Cp2Zr(η2-benzocyclobutadiene)(PMe3), a rare η2-cyclobutadiene complex

Article abstract of DOI:10.1021/om020829h

Cp₂Zr(η²-benzocyclobutadiene)(PMe₃), a rare η²-cyclobutadiene complex, was investigated. The stabilization of a cyclobutadiene by a rare η² coordination mode was described. The coupling reactions of the complex with alkynes, nitriles, and isocyanides yielded zirconocene cyclobutene derivatives, including a unique Dewar-naphthalene structure.

Full text of DOI:10.1021/om020829h

© Copyright 2002
Volume 21, Number 26, December 23, 2002
American Chemical Society
Communications
Cp 2Zr (η2-ben zocyclobu ta d ien e)(P Me3), a Ra r e
η2-Cyclobu ta d ien e Com p lex
T. V. V. Ramakrishna, Svetlana Lushnikova, and Paul R. Sharp*
Department of Chemistry, University of MissourisColumbia, Columbia, Missouri 65211
Received October 3, 2002
Sch em e 1
Summary: Thermal decomposition of Cp2ZrMe(1-
bromobenzocyclobutenyl) yields the η2-cyclobutadiene
complex Cp2Zr(η2-benzocyclobutadiene)(PMe3), which
cycloadds alkenes, nitriles, and t-BuNC, in the last case
giving a novel diamino-Dewar-naphthalene complex.
Zirconocene has been shown to stabilize reactive
organic fragments with retention of substantial reactiv-
ity and the generation of new reactivity patterns.1-3 In
this report we describe the stabilization of a cyclo-
butadiene by a rare η2 coordination mode. As a result,
new reactivity patterns become available and coupling
reactions of the complex with alkynes, nitriles, and
isocyanides yield zirconocene cyclobutene derivatives,
including a unique Dewar-naphthalene structure.
Utilizing the well-known ability of the Cp2ZrMe
fragment to activate C-H bonds,1 Cp2ZrMeCl was
treated with the Grignard reagent prepared from com-
mercially available 1-bromobenzocyclobutene.4 A smooth
reaction occurs, yielding yellow Cp2ZrMe(1-benzocyclo-
butenyl) (1; Scheme 1).
Complex 1 is stable at ambient temperatures but
slowly eliminates methane at 70 °C, producing, in the
presence of PMe3, the yellow η2-benzocyclobutadiene
complex 2 in 82% isolated yield (Scheme 1).5 An X-ray
structure determination6 confirms the identity of 2 and
establishes the rare η2 coordination of the cyclobutadi-
ene.7,8 The structure is similar to that observed for the
cyclobutene complex Cp2Zr(η2-cyclobutene)(PMe3),9 which
was prepared in an analogous manner, and shows a long
* To whom correspondence should be addressed. E-mail: SharpP@
missouri.edu.
(1) (a) Broene, R. D.; Buchwald, S. L. Science 1993, 261, 1696-1701.
(b) Buchwald, S. L.; Nielsen, R. B. Chem. Rev. 1988, 88, 1047-1058.
(2) Erker, G. J . Organomet. Chem. 1977, 134, 189.
(3) (a) Choukroun, R.; Cassoux, P. Acc. Chem. Res. 1999, 32, 494-
502. (b) Fillery, S. F.; Gordon, G. J .; Luker, T.; Whitby, R. J . Pure Appl.
Chem. 1997, 69, 633-638. (c) Majoral, J . P.; Meunier, P.; Igau, A.;
Pirio, N.; Zablocka, M.; Skowronska, A.; Bredeau, S. Coord. Chem. Rev.
1998, 180 (part 1), 145-167. (d) Ohff, A.; Pulst, S.; Lefeber, C.;
Peulecke, N.; Arndt, P.; Burkalov, V. V.; Rosenthal, U. Synlett 1996,
111-118.
(5) Reviews on cyclobutadiene complexes: (a) Baker, P. K.; Silgram,
H. Trends Organomet. Chem. 1999, 3, 21-33. (b) Efraty, A. Chem. Rev.
1977, 77, 691-744.
(4) Aldrich Chemical Co. Catalog.
10.1021/om020829h CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/16/2002

