Metal vapour derived supported rhodium nanoparticles in the synthesis of β-lactams and β-lactones derivatives

Article abstract of DOI:10.1016/j.jorganchem.2011.11.008

Functionalised β-lactones and β-lactams were prepared starting from propargyl alcohols and propargyl tosyl amides by means of efficient silylcarbocyclisation reactions catalysed by rhodium nanoparticles derived from mesitylene-solvated Rh atoms (Metal Vapour Synthesis technique, MVS) and deposited on inorganic (C, γ-Al₂O₃ Fe ₂O₃) and organic matrices (PBI). All the MVS supported nanoclusters resulted more active than the analogous commercial Rh/C and Rh/γ-Al₂O₃, as well as homogeneous Rh ₄(CO)₁₂ used as reference catalyst. In particular, metal vapour derived Rh/C afforded the β-lactones and β-lactams in high yields and chemoselectivity. Preliminary investigations on the nature of the real active metal species involved in the catalytic process showed that rhodium(0) naked nanoparticles are leached by the support. The high catalytic activity encountered with Rh/C could be ascribed to the easy leaching of metal nanoparticles from carbon into solution. Indeed, the presence of a more polar matrix determined a minor catalytic efficiency probably due to a stronger interaction between the metal and the support. Therefore MVS Rh/C species represents a source, stable with ageing at room temperature, of highly active metal nanoparticles.

Full text of DOI:10.1016/j.jorganchem.2011.11.008

Journal of Organometallic Chemistry 700 (2012) 20e28
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Metal vapour derived supported rhodium nanoparticles in the synthesis of
b-lactams and
b-lactones derivatives
,
a
a
b
Laura Antonella Aronica a , Anna Maria Caporusso , Claudio Evangelisti , Maria Botavina ,
*
Gabriele Alberto b, Gianmario Martra b
a Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy
b Dipartimento di Chimica IFM and NIS Centre of Excellence, Università di Torino, Via P. Giuria 7, 10125 Torino, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 May 2011
Accepted 8 November 2011
Functionalised b-lactones and b-lactams were prepared starting from propargyl alcohols and propargyl
tosyl amides by means of efficient silylcarbocyclisation reactions catalysed by rhodium nanoparticles
derived from mesitylene-solvated Rh atoms (Metal Vapour Synthesis technique, MVS) and deposited on
inorganic (C,
more active than the analogous commercial Rh/C and Rh/
used as reference catalyst. In particular, metal vapour derived Rh/C afforded the
in high yields and chemoselectivity. Preliminary investigations on the nature of the real active metal
species involved in the catalytic process showed that rhodium(0) naked nanoparticles are leached by the
support. The high catalytic activity encountered with Rh/C could be ascribed to the easy leaching of metal
nanoparticles from carbon into solution. Indeed, the presence of a more polar matrix determined a minor
catalytic efficiency probably due to a stronger interaction between the metal and the support. Therefore
MVS Rh/C species represents a source, stable with ageing at room temperature, of highly active metal
nanoparticles.
g
-Al2O3 Fe2O3) and organic matrices (PBI). All the MVS supported nanoclusters resulted
-Al2O3, as well as homogeneous Rh4(CO)12
-lactones and -lactams
Keywords:
g
b
b
-Lactones
-Lactams
b
b
Silylcarbocyclisation
Supported rhodium nanoparticles
Metal vapour synthesis
Catalysis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
of b-lactones, a-methylene-b-lactones (3-methyleneoxetan-2-ones)
were nearly unknown molecules until the late ’80s. This is especially
puzzling in view of the high degree of functionality contained in this
heterocyclic system, which should lend itself to diversified synthetic
applications [33e38].
Four-membered monocyclic heterocycles have received little
interest from the chemical community compared to the higher
homologous counter parts. Between
greatest attention was obviously paid to the first ones (also called
azetidine-2-ones) since the -lactam ring is the key of one of the
most widely employed class of antibacterial agents, the -lactam
antibiotics which are distinguished by good tolerance and thera-
peutic safety (e.g. TigemonamÒ) [1e5]. Among
-lactams, the
b-lactams and b-lactones, the
Considering the synthetic value of
b-lactams [39e41] and
b
b-lactones [42], it is not surprising that many methods have been
b
developed to achieve selective and efficient syntheses [43e45]. In
particular, in 1990 Matsuda and co-workers [46] reported the first
example of rhodium-catalysed silylcarbocyclisation of propargyl
b
a-
methylene-
b
-lactam unit is a commune structural feature included
alcohols that generated a-(trialkylsilyl)methylene-b-lactones in
in potent
penicillanic acids [6e17].
b
-lactamase inhibitors such as asparenomycins and
good yields, provided that DBU and a suitable hydrosilane were
used (Scheme 1, route 1). Otherwise, silylformylation by-products,
derived from the direct addition of CO and silane to the triple
bond, may be observed (Scheme 1, route 3).
