Titanium-catalyzed [4+2] and [6+2] cycloadditions of 1,4-bis(trimethylsilyl)buta-1,3-diyne

Article abstract of DOI:10.1135/cccc19961722

The (C₂H₅)₂AlCl/TiCl₄ catalyst induces the [4+2] cycloaddition of butadiene or the [6+2] cycloaddition of 1,3,5-cycloheptatriene (CHT) to individual acetylenic moieties of 1,4-bis(trimethylsilyl)buta-1,3-diyne (BSD). Heating of the 2:1 butadiene adduct, bis(2-trimethylsilylcyclohexa-1,4-dien-1-yl), to 250°C yields 2,2′-bis(trimethylsilyl)biphenyl. The 1:1 adduct of BSD with CHT, 7-trimethylsilyl-8-trimethylsilylethynylbicyclo[4.2.1]nona-2,4-diene, is obtained as virtually the only product if the initial molar ratio CHT:BD equal to 1.86 is used.

Full text of DOI:10.1135/cccc19961722

1722
Kaagman et al.:
TITANIUM-CATALYZED [4+2] and [6+2] CYCLOADDITIONS
OF 1,4-BIS(TRIMETHYLSILYL)BUTA-1,3-DIYNE
a1
a2
b1
c
Jan-Willem F. KAAGMAN , Marco REP , Michal HORACEK , Petr SEDMERA ,
Jiri CEJKA , Vojtech VARGAb1 and Karel MACH
*
b2
b1,
a Inorganic Chemistry Laboratory, J.H. van’t Hoff Institute, University of Amsterdam,
1018 WV Amsterdam, The Netherlands; e-mail: 1 janw@anorg.chem.uva.nl,
2 mrep@anorg.chem.uva.nl
b J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
182 23 Prague 8, Czech Republic; e-mail: 1 mach@jh-inst.cas.cz, 2 cejka@jh-inst.cas.cz
c Institute of Microbiology, Academy of Sciences of the Czech Republic, 142 20 Prague 4,
Czech Republic; e-mail: sedmera@biomed.cas.cz
Received September 5, 1996
Accepted October 23, 1996
The (C2H5)2AlCl/TiCl4 catalyst induces the [4+2] cycloaddition of butadiene or the [6+2] cycloaddi-
tion of 1,3,5-cycloheptatriene (CHT) to individual acetylenic moieties of 1,4-bis(trimethylsilyl)buta-
1,3-diyne (BSD). Heating of the 2 : 1 butadiene adduct, bis(2-trimethylsilylcyclohexa-1,4-dien-1-yl),
to 250 °C yields 2,2′-bis(trimethylsilyl)biphenyl. The 1 : 1 adduct of BSD with CHT, 7-trimethyl-
silyl-8-trimethylsilylethynylbicyclo[4.2.1]nona-2,4-diene, is obtained as virtually the only product if
the initial molar ratio CHT : BD equal to 1.86 is used.
Key words: Titanium catalyst; Cycloadditions; 1,4-Bis(trimethylsilyl)buta-1,3-diyne; 1,3-Butadiene;
1,3,5-Cycloheptatriene.
Bis(trimethylsilyl)acetylene (BTMSA) is a useful reagent in cycloaddition reactions
catalyzed by transition metal complexes1,2. We have found that it is an excellent dieno-
file in the Diels–Alder reactions and a trienofile in the [6+2] addition with 1,3,5-cyclo-
heptatriene (CHT), both having been induced by a classical Ziegler–Natta catalyst
Et2AlCl/TiCl4 (refs3–6). Analogous reactivity was also found for 1-phenyl-2-(trimethyl-
silyl)acetylene7,8. Both these acetylenes were reluctant to catalytically cyclotrimerize
and moreover, they suppressed the property of the catalyst to homooligomerize the
dienes or CHT. Another reagent of interest with respect to cycloaddition reactions is
1,4-bis(trimethylsilyl)buta-1,3-diyne (BSD) whose reactivity in general is rather low9.
It could react as a dieno- or trienofile by one or both triple bonds in a similar way like
* The author to whom correspondence should be addressed.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Titanium Catalyzed [4+2] and [6+2] Cycloadditions
1723
the above mentioned acetylenes, however, its cobalt-catalyzed cyclotrimerization yield-
ing tris(trimethylsilyl)tris(trimethylsilylethynyl)benzene isomers was also reported10.
Cleavage of BSD on titanocenes formed in statu nascendi has been observed yielding
either [(C5HMe4)2Ti(C≡CSiMe3)2]–[Mg(THF)Cl]+ (ref.11) or [(C5H5)2Ti(C≡CSiMe3)]2
(ref.12).
