EQ) is an argument for
configurational stability. A more likely explanation, therefore, is that the absolute
configuration is the same in both 18a and 18b.
Scheme 5

Helvetica Chimica Acta ± Vol. 83 (2000)
2443
The sign of [a] in this class of compounds is determined by the orientation of the
CˆO group with respect to the ortho-substituent. In the majority of cases, an anti-
conformation is preferred in order to reduce A1,3-strain [2a][18]. Phenol complexes are
exceptions because of the establishment of intramolecular H-bridging to the ortho-
carbonyl group. We reason that the Lewis acidity of the Me3Sn group, enhanced by the
arene coordinated to the electrophilic Cr(CO)3 group, may bring about a syn-aldehyde
orientation in this case. This then accounts for the change in sign of the first Cotton
effect in the two complexes (Fig.).
Figure. CD Spectra of ortho-(trimethylsilyl)- and ortho-(trimethyltin)benzaldehyde Cr(CO)3 complexes in
CHCl3 (Ð: (‡)-(1S)-18a; ± ± : (À)-(1S)-18b)
The above argument is supported by both spectral (NMR) and chemical data. The
NOESY spectra of 18a and 18b in C6D6 show the NOE effects indicated in Scheme 5.
This is in keeping with an equilibrium between anti- and syn-aldehyde conformation in
18a, whereas the syn-aldehyde conformation is dominant in 18b. Confirmation of the
same absolute configuration of the two compounds was established by the intercon-
version of enantiomerically enriched (À)-18b into (‡)-18a via the route shown in
Scheme 6.
Conclusions. ± The restriction of conformational freedom in the chiral Li-amide
base in general does not result in an increase in asymmetric induction [11c]. The only
example where a cyclic amide base significantly outperformed the base Li-1 is in the
case of the carbamate complex 11. The chiral dibenzo[c,e]azepine 6 adopts a preferred

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Helvetica Chimica Acta ± Vol. 83 (2000)
Scheme 6
conformation in which the Me groups at the stereogenic center are pseudoequatorial.
Thus, the Me groups, the benzylic C-atoms, and the N-atom are close to lying in a plane,
an arrangement that is also observed in acyclic dibenzylic amines [19]. It remains to be
seen if forcing the benzylic Me groups into pseudoaxial positions, e.g., by introducing
Me groups in the ortho-positions of aryl leads to increased selectivity with this base. The
non-C2-symmetric amines 7 and 8, selected for their efficiency in the enantioselective
deprotonation of ketones, performed poorly in this reaction sequence [13 ± 15]. We
were pleased to find, in the course of these studies, a new, highly enantioselective route
to ortho-substituted benzaldehyde complexes, a class of compounds that have been
most widely used in synthesis. Thus, for both anisole and benzaldehyde complexes
simple procedures are now available that afford ortho-substituted complexes with
enantioselectivities in the range of 75 ± 92%. The hypothesis that different CˆO
conformations are adopted in the Me3Sn-substituted benzaldehyde complex and in the
corresponding Me3Si complex has obvious stereochemical implications in diastereo-
selective reactions. This is presently under investigation.
Experimental Part
1. General. See [9h]. Optically enriched secondary amines 1 [19b], 2 [16], 3 [20], 5 [21], and 6 [22], as well as
Cr complexes 9 [23], 11 [9h], 13 [24], and 16 [17], were synthesized following literature procedures. (‡)-(R)-
Benzyl(1-phenylethyl)amine (7; ee 99%, GC) was obtained from Fluka.
2. Synthesis of Chiral Amines. (‡)-(R,R)-2,6-Diphenylpiperidine (4)3). Under N2, 100 ml of dry THF were
added slowly to a mixture of 5.000 g (19.8 mmol) of 1,5-diphenylpentane-1,5-dione and 13.315 g (41.5 mmol,
2.1equiv.) of ( À)-Ipc2BCl (Ipc ˆ isopinocampheyl). The resulting suspension was stirred at À788 for 2 h, then
brought slowly to r.t. and stirred for 15 h (progressive dissolution of solids). THF was removed under vacuum,
and the oily residue was warmed to 408 at 5 mbar pressure during 24 h to remove pinene. Bis(2-hydroxyethyl)-
3
)
For literature precedent of a synthesis via a different route, see [25].

Helvetica Chimica Acta ± Vol. 83 (2000)
2445
amine (4.800 g, 45.5 mmol, 2.3equiv.) in 100 ml of dry Et2O was added dropwise to the residue
at 08. After complete addition, the mixture was stirred for 45 min. The ice bath was removed, and the stirring
was continued overnight at r.t. (formation of a white precipitate). The solid was eliminated by filtration over
Celite. The solvent was removed by evaporation, and the oily orange residue was purified by CC (silca gel;
hexane/Et2O 1:1to 1:3), to give 4.285 g (16.7 mmol, 84%) of ( À)-(S,S)-1,5-Diphenylpentane-1,5-diol as a white
solid. M.p. 958. [a]2D0 ˆ À19.7 (c ˆ 1.065, MeOH). IR (CH2Cl2): 3597, 3063, 2954, 1948, 1883, 1812, 1485, 1447,
1419, 1382, 1305, 1191, 1060, 1022, 919, 896, 847, 624. 1H-NMR (400 MHz, CDCl3): 1.42 ± 1.52 (quint., J ˆ 7.6,
2 H); 1.68 ± 1.92 (m, 4 H); 1.89 (br. s, 2 H); 4.67 (dd, J ˆ 7.8, 5.4, 2 H); 7.26 ± 7.37 (m, 1 0 H). 13C-NMR
(100.5 MHz, CDCl3): 22.3 (CH2); 38.8 (CH2); 74.5 (CH); 125.8 (CH); 127.6 (CH); 128.5 (CH); 144.8 (C). MS:
‡
239 (1, [M À H2O] ), 238 (5), 147 (2), 133 (6), 132 (16), 120 (20), 117 (5), 107 (36), 105 (19), 104 (100), 91 (8),
‡
‡
79 (26), 77 (17). HR-MS: 238.1348 ([M À H2O] , C17H18O ; calc. 238.1358).
