Applications of 4,4′-Me3Si2-BINAP in transition-metal-catalyzed asymmetric carbon-carbon bond-forming reactions

Article abstract of DOI:10.1021/ol050834s

(Chemical Equation Presented) A recently developed BINAP derivative with trimethylsilyl substituents on the 4- and 4′-positions of the binaphthyl skeleton, 2,2′-bis-(diphenylphosphino)-4,4′-bis(trimethylsilyl)-1, 1′-binaphthyl (tms-BINAP), was used in a variety of transition-metal- catalyzed asymmetric carbon-carbon bond-forming reactions. In π-allylpalladium-mediated reactions, tms-BINAP gave better enantioselectivity than the unsubstituted BINAP, and the origin of the improved enantioselectivity was gained from an X-ray structural study of [Pd(η³-C ₃H₅)((R)-tms-BINAP)]ClO₄.

Full text of DOI:10.1021/ol050834s

ORGANIC
LETTERS
2005
Vol. 7, No. 14
2881-2884
Applications of 4,4
Transition-Metal-Catalyzed Asymmetric
Carbon Carbon Bond-Forming
′-(Me3Si)2-BINAP in
−
Reactions
Masamichi Ogasawara,*,† Helen L. Ngo,‡ Takeshi Sakamoto,†
Tamotsu Takahashi,*,† and Wenbin Lin*,‡
Catalysis Research Center and Graduate School of Pharmaceutical Sciences, Hokkaido
UniVersity, and SORST, Japan Science and Technology Agency (JST), Kita-ku,
Sapporo 001-0021, Japan, and Department of Chemistry, CB# 3290, UniVersity of
North Carolina, Chapel Hill, North Carolina 27599
ogasawar@cat.hokudai.ac.jp; wlin@unc.edu.
Received April 16, 2005
ABSTRACT
A recently developed BINAP derivative with trimethylsilyl substituents on the 4- and 4
(diphenylphosphino)-4,4 -bis(trimethylsilyl)-1,1 -binaphthyl (tms-BINAP), was used in a variety of transition-metal-catalyzed asymmetric carbon
carbon bond-forming reactions. In -allylpalladium-mediated reactions, tms-BINAP gave better enantioselectivity than the unsubstituted BINAP,
and the origin of the improved enantioselectivity was gained from an X-ray structural study of [Pd(
3-C3H5)((R)-tms-BINAP)]ClO4.
′-positions of the binaphthyl skeleton, 2,2′-bis-
′
′
−
π
η
Design of chiral ligands is central to the development of
transition-metal-catalyzed asymmetric reactions. Among
numerous reported chiral ligands, 2,2′-bis(diphenylphos-
phino)-1,1′-binaphthyl (BINAP) is arguably the most suc-
cessful one to date.1 Since the first report of BINAP on the
rhodium-catalyzed asymmetric hydrogenation in 1980,1a
transition-metal/BINAP complexes have been used for a wide
range of asymmetric reactions with good enantioselectivity,
which include Rh-catalyzed isomerization of allylamines,2
Ru-catalyzed hydrogenation of carbonyl groups,3 Pd-
catalyzed Heck reaction,4 Rh-catalyzed conjugate addition
of aryl- or alkenyl-nucleophiles,5 Ir-catalyzed Pauson-Khand
type cyclization,6 and Ag-catalyzed allylation of carbonyl
(2) (a) Tani, K.; Yamagata, T.; Otsuka, S.; Akutagawa, S.; Kumobayashi,
H.; Taketomi, T.; Takaya, H.; Miyashita, A.; Noyori, R. J. Chem. Soc.,
Chem. Commun. 1982, 600. (b) Akutagawa, S. In ComprehensiVe Asym-
metric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer: Berlin, 1999; Vol. 2, Chapter 23.
(3) (a) Ohkuma, T.; Ooka, H.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J.
Am. Chem. Soc. 1995, 117, 2675. (b) Ohkuma, T.; Noyori, R. In
ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yama-
moto, H., Eds.; Springer: Berlin, 1999; Vol. 1, Chapter 6.1.
(4) (a) Sato, Y.; Sodeoka, M.; Shibasaki, M. J. Org. Chem. 1989, 54,
4738. (b) Carpenter, N. E.; Kucera, D. J.; Overman, L. E. J. Org. Chem.
