Highly regio- and stereoselective hydrostannylation of propargyl alcohols and ethers using dibutylchlorostannane and lithium chloride

Article abstract of DOI:10.1055/s-2004-837219

In the presence of LiCl, dibutylchlorostannane (Bu₂SnClH) reacted with γ-unsubstituted propargyl alcohols and ethers to give γ-stannylated allyl alcohols and ethers, respectively, with high regio- and stereoselectivity. These vinylstannane prod

Full text of DOI:10.1055/s-2004-837219

406
LETTER
Highly Regio- and Stereoselective Hydrostannylation of Propargyl Alcohols
and Ethers Using Dibutylchlorostannane and Lithium Chloride
H
ydrostannylation
a
of Proparg
t
yl Alco
s
hols and E
u
thers kiyo Miura,* Di Wang, Akira Hosomi*
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, and CREST, Japan Science and
Technology Corporation (JST), Tsukuba, Ibaraki 305-8571, Japan
Fax +81(29)8536503; E-mail: hosomi@chem.tsukuba.ac.jp
Received 22 November 2004
the internal adduct 3a (entry 1). Aiming at complete regio-
Abstract: In the presence of LiCl, dibutylchlorostannane
control, we examined the effects of additives on the reac-
tion in THF (entries 2–7). As a result, it turned out that
addition of LiCl or Bu4NCl completely suppressed the
(Bu2SnClH) reacted with g-unsubstituted propargyl alcohols and
ethers to give g-stannylated allyl alcohols and ethers, respectively,
with high regio- and stereoselectivity. These vinylstannane prod-
ucts could be successfully utilized for the Pd-catalyzed cross-cou- formation of 3a. MgCl2 and CaCl2 significantly decelerat-
pling reaction.
ed the hydrostannylation. Catalytic use of LiCl was not as
effective in regiocontrol as the stoichiometric use al-
though the regioselectivity was high enough. Judging
from the results shown in Table 1, the reaction conditions
in entry 2 are optimal for highly efficient and selective
conversion of 1a. The hydrostannylation using Bu2SnClH
and LiCl was greatly suppressed by adding galvinoxyl, a
radical scavenger (entry 8). This result suggests that the
reaction mechanism involves a radical chain process.
Key words: alcohols, alkynes, cross-coupling, hydrostannations,
radical reactions
Since simple and functionalized vinylstannanes work as
good vinyl donors particularly in the Pd-catalyzed cross-
coupling reactions, much effort has been directed toward
highly efficient and selective syntheses of these reagents.1
Undoubtedly, hydrostannylation of alkynes with hydro-
stannanes is the most straightforward and convenient
route to a variety of vinylstannanes.1a,d,2 The hydrostan-
nylation is effectively promoted by radical initiators,3
transition metal catalysts,4 Lewis acids,5 and so on.6 In
general, the radical-initiated method is valuable for highly
regioselective hydrostannylation of various terminal
alkynes leading to b-substituted vinylstannanes although
the stereoselectivity is not high.3 We have recently report-
ed highly regio- and stereoselective hydrostannylation of
g-unsubstituted propargyl alcohols with Bu2SnClH.7–9
Unfortunately, this method involves the Et3B-initiated
reaction at low temperatures for the purpose of achieving
both high levels of regiocontrol and high reaction effi-
ciency.10 Bu2SnClH reacted smoothly with these alkynes
at room temperature even in the absence of Et3B; how-
ever, there is some room for improvement of the regio-
selectivity. We herein disclose that the combined use of
Bu2SnClH and LiCl is effective in highly regio- and
stereoselective hydrostannylation of g-unsubstituted
propargyl alcohols and ethers at room temperature, and
that the vinylstannane products can be utilized for the
subsequent Pd-catalyzed cross-coupling reaction by a
one-pot procedure.11
Table 1 Optimization of Reaction Conditionsa
Bu2SnClH
Additive
OH
HO
OH
R
+
SnClBu2
R
THF, r.t., 2 h
R
Bu2ClSn
1a: R = n-C8H17
(Z)-2a
3a
Entry
Additive (equiv) Conversion (%) (Z)-2a:3a
1
2
3
4
5
6
7
8c
None
91
100
90
86:14
>99:1
LiCl (1)
LiCl (0.5)
LiCl (0.2)
Bu4NCl (1)
MgCl2 (1)
CaCl2 (1)
LiCl (1)
98:2
89
96:4
97b
>99:1
29b
49b
10
95:5
89:11
Not determined
a Unless otherwise noted, all reactions were carried out with 1a (0.50
mmol) and Bu2SnClH (0.55 mmol) in THF (1 mL) at r.t. for 2 h. After
the solvent was removed under reduced pressure, 1H NMR analysis of
the residue was performed to determine the conversion of 1a and the
ratio of (Z)-2a to 3a.