In this paper we report on the reactivity of BSD towards several conjugated dienes
and CHT in the presence of the Et2AlCl/TiCl4 catalytic system.
RESULTS AND DISCUSSION
The (C2H5)2AlCl/TiCl4/toluene (Al : Ti = 10) system in the presence of 1,4-bis(tri-
methylsilyl)-1,3-butadiyne (BSD) at the molar ratio BSD : Ti ≥ 5 gives rise to a brown
solution. The system does not induce reactions of BSD alone as it is recovered after
quenching the catalyst with air. Additions of conjugated dienes 1,3-butadiene (BUT),
2,3-dimethyl-1,3-butadiene (DMB) or 1,3-cyclohexadiene (CXD) to the above
(C2H5)2AlCl/TiCl4/BSD/toluene system afford the products of [4+2] addition, where
BSD is a single or double dienofile. In addition, homooligomers and/or polymers of the
dienes are formed as by-products. The products were first separated by liquid chroma-
tography over the silica gel column. The low-molecular weight products including
polydienes were eluted by hexane whereas inorganic products of the catalyst quenching
and high-molecular weight polyolefins were retained. The product mixtures were in
most cases difficult to separate without using e.g., preparative GC chromatography,
nevertheless, the products were identified by GC-MS chromatography. The yields of
the 1 : 1 and 2 : 1 diene-to-BSD cycloadducts depended on the capability of the dienes
to homooligomerize or polymerize. Among the dienes, butadiene is known to easily
cyclotrimerize with the (C2H5)2AlCl/TiCl4 catalytic system to give a mixture of
cis,trans,trans- and trans,trans,trans-1,5,9-cyclododecatrienes13 (CDT). Indeed, a mix-
ture of the 1 : 1 cycloadduct and the CDT isomers was obtained even at the 1 : 1 molar
ratio of BSD and BUT. Their retention times on an SE-30 GC column were close to
each other and thus the separation of the 1 : 1 adduct from the CDT isomers either by
distillation or by liquid chromatography would be difficult. Therefore, a large excess of
BUT (18.3 mmol) with respect to BSD (5.15 mmol) was applied and this resulted in
obtaining a mixture of CDT isomers and the 2 : 1 adduct 1 (Scheme 1).
’
5
’
4
’
’
3
6
TiCl , Et AlCl
4
2
’ ’
2
+
1
Me Si
Me Si
3
SiMe
3
3
toluene
SiMe
2
4
1
5
3
3
6
1
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1724
Kaagman et al.:
More volatile CDT isomers were removed by vacuum distillation at 100 °C and pure
1 was then distilled from a little amount of polymeric residue. The isolated yield 52%
of 1 related to BSD indicated that some part of BSD was also involved in the polymeri-
zation. Compound 1 was identified as bis(2-trimethylsilylcyclohexa-1,4-dien-1-yl) by
1
1H and 13C NMR and mass spectroscopy. The NMR spectra were assigned using H,
1
1
1
13C, H-COSY, H-13C HETCOR, and H-13C HETCOR optimized for the detection of
small coupling constants (long range HETCOR). Carbon atoms are numbered as shown
in formula 1 of Scheme 1 and protons are referred to the number of the carbon atom
which they are attached to. Long range HETCOR proves that the trimethylsilyl group is
coupled to the quatenary carbon atom resonating at 127.58 ppm (C-2). The olefinic
proton showing a long-range coupling to the same carbon is then assigned to H-4,
resonating at 5.717 ppm. The other olefinic proton, H-5, is coupled to the other quater-
nary carbon, denoted (C-1). H-5 is also long-range coupled to C-3, and H-4 is coupled
to C-6. Normal HETCOR shows the short-range couplings. 1H-COSY shows the existence
of a –CH2–CH=CH–CH2– spin system. This leads to a 1,4-cyclohexadiene structure.
The missing substituent at C-1 requires the structure of a symmetric dimer, in agree-
•
ment with the mass spectrum showing the molecular ion M+ m/z 302. The dehydroge-
nation of 1 at 250 °C afforded 2,2′-bis(trimethylsilyl)biphenyl (2) in virtually
quantitative yield (Scheme 2).
Me Si
3
o
250
C
SiMe
+
2 H
2
1
3
SCHEME 2
2
Analogous reactions of BSD with DMB (DMB : BSD = 17.0 : 5.15) afforded the
crude product mixture which contained the hexane-soluble polydiene as a major pro-
•
duct. The GC-MS analysis revealed the presence of 1 : 1 cycloadduct (M+, m/z 276) in
•
ca 80% amount, 5% of 2 : 1 cycloadduct (M+, m/z 358) and the unreacted BSD 10%.
