The Meyer-Schuster rearrangement of ethoxyalkynyl carbinols

Article abstract of DOI:10.1055/s-2007-973885

The combination of electron-rich alkoxyacetylenes and cationic gold catalysts provides excellent reactivity for the Meyer-Schuster rearrangement under mild conditions. Optimization of the reaction conditions with respect to stereoselectivity and investigations into the scope and mechanism of the rearrangement of secondary ethoxyalkynyl carbinols (γ-ethoxy-substituted propargyl alcohols) to α,β-unsaturated esters are described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-973885

LETTER
949
The Meyer–Schuster Rearrangement of Ethoxyalkynyl Carbinols
R
S
earrangemen
u
t
of Ethoxya
s
lkynyl
C
a
a
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, FL 32306-4390, USA
Fax +1(850)6448281; E-mail: gdudley@chem.fsu.edu
Received 15 January 2007
R2
R2
Abstract: The combination of electron-rich alkoxyacetylenes and
cationic gold catalysts provides excellent reactivity for the Meyer–
Schuster rearrangement under mild conditions. Optimization of the
reaction conditions with respect to stereoselectivity and investiga-
tions into the scope and mechanism of the rearrangement of second-
ary ethoxyalkynyl carbinols (g-ethoxy-substituted propargyl
alcohols) to a,b-unsaturated esters are described.
R1
R1
R3
R3
O
path a: Rupe rearrangement
H
H
R2
R2
O
path a
R1
Key words: Meyer–Schuster, gold, alkoxyacetylene, propargyl al-
cohol, rearrangement
R1
H
R3
R3
path b
H
O
H
H
Propargyl alcohols are readily available, versatile tools in
organic synthesis, providing access through different re-
action pathways to alkenes, allenes, alkynes, ketones, etc.1
For example, hydrometalation (syn or anti), substitution
path b: Meyer–Schuster rearrangement
H
R2
H
R2
O
R1
•
R3
OH
(
at the a- or g-centers), hydration, oxidation, hydrogena-
R1
R3
tion, and deoxygenation all may be accomplished through
selective activation of propargyl alcohol substrates. This Scheme 1 Competing Rupe and Meyer–Schuster pathways
2
study focuses on using electron-rich alkoxyacetylenes to
control selectivity so as to access the Meyer–Schuster re-
arrangement, a formal 1,3-hydroxy migration followed
by tautomerization.
3
hard
Lewis acid
H
LA
O
R
(LA)
The Rupe and Meyer–Schuster rearrangements3b
R
R
+
4+
e.g. H , Ti
(
Scheme 1) are not often used in chemical synthesis due
OH
R
to harsh conditions and poor selectivity. The Meyer–
Schuster products (from path b) are especially rare be-
cause the dehydration that leads into the Rupe pathway
R
R
soft
Lewis acid
LA)
LA
(
HO
R
R
(
path a) generally takes precedence under traditional
e.g. Au3+
R
modes of activation that target the substrate through the
alcohol moiety (i.e., acidic catalysts).
Scheme 2 Complementary Lewis acid activation of propargyl alco-
hols
Coordination of the alkyne using soft, late-transition-met-
4
5,6
al Lewis acids, including cationic gold catalysts, pro-
vides a fundamentally different mechanism for activating
propargyl alcohols (Scheme 2). ‘Soft’ activation of the
alkyne is likely to be more tolerant of sensitive function-
ality than ‘hard’ activation of the oxygen atom and can
provide complementary selectivities.
O
1. BuLi, H–≡–OEt, THF
CO2Et
2. AuCl3 (5 mol%)
EtOH (5 equiv), CH2Cl2
99%
1
. BuLi, H–≡–OEt, THF
We recently reported on a two-step strategy for the
7
2
HWE -type olefination of hindered ketones: (1) addition
2. AuCl3 (5 mol%)
8
CO2Et
6% (2:1 E/Z)
EtOH (5 equiv), CH2Cl2
of ethoxyacetylide, then (2) gold-catalyzed Meyer–
O
Schuster rearrangement (Scheme 3). Alkyne addition to
carbonyl groups is relatively insensitive to sterics, where-
as the resulting congested tertiary ethoxyalkynyl
9
Scheme 3 Two-step olefination of hindered ketones2
carbinols are sterically and electronically primed for rear-
rangement.
