DyI2 initiated mild and highly selective silyl radical-catalyzed cyclotrimerization of terminal alkynes and polymerization of MMA

Article abstract of DOI:10.1039/b602883g

An efficient method for the formation of SiCl₃ radicals by the reaction of abundant and cheaper chlorosilanes with DyI₂ has been established, not only demonstrating new distinctive reactivities of solvated DyI₂ but also suggesting that the presence of lanthanide ions can improve the selectivity of some silyl radical-catalyzed reactions. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b602883g

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DyI2 initiated mild and highly selective silyl radical-catalyzed
cyclotrimerization of terminal alkynes and polymerization of MMA{
Zhenyu Zhu,a Chuanfeng Wang,a Xu Xiang,a Chengfu Pia and Xigeng Zhou*ab
Received (in Cambridge, UK) 24th February 2006, Accepted 20th March 2006
First published as an Advance Article on the web 4th April 2006
DOI: 10.1039/b602883g
(MMA) polymerization. The results not only highlight the
unique reactivity of DyI2, but also provide a new method for
the production of silyl radicals from cheaper, unactivated
chlorosilanes.
An efficient method for the formation of SiCl3 radicals by the
reaction of abundant and cheaper chlorosilanes with DyI2 has
been established, not only demonstrating new distinctive
reactivities of solvated DyI2 but also suggesting that the
presence of lanthanide ions can improve the selectivity of some
silyl radical-catalyzed reactions.
Initially, we examined the reaction of DyI2 with an excess of
terminal alkynes in THF to afford 2, along with a small amount of
3, in modest but significant yields between 10 and 24%
(Scheme 1).{ However, 1-phenyl-1-propyne did not form the
benzene derivative under the same conditions. The results indicate
that the dysprosium acetylides might be the active species in the
catalytic cyclotrimerization (Scheme 2).8 Consistent with this
hypothesis, treatment of a THF suspension of DyCl3 with
PhCMCK, followed by reacting with excess PhC;CH, also gave
a small amount of 2a.
Radical reactions have become an important tool in synthetic
chemistry.1 Surprisingly, the C–C bond-forming reactions pro-
moted by silyl radicals have been poorly developed to date, even
though this should be a fertile area, since they have proved to have
a somewhat different reactivity compared to other radicals.2,3 The
reasons are probably the lack of convenient and clean sources of
the free silyl radicals, and the difficulty in controlling the selectivity.
Up to now, silyl radicals are most frequently produced by treating
the corresponding hydrosilanes with other radicals. On the other
hand, the presence of hydrosilanes should make controlling the
C–C bond formation difficult in the free radical reaction due to the
competing H-transfer from hydrosilanes to alkyl radicals.2,4 In
addition, organic initiators may participate in reactions.2c
Therefore, to avoid such obstacles, it is highly desirable to develop
efficient methods of forming silyl radicals from non-hydrosilane
Metal-catalyzed alkyne cyclotrimerization represents a challeng-
ing but attractive strategy for the preparation of substituted
benzenes.9 Despite significant recent advances in this area, no
organolanthanide has been found to catalyze this kind of reaction.
Only linear dimers and/or oligomers were obtained in previously
reported homogeneous lanthanide-mediated reactions of terminal
alkynes.10 The unusual catalytic activity of DyI2 in the present
reaction might be attributed to its high reductive activity and the
fact that it is less sterically demanding compared to other
organolanthanide complexes.6,10
?
precursors. Recent pioneering work shows that the SiCl3 radical,
produced by the thermal cleavage of the Si–Si bond of Si2Cl6, can
catalyze the cyclotrimerization of alkynes.3a However, there are
some problems associated with this methodology. For example,
the regioselectivity is low and the Si2Cl6 precatalyst is expensive.
On the other hand, since the discovery of a new soluble divalent
4f element system by Bochkarev et al. in 1997,5 the chemistry of
molecular complexes of divalent thulium, dysprosium and
neodymium have received considerable attention.6 Despite great
success in their stoichiometric reactions, so far there is no
information about their catalytic reactions. Given that SmI2 can
be used as an initiator in radical reactions of halohydrocarbons,7
we were interested in finding out whether solvated LnI2 (Ln = Nd,
Dy, Tm) are feasible as initiators for radical-catalyzed transforma-
tions that are highly valuable but incontrollable with other
initiating systems. We report now on the first use of catalytic
amounts of DyI2 in organic synthesis and methyl methacrylate
Furthermore, we found that DyI2 is an efficient initiator for
the generation of silyl radicals from chlorosilanes, which can
dramatically improve the regioselectivity of radical-catalyzed
cyclotrimerization of terminal alkynes.3a,b After testing a variety
of silanes, SiCl4 was found to give the best results (Table 1).