On the contrary to what reported for b-lactams, b-lactones (2-
oxetanones) have only recently emerged as important synthetic
targets. Indeed, they have been studied as enzymes inhibitors
[18e29] and have been tested in the treatment of infections gener-
ated by bacteria and viruses (i.e. human cytomegalovirus, human
herpesvirus and Karposis’s sarcoma herpesvirus) [30e32]. Inthe field
A year later the same group published [47] the first application
of the silylcarbocyclisation process to the synthesis of silylated
methylene- -lactams (Scheme 1, route 2) starting from easily
available propargylamine derivatives.
a-
b
The silylcarbocyclisation reactions are generally performed in
the presence of a catalytic amount of rhodium based homogeneous
species such as Rh4(CO)12 [38,48e52]. No examples of the use of
heterogeneous catalysts are known though they would allow
* Corresponding author. Tel.: þ39 (0)502219274; fax: þ39 (0)502219260.
E-mail address: aronica@dcci.unipi.it (L.A. Aronica).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.11.008

L.A. Aronica et al. / Journal of Organometallic Chemistry 700 (2012) 20e28
21
R1
N
R1
O
1
2
R1
R2
SiR3
R2
R2
SiR3
X = O
DBU
XH
X = N-pTs
O
pTs
O
DBU
Rh4(CO)12,
CO,R3SiH,
R1, R2 = H, Me, -(CH2)4-,
-(CH2)5-, -(CH2)6-.
R3SiH= Me2PhSiH, tBuMe2SiH
R1, R2 = H, Me,
nC5H11, iBu,-(CH2)6-
R3SiH= Me2PhSiH, tBuMe2SiH
3
R2
R1
HX
H
OHC
SiR3
Scheme 1.
simpler recovery and easier purification of the product, possible
recycle of the catalyst and could assure a higher thermal stability of
the catalytic species.
deposited on the frozen walls (ꢀ196 ꢁC) of a glass reactor, gener-
ating a red-brown matrix. During the warm-up stage from ꢀ196
to ꢀ30 ꢁC the matrix melted, and nucleation and growth processes
of the metal particles took place leading to rhodium(0) nano-
clusters weakly stabilized by the mesitylene molecules, the so-
called ‘‘solvated metal atoms” (SMA) (Scheme 2, steps 1 and 2)
[74]. The obtained Rh/mesitylene solution was employed directly as
source of metal nanoparticles in the carbonylation reactions.
In the field of heterogeneous catalysis, the use of supported
nanoparticles represents a modern approach which takes advan-
tage from the high reactivity of very small metal clusters. Innova-
tive procedures have been developed to attain effective tailoring of
the size of the metal particles. Successful results have been ob-
tained employing chemical vapour deposition and thermal,
photochemical, or sonochemical decomposition of organometallic
complexes on a suitable matrix. All these methods afford the metal
nanoparticles with good reproducibility and narrow size distribu-
tions, but the precursors, i.e. the organometallic species, may not
always be easily available.
The supported catalysts Rh/C and Rh/g-Al2O3 (Table 1 entries 3
and 4) were prepared by simple addition of the Rh/SMA to the
chosen supports till decolourisation of the solution occurred, which
was indicative of the complete deposition of the rhodium particles
on the matrices (Scheme 2, step 3) [74]. The catalysts thus obtained
were ready for use without any pre-activation step.
An alternative versatile methodology is the so-called Metal
Vapour Synthesis (MVS) technique which is based on the formation
of “solvated metal atoms” (SMA), i.e. solvent stabilized metal
nanoclusters obtained by co-condensation of metal vapours with
organic ligands (mesitylene, toluene, acetone.). SMA have been
employed as catalytic precursors in a wide range of reactions such
as hydrogenation [53e57], hydroformylation [58e60], hydro-
silylation [61e66] and carbonecarbon coupling [67e70].
Recently we have reported a qualitative evaluation of the cata-
lytic properties of rhodium supported nanoparticles, obtained from
Rh/mesitylene SMA solutions, in the silylcarbocyclisation of prop-
argyl amines and alcohols [71].
The morphology of all the supported nanoparticles was inves-
tigated by means of high resolution transition electron microscopy
(HR-TEM) (Fig. 1 and Table 1). The particle size distribution and the
mean diameter of the nanoparticles of Rh/C and Rh/g-Al2O3
prepared via MVS technique were compared with those of
commercially available samples. As reported in Table 1 (entries 1
and 2) and in Fig. 1(a) and (b), the analysis of the metal particles of
the commercial Rh/C and Rh/g-Al2O3 (5% Rh w/w) systems showed
the presence of rhodium clusters with a broad size distribution
lying in the 1.5e14 nm range and average diameters of 3.5 and
7.1 nm.