Propargyl alcohol 1a was initially selected as a substrate
to optimize the reaction conditions for the hydrostannyla-
tion with Bu2SnClH (Table 1). As reported previously,7,12
the hydrostannane reacted spontaneously with 1a at room
temperature to give the terminal adduct (Z)-2a along with
b With 1.3 equiv (0.65 mmol) of Bu2SnClH.
c With galvinoxyl (0.05 mmol).
With the optimized conditions in hand, we next attempted
isolation of the stannylated product. To ease the isolation
by column chromatography, butylation of the crude (Z)-
2a with BuM (M = Li, MgBr) was examined. As a result,
the use of BuMgBr at 0 °C was found to be efficient in the
SYNLETT 2005, No. 3, pp 0406–0410
1
6.
0
2.
2
0
0
5
Advanced online publication: 22.12.2004
DOI: 10.1055/s-2004-837219; Art ID: U32404ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Hydrostannylation of Propargyl Alcohols and Ethers
407
butylation leading to the tributylstannylated product (Z)- Terminal alkynes bearing a hydroxy group at a more
4a. Thus, the successive hydrostannylation–butylation remote position were also used for the present hydrostan-
reaction of 1a gave (Z)-4a in 98% isolated yield with nylation (Scheme 2). The stannylation of 3-butyn-1-ol
complete regio- and stereoselectivity (Scheme 1).
(6a) and 4-pentyn-1-ol (6b) took place only at the termi-
nal sp-carbon; however, they showed low to moderate E-
stereoselectivity and lower reactivity to Bu2SnClH.
1) Bu2SnClH (1.1 equiv)
HO
OH
LiCl (1.0 equiv), THF, r.t., 3 h
R
SnBu3
OH
1) Bu2SnClH (1.1 equiv)
2) BuMgBr, 0 °C, 20 min
R
HO
( )n
LiCl (1.0 equiv), THF, r.t., 3 h
1a: R = n-C8H17
(Z)-4a, 98%
( )n
2) BuMgBr, 0 °C, 20 min
SnBu3
Scheme 1
6a: n = 2
6b: n = 3
7a, 69%, E:Z = 59:41
7b, 28%, E:Z = 89:11
The present method was applied to other propargyl alco-
hols. The results are summarized in Table 2.13 g-Unsubsti-
tuted propargyl alcohols 1b–f underwent highly regio-
and stereoselective hydrostannylation to give only termi-
nal adducts (Z)-4 (entries 2–6). High isolated yields of
(Z)-4 were achieved except the reaction of 1c, which ex-
hibited lower reactivity toward the hydrostannylation
with Bu2SnClH. The Et3B-initiated reaction of 1c with an
increased amount of Bu2SnClH gave (Z)-4c in good yield
(entry 3). Unlike g-unsubstituted propargyl alcohols, g-
substituted propargyl alcohols 1g,h favored b-stannyla-
tion leading to 5 (entries 7 and 8). In the absence of LiCl,
the stannylation of 1g with Bu2SnClH proceeded effi-
ciently with higher b-regioselectivity.7,14 As such, the use
of LiCl raised the ratio of g-stannylation; however, its
ability to control the reaction site is not high enough for
the g-regioselective stannylation of 1g,h.
Scheme 2
We next attempted highly regioselective hydrostannyla-
tion of g-unsubstituted propargyl ethers (Table 3).7,9 The
reaction of methyl propargyl ether 8a with Bu2SnClH fol-
lowed by treatment with BuMgBr formed g- and b-stan-
nylated products, (Z)-9a (85%) and 10a (8%, entry 1). In
contrast, the combined use of Bu2SnClH and LiCl com-
pletely suppressed the formation of 10a to afford only 9a
with high Z-selectivity (entry 2).9 MOM ether 8b and TBS
ether 8c were also stannylated with high g-regioselectivity
(entries 3, 4). Unlike the case of 8a, the reaction of 8b re-
sulted in low Z-selectivity, and 8c was converted into (E)-
vinylstannane 9c with high stereoselectivity. Although 8c
was relatively less reactive than 8a,b, the use of Et3B-dry
air was effective in accelerating the stannylation of 8c.
