Gold(I)-mediated hydrothiolation of conjugated olefins

Article abstract of DOI:10.1055/s-2007-984519

Thiol additions to conjugated olefins were catalyzed by Ph ₃PAuBF₄ with excellent yields at room temperature. A variety of functionalized thiols, with both electron-donating and electron-withdrawing substituents, are compatible with

Full text of DOI:10.1055/s-2007-984519

CLUSTER
1785
Gold(I)-Mediated Hydrothiolation of Conjugated Olefins
H
C
ydrothiolation
o
h
f
Conjugate
d
a
O
lefins d Brouwer, Ronald Rahaman, Chuan He*
Department of Chemistry, University of Chicago, Chicago, IL 60637, USA
Fax +1(773)7020805; E-mail: chuanhe@uchicago.edu
Received 16 March 2007
demonstrate the catalyst’s compatibility with a wide range
of functionalized thiols. The resulting allylic sulfides are
amenable to numerous, orthogonal deprotection proto-
Abstract: Thiol additions to conjugated olefins were catalyzed by
Ph PAuBF with excellent yields at room temperature. A variety of
3
4
functionalized thiols, with both electron-donating and electron-
withdrawing substituents, are compatible with this system. Hy-
13
cols, which are advantageous in synthetic applications.
drothiolations of 1,3-cyclohexadiene were also achieved in similar For example, benzylic sulfide moieties are present in sev-
eral synthetic anticancer inhibitors.14
yields at 50 °C.
Key words: gold, homogeneous catalysis, hydrothiolation, conju-
gated olefin
Inspired by our recent report on the addition of certain
amines to dienes, we were pleasantly surprised to find
5
that this catalyst also facilitated the room-temperature hy-
drothiolation of dienes. Table 1 shows the results of initial
addition trials and establishes that the unoptimized system
is successful with both electron-rich and electron-defi-
cient thiols. Thiophenol, thiobenzoic acid, and diphe-
nylphosphinodithioic acid addition (entries 3, 5, and 6) to
Despite the strong complexation that occurs between sul-
fur and transition-metal ions, metal-catalyzed sulfur trans-
formations are infrequently explored. Indeed, the fact
1
that sulfur is generally poisonous towards transition
metals seems to indicate a general incompatibility be-
3
-methyl-1,3-pentadiene afforded the allylic thioether
2
tween the two. Nonetheless, some novel and exciting
15
product quantitatively at room temperature, whereas
n-propanethiol, thioacetic acid, and benzyl mercaptan
afforded only moderate yields at this stage. Subsequent
trials focused on improving the yields of these latter cases.
metal-catalyzed thiol additions have been reported in the
recent literature. Krause has reported gold-catalyzed in-
tramolecular C–S bond formation in thioallene systems,3
and Duñach has reported indium-catalyzed intermolecular
4
thiol addition to olefins. In continuation of our previous
investigations of gold-catalyzed additions to unactivated
Table 1 Preliminary Survey of Thiolsa
5
,6
C–C bonds,
we now report the gold(I)-mediated
Ph3PAuOTf (5 mol%)
DCE, r.t., overnight
SR
addition of a diverse range of functionalized thiols to con-
jugated olefin systems.
+
RSH
The addition of thiols to conjugated dienes can be simply
Yield (%)b
Entry
1
Thiol
7
–9
achieved by radical initiators or by Brønsted acid catal-
45 (55)c
7
,10
SH
SH
ysis, such as sulfuric and perchloric acids.
However,
these well-known reactions lack considerable generality.
Radical-based mechanisms result in competing telomer-
2
3
100 (90)
42 (69)c
8
ization when using alkyl-substituted thiols, and strong
Brønsted acids are incompatible with many synthetically
useful functional groups. Hydrothiolation of styrene has
also been demonstrated with both radical initiators7 and
Brønsted acid catalysts, such as sulfuric, perchloric, and
SH
,9
O
1
1
4
5
54
p-toluenesulfonic acids, but these reactions have similar
limitations. Furthermore, with styrene substrates, radical
processes produce anti-Markovnikov addition of the thiol,
while acid-based catalysts produce the corresponding
Markovnikov addition products. Earlier attempts to dem-
onstrate Markovnikov addition with Lewis acid catalysts,
such as titanium(IV) chloride and aluminum(III) chloride,
SH
O
100 (96)
Ph
Ph
SH
S
6
100 (90)
P
SH
Ph
12
have required stoichiometric amounts of the metal. Like
Krause’s and Duñach’s recently reported systems, our ad-
a
All reactions were performed in the dark under N with1.0 mmol
2
3
,4
thiol and 1.2 mmol diene at 1.0 M.
dition is successful in truly catalytic quantities. We also
b
1
Yields based on H NMR conversion, using an internal standard.