The HR-TEM analysis of the corresponding MVS catalysts
(Fig.1(c) and (d), Table 1, entries 3 and 4) indicated the presence of
Prompted by the promising data obtained, we decided to deeply
investigate the factors that influence the catalytic performances of
rhodium SMA derived species in synthesis of
a-methylene-b-lac-
tams and -methylene- -lactones. Since the silylformylation is
a
b
Rh vapour
heat
a common side reaction of the silylcarbocyclisation process, for this
paper we started our investigation testing the activity of rhodium
supported nanoparticles in the silylformylation of simple and
functionalised acetylenes. As a matter of fact, we have previously
described [72] that solvated rhodium atoms, prepared by the metal
vapour synthesis technique, promoted the silylformylation reaction
of variously substituted alkynes with catalytic activities comparable
Rh
cold wall
cold
wall
high vacuum
(< 10-4 torr)
-198°C
Bulk
Rh
Mesitylene vapour
matrix of solid mesitylene
and rhodium atoms
Mesitylene vapour
with and even higher than more common species such as Rh4(CO)12
.
1. melting
Subsequently, the activity of the supported Rh-SMA was
exhaustively evaluated in the silylcarbocyclisation of propargyl
derivatives.
2. collection
support
Rh
Rh Rh
2. Results and discussion
liquid
3. deposition
solvent
supported metal
nanoparticles
2.1. Preparation of the catalysts
solvated rhodium
nanocluster
Rh
Rhodium nanoparticles were generated by means of the MVS
technique. According to previously described experimental proce-
dures [73], Rh and mesitylene were vaporised under vacuum and
Scheme 2.

22
L.A. Aronica et al. / Journal of Organometallic Chemistry 700 (2012) 20e28
Table 1
Rhodium catalysts.
No.
Catalyst
Metal
loading
%w/w
Type
dm (mean
Particles size
N
N
diameter, nm)a
distribution (nm)a
1
2
Rh/C
Rh/Al2O3
5
5
Comm.
Comm.
3.5
7.1
1.5e6.0
1.5e14.0
N
H
N
H
n
3
4
5
6
Rh/C
1
1
0.98
0.98
MVS
MVS
MVS
MVS
2.4
2.1
1.2e4.4
1.0e4.0
e
Rh/Al2O3
Rh/Fe2O3
Rh/PBI
Fig. 2. Structure of polybenzoimidazole (PBI).
ꢂ1
2.2
1.7e3.2
Rh/PBI (polybenzoimidazole, Fig. 2) [75], (0.98% w/w), (Fig. 1(e) and
(f), Table 1, entries 5 and 6).
a
Determined by HR-TEM analysis.
For Rh/PBI a statistical evaluation of the size of the Rh particles
was carried out and very narrow particle size distributions were
obtained: 1.7e3.2 nm. Rh particles were rather small and well
supported rhodium particles smaller than the commercial samples,
having a size distribution in a narrower range, 1.0e4.4 nm, and
mean diameters of 2.4 and 2.1 nm, respectively. In these cases,
moreover, two additional considerations have to be made: the
former is that HR-TEM does not have a high sensitivity in revealing
particles smaller than 1 nm in size, and the latter is that the
occurrence of aggregation phenomena of sub-nanometric metal
particles under the electron beam during HR-TEM observation
cannot be excluded. As a consequence the real mean size of MVS Rh
particles could be even smaller.
dispersed with
a more abundant population around 2.2 nm
(Table 1, entry 6 and Fig. 1(f)).
As far as Rh/Fe2O3 the size of the Rh particles could not be
determined since the size of the particles resulted near or less than
the detection limit of the TEM technique e 1 nm. However, the
metal particles seemed to be well dispersed on the support surface.
In order to investigate the effect of a highly polar inorganic or
organic matrix on the reactivity and selectivity of the metal nano-
clusters, two more supported rhodium species were prepared
starting from the Rh/SMA solution: Rh/Fe2O3 (0.98% w/w) and
2.2. Silylformylation of terminal alkynes
The catalytic activity of MVS and commercial supported
rhodium species was initially tested in the silylformylation reaction
Fig. 1. HR-TEM micrographs and histograms of particle size distributions of rhodium supported nanoparticles: (a) Rh/C (comm.); (b) Rh/g-Al2O3 (comm.); (c) Rh/C (MVS); (d) Rh/g-
Al2O3 (MVS); (e) Rh/Fe2O3 (MVS); (f) Rh/PBI (MVS).

L.A. Aronica et al. / Journal of Organometallic Chemistry 700 (2012) 20e28
23
of 1-hexyne (1a) and dimethylphenylsilane (2a) chosen as model
substrates (Scheme 3, Table 2).
reactivity. This observation was confirmed by using Rh/Fe2O3 and
Rh/PBI as catalytic precursors (Table 2, entries 5 and 6). Indeed, in
these cases an even lower conversion of the reagents (around 40%)
was observed.
The presence of a functional group such as OH (Table 2, entries 9
and 10) did not affect the reaction path: the supported Rh/C (MVS)
promoted the formation of the functionalised aldehyde 3ba with
In a typical experiment, equimolar amounts of the alkyne and
Me2PhSiH were reacted in the presence of 0.1 mol% of the rhodium
catalyst with respect to the silane, in a 25 mL stainless steel auto-
clave pressurised to 10 atm of carbon monoxide. The reaction
mixture was stirred at room temperature for the times reported in
Table 2 (entries 1e8).
good selectivity and specific activity similar to Rh4(CO)12
.