Table 2 Hydrostannylation of Propargyl Alcoholsa
1) Bu2SnClH
LiCl
OH
OH
R1
R2
R1
R2
HO
R1
SnBu3
R3
R3
+
2) BuMgBr
R2
Bu3Sn
R3
1
(Z)-4
5
Entry
Substrate
Yield (%)b
(Z)-4:5
R1
R2
R3
1
2
n-C8H17
c-C6H11
Ph
H
H
H
1a
1b
1c
1d
1e
1f
98
98
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
17:83f
31:69f
H
3c,d
4
H
H
H
79
Me
Me
93
5
(CH2)5
H
H
89 (97)e
91
6
H
H
H
H
7
H
Me
n-C6H13
1g
1h
61
8c
Me
63
a For general procedure, see ref.13,
b Isolated yield.
c Et3B (1.0 M in hexane, 0.05 mmol) and dry air (5 mL) were used as radical initiator.
d The reaction was performed with 1.3 equiv (0.65 mmol) of Bu2SnClH for 6 h.
e The yield shown in parentheses was determined by 1H NMR analysis of the crude product.
f (Z)-5g:(E)-5g = 1:1, (Z)-5h:(E)-5h = >99:1.
Synlett 2005, No. 3, 406–410 © Thieme Stuttgart · New York

408
K. Miura et al.
LETTER
Table 3 Hydrostannylation of Propargyl Ethersa
1) Bu2SnClH
OP
R1
R2
PO
R1
OP
LiCl
R1
R2
SnBu3
2) BuMgBr
R2
Bu3Sn
8
9
10
Entry
Substrate
Yield (%)b
(Z)-9:(E)-9
R1
R2
H
P
1c
2
n-C8H17
n-C8H17
n-C8H17
n-C8H17
Ph
Me
8a
8a
8b
8c
8d
8e
85
90
>99:1d
98:2
H
Me
3
H
CH2OMe
SiMe2t-Bu
SiMe2t-Bu
SiMe2t-Bu
88
61:39
2:98
4e
5e
6e
H
84
H
54
6:94
Me
Me
55 (69)f
1:>99
a,b See footnotes a and b in Table 2. The butylation was carried out with 2 equiv of BuMgBr (1.00 M in Et2O, 1.00 mmol) at 0 °C.
c Without LiCl.
d Regioisomer 10a was obtained in 8% yield.
e The hydrostannylation was carried out with Bu2SnClH (0.65 mmol), LiCl (0.65 mmol), Et3B (1.0 M in hexane, 0.05 mmol), and dry air
(5 mL) for 6 h.
f See footnote e in Table 2.
The hydrostannylation of other TBS ethers 8d,e also gave radical chain carrier (Scheme 3). In this mechanism, anion
similar results in regio- and stereoselectivity (entries 5, 6). radical 12 generated from 11 reacts with 1 to give vinyl
radical intermediates 13a.18 Subsequent elimination of
LiCl and internal coordination by the hydroxy oxygen
To gain some insights into the reaction mechanism, NMR
studies on Bu2SnClH and (Z)-2a were performed in the
converts 13a into neutral vinyl radicals 14. No complex-
ation of (Z)-2a with LiCl implies that this step is thermo-
dynamically favored. Hydrogen abstraction of 14 from 11
provides (Z)-2 along with the regeneration of 12. As re-
ported previously,7 the high Z-stereoselectivity is attribut-
able to the internal Sn–O coordination, which controls the
reaction site of 14 and suppresses the isomerization of (Z)-
2 caused by addition–elimination of a stannyl radical. In
the cases of TBS ethers 8c–e, the bulky TBS group would
disturb the formation of the Sn–O coordinate bond to
bring the E-selective hydrostannylation.
absence or presence of LiCl. The 119Sn signal of
Bu2SnClH (0.3 mmol in 0.6 mL of THF-d8) appeared at
1
d = –30.9 ppm (vs Me4Sn).15 Its H NMR and 13C NMR
analyses revealed the coupling constants of tin with the
adjacent proton and carbon [1J (119Sn–1H) = 2208 Hz,
1J (119Sn–13C) = 463 Hz]. In the presence of LiCl, the
119Sn signal of Bu2SnClH shifted to considerably higher
1
field (–115 ppm) and the values of J (119Sn–1H) and
1J (119Sn–13C) increased to 2341 Hz and 519 Hz, respec-
tively. These results clearly indicate the formation of a
trigonal bipyramidal 5-coordinate complex (11 in
Scheme 3).16 The complexation of Bu2SnClH with LiCl There are two possibilities of the reason why 11 shows
was observed even in the presence of EtOH.