Yields in parentheses refer to isolated yields.
c
Isolated yield after optimization.
SYNLETT 2007, No. 11, pp 1785–1789
0
3
.0
7
.2
0
0
7
Advanced online publication: 25.06.2007
DOI: 10.1055/s-2007-984519; Art ID: Y01307ST
©
Georg Thieme Verlag Stuttgart · New York

1
786
C. Brouwer et al.
CLUSTER
We next focused on optimizing the counteranion and sol- Table 2 Optimization of Counteranionsa
vent parameters for the catalyst system (Table 2). BF4–
Ph3PAuCl, cocatalyst
DCE, r.t., overnight
SPr
performed the best and afforded a markedly higher yield
+
PrSH
–
than OTf (Table 2, entry 2 vs. Table 1, entry 1). Other
–
fluorinated anions like PF6 showed moderate activity
Entry
Cocatalyst
None
Yield (%)b
–
(
Table 2, entry 4). Anions such as OCOCF , acetylace-
3
–
–
tonate (acac ), and NO do not support the reaction.
1
2
3
4
5
6
7
8
9
0
72
Trace
44
10
0
3
Numerous other silver and thallium cocatalysts were
AgBF4
also screened, yet none trumped the activity exhibited by
–
BF . A similar trend was seen in our previous investiga-
AgOBz
AgPF6
4
5
tion of the hydroamination reaction. A limited activity
was also seen with AgBF (entry 11), however, other
4
silver salts were catalytically inactive (entries 9 and 10).
In general, like most reported gold-catalyzed processes,
the hydrothiolation reaction required noncoordinating
counteranions. Many solvents were screened and only
chlorinated solvents afforded products. Dichloromethane
and chloroform were the best for the reaction. For experi-
mental ease, dichloromethane (CH Cl ) was chosen for
AgOTs
AgOCOCF3
Tlacac
0
TlNO3
AgFc
0
2
2
0
subsequent trials.
1
0
AgPF c
0
6
Table 3 further exemplifies the scope of the reaction under
optimized conditions. Aliphatic and aromatic thiols were
readily added to 3-methyl-1,3-pentadiene in good to high
yields (entries 1–3). Sterically demanding thiols easily
added as well; however, attempts to add triphenyl-
methanethiol (trityl) were unsuccessful. Quite surprising-
11
AgBF c
27
4
a
All reactions were performed in the dark under N .
2
b
c
1
Yields based on H NMR conversion, using an internal standard.
Without Ph PAuCl.
3
ly, OH, MeO, and NO functionalities were perfectly
2
pendant carbamate to the remaining olefin (80 °C for 4 h)
was fruitless. Other conjugated olefins were amenable to-
wards addition, although in some cases they required
higher temperatures (entries 7–12).
compatible with the catalyst (entries 4, 8, and 9). In
contrast to these examples, there was absolutely no con-
version observed when primary amines were present
(
entry 5). However, protection of the amine with Cbz (car-
bobenzyloxy) permitted reactivity (entry 6). Attempts to
cyclize this product via subsequent addition of the
Table 3 Reaction Scopea
Entry
1
Thiol
Olefin
Product(s)
Yield (%)b
85 (73)
St-Bu
t-BuSH
SAr
2
95 (60)
SH
S
S
3
4
5
SH
89 (29)
63 (30)
0
SiMe3
OH
Me3Si
SH
HO
SH
(No addition product)
H2N
O
S
6
7
70
SH
NHCbz
Ph
Cl
O
N
H
SH
S
Cl
87 (76)
Synlett 2007, No. 11, 1785–1789 © Thieme Stuttgart · New York

CLUSTER
Hydrothiolation of Conjugated Olefins
1787
Table 3 Reaction Scopea (continued)
Entry
8c
Thiol
Olefin
Product(s)
Yield (%)b
NO2
O2N
SH
S
71 (56)
SH
9c,d
S
85
MeO
OMe
SPh
1
1
1
0e
1
PhSH
SPh
86f
Ph
SPh
Ph
PhSH
100 (78)
0
O
2
(Polymerization, no addition product)
Ph
SH
S(n-C8H17)
S(n-C8H17)
1
3g
n-C H SH
As a mixture
8
17
a
All reactions run with Ph PAuBF (5%) as the catalyst, in CH Cl and at r.t., unless otherwise specified. All reactions were performed in the
3
4
2
2
dark, under N .