1989, 54, 5846. (c) Ozawa, F.; Kubo, A.; Hayashi, T. J. Am. Chem. Soc.
1991, 113, 1417. (d) Shibasaki, M.; Vogl, E. M. In ComprehensiVe
Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
Springer: Berlin, 1999; Vol. 1, Chapter 14.
† Hokkaido University.
‡ University of North Carolina.
(1) (a) Miyashita, A.; Yasuda, A.; Takaya, H..; Toriumi, K.; Ito, T.;
Souchi, T.; Noyori, R. J. Am. Chem. Soc. 1980, 102, 7932. (b) Noyori, R.;
Takaya, H. Acc. Chem. Res. 1990, 23, 345.
(5) (a) Takaya, Y.; Ogasawara, M.; Hayashi, T.; Sakai, M.; Miyaura, N.
J. Am. Chem. Soc. 1998, 120, 5579. (b) Hayashi, T.; Yamasaki, K. Chem.
ReV. 2003, 103, 2829. (c) Hayashi, T. Bull. Chem. Soc. Jpn. 2004, 77, 13.
10.1021/ol050834s CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/08/2005

groups.7 Structural modification of BINAP has been exten-
sively examined to improve enantioselectivity of the asym-
metric reactions catalyzed by transition-metal/BINAP com-
plexes. Notable examples include modifications of the phenyl
groups of the -PPh2 moieties of BINAP8 and the develop-
ment of atropisomeric bisphosphines based on modified
biaryls such as H8-BINAP,9 MeO-biphep,10 biphemp,11 and
segphos.12
Among the 4,4′-disubstituted BINAPs, those with Me3Si-
or (HO)2P(O)-substituents have been shown to give the
highest enantioselectivity in Ru-catalyzed asymmetric hy-
drogenation reactions.13 Since some of the reactions exam-
ined here require aprotic conditions, 4,4′-(Me3Si)2-BINAP
(tms-BINAP) was chosen as a representative of the 4,4′-
disubstituted BINAPs for the present study (Figure 1).
Recently, we have reported a novel strategy of BINAP
modification by introducing sterically encumbered substit-
uents at the 4- and 4′-positions of the binaphthyl skeleton,
which drastically enhances enantioselectivity in the Ru-
catalyzed asymmetric hydrogenation of a variety of carbonyl
compounds.13 Herein, we wish to report the effectiveness of
this novel class of modified BINAPs in transition-metal-
catalyzed asymmetric carbon-carbon bond-forming reac-
tions. Four different asymmetric carbon-carbon bond-
forming reactions were examined in this work: (1) Pd-
catalyzed asymmetric synthesis of axially chiral allenes from
2-bromo-1,3-dienes,14 (2) Pd-catalyzed asymmetric allylation
of prochiral nucleophiles,15 (3) Rh-catalyzed conjugate
addition of ArB(OH)2 to R,â-unsaturated carboxylic es-
ters,5b-c,16 and (4) Pd-catalyzed asymmetric allylic alkylation
reactions.17 In the first three reactions, BINAP has shown
superiority over other chiral phosphines; however, the
reported enantioselectivity still has room for further improve-
ment.
Figure 1. BINAP and 4,4′-(Me3Si)2-BINAP (tms-BINAP).
The effect of tms-substituents was first examined in the
Pd-catalyzed asymmetric synthesis of axially chiral allenes.14
For a direct comparison between BINAP and tms-BINAP,
two reactions, one with BINAP and the other with tms-
BINAP, were set up simultaneously and carried out side by
side under identical conditions. The results are summarized
in Table 1. In the previous report on the asymmetric allene
(6) Shibata, T.; Takagi, K. J. Am. Chem. Soc. 2000, 122, 9852.
(7) Yanagisawa, A.; Nakashima, H.; Ishiba, A.; Yamamoto, H. J. Am.
Chem. Soc. 1996, 118, 4723.
(8) (a) Takaya, H.; Mashima, K.; Koyano, K.; Yagi, M.; Kumobayashi,
H.; Taketomi, T.; Akutagawa, S.; Noyori, R. J. Org. Chem. 1986, 51, 629.
