Triphenylphosphine-catalyzed dehydrogenative coupling reaction of carboxylic acids with silanes - A convenient method for the preparation of silyl esters

Article abstract of DOI:10.1002/adsc.200600338

Triphenylphosphine has been found to be an efficient catalyst for the dehydrosilylation of carboxylic acids with silanes. In the presence of 4 mol % of triphenylphosphine (PPh₃), dehydrosilylation reactions in DMF afforded the corresponding silyl esters at 120 °C in good yield.

Full text of DOI:10.1002/adsc.200600338

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DOI: 10.1002/adsc.200600338
Triphenylphosphine-Catalyzed Dehydrogenative Coupling
Reaction of Carboxylic Acids with Silanes – A Convenient
Method for the Preparation of Silyl Esters
Guo-Bin Liu,a,* Hong-Yun Zhao,a and Thies Thiemannb
a
Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, People)s Republic of China
Phone: (+86)-21-513-70613; fax: (+86)-21-513-201-101; e-mail: liuguobin@fudan.edu.cn
Institute of Materials Chemistry and Engineering, Kyushu University, 6-1, Kasuga-koh-en, Kasuga, Fukuoka 816-8580,
b
Japan
Received: July 7, 2006
Abstract: Triphenylphosphine has been found to be
an efficient catalyst for the dehydrosilylation of car-
boxylic acids with silanes. In the presence of 4
mol% of triphenylphosphine (PPh3), dehydrosilyla-
tion reactions in DMF afforded the corresponding
silyl esters at 1208C in good yield.
esters from the corresponding silanes necessitates two
reaction steps. Although some newer synthetic meth-
ods to silyl esters have been published and while a lot
of literature concerns itself with the transition metal-
catalyzed cross-coupling of OH-containing com-
pounds such as water and alcohols with silanes,[5]
there are still but few examples of the dehydrogena-
tive coupling reaction of carboxylic acids with silanes.
Silylating agents such as hexamethyldisilazane, N-tri-
methylsilylalkanamines, N-trimethylsilylacetamide, N-
trimethylsilyl-2-oxazolidinone, aminosilanes, or trial-
kylsilyl methallylsulfinates have been extensively uti-
lized for the the transformation of carboxylic acids
Keywords: carboxylic acids; organocatalysts; phos-
phines; silanes; silylation
Silyl esters are very important intermediates for the into the desired silyl esters.[6a–f] However, some short-
preparation of functional polymeric substrates such as comings have been noted in these reported methods.
easily degradable poly(silyl ester)s, widely employed The silylations of carboxylic acids with hexamethyldi-
as recyclable materials, gene delivery carriers, matri- silazane usually require prolonged reaction times
ces for drug delivery and biodegradable surgical devi- under heating and continuous removal of the ammo-
ces.[1a–j] Simple, economical and practical procedures nia or amine formed thereby and the silylating agents
for the conversion of carboxylic acids into silyl esters, are generally expensive. A few examples have been
are not only needed in normal organic synthesis, but disclosed for dehydrosilylation reactions catalyzed by
are also prerequisities for the accurate performance metal salts such as zinc chloride or, more frequently,
of gas-chromatographic analyses in organic and bio- by transition metals and metal complexes such as
logical chemistry.[1k,l] From the viewpoint of synthetic [(Ph3P)CuH], HPtCl6, Rh, and Pd.[6g–n] Generally, cat-
chemistry, silylation of carboxylic acids is one of the alysts such as transition metals are expensive.
most commonly used protocols for their protection [(Ph3P)CuH] requires a multiple-step synthetic ap-
because deprotection of silyl esters is easily realized proach and in situ generation protocols. Moreover,
