Palladium- and platinum-catalyzed reactions of silacyclobutanes with acid chlorides affording cyclic silyl enol ethers and/or 3-(chlorosilyl)propyl ketones

Article abstract of DOI:10.1021/om950786n

Palladium and platinum complexes catalyze the reaction of silacyclobutanes with acid chlorides in the presence of a large excess of a tertiary amine (triethylamine, diisopropylethylamine) at higher temperatures (～80 °C) to give cyclic silyl enol ethers, 1-sila-2-oxa-3-cyclohexenes, in excellent yields, while the reaction in the presence of a limited quantity of the amine at room temperature forms 3-(chlorosilyl)propyl ketones in good yields.

Full text of DOI:10.1021/om950786n

1524
Organometallics 1996, 15, 1524-1526
P a lla d iu m - a n d P la tin u m -Ca ta lyzed Rea ction s of
Sila cyclobu ta n es w ith Acid Ch lor id es Affor d in g Cyclic
Silyl En ol Eth er s a n d /or 3-(Ch lor osilyl)p r op yl Keton es
Yoshifumi Tanaka,† Hiroshi Yamashita,‡ and Masato Tanaka*,†,‡
Department of Chemistry, University of Tsukuba, and National Institute of Materials and
Chemical Research, Tsukuba, Ibaraki 305, J apan
Received October 4, 1995X
Summary: Palladium and platinum complexes catalyze
the reaction of silacyclobutanes with acid chlorides in
the presence of a large excess of a tertiary amine
(triethylamine, diisopropylethylamine) at higher tem-
peratures (∼80 °C) to give cyclic silyl enol ethers, 1-sila-
2-oxa-3-cyclohexenes, in excellent yields, while the reac-
tion in the presence of a limited quantity of the amine
at room temperature forms 3-(chlorosilyl)propyl ketones
in good yields.
29Si NMR revealed that 1,1-dimethyl-3-phenyl-1-sila-
2-oxa-3-cyclohexene (3a )6 was formed in 43 (0.5 h), 71
(1 h), 93 (2 h), and 97% (4 h) yields.7 After removal of
the solvent under reduced pressure, the residue was
mixed with hexane and the ammonium salt was filtered
off. The filtrate was concentrated in vacuo, and the
residual oil was purified by Kugelrohr distillation to give
analytically pure 3a as a colorless oil (eq 1).
Silacyclobutanes (1) are an interesting class of com-
pounds that are able to undergo a variety of reactions
based on their ring strain1 and/or Lewis acidity.2
Particularly useful reactions are those promoted by
metal complex catalysts such as ring-opening polymer-
ization3 and cycloaddition reactions with acetylenes and
allenes.4 We also reported previously that Pt(0) species
catalyzed dimerization and/or polymerization of 1,1-
dimethyl-1-silacyclobutane (1a ), depending on the struc-
ture of the catalyst. In addition, 1 was found to undergo
oxidative-addition reactions with Pt(0) complexes to
form 1-platina-2-silacyclopentane complexes.5 Contin-
ued research along these lines has revealed that the
palladium-catalyzed reaction of 1 with acid chlorides (2)
proceeds smoothly in the presence of tertiary amines
to give unique cyclic silyl enol ethers (3) in high yields.
To a stirred solution of PdCl2(PhCN)2 (0.08 mmol) in
toluene (2 mL) were added, under nitrogen, benzoyl
chloride (2a , 2 mmol), triethylamine (4 mmol), and then
1a (2 mmol), and the resulting mixture was stirred at
PdCl2(PPh3)2 also catalyzed the foregoing reaction to
give 3a , although the reaction was slower (0.5 h, 9%
yield; 2 h, 46%; 4 h, 83%).7,8
As anticipated from mechanistic considerations (vide
infra), the reaction could be promoted by a platinum
complex, Pt(CH2dCH2)(PPh3)2, but its performance was
inferior to that of the palladium complexes. Thus, the
reaction between 1a and 2a by the same procedure at
80 °C (for the first 2 h) up to 100 °C (thereafter) gave
3a and 4a in 6 and 4% (80 °C, 2 h), 29 and 4% (100 °C,
additional 2 h), 42 and 4% (100 °C, additional 2 h), and
87 and 5% (100 °C, additional 10 h) yields,7 respectively.