A soln. of 2.4 ml (30.8 mmol, 2.5 equiv.) of MsCl in 100 ml of dry CH2Cl2 was cooled to À208 and then
treated dropwise with a soln. of 3.000 g (12.5 mmol, 1 equiv.) of (À)-(S,S)-1,5-Diphenylpentane-1,5-diol and
5.2 ml of Et3N in 100 ml of dry CH2Cl2. After complete addition, the mixture was stirred for 2.5 h at À208.
Allylamine (89 ml, 118 mmol, 9.5 equiv.) was then added, and the resulting soln. was stirred at r.t. overnight.
Volatile products were removed under vacuum, and the residue was diluted with 350 ml of Et2O. The org. phase
was washed with 2 Â 75 ml of an aq. NaHCO3 soln. and 75 ml of brine. The aq. phases were combined and
extracted with 2 portions of Et2O. The combined org. phases were dried (MgSO4). After filtration and
evaporation of solvents, the residue was purified by CC (silica gel; hexane/Et2O 30 :1) to give 2.303 g (8.4 mmol,
67%) of (R,R)-1-allyl-2,6-diphenylpiperidine as a colorless oil. IR (CH2Cl2): 3074, 3019, 2932, 2867, 1948, 1887,
1807, 1638, 1594, 1491, 1447, 1365, 1256, 1223, 1125, 1028, 918. 1H-NMR (400 MHz, CDCl3): 1.65 ± 2.10 (m, 6 H);
2.89 (dd, J ˆ 14.4, 6.2, 1 H); 3.14 (dd, J ˆ 14.4, 6.2, 1 H); 4.21 (dd, J ˆ 6.4, 4.8, 2 H); 4.95 (s, 1H); 4.98 ( d, J ˆ 8.8,
1H); 5.64 ± 5.75 ( m, 1H); 7.22 ± 7.52 ( m, 1 0 H).13C-NMR (100.5 MHz, CDCl3): 19.8 (CH2); 27.9 (CH2); 51.0
‡
(CH2); 58.7 (CH); 115.8 (CH2); 126.4 (CH); 128.0 (CH); 128.1 (CH); 137.5 (CH); 144.4 (C). MS: 277 (36, M ),
276 (12), 236 (9), 201 (16), 200 (100), 146 (21), 144 (44), 118 (13), 117 (65), 115 (12), 104 (56), 91 (43), 77 (13).
‡
‡
HR-MS: 277.1828 (M , C20H23N ; calc. 277.1830).
(R,R)-1-Allyl-2,6-diphenylpiperidine (2.303 g, 8.3 mmol) and 0.369 g (0.43 mmol, 5 mol%) of RhCl(PPh3)3
were placed in a three-neck round-bottomed flask equipped with a N2 inlet, a dropping funnel, and a Claisen
bridge. MeCN/H2O 84 :16 (250 ml) was added, and the resulting soln. was heated to boiling point. The solvent
level was maintained by continuous addition of solvents. After 3.5 h, the mixture was cooled to r.t. and diluted
with Et2O (400 ml). H2O (100 ml) was added, and the phases were separated. The org. phase was washed with
2 Â 200 ml of brine, and aq. phases were back-extracted with 2 Â 100 ml of Et2O. The org. phases were combined,
dried (MgSO4), and filtered over Celite. Evaporation of solvents left an oily residue, which was purified by FC
(silica gel; hexane/Et2O 1:1) to afford 1.411 g (5.95 mmol, 72%) of 4. Yellow oil. [a]2D0 ˆ ‡70.0 (c ˆ 1.205,
CHCl3). IR (CH2Cl2): 3086, 3028, 2937, 2862, 1951, 1887, 1812, 1600, 1494, 1447, 1326, 1210. 1H-NMR (200 MHz,
CDCl3): 1.63 ± 1.78 (m, 2 H); 1.85 ± 2.10 (m, 4 H); 2.10 ± 2.20 (br. s, 1 H); 4.10 ± 4.18 (m, 2 H); 7.20 ± 7.50
(m, 1 0 H).13C-NMR (100.5 MHz, CDCl3): 20.8 (CH2); 31.4 (CH2); 54.8 (CH); 126.6 (CH); 126.7 (CH); 128.5
‡
(CH); 144.3 (C). MS: 237 (61, M ), 236 (19), 194 (17), 160 (15), 133 (21), 132 (41), 120 (46), 117 (37), 106 (30),
‡
‡
105 (17), 104 (100), 103 (13), 91 (35), 78 (11), 77 (13). HR-MS: 237.1508 (M , C17H19N ; calc. 237.1518).
(‡)-(R)-(1-Phenylethyl)(2,2,2-trifluoroethyl)amine (8). NaH (2.2 g, 55 mmol; 55 ± 65% in mineral oil) was
placed in a two-neck 250-ml round-bottomed flask. Dry Et2O (150 ml) was added under N2, and the resulting
suspension was cooled to 08. (‡)-(R)-1-Phenylethylamine (6.36 ml, 50 mmol) was added, and the mixture was
first left 20 min at 08, and then 2 h at r.t. After cooling to 08, 7.16 ml (60 mmol) of CF3COOEt were added, and
the mixture was stirred for 1h at this temp. and then overnight at r.t. Awhite precipitate formed, and the starting
material was no longer visible by TLC. The reaction was quenched by careful addition of 1m HCl (60 ml). The
resulting phases were separated, and the aq. phase extracted with 2 Â 30 ml of Et2O. The combined org. phases
were dried (MgSO4), and the solvent was evaporated to give 10.844 g (49.9 mmol, 100%) of (‡)-(R)-2,2,2-
Trifluoro-N-(1-phenylethyl)acetamide as a white solid. The crude product was used without purification in the
next step. M.p. 888. [a]2D0 ˆ ‡115.3 (c ˆ 1.24,CH2Cl2). IR (CH2Cl2): 3420, 2932, 1713, 1523, 1447, 1207, 1164.