2
b
1
Yields based on H NMR conversion, using an internal standard. Yields in parentheses refer to isolated yields.
Reaction was run at 50 °C.
Reaction was run in CHCl3.
c
d
e
f
Reaction used Ph PAuNTf (5%) as the catalyst.
3
2
A 9:1 Markovnikov–anti-Markovnikov product distribution.
g
Reaction used Ph PAuOTf (5%) as the catalyst at 80 °C.
3
Styrene was a more challenging substrate for thiol addi- that only vinylcyclohexane permitted thiol addition. We
tion because of its strong tendency to polymerize under currently have no explanation for the discrepancy be-
acidic conditions, as well as competing anti-Markovnikov tween these two cases.
radical additions. Indeed, when adding unpurified
The mechanism of Au(I)-catalyzed additions is still not
thiophenol to styrene, the yield of the Markovnikov prod-
completely understood. The most frequently invoked cat-
uct was low, and most of the substrate either polymerized
alytic cycle to explain pronucleophile additions involves,
or underwent radical-mediated anti-Markovnikov addi-
first, cationic gold(I) activation of the p-bond. This polar-
izes the p-bond to enable nucleophilic attack and product
tion. However, using freshly distilled thiophenol pro-
duced a much higher yield of the desired Markovnikov
formation upon protodemetalation. To elucidate the direc-
product, with only small amounts of the anti-Markovnik-
ov product present (entry 10). Thiophenol also added to
indene, affording an even higher yield of the benzylic-
substituted sulfide (entry 11). Thiobenzoic acid, however,
gave only polymerization of the styrene (entry 12).
16
tion of attack in our reactions, PhSD was added to
17
1
indene in CDCl and the H NMR was measured. An
3
approximately 1:1 ratio of syn and anti products was
detected. This seems to indicate that a late transition state
or even an open benzylic cationic intermediate is formed
We also investigated the possibility of thiol addition to before addition of thiol.
4
simple unactivated olefins. Attempts to react n-octane-
To conclude, we have demonstrated the use of gold(I) to
thiol with 1-octene at 100 °C resulted in isomerization to
the internal olefin, 2-octene, with no addition occurring.
Vinylcyclohexane also showed evidence of double-bond
isomerization; however, in this substrate, addition did
occur, and resulted in a 1:1 mixture of two products: addi-
tion at the secondary carbon and addition at the tertiary
carbon (entry 13). It is peculiar that both substrates
studied underwent isomerization to internal olefins but
mediate the addition of thiols to conjugated olefins. The
reaction conditions are quite mild (r.t.), and the catalyst
loadings low (5 mol%). A large scope of sulfur nucleo-
philes is tolerated, offering a number of mutually ortho-
gonal protecting groups. This presents an attractive
protocol to generate allylic thiols.
Synlett 2007, No. 11, 1785–1789 © Thieme Stuttgart · New York

1
788
C. Brouwer et al.
CLUSTER
Representative Procedure for Synthesis of Sulfides
(s, 2 H). 13C NMR (125 MHz, CDCl ): d = 11.42, 13.28, 19.20,
3
An oven-dried amber vial was charged with chlorotriphenylphos-
phinegold(I) (25 mg, 0.050 mmol) and silver tetrafluoroborate (9.7
19.39, 20.90, 30.24, 40.17, 50.00, 120.54, 128.86, 136.16, 136.83.
