Rhodium-catalyzed synthesis of terminal alkenes

Article abstract of DOI:10.1055/s-2004-834920

Terminal alkenes have been efficiently prepared via a rhodium-catalyzed olefination procedure using Wilkinson's catalyst in the presence of triphenylphosphine, 2-propanol and trimethylsilyldiazomethane. Optimized reaction conditions are described for aldehydes and ketones, as well as alternative work up procedures. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2004-834920

PRACTICAL SYNTHETIC PROCEDURES
1901
Rhodium-Catalyzed Synthesis of Terminal Alkenes
R
hodium-Catalyze
a
d
Synthesis
l
of Ter
é
minal
A
lke
r
ne
s
ie Paquet, Hélène Lebel*
Département de Chimie, Université de Montréal, Montréal, Québec, H3C 3J7, Canada
Fax +1(514)3432177; E-mail: helene.lebel@umontreal.ca
Received 5 October 2004
PSP
No49
Abstract: Terminal alkenes have been efficiently prepared via a rhodium-catalyzed olefination procedure using Wilkinson’s catalyst in the
presence of triphenylphosphine, 2-propanol and trimethylsilyldiazomethane. Optimized reaction conditions are described for aldehydes and
ketones, as well as alternative work up procedures.
Key words: terminal alkene, methylenation, trimethylsilyldiazomethane, triphenylphosphine, Wilkinson’s catalyst
RhCl(PPh3)3 (0.25 mol%)
PPh3, TMSCHN2
Procedure 1
Procedure 2
TBSO
TBSO
O
i-PrOH, THF, 25 °C
1
2
88% yield
O
RhCl(PPh3)3 (0.5 mol%)
i-PrOH, TMSCHN2
O
O
O
O
PPh3, 1,4-dioxane, 60 °C
3
4
84% yield
O
1- RhCl(PPh3)3 (0.25 mol%)
PPh3, TMSCHN2
Procedure 3
i-PrOH, THF, 25 °C
2- Oxone
5
6
91% yield
O
1- RhCl(PPh3)3 (2.5 mol%), THF
i-PrOH, TMSCHN2, 25 °C
TMS
Procedure 4
Ph2P
CO2
(7, DPPBE)
2- TBAF / THF
3- NaOH (aq)
5
6
79% yield
Scheme 1
years.1 Besides the Wittig reaction,2 there are several stoi-
chiometric transition metal-promoted olefination sys-
Introduction
Terminal alkenes are very important precursors for nu- tems.3 In contrast, only one catalytic methylenation
merous organic transformations; thus easy and reliable reaction has been disclosed so far. Indeed, we have shown
olefination methodologies became instrumental over the that the Wilkinson’s complex could catalyze the forma-
tion of terminal alkenes from a variety of carbonyl precur-
sors in the presence of triphenylphosphine, 2-propanol
and trimethylsilyldiazomethane.4 This method is compat-
ible with numerous functionalities and enolizable sub-
SYNTHESIS 2005, No. 11, pp 1901–1905
Advanced online publication: 24.11.2004
DOI: 10.1055/s-2004-834920; Art ID: M07604SS
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x
.
2
0
0
4

1902
V. Paquet, H. Lebel
PRACTICAL SYNTHETIC PROCEDURES
strates. In addition, we showed that this reaction could be gives superior or similar results than the standard Wittig
combined in a multicatalytic process allowing the synthe- reaction. Moreover, a very large range of functional
sis of alkenes directly from alcohols.5 In this communica- groups is tolerated in the reaction with the exception of
tion, we wish to report optimized large-scale phenol and aromatic nitro groups that are problematic.
methylenation procedures using both aldehydes and ke-
As the reaction procedure was scaled up, the catalyst load-
tones. Furthermore, alternative work-up procedures are
ing was studied (Table 2). Indeed, the methylenation reac-
also available to facilitate the isolation of the desired alk-
tion could be performed with only 0.25 mol% of
ene in good yields.
