Highly regioselective [2 + 2 + 2] cycloaddition of terminal alkynes catalyzed by η6-arene complexes of titanium supported by dimethylsilyl-bridged p-tert-butyl calix[4]arene ligand

Article abstract of DOI:10.1021/ja994543o

Two new Ti-η⁶-arene complexes [(DMSC)Ti{η⁶-1,2,4-C₆H₃(SiMe ₃)₃}] (6) and [(DMSC)Ti{η⁶-1,3,5-C₆H₃Bu^t ₃}] (7) containing 1,2-alternate, Me₂Si-bridged p-tert-butylcalix[4]arene (DMSC) ancillary ligand have been synthesized. The solid-state structure of 6 revealed a highly folded arene ligand [with a dihedral angle of 29.7(7)○] and suggests that 6 is better described as a 7-titananorbornadiene species. Both 6 and 7 are efficient catalysts for highly regioselective [2 + 2 + 2] cycloaddition of terminal alkynes to yield 1,2,4-substituted benzenes. Kinetic studies of the catalytic [2 + 2 + 2] cycloaddition of Me₃SiC≡CH revealed first-order dependence on [6] and [Me₃SiC≡CH]; and activation parameters, ΔH? = 14 kcal/mol, and ΔS? = - 11 cal/mol K, that are consistent with an associative mechanism. The reaction rate is influenced by the steric requirements of both the alkyne and the η⁶-arene compound. The high selectivity for 1,2,4-substituted benzene may be understood in terms of the directing influence of the DMSC ligand.

Full text of DOI:10.1021/ja994543o

J. Am. Chem. Soc. 2000, 122, 6423-6431
6423
Highly Regioselective [2 + 2 + 2] Cycloaddition of Terminal
6
Alkynes Catalyzed by η -Arene Complexes of Titanium Supported by
Dimethylsilyl-Bridged p-tert-Butyl Calix[4]arene Ligand
‡
Oleg V. Ozerov, Brian O. Patrick, and Folami T. Ladipo*
Contribution from the Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055
ReceiVed December 30, 1999
6
6
6
Abstract: Two new Ti-η -arene complexes [(DMSC)Ti{η -1,2,4-C6H3(SiMe3)3}] (6) and [(DMSC)Ti{η -
t
1
,3,5-C6H3Bu 3}] (7) containing 1,2-alternate, Me2Si-bridged p-tert-butylcalix[4]arene (DMSC) ancillary ligand
have been synthesized. The solid-state structure of 6 revealed a highly folded arene ligand [with a dihedral
angle of 29.7(7)°] and suggests that 6 is better described as a 7-titananorbornadiene species. Both 6 and 7 are
efficient catalysts for highly regioselective [2 + 2 + 2] cycloaddition of terminal alkynes to yield 1,2,4-
substituted benzenes. Kinetic studies of the catalytic [2 + 2 + 2] cycloaddition of Me3SiCtCH revealed
q
q
first-order dependence on [6] and [Me3SiCtCH]; and activation parameters, ∆H ) 14 kcal/mol, and ∆S )
11 cal/mol K, that are consistent with an associative mechanism. The reaction rate is influenced by the steric
-
6
requirements of both the alkyne and the η -arene compound. The high selectivity for 1,2,4-substituted benzene
may be understood in terms of the directing influence of the DMSC ligand.
Introduction
past few years, Floriani has been studying the use of p-tert-
butylcalix[4]arene and its O-methylated derivatives (in cone
The organometallic chemistry of the group 4 metals has been
studied using mainly complexes supported by cyclopentadienyl
(3) See for example: (a) Balaich, G. J.; Hill, J. E.; Waratuke, S. A.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 1995, 14, 656. (b) Durfee,
L. D.; Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics, 1990, 9,
1
(Cp) or substituted Cp ligands. Since the electronic and steric
properties of an ancillary ligand can have a pronounced effect
on the reactivity of a transition metal complex, non-Cp ligand
75. (c) Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1992,
11, 1775. (d) Hill, J. E.; Fanwick, P. E.; Rothwell, I. P. Organometallics
1992, 11, 1771. (e) Hill, J. E.; Balaich, G. J.; Fanwick, P. E.; Rothwell, I.
P. Organometallics 1991, 10, 3428. (f) Hill, J. E.; Fanwick, P. E.; Rothwell,
I. P. Organometallics 1990, 9, 2211. (g) Durfee, L. D.; Hill, J. E.; Kerschner,
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Covert, K. J.; Mayol, A. R.; Wolczanski, P. T. Inorg. Chim. Acta 1997,
2
3
4
arrays such as chelating diamides, aryloxides, carboranes,
5
6
7
amidinates, porphyrins, tetradentate Schiff bases, borataben-
zenes,8 and calixarenes
9,10a-f
have recently been attracting
increased attention, especially with regard to Ziegler-Natta
R-olefin polymerization. The use of calixarenes as ancillary
ligands in organotransition metal chemistry has not been
adequately explored.9 The degree of steric shielding and
electronic stabilization provided by a calix[4]arene ligand may
be influenced by which conformation it adopts.11 During the
263, 263. (i) Thorn, M. G.; Hill, J. E.; Waratuke, S. A.; Johnson, E. S.;
Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1997, 119, 8630. (j)
Johnson, E. S.; Balaich, G. J.; Fanwick, P. E.; Rothwell, I. P. J. Am. Chem.
Soc. 1997, 119, 11086. (k) Corazza, F.; Floriani, C.; Chiesi-Villa, A.;
Guastini, C. Inorg. Chem. 1991, 30, 145. (l) Okuda, J.; Fokken, S.; Kang,
H.-C.; Massa, W. Chem. Ber. 1995, 128, 221. (m) Fokken, S.; Spaniol, T.
P.; Kang, H.-C.; Massa, W.; Okuda, J. Organometallics 1996, 15, 5069.
,10
(
n) Eilerts, N. W.; Heppert, J. A. Polyhedron 1995, 14, 3271. (o) Thorn,
‡
Current address: Department of Chemistry, The University of British
M. G.; Villardo, J. S.; Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc., Chem.
Comunn. 1998, 2127.
Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T 1Z1.
(
1) (a) Metallocenes: Synthesis. ReactiVity. Applications; Togni, A.,
(4) See for example: Crowther, D. J.; Baenziger, N. C.; Jordan, R. F. J.
Am. Chem. Soc. 1991, 113, 1455.
Halterman, R. L., Eds.: Wiley-VCH: Weinheim. 1998: Vol 1 and 2. (b)
ComprehensiVe Organometallic Chemistry II: Abel, E. W., Stone, F. G.
A., Wilkinson, G., Eds.: Pergamon: New York. 1995: Vol 4.
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1998, 17, 1355. (b) Hagadorn, J. R.; Arnold, J. J. Chem. Soc., Dalton Trans.
1997, 3087. (c) Hagadorn, J. R.; Arnold, J. Organometallics, 1993, 13, 4670.
(d) Hagadorn, J. R.; Arnold, J. J. Am. Chem. Soc. 1996, 118, 893. (e) Hao,
S.; Feghali, K.; Gambarotta, S. Inorg. Chem. 1997, 36, 1745. (f) Schmidt,
J. A. R.; Arnold, J. J. Chem. Soc. Chem. Commun. 1999, 2149.
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Kadish, K. M.; Woo, K. L. Inorg. Chem. 1998, 37, 1 (b) Wang, X.; Gray,
S. D.; Chen, J.; Woo, K. L. Inorg. Chem. 1998, 37, 5. (c) Woo, K. L.;
Hays, J. A. Inorg. Chem. 1993, 32, 2228. (d) Woo, K. L.; Hays, J. A.;
Jacobson, R. A.; Day, C. L. Organometallics 1991, 10, 2102.
(7) See for example: (a) Tjaden, E. B.; Sewnson, D. C.; Jordan, R. F.;
Petersen, J. L. Organometallics, 1995, 14, 371. (b) Solari, E.; Floriani, C.;
Chiesi-Villa, A.; Rizzoli, C. J. Chem. Soc., Dalton Trans. 1992, 367.
(8) See for example: Bazan, G. C.; Rodriguez, G.; Ashe, A. J., III; Al-
Ahmad, S.; Muller, C. J. Am. Chem. Soc. 1996, 118, 2291. (b) Bazan, G.
C.; Rodriguez, G.; Ashe, A. J., III; Al-Ahmad, S.; Kampf, J. W.
Organometallics 1997, 16, 2492. (c) Barnhart, R. W.; Bazan, G. C. J. Am.
Chem. Soc. 1998, 120, 1082. (d) Rogers, J. S.; Bazan, G. C.; Sperry, C. K.
J. Am. Chem. Soc. 1997, 119, 9305. (e) Ashe, A. J., III; Al-Ahmad, S.;
Fang, X.; Kampf, J. W. Organometallics 1998, 17, 3883.
(2) See for example: (a) Warren, T. H.; Schrock, R. R.; Davis, W. M.
Organometallics 1996, 15, 562. (b) Tsuie, B.; Swenson, D. C.; Jordan, R.
F. Organometallics 1997, 16, 1392. (c) Warren, T. H.; Schrock, R. R.; Davis,
W. M. Organometallics 1998, 17, 308. (d) Guerin, F.; McConville, D. H.;
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With, J. J. Chem. Soc., Chem. Commun. 1996, 1375. (g) Horton, A. D.; de
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2
672. (h) Scollard, J. D.; McConville, D. H., Payne, N. C.; Vittal, J. J.
