Trimerization of tert-butylacetylene to 1,3,6-tri(tert-butyl)fulvene catalyzed by titanium aryloxide compounds

Full text of DOI:10.1021/ja972306k

11086
J. Am. Chem. Soc. 1997, 119, 11086-11087
Communications to the Editor
Scheme 1
Trimerization of tert-Butylacetylene to
1,3,6-Tri(tert-butyl)fulvene Catalyzed by Titanium
Aryloxide Compounds
Eric S. Johnson, Gary J. Balaich, Phillip E. Fanwick, and
Ian P. Rothwell*
Department of Chemistry
1393 Brown Building, Purdue UniVersity
West Lafayette, Indiana 47907-1393
ReceiVed July 11, 1997
The [2 + 2 + 2] cycloaddition of alkyne segments to produce
the arene nucleus is one of the most ubiquitous and studied
reactions in organo-transition-metal chemistry.1,2 In contrast
there are only scattered reports of the stoichiometric conversion
of three alkyne units into the fulvene nucleus via a formal
[2+2+1] cycloaddition.3,4 In this paper, we report on our initial
synthetic and mechanistic studies of a titanium aryloxide catalyst
system which selectively converts tert-butylacetylene into the
corresponding fulvene.
Scheme 2
The titanacyclopentadiene compound [(2,6-Ph2C6H3O)2Ti-
(C4H2But2)] 1 (Table 1; 2,6-Ph2C6H3O ) 2,6-diphenylphenox-
ide) has been demonstrated to be a catalyst for the slow
cyclotrimerization of tert-butylacetylene into 1,3,5-tri-(tert-
butyl)benzene (2).5 When one or more equivalents of LiC≡CBut
is mixed with 1 in benzene solution the resulting system causes
the catalytic production of 1,3,6-tri(tert-butyl)fulvene (3) along
with dimer 4 and smaller amounts of an as yet unidentified
alkyne oligomer (5) (Table 1).6 This catalysis can be more
conveniently carried out without isolation of 1 by activating
one of the dichlorides [(ArO)2TiCl2] 6 (ArO ) 2,6-diphenylphe-
noxide,7 a; 2,6-diisopropylphenoxide,6,8 b; 2,6-dimethylphe-
noxide,6 c) with >2 equiv of LiC≡CBut (Table 1). The result
of heating (100 °C sealed vessel) a mixture of these components
with HC≡CBut in benzene can be monitored by GC to show
the catalytic buildup of products over time (Figure 1). Although
a small amount of arene 2 is initially produced, it is rapidly
exceeded by fulvene 3. The ratio of 3:4:5 produced throughout
the reaction remains almost constant. The purification of dimer
4 can be achieved by vacuum distillation, while bright yellow
fulvene 3 can be separated by chromatography. The potential
utility of fulvene 3 is demonstrated by its reaction with alkylating
Figure 1. Plot showing the buildup with time of 2 (circle), fulvene 3
(square), 4 (diamond), and 5 (triangle) produced at 100 °C from
HC≡CBut (30 mL, 244 mmol) using 6b (1.00 g, 2.1 mmol) activated
with LiC≡CBut (0.74 g, 8.40 mmol) in benzene (6 mL).
(1) (a) Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539.
(b) Vollhardt, K. P. C. Pure Appl. Chem. 1993, 65, 153. (c) Bose, R.;
Matzger, A. J.; Mohler, D. L.; Vollhardt, K. P. C. Angew. Chem., Int. Ed.
Engl. 1995, 34, 1478. (d) Bianchini, C.; Caulton, K. G.; Chardon, C.;
Doublet, M.-L.; Eisenstein, O.; Jackson, S. A.; Johnson, T. J.; Meli, A.;
Peruzzini, M.; Streib, W. E.; Vacca, A.; Vizza, F. Organometallics 1994,
13, 2010.
agents (eq 1) to produce (after workup) the corresponding bulky
cyclopentadienes 7 and 8 (structurally characterized).6,9
A variety of mechanistic pathways leading to fulvene
formation can be envisaged including those involving vinylidene
intermediates.3,4 The stoichiometric reaction of 6a with 2 equiv
of LiC≡CBut produces the bis(alkynyl) 9 which is then
converted by an extra equivalent of LiC≡CBut to the mixed
(9) Crystallographic data for C25OH38 8a at 296 K: a ) 13.702(3), b )
10.326(1), and c ) 17.255(3) Å, â ) 111.74(2)°, V ) 2267(1) Å3, Z ) 4
in space group P21/c. Data for TiLiC54O2H53 10 at 296 K: a ) 10.800(4),
b ) 40.398(9), and c ) 11.646(4) Å, â ) 117.41(3)°, V ) 4510(5) Å3, Z
) 4 in space group P21/n.
(2) Smith, D. P.; Strickler, J. R.; Gray, S. D.; Bruck, M. A.; Holmes, R.
S.; Wigley, D. E. Organometallics 1992, 11, 1275.
(3) O’Connor, J. M.; Hiibner, K.; Merwin, R.; Gantzel, P. K.; Fong, B.
S.; Adams, M.; Rheingold, A. L. J. Am. Chem. Soc. 1997, 119, 3631.
(4) (a) Moran, G.; Green, M.; Orpen, A. G. J. Organomet. Chem. 1983,
250, C15. (b) Moreto, J.; Maruya, K.; Bailey, P. M.; Maitlis, P. M. J. Chem.
Soc., Dalton Trans. 1982, 1341.
(5) Balaich, G. J.; Hill, J. E.; Waratuke, S. A.; Fanwick, P. E.; Rothwell,
I. P. Organometallics 1995, 14, 656.
(6) Experimental details and characterization data are contained in the
Supporting Information.
(7) Dilworth, J. R.; Hanich, J.; Krestel, M. J. Organomet. Chem. 1986,
315, 9.
(8) No¨th, H.; Schmidt, M. Organometallics 1995, 14, 4601.
S0002-7863(97)02306-8 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 45, 1997 11087
Table 1. Product Distribution (%) from the Oligomerization of HC≡CBut using Various Titanium Catalysts6
followed by liberation of fulvene by CH bond activation (either
σ-bond metathesis or insertion) on an alkyne unit (Scheme 2).
In the case of the [(ArO)2TiCl2]/3LiC≡CBut systems, initial
coupling of acetylides to a diyne preceed titanacyclopentadiene
formation. The production of dimer 4 can be similarly
accounted for by η2-alkyne/acetylide coupling. Attempts to
utilize less bulky terminal alkynes has been found to lead to
oligomeric products. Further studies of this reactivity are
underway.
(1)
Acknowledgment. We thank the National Science Foundation
(Grant CHE-9700269) for financial support of this research.
lithium/titanium ate species 10 (Scheme 1).6 The formulation
of 10 is based upon structural studies9 as well as its hydrolysis
to yield the phenol 11. The production of 10 may arise via
initial coupling of acetylide units to a diyne followed by
cyclometalation and reductive coupling. Compound 10 acts as
a single component catalyst producing a similar blend of
products as produced by [(ArO)2TiCl2]/LiC≡CBut mixtures.
These observations lead us to propose that the catalytic
reaction proceeds via titanacyclopentadiene/acetylide coupling
Supporting Information Available: Experimental details of the
synthesis of new compounds, description of the experimental procedures
for X-ray diffraction studies, ORTEP drawings and tables of thermal
parameters, bond distances and angles, intensity data, torsion angles,
and mutiplicities for 10 and 8a (33 pages). See any current masthead
page for ordering and Internet access instructions.
JA972306K