SYNLETT 2007, No. 6, pp 0949–0953
0
3.
0
4.
2
0
0
7
Advanced online publication: 26.03.2007
DOI: 10.1055/s-2007-973885; Art ID: S00207ST
Having identified this important two-stage tactical appli-
cation, we focused our attention on step two, the Meyer–
©
Georg Thieme Verlag Stuttgart · New York

9
50
S. S. Lopez et al.
LETTER
9
Schuster rearrangement. In our earlier study, which fea-
tured highly reactive tertiary propargyl alcohol substrates,
rearrangement occurred immediately upon addition of the
gold catalyst. Main goals for this effort included (1) to in-
crease the scope of the rearrangement, and (2) to identify,
through rigorous experimentation, conditions that afford
the enoate products stereoselectively.
catalyst (10 mol%)
additive (10 equiv)
OH
O
OEt
OEt
solvent, r.t.
1a
2a
catalyst: AuCl⋅AgSbF6 > AuCl3, AuCl, Ph3P⋅AuCl >> AgSbF6
additive: EtOH >> CF3CH2OH, AcOH, PhOH, morpholine, none
solvent: THF–CH2Cl2 > THF, CH2Cl2, EtOH, H2O
Herein we report new reaction protocols and observations
with respect to the rearrangement of secondary alcohol
substrates (prepared by addition of lithium ethoxyacetyl-
ide to the corresponding aldehydes). In particular, our ef-
forts focused on the rearrangement of secondary
propargyl alcohols with simple alkyl substituents. These
aliphatic substrates (e.g., 1a, Equation 1) are less reactive
Equation 1 Qualitative screening for optimal conditions
distinction between the alkyl substituent and a hydrogen
atom) provides greater control and the opportunity to en-
hance stereoselectivity in the formation of the a,b-unsat-
urated ester products 2.
towards the Meyer–Schuster reaction than tertiary propar- Equation 1 summarizes observations from extensive
gyl alcohols, which ionize more easily. However, the qualitative experiments on the rearrangement of 1a (de-
dampened reactivity of secondary alcohols (and the steric rived from pivaldehyde) to 2a. We examined three main
Table 1 Meyer–Schuster Reaction of Ethoxyalkynyl Carbinolsa
5
mol% AuCl⋅AgSbF6
0 equiv EtOH
OH
1
O
R
THF–CH2Cl2 (1:1)
r.t., 30–60 min
R
OEt
OEt
1
2
Entry
Substrate
Product
Yield (%, E/Z)
1
1
aa
b
OH
Me
Me
CO Et
91 (97:3)
93 (E)
2
b
Me
Me
Me
Me
OEt
2a
1
a
2
2
aa
b
OH
CO Et
80 (94:6)
82 (99:1)
2
b
OEt
2
2
b
c
1
1
b
c
3
3
aa
b
OH
CO Et
91 (63:37)
90 (86:14)
2
b
5
5
OEt
OEt
4
4
aa
OH
OH
OH
CO Et
75 (57:43)
78 (70:30)
2
b
b
2
2
2
d
e
f
1
1
1
d
e
f
5
5
aa
CO Et
72 (40:60)
70 (40:60)
2
b
b
OEt
OEt
6
6
6
aa
CO Et
90 (47:53)
92 (91:9)
90 (53:47)
2
b
b
c
c
a
13
Solutions of AgSbF (5 mol%) and AuCl (5 mol%) in 1:1 THF–CH Cl added sequentially to ethoxyalkynyl carbinol (1, 1.0 equiv) and EtOH
6
2
2
(
10 equiv) in 1:1 THF–CH Cl . No camphorsulfonic acid (CSA) was included unless otherwise inidcated.
2 2
b
1.0 Equiv CSA.
c
14
Reaction conducted in 2-butanone instead of 1:1 THF–CH Cl .