Macromolecules 1996, 29, 5241. (i) Scollard, J. D.; McConville, D. H. J.
Am. Chem. Soc. 1996, 118, 10008. (j) Baumann, R.; Davis, W. M.; Schrock,
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998, 313. (m) Jeon, Y.-M.; Park, S. J.; Heo, J.; Kim, K. Organometallics
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M. A.; Manzoni, M. R.; Baumann, R.; Davis, W. M.; Schrock, R. R.
Organometallics 1999, 18, 3220.
1
1
1
0.1021/ja994543o CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/21/2000

6424 J. Am. Chem. Soc., Vol. 122, No. 27, 2000
OzeroV et al.
conformation) to support organometallic chemistry at early
transition metal centers.9
We have been investigating the potential of bis(aryloxide)
ligands derived from p-tert-butylcalix[4]arene in early transition
metal chemistry. Recently, we described the synthesis of Ti-
in excellent yield.13 Although many transition metals catalyze
[2 + 2 + 2] cycloaddition of alkynes to yield substituted
,10g-k
1
4,15
benzenes,
the reaction rarely proceeds with high regio-
14d
selectivity. Highly regiocontrolled synthesis of arenes is very
attractive since arenes are important building blocks in organic
synthesis. In this paper, we present results from our investigation
of the scope and mechanism of the [2 + 2 + 2] cycloaddition
of terminal alkynes mediated by titanium complexes supported
by ancillary DMSC ligation.
(
[
IV) complexes supported by proximally bridged p-tert-butylcalix-
4]arenes in 1,2-alternate conformation.1 In this conformation,
2,13
the calix[4]arene ligand imposes a unique stereochemical
environment at titanium. For example, the two chloride ligands
in (DMSC)TiCl2 (1) (DMSC ) 1,2-alternate Me2Si-bridged
p-tert-butylcalix[4]arene) exist in different stereochemical en-
vironments.13 The endo-chloride is located inside the calix[4]-
arene cavity, above the centers of two aromatic rings while the
exo-chloride is located outside of the calix[4]arene cavity. The
difference between the endo and exo coordination sites is also
evident in the reactivity of dialkyl derivatives (DMSC)TiR2 (2,
R ) Me; 3, R ) CH2Ph). Thus, 2 and 3 give alkyl-triflato
complexes 4 and 5, respectively; formed by exclusive abstraction
of the more exposed exo-alkyl (eq 1).12
Experimental Section
General. All experiments were performed under dry nitrogen
atmosphere using standard Schlenk techniques or in a Vacuum
Atmospheres, Inc. glovebox. Solvents were dried and distilled by
standard methods before use. Alkynes were purchased from Aldrich
2
or Farchan and were distilled from CaH prior to use. Mg(C14H10)-
16
1
(
thf)
3
was prepared by modification of the reported method. H (200
MHz) and 13C (50.3 MHz) NMR spectra were recorded on a Varian
1
13
Gemini-200 spectrometer at ∼22 °C. H and C chemical shifts were
referenced to residual solvent peaks. GC-MS analyses were performed
on a Hewlett-Packard 5890 series II gas chromatograph with a Hewlett-
Packard 5972 series mass selective detector at an ionizing potential of
7
0 eV. HR-MS analysis was performed in the University of Kentucky
Mass-Spectrometry Center. Elemental analyses were performed by E+R
Microanalytical Laboratory, Inc., Ithaca, NY.
6
[
(DMSC)Ti{η -C
3
6
H
3
(SiMe
3 3
)
}] (6). A suspension of C14
H
10Mg-
16
(
THF)
(1.31 g, 3.13 mmol) in 50 mL of toluene was heated under
2
N atmosphere at 90 °C for 30 min, during which time Mg* precipitated.
The toluene solution was removed via suction through a glass tube
equipped with a glass frit. Mg powder was washed with ether until all
of the anthracene was removed. Toluene (30 mL) was added to the
We have found that (DMSC)TiCl2 (1) catalyzed the [2 + 2
+
2] cycloaddition of terminal alkynes RCtCH (R ) Me3Si,
flask followed by Me
3
SiCtCH (1.78 mL, 12.5 mmol) and (DMSC)-
Ph, or p-tolyl) at 80 °C and in the presence of an excess of
sodium (with respect to Ti) to yield the corresponding 1,2,4-
substituted benzene with excellent regioselectivity (g97%) and
13
TiCl
2
(1) (2.06 g, 2.50 mmol). THF (1 mL) was added, while the
reaction mixture was vigorously stirred. The mixture quickly turned
brown-yellow, it was stirred for 10 min, and then the volatiles were
removed in vacuo. The remaining solids were triturated with heptane,
extracted with pentane until the washings were colorless, and then
filtered. The filtrate was stripped to dryness, and the residue was treated
with 15 mL of ether. The resulting suspension was stirred for 30 min
and then cooled at -15 °C for 24 h. The suspension was then filtered,
and the solids on the filter were washed with 15 mL of cold ether. The
yellow product was dried in vacuo (1.80 g, 69%) and identified as 6
(
9) See for example: (a) Zanotti-Gerosa, A.; Solari, E.; Giannini, L.;
Floriani, C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. Inorg. Chim. Acta 1998,
70, 298. (b) Castellano, B.; Zanotti-Gerosa, A.; Solari, E.; Floriani, C.
2
Organometallics, 1996, 15, 4894. (c) Corazza, F.; Floriani, C.; Lesueur,
W.; Chiesi-Villa, A.; Guastini, C. J. Chem. Soc., Chem. Commun. 1990,
083. (d) Corazza, F.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Chem.
Soc., Chem. Commun. 1990, 640. (e) Giannini, L.; Caselli, A.; Solari, E.;
Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.; Re, N.; Sgamellotti, A. J. Am.
Chem. Soc. 1997, 119, 9198. (f) Giannini, L.; Caselli, A.; Solari, E.; Floriani,
C.; Chiesi-Villa, A.; Rizzoli, C.; Re, N.; Sgamellotti, A. J. Am. Chem. Soc.
997, 119, 9709. (g) Giannini, L.; Solari, E.; Zanotti-Gerosa, A.; Floriani,
C.; Chiesi-Villa, A.; Rizzoli, C. Angew. Chem., Int. Ed. Engl. 1996, 36,
5. (h) Zanotti-Gerosa, A.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli,
1
on the basis of the following data: 1H NMR (C
arom CH), 7.30 (d, J ) 2.5 Hz, 1H, arom CH), 7.26 (d, J ) 2.5 Hz,
H, arom CH), 7.01 (d, J ) 2.5 Hz, 1H, arom CH), 6.96 (d, J ) 2.5
Hz, 1H, arom CH), 6.80 (br, 1H, arom CH), 6.60 (br, 1H, arom CH),
.39 (br d, 1H, C {SiMe ), 5.14 (br d, 1H, C {SiMe ), 4.73
6
6
D ) δ 7.83 (br, 2H,
1
1
8
5
H
6 3
3
}
3
6
H
3
3 3
}
C. J. Chem. Soc., Chem. Commun. 1997, 183. (i) Caselli, A.; Giannini, L.;
Solari, E.; Floriani, C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. Organometallics
1
997, 16, 5457. (j) Floriani, C. Chem. Eur. J. 1999, 5, 19. (k) Giannini, L.;
(14) 14. See for example: (a) Ojima, I.; Tzamarioudaki, M.; Zhaoyang,
Solari, E.; De Angelis, S.; Ward, T. R.; Floriani, C.; Chiesi-Villa, A.; Rizzoli,
C. A. J. Am. Chem. Soc. 1995, 117, 5801.
L.; Donovan, R. J. Chem. ReV. 1996, 96, 635. (b) Negishi, E. In
ComprehensiVe Organic Synthesis; Trost, B. M., Ed.; Permagon: Oxford,
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1992, 539. (d) Schore, N. E. Chem. ReV. 1988, 88, 1081. (e) Trost, B. M.
Angew. Chem., Int. Ed. Engl. 1986, 25, 1.
(10) (a) Olmstead, M. M.; Sigel, G.; Hope, H.; Xu, X.; Power, P. P. J.
Am. Chem. Soc. 1985, 107, 8087. (b) Delaigue, X.; Hosseini, M. W.; Leize,
E.; Kieffer, S.; Van Doersselaer, A. Tetrahedron Lett. 1993, 34, 7561. (c)
Acho, J. A.; Lippard, S. J. Inorg. Chim. Acta 1995, 229, 5. (d) Hampton,
P. D.; Daitch, C. E.; Alam, T. M.; Bencze, Z.; Rosay, M. Inorg. Chem.
(15) See for example: (a) McAllister, D. R.; Bercaw, J. E.; Bergman,
R. G. J. Am. Chem. Soc. 1977, 99, 1666. (b) Volhardt, K. P. C. Angew.
Chem., Int. Ed. Engl. 1984, 23, 539. (c) Strickler, J. R.; Bruck, M. A.;
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P. E.; Rothwell, I. P. Organometallics 1990, 9, 2211. (f) Smith, D. P.;
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Organometallics 1992, 11, 1275. (g) Hill, J. E.; Balaich, G.; Fanwick, P.
E.; Rothwell, I. P. Organometallics 1993, 12, 2911. (h) van der Linden,
A.; Schaverien, C. J.; Meijboom, N.; Ganter, C.; Orpen, A. G. J. Am. Chem.
Soc. 1995, 117, 3008. (i) Sigman, M. S.; Fatland, A. W.; Eaton, B. E. J.