Wilkinson’s catalyst (a tenth compared to the original pro-
cedure) without jeopardizing the isolated yield. The al-
kene 2 could be consistently isolated in very good yields
(86–90%) without a significant change in the reaction
Scope and Limitations
time (2–6 h). However, when 0.10 mol% of the catalyst
The catalytic methylenation reaction is very general with
was used, the methylenation never reached completion
a wide range of aldehydes providing the terminal alkene
even after 48 hours.
products in excellent yields with readily available re-
agents. The standard reaction conditions for the olefina-
tion of aldehydes consist in using 2.5 mol% of the
rhodium catalyst in combination with 1.1 equivalent of
triphenylphosphine, 1.1 equivalent of 2-propanol and 1.4
equivalent of trimethylsilyldiazomethane in THF at 25
°C. A number of terminal alkenes were synthesized ac-
cording to this procedure (Table 1).
Table 2 Synthesis of Alkene 2 Using Procedure 1: Catalyst Loading
Study
Entry
Loading
Time (h)
Yield (%)
1
2
3
4
5
2.50 mol%
1.00 mol%
0.50 mol%
0.25 mol%
0.10 mol%
2
86
2
88
5
90
Table 1 Synthesis of Terminal Alkenes Using Olefination Proce-
dure 1
6
88
48
76% conv.a
Entry
1
Product
Yield (%)
98
a Conversion measured by 1H NMR.
OBn
Ph
An excellent chemoselectivity between aldehydes and ke-
tones is also observed, because of the difference in the
electronic and steric character between both carbonyl de-
rivatives (Equation 1). In comparison, up to 20% of the
corresponding diene was isolated using standard Wittig
reaction conditions (Ph3PCH3Br + NaHMDS–THF) com-
bined with 59% yield of the terminal alkene 15.
8
2
3
79
OTBDPS
9
90
86
O
O
RhCl(PPh3)3
PPh3, i-PrOH
10
BnO
BnO
(1)
4
5
O
3
Ph
O
TMSCHN2
THF, 25 °C
3
14
15
87% yield
11
89a
(95% ee)
Equation 1
OTBDPS
The methylenation of highly reactive fluoromethylke-
tones proceeded equally well to lead to the corresponding
alkenes in good yields, when Procedure 1 was used.6 Fur-
ther optimization was required to perform the methylena-
tion of less reactive a-alkoxymethyl and aliphatic ketones.
The use of an excess of 2-propanol (10 equiv) was instru-
mental to suppress the undesired formation of a ketone-
derived silylated byproduct. Complete conversion and
good yields were obtained for the methylenation of a-
alkoxymethyl ketones when 1.6 equivalent of trimethyl-
silyldiazomethane was used (Table 3).
12
6
93
THPO
13
a The starting material had 95% ee.
Since the methylenation does not require the use of a base,
these reaction conditions are mild enough to be compati-
ble with sensitive and enolizable substrates. In fact, the
methylenation of enantioenriched a-chiral aldehydes led
to the isolation of alkene products without significant ero-
sion of the enantiomeric excess. In general, this procedure
In the case of aliphatic and aromatic ketones, the solvent
was changed to 1,4-dioxane, thus allowing heating the re-
action mixture to 60 °C without decomposing the rhodium
Synthesis 2005, No. 11, 1901–1905 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Rhodium-Catalyzed Synthesis of Terminal Alkenes
1903
Table 3 Methylenation of a-Alkoxymethyl Ketones
from the crude mixture containing the phosphorus
byproducts. It is possible to first eliminate the remaining
traces of triphenylphosphine by an oxidative work up,
thus facilitating the isolation of non-polar alkenes by flash
chromatography. Because the purification step is compli-
cated by these byproducts, the alkene 6 was isolated in
only 75% yield following the Procedure 1. By adding one
equivalent of oxone at the end of the reaction,8 the purifi-
cation of the alkene product by flash chromatography is
simplified and the triene 6 could be isolated in 91% yield
with the Procedure 3 (Scheme 1). The possibility of using
of an easily removable phosphine reagent was also exam-
ined. 4-Diphenylphosphanylbenzoic acid 2-trimethylsila-
nyl-ethyl ester (DPPBE) (7) was the only substitute as
efficient as triphenylphosphine. This phosphine devel-
oped by Yoakim and coauthors greatly simplifies the pu-
rification by allowing complete removal of the
phosphorus byproducts from the reaction mixture.9 Once
the olefination of perillaldehyde (5) was complete, the so-
lution was treated with TBAF in THF to cleave the tri-
methylsilanylethyl ester moiety. The corresponding
carboxylate phosphorus byproduct was then removed us-
ing a basic aqueous solution of NaOH to leave the organic
phase with only the triene 6, which was isolated in 79%
yield (Scheme 1).