All the MVS rhodium species were able to catalyse the silylfor-
mylation of 1-hexyne with high chemo- and stereoselectivity,
2.3. Silylcarbocyclisation of propagyl alcohols: synthesis of
lactones
b-
affording
(Z)-1-(dimethylphenylsilyl)-formyl-1-hexene
3aa
exclusively.
The specific activity of the catalysts SA (calculated as [mmol(-
silane)/mgat Rh$time of reaction(h)]$conv.%) resulted however
dependent on the nature of the support.
At the beginning of this study a preliminary investigation on the
reactivity of MVS rhodium catalysts in the silylcarbocyclisation of
a monosubstituted propargyl alcohol was carried out (Scheme 4,
Table 3, entries 1e5). The reactions were performed at 100 ꢁC for
4 h, employing 3 mmol of hydrosilane, 3 mmol of alkyne,
3 ꢃ 10ꢀ3 mmol of Rh, 3 mL of CH2Cl2 and 0.3 mmol of DBU under
CO atmosphere (30 atm). Initially, t-BuMe2SiH 2b was reacted with
the alcohol since it is known that the silylcarbocyclisation reaction
is favoured by the presence of bulky substituents on the reagents.
Rh/mesitylene solution showed an excellent catalytic perfor-
While both commercial Rh/C and Rh/g-Al2O3 where almost
inert (Table 2, entries 7 and 8), the supported MVS catalysts showed
appreciable to excellent SA (Table 2, entries 3e6). In particular the
catalytic activity of Rh/C (MVS) was comparable to the Rh/mesity-
lene precursor solution and even higher than Rh4(CO)12 (Table 2,
entry 3 vs. entries 2 and 1), commonly used as homogeneous
catalyst in this reaction [72].
The particle sizes obtained from the HR-TEM analysis (Table 1,
entries 1,2 vs. 3,4) can possibly account for the different catalytic
behaviour: the smaller the nanoparticles are, the higher the specific
activity is (Table 2 entries 3,4 vs. 7,8). Moreover, the presence of the
carbon instead of alumina as inert support determines an
improvement in the catalyst efficiency, as it is evident from the data
reported in Table 2 (entry 3 vs. 4). This effect could be reasonably
mance in the synthesis of the b-lactone 4cb which was generated
with high chemoselectivity (Table 3, entry 2). When the same
reactions was carried out in the presence of Rh/C, as expected,
a lower reaction rate was observed (Table 3, entry 3); however the
supported catalyst resulted more active and selective towards the
formation of the lactone ring than Rh4(CO)12 used as reference
catalyst (Table 3, entry 3 vs. 1).
It is important to notice that the supported derived catalyst Rh/C
is stable at room temperature and can be handled at open air,
allowing an easy experimental procedure, while Rh/mesitylene
related to the polarity of g-Al2O3 which could cause an increased
stabilization of rhodium particles and a subsequent reduction of
2
R
X
R1
2
X
R
SiMe2R
CO, DBU
1
[Rh]
CO
C
H
R
R2
HO
C
C
CH
RMe2SiH
2
+
R1
C
C
1
CH
+ Me2PhSiH
2a
[ Rh ]
1
R
1
O
R2
OHC
SiMe2Ph
3
O
4
Scheme 3.
Scheme 4.
Table 3
Silylcarbocyclisation of propargyl alcohols.
Table 2
Rhodium catalysts in the silylformylation of terminal alkynes.
Entrya
1
R1
R2
2
R
[Rh]
t (h)
4
Conv.b Sel.b SAc
Entry
1
R1
R2
X
[Rh]
t (h)
3
Conv. Sel.a SAb
(%)
(%)
(%)a
(%)
1
2
3
4
5
c
c
c
c
c
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
H
H
H
H
H
b
b
b
a
a
tBu Rh4(CO)12
4
4
4
4
4
cb 80
cb 95
cb 53
cb 95
cb 100
66
87
71
48
36
50
237
132
237
62
1c
2c
3c
4c
5c
6c
7c
8c
a
a
a
a
a
a
a
a
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
n-Pr
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Rh4(CO)12
6
6
6
6
6
6
8
8
aa 98
aa 91
aa 81
aa 62
aa 43
aa 41
96
92
94
98
97
96
0
41
152
135
103
72
tBu Rh/mesitylene
tBu Rh/C (MVS)
Ph Rh/C (MVS)
Ph Rh4(CO)12
d
Rh/mesitylened,e
Rh/C (MVS)e
Rh/g
-Al2O3 (MVS)e
Rh/Fe2O3 (MVS)e
Rh/PBI (MVS)e
6
7
8
9
10
11
12
13
14
15
16
b
b
b
b
b
b
b
b
d
d
e
Me Et
Me Et
Me Et
Me Et
Me Et
Me Et
Me Et
Me Et
Me t-Bu
Me t-Bu
e(CH2)6e
a
a
a
a
a
a
a
a
a
a
a
Ph Rh4(CO)12
Ph Rh/C (MVS)
Ph Rh4(CO)12
Ph Rh/C (MVS)
Ph Rh/Fe2O3 (MVS)
Ph Rh/PBI (MVS)
4
4
ba 100
ba 100
93
90
95
92
86
89
87
81
96
97
96
e
68
e
e
Rh/
g
-Al2O3 (comm.)