Am. Chem. Soc. 1998, 120, 5130. (j) Musso, F.; Solari, E.; Floriani, C.;
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A. S.; Wigley, D. E. J. Am. Chem. Soc. 1987, 109, 6525. (l) Solari, E.;
Floriani, C.; Schenk, K.; Chiesi-Villa, A.; Rizzoli, C.; Rosi, M.; Sgamellotti,
A. Inorg. Chem. 1994, 33, 2018.
1
994, 33, 4750. (e) Clegg, W.; Elsegood, M. R. J.; Teat, S. J.; Redshaw,
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Hofmeister, G. E.; Alvarado, E.; Leary, J. A.; Yoon, D. I.; Pedersen, S. F.
J. Am. Chem. Soc. 1990, 112, 8843. (g) Castellano, B.; Solari, E.; Floriani,
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E.; Scopelliti, R.; Floriani, C. Angew. Chem., Int. Ed. 1999, 38, 2388. (i)
Caselli, A.; Solari, E.; Scopelliti, R.; Floriani, C. J. Am. Chem. Soc. 1999,
1
21, 8296. (j) Giannini, L.; Solari, E.; Floriani, C.; Re, N.; Chiesi-Villa,
A.; Rizzoli, C. Inorg. Chem. 1999, 38, 1438. (k) Giannini, L.; Solari, E.;
Dovesi, S.; Floriani, C.; Re, N.; Chiesi-Villa, A.; Rizzoli, C. J. Am. Chem.
Soc. 1999, 121, 2784.
(
11) Gutsche, C. D. Calixarenes; The Royal Society of Chemistry:
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999, 586, 223.
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999, 121, 7941.
(
1
1
(16) (a) Bogdanovic, B.; Liao, S.; Schwickardi, M.; Sikorsky, P.;
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B. Acc. Chem. Res. 1988, 21, 261.
(

Cycloaddition of Terminal Alkynes
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6425
(
)
d, 1H, C
17 Hz, 1H, calix-CH
6
H
3
{SiMe
3
}
3
), 4.41 (d, J ) 16 Hz, 1H, calix-CH
), 4.13 (d, J ) 14 Hz, 2H, calix-CH
), 3.79 (d, J ) 14 Hz, 1H, calix-CH
d, J ) 17 Hz, 1H, calix-CH ), 3.36 (d, J ) 16 Hz, 1H, calix-CH
2
), 4.21 (d, J
), 3.84 (d,
), 3.59
),
.51 (s, 9H, Bu ), 1.45 (s, 9H, Bu ), 1.34 (s, 9H, Bu ), 1.24 (s, 9H,
for 30 min and was then passed through a short silica column. Following
removal of the volatiles in vacuo, the crude product (3.68 g, 95%) was
obtained as a colorless oil that was essentially pure (a small amount of
1,2,4-(Me Si) C H was present). Analytically pure product could be
3 3 6 3
isolated by fractional vacuum distillation. The product is a mixture of
2
2
J ) 14 Hz, 1H, calix-CH
(
1
2
2
2
2
t
t
t
t
Bu ), 0.20 (s, 3H, exo-SiCH
s, 9H, C {SiMe ), -0.35 (s, 9H, C
endo-SiCH ); C NMR (75 °C, C ) 161.2(TiOC), 160.1(TiOC),
52.0(SiOC), 150.7(SiOC), 142.8, 142.4, 142.3, 141.7, 139.2, 139.0,
3
), -0.04 (s, 9H, C
6
H
3
{SiMe
3
}
3
), -0.27
1,2,4- and 1,3,5-regioisomers in 95:5 ratio. Spectroscopic data for 1,2,4-
1
(
6
H
3
3
}
3
6
H
3
{SiMe
3
}
3
), -0.77 (s, 3H,
C
6
3
H (CH
2
OSiMe
3
)3 (16): H NMR (C
6 6
D ) δ 7.65 (s, 1H), 7.49 (d,
1
3
3
D
6 6
1H), 7.29 (d, 1H), 4.80 (s, 2H), 4.75 (s, 2H), 4.62 (d, 2H), 0.12 (s,
9H), 0.10 (br s, 18H); 13C NMR (C
) δ 140.7, 138.8, 137.3, 127.6,
125.7, 125.3, 64.7, 62.6 (br), -0.5 (br). GC-MS(EI) M (384), M -
CH (369). Anal. Calcd for: C, 56.19; H, 9.43. Found: C, 55.81; H,
9.20.
1
1
1
6 6
D
+
+
36.8, 135.6, 132.8(br), 131.3, 130.9, 129.3, 129.1, 128.9, 128.3, 127.8,
27.3, 127.1, 126.9, 126.8, 126.6, 126.2, 126.0, 125.7, 125.7, 114.3-
br), 41.1 (calix-CH
3
(
2
), 38.4 (calix-CH
2
), 38.1 (calix-CH
), 34.2 (C(CH ), 34.0 (C(CH
), 31.9 (C(CH ), 31.8 (C(CH ), 2.3 (exo-
), -1.9 (SiMe ), -2.0 (endo-SiMe).
Ti: C, 70.07; H, 8.48; Cl, 0.00. Found: C,
2
), 36.7 (calix-
CH
2
), 34.3 (br, C(CH
), 32.0 (C(CH
SiMe), 1.3 (SiMe
Anal. Calcd for C61
9.76; H, 8.54; Cl, <0.005.
3
)
3
3
3
)
3
3
)
3
), 32.1
Characterization of Products from [2 + 2 + 2] Cycloaddition of
1-(Trimethylsilyl)-4-thiahepta-1,6-diyne. 1-(Trimethylsilyl)-4-thia-
hepta-1,6-diyne18 (0.88 g, 4.84 mmol) was dissolved in 3 mL of heptane
and 4 portions of 6 (21 mg, 0.02 mmol each) were added at 12 h
intervals. The reaction was monitored by analyzing aliquots of the
(
3
C(CH )
3
3
)
3
)
3
3 3
)
3
), 0.3 (SiMe
Si
3
3
H
88
O
4
4
6
6
But
1
[
(DMSC)Ti(η -1,3,5-C
mmol) was treated with a solution of Me
in toluene (20 mL). C14 10Mg(THF) (0.63 g, 1.50 mmol) was added
to the resulting suspension with rapid stirring. The mixture quickly
turned brown-yellow. After 10 min, the volatiles were removed in
vacuo. The residue was triturated with heptane and extracted with
pentane until extracts became colorless. After the suspension was
filtered, all of the extracts were combined and stripped in vacuo. The
6
H
3
3
)] (7). (DMSC)TiCl
2
(1.23 g, 1.50
solution by H NMR. The completion of the reaction was signaled by
SiCtCH (1.28 mL, 9 mmol)
the absence of the starting diyne peaks. The solution was transferred
3
H
3
onto a 15 × 1 cm SiO
solution was stripped, and the residue was poured onto a similar column
of SiO . Pentane was used to elute 1,2,4-C (SiMe . The rest of the
2
-packed column and eluted with ether. The ether
2
H
6 3
3 3
)
material was eluted from the column with ether, and the volatiles were
removed in vacuo. The product (0.80 g, 91%) was obtained as a
yellowish oil. The oil was somewhat cloudy due to the presence of
t
t
resulting residue was then dissolved in pentane and filtered. Bu Ct
trace amounts of p-Bu -calix[4]arene, which were removed by filtering
1
CH (1.0 mL, 8 mmol) was added to the filtrate, and the mixture was
stirred for 72 h. After removal of the volatiles in vacuo, the solid residue
was washed with pentane (10 mL) and filtered (filtrate discarded). The
precipitate was extracted with toluene. After filtration, the toluene
extract was concentrated to dryness under reduced pressure. The residue
was washed with pentane (10 mL) and dried under vacuum to give an
the oil neat. H NMR and GC-MS analysis of the resulting oil revealed
a 91:9 ratio of 1,2,3,4- and 1,2,3,5-regioisomers of 1,3-dihydroben-
zothiophene derivative 17 (see text). All of the above manipulations
were performed in a glovebox, as 17 is susceptible to decomposition
1
in air. Spectroscopic data for 1,2,3,4-regioisomer (17a): H NMR
(C D
6 6
) δ 7.18 (d, J ) 8 Hz, 1H), 6.82 (d, J ) 8 Hz, 1H), 4.19 (br t,
),
) δ 7.21 (s, 2H), 4.31 (br t, 2H),
4.15 (br t, 2H), 4.00 (s, 2H), 3.22 (s, 2H), 0.45 (s, 9H, SiMe ), 0.19 (s,
) δ 146.8, 141.7, 139.1, 134.6, 129.2,
125.1, 101.7, 87.8, 39.4, 36.5, 36.1, 20.3, 2.8, -0.2. MS (EI-GC-
orange product (0.50 g, 33%) which was identified as 7 on the basis
of the following data: H NMR (C
2H), 3.96 (s, 2H), 3.89 (br t, 2H), 2.98 (s, 2H), 0.32 (s, 9H, SiMe
0.19 (s, 9H, SiMe ); H NMR (CD Cl
3 2 2
3
1
1
6 6
D ) δ 7.84 (d, J ) 2.5 Hz, 2H,
arom CH), 7.36 (d, J ) 2.5 Hz, 2H, arom CH), 6.88 (d, J ) 2.5 Hz,
H, arom CH), 6.74 (d, J ) 2.5 Hz, 2H, arom CH), 4.87 (s, 3H, C
3
13
2
6
H
3
-
3 3
9H, SiMe ); C NMR (CDCl
t
Bu
calix-CH
H, calix-CH
Hz, 1H, calix-CH
3
), 4.76 (d, J ) 15 Hz, 1H, calix-CH
), 4.12 (d, J ) 17 Hz, 2H, calix-CH
), 3.85 (d, J ) 17 Hz, 2H, calix-CH
2
), 4.53 (d, J ) 13 Hz, 1H,
), 3.87 (d, J ) 13 Hz,
), 3.37 (d, J ) 15
+
+
MS): 364 (M ), 349 (M - CH
of four scans): 364.1177 Calculated for C18
regioisomer (17b): 1H NMR (CD
Cl ) δ 7.29 (br s, 1H), 7.22 (br s,
1H), 4.29 (br, 2H), 4.20 (br, 2H), 3.83 (s, 2H), 3.10 (s, 2H), 0.31 (s,
9H, SiMe ), 0.20 (s, 9H, SiMe ).