Entry
1
Product
Yield (%)
84
OBn
BnO
16
2
3
84
97
OTBS
MeO
17
OTiPS
Ph
18
catalyst.7 The best reaction conditions for unactivated ke-
tones utilized 1.1 equivalent of triphenylphosphine, 15
equivalents of 2-propanol and 2.4 equivalents of trimeth-
ylsilyldiazomethane, thus producing the alkene 4 in 84%
yield (Scheme 1). These new reaction conditions are also
compatible with enolizable ketones and no epimerization
of chiral centers was observed during the reaction
(Table 4). In contrast, cyclic a,b-unsaturated ketones did
not lead to the desired alkene product.
In summary, terminal alkenes can be efficiently prepared
by a rhodium-catalyzed methylenation reaction using
readily available reagents. This methodology is compati-
ble with a wide range of functionalized aldehydes and ke-
tones. In some cases, the use of an easily removable
phosphine or an oxidative work-up improves the purifica-
tion procedure and higher yields of the desired alkene are
obtained.
Table 4 Methylenation of Ketones Using Procedure 2
Entry
1
Product
Yield (%)
93
Ph
19
20
2
89
97
OTiPS
Procedures
Herein, we describe four typical synthetic procedures for
the catalytic conversion of carbonyl compounds into al-
kene products. First, the reaction can be performed with
aldehydes using THF as a solvent at 25 °C. This procedure
was done on a 10 mmol scale using only 0.25 mol% of the
Wilkinson’s catalyst (Procedure 1). The terminal alkene 2
was isolated with 88% yield after distillation. In order to
obtain complete conversion with ketone substrates, the
olefination reaction must be done in 1,4-dioxane at 60 °C.
This second procedure enabled us to isolate the alkene 4
in 84% yield (Procedure 2). Alternatively, the purification
step can be simplified by using DPPBE (7) as the phos-
phine reagent and subsequent treatment of the mixture
with TBAF followed by a basic wash to remove the phos-
phorus byproducts (Procedure 4). A procedure using ox-
one to achieve an oxidative work up is also possible to
facilitate the isolation of some alkenes (Procedure 3). Fi-
nally, because there is some inconsistency in the purity of
the solution of trimethylsilyldiazomethane available from
Aldrich,10 we also describe the procedure used for the
preparation of this reagent.
3
4
Ph
21
90a
(95% ee)
OTBS
Ph
22
5
87
Ph
23
a The starting material had 95% ee.
An apparent drawback of the methodology is the use of
triphenylphosphine and the formation of stoichiometric
amount of triphenylphosphine oxide during the process.
To facilitate the purification procedure, several options
were examined and two alternative work-up procedures
were designed. Both are an option to the standard distilla-
tion procedure used to isolate the desired alkene product
Synthesis 2005, No. 11, 1901–1905 © Thieme Stuttgart · New York

1904
V. Paquet, H. Lebel
PRACTICAL SYNTHETIC PROCEDURES
Trimethylsilyldiazomethane11
Anal. Calcd for C12H26OSi: C, 67.22; H, 12.22. Found: C, 65.94; H,
12.41.
A dry, 1 L, three-necked round-bottomed flask was equipped with
a 250 mL pressure-equalizing dropping funnel, a stirring bar and
two septums. The apparatus was flame-dried under Ar. When the
system had cooled to r.t., a solution of diphenylphosphorazidate12
(46.0 g, 167 mmol) in anhyd Et2O (200 mL) was transferred to the
flask via a cannula. The Et2O solution of the trimethylsilylmethyl-
magnesium chloride (200 mL, 200 mmol) was measured in a grad-
uate cylinder via cannula under Ar and then transferred to the
dropping funnel. The flask was cooled with a dry-ice–acetone bath.