5686 Organometallics, Vol. 21, No. 26, 2002
Communications
Sch em e 2
Sch em e 4
Sch em e 3
An unfortunate feature of the previously reported
cyclobutene complex9 and other zirconocene alkene
complexes1b,10 is facile displacement of the coordinated
alkene by alkynes. This limits their synthetic usefulness
in insertion reactions. The cyclobutadiene complex 2
does not suffer from this, most probably because dis-
placement would result in the liberation of the unstable
antiaromatic cyclobutadiene.11 Thus, yellow solutions of
2 react with acetylenes at 40 °C (4 h) to yield the yellow
12
insertion products 3 and 4 (88% yield) and free PMe3
(Scheme 2). For the terminal alkyne product 4, the
alkyne R group is in the R-position. This is established
by a triplet signal in the 1H NMR spectrum for the
cyclobutane hydrogen atom at the â′-position, indicating
coupling to both the benzocyclobutene hydrogen atom
at the R′-position and the former alkyne hydrogen atom.
An X-ray crystal structure determination13 of 3 confirms
the cycloaddition of the alkyne.
Complex 2 also reacts with nitriles (Scheme 3).
Benzonitrile gives a mixture of the two possible regio-
isomers 5 and 6 (35:65) with what is believed to be the
nitrogen-bonded isomer 6 (R ) Ph) predominating. The
C-C bond for the Zr-bonded ene (d(C1-C2) ) 1.526(4)
Å). This indicates a major contribution from a zircona-
cyclopropane10 resonance form, the likely source of the
stabilization of the cyclobutadiene.
(10) (a) Negishi, E.; Takahashi, T. Rev. Heteroat. Chem. 1992, 6,
177-201. (b) Steigerwald, M. L.; Goddard, W. A., III. J . Am. Chem.
Soc. 1985, 107, 5027-5035.
(11) (a) Chapman, O. L.; Chang, C. C.; Rosenquist, N. R. J . Am.
Chem. Soc. 1976, 98, 261-262. (b) Winter, W.; Straub, H. Angew.
Chem., Int. Ed. Engl. 1978, 17, 127-128 and references therein.
(12) Periodic removal of the free PMe3 by exposing the reaction
mixture to vacuum maximizes the yield and purity of 3.
(6) X-ray data for 2 (173 K): C21H25PZr, a ) 8.5796(7) Å, b )
14.8697(12) Å, c ) 14.2085(12) Å, â ) 91.411(2)°, monoclinic, P21/n, Z
) 4. Data were collected using a Siemens SMART CCD diffractometer.
The data were refined on F2 (SHELXL) to R1 ) 0.0413. Selected
distances (Å) and angles (deg): Zr1-P1 ) 2.7022(9), Zr1-C1 )
2.346(3), Zr1-C2 ) 2.339(3), C1-C2 ) 1.526(4), C1-C7 ) 1.502(4),
C2-C8 ) 1.510(4), C3-C8 ) 1.376(4), C6-C7 ) 1.377(4), C7-C8 )
1.412(4); C2-Zr1-C1 ) 38.01(11), C2-C1-Zr1 ) 70.75(16), C2-C1-
C7 ) 87.9(2), C7-C1-Zr1 ) 113.27(19), C1-C2-C8 ) 87.8(2), C1-
C2-Zr1 ) 71.24(16), C8-C2-Zr1 ) 113.14(19), C4-C3-C8 ) 116.4(3),
C3-C4-C5 ) 121.8(3), C4-C5-C6 ) 121.7(3), C5-C6-C7 ) 116.4(3),
C1-C7-C6 ) 145.8(3), C1-C7-C8 ) 92.4(2), C6-C7-C8 ) 121.8(3),
C2-C8-C3 ) 146.2(3), C2-C8-C7 ) 91.9(2), C3-C8-C7 ) 121.9(3).
(7) Only one structurally characterized η2-cyclobutadiene complex
has been reported: Winter, W.; Stra¨hle, J . Angew. Chem., Int. Ed. Engl.
1978, 17, 128-129.
(13) X-ray data for 3 (173 K): C32H26Zr‚0.5C7H8, a ) 8.5915(16) Å,
b ) 19.945(4) Å, c ) 15.720(3) Å, â ) 98.407(4)°, monoclinic, P21/n, Z
) 4. Data were collected using a Siemens SMART CCD diffractometer.
The data were refined on F2 (SHELXL) to R1 ) 0.1101. Selected
distances (Å) and angles (deg): Zr1-C11 ) 2.250(12), Zr1-C26 )
2.298(10), C11-C12 ) 1.527(17), C11-C18 ) 1.631(15), C12-C13 )
1.47(2), C12-C17 ) 1.475(19), C13-C14 ) 1.26(2), C14-C15 ) 1.33(2),
C15-C16 ) 1.35(2), C16-C17 ) 1.51(2), C17-C18 ) 1.482(17), C18-
C19 ) 1.535(15), C19-C26 ) 1.347(14), C11-Zr1 ) C26 87.9(4); C12-
C11-C18 ) 86.1(8), C12-C11-Zr1 ) 119.0(9), C18-C11-Zr1 )
100.0(6), C13-C12-C17 ) 114.1(14), C13-C12-C11 ) 153.0(14),
C17-C12-C11 ) 92.2(10), C14-C13-C12 ) 119.4(16), C13-C14-
C15
) 127.8(17), C14-C15-C16 ) 119.1(18), C15-C16-C17 )
(8) Proposed η2-cyclobutadiene complexes: (a) Sanders, A.; Giering,
W. P. J . Organomet. Chem. 1976, 104, 49-65. (b) Sanders, A.; Magatti,
C. V.; Giering, W. P. J . Am. Chem. Soc. 1974, 96, 1610-1611.
(9) Fisher, R. A.; Buchwald, S. L. Organometallics 1990, 9, 871-
873.
116.8(18), C12-C17-C18 ) 93.7(11), C12-C17-C16 ) 119.0(14),
C18-C17-C16 ) 147.1(15), C17-C18-C19 ) 114.6(10), C17-C18-
C11 ) 87.9(9), C19-C18-C11 ) 121.7(9), C26-C19-C20 ) 124.2(9),
C26-C19-C18 ) 123.9(9), C20-C19-C18 ) 111.8(9), C19-C26-C27
) 122.6(9), C19-C26-Zr1 ) 106.3(7), C27-C26-Zr1 ) 131.0(7).