2
2
Synlett 2007, No. 6, 949–953 © Thieme Stuttgart · New York

LETTER
Rearrangement of Ethoxyalkynyl Carbinols
951
variables: gold catalyst, additive, and solvent. Among the Scheme 4 shows one reasonable mechanistic sequence for
protic additives, which are envisioned to assist in the the Meyer–Schuster reaction (1 → 2). Coordination be-
formal 1,3-hydroxy migration, ethanol was significantly tween the ethoxyalkyne and the cationic gold catalyst pro-
more effective than other agents tested. Reactions con- motes g-substitution to generate a 1,1-diethoxyallene. The
ducted in a mixed system of THF and CH Cl were most immediate expulsion of water (I → II) accounts for the
2
2
efficient (qualitatively) and selective for the E-alkene iso- lack of b-hydroxy ester byproducts, which stands in con-
mer (quantitatively). Finally, only minor differences were trast to acidic hydrolysis of ethoxyalkynyl carbinols.1
observed among the various gold catalysts; all of the The gold catalyst may or may not promote the subsequent
cationic gold catalysts that we screened were more or less reincorporation of water to produce III; this step (II →
effective. Silver(I) hexafluoroantimonate (AgSbF6) III) determines the stereochemistry of the eventual prod-
1a,c
16
showed little activity on its own, but when employed in uct. Rapid collapse of intermediate III yields enoate 2.
conjunction with the gold catalysts it exerted a positive
effect on the E/Z-selectivity of the reaction.
The mechanism outlined in Scheme 4 implies that the ex-
ternal alcohol incorporates into the enoate ester product to
Further experimentation indicated that a catalyst loading a partial (but not statistical) degree, based on competitive
1
7
of 5 mol% was optimal. We observed no significant ef- collapse of tetrahedral intermediate III. Alcohol 1a was
fects of activated silica gel on these small-scale reac- subjected to the rearrangement conditions using n-pro-
1
0
tions.
Addition of camphorsulfonic acid (CSA) panol as the external alcohol promoter (Equation 2) under
accelerated the reaction, whereas an acid scavenger [2,6- the assumption that n-propanol and ethanol would be ex-
di-(tert-butyl)-4-methylpyridine, DTBMP] inhibited the pelled from the tetrahedral intermediate at similar rates.
reaction. These results, along with earlier experiments,11 The corresponding a,b-unsaturated ethyl and n-propyl es-
indicate that exchangeable protons play an important sup- ters (2a and 2a¢) were obtained in an approximately 1:1 ra-
1
8
porting role in the gold-catalyzed rearrangement.
tio, which is consistent with our mechanistic hypothesis.
We tested the rearrangement protocol on a series of repre-
sentative secondary alcohol substrates (1a–f, Table 1).12
Neopentyl alcohol (1a) gave rise to nonenolizable enoate
5 mol% AuCl⋅AgSbF6
OH
O
10 equiv n-PrOH
Me
Me
Me
Me
OR
THF–CH2Cl2 (1:1)
r.t., 30 min
2
a with nearly complete stereoselectivity (entry 1a).
Alkyl-substituted alcohols 1b–d afforded enoates 2b–d
entries 2a–4a) to the complete exclusion of dehydration
products (cf. path a of Scheme 1).
Me
OEt
Me
1
a
2a, 2a'
(
(R = Et, n-Pr)
Equation 2 Partial incorporation (ca. 50%) of external alcohol
Inclusion of camphorsulfonic acid (CSA) in the reaction additive is consistent with postulated dialkoxyallene intermediate (II)
mixture improved the stereoselectivity of most reactions
(
entries 1b–6b, cf. 6a vs. 6b); however, in this protocol the
Finally, the experiment illustrated in Equation 3 de-
monstrates the applicability of these conditions to the
substrates must tolerate more acidic conditions.
Sequential addition of the silver and gold precatalysts in synthesis of a,b-unsaturated esters from tertiary alcohols
1
2
solution to the reaction mixture provided optimal stereo- (3 → 4).
1
5
selectivity and reproducibility. Premixing the gold and
silver salts gave poorer results with respect to selectivity,
as did addition of the precatalysts as solids. Reactions
were typically conducted under an inert atmosphere of ar-
gon using anhydrous THF and CH Cl , but similar results
AuCl⋅AgSbF6 (5 mol%)
OH
EtOH (10 equiv)
O
THF–CH2Cl2 (1:1)
r.t., < 5 min, 92%
OEt
OEt
2
2
3
4
were obtained in ‘open-flask’ reactions using reagent-
grade solvents. The small amount of water present in re-
agent-grade ethanol does not interfere with (and may fa-
cilitate) the reaction.