When the internal temperature reached –10 °C, the Grignard re-
agent was added dropwise at such a rate that the internal tempera-
ture was maintained below 0 °C (some Et2O can be added for better
stirring). After the addition was completed, the reaction was stirred
for 2 h in an ice bath. The funnel was replaced by a septum and the
flask was then placed in the fridge for 14 h. The reaction mixture
was opened to air and cooled again to –15 °C with a dry-ice–acetone
bath. Water (50 mL) was then carefully added dropwise at such a
rate that the internal temperature was maintained below 0 °C (stir-
ring can be done manually). The reaction mixture was filtered with
a Buckner filter and the white solid was washed with Et2O (2 × 100
mL). The yellow filtrate was washed with water (100 mL) and dried
over MgSO4. The filtrate was placed in a 1 L round-bottomed flask
with a magnetic stirrer and equipped with a distillation apparatus
(the Vigreux column must be 5–10 cm). The solution was slowly
distilled under Ar until no more volatile came over (oil bath temper-
ature was maintained below 55 °C). The oil bath was removed and
the distillation apparatus was replaced by a ‘U’ glass tube connected
to a 250 mL round-bottomed flask placed in a dry-ice–acetone bath.
The solution was transferred under vacuum (0.1 mmHg). Finally,
the oil bath at 55 °C was used to transfer the remaining trimethyl-
silyldiazomethane. The yellow solution was dried over Na2SO4 at
r.t. After the filtration in a 100 mL flask, the solution was concen-
trated slowly, until the vapor temperature reached 90 °C. The re-
maining trimethylsilyldiazomethane (14.86 g, 78%) was used as it
was and transferred in an amber (the product was light sensitive). It
was also possible to use trimethylsilyldiazomethane as a solution in
THF and the concentration was determined by 1H NMR with an in-
ternal standard.13
5-Isopropenylbenzo[1,3]dioxole (4); Procedure 2
To a solution of chlorotris(triphenylphosphine)rhodium (0.093 g,
0.10 mmol) and triphenylphosphine (5.77 g, 22.0 mmol) in 1,4-di-
oxane (200 mL) at 60 °C, was added 2-propanol (23.0 mL, 300
mmol) followed by 3¢,4¢-(methylenedioxy)acetophenone (3.28 g,
20.0 mmol). To the resulting red mixture, was then added a solution
of trimethylsilyldiazomethane (7.50 mL, 48.0 mmol). Gas evolu-
tion was observed and the resulting dark orange mixture was stirred
at 60 °C. When the reaction was completed (determined by GC or
TLC analysis), oxone (12.3 mg, 20.0 mmol) was added and the re-
sulting mixture was then stirred for an other 6 h at 60 °C. The sol-
vent was removed under reduced pressure. The desired alkene 4
(2.71 g, 84%) was obtained as a colorless oil after flash chromatog-
raphy with a pre-absorption on silica (1% EtOAc–hexanes); Rf 0.27
(1% EtOAc–hexanes).
IR (neat): 3077, 2966, 2892, 1602, 1502, 1489, 1296, 1228, 1038,
932, 885, 811, 629 cm–1.
1H NMR (400 MHz, CDCl3): d = 7.02–6.96 (m, 2 H, Ar), 6.81–6.79
(m, 1 H, Ar), 5.98 (s, 2 H, OCH2O), 5.28 (s, 1 H, C=CHH), 5.02 (s,
1 H, C=CHH), 2.13 (s, 3 H, CH3C=CH2).
13C NMR (100 MHz, CDCl3): d = 148.0, 147.4, 143.1, 136.0, 119.5,
111.7, 108.3, 106.4, 101.4, 22.5.
HMRS (MAB): m/z [M]+ calcd for C10H10O2: 162.068080; found:
162.068231.
(S)-4-Isopropenyl-1-vinylcyclohexene (6); Procedure 3
To a solution of chlorotris(triphenylphosphine)rhodium (0.012 g,
0.013 mmol) and triphenylphosphine (1.44 g, 5.50 mmol) in THF
(50 mL) was added 2-propanol (0.42 mL, 5.5 mmol) followed by
(S)-(–)-perillaldehyde (750 mg, 5.00 mmol). To the resulting red
mixture, was then added a solution of trimethylsilyldiazomethane
(1.20 mL, 7.00 mmol). Gas evolution was observed and the result-
ing dark orange mixture was stirred at 25 °C. When the reaction is
completed (determined by GC or TLC analysis), oxone (3.09 g,
5.00 mmol) was added and the resulting mixture was stirred for 3 h
at 60 °C. The solvent was removed under reduced pressure. The de-
sired alkene 6 (670 mg, 91%) was obtained as a colorless oil after
flash chromatography with a pre-absorption on silica (1% Et2O–
pentane); Rf 0.60 (1% EtOAc–hexanes).