1H-NMR (400 MHz, CDCl3): 1.60 (d, J ˆ 7.0, 3 H); 5.16 (q, J ˆ 7.0, 1H); 6.50 (br. s, 1H); 7.27 ± 7.42 ( m, 5 H).
13C-NMR (100.5 MHz, CDCl3): 21.0 (Me); 49.8 (CH); 116.0 (d, CF3); 126.2 (CH); 128.2 (CH); 129.1 (CH);
‡
140.9 (C); 157.0 (q, CˆO). MS: 217 (100, M ), 216 (71), 215 (10), 203 (9), 202 (91), 148 (10), 132 (11), 107 (26),
‡
105 (61), 104 (42), 103 (31), 96 (12), 79 (53), 78 (18), 77 (39), 69 (37), 51 (26), 50 (11). HR-MS: 217.0700 (M ,
‡
C10H10F3NO ; calc. 217.0714).
The amide (4.366 g, 20 mmol) was placed in a 250-ml round-bottomed flask; 40 ml of a 1m BH3 ´ THF soln.
were added, followed by 40 ml of THF. The resulting soln. was stirred at 408 for 60 h (the reaction was monitored

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Helvetica Chimica Acta ± Vol. 83 (2000)
by IR; disappearance of the amide band). The reaction was then quenched with conc. HCl, the solvent was
removed, and the resulting aq. phase was neutralized by addition of solid NaOH and extracted with 4 portions of
Et2O. The combined org. phases were dried (MgSO4), and the solvent was evaporated. The yellow oil, which was
purified by bulb-to-bulb distillation (608/0.8 mbar), afforded 3.355 g (16.5 mmol, 82%) of 8. Colorless liquid.
[a]2D0 ˆ ‡46.7 (c ˆ 1.305, CH2Cl2). IR (CH2Cl2): 3028, 2965, 2869, 1493, 1451, 1402, 1373, 1278, 1252, 1215, 1146,
1097, 971, 822. 1H-NMR (200 MHz, CDCl3): 1.56 (d, J ˆ 6.6, 3 H); 3.02 ± 3.21( m, 2 H); 5.16 (q, J ˆ 6.6, 1H);
7.25 ± 7.48 (m, 5 H). 13C-NMR (100.5 MHz, CDCl3): 23.0 (Me); 47.3 (m, CH2); 58.2 (CH); 127.1 (CH); 128.7
‡
(C); 129.1 (CH); 129.3 (CH). MS: 203 (1, M ), 202 (4), 189 (14), 188 (100), 126 (21), 110 (24), 106 (15), 105
‡
‡
(38), 91(9), 77 (16). HR-MS: 203.0894 ( M , C10H12NF3 ; calc. 203.0922).
(À)-2,6-Dimethyl-2,6-diphenylpiperidine (5). Compound 5 was prepared according to the procedure
reported by one of us (J. E. ) [21]. Colorless oil. [a]2D0 ˆ À44 (c ˆ 1.5, AcOEt). IR (CHCl3): 3318, 3154, 2938,
2864, 2252, 1793, 1732, 1600, 1493, 1444, 1370, 1078. 1H-NMR (200 MHz, CDCl3): 1.12 (s, 6 H); 1.56 (br. s, 1 H);
1.68 ± 1.82 (m, 2 H); 1.88 ± 2.20 (m, 2 H); 2.17 ± 2.31 (m, 2 H); 7.24 ± 7.33 (m, 2 H); 7.36 ± 7.48 (m, 4 H); 7.71± 7.79
(m, 4 H). 13C-NMR (50.3 MHz, CDCl3): 19.3 (CH2); 33.7 (Me); 37.0 (CH2); 55.6 (C); 126.2 (CH); 126.8 (CH);
‡
128.5 (CH); 151.1 (C). MS: 265 (3, M ), 264 (1), 252 (2), 251 (20), 250 (100), 188 (11), 146 (7), 132 (23), 131
‡
‡
(76), 120 (22), 118 (32), 103 (10), 91 (22), 77 (11). HR-MS: 265.1835 (M , C19H23N ; calc. 265.1830).
3. Determination of Enantiomeric Excess of Chiral Amines 1 ± 6. (‡)-(R,R)-Bis(1-phenylethyl)amine (1).
The trifluoroacetamide derivative was formed, and the two enantiomers were separated by chiral HPLC.
Chiracel OD-H, hexane/i-PrOH 99.5 :0.5, flow: 1ml/min, UV detection: 254 nm, tR ((R,R)) 5.6 min, tR ((S,S))
7.7 min, ee > 98% (only one enantiomer detected).
(‡)-(R,R)-2,5-Diphenylpyrrolidine (2). The amide derivative was formed with (À)-(S)-3,3,3-Trifluoro-2-
methoxy-2-phenylpropanoic acid (Mosherꢁs acid) [26], and de value was determined by integration of the H-
atoms at the stereogenic centers (500 MHz, CDCl3). With rac-2: d 4.55 and 4.85 (t, J ˆ 7.6). With (‡)-(R,R)-2:
d ˆ 4.83, de > 96% (only one diastereoisomer detected).
(À)-(S,S)-2,5-Dimethyl-2,5-diphenylpyrrolidine (3). Determination of the ee via chiral HPLC of the
nitroxide derivative [20].
(‡)-(R,R)-2,6-Diphenylpiperidine (4). The amide derivative was formed with (À)-(S)-3,3,3-Trifluoro-2-
methoxy-2-phenylpropanoic acid (Mosherꢁs acid) [26], and the de value was determined by integration of the H-
atoms at the stereogenic centers of the piperidine (500 MHz, CDCl3). With rac-4: d 4.26 and 4.32 (dd, J ˆ 7.0,
3.9). With (‡)-(R,R)-4: d 4.22, de > 96% (only one diastereoisomer detected).