3
). EI-HRMS: Measured mass (average
2
2
1
2
2
H
28
S
2
Si : 364.1171. 1,2,3,5-
2
t
t
2
), 1.50 (s, 18H, calix-Bu ), 1.22 (s, 18H, calix-Bu ),
.44 (s, 3H, exo-SiCH ), 0.33 (s, 27H, C {CMe }3), -0.51 (s, 3H,
); C NMR (C ) δ 160.3(TiOC), 151.0(SiOC), 142.3,
41.3, 140.9, 135.7, 130.1, 129.1, 128.3, 126.9, 126.8, 126.5, 126.2,
15.5, 39.9 (calix-CH
2
2
0
3
6
H
3
3
1
3
endo-SiCH
1
1
3
D
6 6
3
3
Synthesis of 1,3-Bis(trimethylsilyl)-6-(4-chlorophenyl)cyclohexa-
1,3-diene (19). 6 (0.125 g, 0.12 mmol) was added to a mixture of
4-chlorostyrene (3.2 g, 23 mmol) and Me SiCtCH (1.65 mL, 11.6
3
mmol). The reaction mixture was stirred at ambient temperature and
monitored by analyzing aliquots of the solution by 1H NMR. The
following additions were performed: (i) 0.04 mmol of catalyst was
added after 9 h; (ii) 0.02 mmol of catalyst was added after 27 h; (iii)
2
), 38.4 (calix-CH
), 33.7 (C(CH
), 4.1 (exo-SiMe), -1.1 (endo-SiMe). Anal.
2
), 36.4 (calix-CH
2
), 34.25
(
(
C(CH
C{CH
3
)
3
), 34.20 (C(CH
), 29.5 (C{CH
3
)
3
3
)
3 3
), 31.9 (C{CH
3
}
3
), 31.5
3
}
3
3
}
Calcd for: C, 77.07; H, 8.89; Cl, 0.00. Found: C, 76.86; H, 8.51; Cl,
<
0.03.
General Procedure for [2 + 2 + 2] Cycloaddition of Terminal
Alkynes in an NMR Tube. Catalyst (5 µmol) was dissolved in 0.6
mL of C
practically all of Me
1.22 mL (8.6 mmol) of Me
catalyst was added after 95 h; (v) all of Me
after 100 h, and the reaction was quenched with 0.5 mL of Pr
Excess 4-chlorostyrene and other volatiles were distilled off in vacuo.
The residue was passed through a short SiO column using pentane as
3
SiCtCH was consumed after 50 h; hence, another
SiCtCH was added; (iv) 0.02 mmol of
SiCtCH was consumed
OH.
6
D
6
in a screw-capped 5-mm NMR tube and 500 µmol of the
3
alkyne was added to it. The course of the reaction was monitored by
3
1
i
H NMR until the starting alkyne was completely consumed. At this
1
3
point the C NMR spectrum of the product could be obtained. The
solution was poured into pentane (15 mL) and treated with MeOH
0.5 mL). This solution was allowed to stand for 20 min in the air and
C
(
D
6 6
2
eluent. The pentane solution was concentrated under reduced pressure
to yield 3.0 g of a colorless oil. NMR and GC-MS analysis revealed
that the oil consisted of 80% 1,3 bis(trimethylsilyl)-6-(4-chlorophenyl)-
was then passed through a plug of silica gel to remove Ti byproducts
and p-tert-butylcalix[4]arene. An appropriate aliquot of the solution
was subjected to GC-MS analysis. Characterization data for 8-15
and 18 are given in the Supporting Information.
cyclohexa-1,3-diene (19), 12% 1,2,4-(Me
of 19. Attempts to purify the product oil by chromatography or
3
Si)
3
C
6
H
3
and ∼8% of isomers
recrystallization were completely unsuccessful, and apparent isomer-
Synthesis Characterization of 1,2,4-Synthesis and Characteriza-
ization occurred upon an attempt to distill the product oil at 0.3 Torr.
tion of 1,2,4-C
mmol) was dissolved in 30 mL of heptane. Me
mL, 30.0 mmol) was added in 6 portions (0.77 mL every 2 min).
Ten minutes after addition of the last portion, the reaction mixture was
treated with MeOH (2 mL). The solution was allowed to stand in air
6
H
3
(CH
2
OSiMe
3
)
3
(16). Compound 6 (0.157 g, 0.15
1
1
9 was therefore characterized as a part of the mixture. H NMR (C
6 6
D )
3
SiOCH CtCH (4.62
2
17
δ 7.10 (pseudo d, J ) 8 Hz, 2H, Cl-C H ), 6.92 (pseudo d, J ) 8 Hz,
6
4
2
H, Cl-C
6
H
B
4
), 6.77 (s, 1H, H
), 3.33 (dd, JCE ) 2.5 Hz, JDE ) 9.2 Hz, 1H, H
ddd, JBD ) 2.8 Hz, JDE ) 9.2 Hz, JCD ) 17.2 Hz, 1H, H ), 2.12 (ddd,
), 0.12 (s, 9H,
) δ 141.5, 139.3, 136.4,
35.6, 134.2, 132.4, 129.8, 128.5, 38.7, 32.8, -1.4, -2.0.
(18) The preparation is given in the Supporting Information.
A
), 5.90 (dd, JBD ) 2.8 Hz, JBC ) 5.8
Hz, 1H, H
(
E
), 2.48
D
(17) The alkyne was added in portions to overcome deactivation of 6
J
BC ) 5.8 Hz, JCE ) 2.5 Hz, JCD ) 17.2 Hz, 1H, H
C
during the reaction, due probably to oxidative cleavage of the C-O bond.
For an example of [2 + 2 + 2] cycloaddition followed by C-O bond
cleavage mediated by titanium-aryloxide species, see: Balaich, G. J.;
Rothwell, I. P. Tetrahedron 1995, 51, 4463.
13
3 3 6 6
SiMe ), 0.02 (s, 9H, SiMe ); C NMR (C D
1

6426 J. Am. Chem. Soc., Vol. 122, No. 27, 2000
OzeroV et al.
structure of 6 were established by microanalysis, H and 13C
NMR and X-ray crystallography. NMR data demonstrate that
6 is C1-symmetric in solution, three singlets are observed in
1
Table 1. Summary of Crystal Data and Structure Refinement for
6
6 3 3 3
[(DMSC)Ti{η -1,2,4-C H (SiMe ) }] (6)
empirical formula
formula weight
temperature
wavelength
crystal system
space group
64 95 4 4
C H O Si Ti
1088.66
173(2) K
0.71073 Å
triclinic
P-1
1
6
the H NMR spectrum for the inequivalent η -arene SiMe3
groups while the calix[4]arene Bu groups show as four singlets.
That the calix[4]arene ligand exists in 1,2-alternate conformation
is apparent from the NMR resonances of the endo -and exo-
methyls of the bridging SiMe2 unit, which are observed as
separate signals at δ -0.77 and 0.20, respectively ( H NMR),
and at δ -2.0 and 2.3, respectively ( C NMR). Invariably, H
and C NMR resonances for the endo-Me are strongly shielded
compared to corresponding signals for the exo-Me, due most
t
unit cell dimensions
1
a ) 12.0956(6) Å R )97.080(10)°
b ) 16.2223(8) Å â ) 100.920(10)°
c ) 17.3931(9) Å γ ) 91.990(10)°
13
1
13
3
volume
3319.6(3) Å
Z
2
3
19,20
6
density (calculated)
absorption coefficient
F(000)
1.089 mg/m
0.243 mm
1178
probably to ring current effect.