Selected results are summarized in Table 1. In the
presence of triethylamine and PdCl2(PhCN)2, aromatic
and heteroaromatic acid chlorides 2a -e reacted with
1a to afford 3a -e6 in high yields. Cinnamoyl chloride
(2f) smoothly reacted with 1a to give the cyclic silyl
dienol ether (3f)6 in 97% yield. Cyclohexanecarbonyl
chloride (2g) also reacted with 1a to give 3g6 in 94%
yield. The only acid chloride that did not afford 3 under
the same conditions was heptanoyl chloride (2h ), a
linear acid chloride. When treated with triethylamine
in toluene at room temperature, it was readily converted
to the corresponding ketene dimer, which was confirmed
to be unreactive toward 1a under the catalytic reaction
conditions. The ketene dimer formation is expected to
be less serious when more hindered amines are used.9
Reactions of linear acid chlorides run in the presence
of diisopropylethylamine in place of triethylamine in-
1
80 °C for 4 h. Monitoring the reaction by H, 13C, and
† University of Tsukuba.
‡ National Institute of Materials and Chemical Research.
X Abstract published in Advance ACS Abstracts, February 15, 1996.
(1) The strain energies of silacyclobutane, silacyclopentane, and
silacyclohexane are 102.5, 20.1, and 13.0 kJ mol-1, respectively. See:
Gordon, M. S.; Boatz, J . A.; Walsh, R. J . Phys. Chem. 1989, 93, 1584.
(2) (a) For “strain release Lewis acidity” of silacyclobutanes, see:
Denmark, S. E.; Griedel, B. D.; Coe, D. E.; Schnute, M. E. J . Am. Chem.
Soc. 1994, 116, 7026. (b) See also: Matsumoto, K.; Oshima, K.;
Utimoto, K. J . Org. Chem. 1994, 59, 7152.
(3) (a) Weyenberg, D. R.; Nelson, L. E. J . Org. Chem. 1965, 30, 2618.
(b) Nametkin, N. S.; Ushakov, N. V.; Vdovin, V. M. Vysokomol. Soedin.,
Ser. A 1971, 13, 29; Chem. Abstr. 1971, 74, 88325w. (c) Cundy, C. S.;
Eaborn, C.; Lappert, M. F. J . Organomet. Chem. 1972, 44, 291 and
references cited therein. (d) Poletaev, V. A.; Vdovin, V. M.; Nametkin,
N. S. Dokl. Akad. Nauk SSSR 1973, 208, 1112; Chem. Abstr. 1973,
79, 19191r. (e) Finkel’shtein, E. Sh.; Ushakov, N. V.; Pritula, N. A.;
Andreev, E. A.; Plate, N. A. Izv. Akad. Nauk SSSR, Ser. Khim. 1992,
223; Chem. Abstr. 1992, 116, 256191f. (f) Ushakov, N. V.; Yarysheva,
A. Yu.; Tal’roze, R. V.; Finkel’shtein, E. Sh.; Plate, N. A. Dokl. Akad.
Nauk 1992, 325, 964; Chem. Abstr. 1993, 118, 102788k. (g) Bialecka-
Florjanczyk, E.; Ganicz, T.; Stanczyk, W.; Sledzinska, I. Polimery
(Warsaw) 1993, 38, 424; Chem. Abstr. 1994, 121, 36351w. (h) Liao, C.
X.; Chen, M. W.; Sun, L.; Weber, W. P. J . Inorg. Organomet. Polym.
1993, 3, 231.
(6) Compounds 3a -j, 3a ′, and 3b′ were isolated by Kugelrohr
distillation. Compounds 3a -j, 3a ′, and 3b′ showed satisfactory 1H, 13C,
and 29Si NMR, IR, (HR)MS, and/or analytical data (see Supporting
Information). For example, 1H and 29Si NMR spectral data for 3a in
C6D6 are as follows. 3a : 1H NMR δ 0.15 (s, 6H, SiCH3), 0.63 (t, J )
7.1 Hz, 2H, CH2Si), 2.27 (dt, J ) 4.9 and 7.1 Hz, 2H, CH2), 5.39 (t, J
) 4.9 Hz, 1H, dCH), 7.07-7.80 (m, 5H, C6H5); 29Si NMR δ 18.2.
(7) Quantification is based on integration of 1H NMR signals
assignable to Si-CH3 protons of 1a (0.18 ppm), 3a (0.15 ppm), and/or
4a (0.22 ppm); error limits are (2%.
(4) Sakurai, H.; Imai, T. Chem. Lett. 1975, 891. Takeyama, Y.;
Nozaki, K.; Matsumoto, K.; Oshima, K.; Utimoto, K. Bull. Chem. Soc.
J pn. 1991, 64, 1461.
(5) Yamashita, H.; Tanaka, M.; Honda, K. J . Am. Chem. Soc. 1995,
117, 8873.
(8) The reaction was accompanied by the formation of unknown
compounds (4 h, ∼9% yield).
(9) We thank one of the reviewers for this suggestion.