1H NMR (400MHz, CDCl3): d = 2.58 [s, 1 H, (CH3)3CHN2], 0.16 [s,
9 H, (CH3)3CHN2)].
6-(tert-Butyldimethylsilyloxy)-1-hexene (2); Procedure 1
To a solution of chlorotris(triphenylphosphine)rhodium (0.023 g,
0.025 mmol) and triphenylphosphine (2.88 g, 11.0 mmol) in THF
(100 mL) at 25 °C, was added 2-propanol (0.84 mL, 11.0 mmol) fol-
lowed by 6-(tert-butyldimethylsilyloxy)-1-pentanal (2.16 g, 10.0
mmol). To the resulting red mixture, was then added a solution of
trimethylsilyldiazomethane (3.40 mL, 14.0 mmol). Gas evolution
was observed and the resulting orange mixture was stirred at 25 °C.
When the reaction was completed (determined by GC or TLC ana-
lysis), the solvent was removed under reduced pressure and the
crude alkene was purified by Kugelrhor distillation (110–120 °C;
0.1 mmHg) to afford 2 (1.89 g, 88%) as a colorless oil.
IR (neat): 3066, 3086, 2920, 1645, 1436, 989, 890, 835 cm–1.
1H NMR (400 MHz, CDCl3): d = 6.38 (dd, J = 17, 11 Hz, 1 H,
CH=CH2), 5.78–5.76 [m, 1 H, CH=C(CH=CH2)], 5.08 (d, J = 17
Hz, 1 H, CH=CHH-trans), 4.92 (d, J = 11 Hz, 1 H, CH=CHH-cis),
4.75 (s, 1 H, MeC=CH2), 4.74 (s, 1 H, MeC=CH2), 2.36–2.07 (m, 6
H, CH2-cycle), 1.94–1.89 (m, 1 H, CHMeC=CH2), 1.76 (s, 3 H,
CH3C=CH2).
13C NMR (100 MHz, CDCl3): d = 149.6, 139.5, 135.6, 129.0, 109.9,
108.6, 41.1, 31.1, 27.2, 24.1, 20.6.
HMRS (MAB): m/z [M]+ calcd for C11H16: 148.125201; found:
148.125212.
IR (NaCl, film): 3080, 2935, 1640, 1470, 1390, 1360, 1255, 1110,
910, 840, 775, 660 cm–1.
1H NMR (400 MHz, CDCl3): d = 5.84–5.78 (m, 1 H, CH=CH2),
5.00 (d, J = 17 Hz, 1 H, CH=CHH-trans), 4.94 (d, J = 11 Hz, 1 H,
CH=CHH-cis), 3.61 (t, J = 6 Hz, 2 H, CH2OTBS), 2.09–2.04 (m, 2
H, CH2CH=CH2), 1.55–1.50 (m, 2 H, CH2CH2OTBS), 1.47–1.41
(m, 2 H, CH2CH2CH2OTBS), 0.90 [s, 9 H, SiC(CH3)3], 0.05 [s, 6 H,
Si(CH3)2].
(S)-4-Isopropenyl-1-vinylcyclohexene (6); Procedure 4
To a solution of chlorotris(triphenylphosphine)rhodium (0.116 g,
0.125 mmol) and DPPBE (5.50 mL, 5.50 mmol) in THF (50 mL)
was added 2-propanol (0.42 mL, 5.5 mmol) followed by (S)-(–)-pe-
rillaldehyde (750 mg, 5.00 mmol). To the resulting red mixture, was
then added a solution of trimethylsilyldiazomethane (1.40 mL, 7.00
mmol). Gas evolution was observed and the resulting dark orange
mixture was stirred at 25 °C. When the reaction was completed (de-
termined by GC or TLC analysis), the mixture was cooled to 0 °C,
prior to the addition of a solution of TBAF in THF (5.00 mL, 5.00
13C NMR (100 MHz, CDCl3): d = 138.8, 114.2, 62.9, 33.4, 32.1,
25.8, 25.0, 18.2.
Synthesis 2005, No. 11, 1901–1905 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Rhodium-Catalyzed Synthesis of Terminal Alkenes
1905
mmol). The resulting mixture was warmed to r.t. and stirred for 12
h. The mixture was then diluted with CH2Cl2 (30 mL), washed with
10% aq NaOH (15 mL) and brine (15 mL). The organic layer was
dried over MgSO4 and the solvent was removed under reduced pres-
sure. The desired alkene 6 (585 mg, 79%) was obtained as a color-
less oil after a Kugelrohr distillation (95 °C, 0.1 mmHg).