The η -arene ring-protons
-
1
are observed as a singlet and two doublets, consistent with 1,2,4-
substitution. These resonances are slightly broad at 25 °C but
become sharp at < -30 °C and > 40 °C. The broad peaks
could be a result of the arene oscillating or rotating about the
Ti-C6(ring) vector.
crystal size
θ range for data collection
index ranges
0.47 × 0.26 × 0.105 mm
3.05 to 27.92°
0 e h e 15, -21 e k e 21,
-
22 e l e 22
11600
reflections collected
6
Single-crystals of [(DMSC)Ti{η -1,2,4-C6H3(SiMe3)3}] (6)
independent reflections
refinement method
data/restraints/parameters
goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
15328 [R(int) ) 0.0000]
Full-matrix least-squares on F2
11590/17/685
suitable for an X-ray diffraction study were obtained by slow
evaporation from heptane at ambient temperature. The molecular
structure of 6 (Figure 1) confirmed that the calix[4]arene ligand
exists in 1,2-alternate conformation and that the SiMe3 groups
are 1,2,4-substituted about the η -arene. Selected metrical
parameters are listed in Table 2. The η -arene ligand is
characterized by substantial folding and a loss of aromatic
character; the dihedral angle between the C48-C49-C50-C51
plane and the C48-C47-C52-C51 plane is 29.7(7)°. This
folding suggests a strong contribution from the highly reduced
1.125
R1 ) 0.0783, wR2 ) 0.1730
R1 ) 0.0939, wR2 ) 0.1841
0.0000(9)
6
extinction coefficient
largest diff. peak and hole
6
-
3
0.788 and -0.580 e‚A
2
1
Scheme 1
cyclohexadiene dianion resonance structure (Figure 2). Although
6
6
may be conceived as a Ti(II)-η -arene complex, it possesses
significant Ti(IV) character and is probably more precisely
described as a 7-titananorbornadiene complex. The Ti-C bond
distances are reflective of this structure, C48 and C51 approach
Ti more closely (2.142(3) and 2.150(3) Å, respectively) than
2
2
the other four arene carbons (2.324-2.422 Å). The C-C
6
bonds in the η -arene ring also reflect the apparent loss of
Typical Procedure for Kinetic Study of the [2 + 2 + 2]
Cycloaddition of Trimethylsilylacetylene. A stock solution of [(DM-
aromaticity, having short C47-C52 and C49-C50 bonds and
longer C47-C48, C48-C49, C50-C51, and C51-C52 interac-
6
SC)Ti{η -1,2,4-C
6 3
H (SiMe
3
)
3
}] (6) (0.360 mL, 0.040 M) in C
D
6 6
2
3
(
0.0144 mmol of 6) was added into an NMR tube, followed by 0.057
tions (Table 2). As shown in Figure 1, the environment about
mL (0.403 mmol) of Me SiCtCH and then 0.183 mL of C . This
3
6 6
D
the titanium center in 6 may be described as pseudo-tetrahedral
with O1, O2, C48, and C51 as vertexes of the tetrahedron, see
resulted in 0.600 mL of a 0.024 M solution of 6. The tube was
(
vigorously shaken and placed into the spectrometer at a set temperature.
Table 2 for bond angles). One tetrahedral face is sterically
protected by the highly distorted 1,2-alternate DMSC ligand.
The severe distortion of the DMSC ligand is evidently due to
the steric requirements of the bulky SiMe3 groups of the 1,2,4-
substituted arene ring.
1
The first H NMR spectrum (at time ) 0) was recorded after the
temperature had stabilized. Spectra were recorded at various time
intervals thereafter. The time was measured with a timer. The
dependence of [2 + 2 + 2] cycloaddition of Me
3
SiCtCH on catalyst
concentration was obtained by varying the concentration of [(DMSC)-
6
Ti{η -1,2,4-C
6
H
3
(SiMe
3
)
3
}] (6) while conducting each experiment in
SiCt
Arene complexes of titanium are quite rare, and two main
classes of such species are known: Ti(0) sandwich complexes
C
6
D
6
at the same temperature, using an identical amount of Me
3
CH (0.071 mL, 0.500 mmol) and the same total volume (0.6 mL).
6
24-27
6
28-34
[
(η -arene)2Ti]
and Ti(II)-η -arene complexes.
In
Crystallographic Study. Crystal data and data collection parameters
are collected in Table 1. Further details of the crystallographic study
are given in the Supporting Information.
(
19) Fan, M.; Zhang, H.; Lattman, M. Organometallics 1996, 15, 5216.
(20) Gunther, H. NMR Spectroscopy: An Introduction; Wiley: New
York, 1980; pp 77-86.
21) For a discussion of structurally folded arenes, see: Arney, D. J.;
Wexler, P. A.; Wigley, D. E. Organometallics 1990, 9, 1282.
(
Results and Discussion
(
22) In this context, it is interesting to note that Ti-C48 and Ti-C51
6
Synthesis and Characterization of η -Arene Complexes.
distances are in the range expected for Ti(IV)-C(sp3) bonds. See for
example: Chesnut, R. W.; Durfee, L. D.; Fanwick, P. E.; Rothwell, I. P.;
Folting, K.; Huffman, J. C. Polyhedron 1987, 6, 2019.
(23) The short C-C bonds (C47-C52 and C49-C52) are slightly
elongated from C(sp2)-C(sp2) bond distance of 1.34 Å, while the longer
C-C bonds have lengths between those of benzene (1.395 Å) and a C(sp3)-
C(sp3) bond ) 1.54 Å. See: Loudon, G. M. Organic Chemistry, 3rd ed.;
Benjamin/Cummings: Redwood City, 1995; p 717.
1
6
The reduction of (DMSC)TiCl2 (1) with Mg(C14H10)(thf)3 or
activated Mg, in the presence of an excess of Me3SiCtCH led
to isolation of [(DMSC)Ti{η -1,2,4-C6H3(SiMe3)3}] (6) as a
diamagnetic yellow solid (70% yield with Mg* as reductant,
Scheme 1). When 1 was reduced with Mg(C14H10)(thf)3 in the
presence of <3 equiv of Me3SiCtCH, 6 was the only detectable
product. Reduction of 1 by Mg(C14H10)(thf)3 in the absence of
alkyne did not yield a clean product. The formulation and
6
(24) Anthony, M. T.; Green, M. L. H.; Young, D. J. Chem.. Soc. Dalton
Trans. 1975, 1419.
(25) Morand, P. D.; Francis, C. G. Inorg. Chem. 1985, 24, 56.

Cycloaddition of Terminal Alkynes
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6427
6
Figure 1. Molecular structure of [(DMSC)Ti{η -1,2,4-C
6 3 3 3
H (SiMe ) }] (6).
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 6
puckered arene structure (analogous to 6) with a dihedral angle
6
t
of 20.0°. Ti(0)-naphthalene complex [(η -C10H8)Ti{Bu Si(CH2-
Ti(1)-O(1)
1.829(2)
1.841(2)
2.142(3)
2.150(3)
2.340(3)
2.354(3)
2.379(3)
2.422(3)
1.388(5)
1.490(5)
1.476(5)
1.365(5)
1.460(5)
1.447(5)
O(1)-Ti(1)-O(2)
O(1)-Ti(1)-C(48)
O(2)-Ti(1)-C(48)
O(1)-Ti(1)-C(51)
O(2)-Ti(1)-C(51)
C(48)-Ti(1)-C(51)
100.41(11)
117.09(12)
113.53(12)
114.86(12)
127.67(12)
84.10(13)
27
Ti(1)-O(2)
PMe2)3}] was reported to have a dihedral angle of 12.4°. Also,
6
Ti(1)-C(48)
Ti(1)-C(51)
Ti(1)-C(52)
Ti(1)-C(49)
Ti(1)-C(47)
Ti(1)-C(50)
C(47)-C(52)
C(47)-C(48)
C(48)-C(49)
C(49)-C(50)
C(50)-C(51)
C(51)-C(52)
Wigley and colleagues have reported several (η -C6Me6)TaX3
i
(
X ) O-2,6-C6H3 Pr2 or Cl) complexes with dihedral angles in
21
the range of 26-34°. The folding of the arene ligand in these
complexes has been attributed to a strong metal to ligand δ
back-bonding interaction from a filled dx -y orbital into the
LUMO of the arene.
The [2 + 2 + 2] cycloaddition of Bu CtCH by [(DMSC)-
Ti{η -1,2,4-C6H3(SiMe3)3}] (6) gave [(DMSC)Ti{η -1,3,5-
C6H3Bu 3}] (7) as a diamagnetic orange solid in fair yield
2
2
2
1,36
t
6
6
t
(Scheme 1). In contrast to 6, a significant amount of 7 was not
produced when (DMSC)TiCl2 (1) was reduced with Mg-
t
(
C14H10)(thf)3 in the presence of an excess of Bu CtCH.
6
t
Presumably, the formation of η -1,2,4-C6H3Bu 3 is disfavored
because it results in increased steric repulsion between Bu
groups in comparison to η -1,3,5-C6H3Bu 3. In comparison
t
6
t 37
with 1,2,4-C6H3(SiMe3)3, greater repulsive interaction between
t
t
Bu groups should occur in 1,2,4-C6H3Bu 3 because the C-C
bond (1.54 Å) is significantly shorter than the C-Si bond (1.85
6
Figure 2. Cyclohexadiene dianion resonance structure of the η -arene
ligand of 6.
3
8
Å). Microanalysis data confirmed the formulation given for
13
7
. 1H and C NMR studies established that 7 is Cs-symmetric
6
comparison with the arene ligand in 6, Ti(II)-η -arene com-
in solution. The data are consistent with a symmetrically
6
plexes generally show nearly planar arene rings with longer
substituted η -arene and a 1,2-alternate calix[4]arene ligand.
2
8-34
35
t
Ti-C bonds (∼2.50 Å).
Arnold and co-workers have
recently reported a Ti-η -toluene complex supported by a
cyclohexane-linked bis(amidinate) ligand, which displayed a
Thus, the Bu groups of the calix[4]arene ligand are observed
6
as two singlets, and the bridging methylene protons show as
two pairs of doublets and an AB system (integrating as four
1
13
protons) in the H NMR spectrum. In the C NMR spectrum,
(
26) Hawker, P. N.; Kundig, E. P.; Timms, P. L. J. Chem.. Soc. Chem.