(‡)-(S,S)-2,6-Dimethyl-2,6-diphenylpiperidine (5). The amide derivative was formed using (À)-(S)-3,3,3-
Trifluoro-2-methoxy-2-phenylpropanoic acid (Mosherꢁs acid) [26], and the de value was determined by
integration of singlets associated with the Me groups of the piperidine ring (500 MHz, CD3OD). With rac-5:
d 1.24 and 1.68. With (‡)-(S,S)-5: d 1.20, de > 96% (only one diastereoisomer detected).
(À)-(R,R)-5,7-Dimethyl-6,7-dihydro-5H-dibenzo[c,e]azepine (6).
A
salt was formed with (À)-(R)-
mandelic acid, and the de value was determined by integration of doublets associated with the benzylic Me
groups (400 MHz, C6D6). With rac-6: d 1.42 and 1.49 (d, J ˆ 7.0). With (À)-(R,R)-6: d 1.42, de > 96% (only one
diastereoisomer detected).
4. Synthesis of Tricarbonyl[(1,2,3,4,5,6-h-benzylidene)phenylamine]chromium (17). (h6-Benzaldehyde)tri-
carbonyl chromium (4.840 g, 20 mmol) [17] was dissolved in 100 ml of Et2O. Aniline (2.2 ml, 24 mmol,
1.2 equiv.) and 6.2 g of 4-Š molecular sieves were added, and the mixture was stirred at r.t. during 8 h. After
filtration over Celite and concentration of the soln., the product was precipitated with hexane and dried under
vacuum, affording 6.397 g (20 mmol, 100%) of 17. Red solid. M.p. 1108. 1H-NMR (200 MHz, C6D6): 0.44
(s, 1H); 4.30 ± 4.48 ( m, 3 H); 5.27 ± 5.34 (m, 2 H); 7.00 ± 7.20 (m, 5 H). 13C-NMR (50.3 MHz, C6D6): 91.6 (CH);
93.7 (CH); 93.8 (CH); 100.2 (C); 121.7 (CH); 127.1 (CH); 128.6 (C); 129.9 (CH); 156.8 (CH); 232.8 (C). MS:
‡
‡
317 (3, M ), 289 (1), 261 (16), 234 (26), 233 (100), 181 (24), 180 (28), 77 (27). HR-MS: 317.0131 (M ,
‡
C16H11NO3Cr ; calc. 317.0144).
5. Lithiation According to the ISQ Method: General Procedure. A 0.02m soln. of the chiral base was
prepared by adding 1.1 equiv. of BuLi (ca. 1 .6m in hexane) to 1.1 equiv. of the chiral amine in THFatÀ788. After
1h, 3 equiv. of Me 3SiCl were added, immediately followed by the complex (in solid form, or dissolved in a
minimum of THF). After complete reaction (TLC), THF was evaporated, the residue was diluted in 15 ml of
Et2O and 15 ml of 1m HCl. The phases were separated, and the aq. phase was extracted twice with 10 ml of Et2O.
The combined org. phases were dried (MgSO4), the solvent was evaporated, and the residue was purified by FC
(silica gel) or analyzed directly by chiral HPLC. The chiral base was recovered by basification of the aq. phase
with 1m NaOH, extraction with 3 portions of Et2O, drying (MgSO4), and evaporation of solvent.

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2447
Chiral Base Li-(‡)-(R,R)-1/Complex 9. The reaction was carried out with 1.480 g (6.07 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/Et2O 95 :5) to yield 1.725 g (5.46 mmol, 90%) of (‡)-(1S)-
10, ee 90%.
Chiral Base Li-(À)-(S,S)-2/Complex 9. The reaction was carried out with 165 mg (0.68 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/Et2O 20 :1), affording 133 mg (0.47 mmol, 69%) of rac-10.
Chiral Base Li-(À)-(S,S)-3/Complex 9. The reaction was carried out with 142 mg (0.57 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/AcOEt 98 :2), affording 131 mg (0.41 mmol, 72%) of (À)-
(1R)-10, ee 73%.
Chiral Base Li-(‡)-(R,R)-4/Complex 9. The reaction was carried out with 209 mg (0.86 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/Et2O 20 :1), affording 183 mg (0.58 mmol, 67%) of (‡)-
(1S)-10, ee 9%.
Chiral Base Li-(‡)-(S,S)-5/Complex 9. The reaction was carried out with 134 mg (0.55 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/Et2O 20 :1), affording 68 mg (0.22 mmol, 40%) of (À)-
(1R)-10, ee 22%.
Chiral Base Li-(À)-(R,R)-6/Complex 9. The reaction was carried out with 106 mg (0.44 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/Et2O 20 :1), affording 60 mg (0.19 mmol, 43%) of (‡)-
(1S)-10, ee 14%.
Chiral Base Li-(‡)-(R)-7/Complex 9. The reaction was carried out with 210 mg (0.86 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/Et2O 20 :1), affording 252 mg (0.80 mmol, 93%) of (‡)-
(1S)-10, ee 48%.
Chiral Base Li-(‡)-(R)-8/Complex 9. The reaction was carried out with 290 mg (1.18 mmol) of 9. The
yellow residue was purified by FC (silica gel; hexane/Et2O 20 :1), affording 301 mg (0.95 mmol, 81%) of (‡)-
(1S)-10, ee 13%.
Tricarbonyl[(1R,2S)-[(1,2,3,4,5,6-h-2-Methoxyphenyl)trimethylsilane]chromium) (10)4) ((‡)-(1S)-10):
Yellow solid. M.p. 1288. [a]2D0 ˆ ‡220 (c ˆ 0.62, CHCl3). IR (hexane): 2930, 2846, 1974, 1904, 1565, 1472,
1451. 1H-NMR (200 MHz, C6D6): 0.30 (s, 9 H); 2.91( s, 3 H); 3.98 (dm, J ˆ 7.1, 1 H); 4.02 (tm, J ˆ 7.1, 1 H); 4.86
(tm, J ˆ 7.1, 1 H); 5.12 (dd, J ˆ 6.1, 1.5, 1 H).13C-NMR (50.3 MHz, C6D6): 0.2 (Me); 55.2 (Me); 73.8 (CH); 85.6
‡
(CH); 88.6 (C); 96.2 (CH); 102.0 (CH); 147.9 (C); 234.6 (C). MS: 316 (14, M ), 260 (6), 233 (27), 232 (100), 217
‡
‡
(5), 202 (6), 201 (7), 187 (8), 165 (16), 135 (44), 91 (6), 52 (32). HR-MS: 316.0252 (M , C13H16O4SiCr ; calc.