However, SmI2, DyCl3 and Na in presence of excess phenyl
acetylene, with or without the addition of SiCl4, did not afford any
cyclotrimer under the same conditions. Additionally, we proved
that SiCl4 by itself is ineffective as a catalyst in the reaction. All the
results indicate that DyI2 is essential, and its strong reductive
power may play a key role in initiating the silyl radical-catalyzed
reaction. Furthermore, we found the reaction achieved the highest
aDepartment of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Fudan University, Shanghai 200433,
People’s Republic of China. E-mail: xgzhou@fudan.edu.cn;
Fax: (+86) 21 65641740; Tel: (+86) 21 65643769
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute
of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032,
People’s Republic of China
{ Electronic Supplementary Information (ESI) available: Full procedures
and characterization data. See DOI: 10.1039/b602883g
Scheme 1
2066 | Chem. Commun., 2006, 2066–2068
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whereas electron-withdrawing groups, like bromide and fluoride in
the para-position, seem to negatively affect the yield of the
reaction. However, the percentage distribution of isomers remains
essentially constant. Noticeable for p-MeOC6H4CMCH, the
apparent intramolecular trapping of the resulting alkenyl radical
by a methoxy group led to a remarkable decrease in the yield. In
addition, the steric effect of the substituent in the ortho-position
also lowers the reaction yield and selectivity.
The reaction pathway shown in Scheme 3 is proposed to
account for the DyI2/SiCl4 co-catalyzed cyclotrimerization of
?
alkynes. Namely, SiCl3 radicals, produced by the single electron
reduction of SiCl4 by DyI2, mediate the alkyne cyclotrimerization
via a tandem radical addition/cyclization/elimination pathway. The
radical mechanism was substantiated by the lack of detectable
products when 1,4-benzoquinone or hydroquinone were added to
the reaction mixture as a radical trap. Additional evidence for the
?
presence of SiCl3 radicals was obtained from the formation of
Scheme 2 Proposed pathway for the formation of 2.
small amounts of authentic disilane derivatives such as hexachlor-
odisilane. Apparently, the function of the metal species in the
present reaction can be distinguished from that in Me3SiCl–Pd/C-
cocatalyzed reactions of alkynes, where the resulting Pd complex
might be the actual catalyst.11 Furthermore, it is evident that Pd/C
and PdCl2 act as radical traps at 140 uC.3a In addition, the present
chemoselectivity is in sharp contrast to that in the Me3SiCl–Pd/C
catalytic system, where internal alkynes gave cyclotrimers in
excellent yield but phenylacetylene did not afford any cyclotrimer
product.11 This different behavior could be of great importance in
a synthetic strategy, as the two reagents could complement each
other.
Table 1 Comparison of the cyclotrimerization of phenylacetylene in
different catalytic systemsa
Entry Catalyst
T/uC
Yield (%)b
2/3
1
2
3
4
5
6
7
8
9
DyI2/Me3SiCl
DyI2/Me2 BuSiCl
70
70
70
70
35
70
70
70
70
70
200
70
36
20
95
90
90
—
—
—
—
—
100
—
90/10
91/9
97/3
94/6
91/9
—
—
—
—
—
t
DyI2/SiCl4
DyI2/SiCl4
DyI2/SiCl4
DyI2/CCl4
DyCl3/SiCl4
SmI2/SiCl4
Na/SiCl4
The present results not only provide a convenient method for
the formation of silyl radicals from cheaper chlorosilanes such as
SiCl4 and Me3SiCl, which are ineffective as precursors in terms of
10
11
12
SiCl4
Si2Cl6
Cl3SiH/(PhCO2)2
c
50/50
—
d
a
All the reactions were carried out in THF, except entries 3 and 12,
which were carried out in DME and cyclohexane, respectively.
b
c
Yields of isolated products. Reported in ref. 3a and ref. 3b.
Reported in ref. 4a.
d
yield with excellent regioselectivity in DME, but led to complex
mixtures in toluene.