The presence of BSD indicates that at least 12 mmoles of DMB out of 17 were in-
volved in the polymerization. When a higher excess of DMB was used (BSD 2.6 mmol
and DMB 17.0 mmol) all BSD was consumed, the 1 : 1 and 2 : 1 adducts were formed
•
in comparable quantities, and a small amount of the DMB dimer (M+, m/z 164) was
also found. The reaction of BSD with CXD (BSD 5.15 mmol, CXD 16.0 mmol) pro-
duced mainly the polymer which was not retained in the column. The volatile products
•
consisted mainly of a dimer of CXD (M+, m/z 160) and minor amounts of the 1 : 1 and
2 : 1 adducts in approximately 1 : 2 ratio. Since no free BSD was detected it has to be
assumed that it copolymerized with CXD. No attempts were, however, made to estab-
lish the polymer composition or its structure.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Titanium Catalyzed [4+2] and [6+2] Cycloadditions
1725
The reaction of BSD (5.15 mmol) with 1,3,5-cycloheptatriene (CHT, 9.6 mmol) af-
fords a high yield of 1 : 1 cycloadduct 3 arising from the [6+2] addition (Scheme 3).
1
The structure of 3 follows from fully assigned H and 13C NMR spectra. Long-range
HETCOR shows one TMS-group to be coupled to an sp2-carbon atom (C-7, 146.84 ppm),
the other TMS-group coupled to an acetylenic carbon (C-11, 95,77 ppm) and the up-
field methylene proton to be coupled with both quaternary olefinic carbons. One meth-
ine proton shows a coupling with the acetylenic C-10 carbon atom and is therefore
1
assigned to H-1. With this starting point and the results of the H-COSY, the other
1
protons could be assigned. The assignment of other carbon atoms is based on the H-13C
HETCOR experiments. Minor volatile impurities are the pentacyclic dimers of CHT 4
and 5 which nearly quantitatively arise from the catalytic action of the (C2H5)2AlCl/TiCl4
system on cycloheptatriene14,15. The mechanism of their formation involves the initial
[6+2] addition step followed by the [4+2] intramolecular cyclization14.
11
10
SiMe
3
Me Si
3
8
3
7
4
9
TiCl , Et AlCl
4
2
6
5
1
+
Me Si
3
SiMe
3
toluene
2
SCHEME 3
3
At lower excess of CHT, a portion of BSD remains unreacted while a mixture of 3
with approximately 6% of 4 and 5 is obtained, as in the above optimized experiment.
Their GC retention times on SE-30 column are very similar, and this precludes the easy
separation of 3 from 4 and 5. At higher excess of CHT, largely the dimers are obtained
as the 2 : 1 cycloadduct of CHT and BSD is formed only reluctantly. Using 19.3 mmol
of CHT and 5.5 mmol of BSD, the GC analysis afforded following yields: 2 : 1 adduct
6%, 3 16%, and dimers of CHT 70%.
4
5
The cycloaddition reactions undoubtedly proceed in the coordination sphere of a
low-valent titanium complex3,4,16, as it has been outlined for the addition reactions of
BTMSA. The present results, however, show that BSD is so weakly coordinated that
the catalytic activity of the (C2H5)2AlCl/TiCl4 system in the polymerization of DMB
and CXD, the cyclotrimerization of BUT or the dimerization of CHT are not signifi-
cantly hindered. This is the main disadvantage of BSD compared to BTMSA whose
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1726
Kaagman et al.:
apparently stronger complexation to the titanium species suppressed its polymerization
and homooligomerization activities.
EXPERIMENTAL
Chemicals
1,4-Bis(trimethylsilyl)buta-1,3-diyne (BSD) was obtained by the reaction of perchlorobutadiene with
trimethylsilyl chloride in the presence of a large excess of magnesium turnings according to de-
scribed procedure9. Recrystallization was carried out from ethanol and finally from hexane. The
purity was checked by GC-MS. 1,3-Butadiene and 1,3,5-cycloheptatriene (both Fluka) and toluene
(Lachema, Brno) were distilled in vacuo and stored for several days as the solutions of dimeric tita-
nocene17. They were then distributed into ampoules by vacuum distillation. TiCl4 (International
Enzymes) was refluxed with copper wire and then distilled in vacuo and diluted with toluene to give
0.1 M solution. (C2H5)2AlCl (Fluka) was purified from traces of (C2H5)AlCl2 by heating with anhydrous
sodium chloride to 180 °C. Then it was distilled in vacuo and diluted with toluene to give 1.0
M
solution. Authentic compounds 4 and 5 (ref.15 were used for their identification in the reaction pro-
ducts by gas chromatography.