316.0223). Chiral HPLC: Chiracel OD, hexane/i-PrOH 90 :10, flow: 0.5 ml/min, UV detection: 254 nm, tR ((À))
13.3 min, tR ((‡)) 18.0 min.
Chiral Base Li-(‡)-(R,R)-1/Tricarbonyl(1,2,3,4,5,6-h-phenyl N,N-diisopropylcarbamate)chromium (11).
The reaction was carried out with 204 mg (0.57 mmol) of 11. The yellow residue was purified by FC (silica gel;
hexane/Et2O 7:3), affording 136 mg (0.32 mmol, 56%) of (‡)-(1S)-12, ee 39%.
Chiral Base Li-(‡)-(R,R)-2/Complex 11. The reaction was carried out with 211 mg (0.59 mmol) of 11. The
yellow residue was purified by FC (silica gel; hexane/Et2O 7 :3), affording 129 mg (0.30 mmol, 51%) of rac-12.
Chiral Base Li-(‡)-(R,R)-3/Complex 11. The reaction was carried out with 275 mg (0.77 mmol) of 11. The
yellow residue was purified by FC (silica gel; hexane/Et2O 7:3), affording 322 mg (0.75 mmol, 97%) of (‡)-
(1S)-12, ee 27%.
Chiral Base Li-(‡)-(R,R)-4/Complex 11. The reaction was carried out with 306 mg (0.86 mmol) of 11. The
yellow residue was purified by FC (silica gel; hexane/Et2O 7:3), affording 202 mg (0.56 mmol, 65%) of (‡)-
(1S)-12, ee 29%.
Chiral Base Li-(‡)-(S,S)-5/Complex 11. The reaction was carried out with 214 mg (0.60 mmol) of 11. The
yellow residue was purified by FC (silica gel; hexane/Et2O 7:3), affording 185 mg (0.43 mmol, 72%) of (À)-
(1R)-12, ee 24%.
Chiral Base Li-(À)-(R,R)-6/Complex 11. The reaction was carried out with 150 mg (0.42 mmol) of 11. The
yellow residue was purified by FC (silica gel; hexane/Et2O 7:3), affording 85 mg (0.20 mmol, 48%) of (‡)-(1S)-
12, ee 62%.
Chiral Base Li-(‡)-(R)-7/Complex 11. The reaction was carried out with 307 mg (0.86 mmol) of 11. The
yellow residue was purified by FC (silica gel; hexane/Et2O 7:3), affording 181 mg (0.42 mmol, 49%) of (‡)-
(1S)-12, ee 13%.
Chiral Base Li-(‡)-(R)-8/Complex 11. The reaction was carried out with 448 mg (1.25 mmol) of 11. The
yellow residue was purified by FC (silica gel; hexane/Et2O 7:3), affording 476 mg (1.11 mmol, 88%) of rac-12.
4
)
The numbering of the benzene moiety differs from that used in the text.

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Helvetica Chimica Acta ± Vol. 83 (2000)
Tricarbonyl[(1R,2S)-(1,2,3,4,5,6-h-2-(trimethylsilyl)phenyl N,N-diisopropylcarbamate]chromium ((À)-
(1R)-12). Yellow solid. M.p. 1228. [a]D20 ˆ À104.4 (c ˆ 0.20, CH2Cl2). IR (CH2Cl2): 2973, 1968, 1892, 1721,
1427, 1405, 1372, 1313, 1274, 1191, 1152, 1039, 979, 842. 1H-NMR (200 MHz, C6D6): 0.29 (s, 9 H); 0.92 ± 1.02
(m, 6 HA); 1.20 ± 1.30 (m, 6 HB); 3.22 (quint., J ˆ 13.3, 1 HA); 4.34 (quint., J ˆ 13.3, 1 HB); 3.96 ± 4.04 (m, 1 H);
4.75 ± 4.86 (m, 2 H); 4.98 ± 5.03 (dm, J ˆ 5.5, 1H). 13C-NMR (50.3 MHz, C6D6): 0.0 (Me); 21.0 (Me); 46.4 (CH);
48.3 (CH); 87.3 (CH); 88.3 (CH); 92.4 (C); 96.3 (CH); 100.2 (CH); 138.5 (C); 152.1 (C); 234.3 (C). MS: 429 (3,
‡
‡
‡
M ), 373 (12), 345 (100), 187 (15), 128 (26), 86 (38), 52 (18). HR-MS: 429.1111 (M , C19H27NO5SiCr ; calc.
429.1064). Chiral HPLC: Chiracel OD, hexane/i-PrOH 99 :1, flow: 0.6 ml/min, UV detection: 254 nm, tR ((À))
19 min, tR ((‡)) 24 min.
Chiral Base Li-(‡)-(R,R)-1/Tricarbonyl[1,2,3,4,5,6-h-1-(1,3-dioxolan-2-yl)phenyl]chromium 13. The reac-
tion was carried out with 230 mg (0.80 mmol) of 13. The yellow residue was analyzed by 1H-NMR, showing a 14/
15 ratio of 48 :52. Separation by FC (silica gel, hexane/Et2O 7:3) afforded 149 mg (0.42 mmol) of 15 and 115 mg
(0.32 mmol, 40%) of (‡)-(1S)-14, ee 81%.
Chiral Base Li-(‡)-(R,R)-2/Complex (13). The reaction was carried out with 217 mg (0.76 mmol) of 13.