Rh/C (comm.)
aa
aa 12
0
1.5 ba 94
1.5 ba 87
1.5 ba 60
1.5 ba 51
157
580
400
340
240
227
500
e
97
15
9f
b
b
Me
Me
Et OH Rh4(CO)12
24
24
ba 86
ba 30
85
85
9
12
10f
Et OH Rh/C (MVS)e
Ph Rh/g-Al2O3 (MVS) 1.5 ba 36
a
Determined by GLC and 1H NMR spectroscopic analysis of the reaction mixture
Ph Rh/C (comm.)
Ph Rh/C (MVS)
Ph Rh/C (MVS)
Ph Rh/C (MVS)
1.5 ba 34
1.5 da 75
4
6
after work up.
b
c
SA: [mmol(silane)/mgat Rh$time of reaction(h)] conv.%.
da 96
ea 100
Reaction conditions: 2 mmol of Me2PhSiH, 2 mmol of 1-hexyne, 2 ꢃ 10ꢀ3 mmol
e
of Rh, 2 mL of toluene, rt, 10 atm CO; hydrosilylation by-products were detected by
1H NMR spectroscopy.
a
Reaction conditions: 3 mmol of RMe2SiH, 3 mmol of alkyne, 3 ꢃ 10ꢀ3 mmol of
Rh, 3 mL of CH2Cl2 0.3 mmol of DBU, 100 ꢁC, 30 atm CO; silylformylation by-
products were detected by 1H NMR spectroscopy.
d
See Ref. [72].
e
Catalysts prepared according to the metal vapour synthesis technique.
f
b
Reaction conditions: 3 mmol of Me2PhSiH, 3 mmol of alkyne, 3 ꢃ 10ꢀ3 mmol of
Determined by GLC and 1H NMR spectroscopic analysis of the reaction mixture
Rh, 3 mL of CH2Cl2, 100 ꢁC, 20 atm CO; small amounts of (E)-isomers were detected
after work up.
c
by 1H NMR spectroscopy.
SA: [mmol(silane)/mgat Rh$time of reaction(h)] conv.%.

24
L.A. Aronica et al. / Journal of Organometallic Chemistry 700 (2012) 20e28
2
2
R
R
R
SiMe2Ph
Ph
SiMe2Ph
TBAF
R
Me
1
1
R
CO, DBU
R
TsHN
Me
C
C
CH
N
[ Rh ] PhMe2SiH
2a
O
THF, rt
O
O
Ts
7
6
O
O
4
5
Scheme 6.
Scheme 5.
Table 4
Silylcarbocyclisation of p-toluensulphonamides with Me2PhSiH (2a).
solution must be kept at ꢀ40 ꢁC to avoid decomposition of the SMA
and must be manipulated under argon.
Finally, even if t-BuMe2SiH seems to favour the cyclisation
process (Table 3, entries 1e3 vs. 4,5), the use of Me2PhSiH is to be
Entrya
R
6
[Rh]
t (h)
7
Conv.
(%)b
Sel.
SAc
(%)b
1
2
3
4
Et
Et
Et
Et
a
a
a
a
Rh4(CO)12
Rh/C (MVS)
Rh/Fe2O3 (MVS)
4
4
4
4
aa
aa
aa
aa
100
98
96
96
90
86
89
e
e
e
e
preferred in the synthesis of the
moiety can be easily removed by fluoride promoted rearrangements
generating -methylphenyl- -lactones (Scheme 5) [38].
b-lactones since in this case the silyl
Rh/
g-Al2O3 (MVS)
27
a
b
5
6
7
8
9
10
11
Et
Et
Et
Et
Et
Et
Et
a
a
a
a
a
a
a
Rh4(CO)12
Rh/C (MVS)
Rh/Fe2O3 (MVS)
Rh/g-Al2O3 (MVS)
Rh/PBI (MVS)
Rh/C (comm.)
1.5
1.5
1.5
1.5
1.5
1.5
1.5
aa
aa
aa
aa
aa
aa
aa
28
17
16
12
12
6
81
89
78
92
89
95
e
47
113
107
80
80
40
Unfortunately, the silylcarbocyclisation reactions performed
with dimethylphenylsilane were characterised by a loss of che-
moselectivity towards the
b-lactone ring and relevant yield of the
corresponding -silylalkenals was observed (Table 3, entries 4 and
b
5). However, MVS Rh/C catalyst confirmed its better chemo-
selectivity and higher specific activity than the Rh4(CO)12 (Table 3,
entry 4 vs. 5).
Rh/
g-Al2O3 (comm.)