Commun. 1978, 730.
27) Gardner, T. G.; Girolami, G. S. Angew. Chem., Int. Ed. Engl. 1988,
7, 1693.
the bridging methylene carbons show as three peaks at δ 36.8,
(
2
(36) (a) Green, J. C.; Kelly, M. R.; Grebenik, P. D.; Briant, C. E.;
McEvoy, N. A.; Mingos, D. M. P. J. Organomet. Chem. 1982, 228, 239.
(b) Jorgensen, W. L.; Salem, L. The Organic Chemist’s Book of Orbitals;
Academic Press: New York, 1973; pp 257-259.
(
(
(
(
(
28) Fischer, E. O.; Rohrschied, F. J. Organomet. Chem. 1966, 6, 53.
29) Martin, F.; Vohwinkel, F. Chem. Ber. 1961, 94, 2416.
30) Thewalt, U.; Osterle, F. J. Organomet. Chem. 1979, 172, 317.
31) Thewalt, U.; Stollmaier, F. J. Organomet. Chem. 1982, 228, 149.
32) Calderazzo, F.; Ferri, I.; Pampaloni, G.; Englert, U.; Green, M. L.
(37) Molecular mechanics calculations (using CS Chem3D Pro version
3.2, CambridgeSoft Corporation) of the steric energies of 1,3,5- and 1,2,4-
t
H. Organometallics 1997, 16, 3100.
(
(
R3C6H3 (R ) Bu , SiMe3, or Ph) reveal that 1,3,5-substituted benzenes
33) Troyanov, S. J. Organomet. Chem. 1994, 475, 139.
34) Troyanov, S.; Polacek, J.; Antropiusova, H.; Mach, K. J. Organomet.
Chem. 1992, 431, 317.
35) Hagadorn, J. R.; Arnold, J. Angew. Chem., Int. Ed. 1998, 37, 1729.
possess lower steric energy in each case [∆∆H° ) ∆H (1,2,4-R3C6H3) -
∆H (1,3,5-R3C6H3)
(38) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry, 4th
ed.: Harper-Collins: New York, 1993.
(

6
3
428 J. Am. Chem. Soc., Vol. 122, No. 27, 2000
OzeroV et al.
8.8, and 39.9 (doubly intense), consistent with chemical shifts
Table 3. Catalytic [2 + 2 + 2] Cycloaddition of Terminal
Alkynes
previously reported by us for corresponding carbons of 1,2-
alternate DMSC-based titanium complexes.12,13 Analogous to
yield(%)c
entry
no.
6
and consistent with the structural characterization by Wigley
alkyne
1,2,4-isomer 1,3,5-isomer
15f
6
t
et al. of a folded arene for [(η -1,3,5-C6H3Bu 3)Ta(CH3)(O-
2
However, the NMR data does not permit such a structure to be
unambiguously established. The Bu groups of the η -1,3,5-C6H3-
a or b
99 (98)d
1
2
3
4
5
6
7
8
9
trimethylsilylacetylene
1-pentyne
1
4
1
1
i
,6-C6H3 Pr2)2], the arene ligand in 7 is most likely folded.
a or b
96
a or b
99 (95)d
Phenylacetylenea
t
6
p-tolylacetylene
99
100
100
100
t
1
1,6-heptadiynea
propargyl ethera
Bu 3 ligand are observed in the H NMR spectrum as a singlet
3
9
at δ 0.33, and the ring protons resonate as a singlet at δ 4.87.
Also, no new peaks were observed in the spectrum down to
70 °C in toluene-d8. The equivalence of these groups suggest
a
propargyl sulfide
N,N-dimethylpropargylaminea
90
10
<5
9 (17b)
-
O-trimethylsilylpropargyl alcohola
>95 (70)d
that the arene ligand does not adopt a static folded structure in
solution; and that interconversion among folded structures
10 1-(trimethylsilyl)-4-
91 (17a)
a or b
thiahepta-1,6-diyne
11 trimethylsilylacetylenee
95
5
4
0
probably occurs. Unfortunately, all attempts to obtain single
crystals of 7 suitable for an X-ray diffraction study have thus
far proven unsuccessful.
a
6
Catalyzed by [(DMSC)Ti{η -1,2,4-C
6
H
Bu
3
(SiMe
3
)
3
}] (6) in C
6
D
6
.
b
6
t
c
Catalyzed by [(DMSC)Ti{η -1,3,5-C
H
6 3
3
}] (7) in C
6
D
6
;. Ratios
1
d
e
6
determined from GC-MS and H NMR data. Isolated yield. Cata-
The η -arene complexes 6 and 7 are extremely sensitive to
6
6 3 3 3 6 6
lyzed by [(DMSC)Ti{η -1,2,4-C H (SiMe ) }] (6) in C D in the
oxidation and hydrolysis. Both compounds are soluble in
hydrocarbon solvents, especially aromatic hydrocarbon solvents
presence of THF.
(although 7 is not as soluble as 6). The compounds are unstable
in halogenated organic solvents: 6 is somewhat stable in CH2-
Cl2 with a half-life of several hours, while 7 undergoes fast
decomposition in CH2Cl2. The decomposition of 7 in CDCl3 is
t
extremely fast, yielding (DMSC)TiCl2 (1) and 1,3,5-C6H3Bu 3
quantitatively. 7 is generally less stable than 6. Thus, 6 is
thermally more stable than 7; decomposing slowly at 80 °C in
C6D6 (t1/2 ≈ 6 days) with release of 1,2,4-C6H3(SiMe3)3. The
decomposition rate was not affected by addition of either PMe3
or THF. Neither 6 nor 7 undergo arene ligand exchange with
C6D6 or toluene. Furthermore, 6 was not produced when 1 was
reduced with Mg(C14H10)(thf)3 in the presence of 1,2,4-C6H3-
conversion after 13 h; no decrease in the intensity of the signals
of 6 was observed during the course of the catalysis. H NMR
and GC-MS analysis of the reaction mixture revealed formation
of 1,2,4-C6H3(SiMe3)3 (8a) and 1,3,5-C6H3(SiMe3)3 (8b) in 99:1
ratio. Terminal diynes generally undergo [2 + 2 + 2] cycload-
dition at a faster rate than monoynes. For example, catalytic [2
1
+
2 + 2] cycloaddition of 1,6-heptadiyne (100 equiv) by 6 (5
(SiMe3)3.
µmol) is extremely exothermic and was complete in <5 min
with 12 as the sole product (eq 2).
Catalytic [2 + 2 + 2] Cycloaddition of Terminal Alkynes.
6
The formation of η -arene complexes by [2 + 2 + 2]
cycloaddition of alkynes is known;1
have been implicated in cyclotrimerization of alkynes. Both
and 7 are highly efficient catalysts for [2 + 2 + 2]
5f,k,21
and η -arene species
6
1
5
6
cycloaddition of terminal alkynes at 25 °C, producing 1,2,4-
substituted benzenes with excellent regioselectivity and in
excellent yield (Table 3). Even though 7 likely possesses less
Ti(IV) character than 6,41 it is generally less reactive toward
alkynes than 6 for steric reasons (vide infra) The catalytic [2 +
The rate and regioselectivity of the cyclotrimerization reac-
tions are influenced by the steric properties of the alkyne
substituent, although modest variation in the steric size of the
substituent does not adversely affect the regioselectivity (Table
2
+ 2] cycloaddition of various alkynes by 6 was investigated.
Typically, a mixture of 6 (5 µmol) and alkyne (100 equiv) in
1
C6D6 was monitored at 25 °C by H NMR. Generally, aliphatic
3
). The influence of the steric size of the alkyne substituent on
and aromatic terminal alkynes are cyclotrimerized in rapid and
exothermic fashion by 6. However, the reaction occurred more
slowly for Me3SiCtCH (200 equiv), proceeding to 98%
rate and regioselectivity is demonstrated by the reaction of 6
t
6
with an excess of Bu CtCH, which gave [(DMSC)Ti{η -1,3,5-
t
C6H6Bu 3}] (7) and no evidence of catalytic [2 + 2 + 2]
6
t
i
(
39) Arene protons of [(η -1,3,5-C6H3Bu 3)Ta(CH3)(O-2,6-C6H3 Pr2)2]
t
cycloaddition of Bu CtCH over 14 days at 25 °C. Also, no
1
5f
also show at δ 4.87 in C6D6.
(
i
reaction occurred between 6 and more bulky Pr 3SiCtCH over
40) Rapid ring rotation alone cannot account for the equivalence of these
groups in a static, folded structure. Wigley et al. have proposed intercon-
several hours at 80 °C. Furthermore, internal alkynes are rarely
cyclotrimerized, even at elevated temperatures. For instance,
93 h was required for 94% conversion of 65 equiv of 2-butyne
into C6Me6 at 25 °C, with 21% of the original catalyst (6)
6
version among folded structures, via planar η -arene intermediates, to
6
t
account for the equivalence of the arene protons in [(η -1,3,5-C6H3Bu 3)-
i
15f,k
Ta(CH3)(O-2,6-C6H3 Pr2)2]:
1
remaining intact (by H NMR). This suggests that the rate-
determining step of this cycloaddition proceeds at a faster rate
4
2
than the reaction between 6 and 2-butyne and that the
(42) We have isolated a product tentatively identified as [(DMSC)TiC4-
Me4Ti(DMSC)] from the reaction between 6 and 2-butyne. Complete
characterization and the role of this product in [2 + 2 + 2] cycloaddition
of 2-butyne are under investigation. For related compounds, see: Veldman,
M. E. E.; Van der Wal, H. R.; Veenstra, S. J.; De Liefde Meijer, H. J. J.
Organomet. Chem. 1980, 197, 59.