The yellow residue was analyzed by 1H-NMR, showing a 13/15 ratio of 1:1. Complex 15 could be separated from
unreacted 13 by FC (silica gel; hexane/Et2O 10 :1), affording 124 mg (0.26 mmol, 46%) of 15.
Chiral Base Li-(À)-(S,S)-3/Complex 13. The reaction was carried out with 139 mg (0.49 mmol) of 13. The
1
yellow residue was not purified, but analyzed by H-NMR to determine the conversion (40%), and by chiral
HPLC to determine the enantiomeric excess ((À)-(1R)-14, 51 % ee).
Chiral Base Li-(‡)-(R,R)-4/Complex 13. The reaction was carried out with 263 mg (0.92 mmol) of 13. The
yellow residue (298 mg) was not purified, but analyzed by 1H-NMR (ratio 14/15 30 :70). The chiral HPLC
showed an ee of 5% ((‡)-(1S)-14).
Chiral Base Li-(À)-(R,R)-5/Complex 13. The reaction was carried out with 267 mg (0.93 mmol) of 13. The
yellow residue was not purified, but analyzed by 1H-NMR (ratio 14/15 57:43). The chiral HPLC showed an ee of
4% ((‡)-(1S)-14).
Chiral Base Li-(À)-(R,R)-6/Complex 13. The reaction was carried out with 159 mg (0.55 mmol) of 13. The
yellow residue was not purified, but analyzed by 1H-NMR (ratio 14/15 57:43). The chiral HPLC showed an ee of
11% ((‡)-(1S)-14).
Chiral Base Li-(‡)-(R)-7/Complex 13. The reaction was carried out with 246 mg (0.86 mmol) of 13. The
yellow residue (253 mg) was not purified, but analyzed by 1H-NMR, showing unreacted 13 and substitution in
benzylic position (15) in a 2 :1ratio.
Chiral Base Li-(‡)-(R)-8/Complex 13. The reaction was carried out with 333 mg (1.16 mmol) of 13. The
1
yellow residue was not purified, but analyzed by H-NMR, showing only unreacted 13.
Tricarbonyl{(1S)-[1,2,3,4,5,6-h-2-(1,3-dioxolan-2-yl)phenyl]trimethylsilane]chromium)4)
((‡)-(1S)-14).
Yellow solid. M.p. 688. [a]2D0 ˆ ‡24.8 (c ˆ 0.25, CHCl3). IR (CHCl3): 3000, 2931, 2886, 2354, 1970, 1888, 1098,
1
848. H-NMR (200 MHz, C6D6): 0.32 (s, 9 H); 3.25 ± 4.02 (m, 4 H); 4.36 (t, J ˆ 6.3, 1H); 4.79 ( t, J ˆ 6.3, 1H);
4.96 (d, J ˆ 6.3, 1H); 5.12 ( d, J ˆ 6.3, 1H); 5.67 ( s, 1 H). 13C-NMR (50.3 MHz, C6D6): 1.0 (Me); 65.6 (CH2); 65.9
(CH2); 88.9 (CH); 91.1 (CH); 94.7 (CH); 98.8 (C); 100.1 (CH); 101.8 (CH); 114.3 (C); 233.8 (C). MS: 358 (37,
‡
M ), 302 (6), 274 (100), 231 (39), 212 (44), 163 (73), 126 (39), 96 (10), 73 (10). Anal. calc. for C15H18O5CrSi:
C 50.27, H 5.06; found: C 50.29, H 5.01. Chiral HPLC: Chiracel OD, hexane/i-PrOH 99.5 :0.5, flow: 0.8 ml/min,
UV detection: 254 nm, tR ((À)) 22 min, tR ((‡)) 24 min.
Tricarbonyl{1,2,3,4,5,6-hÀ1-[2-(trimethylsilyl)-1,3-dioxolan-2-yl]phenylchromium} (15). Yellow solid. M.p.
1478. IR (CHCl3): 3023, 2953, 2872, 1970, 1883, 837, 662. 1H-NMR (200 MHz, C6D6): À0.08 (s, 9 H); 3.44 (t, J ˆ
7.6, 2 H); 4.10 ± 4.28 (m, 4 H); 4.56 (t, J ˆ 7.6, 1H); 5.21( d, J ˆ 7.6, 2 H). 13C-NMR (100.5 MHz, C6D6): À4.41
‡
(CH3); 65.9 (CH2); 88.1 (CH); 93.0 (CH); 95.2 (CH); 122.6 (C); 126.6 (C); 233.6 (C). MS: 358 (13, M ), 302
(8), 274 (25), 149 (100), 126 (40), 105 (37), 73 (26). Anal. calc. for C15H18O5CrSi: C 50.27, H 5.06; found:
C 50.34, H 5.05.
Chiral Base Li-(‡)-(R,R)-1/Tricarbonyl[1,2,3,4,5,6-h-(cyclohexylimino)benzyl]chromium (16). The reac-
tion was carried out with 358 mg (1.1 mmol) of 16. The red residue was purified by FC (silica gel; cyclohexane/
Et2O 10 :1), affording 260 mg (0.92 mol, 84%) of (‡)-(1S)-18a, ee 84%.
Chiral Base Li-(‡)-(R,R)-1/Tricarbonyl[1,2,3,4,5,6-h-(phenylimino)benzyl]chromium (17). The reaction
was carried out with 317 mg (1 mmol) of 17. The red residue was purified by FC (silica gel; cyclohexane/Et2O
10 :1), affording 213 mg (0.76 mmol, 76%) of (‡)-(1S)-18a, ee 88%.
Tricarbonyl(1S)-[1,2,3,4,5,6-h-2-(trimethylsilyl)benzaldehyde]chromium (18a) ((‡)-(1S)-18a). Red oil.