0
e
12
13
14
15
16
17
Me
Me
Me
Me
t-Bu
t-Bu
b
b
b
b
c
Rh4(CO)12
Rh/C (MVS)
Rh/C (MVS)
4
4
1.5
4
4
ba
ba
ba
ba
ca
ca
94
94
32
33
65
75
94
91
99
96
100
100
e
e
The steric requirements of the silylcarbocyclisation reactions
were respected using 3-dialkylpropargyl alcohols 1b,d,e as re-
ported in entries 6e16 of Table 3. Initially, the rhodium catalysts
were tested in the silylcarbocyclisation of 3-methyl-1-pentyn-3-ol
1b that had already been successfully submitted to the silylfor-
mylation process in the presence of Me2PhSiH (Table 2, entries 9
and 10). By comparing Rh/C (MVS) with Rh4(CO)12 (Table 3, entries
6 and 7) we could observe that quantitative conversion of the
reagents was obtained after 4 h and that both catalysts yielded the
213
81
41
187
Rh/
g-Al2O3 (MVS)
Rh4(CO)12
Rh/C (MVS)
c
4
a
Reaction conditions: 3 mmol of Me2PhSiH, 3 mmol of alkyne, 3 ꢃ 10ꢀ3 mmol of
Rh, 3 mL of CH2Cl2, 0.3 mmol of DBU, 100 ꢁC, 30 atm CO; small amounts of (Z)-
b-
silylalkenals were detected by 1H NMR spectroscopy.
b
Determined by GLC and 1H NMR spectroscopic analysis of the reaction mixture
after work up.
c
SA: [mmol(silane)/mgat Rh$time of reaction(h)] conv.%.
expected b-lactones with high chemoselectivity (>90%).
Subsequently, in order to investigate the catalytic performances of
thesupported catalysts, the reactionswere stopped after 1.5 h and the
specific activity of the rhodium species was calculated. As it is evident
from the data reported in Table 3 (entries 9e12), all the supported
rhodium species prepared according to the MVS methodology dis-
played specific activities higher than Rh4(CO)12. The nature of the
support plays a fundamental role on the catalytic behaviour: while
(Table 3, entries 14e16) from their precursors 3,4,4-trimethyl-1-
pentyn-3-ol 1d and 1-ethynylcyclohexanol 1e which yielded the
desired products almost quantitatively and with excellent chemo-
selectivity (>95%).
2.4. Silylcarbocyclisation of propagyl amides: synthesis of
lactams
b-
Rh/C (MVS) was indubitably the best catalytic species, Rh/g-Al2O3
(MVS) promoted the cyclisation reaction more slowly, but resulted
a better catalyst than the commercial Rh/C in terms of conversion and
selectivity (Table 3, entries 9 and 12 vs. entry 13).
Particularly interesting resulted the catalytic efficiency showed
by the rhodium nanoparticles deposited on the polymeric matrix
PBI (Table 3, entry 11), a thermooxidatively stable material (up to
600 ꢁC) encouraging support for catalytic species [75]. Indeed, Rh/
Prompted by the good results obtained in the synthesis of
lactones, we tested our supported catalysts in the silylcarbocycli-
sation of p-toluensulphonamides in order to prepare -lactam
rings. Initially N-(1-methyl-1-ethyl-2-propynyl)-p-toluensulpho-
namide 6a was chosen as model substrate and reacted with equi-
molar amount of Me2PhSiH 2a, 10 mol% of DBU, in the presence of
0.1 mol% of rhodium catalyst, under 30 atm of CO, at 100 ꢁC for 4 h
(Scheme 6, Table 4, entries 1e11).
b-
b
PBI showed an SA more than twice the specific activity of Rh4(CO)12
.
The promising results of specific activity obtained for the MVS
nanostructured catalysts can be reasonably related to the morpho-
logical properties of the metal nanoparticles obtained by means of
this technique, i.e. very small mean diameter and narrow size
distribution (dm z 1e2 nm, Table 1, entries 3e6). Indeed, in the case
of Rh/C purchased by Engelhardt and characterised by rhodium
nanoparticles with a larger mean diameter and a wider size distri-
bution (Table 1, entry 1 vs. 3) a dramatic reduction of catalytic
activity in the silylcarbocyclisation reaction of 3-methyl-1-pentyn-3-
ol 1b was observed (less than an half than the corresponding MVS
species), together with a remarkable loss of chemoselectivity
Supported species Rh/C, Rh/Fe2O3 and Rh/
according to MVS technique, were able to promote the cyclisation
reaction with almost total chemoselectivity towards the -lactam
7aa (Table 4, entries 2e4). The reaction rate resulted dependent on
the nature of the support: after 4 h, Rh/ -Al2O3 afforded the
g-Al2O3, prepared
b
g
b-
lactam derivative with a conversion much lower than Rh/C and Rh/
Fe2O3 which showed catalytic performances comparable to
Rh4(CO)12, taken as homogeneous reference catalyst (Table 4, entry
1 vs. entries 2e4).
To get a deeper insight into the catalytic behaviour of the sup-
ported rhodium species, we compared the specific activity (SA) of
the catalysts at low conversion of the reagents (i.e. after 1.5 h).