under the mild reaction conditions.[2] Much attention the transition metal-catalyzed dehydrocoupling of un-
has been devoted to the development of a simple, saturated carboxylic acids and silanes always results
practical and atom-economical method for the prepa- in the formation of by-products due to the known
ration of stable and easily isolable silyl esters.
ability of Rh, Pd or Pd/C to act as hydrogenation cat-
Usually, silyl esters are prepared by cross-coupling alysts in the reduction of carbon-carbon double bonds
reactions of carboxylic acids and chlorosilanes.[3] Hy- in the desired olefinic silyl esters, leading to a trouble-
drogen chloride is unavoidably formed in these proce- some purification.
dures, and a stoichiometric or even an excess amount
In this communication, we report that triphenyl-
of bases such as amines or ammonia is needed to con- phosphine (Ph3P) effectively catalyzes the dehydro-
sume the HCl gas formed thereby. Since chlorosilanes genative coupling of carboxylic acids with silanes,
themselves are prepared by the chlorination of si- yielding the corresponding silyl esters selectively with-
lanes, either with chlorine gas[1g] or with hydrochloric out formation of any reduced by-products in the case
acid under Pd/C catalysis,[4] the synthesis of silyl
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Guo-Bin Liu et al.
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Table 2. Ph3P-catalyzed dehydrocoupling of propionic acid
with triethylsilane in different solvents.[a]
Scheme 1.
of unsaturated silyl esters. The results are summarized
in Scheme 1 and Tables 1–4.
Dehydrogenative coupling reactions were carried
out by heating a mixture of carboxylic acid, silane
and a catalytic amount of phosphine in solvents under
a nitrogen atmosphere for several hours (Scheme 1,
Tables 1–3, the dehydrocoupling reaction was moni-
tored by GC). The transformation of propionic acid
with triethylsilane was used as a model to optimize
the reaction conditions.
The dehydrogenative coupling was found to be fin-
ished after 15 h at 1208C, in the presence of 4 mol%
PPh3, affording the corresponding triethylsilyl propio-
nate in 87% yield (Table 1, run 3). When the amount
of catalyst was increased to 8, 16, or even 32%, the
product yields were 85–88% (Table 1, runs 4–6). The
reaction proceeded more slowly, however, when the
amount of PPh3 was decreased (to 1 or 2 mol%),
when 48% and 30% of triethylsilane remained un-
reacted (GC ratio), even after 48 h at 1208C (Table 1,
runs 1 and 2). The reaction progressed more slowly
when carried out at 50, 80 or 1008C. Thus, a signifi-
cant amount of Et3SiH was found to be unreacted
(88% at 508C, 72% at 808C and 51% at 1008C) after
being heated for 48 h (Table 1, runs 8–10). No silyl
[a]
Propionic acid (20 mmol), triethylsilane (20 mmol).
GC ratio.
Isolated yield in parenthesis.
[b]
[c]
Table 3. Catalyst-screening for the dehydrocoupling of pro-
pionic acid with triethylsilane in DMF at 1208C.[a]
Table 1. Ph3P-catalyzed dehydrocoupling of propionic acid
with triethylsilane in DMF.[a]
[a]
[a]
Propionic acid (20 mmol), triethylsilane (20 mmol).
GC ratio.
Isolated yield in parenthesis.
Propionic acid (20 mmol), triethylsilane (20 mmol).
GC ratio.
Isolated yield in parenthesis.
[b]
[b]
[c]
[c]
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Dehydrogenative Coupling Reaction of Carboxylic Acids with Silanes
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Table 4. Ph3P-catalyzed dehydrocoupling of carboxylic acids with silanes.[a]
[a]
Propionic acid (20 mmol), triethylsilane (20 mmol).
GC ratio.
Isolated yield in parenthesis.
[b]
[c]
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809