Equation 3 Facile rearrangement of a tertiary ethoxyalkynyl
carbinol
OH
O
+
[
Au ], EtOH
R
R
OEt
OEt
1
2
[
Au+]
–
EtOH
HO
Au+
OH
+
EtOH
H2O
+ H2O
– [Au+]
OEt
R
•
R
R
OEt
–
OEt
OEt
OEt
H
I
II
III
Scheme 4 Hypothesis on the Meyer–Schuster reaction pathway
Synlett 2007, No. 6, 949–953 © Thieme Stuttgart · New York

9
52
S. S. Lopez et al.
LETTER
In summary, a,b-unsaturated esters were prepared from through a plug of SiO
with the aid of 1:7 Et
was concentrated and purified using silica gel column chromatogra-
O–hexane. The filtrate
2
2
ethoxyalkynyl carbinols using cationic gold catalysts.
Substitution on the alcohol substrate, including aryl,
alkyl, and vinyl groups, is well tolerated, with aliphatic
substituents providing the highest stereoselectivity. Nei-
phy (Et O–hexane, 1:50) to give ethyl (E)-4,4-dimethylpent-2-
enoate (2a); yield 0.093 g (93%).
2
ther Rupe-type elimination products (from loss of water) Acknowledgment
nor b-hydroxy ester products (from addition of water)
This research was supported by the James and Ester King Biomedi-
were observed. Mechanistic observations and important
factors that influence the stereochemical course of the re-
action are reported.
cal Research Program, Florida Department of Health, the Donors of
the American Chemical Society Petroleum Research Fund, and by
the FSU Department of Chemistry and Biochemistry. D.A.E. is the
recipient of graduate fellowships from FSU and the MDS Research
Foundation. We are profoundly grateful to all of these agencies for
their support. We thank Dr. Umesh Goli (FSU) for providing the
mass spectrometry data, the Krafft Lab for the use of their FT-IR in-
The mild, efficient, and convenient reaction conditions
should find use in chemical synthesis. This work illus-
trates the potential role of activated, electron-rich alkyne
substrates in the rapidly emerging field of catalysis using strument, and Professor Liming Zhang for helpful discussions.
1
9
soft, late-transition-metal cations.
References and Notes
(
1) (a) Brandsma, L. Acetylenes, Allenes and Cumulenes;
Elsevier: Amsterdam, 2003. (b) Modern Acetylene
Chemistry; Stang, P. J.; Diederich, F., Eds.; VCH:
Weinheim, 1995.
2) Preliminary communication: Engel, D. A.; Dudley, G. B.
Org. Lett. 2006, 8, 4027.
Typical Procedure for the Addition of Ethoxyacetylene of Alde-
hydes: 1-Ethoxy-3-tert-butyl-1-propyn-3-ol (1a)
To a THF solution (5 mL) of ethyl ethynyl ether (0.4 g, ca. 40% by
weight in hexane, ca. 5.7 mmol) was added n-BuLi (2.5 M in hex-
ane, 1.4 mL, 3.4 mmol) dropwise over 5 min at –78 °C under argon
atmosphere. The solution was allowed to warm to 0 °C over 1 h and
held at 0 °C for an additional 30 min, then recooled to –78 °C. Piv-
aldehyde (0.28 mL, 3.2 mmol) was added in one portion dropwise.
The solution was allowed to warm to r.t. over 1 h and held at r.t. for
(
(
3) (a) Kürti, L.; Czakó, B. Strategic Applications of Named
Reactions in Organic Synthesis; Elsevier: New York, 2003,
284–285. (b) Swaminathan, S.; Narayanan, K. V. Chem.
Rev. 1971, 71, 429.
an additional 3 h. A sat. aq NH Cl solution was added to quench the
4
(
(
4) Lewis Acids in Organic Synthesis; Yamamoto, H., Ed.;
Wiley-VCH: New York, 2000.
reaction and the mixture was extracted with EtOAc. The organic
layer was washed with H O, sat. aq NaHCO , and brine, then dried
2
3
5) Recent reviews with leading references: (a) Dyker, G.
Angew. Chem. Int. Ed. 2000, 39, 4237. (b) Hashmi, A. S. K.
Gold Bull. 2003, 36, 3. (c) Ma, S.; Yu, S.; Gu, Z. Angew.