Analogously to what observed in the silylcarbocyclisation of
towards the
The good catalytic efficiency of Rh/C (MVS) was confirmed by
the results obtained in the synthesis of -lactones 4da and 4ea
b-lactone product 4ba (Table 3, entry 9 vs. 13).
b

L.A. Aronica et al. / Journal of Organometallic Chemistry 700 (2012) 20e28
25
propargyl alcohols 1b,d,e (Table 3), it is clear from the data reported
in Table 4 (entries 7e9) that all the MVS catalysts resulted more
reactive than Rh4(CO)12 and that MVS Rh/C appeared to be the best
catalytic system (entry 6). Rhodium nanoparticles deposited on
presence of 0.1 mol% of Rh/C (MVS). After 4 h, the CO pressure was
discharged and the reaction mixture was filtered through a Teflon
filter (0.2 mm) in order to remove all suspended metal and/or
carbon particles. Then, the clear solutions obtained were added
with fresh reagents (3 mmol of acetylenes, 3 mmol of Me2PhSiH,
0.3 mmol of DBU and 3 mL of CH2Cl2) and reintroduced into auto-
claves. The reaction mixtures were reacted for further 4 h at 100 ꢁC
under 30 atm of carbon monoxide (Scheme 7, path a). Contempo-
raneously, two blank experiments of silylcarbocyclisations of 1b
and 6a were carried out under the usual experimental conditions
Fe2O3 and
g-Al2O3 and on the organic matrix PBI showed good
specific activities too (Table 4, entries 7 and 9), thus confirming the
importance of the catalyst morphology, i.e. small particles homo-
geneously dispersed on the support. Indeed, commercial Rh/C 5%
characterised by larger rhodium clusters promoted the formation of
7aa three times slower than the corresponding MVS species
(Table 4, entry 6 vs.10) and commercial Rh/
g
-Al2O3 resulted totally
affording the corresponding
b-lactone and b-lactam in high yields
inert (Table 4, entry 11).
and selectivity (Scheme 7, path b).
The good efficiencies and chemoselectivities towards the
synthesis of -lactam ring showed by the MVS derived catalysts
was confirmed by the reactions of Me2PhSiH 2a with alkynes 6b
and 6c (Table 4, entries 13e17). Again, Rh/C resulted the best choice
in terms of specific activity (entries 13 and 17) affording the desired
The obtained data are described in Fig. 3. The conversions of the
alkyne derivatives (98e100%) 1b and 6a in the blank experiments
are reported in columns 1 and 3, respectively, while columns 2 and
4 indicate the data obtained in the leaching tests.
In the leaching procedures (i.e. recharging tests), after usual
work up, both chromatographic and spectroscopic analyses indi-
cated a complete consumption of the reagents (100%) and a high
b
b
-lactams in high yields, regardless the structural features of the p-
toluensulphonamides.
selectivity towards the b-lactone 4ba and the b-lactam 7aa. Since
all rhodium particles supported on carbon should have been
removed from the crude products during the filtration step
(Scheme 7, path a, step 2), the obtained results clearly indicate the
presence of soluble catalytically active metal nanoparticles in the
filtered solutions. We can conclude that the Rh/C (MVS) system
behaves not as a heterogeneous catalyst but as a source of homo-
geneous rhodium nanoparticles that represent the real active
species. The lower catalytic performances observed for the solvated
2.5. Preliminary investigation on the nature of the catalytically
active species
To obtain more information on the characteristics of the real
species catalytically active in the silylcarbocyclisation reactions, we
performed two experiments reacting, respectively, 3-methyl-1-
pentyn-3-ol 1b and N-(1-methyl-1-ethyl-2-propinyl)-p-toluensul-
phonamide 6a under the usual experimental conditions (equimolar
amount of Me2PhSiH, 10 mol% of DBU, 100 ꢁC, 30 atm of CO) in the
rhodium atoms deposited on the metal oxides (Rh/g-Al2O3 and Rh/
Scheme 7.

26
L.A. Aronica et al. / Journal of Organometallic Chemistry 700 (2012) 20e28
electrochemically heated graphite furnace with a Perkin Elmer
4100ZL instrument. The limit of detection calculated for rhodium
was 2 ppb.
Me2PhSiH (2a), t-BuMe2SiH (2b), 1-hexyne (1a) and propargyl
alcohols 1bee were purchased by SigmaeAldrich and distilled
before use. N-propargyl-p-toluensulphonamides 5aec were
prepared as previously reported [50]. Solvents were purified by
conventional methods, distilled and stored under argon.
100
80
60
40
20
0
Commercial
110 m2/g, mean particle diameter 3.1
surface area 890 m2 gꢀ1), Fe2O3 (Aldrich product, powder, 99.5%,
1.0
m average particle size, surface area 2.1 m2 gꢀ1), were dried in
a static oven before use. Polybenzoimidazole (PBI) in beaded form
(250e500
m, surface area 20 m2 gꢀ1) was gently provided from
Prof. B. Corain (University of Padova).
Commercial Rh/
g-Al2O3 (Chimet product, type 49, surface area
mm), C (Chimet product,
m
m
1b, blank
1b,
recharged
6a, blank
6a,
recharged
g
-Al2O3 (5 wt.% of Rh, surface area 130 m2/g)
and Rh/C (5 wt.% of Rh, surface area 900 m2/g) were Engelhardt
products. Rh4(CO)12 was prepared as previously described [76].