higher regioselectivity than Bu2SnClH. One is a kinetic
reason: the relative rate of terminal addition to internal ad-
dition in the reaction of 12 is larger than that regarding
Bu2ClSn• because the sterically more demanding struc-
ture of 12 hinders the internal addition more severely. The
other is a thermodynamic reason: the energy difference
between 13a and 13b is larger than that between terminal
and internal adducts of Bu2ClSn•, which would be posed
by the fact that distribution of negative charge from the
pentacoordinate tin to the internal sp2-carbon in 13b is
unfavorable in comparison with that to the terminal sp2-
carbon in 13a.
On the other hand, addition of LiCl hardly affected the 1H
NMR and 13C NMR spectra of (Z)-2a, which proved no
complexation of (Z)-2a with LiCl. This observation sug-
gests the presence of a tight internal Sn–O coordination.
The coupling constant between the allylic proton and the
adjacent olefinic proton in (Z)-2a (2.3 Hz) is smaller than
that in (Z)-4a (7.5 Hz). The dihedral angle predicted from
the smaller value for (Z)-2a supports the formation of a
chelate ring by Sn–O coordination.17
The present hydrostannylation is accelerated by Et3B-air
and decelerated by galvinoxyl. In addition, Bu2SnClH and
LiCl readily form 5-coordinate tin complex 11. On the ba-
sis of these facts, a possible mechanism for the present hy-
drostannylation of g-unsubstituted propargyl alcohols 1
involves a radical chain process in which 11 works as the
Under the regeneration of LiCl (Scheme 3), a proper con-
trol of the concentration of Bu2SnClH, which prevents its
direct reaction with a substrate, is expected to achieve
complete regiocontrol of the hydrostannylation with a
Synlett 2005, No. 3, 406–410 © Thieme Stuttgart · New York

LETTER
Hydrostannylation of Propargyl Alcohols and Ethers
409
catalytic amount of LiCl. Indeed, after Bu2SnClH (0.22 rate was also examined.22 The absence of LiCl brought no
equiv) was added to a THF solution of 1a and LiCl (0.20 distinct difference in the coupling rate although the regio-
equiv) five times every 2 hours, butylation of the reaction isomer of 15a was formed as a minor product (entry 3).
mixture gave (Z)-4a with excellent regioselectivity [95%
yield, (Z)-4a:5a = 99:1]. The same reaction without the
portionwise addition of Bu2SnClH resulted in slightly
lower regioselectivity [95% yield, (Z)-4a:5a = 96:4].
The reaction conditions employed in entry 2 are effective
in the coupling with 4-iodoanisole (entry 4). In contrast,
bromobenzene did not react with (Z)-2a under the same
conditions. The coupling with bromobenzene could be
promoted by using Ph3P and an increased amount of the
+
Bu2SnClH
LiCl
[Bu2SnCl2H] Li
Pd catalyst (entry 5). Further optimization of the reaction
conditions is needed for improvement of the reaction effi-
ciency. The successive hydrostannylation–cross-coupling
reaction of TBS ether 8c with Bu2SnClH–LiCl and iodo-
or bromobenzene gave allyl ether (E)-16 (P = TBS,
Ar = Ph, entries 6 and 7). Thus, the protection with a TBS
group reversed the stereochemical outcome of the arylat-
ed products.