mg, 0.050 mmol) under an atmosphere of N . The mixture was
[2-(1,2-Dimethylbut-2-enylsulfanyl)ethyl]trimethylsilane
2
stirred in anhydrous CH Cl (1.0 mL) for several minutes. To this
(Table 3, Entry 3)
Colorless oil. H NMR (500 MHz, CDCl ): d = 0.01 (s, 9 H), 0.77
2
2
1
was added benzenethiol (0.10 mL, 1.0 mmol), then 3-methyl-1,3-
pentadiene (cis/trans mixture, 0.14 mL, 1.2 mmol) and the mixture
was stirred overnight. The mix was then filtered through a Celite®
3
(dt, 1 H, J = 6.7, 5.5 Hz), 0.85 (dt, 1 H, J = 7.3, 5.1 Hz), 1.28 (d, 3
H, J = 7.2 Hz), 1.62 (d, 3 H, J = 6.5 Hz), 1.63 (s, 3 H), 2.35 (tm and
dm, 2 H, J = 16.0, 5.5 Hz), 3.43 (q, 1 H, J = 7.1 Hz), 5.33 (q, 1 H,
5
45 pad and the solvent removed via rotavap. The crude material
1
3
was then columned on silica gel (hexane) and solvent removed to
yield an off-white oil (0.17 g, 90%).
J = 6.0 Hz). C NMR (125 MHz, CDCl ): d = –1.80, 10.91, 13.17,
3
17.31, 19.26, 48.16, 120.35, 136.28.
Spectra of All Previously Unreported Compounds
2-(1,2-Dimethylbut-2-enylsulfanyl)ethanol (Table 3, Entry 4)
1
3
-Methyl-4-propylsulfanylpent-2-ene (Table 1, Entry 1)
Yellow oil. H NMR (500 MHz, CDCl ): d = 1.29 (d, 3 H, J = 7.0
3
1
Colorless oil. H NMR (500 MHz, CDCl ): d = 0.95 (t, 3 H, J = 7.3
Hz), 1.62 (d, 3 H, J = 8.2 Hz), 1.64 (s, 3 H), 2.09 (br s, 1 H), 2.57
(m, 2 H, J = 6.1, 3.0 Hz), 3.42 (q, 1 H, J = 7.0 Hz), 3.64 (br s, 2 H),
3
Hz), 1.27 (d, 3 H, J = 7.1 Hz), 1.55 (m, 2 H), 1.61 (d, 3 H, J = 6.1
Hz), 1.63 (s, 3 H), 2.29 (m, 2 H), 3.38 (q, 1 H, J = 7.1 Hz), 5.32 (q,
1
3
5.35 (q, 1 H, J = 6.5 Hz). C NMR (125 MHz, CDCl ): d = 10.85,
3
1
1
H, J = 6.5 Hz). 13C NMR (125 MHz, CDCl ): d = 10.91, 13.21,
3.63, 22.85, 32.97, 38.62, 48.52, 120.40, 136.38.
13.26, 19.40, 34.10, 48.32, 60.45, 121.38, 135.97.
3
1
-Chloro-3-(cyclohex-2-enylsulfanyl)benzene (Table 3, Entry 6)
1
(
1,2-Dimethylbut-2-enylsulfanyl)benzene (Table 1, Entry 2)
Cloudy oil. H NMR (500 MHz, CDCl ): d = 1.53 (m, 1 H), 1.69 (m,
3
1
Off-white oil. H NMR (500 MHz, CDCl ): d = 1.36 (d, 3 H, J = 7.0
1 H), 1.80 (m, 1 H), 1.86 (m, 1 H), 1.95 (m, 1 H), 5.65 (qd, 1 H,
J = 8.1, 1.8 Hz), 5.77 (td, 1 H, J = 7.9, 1.4 Hz), 7.08 (td, 1 H, J = 1.7
Hz), 7.11 (t, 1 H, J = 7.8 Hz), 7.17 (td, 1 H, J = 3.9, 1.5 Hz), 7.28 (t,
3
Hz), 1.50 (d, 3 H, J = 6.3 Hz). 1.69 (s, 3 H), 3.76 (q, 1 H, J = 7.0
Hz), 5.19 (q, 1 H, J = 6.8 Hz), 7.21–7.35 (m, 5 H). C NMR (125
1
3
1
3
MHz, CDCl ): d = 12.12, 13.29, 19.58, 43.67, 52.63, 121.34,
1 H, J = 1.8 Hz). C NMR (125 MHz, CDCl ): d = 19.34, 24.91,
3
3
1
26.83, 128.46, 132.66, 132.98, 135.65.
28.68, 29.73, 43.67, 126.31, 126.50, 128.66, 129.84, 130.18,
1
30.98, 134.52, 138.22.