Methods
1H and 13C NMR spectra (δ, ppm; J, Hz) of the products were measured on a Varian VXR-400 spec-
trometer (400 and 100 MHz, respectively) in CDCl3 at 25 °C using the residual signal of the solvent
as a reference. Infrared spectra (wavenumbers in cm–1) of the liquid films were obtained on a UR-75
instrument (Zeiss, Jena). The GC analyses were performed on a CHROM 5 gas chromatograph (La-
boratorni pristroje, Prague) using 10% SE-30 on a Chromaton N-AW-DMCS column. Analogous
GC-MS analyses were carried out on a Hewlett–Packard gas chromatograph (5890 series II) equipped
with a capillary column SPB-1 (length 30 m; Supelco) and a mass spectrometric detector (5971 A).
General Procedure for Cycloadditions of BSD
In a typical experiment, crystalline BSD (1.0 g, 5.4 mmol) was charged into an ampoule and was
degassed on a vacuum line. The 1 M toluene solution of (C2H5)2AlCl (4 ml) was added from an at-
tached ampoule. After BSD had dissolved, the solution was cooled by liquid nitrogen. Then, 0.1
M
TiCl4 in toluene (4 ml) was added from another ampoule. The reaction mixture was warmed to room
temperature with shaking and then heated to 60 °C for 15 min to give a brown, homogeneous solu-
tion. Then, the solution was cooled by liquid nitrogen and the conjugated diene or CHT was distilled
into the reaction mixture. The system of dosing ampoules was then sealed off and the reaction am-
poule was heated to 60 °C for 8 h. After cooling to room temperature, the ampoule was opened and
a brown, heterogeneous reaction mixture was poured onto a silica gel column (15 mm diameter,
length 100 mm). An exothermic reaction occurred as the organometallic compounds were hydrolyzed
by the water contained in the silica gel. The organic products were eluted from the column by hexane
whereas inorganic products of hydrolysis remained in the column. The solvents were evaporated on
a vacuum rotary evaporator and the crude product was analyzed by GC-MS or GC. The estimates of
the product composition were obtained from integrated areas of GC peaks using an integrator and
considering the number of C atoms in the molecule of the product components.
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

Titanium Catalyzed [4+2] and [6+2] Cycloadditions
1727
Bis(2-trimethylsilylcyclohexa-1,4-dien-1-yl) (1)
Butadiene (1.6 ml, 18.3 mmol) was applied in the above experiment. The crude product (1.4 g) con-
tained according to GC analysis 77% of 1 and 21% of a mixture of cis,trans,trans- and
trans,trans,trans-1,5,9-cyclododecatrienes and traces of unidentified compounds. The cyclododeca-
trienes were removed by vacuum distillation at 100 °C. The distillation at higher temperatures (100–
140°C) afforded a nearly colourless 1 of 99% purity. Yield 0.8 g (52% related to BSD). The
1
compound was identified on the basis of MS, H and 13C NMR spectra. IR spectrum (neat): 3 019 m,
2 948 s, 2 888 m, 2 866 m, 2 843 m, 2 803 m, 1 667 m, 1 592 m, 1 445 w, 1 420 s, 1 400 m, 1 246 vs,
1 168 vw, 1 090 m, 1 012 m, 908 m, 868 s, 835 vs, 755 s, 729 w, 685 m, 663 s, 609 w. Mass
•
spectrum, m/z (%): 302 (M+, 2); 211 (7); 197 (7); 195 (12); 160 (7); 159 (13); 155 (29); 154 (34);
1
135 (5); 74 (8); 73 (100); 59 (14); 45 (19). H NMR spectrum (CDCl3): 0.08 s, 9 H (TMS); 2.54 m,
1 H (H-6u); 2.71 m, 2 H (H-3); 2.83 m, 1 H (H-6d); 5.71 m, 1 H (H-5); 5.72 m, 1 H (H-4). 13C NMR
spectrum: –0.30 q, 3 C (TMS); 28.90 t (C-3); 32.37 t (C-6); 123.81 d (C-5); 128.69 d (C-4); 127.58 s
(C-2); 147.39 s (C-1).