0276-7333/96/2315-1524$12.00/0 © 1996 American Chemical Society

Communications
Organometallics, Vol. 15, No. 6, 1996 1525
Ta ble 1. Rea ction of Sila cyclobu ta n es 1 w ith Acid
react even at a higher temperature (120 °C), presumably
because of their nearly strain-free ring system.1
a
Ch lor id es 2 Ca ta lyzed by P d Cl2(P h CN)2
As described in the previous section, the catalysis
worked efficiently in the presence of a large excess of a
tertiary amine. In the absence of the amines, an
attempted catalytic reaction between 1a and 2a under
the standard catalytic reaction conditions gave only a
2.5% yield of 3a ,7 indicating that the reaction was, in
reality, not catalytic; note that the quantity of the
catalyst was 4 mol % relative to 1a .11 However, the
reaction did proceed catalytically even at room temper-
ature when it was effected in the presence of a minute
quantity of triethylamine (0.1 equiv). Under the modi-
fied conditions 1-phenyl-4-(chlorodimethylsilyl)-1-bu-
tanone (4a )12 was formed as the major product (86%
NMR yield) in 2 h along with a small amount of 3a (7%
NMR yield),7 as shown in eq 3. This modified procedure
b
acid chloride (
)
3
yield of /%
2
(3)
thus provides another useful variation of the catalytic
reaction between 1a and acid chlorides to synthesize
γ-silylalkyl ketones in good yields. Note that hydro-
silylation of â,γ- or γ,δ-unsaturated carbonyl compounds
does not appear to selectively form γ-silylalkyl ketones
but usually results in nonselective formation of a
mixture of products arising from 1,2-addition to both
CdC and CdO double bonds (and also 1,4-addition).13
When the time course of the catalytic reaction be-
tween 1a and 2a (2 equiv) under normal conditions (i.e.,
in the presence of a large excess of triethylamine) was
1
carefully followed by H NMR, compound 4a could be
detected as a transient intermediate that appeared in
the early stages (0.5 h, 50%; 1 h, 26%; 2 h, 5%; 4 h, 0%).7
The base-assisted reaction between chlorosilanes and
enolizable carbonyl compounds is the standard method
to prepare silyl enol ethers. Indeed, 4a cyclized to give
3a in a nearly quantitative yield when heated in the
presence of 2 equiv of triethylamine in toluene at 80 °C
for 1 h. Accordingly, the catalytic formation of 3a is
envisioned to occur through the cyclization of 4a .
The intermediates 4 were also observed in the reac-
tions with other acid chlorides. With regard to the
formation mechanism of 4, one can envision two alter-
native intermediates; one is a 1-pallada-2-silacyclopen-
tane species (5),4 and the other is a chloro(acyl)-
palladium species (6; Scheme 1). On the basis of the
following observations, the latter (path B) appears more
likely. Thus, PhCOPdCl(PPh3)2 (6a ; 0.1 M in benzene-
d6) was treated with 1a (1.1 equiv) in the absence of
deed worked nicely to give isomeric mixtures of the
corresponding silyl enol ethers in good total yields (eq
2).
(10) Yields were determined by integration of 1H NMR signals
assignable to CH2Si of 1b (1.49 ppm) and the vinylic CH of 3a ′ (5.43
ppm) or 3b′ (5.44 ppm); error limits are (4%.
(11) It is well recognized that palladium-catalyzed hydrogen halide
generating reactions such as the Heck reactions do not work unless
amines (or other hydrogen halide acceptors) are present in the reaction
system. This is because the catalyst is regenerated by the reaction of
amines that re-form palladium(0) species. For instance, see: (a)
Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
Wiley-Interscience: New York, 1994; p 396. (b) Heck, R. F. Palladium
Reagents in Organic Synthesis; Academic Press: London, UK, 1985; p
276.
1,1-Diphenyl-1-silacyclobutane (1b) also reacted with
2a and 2b under identical conditions to give the cyclic
silyl enol ethers 3a ′ 6 (0.5 h, 54%; 2 h, 98%) and 3b′ 6 (4
h, 95%),10 respectively, in spite of its increased steric
hindrance. On the other hand, 1,1-dimethyl-1-silacyclo-
pentane and 1,1-dimethyl-1-silacyclohexane did not
(12) 4a : 1H NMR (C6D6) δ 0.22 (s, 6H, SiCH3), 0.60-0.69 (m, 2H,
CH2Si), 1.67-1.82 (m, 2H, CH2), 2.58 (t, J ) 7.0 Hz, 2H, COCH2), 7.10-
7.88 (m, 5H, C6H5); 29Si NMR (C6D6) δ 31.6.
(13) Marciniec, B., Ed. Comprehensive Handbook on Hydrosilylation;
Pergamon: Headington Hill Hall, UK, 1992; pp 145-150.