(
1,2-Dimethylbut-2-enylsulfanylmethyl)benzene (Table 1, En-
try 3)
1-(Cyclohex-2-enylsulfanylmethyl)-3-nitrobenzene (Table 3,
1
Light-yellow oil. H NMR (500 MHz, CDCl ): d = 1.25 (d, 3 H,
J = 7.1 Hz), 1.63 (s, 3 H), 1.64 (d, 3 H, J = 6.2 Hz), 3.31 (q, 1 H,
J = 7.1 Hz), 3.52 (s, 2 H), 5.31 (q, 1 H, J = 6.6 Hz), 7.22 (m, 1 H,
Entry 7)
3
1
Yellow oil. H NMR (500 MHz, CDCl ): d = 1.60 (m, 1 H), 1.74 (m,
3
1 H), 1.84 (m, 1 H), 1.93 (m, 1 H), 2.02 (m, 2 H), 3.29 (br s, 1 H),
3.83 (d, 2 H, J = 3.3 Hz), 5.63 (dd, 1 H, J = 9.7, 1.4 Hz), 5.81 (md,
1 H), 7.49 (t, 1 H, J = 8.0 Hz), 7.68 (d, 1 H, J = 7.5 Hz), 8.10 (td, 1
1
3
J = 4.4 Hz), 7.29 (m, 4 H). C NMR (125 MHz, CDCl ): d = 11.10,
3
1
1
3.32, 19.22, 35.52, 38.80, 48.39, 121.23, 126.64, 128.32, 128.87,
36.04, 139.07.
1
3
H, J = 8.2, 0.9 Hz), 8.21 (s, 1 H). C NMR (125 MHz, CDCl ): d =
3
1
9.71, 24.87, 29.06, 34.73, 40.56, 122.01, 123.71, 126.93, 129.40,
Thiobenzoic Acid (S)-(1,2-Dimethylbut-2-enyl) Ester (Table 1,
130.57, 135.00, 141.19, 148.33.
Entry 5)
1
Off-white oil. H NMR (500 MHz, CDCl ): d = 1.51 (d, 3 H, J = 7.2
1-(Cyclohex-2-enylsulfanylmethyl)-4-methoxybenzene (Table
3
Hz), 1.64 (d, 3 H, J = 6.8 Hz), 1.72 (s, 3 H), 4.41 (q, 1 H, J = 7.0
Hz), 5.64 (q, 1 H, J = 6.8 Hz), 7.43 (t, 2 H, J = 8.0 Hz), 7.56 (t, 1 H,
J = 6.2 Hz), 7.97 (d, 2 H, J = 7.3 Hz). C NMR (125 MHz, CDCl3):
d = 13.49, 14.01, 20.27, 39.32, 46.85, 121.34, 127.19, 128.51,
3, Entry 8)
1
Colorless oil. H NMR (500 MHz, CDCl ): d = 1.59 (m, 1 H), 1.73
3
1
3
(m, 1 H), 1.87 (m, 2 H), 2.00 (m, 2 H), 3.26 (br s, 1 H), 3.71 (d, 2 H,
J = 2.1 Hz), 3.79 (s, 3 H), 5.65 (td, 1 H, J = 9.9, 1.7 Hz), 5.76 (m, 1
H), 6.83 (d, 2 H, J = 8.6 Hz), 7.24 (d, 2 H, J = 8.6 Hz). 1 C NMR
3
1
33.14, 135.47, 137.31, 191.84.
(
125 MHz, CDCl ): d = 19.81, 24.95, 29.02, 34.82, 39.95, 55.27,
3
Diphenylphosphinodithioic Acid 1,2-dimethylbut-2-enyl Ester
Table 1, Entry 6)
113.89, 127.56, 129.74, 129.90, 130.67, 158.53.
(
1
Thick, off-white syrup. H NMR (500 MHz, CDCl ): d = 1.22 (d, 3
3
Acknowledgment
H, J = 6.8 Hz), 1.40 (d, 3 H, J = 7.0 Hz), 1.49 (s, 3 H), 4.25 (q, 1 H,
2
JPH = 7.1, 5.0 Hz), 5.36 (q, 1 H, J = 6.4 Hz), 7.44 (m, 6 H), 7.87
dd, 2 H, J = 6.9, 1.5 Hz), 7.95 (ddd, 2 H, J = 5.8, 2.2 Hz). C NMR
We thank the A. P. Sloan Foundation and the University of Chicago
for financial supports.