(
41) The greater electron-releasing character of the But group (in
comparison with SiMe3) should result in reduced metal-to-ligand σ back-
bonding.

Cycloaddition of Terminal Alkynes
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6429
6
unisolated intermediate [(DMSC)Ti(η -1,2,4-C6Me6)] reacts
faster with 2-butyne than 6. More bulky internal alkynes such
as EtCtCEt, and Me3SiCtCMe did not react with 6 at 80 °C
in benzene, nor did any reaction occur between 6 and
MeOCH2CtCCH2OMe, or Me3SiCtC(CH2)3CtCSiMe3 at 25
Scheme 2
°
C over several days. Similarly, catalytic [2 + 2 + 2]
cycloaddition was not observed when the reduction of (DMSC)-
TiCl2 (1) with Mg(C14H10)(thf)3 was carried out in the presence
of an excess of any of these internal alkynes.
6
[
(DMSC)Ti{η -1,2,4-C6H3(SiMe3)3}] (6) exhibits modest
functional group tolerance. For instance, 6 catalyzed [2 + 2 +
] cycloaddition of propargylic compounds, bearing oxygen,
selectivity for 18 probably reflects the fact that Me3SiCtCH
is a better dienophile than 1,6-heptadiyne and effectively
participates in reactions after rate-limiting displacement of arene
despite its steric bulk. However, analogous reactions with other
terminal alkynes generally yielded a distribution of products.
We also investigated cross-coupling reactions between Me3SiCt
2
nitrogen, or sulfur functionality HCtCCH2R (R ) NMe2,
OSiMe3, SCH2CtCH, OCH2CtCH, or SCH2CtCSiMe3), with
high regioselectivity (Table 3). The reaction of 6 with HCt
CCH2NMe2 (250 equiv) was complete in <3 min, giving a 90:
10 ratio of 1,2,4-C6H3(CH2NMe2)3 (15a) and 1,3,5-C6H3(CH2-
6
CH and olefins. At 25 °C, [(DMSC)Ti{η -1,2,4-C6H3(SiMe3)3}]
NMe2)3 (15b). The reduced regioselectivity is probably due to
coordination of the amine moiety to titanium, altering the
stereochemical environment at titanium (vide infra). Similarly,
the cyclotrimerization of HCtCCH2OSiMe3 by 6 gave 1,2,4-
C6H3(CH2OSiMe3)3 (16) in 95% yield. It is worth noting that
HCtCCH2OSiMe3 is similar to 1-pentyne in terms of its steric
influence on the course of the reaction since the Me groups are
three atoms away from the CtC bond; the SiMe3 group thus
serves mainly to protect the oxygen atom. The reaction of 6
with Me3SiCtCCH2SCH2CtCH in heptane gave the 1,3-
dihydrobenzothiophene derivative 17 as 1,2,3,4- and 1,2,3,5-
substitutional isomers (17a and 17b, respectively; eq 3) in 91:9
ratio and in 91% yield. The formation of 17b is likely due to
prior coordination of the sulfur atom to titanium (vide infra) or
the flexible three-atom linker group which permitted the required
orientation of the CtC bond.
(
6) catalyzed [2 + 2 + 2] cycloaddition of 4-chlorostyrene with
Me3SiCtCH to give the 1,3-cyclohexadiene 19 as the major
product (80%, GC-MS). Minor isomeric cyclohexadiene
products (∼8%, GC-MS) and 1,2,4-C6H3(SiMe3)3 (∼12%)
were also formed (eq 5). Cyclohexadiene isomers can differ by
double bond positions as well as substitution pattern.43,44 Hence
some of the minor cyclohexadiene isomers may have substit-
uents that are positioned “1,2,4” relative to one another but
possess different double bond positions. In this regard, several
studies have demonstrated that titanium aryloxide species can
44
isomerize 1,3-cyclohexadienes. Attempts to purify and isolate
9 (see Experimental Section) were unsuccessful. Similar
1
investigation of the cross-coupling reaction between Me3SiCt
CH and ethylene catalyzed by 6 produced a mixture of
4
5
products. Only cyclotrimerization of Me3SiCtCH occurred
in the presence of more bulky terminal or disubstituted olefins
including Me3SiCHdCH2, Ph2CdCH2, Cl2CdCCl2, and H2Cd
CMe-CMe)CH2.
Mechanistic Considerations. Much discussion has centered
on the nature of the intermediates involved in the [2 + 2 + 2]
cycloaddition reaction. Many studies have implicated metalla-
t
t
Interestingly, added Et2O, Et3N, Bu NH2, PCy3, PPh3, BuCtN,
t
Bu Me2SiCtN, SiMe3Cl, or SiMe3I did not affect the rate or
14,15
6
15
regioselectivity of [2 + 2 + 2] cycloaddition of Me3SiCtCH
catalyzed by 6. Perhaps in part, because of the relatively
crowded environment at titanium. Added THF or TMEDA
decreased the selectivity for 1,2,4-C6H3(SiMe3)3 during catalytic
cyclotrimerization of Me3SiCtCH by 6 (Table 3, entry 11),
although neither THF nor TMEDA independently react with 6.
This suggests that THF and TMEDA influence alkyne cyclo-
trimerization by reacting with coordinatively unsaturated inter-
mediates generated from 6.
Catalytic Cross-Coupling Reactions. The catalytic [2 + 2
2] cycloaddition of Me3SiCtCH with a variety of terminal
alkynes was investigated. The reaction of 6 (2 µmol) with 1,6-
heptadiyne (250 equiv) and Me3SiCtCH (250 equiv) at 25 °C
in benzene was found to be exothermic, producing the benzo-
cyclopentene derivative 18 almost quantitatively (eq 4). The
cyclopentadiene
or η -arene intermediates in the reaction.
Two pathways have been proposed to account for the reaction
of a metallacyclopentadiene intermediate with 1 equiv of alkyne
to produce the free arene product (Scheme 2): (i) the two new
C-C bonds are formed in a stepwise manner via a metallacy-
cloheptatriene intermediate and (ii) the two new C-C bonds
are formed in a concerted fashion (as in the Diels-Alder
reaction) via a metallanorbornadiene intermediate. Although the
(43) For a discussion of the NMR of 1,3-cyclohexadienes, see: The
Conformational Analysis of Cyclohexenes, Cyclohexadienes, and Related
Hydroaromatic Compounds; Rabideau, P. W., Ed.; VCH Publishers: New
York, 1989; Chapter 3.
+
(44) See for example: (a) Johnson, E. S.; Balaich, G. J.; Rothwell, I. P.
J. Am. Chem. Soc. 1997, 119, 7685. (b) Warantuke, S. A.; Johnson, E. S.;
Thorn, M. G.; Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc. Chem. Commun.
1
996, 2617. (c) Balaich, G. J.; Rothwell, I. P. J. Am. Chem. Soc. 1993,
115, 1581. (d) Warantuke, S. A.; Thorn, M. G.; Fanwick, P. E.; Rothwell,
A. P.; Rothwell, I. P. J. Am. Chem. Soc. 1999, 121, 9111.
(
45) GC-MS analysis of the reaction mixture following protonolysis
revealed isomeric mixtures of cyclohexadiene products as well as products
of cross-coupling between ethylene and cyclohexadiene species, and among
cyclohexadienes. See ref 44d for similar results.

6430 J. Am. Chem. Soc., Vol. 122, No. 27, 2000
OzeroV et al.
Scheme 3
Figure 3. Plot showing the disappearance of Me
during [2 + 2 + 2+] cycloaddition.
3
SiCtCH with time
steps in each mechanism have precedence, neither pathway has
been unambiguously established. Rothwell et al. have exten-
sively studied reactions of aryloxide-based titanacyclopenta-
dienes with alkynes and olefins;15g,44 and their results are best
interpreted in terms of a concerted pathway for the reactions.
Studies by Wigley et al. also suggest a concerted addition
pathway for the reactions of tantalacyclopentadiene and tanta-
knowledge, this study presents the first unambiguous demon-
stration of the involvement of an η -arene (7-titananorborna-
diene) species in the rate-limiting step of [2 + 2 + 2]
cycloaddition of alkynes.
6
15c,f
lanorbornadiene complexes supported by aryloxide ligation.
Bianchini and Caulton et al.46 also favored a concerted pathway
The product regiochemistry can be accounted for by a
mechanistic pathway in which the stereochemical environment
imposed at titanium by the DMSC ligand plays an important
role. Four different disubstituted titanacyclopentadiene inter-
mediates are possible (R,R′, R′,â, R,â′, â,â′) each possessing
different steric properties (Scheme 3). Both the R,R′- and R′,â-
substituted species would probably be disfavored because
incorporation of a bulky substituent in the endo-R′-position
would likely lead to unfavorable steric interactions with nearby
for the [2 + 2 + 2] cycloaddition of acetylene at iridium centers.
Recently, a theoretical study of the mechanism of the cyclo-
trimerization of acetylene by CpCoL2 (L ) CO, PR3, olefin)
concluded that the intermediacy of a cobaltocycloheptatriene
was energetically prohibitive and favored a concerted addition
4
7
pathway.