11
R• – RH (R• = 14 or other radicals)
–
·
[Bu2SnCl2]
Li
12
1 (R3 = H)
1 (R3 = H)
OH
Li
R1
R2
OH
R1
R2
Table 4 Hydrostannylation Followed by Pd-Catalyzed Cross-
SnCl2Bu2
Li
Couplinga
Bu2Cl2Sn
1) Bu2SnClH–LiCl
PO
OP
13b
13a
THF, r.t., 3 h
– LiCl
n-C8H17
Ar
2) Pd2(dba)3·CHCl3
Ph3P, TBAF (3 equiv)
ArX (1.1 equiv), 60 °C
n-C8H17
H
15
16
1a: P = H
8c: P = TBS
O
R1
R2
SnClBu2
Entry Reactants
Pd cat., Ph3P Time Yield Z:E
(mol%)b
(h)
(%)c
14
11 – 12
(Z)-2
1
2
3d
4
5
6
7
1a, PhI
1a, PhI
1a, PhI
4, 5
5
97
>99:1
>99:1
>99:1
>99:1
99:1
0.5, 0
0.5, 0
1
93
Scheme 3 A plausible mechanism for hydrostannylation with
Bu2SnClH and LiCl
1
91e
82
1a, 4-MeOC6H4I 0.5, 0
1
1a, PhBr
8c, PhI
4, 10
2, 0
20
10
20
58
To enhance the synthetic utility of the present hydrostan-
nylation, we investigated the use of the vinylchlorostan-
nane products for the Pd-catalyzed cross-coupling
reaction by a one-pot procedure.1,11 It is known that aryl-
halostannanes are less reactive toward the Pd-catalyzed
coupling with haloarenes than the corresponding aryl-
trialkylstannanes,1c,e however, that the biaryl coupling
with arylhalostannanes is effectively promoted by TBAF
(Bu4NF).19,20 According to the procedure reported by
Kosugi et al.,19a we initially attempted the coupling reac-
tion of (Z)-2a with iodobenzene using Ph3P,
Pd2(dba)3·CHCl3, and TBAF, in which the (Z)-2a-con-
taining reaction mixture obtained from 1a and
Bu2SnClH–LiCl was directly subjected to the cross-cou-
pling.21 The successive reaction of 1a gave the coupling
product (Z)-15a (P = H, Ar = Ph) in high yield with com-
plete regio- and stereoselectivity (entry 1 in Table 4). The
optimization of the amount of Ph3P revealed that Ph3P was
not necessary, or rather, it decelerated this coupling. A
comparable yield of (Z)-15a was attained with no Ph3P
and a less amount of the Pd catalyst (entry 2). On the other
hand, TBAF was found to be essential for the successful
C-C bond formation. The effect of LiCl on the coupling
79
1:>99
1:>99
8c, PhBr
4, 10
63
a For the reaction conditions of the hydrostannylation step, see
Table 2 (for 1a), Table 3 (for 8c), and ref.13 General procedure for the
subsequent cross-coupling is shown in ref.20
b With respect to the Pd catalyst, the mole percentage of Pd, not the
dinuclear complex, is shown.
c Isolated yield.
d Without LiCl.
e The reaction in the absence of LiCl gave a mixture of (Z)-15a and its
regioisomer [(Z)-15a:regioisomer = 86:14]. The value shown is the
combined yield of both isomers.
In conclusion, we have developed highly regio- and
stereoselective hydrostannylation of g-unsubstituted pro-
pargyl alcohols and ethers with Bu2SnClH–LiCl at room
temperature. Complexation of Bu2SnClH with LiCl is
attributable to the compete regiocontrol. The hydrostan-
nylation products can be used for the subsequent Pd-cata-
lyzed cross-coupling with haloarenes by a one-pot
procedure.
Synlett 2005, No. 3, 406–410 © Thieme Stuttgart · New York

410
K. Miura et al.
LETTER
(13) General Procedure for the Hydrostannylation of 1 with
Bu2SnClH-LiCl Followed by Butylation:
Acknowledgment
This work was partly supported by Grants-in-Aid for Scientific Re-
search from the Ministry of Education, Culture, Sports, Science,
and Technology, Government of Japan.
Under N2, THF (1 mL) was added to a mixture of Bu2SnCl2
(83.5 mg, 0.275 mmol) and LiCl (21.2 mg, 0.500 mmol) at
0 °C. Bu2SnH2 (64.6 mg, 0.275 mmol) was added to the THF
solution, and the mixture was stirred for 10 min at 0 °C. A
propargyl alcohol 1 (0.500 mmol) was added to the resultant
Bu2SnClH–LiCl solution. The mixture was warmed to r.t.
and stirred for 3 h. The resultant mixture was cooled to 0 °C
and treated with BuMgBr (1.00 M in Et2O, 1.30 mmol).
After 20 min, the resultant mixture was poured into a 1:1
mixture of sat. aq NH4Cl and H2O. The extract with
t-BuOMe was dried over Na2SO4 and evaporated.
Purification of the residual oil was performed by silica gel
column chromatography (hexane–t-BuOMe 20:1).