Chem. Int. Ed. 2006, 45, 200. (d) Asao, N. Synlett 2006,
over MgSO , filtered, and concentrated. The residue was purified
4
using silica gel column chromatography (EtOAc–hexane) to give 1-
ethoxy-3-tert-butyl-1-propyn-3-ol (1a) as a light yellow oil; yield
0
.48 g (97%).
1
645.
6) Selected recent examples: (a) Asao, N.; Sato, K. Org. Lett.
006, 8, 5361. (b) Staben, S. T.; Kennedy-Smith, J. J.;
(
Typical Procedure for the Gold-Catalyzed Meyer–Schuster Re-
arrangement of Ethoxyalkynyl Carbinols: Ethyl 4,4-Dimethyl-
pent-2-enoate (2a)
2
Huang, D.; Corkey, B. K.; LaLonde, R. L.; Toste, F. D.
Angew. Chem. Int. Ed. 2006, 45, 5991. (c) Belting, V.;
Krause, N. Org. Lett. 2006, 8, 4489. (d) Wang, S.; Zhang,
L. Org. Lett. 2006, 8, 4585. (e) Huang, B.; Yao, X.; Li, C.-J.
Adv. Synth. Catal. 2006, 348, 1528. (f) Seregin, I. V.;
Gevorgyan, V. J. Am. Chem. Soc. 2006, 128, 12050.
A mixture of AuCl (7.4 mg, 0.032 mmol) in 1:1 CH Cl –THF (5
2
2
mL) was prepared and allowed to stir for 20 min to give a homoge-
neous solution with an insoluble residue. A separate 25 mL round-
bottomed flask under argon was charged with 1a (100 mg, 0.64
mmol), EtOH (95%, 0.36 mL, 6.4 mmol) and a solution of AgSbF6
(
g) Carretin, S.; Blanco, M. C.; Corma, A.; Hashmi, A. S. K.
(
11 mg, 0.032 mmol) in 1:1 CH Cl –THF (5 mL). The mixture of
2 2
Adv. Synth. Catal. 2006, 348, 1283. (h) Park, S.; Lee, D. J.
Am. Chem. Soc. 2006, 128, 10664. (i) Nakamura, I.; Sato,
T.; Yamamoto, Y. Angew. Chem. Int. Ed. 2006, 45, 4473.
AuCl in 1:1 CH Cl –THF (5 mL) was then added dropwise. After
2
2
4
0 min, the reaction mixture was filtered through a plug of SiO2
with the aid of 1:7 Et O–hexane. The filtrate was concentrated and
2
(
2
j) Kang, J.-E.; Kim, H.-B.; Lee, J.-W.; Shin, S. Org. Lett.
006, 8, 3537. (k) Sun, J.; Conley, M. P.; Zhang, L.;
purified using silica gel column chromatography (Et O–hexane,
2
1
:50) to give ethyl 4,4-dimethylpent-2-enoate (2a); yield 0.091 g
Kozmin, S. A. J. Am. Chem. Soc. 2006, 128, 9705. (l) Liu,
Y.; Liu, M.; Guo, S.; Tu, H.; Zhou, Y.; Gao, H. Org. Lett.
(91%, 97:3 E/Z ratio).
2
006, 8, 3445. (m) Robles-Machin, R.; Adrio, J.; Carretero,
Typical Procedure for the Gold-Catalyzed Meyer–Schuster Re-
arrangement of Ethoxyalkynyl Carbinols in the Presence of
Camphorsulfonic Acid (CSA)
J. C. J. Org. Chem. 2006, 71, 5023. (n) Harrison, T. J.;
Kozak, J. A.; Corbella-Pane, M.; Dake, G. R. J. Org. Chem.
2
2
006, 71, 4525. (o) Fürstner, A.; Hannen, P. Chem. Eur. J.
006, 12, 3006 . Relevant earlier examples: (p) Georgy, M.;
A mixture of AuCl (7.4 mg, 0.032 mmol) in 1:1 CH Cl –THF (5
2
2
mL) was prepared and allowed to stir for 20 min to give a homoge-
neous solution with an insoluble residue. A separate 25 mL round-
bottomed flask under argon was charged with 1a (100 mg, 0.64
mmol), EtOH (95%, 0.36 mL, 6.4 mmol), CSA (0.15 g, 0.64 mmol)
and a solution of AgSbF (11 mg, 0.032 mmol) in 1:1 CH Cl –THF
Boucard, V.; Campagne, J.-M. J. Am. Chem. Soc. 2005, 127,
14180. (q) Fukuda, Y.; Utimoto, K. Bull. Chem. Soc. Jpn.