1
3
(
(
125 MHz, CDCl ): d = 12.22, 13.01, 21.39, 21.44, 43.27, 51.56,
3
1
1
6
22.7, 128.07, 128.17, 128.42, 128.53, 131.07, 131.16, 131.41,
3
1
31.62, 131.65, 131.83, 131.92. P NMR (202 MHz, CDCl ): d =
3
References and Notes
2.61.
(1) Kondo, T.; Mitsudo, T. Chem. Rev. 2000, 100, 3205.
4
-tert-Butylsulfanyl-3-methylpent-2-ene (Table 3, Entry 1)
(2) Hegedus, L. L.; McCabe, R. W. Catalyst Poisoning; Marcel
Dekker: New York, 1984.
1
Colorless oil. H NMR (500 MHz, CDCl ): d = 1.27 (s, 3 H), 1.31
3
(
s, 9 H), 1.58 (d, 3 H, J = 6.2 Hz), 1.67 (s, 3 H), 3.55 (q, 1 H, J = 7.1
(3) Morita, N.; Krause, N. Angew. Chem. Int. Ed. 2006, 45,
1897.
(4) Weïwer, M.; Coulombel, L.; Duñach, E. Chem. Commun.
1
3
Hz), 5.41 (q, 1 H, J = 6.5 Hz). C NMR (125 MHz, CDCl ): d =
1
3
1.88, 13.37, 21.84, 29.70, 31.12, 46.42, 119.11, 138.96.
2006, 332.
2
-(1,2-Dimethylbut-2-enylsulfanylmethyl)-1,3,5-trimethylben-
(5) Brouwer, C.; He, C. Angew. Chem. Int. Ed. 2006, 45, 1744.
(6) (a) Li, Z.; Ding, X.; He, C. J. Org. Chem. 2006, 71, 5876.
(b) Reich, N. W.; Yang, C.-G.; Shi, Z.; He, C. Synlett 2006,
1278. (c) Zhang, J.; Yang, C.-G.; He, C. J. Am. Chem. Soc.
2006, 128, 1798.
zene (Table 3, Entry 2)
Colorless oil. H NMR (500 MHz, CDCl ): d = 1.32 (d, 3 H, J = 7.0
Hz), 1.67 (d, 3 H, J = 6.0 Hz), 1.72 (s, 3 H), 2.23 (s, 3 H), 2.33 (s, 6
H), 3.50 (m, 3 H, J = 17.4, 8.8 Hz), 5.41 (q, 1 H, J = 6.9 Hz), 6.80
1
3
Synlett 2007, No. 11, 1785–1789 © Thieme Stuttgart · New York

CLUSTER
Hydrothiolation of Conjugated Olefins
1789
(
(
7) Posner, T. Ber. Dtsch. Chem. Ges. 1905, 38, 646.
8) Oswald, A. A.; Griesbaum, K.; Thaler, W. A.; Hudson, B. E.
Jr. J. Am. Chem. Soc. 1962, 84, 3897.
(14) For recent examples, see: (a) Lippa, B.; Morris, J.; Corbett,
M.; Kwan, T. A.; Noe, M. C.; Snow, S. L.; Gant, T. G.;
Mangiaracina, M.; Coffey, H. A.; Foster, B.; Knauth, E. A.;
Wessel, M. D. Bioorg. Med. Chem. Lett. 2006, 16, 3444.
(b) Tremblay, M. R.; Boivin, R. P.; Luu-The, V.; Poirier, D.
J. Enzyme Inhib. Med. Chem. 2005, 2, 153.
(15) For thoroughness, all reactions were conducted overnight,
but complete conversions were typically achieved in one
hour.
(
9) Griesbaum, K. Angew. Chem. Int. Ed. 1970, 9, 273.
(
(
10) Saville, B. J. Chem. Soc. 1962, 5040.
11) (a) Wolf, F.; Finke, H. Z. Chem. 1972, 12, 180. (b)Screttas,
C. G.; Micha-Screttas, M. J. Org Chem. 1979, 44, 713.
12) (a) Mukaiyama, T.; Izawa, T.; Saigo, K.; Takei, H. Chem.
Lett. 1973, 355. (b) Belley, M.; Zamboni, R. J. Org. Chem.
(
1
989, 54, 1230.
(16) Shono, T.; Matsumara, Y.; Tsubata, K. Chem. Lett. 1979, 8,
1051.
(17) Indene was chosen because the two benzylic protons at C1
are enantiotopic with respect to added deuterium.
(
13) Green, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis; Wiley: New York, 1999.
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