Ring-closing alkyne metathesis with simple catalyst systems: An access to molecular triangles and rhomboids

Article abstract of DOI:10.1039/a908638b

Treatment of the siloxane monomer 2 with a mixture of molybdenum hexacarbonyl and 4-chlorophenol at 140 °C furnished the corresponding cyclotrimer 3 and the cyclotetramer 4.

Full text of DOI:10.1039/a908638b

Ring-closing alkyne metathesis with simple catalyst systems: an access to
molecular triangles and rhomboids
Neil G. Pschirer, Wei Fu, Richard D. Adams* and Uwe H. F. Bunz*
Department of Chemistry and Biochemistry, The University of South Carolina, Columbia, South Carolina 29208
USA. E-mail: bunz@psc.sc.edu
Received (in Columbia, MO, USA) 26th October 1999, Accepted 30th November 1999
Treatment of the siloxane monomer 2 with a mixture of
molybdenum hexacarbonyl and 4-chlorophenol at 140 °C
furnished the corresponding cyclotrimer 3 and the cyclo-
tetramer 4.
Ring-closing alkene metathesis utilizing well-defined organo-
metallic catalysts1–4 has developed into a powerful synthetic
tool. A variety of rings have been prepared by this method.
Cyclic polyynes are also of great interest,5 but their synthesis
via ring-closing alkyne metathesis is much less explored.6
Scheme 1 Reagents and conditions: i, 4-IC6H4OH, Et3N, THF; ii, propyne,
Fu¨rstner7 has reported the preparation of large-ring alkynes by
Pd(PPh3)2Cl2, CuI, piperidine; iii, Mo(CO)6, 4-ClC6H4OH.
ring-closure of suitable dipropynylated precursors utilizing
Schrock’s tungsten carbyne.8
We have recently developed an efficient and simple protocol
for alkyne metathesis9,10 and we now report the first examples
of ring-closing metathesis using our ‘instant’ catalyst. This
powerful catalyst is formed in situ from Mo(CO)6 and
4-chlorophenol using off-the-shelf quality solvents.
The synthesis of shape-persistent macrocycles, potentially
useful as molecular boxes, presents a new and interesting
challenge.11,12 We have chosen diyne 2 to be a convenient
precursor to the novel molecular siloxane triangles/boxes 3 and
4. Compound 2 was prepared in two steps from 1, which was,
itself, obtained from 4-iodophenol by treatment with Pri2SiCl2
in Et3N (Scheme 1). A palladium-catalyzed coupling of 1 with
propyne affords the monomer 2 in 61% yield.13 Alkyne
metathesis of 2 with Mo(CO)6 and 4-chlorophenol at 140 °C in
1,2-dichlorobenzene under a steady stream of N2 (Scheme 1)
furnished the novel cycles 3 and 4.† Sufficient dilution is an
important factor for obtaining cyclic rather than polymeric
products, although minor changes in concentration did not
result in decreased yields of the macrocycles. Separation was
difficult due to the similar retention times of 3 and 4, and their
corresponding open congeners. However, we were able to
isolate 3 (14% yield) and 4 (18% yield). GC-MS confirmed that
the isolated fractions of 3 and 4 were uncontaminated by other
cycloisomers. The molecular structures of 3 and 4 were
established by single crystal X-ray diffraction analyses.‡
The crystal of 3 contains three symmetry-independent
molecules. All three display similar conformations and the
molecular structure of one of these is shown in Fig. 1. The
cyclotetramer 4 is crystallographically centrosymmetric. It
exhibits a rhomboidal structure with a large interior cavity that
contains two molecules of hexane as solvate, from which it was
crystallized (Fig. 2). This feature indicates the potential of these
box-shaped molecules to engage in important host–guest
chemistry.
Fig. 1 A structural diagram of the cyclic triyne 3. Selected bond distances
(Å) and angles (°) are C·C = 1.22(2), 1.20(2) and 1.16(2), Si(1)–O(1) =
1.616(9), Si(1)–O(2) = 1.624(9), Si(2)–O(5) = 1.67(1), Si(2)–O(6) =
1.63(1), Si(3)–O(3) = 1.63(1), Si(3)–O(4) = 1.66(1); O(1)–Si(1)–O(2) =
109.8(5), O(5)–Si(2)–O(6) = 106.9(5), O(3)–Si(3)–O(4) = 108.1(6).
Chem. Commun., 2000, 87–88
This journal is © The Royal Society of Chemistry 2000
87