1991, 64, 2013.
(
(
7) Maryanoff, B. E.; Reitz, A. B. Chem. Rev. 1989, 89, 863.
8) Ethoxyacetylene was purchased from Sigma-Aldrich as a
6
2
2
(
5 mL). The mixture of AuCl in 1:1 CH Cl –THF (5 mL) was then
2 2
40% solution in hexane and used as received.
added dropwise. After 40 min, the reaction mixture was filtered
Synlett 2007, No. 6, 949–953 © Thieme Stuttgart · New York

LETTER
9) For selected recent examples, see ref. 3a and:
Rearrangement of Ethoxyalkynyl Carbinols
953
(
private communication; Yu, M.; Li, G.; Wang, S.; Zhang, L.
Adv. Synth Catal. 2007, in press, DOI: 10.1002/
adsc.200600579).
(a) Chabardes, P. Tetrahedron Lett. 1988, 29, 6253.
b) Narasaka, K.; Kusama, H.; Hayashi, Y. Chem. Lett.
(
1
991, 1413. (c) Yoshimatsu, M.; Naito, M.; Kawahigashi,
M.; Shimizu, H.; Kataoka, T. J. Org. Chem. 1995, 60, 4798.
d) Lorber, C. Y.; Osborn, J. A. Tetrahedron Lett. 1996, 37,
53.
(15) In fact, simultaneous addition of the solutions of the gold and
silver salts to the reaction mixture provided the enoate
products with slightly better selectivity, but we consider the
sequential addition protocol to be more easily duplicated and
thus preferable.
(16) Isomerization of the Z-enoates to the E-enoates does not
occur under the reaction conditions: extending the reaction
time does not have a significant effect on the product ratio,
and resubjecting the enoate mixtures to the rearrangement
conditions does not change the ratio of stereoisomers.
Therefore, we assume that the nonthermodynamic product
distribution is purely the result of kinetic control.
(17) To the extent that the additive is incorporated into the
product, the Meyer–Schuster reaction is not a true
‘rearrangement’, although it is generally referred to as such
in a formal sense for the sake of simplicity.
(
8
(
(
10) Kropp, P. J.; Breton, G. W.; Craig, S. L.; Crawford, S. D.;
Durland, W. F. Jr.; Jones, J. E. III; Raleigh, J. S. J. Org.
Chem. 1995, 60, 4146.
11) Protic acids in the absence of gold are significantly less
effective (ref. 2). Examples of the protic-acid-catalyzed
Meyer–Schuster rearrangement of ethoxyalkynyl carbinols:
(
a) Welch, S. C.; Hagan, C. P.; White, D. H.; Fleming, W. P.;
Trotter, J. W. J. Am. Chem. Soc. 1977, 99, 549.
b) Duraisamy, M.; Walborsky, H. M. J. Am. Chem. Soc.
983, 105, 3252. (c) Crich, D.; Natarajan, S.; Crich, J. Z.
(
1
Tetrahedron 1997, 53, 7139.
1
(
(
12) Satisfactory characterization data ( H NMR, 13C NMR, IR,
HRMS) were obtained for all compounds. Yields refer to at
least 50 mg of material isolated in >95% purity.
(18) Control experiments confirmed that the rearrangement
conditions do not promote transesterification between the
ethyl and propyl esters, so the incorporation of the propanol
additive is most likely occurring during the course of the
rearrangement.
13) An insoluble residue, which has no apparent impact on the
course of the reaction, was observed in the solutions of AuCl
(
Aldrich, 99.9%) in 1:1 THF–CH Cl . Identical results were
2 2
obtained when this catalyst solution was filtered, decanted,
or used with the unknown residue in place.
14) 2-Butanone is the recommended solvent in a forthcoming
study on the transpositional hydrolysis of propargylic
acetates (Prof. Liming Zhang, University of Nevada, Reno,
(19) For a previous report on the combined use of oxygen-
activated alkynes and cationic gold catalysts, see:
Zhang, L.; Kozmin, S. A. J. Am. Chem. Soc. 2004, 126,
11806.
(
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