(14) g-Substituted propargyl alcohols are known to undergo
b-regioselective hydrostannylation with Bu3SnH in the
presence of a radical initiator. See: (a) Ensley, H. E.;
Buescher, R. R.; Lee, K. J. Org. Chem. 1982, 47, 404.
(b) Konoike, T.; Araki, Y. Tetrahedron Lett. 1992, 33, 5093.
(15) The 119Sn signal of Bu2SnClH (8 mmol in 1 mL of THF-d8)
was reported to appear at d = –18.3 ppm. See: Kawakami, T.;
Sugimoto, T.; Shibata, I.; Baba, A.; Matsuda, H.; Sonoda, N.
J. Org. Chem. 1995, 60, 2677.
(16) Shibata and Baba et al. have reported that halide ions and
HMPA easily complex Bu2SnXH (X = F, Cl, Br, I), and that
the resulting 5-coordinate tin compounds can be
successfully used as reducing and hydrostannylating agents.
See ref. 6, ref.14, and the following references: (a) Shibata,
I.; Kato, H.; Kanazawa, N.; Yasuda, M.; Baba, A. Synlett
2004, 137. (b) Shibata, I.; Baba, A. Curr. Org. Chem. 2002,
6, 665; and references therein.
(17) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C.
Spectrometric Identification of Organic Compounds, 5th
ed.; Wiley: New York, 1991.
(18) The formation of Bu2SnClH from Bu2SnCl2 and Bu2SnH2 is
known to be reversible. The initiation of the present
homolytic hydrostannylation may be caused by electron
transfer from Bu2SnH2 to Bu2SnCl2, which forms the anion
radical of Bu2SnCl2. See ref.12b
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Subramanian, L. R. Synlett 1994, 1047. (b) Vedejs, E.;
Haight, A. R.; Moss, W. O. J. Am. Chem. Soc. 1992, 114,
6556.
(7) Miura, K.; Wang, D.; Matsumoto, Y.; Fujisawa, N.; Hosomi,
A. J. Org. Chem. 2003, 68, 8730.
(8) For a related work by us, see: Miura, K.; Saito, H.;
Uchinokura, S.; Hosomi, A. Chem. Lett. 1999, 659.
(9) Quite recently, Mitchell et al. have reported results similar to
those reported previously by us. See: (a) Thiele, C. M.;
Mitchell, T. N. Eur. J. Org. Chem. 2004, 337. (b) Highly
regio- and stereoselective hydrostannylation of g-
unsubstituted propargyl ethers with Bu2SnClH was also
reported by them; however, the high regioselectivity could
not be reproduced by our hands. See: Mitchell, T. N.;
Moschref, S.-N. Chem. Commun. 1998, 1201.
(21) General Procedure for the Subsequent Pd-Catalyzed
Cross-Coupling:
PPh3, Pd2(dba)3·CHCl3, TBAF (1.0 M in THF, 1.5 mmol),
and a haloarene (0.550 mmol) were added successively to
the reaction mixture obtained by the hydrostannylation of 1a
or 8c (0.500 mmol). After being stirred at 60 °C for a given
time, the resultant mixture was cooled to r.t. and diluted with
t-BuOMe (10 mL). After addition of DBU (0.5 mL), the
mixture was stirred for 5–10 min, passed through a short
silica gel column, and evaporated. Purification of the
residual oil was performed by silica gel column
(10) For the Et3B-initiated reaction, see ref.3b
(11) (a) Gallagher, W. P.; Terstiege, I.; Maleczka, R. E. Jr. J. Am.
Chem. Soc. 2001, 123, 3194. (b) Maleczka, R. E. Jr.;
Gallagher, W. P. Org. Lett. 2001, 3, 4173. (c) Maleczka, R.
E. Jr.; Gallagher, W. P.; Terstiege, I. J. Am. Chem. Soc.
2000, 122, 384. (d) Maleczka, R. E. Jr.; Lavis, J. M.; Clark,
D. H.; Gallagher, W. P. Org. Lett. 2000, 2, 3655.
(12) (a) Neumann, W. P.; Pedain, J. Tetrahedron Lett. 1964,
2461. (b) Davies, A. G.; Kinart, W. J.; Osei-Kissi, D. K. J.
Organomet. Chem. 1994, 474, C11.
chromatography.
(22) Espinet, P.; Echavarren, A. M. Angew. Chem. Int. Ed. 2004,
43, 4704.
Synlett 2005, No. 3, 406–410 © Thieme Stuttgart · New York