[a]2D0 ˆ ‡104.3 (c ˆ 0.12, CHCl3). IR (CHCl3): 3019, 1982, 1915, 1695, 1198, 847, 780. 1H-NMR (200 MHz,
C6D6): 0.24 (s, 9 H); 4.38 ± 4.52 (m, 2 H); 4.75 ± 4.84 (m, 2 H); 9.13 (s, 1 H). 13C-NMR (100.5 MHz, C6D6):

Helvetica Chimica Acta ± Vol. 83 (2000)
2449
À0.02 (Me); 92.2 (CH); 92.4 (CH); 94.2 (CH); 97.7 (CH); 100.7 (C); 101.0 (C); 190.6 (CH); 231.3 (C). MS: 314
‡
(16, M ), 258 (5), 243 (4), 231 (16), 230 (58), 215 (10), 164 (15), 163 (100), 126 (27), 96 (8), 52 (69). HR-MS:
‡
‡
314.0054 (M , C13H14O4CrSi ; calc. 314.0067). Chiral HPLC: Chiracel OD-H, hexane/i-PrOH 98 :2, flow: 1ml/
min, UV detection: 254 nm, tR ((À)) 10.7 min, tR ((‡)) 12.2 min.
6. Lithiation According to the EQ Method: General Procedure. The chiral Li-amide base was prepared by
adding 1.1 equiv. of BuLi (ca. 1 .6m in hexane) to 1.1 equiv. of the chiral amine (‡)-(R,R)-1 in 10 ± 20 ml of THF
at À788. After 1h at À788, 1equiv. of imine complex 16 or 17 was added as a solid. After 2 h at À788, 3 equiv. of
electrophile were added. After 1h at À788, the mixture was quenched with 1ml of HCl (2 m) and stirred at r.t.
for 1h. THF was evaporated, and the residue was extracted with 15 ml of Et 2O and 15 ml of 1m HCl. The phases
were separated, and the aq. phase was extracted twice with 10 ml of Et2O. The combined org. phases were dried
(MgSO4), concentrated, and the residue was purified by FC (silica gel).
Complex 16/Me3SiCl. The reaction was carried out with 660 mg (2.04 mmol) of 16. The red residue was
purified by FC (silica gel; cyclohexane/Et2O 10 :1), affording 473 mg (1.68 mmol, 82%) of (‡)-(1S)-18a, ee
78%.
Complex 17/Me3SiCl. The reaction was carried out with 330 mg (1.04 mmol) of 17. The red residue was
purified by FC (silica gel; cyclohexane/Et2O 10 :1), affording 197 mg (0.70 mmol, 67%) of (‡)-(1S)-18a, ee 92%.
Complex 16/Me3SnCl. The reaction was carried out with 199 mg (0.62 mmol) of 16. The red residue was
purified by FC (silica gel; pentane/Et2O 9 :1), affording 168 mg (0.41 mmol, 67%) of (À)-(1S)-18b, ee 78%.
Complex 17/Me3SnCl. The reaction was carried out with 317 mg (1 mmol) of 17. The red residue was
purified by FC (silica gel; pentane/Et2O 7 :1), affording 263 mg (0.65 mmol, 65%) of (À)-(1S)-18b, ee 89%.
Tricarbonyl[(R)-1,2,3,4,5,6-h-2-(trimethylstannanyl)benzaldehyde]chromium (18b) ((À)-(1S)-18b). Red
oil. [a]2D0 ˆ À354 (c ˆ 0.185, CHCl3). IR (CH2Cl2): 1978, 1909, 1698, 1260, 1196. 1H-NMR (200 MHz, C6D6): 0.35
(s, 9 H); 4.37 (t, J ˆ 6.3, 1H); 4.51± 4.62 ( m, 2 H); 4.86 (d, J ˆ 6.2, 1H); 9.73 ( s, 1 H). 13C-NMR (100.5 MHz,
C6D6): À7.1(Me); 91.1(CH); 95.2 (CH); 97.8 (CH); 98.1(CH); 99.6 (C); 101.6 (C); 128.3 (CH); 231.6 (C). MS:
410 (1), 408 (1), 406 (7), 405 (3), 404 (5), 403 (3), 402 (3), 391 (3), 322 (21), 320 (16), 318 (10), 255 (18), 253
(14), 225 (13), 223 (10), 172 (33), 158 (20), 157 (100), 52 (57). HR-MS: 403.9481 (M , C13H14O4118CrSn ; calc.
‡
‡
403.9313), 405.9492 (M , C13H14O4120CrSn ; calc. 405.9319). Chiral HPLC: Chiracel OD, hexane/i-PrOH 98 :2,
flow: 0.5 ml/min, UV detection: 254 nm, tR ((‡)) 15.8 min, tR ((À)) 17.7 min.
‡
‡
Complex 16/MeI. The reaction was carried out with 162 mg (0.5 mmol) of 16. The red residue was purified
by FC (silica gel; pentane/Et2O 9 :1), affording 79 mg (0.31 mmol, 62%) of (‡)-(1S)-18c, ee 72%.
Tricarbonyl[(R)-1,2,3,4,5,6-h-2-methylbenzaldehyde]chromium (18c) ((‡)-(1S)-18c). Red oil. [a]D20
ˆ
‡464 (CHCl3, c ˆ 0.125). IR (CH2Cl2): 1982, 1913, 1694, 1268, 1203. 1H-NMR (400 MHz, C6D6): 1.88
(s, 3 H); 3.99 (d, J ˆ 6.4, 1H); 4.21( t, J ˆ 6.4, 1H); 4.72 ( t, J ˆ 6.2, 1H); 5.49 ( d, J ˆ 6.5, 1H); 9.28 ( s, 1 H).
13C-NMR (100.5 MHz, C6D6): 17.3 (Me); 87.5 (CH); 91.2 (CH); 94.6 (CH); 93.8 (C); 95.0 (CH); 116.7 (C); 186.7
‡
‡
(CH); 230.9 (C). MS: 256 (13, M ), 200 (4), 173 (8), 172 (42), 53 (12), 52 (100). HR-MS: 255.9834 (M ,
C11H8CrO4 ; calc. 255.9828). Chiral SFC: Chiracel OD-H, 10% MeOH, flow: 2 ml/min, UV detection: 220 nm,
‡
tR ((À)) 21.4 min, tR ((‡)) 23.3 min, ee 72%. Formation of the chiral aminal with (R,R)-cyclohexane-1,2-diamine
1
and analysis by H-NMR showed a de > 70% and confirmed the absolute configuration [5c].