6
In the present work, we found that [(DMSC)Ti{η -1,2,4-C6H3-
SiMe3)3}] (6) is the resting state of the catalyst in the [2 + 2
(
+
2] cycloaddition of Me3SiCtCH. The catalysis occurs at a
t
aromatic rings and the attached Bu groups. Wigley and
convenient rate over a broad temperature range and yields 1,2,4-
C6H3(SiMe3)3 almost exclusively (vide supra). Kinetic studies
were conducted by adding an excess of Me3SiCtCH to a C6D6
solution of 6, and monitoring the reactions at various intervals
1
5f
colleagues have shown in a related tantalacyclopentadiene
system that the thermodynamic stability and kinetic accessibility
of the R,R′-substituted regioisomer is lost as steric congestion
at the metal center increases. We believe that the 1,2-alternate,
DMSC ligand promotes predominant (or quite possibly exclu-
sive) formation of R,â′-substituted titanacyclopentadiene. This
belief finds strong support in the cross-coupling reaction of
trimethylsilylacetylene with 4-chlorostyrene, which gave 19 as
the major cyclohexadiene product (∼91% of the cyclohexadiene
1
by H NMR spectroscopy. The reaction showed first-order
dependence on both [6] and [Me3SiCtCH], confirming that the
rate-limiting step is the displacement of 1,2,4-C6H3(SiMe3)3
from 6. A plot of the disappearance of Me3SiCtCH with time
is shown in Figure 3. Analysis of the kinetic data gave activation
q
q
parameters, ∆H ) 14 kcal/mol, and ∆S ) -11 cal/mol K,
consistent with an associative mechanism. That 6 did not react
1
products). H NMR data established that the SiMe3 groups in
1
9 are arranged in 1,3-fashion about the cyclohexadiene ring.43
i
with bulky terminal alkynes such as Pr 3SiCtCH, and most
internal alkynes may therefore be explained by the inability of
6
the substrates to displace the η -arene ligand. In fact, the reaction
rate is influenced by the steric requirements of both the alkyne
6
6
and the η -arene compound. Thus, [(DMSC)Ti{η -1,3,5-C6H3-
t
t
Bu 3}] (7) did not cyclotrimerize Bu CtCH but acts as a catalyst
precursor for [2 + 2 + 2] cycloaddition of less bulky alkynes
such as phenylacetylene and 1-pentyne, resulting in identical
regioselectivities as 6 (Table 3, entries 2 and 3). 7 apparently
The SiMe3 groups show as different singlets at δ 0.02 and 0.12,
and HA shows as a singlet at δ 6.77. HB and HE each show as
6
doublet of doublets at δ 5.90 and 3.33, respectively, while HD
reacts with these substrates to generate the corresponding η -
and HC each show as a doublet of doublets of doublets at δ
arene complex which does the catalysis. To the best of our
2
.48 and 2.12, respectively. This product can only be derived
(46) Bianchini, C.; Caulton, K. G.; Chardon, C.; Eisenstein, O.; Folting,
from reaction of an R,â′-substituted titanacyclopentadiene with
K.; Johnson, T. J.; Meli, A.; Peruzzini, M.; Rauscher, D. J.; Streib, W. E.;
Vizza, F. J. Am. Chem. Soc. 1991, 113, 5127.
4
-chlorostyrene.
Unlike â,â′-substituted titanacyclopentadiene, which would
exclusively yield 1,2,4-substituted benzene regardless of the
(47) Hardesty, J. H.; Koerner, J. B.; Albright, T. A.; Lee, G.-Y. J. Am.
Chem. Soc. 1999, 121, 6055.

Cycloaddition of Terminal Alkynes
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6431
Scheme 4
Table 10
R
1,3,5- R3C
H
6 3
kcal/mol 1,2,4- R3C
6
H
3
kcal/mol ∆∆H°
But
SiMe
Ph
11.0
-16.5
-18.0
33.2
-8.3
-16.0
22.5
8.2
2
3
titanium from pseudo-tetrahedral. Finally, a concerted pathway
best accounts for products of [2 + 2 + 2 ] cycloaddition
catalyzed by 6 and 7. Support for a concerted pathway can be
found in the results of the cross-coupling reaction of 4-chlo-
rostyrene with Me3SiCtCH. We did not observe products that
can be attributed to â-H migration in a titanacyclohepta-2,4-
4
9
diene intermediate. In contrast, the related complex [(2,6-
Ph2C6H3O)2Ti(CH2CMe)CMeCH2)] has been shown to cata-
lyze cross-coupling of 2,3-dimethyl-1,3-butadiene and ethylene
via â-H elimination from an isolable titanacyclohept-3-ene.3e
Conclusions
The effect of the DMSC ligand on the mechanism of Ti-
catalyzed [2 + 2 + 2] cycloaddition of alkynes is remarkable:
6
DMSC ligation resulted in isolation of Ti-η -arene complexes
(6 and 7) from [2 + 2 + 2] cycloaddition of terminal alkynes;
and 6 was the resting state of the catalyst in the [2 + 2 + 2]
cycloaddition of Me3SiCtCH. Furthermore, 1,2,4-substituted
benzenes are formed with excellent regioselectivity. These
results represent a sharp departure from those previously
observed for related titanium-aryloxide systems, where titana-
cyclopentadienes are usually formed as stable species and high
regioselectivity is rare. In 1,2-alternate conformation, the DMSC
ligand sterically defines the reaction sites at titanium; the
formation of R,â′-substituted titanacyclopentadiene intermediate
is apparently favored. The DMSC ligand also exerts steric
control over approach of a substrate to the R,â′-substituted
titanacyclopentadiene; the less hindered end of an olefin or
alkyne is directed into the calixarene cavity. Thus, the DMSC
ligand results in a dramatic change in the reaction mechanism
and regioselectivity in comparison with that previously observed
for Ti-aryloxide systems. Further reactivity studies of these
and related complexes are underway in our laboratory.
approach of the third alkyne molecule, R,â′-substituted titana-
cyclopentadiene could give either 1,2,4- or 1,3,5-substituted
benzene.48 The very high regioselectivity for 1,2,4-substituted
benzene presented in this report for catalytic [2 + 2 + 2]
cycloaddition of terminal alkynes suggests that the DMSC ligand
directs the reaction through the stereochemical environment it
imposes at titanium. For instance, the related titanium aryloxide,
t
[
+
(2,6-Ph2C6H3O)2Ti(C4H2Bu 2)], was shown to catalyze [2 + 2
2] cycloaddition of Me3SiCtCH to yield 1,3,5- and 1,2,4-
C6H3(SiMe3)3 in ∼95:5 ratio; and aliphatic alkynes such as
1
1
-pentyne and 1-hexyne, were cyclotrimerized to 1,3,5- and
,2,4-substituted arene products in ∼1:3 ratio. We believe
15g
the DMSC ligand promotes regioselective formation of 1,2,4-
substituted benzenes by exerting steric control over the approach
of the third alkyne molecule to the titanacyclopentadiene
intermediate. Thus, the preferred orientation for the third alkyne
molecule is one in which the bulky R group is pointing out of
the calix[4]arene cavity. The product distribution obtained from
catalytic [2 + 2 + 2] cycloaddition of Me3SiCtCCH2SCH2Ct
CH by 6 is consistent with this proposal. As previously
mentioned, 17a and 17b were produced in 91:9 ratio and in
excellent yield. This result clearly demonstrates that the reaction
of Me3SiCtCCH2SCH2CtCH with the titanacyclopentadiene
intermediate selectively occurs through the less-hindered ter-
minal alkyne end (Scheme 4, R ) CH2SCH2CtCSiMe3).
It is important to note that the regiochemistry of the arene
product is in fact determined by (at least) two competing
effects: (i) the directing influence of the DMSC ligand, and
Acknowledgment. Thanks are expressed to the Kentucky
National Science Foundation EPSCoR Program (Grant Number
EPS-9452895) for partial support of this work, as well as to
the Chemistry Department and the Graduate School of the
University of Kentucky for the award of fellowships to O.V.O.
Also, acknowledgment is made to the donors of the Petroleum
Research Fund, administered by the ACS, for partial support
of this research (Grant Number 33878-G3). The authors also
thank Professors Jack Selegue and Carol Brock for helpful
discussions.
(
ii) the steric interaction that would result from ortho-substitution
Supporting Information Available: A summary of crystal-
lographic parameters, atomic coordinates and equivalent iso-
tropic displacement parameters, bond lengths and angles,
anisotropic displacement parameters, and hydrogen coordinates
and isotropic displacement parameters for 6. Experimental and
characterization data for 8-15 and 18, as well as 1-(trimeth-
ylsilyl)-4-thia-1,6-heptadiyne (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
t
in the arene product. When the arene substituents are Bu groups,
the second effect apparently wins out, resulting in the formation
1
regioselectivity was noted when alkyne cyclotrimerization
reactions were catalyzed by 6 in the presence of a donor ligand
such as THF (vide supra). This is presumably due to attenuation
of the stereochemical influence of the DMSC ligand on the
reactivity of the intermediates generated from 6, as coordination
of THF (or other donor molecule) distorts the environment about
,3,5-substituted arene as in 7. As indicated earlier, a loss of
JA994543O
(49) This pathway would have to be a very minor one even if one allowed
(
48) See for example: Wielstra, Y.; Gambarotta, S.; Meetsma, A.; de
that the mixture of isomeric cyclohexadiene products (∼8% by GC-MS)
Boer, J. L.; Chiang, M. Y. Organometallics 1989, 8, 2696.
also contained a product derived a titanacyclohepta-2,4-diene intermediate.