Guo-Bin Liu et al.
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ester was formed at 308C even for 48 h and triethylsi- of carboxylic acids with silanes. The dehydrogenative
lane was recovered quantatively (Table 1, run 7). coupling reactions proceed efficiently to give the cor-
The dehydrocoupling in different solvents was in- responding silyl esters in good yields. No over-re-
vestigated (Table 2). In aprotic dipolar solvents such duced silyl esters are formed in the case of coupling
as N,N-dimethylacetamide (DMAc), 1,3-dimethyl-2- unsaturated carboxylic acids, bromo-, chloro- or nitro-
imidazolidinone
(DMI),
N-methylpyrrolidinone benzoic acids, and unsaturated acids with silanes. We
(NMP), and hexamethylphosphoramide (HMPA), the believe that the PPh3-catalyzed dehydrogenative cou-
dehydrogenative coupling is slightly slower compared pling reaction of carboxylic acids with silanes provides
with that in DMF and small amounts of Et3SiH were another important protocol for a one-step, metal-free,
detected (8–12%) (Table 2, runs 2–5). In xylene, tert- highly selective, atom-economical and efficient syn-
butylbenzene, mesitylene, chlorobenzene, 1,2-dichloro- thetic method. Also, the PPh3-catalyzed dehydrocou-
benzene, n-octane, anisole, di(ethylene glycol) di- pling reaction of carboxylic acids with silanes offers a
methyl ether, and benzonitrile, the dehydrocoupling useful method for protecting carboxylic acid because
could not be finished and even when heated at 1208C the silyl ester is easily hydrolyzed to give the corre-
for 72 h significant amounts of Et3SiH were detected sponding carboxylic acid. Further work on the dehy-
(62–90%) (Table 2, runs 6–11).
drosilylation of carboxylic acids and silanes is current-
Phosphines other than Ph3P were also tested as cat- ly in progress in our laboratory and the results will be
alysts for the dehydrogenative coupling and the re- published in due course.
sults are summarized in Table 3 (all at 4 mol%, DMF,
1208C). Only a trace amount of silyl ester was formed
when triphenylphosphine oxide (Ph3PO) was used as Experimental Section
a catalyst (Table 3, run 2). In the case of using tri-n-
butylphosphine and tri-tert-butylphosphine as cata- Typical Procedure
lysts, 14% and 16% of Et3SiH were still detected
even being heated for 24 h at 1208C (Table 3, runs 3
To a mixture of propionic acid (40 mmol, 2.96 g), and tri-
ethylsilane (40 mmol, 4.64 g) in DMF (20 mL) was added
and 4). Under the catalysis of 1,2-bis(diphenylphos-
triphenylphosphine (1.6 mmol, 0.42 g, 0.04 equivs.) at room
phino)ethane,
1,3-bis(diphenylphosphino)propane,
temperature under a nitrogen atmosphere. The reaction
mixture was stirred at 1208C for 15 h (monitored by GC).
The desired triethylsilyl propionate was obtained as a col-
ourless oil (yield: 87%) after distillation under reduced
pressure (run 3).
1,4-bis(diphenylphosphino)butane, tricyclohexylphos-
phine, tris(2-methylphenyl)phosphine, and rac-2,2’-
bis(diphenylphosphino)-1,1’-naphthyl
(rac-BINAP),
the dehydrocoupling was sluggish (Table 3, runs 5–10)
and some amounts of Et3SiH were detected even
after being heated for 24 h or 48 h at 1208C.
Triethylsilyl propionate:[6f] IR (neat): n=684, 740, 824,
1
998, 1062, 1241, 1410, 1464, 1714, 2870, 2954 cmÀ1. H NMR
3
(400 MHz, CDCl3): d=0.72 (6H, q, J=7.8 Hz), 0.95 (9H, t,
A number of carboxylic acids were reacted with
triethylsilane, tri-n-propylsilane [(n-Pr)3SiH], tri-n-bu-
tylsilane [(n-Bu)3SiH] or tert-butyldimethylsilyl (t-Bu-
Me2SiH) and all afforded the corresponding silyl
esters in good yields (all with 4 mol% PPh3, DMF,
1208C) (Table 4). In the case of the reaction of
bromo- and chlorobenzoic acids, the desired silyl
esters were obtained in 77–85% yield, free of dehalo-
genated by-products (Table 4, runs 8–12). Also, the
dehydrocoupling of 4-nitrobenzoic acid with tert-bu-
tyldimethylsilane afforded the corresponding silyl
ester in good yield without any over-reduced by-prod-
uct (Table 4, run 13). In the case of 2-phenylacetic
acid and 3-furylpropionic acid, the corresponding silyl
esters were obtained in good yields (Table 4, runs 14
and 15). Under similar reaction conditions, the de-
sired unsaturated silyl esters were produced by dehy-
drogenative coupling reaction of silanes with unsatu-
rated acids such as cinnamic acid, furylacrylic acid,
1,3-hexadienoic acid, and methacrylic acid. No forma-
tion of over-reduced by-products was observed
(Table 4, runs 16–23).
3J=7.8 Hz), 1.14 (3H, t, 3J=7.6 Hz), 2.36 (2H, q, 3J=
7.6 Hz); 13C NMR (100 MHz, CDCl3): d=4.42, 6.46, 9.34,
28.46, 175.20.
All of the silyl esters are known compounds and were
compared with authentic samples [prepared by cross-cou-
pling of carboxylic acids and chlorosilanes in the presence of
a base such as triethylamine or imidazole (tert-butylsilyl
esters) in dichloromethane] and were identified on the basis
1
of their IR, H NMR, 13C NMR and GC-mass spectral data.
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