Complex 16/ClCO2Me. The reaction was carried out with 162 mg (0.5 mmol) of 16. The dark-red residue
was purified by FC (silica gel; pentane/Et2O 4 :1), affording 99 mg (0.33 mmol, 66%) of (‡)-(1S)-18d, ee 74%.
Complex 17/ClCO2Me. The reaction was carried out with 317 mg (1.0 mmol) of 17. The dark-red residue
was purified by FC (silica gel; pentane/Et2O 4 :1), affording 205 mg (0.68 mmol, 68%) of (‡)-(1S)-18d, ee 90%.
Tricarbonyl[methyl (S)-1,2,3,4,5,6-h)-2-formylbenzoate]chromium (18d)4) ((‡)-(1S)-18d). Dark-red solid.
[a]2D0 ˆ ‡432 (CHCl3, c ˆ 0.053). IR (CH2Cl2): 1997, 1937, 1725, 1689, 1602, 1272.1H-NMR (200 MHz, C6D6):
3.27 (s, 3 H); 4.26 (t, J ˆ 6.3, 1H); 4.40 ( t, J ˆ 6.4, 1H); 5.11 ( d, J ˆ 6.6, 1H); 5.51 ( d, J ˆ 6.6, 1H); 10.32
(s, 1 H). 13C-NMR (100.5 MHz, C6D6): 52.4 (Me); 90.7 (CH); 90.8 (CH); 91.8 (CH); 91.9 (CH); 94.8 (C); 97.2
‡
(C); 165.8 (CH); 188.2 (CH); 229.8 (C). MS: 300 (7, M ), 244 (4), 216 (24), 188 (14), 158 (25), 53 (13), 52 (100).
‡
‡
HR-MS: 299.9729 (M , C12H8CrO6 ; calc. 299.9726). Chiral HPLC: Chiracel OD, hexane/i-PrOH 90 :10, flow:
0.3 ml/min, UV detection: 245 nm, tR ((À)) 60.3 min, tR ((‡)) 66.8 min.
7. Transformation of (À)-(1S)-18b into (‡)-(1S)-18a. Tricarbonyl{[1,2,3,4,5,6-h-2-(1,3-dioxolan-2-yl)phe-
nyl]trimethylstannane}chromium (20)4) ((1S)-20). Complex (À)-(1S)-18b (73% ee) (258 mg, 0.64 mmol) was
dissolved in 2 ml of THF. Bis(trimethylsilyl)ethylene glycol (1.1 equiv.) was added, followed by 10 mol-% of
Me3SiOTf [27]. After 1h, the mixture was diluted with Et 2O and filtered through a plug of silica, affording
236 mg (0.52 mmol, 81%) of (1S)-20 as an orange oil. IR (CHCl3): 2977, 2872, 1965, 1888, 1108. 1H-NMR
(200 MHz, C6D6): 0.32 (s, 9 H); 3.15 ± 3.43 (m, 4 H); 4.40 (td, J ˆ 6.2, 1.1, 1 H); 4.71 (td, J ˆ 6.5, 1.1, 1 H); 4.97
(dd, J ˆ 6.2, 1.1, 1 H); 5.20 (dd, J ˆ 6.6, 1.0, 1 H); 10.32 (s, 1 H). 13C-NMR (50.3 MHz, C6D6): À6.3 (Me); 65.4

2450
Helvetica Chimica Acta ± Vol. 83 (2000)
(CH2); 65.7 (CH2); 91.1 (CH); 92.4 (CH); 94.0 (CH); 100.7 (CH); 102.5 (CH); 115.8 (C), 108.2 (C), 234.1 (C).
MS: 452 (2), 451 (2), 450 (9), 449 (4), 448 (7), 447 (3), 446 (4), 406 (4), 366 (14), 364 (11), 338 (11), 320 (12),
307 (11), 306 (14), 305 (11), 304 (12), 291 (10), 225 (16), 253 (13), 225 (15), 223 (12), 188 (10), 186 (10), 172
‡
‡
(21), 158 (22), 157 (100), 53 (12), 52 (98). HR-MS: 447.9622 (M , C15H18O5Cr118Sn ; calc. 447.9576) and
‡
‡
449.9615 (M , C15H18O5Cr120Sn ; calc. 449.958 (1).
Complex (1S)-20 (205 mg, 0.46 mmol) was dissolved in 5 ml of THF. The soln. was cooled to À788, and
1equiv. of BuLi was added. After 1h, 3 equiv. of Me 3SiCl were added. The mixture was left 30 min at À788,
quenched with 1ml of sat. NaHCO 3 soln. and filtered through a plug of silica, affording 149 mg (0.42 mmol,
91%) of (‡)-(1S)-14.
Complex (‡)-(1S)-14 (120 mg, 0.34 mmol) was dissolved in 5 ml of THF, and 1 ml of conc. HCl was added.
After 2 h, the reaction was complete (followed by TLC). The mixture was put directly on a column and purified
by FC (silica gel; pentane/Et2O 9 :1to 4 :1), affording 91mg (0.32 mmol, 96%) of ( ‡)-(1S)-18a, [a]2D0 ˆ ‡76
(c ˆ 0.185, CHCl3), ee 67% (by chiral HPLC).
We thank the Swiss National Science Foundation for financial support. S. P. acknowledges a scholarship
from the Roche Research Foundation. We are grateful to Mr. P. Romanens for technical assistance and to Mr.
J. P. Saulnier, Mr. A. Pinto, and Mrs. D. Klink for NMR and MS measurements. We thank Prof. A. Alexakis and
A. Tomassini for help with the chiral SFC measurements.
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Received May 15, 2000