(b) Mashima, K.; Kusano, K.; Sato, N.; Matsumura, Y.; Nozaki, K.;
Kumobayashi, H.; Sayo, N.; Hori, T.; Ishizaki, T.; Akutagawa, S.; Takaya,
H. J. Org. Chem. 1994, 59, 3064. (c) Gladiali, S.; Dore, A.; Fabbri, D.;
Medici, S.; Pirri, G.; Pulacchini, S. Eur. J. Org. Chem. 2000, 2861.
(9) (a) Zhang, X.; Mashima, K.; Koyano, K.; Sayo, N.; Kumobayashi,
H.; Akutagawa, S.; Takaya, H. Tetrahedron Lett. 1991, 32, 7283. (b) Zhang,
X.; Mashima, K.; Koyano, K.; Sayo, N.; Kumobayashi, H.; Akutagawa,
S.; Takaya, H. J. Chem. Soc., Perkin Trans. 1 1994, 2309.
(10) Schmid, R.; Foricher, J.; Cereghetti, M.; Scho¨nholzer, P. HelV. Chim.
Acta 1991, 74, 370.
(11) (a) Schmid, R.; Cereghetti, M.; Heiser, B.; Scho¨nholzer, P.; Hansen,
H. J. HelV. Chim. Acta 1988, 71, 897. (b) Frejd, T.; Klingstedt, T. Acta
Chem. Scand. 1989, 43, 670.
(12) Saito, T.; Yokozawa, T.; Ishizaki, T.; Moroi, T.; Sayo, N.; Miura,
T.; Kumobayashi, H. AdV. Synth. Catal. 2001, 343, 264.
(13) (a) Hu, A.; Ngo, H. L.; Lin, W. Angew. Chem., Int. Ed. 2004, 43,
2501. (b) Hu, A.; Ngo, H. L.; Lin, W. Org. Lett. 2004, 6, 2937. (c) Hu, A.;
Lin, W. Org. Lett. 2005, 7, 455. (d) Ngo, H. L.; Lin, W. J. Org. Chem.
2005, 70, 1177.
(14) (a) Ogasawara, M.; Ikeda, H.; Hayashi, T. Angew. Chem., Int. Ed.
2000, 39, 1042. (b) Ogasawara, M.; Ikeda, H.; Nagano, T.; Hayashi, T.
Org. Lett. 2001, 3, 2615. (c) Ogasawara, M.; Ikeda, H.; Nagano, T.; Hayashi,
T. J. Am. Chem. Soc. 2001, 123, 2089. (d) Ogasawara, M.; Ueyama, K.;
Nagano, T.; Mizuhata, T.; Hayashi, T. Org. Lett. 2003, 5, 217.
(15) (a) Kuwano, R.; Ito, Y. J. Am. Chem. Soc. 1999, 121, 3236. (b)
Kuwano, R.; Nishio, R.; Ito, Y. Org. Lett. 1999, 1, 837. (c) Kuwano, R.;
Uchida, K.; Ito, Y. Org. Lett. 2003, 5, 2177.
Table 1. Pd-Catalyzed Asymmetric Synthesis of Axially Chiral
Allenesa
% eec
entry
1
2
base
L*
solvent yieldb/% (config)d
1a
1b
2a
2b
1a 2m CsOtBu (R)-BINAP
CH2Cl2 70 (3am) 74 (R)
72 (3am) 85 (R)
(R)-tms-BINAP
(R)-BINAP
(R)-tms-BINAP
1a 2n NaH
THF
THF
THF
80 (3an) 70 (R)
82 (3an) 80 (R)
73 (3bo) 53 (R)
83 (3bo) 61 (R)
76 (3cp) 62 (R)
98 (3cp) 77 (R)
3ae,f 1b 2o CsOtBu (R)-BINAP
3be,f
4a
4b
(R)-tms-BINAP
(R)-BINAP
1c 2p KH
(R)-tms-BINAP
a All the reaction were carried out with 1 (0.50 mmol), 2 (0.55 mmol),
and base (0.55 mmol) in a given solvent (5.0 mL) for 24 h in the presence
of a Pd catalyst (10 mol %) generated from Pd(dba)2 and the chiral
phosphine. b Isolated yield by chromatography on alumina. c Determined
by chiral HPLC (Chiralpak AD-H (3am and 3an), Chiralcel OD-H (3bo
and 3cp)). d The absolute configurations were deduced by the Lowe-
Brewster rule (ref 19). e With 5 equiv of 2o with respect to 1b. f At 0 °C.