1526 Organometallics, Vol. 15, No. 6, 1996
Communications
Sch em e 1
(5)
metal complexes than are acid chlorides provide strong
evidence against path A. Thus, when a mixture of 1a
or 1b (0.125 mmol), 2a (0.125 mmol), and benzene-d6
(0.25 mL) was added to Pd(PPh3)4 (0.125 mmol) in a
sealed NMR tube, PhCOPdCl(PPh3)2 was nearly quan-
titatively formed as the sole product in 15 min at
ambient temperature, while 1a or 1b remained intact.16
Pt(PEt3)3 (0.125 mmol) used in place of Pd(PPh3)4
behaved very similarly to give only PhCOPtCl(PEt3)2.17
In strong contrast with these stoichiometric reactions
of Pd(0) and Pt(0) complexes, compound 4a could be
catalytically formed even at room temperature (eq 3).
Finally, it is interesting to note that, in any of the
catalytic reactions with acid chlorides, we did not find
1,1,5,5-tetramethyl-1,5-disilacyclooctane, a dimer of 1a ,
formed as a byproduct. Since Pt(CH2dCH2)(PPh3)2 is
quite active in the dimerization of 1a ,5 the dimer
formation could have resulted at least in the Pt-
(CH2dCH2)(PPh3)2-catalyzed reaction if the catalysis
were initiated by the generation of a five-membered
intermediate such as 5′. Accordingly, the possibility of
path A playing an important role in the catalysis
appears less likely, even though it is not rigorously
excluded.
Sch em e 2
the amine at 60-120 °C in a sealed NMR tube. NMR
of the reaction mixture indeed revealed formation of 3a ,
via the intermediate 4a : 60 °C, 2 h, 3a 19%, 4a 50%;
80 °C, additional 2 h, 3a 38%, 4a 50%; 120 °C, additional
1 h, 3a 87%, 4a 7% (eq 4),7 suggesting catalysis via path
B.
In conclusion, the reaction of silacyclobutanes with
acid chlorides offers a novel and high-yield synthesis of
a new class of cyclic silyl enol ethers (1-sila-2-oxa-3-
cyclohexenes) or γ-(chlorosilyl)propyl ketones.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details for the synthesis of 3a and 4a , synthetic
procedures to prepare authentic samples of 7 and 8, and 1H,
13C, and 29Si and/or 31P NMR, IR, MS, and analytical data for
3a -j, 3a ′, 3b′, 4a , 7, and 8 (7 pages). Ordering information
is given on any current masthead page.
On the other hand, to look into the possibility of the
alternative mechanism (path A), we attempted to syn-
thesize the 1-pallada-2-silacyclopentane species (5). We
have been unsuccessful at obtaining it to date (vide
infra). Instead, we treated a toluene-d8 solution of the
1,1-bis(triethylphosphine)-2,2-diphenyl-1-platina-2-sila-
cyclopentane complex5 (5′; 0.1 M) with 2b (2 equiv) in
the absence of the amine at 60 °C for 1 h in a sealed
NMR tube; note that a platinum complex was also
catalytically active (vide supra). 1H, 13C, 29Si, and 31P
NMR of the reaction mixture showed formation of the
corresponding cyclic silyl enol ether (3b′) in 50% yield
along with several platinum complexes (eq 5).14 The
result is reasonably explained by the sequence of events
depicted in Scheme 2 and is not totally at odds with path
A. However, the following observations that indicate
1a is much less reactive toward low-valent transition
OM950786N
(14) A mixture of 5′8 (0.03 mmol), 2b (0.06 mmol), and toluene-d8
(0.3 mL) in a nitrogen-purged, sealed NMR tube was heated at 60 °C
for 1 h. The resulting mixture did not exhibit 31P NMR signals centered
at 7.5 and 16.7 ppm due to 5′, indicative of completion of the reaction;
instead, new signals emerged at 13.5, 16.4, and 23.2 ppm. Preparation
of authentic samples confirmed that the first two signals were due to
7 and 8, and the last one was safely assigned to 9 by comparison with
literature data (including of 1H NMR).15 1H NMR showed that 7 (47%,
integration of CH2Si signal at 1.07 ppm), 8 (46%, integration of p-CH3
signal at 2.04 ppm), and 9 (5%, integration of HsPt sigral at -16.91
ppm) were formed in addition to 3b′ (50%, integration of the vinylic H
signal at 5.44 ppm).
(15) (a) Socrates, G. J . Inorg. Nucl. Chem. 1969, 31, 1667. (b)
Parshall, G. W. Inorg. Synth. 1970, 12, 26.
(16) No reaction took place, even when
a mixture of 1b and
Pd(PPh3)4 in benzene-d6 was heated at 140 °C.
(17) Note that the reactions of 1a and 1b with Pt(PEt3)3 in the
absence of acid chlorides form the five-membered complexes only when
the temperature is raised to 90 °C.5