References
(1) For a review on methylenations, see: Kelly, S. E. Alkene
Synthesis, In Comprehensive Organic Synthesis, Vol. 1;
Trost, B. M.; Fleming, I., Eds.; Pergamon Press: Oxford,
1991, 729.
(2) Wittig, G.; Geissler, G. Liebigs Ann. Chem. 1953, 580, 44.
(3) (a) Williams, J. M. J. Preparation of Alkenes: A Practical
Approach.; Oxford University Press: Oxford UK., 1996.
(b) Takeda, T. Modern Carbonyl Olefination; Wiley-VCH:
UK, 2004.
(4) (a) Lebel, H.; Paquet, V. J. Am. Chem. Soc. 2004, 126, 320.
(b) Lebel, H.; Paquet, V. Organometallics 2004, 23, 1187.
(c) Grasa, G. A.; Moore, Z.; Martin, K. L.; Stevens, E. D.;
Nolan, S. P.; Paquet, V.; Lebel, H. J. Organomet. Chem.
2002, 658, 126. (d) Lebel, H.; Paquet, V.; Proulx, C. Angew.
Chem. Int. Ed. 2001, 40, 2887.
(5) Lebel, H.; Paquet, V. J. Am. Chem. Soc. 2004, 126, 11152.
(6) Lebel, H.; Paquet, V. Org. Lett. 2002, 4, 1671.
(7) Lebel, H.; Guay, D.; Paquet, V.; Huard, K. Org. Lett. 2004,
6, 3047.
(8) Oxone is known to oxidize organophosphorus derivatives,
see: Wozniak, L. A.; Stec, W. J. Tetrahedron Lett. 1999, 40,
2637.
(9) Yoakim, C.; Guse, I.; O’Meara, J. A.; Thavonekham, B.
Synlett 2003, 473.
(10) Some bottles purchased from Aldrich contain as much as
50% of chloromethyltrimethylsilane by 1H NMR.
(11) (a) This is a modified procedure according to: Shioiri, T.;
Aoyama, T.; Mori, S. Org. Synth. 1990, 68, 1. (b) As
trimethylsilyldiazomethane is non-explosive and non-
mutagenic, the very careful operations used for the
preparation of diazomethane are not necessary.
(12) Shioiri, T.; Yamada, S. Org. Synth. 1984, 62, 187.
(13) See ref. 11 for analytical procedure.
IR (neat): 3066, 3086, 2920, 1645, 1436, 989, 890, 835 cm–1.
1H NMR (400 MHz, CDCl3): d = 6.38 (dd, J = 17, 11 Hz, 1 H,
CH=CH2), 5.78–5.76 [m, 1 H, CH=C(CH=CH2)], 5.08 (d, J = 17
Hz, 1 H, CH=CHH-trans), 4.92 (d, J = 11 Hz, 1 H, CH=CHH-cis),
4.75 (s, 1 H, MeC=CH2), 4.74 (s, 1 H, MeC=CH2), 2.36–2.07 (m, 6
H, CH2-cycle), 1.94–1.89 (m, 1 H, CH-MeC=CH2), 1.76 (s, 3 H,
CH3C=CH2).
13C NMR (100 MHz, CDCl3): d = 149.6, 139.5, 135.6, 129.0, 109.9,
108.6, 41.1, 31.1, 27.2, 24.1, 20.6.
HMRS (MAB): m/z [M]+ calcd for C11H16: 148.125201; found:
148.125212.
Acknowledgment
This research was supported by NSERC (Canada), F. C. A. R (Qué-
bec, Canada), the Canadian Foundation for Innovation, the Re-
search Corporation, Boehringer Ingelheim (Canada) Ltée, Merck
Frosst Canada and the Université de Montréal. V. P. would like to
thank Boehringer Ingelheim (Canada) Ltée for a graduate scholar-
ship. Christiane Yoakim from Boehringer Ingelheim (Canada) Ltée
is gratefully acknowledged for useful discussions and for providing
the 4-diphenylphosphanylbenzoic acid 2-trimethylsilanylethyl ester
(DPPBE).
Synthesis 2005, No. 11, 1901–1905 © Thieme Stuttgart · New York