(16) Takaya, Y.; Senda, T.; Kurushima, H.; Ogasawara, M.; Hayashi,
T. Tetrahedron: Asymmetry 1999, 10, 4047.
(17) For reviews: (a) Trost, B. M.; Chulbom, L. In Catalytic Asymmetric
Synthesis, 2nd ed.; Ojima, I., Ed.; VCH: New York, 2000; p 593. (b) Pfaltz,
A.; Lautens, M. In ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N.,
Pfaltz, A., Yamamoto, H., Eds.; Springer: Berlin, 1999; Vol. 2, Chapter
24. (c) Trost, B. M.; Van Vranken, D. L. Chem. ReV. 1996, 96, 395. (d)
Hayashi, T. In Catalytic Asymmetric Synthesis; Ojima, I., Ed.; VCH: New
York, 1993; p 325. (e) Frost, C. G.; Howarth, J.; Williams, J. M. J.
Tetrahedron: Asymmetry 1992, 3, 1089.
synthesis, malonate derivatives, such as 2m and 2n, were
used as pronucleophiles.14c,d While the Pd/BINAP catalyst
gave the axially chiral allene 3am with 74% ee for the
reaction of the tBu-substituted bromodiene 1a with 2m (entry
2882
Org. Lett., Vol. 7, No. 14, 2005

1a), the Pd/tms-BINAP catalyst afforded 3am in 85% ee
under the identical conditions except the ligand (entry 1b).
Similarly, tms-BINAP gave higher ee’s than BINAP for the
reaction of 1a and 2n to give 3an (80% ee vs 70% ee; entries
2a and 2b) and 1b and 2o to give 3bo (61% ee vs 53% ee;
entries 3a and 3b). An N-nucleophile generated from HN-
(boc)2 (2o) and KH was found to be effective for the
asymmetric reaction.18 The asymmetric amination product
3cp of 62% ee was obtained in 76% yield using BINAP
(entry 4a). In comparison, the Pd/tms-BINAP system af-
forded 3cp with 77% ee in 98% yield under the same
conditions (entry 4b).
vs 90% ee; entries 2a and 2b), and 4b and 5m, giving 6bm
(84% ee vs 72% ee; entries 3a and 3b).
Contrary to the above-mentioned Pd-catalyzed reactions,
tms-BINAP did not give ee enhancement for the Rh-
catalyzed conjugate addition of arylboronic acids to R,â-
unsaturated carboxylic esters.16 A representative example was
shown in Scheme 1. For the reaction of methyl 2-hexenoate
(7) with PhB(OH)2 (8), the Rh/BINAP catalyst gave methyl
3-phenylhexanoate (9) of 86% ee in 96% yield. The
enantioselectivity of the Rh/tms-BINAP was slightly lower
for the same reaction, and the addition product 9 was
obtained in 98% yield with 84% ee.
It is known that the key intermediates for the two Pd-
catalyzed reactions in which tms-BINAP are effective are
quite similar to each other. The reactions in Table 2
proceeded via a well-known π-allylpalladium intermediate.15a
The intermediate for the Pd-catalyzed allene formation
process (Table 1) is a (1,2,3-η3-butadien-3-yl)palladium
species,14a,c which possesses a π-allylpalladium substructure
as well. Apparently, the effect of the Me3Si substituents was
not operative in the Rh-catalyzed reaction (Scheme 1), of
Encouraged by these results, we have examined the effects
of tms-BINAP on Pd-catalyzed allylations of prochiral
nucleophiles.15 BINAP was reported to be a particularly
effective chiral ligand for the reactions using R-acetamido-
â-ketoesters as pronucleophiles. As shown in Table 2, the
Table 2. Pd-Catalyzed Asymmetric Allylation of Prochiral
Nucleophilesa
Scheme 1
% eec
entry
4
5
L*
yieldb/%
(config)d
1a
1b
2a
2b
3a
3b
4a
5m
(R)-BINAP
(R)-tms-BINAP
(R)-BINAP
(R)-tms-BINAP
(R)-BINAP
(R)-tms-BINAP
87 (6am)
75 (6am)
78 (6an)
68 (6an)
93 (6bm)
90 (6bm)
68 (R)
77 (R)
90 (R)
93 (R)
72 (R)
84 (R)
4a
4b
5n
which the catalytic cycle21 as well as the structure of the
suggested stereodetermining intermediates5a,16 were different
from the above-mentioned π-allylpalladium-mediated reac-
tions.