The GLC analyses were performed on a PerkineElmer Auto
System gas chromatograph, equipped with a flame ionization
entries
Fig. 3. Results of leaching tests.
detector (FID), using a SiO2 column (DB1, 30 m ꢃ 0.52 mm, 5
mm)
Fe2O3) and on polybenzoimidazole (Rh/PBI) could be related to
a minor amount of rhodium leached from these matrices due to
their stronger bonds with the metal particles. Moreover, while very
small “naked” particles of rhodium are leached from the MVS
catalysts, the larger dimensions of commercial rhodium deposited
on carbon and alumina could account for the lower specific activ-
ities observed in these cases.
and helium as carrier gas.
4.2. Preparation of rhodium catalysts
4.2.1. Synthesis of solvated Rh atoms
In a typical experiment, rhodium vapour, generated by resistive
heating of a tungsten wire surface coated with electrodeposited
rhodium (110 mg), was co-condensed at liquid nitrogen tempera-
ture with mesitylene (45 ml) in the glass reactor chamber of the
MVS apparatus in ca. 45 min. The reactor chamber was warmed to
the melting point of the solid matrix (ca. ꢀ30/ꢀ40 ꢁC) and the
resulting red-brown solution was siphoned at low temperature in
a Schlenk tube and kept in a refrigerator at ꢀ20 ꢁC. The content of
the metal solution was 1 mg rhodium/ml (0.097 mg atom/ml).
3. Conclusions
In this paper we have demonstrated that MVS derived
rhodium nanoparticles supported on inorganic and organic
matrices are able to promote silylcarbocyclisation reactions thus
generating
b-lactones and b-lactams in high yields. Among the
prepared catalysts, MVS Rh/C resulted the best choice in terms of
activity and selectivity. Besides, all the other MVS species showed
a specific activity much better than the corresponding commer-
4.2.2. Preparation of Rh/
The Rh/mesitylene reaction solution (5 ml, 5 mg Rh) was added
to a suspension of -Al2O3 (500 mg) in mesitylene (10 ml). The
g-Al2O3 catalyst (1% w/w)
cial catalysts Rh/C and Rh/g-Al2O3 and also than Rh4(CO)12
chosen as homogeneous reference species. The good efficiencies
observed with the MVS catalysts seem to be related to their
structural features, i.e. very small rhodium particles well
dispersed on the support. The minor specific activity observed in
the cases of metal particles deposited on polar matrices such as
PBI and Fe2O3 could be due to stronger interactions between the
rhodium nanoclusters and the support. As a matter of fact,
preliminary experiments on Rh/C aimed to investigate the real
nature of the catalytic species unfortunately showed a remark-
able leaching of rhodium from carbon into solution. We can
conclude that Rh/C, and reasonably all MVS supported catalysts,
acts as a reservoir, stable with ageing at room temperature, of
very active nanoparticles that are released from the support
under the reaction conditions.
g
mixture was stirred for 24 h at room temperature. The colourless
solution was removed and the light-brown solid, containing 1 wt.%
Rh, was washed with n-pentane and dried under reduced pressure.
4.2.3. Preparation of Rh/C catalyst (1% w/w)
The Rh/mesitylene reaction solution (5 ml, 5 mg Rh) was added
to a suspension of carbon (500 mg) in mesitylene (10 ml). The
mixture was stirred for 24 h at room temperature. The colourless
solution was removed and the light-brown solid, containing 1 wt.%
Rh, was washed with n-pentane and dried under reduced pressure.
4.2.4. Preparation of Rh/Fe2O3 catalyst (0.98% w/w)
The Rh/mesitylene reaction solution (4.9 ml, 4.9 mg Rh) was
added to a suspension of Fe2O3 (503 mg) in mesitylene (10 ml). The
mixture was stirred for 5 days at room temperature. The colourless
solution was removed and the solid, containing 0.98 wt.% Rh, was
washed with n-pentane and dried under reduced pressure.
4. Experimental
4.1. General
All operations involving the MVS products were performed
under a dry argon atmosphere. The co-condensation of rhodium
and mesitylene was carried out in a static reactor previously
described [73]. The “mesitylene-solvated Rh atoms” solution was
worked up under argon atmosphere with the use of the standard
Schlenk techniques. The amount of rhodium in the solutions was
determined by atomic absorption spectrometry in an
4.2.5. Preparation of Rh/PBI catalyst (0.98% w/w)
The Rh/mesitylene reaction solution (3.9 ml, 3.9 mg Rh) was
added to a suspension of PBI (398 mg) in mesitylene (10 ml). The
mixture was stirred for 5 days at room temperature then treated
with hydrogen. The colourless solution was then removed and the
solid, containing 0.98 wt.% Rh, was washed with n-pentane and
dried under reduced pressure.

L.A. Aronica et al. / Journal of Organometallic Chemistry 700 (2012) 20e28
27
4.3. Characterization of the catalysts
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b
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University and Scientific Research (MIUR) under the PRIN 2008
program (2008SXASBC_001).

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