dH(400 MHz, CDCl3) 7.35 (d, J 8.8, 16H), 6.88 (d, J 8.8, 16H), 1.40 (m,
8H), 1.09 (d, J 7.3, 48H); dH(400 MHz, CDCl3) 154.4, 132.9, 119.7, 116.8,
88.1, 17.2, 12.7.
‡ Crystal data: for 3: Si3O6C60H68, triclinic, M = 936.12 g cm23, space
¯
group = P1 (No. 2). T = 293 K, Mo-Ka, a = 23.76(2), b = 24.07(3), c =
18.7192(5) Å, a = 92.0(1), b = 96.25(7), g = 93.70(9)°, Z = 6, D = 1.083
g cm23, m = 0.13 cm21, 15889 measured reflections, 7703 independent
reflections, R (Rw) = 0.091 (0.126), hydrogen atoms calculated, not refined.
¯
For 4: Si4O8C80H176·2(C6H14), triclinic, M = 1378.60, Space group = P1
(No. 2), T = 293 K, Mo-Ka, a = 13.977(8), b = 14.25(1), c = 12.144(7)
Å, a = 106.29(5), b = 108.21(5), g = 69.68(5)°, Z = 1, D = 1.083 g cm23
,
m = 0.12 cm21, 4534 measured reflections, 2740 independent reflections,
R (Rw) = 0.0599 (0.0814). Hydrogen atoms calculated, not refined. CCDC
182/1494. See http://www.rsc.org/suppdata/cc/a9/a908638b/ for crystallo-
graphic data in .cif format.
1 R. R. Schrock, Polyhedron, 1995, 14, 3177.
2 R. H. Grubbs and S. Chang, Tetrahedron, 1998, 54, 4413 and references
therein.
3 R. H. Grubbs, S. J. Miller and G. C. Fu, Acc. Chem. Res., 1995, 28,
446.
4 M. Schuster and S. Blechert, Angew. Chem., Int. Ed. Engl., 1997, 36,
2036; A. Fu¨rstner, Top. Catal., 1997, 4, 285.
5 R. Gleiter and R. Merger, in Modern Acetylene Chemistry, ed. P. J.
Stang and F. Diederich, VCH, Weinheim, 1995, ch. 8; L. T. Scott and
M. J. Cooney, in Modern Acetylene Chemistry, ed. P. J. Stang and F.
Diederich, VCH, Weinheim, 1995, ch. 9.
6 U. H. F. Bunz and L. Kloppenburg, Angew. Chem., Int. Ed., 1999, 38,
478.
7 A. Fu¨rstner and G. Seidel, Angew. Chem., Int. Ed., 1998, 37, 1734.
8 R. R. Schrock, D. N. Clark, J. Sancho, J. H. Wengrovius and S. F.
Pedersen, Organometallics, 1982, 1, 1645; X.-I. Zhang and G. C. Bazan,
Macromolecules, 1994, 27, 4627.
9 L. Kloppenburg, D. Song and U. H. F. Bunz, J. Am. Chem. Soc., 1998,
120, 7973; L. Kloppenburg, D. Jones and U. H. F. Bunz, Macromole-
cules, 1999, 32, 4194.
Fig. 2 A structural diagram of the cyclic tetrayne 4 showing two molecules
of occluded hexane in the interior of the ring. Selected bond distances (Å)
and angles (°) are C·C = 1.179(9) and 1.213(9), Si(1)–O(1) = 1.635(4),
Si(1)–O(2) = 1.646(4), Si(2)–O(3) = 1.654(4), Si(2)–O(4) = 1.652(4);
O(1)–Si(1)–O(2) = 110.6(2), O(3)–Si(2)–O(4) = 109.9(2).
We have now demonstrated that ring-closing alkyne met-
athesis with our ‘instant’ catalyst system provides a convenient
route to novel alkyne-containing macrocycles. Functionalized
rings should also be accessible via this methodology as we have
already shown that the mixtures of Mo(CO)6 and 4-chloro-
phenol are metathesis-active in the presence of a variety of
different functional groups.8,9
This work was generously supported by the NSF (CHE
9814118, PI Bunz) and the Research Corporation (PI Bunz).
10 N. G. Pschirer and U. H. F. Bunz, Tetrahedron Lett., 1999, 40, 2481.
11 P. J. Stang and B. Olenyuk, Acc. Chem. Res., 1997, 30, 502; P. J. Stang,
Chem. Eur. J., 1998, 4, 19; B. Olenyuk, A. Fechenko¨tter and P. J. Stang,
J. Chem. Soc., Dalton Trans., 1998, 1707; B. Olenyuk, A. J. A.
Whiteford, A. Fechenko¨tter and P. J. Stang, Nature, 1999, 398, 796.
12 For similar, conjugated silicon containing macrocycles, see F. Q. Liu, G.
Harder and T. D. Tilley, J. Am. Chem. Soc., 1998, 120, 3271; S. S. H.
Mao, F. Q. Liu and T. D. Tilley, J. Am. Chem. Soc., 1998, 120, 1193.
13 M. Altmann and U. H. F. Bunz, Angew. Chem., Int. Ed. Engl., 1995, 34,
569.
Notes and references
† Sample cyclization: 2 (1.00 g, 2.66 mmol), Mo(CO)6 (0.66 g, 0.27 mmol),
4-chlorophenol (0.206 g, 1.60 mmol) and 1,2-dichlorobenzene (100 ml)
were held at 140 °C under a steady stream of N2 for 17 h. The solution was
allowed to cool, then dissolved in hexanes and washed with dilute acid and
base. Separation of the resulting products was achieved by chromatography
over silica gel (Merck silica gel 60, 40–63 mm particle size; eluent 3+1
hexanes–CH2Cl22). Selected data for 3: dH(400 MHz, CDCl3) 7.31 (d, J
8.6, 12H), 6.71 (d, J 8.6, 12H), 1.30 (m, J 7.1, 6H), 1.16 (d, J 7.1, 36H);
dC(400 MHz, CDCl3) 154.1, 132.7, 119.9, 116.8, 88.2, 17.2, 12.7. For 4:
Communication a908638b
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Chem. Commun., 2000, 87–88