5m
a All the reactions were carried out with 4 (0.50 mmol) and 5 (0.80 mmol)
at -25 °C20 in toluene (2.5 mL) in the presence of a Pd catalyst (1 mol %)
generated from [PdCl(π-allyl)]2 and the chiral phosphine. b Isolated yield
by silica gel chromatography. c Determined by chiral HPLC (Chiralcel
OD-H (6am and 6an), Chiralcel OJ-H (6bm)). d Determined on the basis
of the sign of the specific rotations of the products.
To gain insight into the origin of the interesting enantio-
enhancement of the tms-BINAP ligand in the π-allylpalla-
dium-mediated asymmetric reactions, we have prepared
[Pd(η3-C3H5)((R)-tms-BINAP)]ClO4 and determined its struc-
ture by single-crystal X-ray diffraction studies.22 As shown
in Figure 2 for the space-filling model of the [Pd(η3-C3H5)-
((R)-tms-BINAP)]+ ion, there is steric interaction between
the SiMe3 substituents on the binaphthyl skeleton and the
phenyl groups of the diphenylphosphino moieties. Conse-
quently, the average dihedral angle of 76.7° between the
naphthyl rings in [Pd(η3-C3H5)((R)-tms-BINAP)](ClO4) is
smaller than that between the naphthyl rings in the BINAP
analogue (79.9°).15a This change in the dihedral angle can
in effect tilt the equatorial phenyl groups toward the
coordinating η3-allyl moiety to presumably lead to a better
stereodiscrimination between the favored and disfavored
Pd-catalyst with tms-BINAP gave higher ee’s for these
reactions. For instance, the allylation product 6am of 68%
ee was obtained by a Pd/BINAP-catalyzed reaction of 4a
and 5m in toluene at -25 °C (entry 1a).20 In comparison,
the Pd/tms-BINAP catalyst afforded 6am of 77% ee under
the same conditions (entry 1b). Analogously, the Pd/tms-
BINAP catalyst exhibited higher ee’s than the Pd/BINAP
catalyst for the reactions of 4a and 5n, giving 6am (93% ee
(18) Recently, Imada et al. reported dynamic kinetic resolution of racemic
allenylmethyl esters by the Pd-catalyzed amination giving allenic amines;
see Nishida, M.; Kutsuwa, K.; Imada, Y.; Murahashi, S.-I. Naota, T. In
Abstracts; 51st Symposium on Organometallic Chemistry, October 2-3,
2004, Toyko, Japan; Kinki Chemical Society: Japan, 2004; PB153. Also
see Imada, Y.; Ueno, K.; Kutsuwa, K.; Murahashi, S.-I. Chem. Lett. 2002,
140.
(21) Hayashi, T.; Takahashi, M.; Takaya, Y.; Ogasawara, M. J. Am.
Chem. Soc. 2002, 124, 5052.
(22) η3-C3H5)((R)-tms-BINAP)]ClO4 was prepared according to the
literature procedure for its BINAP analogue and crystallizes (from slow
evaporation of a CH2Cl2/2-propanol solution) in the chiral space group P21
with two molecules in each asymmetric unit. See Pregosin, P. S.; Ruegger,
H.; Salzmann, R.; Albinati, A.; Lianza, F.; Kunz, R. W. Organometallics
1994, 13, 83.
(19) (a) Lowe, G. Chem. Commun. 1965, 411. (b) Brewster, J. H. Top.
Stereochem. 1967, 2, 1.
(20) In the original report by Kuwano and Ito,15a the reactions were
performed at -30 °C. Because of a limitation of the equipment available
in our laboratory, we carried out the reactions at -25 °C.