A series of aryl-substituted acetylenes were subjected to the
above optimal reaction conditions (Table 2). All reactions gave 2
as major products. The reactivity of alkynes is controlled by the
steric and electronic properties of the substituents. Electron-neutral
aromatic acetylenes, such as phenylacetylene and para-tolylacety-
lene, trimerize most readily to the corresponding 1,2,4-trisubsti-
tuted benzenes in high yield and with excellent regioselectivity,
Table 2 DyI2/SiCl4 catalyzed cyclotrimerization of alkynesa
Entry Ar
[Sub]/[Cat]
Yield (%)b
2/3
1
2
3
4
5
6
7
8
C6H5
C6H5
p-MeC6H5
p-MeC6H5
p-BrC6H5
p-FC6H5
3 : 1
10 : 1
3 : 1
10 : 1
3 : 1
3 : 1
3 : 1
3 : 1
95
73
87
70
60
67
50
25
97/3
91/9
97/3
91/9
95/5
97/3
87/13
.99/1
o-MeC6H5
p-MeOC6H5
a
All reactions were carried out in DME at 70 uC or refluxing
temperature for 3 d, except entry 8, which was carried out in THF.
Yields of isolated products.
Scheme 3 Proposed mechanism for the silyl radical-catalyzed cyclotri-
b
merization of alkynes.
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bicarbonate. The mixture was extracted with ether, the organic layer
the thermolytic or photochemical production of silyl radicals due
to their exceedingly high first dissociation energies (e.g. for SiCl4
ca. 111 kcal mol21), but also achieve success in both controlling
the regioselectivity and reducing reaction conditions.
separated and dried over anhydrous MgSO4, concentrated under reduced
pressure, and purified by flash column chromatography to afford desired
substituted benzenes.
3. Polymerization.
To a dark green solution of DyI2 (0.210 g, 0.5 mmol) in THF was added
SiCl4 (0.06 ml, 0.5 mmol). Then the MMA monomer (5 ml) was injected
into the suspension. The reaction mixture was allowed to stir at 0 uC for
10 h and was filtered, the filtrate quenched, and washed three times with
methanol.
Moreover, we found DyI2/SiCl4 could effectively initiate the
polymerization of MMA. However, in related reactions initiated
by Cl3SiH/organic peroxide systems, only telomers are obtained
due to H-abstraction from Cl3SiH competing with the formation
of the adduct radicals.2c Addition of 1,4-benzoquinone inhibited
the polymerization completely, indicating that the reaction took
place in a radical fashion. Both the average molecular weight
(Mn . 178 000) and the syndiotactic content (rr triad, 71%) are
higher than those usually reported for poly(methyl methacrylate)
(PMMA) initiated by other conventional free radicals, while the
molecular weight (Mw) distribution (Mw/Mn = 1.74) is smaller.3c,12
These values probably provide unique information about the
influence of the resulting Dy3+ ion on the silyl radical-catalyzed
reaction. The fact which supports the assumption is that the
presence of DyCl3 could narrow significantly the molecular weight
distribution of PMMA promoted by AIBN.
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In summary, we have developed an efficient method for the
production of silyl radicals under mild conditions from cheaper
unactivated chlorosilanes via the reduction of DyI2. Significant
versatility and excellent control of the selectivity presented here
illustrates that this should be an attractive strategy for the
development of the silyl radical-catalyzed C–C bond-forming
reactions due to the avoidance of hydrosilylation and the probable
cooperative action of trivalent lanthanide ions. On the other hand,
the results further highlight the distinctive reactivity of DyI2,
beyond that possible with the traditional divalent lanthanide halide
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This work is supported by the NNSF of China, the NSF of
Shanghai, the Research Funds for the New Century Distinguished
Scientist and the Doctoral Program of the National Education
Ministry of China.
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Notes and references
{ All manipulations involving air- and moisture-sensitive compounds were
carried out under purified argon. Full procedures and characterization data
are given in the ESI.{
General Procedures
1. DyI2-catalyzed cyclotrimerization of terminal alkynes.
In general, the reactions were performed in THF at ambient or refluxing
temperature with a molar ratio of substrate/DyI2 = 3 : 1 or 10 : 1. The yield
of the products and their isomeric distribution were determined by GC–
MS, chromatography on silica gel columns or by fractional crystallization.
2. DyI2/SiCl4-catalyzed cyclotrimerization of terminal alkynes.
To a dark green solution of DyI2 (0.5 mmol) in DME was added SiCl4
(0.5 mmol). The solution changed immediately into a pale grey suspension,
to which was added alkyne (1.5 mmol). After stirring at 70 uC or refluxing
for 3 d, the reaction mixture was quenched with saturated sodium
11 A. K. Jhingan and W. F. Maier, J. Org. Chem., 1987, 52, 1161.
12 Q. Shao, H. M. Sun, X. G. Pang and Q. Shen, Eur. Polym. J., 2004, 40,
97.
2068 | Chem. Commun., 2006, 2066–2068
This journal is ß The Royal Society of Chemistry 2006