2,2′-Bis(trimethylsilyl)biphenyl (2)
Compound 1 (1.1 g) was degassed in an ampoule (200 ml) and this was sealed out. The ampoule was
heated to 250 °C for 15 h and then was opened to air (Attention: do not use flame as the ampoule
contains hydrogen). The yellowish liquid was distilled in vacuo to give colourless 2. GC-MS analysis
revealed the presence of 1 to be less than 0.5%. Yield 0.9 g (82%). IR spectrum (neat): 3 047 m, 2 944 s,
2 892 m, 1 920 w, 1 853 vw, 1 814 vw, 1 580 m, 1 449 m, 1 416 s, 1 306 vw, 1 247 vs, 1 119 s, 1 090 m,
1073 w, 1 042 w, 999 w, 843 sh, 833 vs, 770 sh, 754 s, 727 s, 686 m, 619 m, 557 w, 465 m. Mass
•
spectrum, m/z (%): 298 (M+, 6); 224 (12); 196 (19); 195 (100); 165 (17); 74 (7); 73 (81); 45 (16).
1H NMR spectrum (CDCl3): 0.04 s, 18 H (2 × SiMe3); 7.187 m, 2 H; 7.355 m, 4 H; 7.609 m, 2 H.
13C NMR spectrum: 0.35 q, 6 C (2 × SiMe3); 126.39 d, 2 C; 127.69 d, 2 C; 129.89 d, 2 C; 134.29 d,
2 C; 138.83 s, 2 C (C-2, C-2’); 149.90 s, 2 C (C-1, C-1’). The NMR and mass spectra are in agree-
ment with ref.18.
7-Trimethylsilyl-8-trimethylsilylethynylbicyclo[4.2.1]nona-2,4-diene (3)
1,3,5-Cycloheptatriene (1 ml, 9.6 mmol) was applied in the general procedure described above. The
isolated crude product (1.4 g) contained, according to GC analysis, 3 (91%), dimers of CHT 4 and 5
(6%) and traces of more volatile unidentified by-products (3%). The crude product was distilled
under dynamic vacuum (ca 1 Pa) with the temperature rising up to 140 °C. The first 6 drops were
separated. The main fraction (0.9 g) contained 3 97% and 4 (2%) and 5 (1%). Yield of 3 was ca 60%
related to BSD. This fraction was further distilled in vacuo, and the middle fraction (0.3 g) contained
3 in 99% purity. This was used for the structure identification by NMR spectroscopy. IR spectrum
(neat): 3 013 m, 2 951 s, 2 930 sh, 2 893 m, 2 133 s, 1 532 w, 1 438 w, 1 403 m, 1 380 m, 1 313 m,
1 285 vw, 1 246 vs, 1 213 vw, 1 177 vw, 1 137 m, 1 073 w, 1 050 vw, 1 024 m, 1 004 w, 942 m,
896 w, 873 s, 859 sh, 837 vs, 755 s, 721 s, 698 m, 660 vw, 627 m, 613 m, 577 vw, 545 w, 524 w,
•
489 m, 461 vw, 430 w. Mass spectrum, m/z (%): 286 (M+, 17); 271 (5); 213 (5); 212 (6); 199 (5);
198 (23); 197 (31); 184 (5); 183 (24); 181 (5); 179 (9); 155 (8); 145 (5); 121 (5); 116 (7); 107 (5);
1
97 (10); 93 (5); 92 (27); 91 (33); 83 (7); 79 (5); 74 (9); 73 (100); 59 (9); 45 (18); 43 (10). H NMR
spectrum (CDCl3): 0.18 s, 9 H (TMS, C-11); 0.19 s, 9 H (TMS, C-7); 1.50 d, 1 H, J = 11.6 (H-9
endo); 2.08 m, 1 H (H-9 exo); 3.21 dd, 1 H, J = 7.5, 6.7 (H-6); 3.34 dd, 1 H, J = 7.6, 6.9 (H-1); 5.87 ddd,
1 H, J = 11.0, 7.4, 1.2 (H-4), 5.89 ddd, 1 H, J = 11.0, 7.4, 1.2 (H-3); 6.07 dddd, 1 H, J = 11.0, 7.5,
1.2, 1.1, 1.1 (H-5); 6.17 dddd, 1 H, J = 11.0, 7.6, 1.2, 1.2, 1.1 (H-2). 13C NMR spectrum: –0.67 q,
Collect. Czech. Chem. Commun. (Vol. 61) (1996)

1728
Kaagman et al.:
3 C (TMS, C-11); –0.12 q, 3 C (TMS, C-7); 30.62 t (C-9), 48.32 d (C-6); 49.72 d (C-1); 95.77 s
(C-11); 103.19 s (C-10); 124.49 d (C-4); 124.81 d (C-3); 125.32 s (C-8); 137.42 d (C-5); 137.64 d
(C-2); 146.84 s (C-7).
This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic
(Grant No. A440403).
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