Org. Lett., Vol. 7, No. 14, 2005
2883

Table 3. Pd-Catalyzed Asymmetric Allylic Alkylationsa
% eec
conditions yieldb/% (config)d
entry 10 11
L*
1a 10a 11m (R)-BINAP
rt, 12 h
85 (12am)
96 (12am)
25 (R)
80 (R)
84 (R)
1b
(R)-tms-BINAP
Figure 2. Capped stick and space-filling models of single-crystal
X-ray structure of [Pd(η3-allyl)((R)-tms-BINAP)]ClO4. Red, Ru;
orange, P; green, Si; gray, C; white, H. The allyl carbon atoms are
2a 10a 11n (R)-BINAP
0 °C, 24 h 89 (12an)
2b
(R)-tms-BINAP
>99 (12an) 94 (R)
3a 10b 11m (R)-BINAP
0 °C, 24 h 91 (12bm)
40 (S)
57 (S)
-
highlighted in light blue color. The ClO4 anion is omitted for
3b
(R)-tms-BINAP 94 (12bm)
clarity.
a All the reactions were carried out with 10 (0.50 mmol) and 11 (0.55
mmol) in THF (5.0 mL) in the presence of a palladium catalyst (2 mol %)
generated from [PdCl(π-allyl)]2 and the chiral phosphine. b Isolated yield
by silica gel chromatography. c Determined by chiral HPLC on a Chiralpak
AD-H column. d Determined on the basis of the sign of the specific rotations
of the products.
diastereomeric transition states during the subsequent step
of nucleophilic attack on the π-allyl group.
The X-ray structure study as well as the results in Tables
1 and 2 prompted us to explore the effect of tms-BINAP on
Pd-catalyzed asymmetric allylic alkylations.17 The allylic
alkylation reactions shown in Table 3 proceed via an
essentially identical intermediate to that of the reactions in
Table 2, with the key difference of generating a stereogenic
center on the electrophile fragment. Previous studies indi-
cated that BINAP was not a suitable ligand for this
asymmetric reaction, and the Pd/BINAP catalyst showed only
modest enantioselectivity for most cases.23 As shown in Table
3, while the Pd/BINAP catalyst gave the alkylated product
12am with only 25% ee for the reaction of 1,3-diphenyl-2-
propenyl acetate (10a) with sodium dimethyl methylmalonate
(11m) (entry 1a), the Pd/tms-BINAP catalyst afforded 12am
of a much higher 80% ee under the same conditions (entry
1b). Similarly, tms-BINAP showed a better enantioselectivity
than BINAP for reactions of 10a and 11n, giving 12an (94%
ee vs 84% ee; entries 2a and 2b), and 10b and 11m, giving
12bm (57% ee vs 40% ee; entries 3a and 3b).
bond-forming reactions. It was found that tms-BINAP was
more enantioselective than the unsubstituted BINAP in the
π-allylpalladium-mediated reactions; however, the effect of
the Me3Si-substituents was not operative in the Rh-catalyzed
conjugate addition of phenyboronic acid to an R,â-unsatur-
ated ester. A comprehensive survey of the scope and the
limitation of the 4,4′-substituted-BINAP in other transition-
metal-catalyzed asymmetric transformations is currently
under investigation.
Acknowledgment. H.L.N. was a recipient of the EAPSI
Summer Fellowship from NSF/JSPS. This work was partly
supported by a Grant-in-Aid for Scientific Research on
Priority Areas (no. 15036202, “Reaction Control of Dynamic
Complexes”) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (to M.O.) and by the National
Science Foundation of the U.S.A. (to W.L.).
Supporting Information Available: Detailed experi-
mental procedures, compound characterization data, and
crystallographic data (CIF file). This material is available
free of charge via the Internet at http://pubs.acs.org.
In summary, we have applied 4,4′-(Me3Si)2-BINAP in
several Pd- and Rh-catalyzed asymmetric carbon-carbon
(23) (a) Yamaguchi, M.; Shima, T.; Yamagishi, T.; Hida, M. Tetrahedron
Lett. 1990, 31, 5049. (b) Yamaguchi, M.; Shima, T.; Yamagishi, T.; Hida,
M. Tetrahedron: Asymmetry 1991, 2, 663.
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