Communications
Organometallics, Vol. 21, No. 26, 2002 5687
formation of the carbon-bonded isomer 5 is exceptional
for reactions of zirconocene complexes with nitriles.1,14
With the bulkier t-BuCN a single regioisomer is pro-
duced in 91% yield, which an X-ray crystal structure15
study shows is the usual nitrogen-bonded isomer 6 (R
) t-Bu).
Two different products are obtained from the reaction
of 2 with t-BuNC. Which product is obtained depends
on the conditions of the reaction. Insertion product 7
(Scheme 4) is formed as the sole product when 2 is
added to a solution of 2 equiv of t-BuNC. The structure
of 7 was established by a single-crystal X-ray diffraction
study16 and reveals an unprecedented Dewar-naphtha-
lene structure17 with a new four-membered ring formed
by isocyanide insertion and coupling.18,19 Alternatively,
the ligand may be viewed as a diaza diene complex.20
The second product, 8, is obtained by slow addition of 2
equiv of t-BuNC to a solution of 2. The structure of 8
has not yet been established, but equivalent Cp groups
and inequivalent t-Bu groups are indicated by NMR
spectroscopy.
In conclusion, zirconocene is effective at stabilizing
benzocyclobutadiene by a rare η2-cyclobutadiene coor-
dination mode. The complex shows excellent reactivity,
allowing coupling of alkynes and nitriles with the cyclo-
butadiene moiety. Double insertion and coupling of
tert-butyl isocyanide yields a novel diamino-Dewar-
naphthalene complex. Many more cyclobutadiene moi-
ety coupling products may be anticipated with this
system.
(14) (a) Fagan, P. J .; Nugent, W. A. J . Am. Chem. Soc. 1988, 110,
2310-2312. (b) Takahashi, T.; Xi, C.; Xi, Z.; Kageyama, M.; Fischer,
R.; Nakajima, K.; Negishi, E. J . Org. Chem. 1998, 63, 6802-6806.
(15) X-ray data for 6 (173 K):
C21H25PZr, a ) 13.396(2) Å, b )
16.028(3) Å, c ) 9.6200(16) Å, â ) 108.487(3)°, monoclinic, P21/c, Z )
4. Data were collected using a Siemens SMART CCD diffractometer.
The data were refined on F2 (SHELXL) to R1 ) 0.0310. Selected
distances (Å) and angles (deg): Zr1-N1 ) 2.0509(17), Zr1-C2 )
2.349(2), N1-C9 ) 1.273(3), C1-C9 ) 1.523(3), C1-C8 ) 1.535(3),
C1-C2 ) 1.611(3), C2-C3 ) 1.499(3), C3-C4 ) 1.388(3), C3-C8 )
1.394(3), C4-C5 ) 1.391(3), C5-C6 ) 1.393(4), C6-C7 ) 1.402(4),
C7-C8 ) 1.377(3); N1-Zr1-C2 ) 78.29(8), C9-N1-Zr1 ) 122.83(15),
C9-C1-C8 ) 117.70(17), C9-C1-C2 ) 113.00(17), C8-C1-C2 )
85.56(16), C3-C2-C1 ) 86.15(15), C3-C2-Zr1 ) 123.21(14), C1-
C2-Zr1 ) 104.94(13), C4-C3-C8 ) 121.1(2), C4-C3-C2 ) 143.7(2),
C8-C3-C2 ) 95.18(18), C3-C4-C5 ) 116.9(2), C4-C5-C6 ) 121.7(2),
C5-C6-C7 ) 121.4(2), C8-C7-C6 ) 116.3(2), C7-C8-C3 ) 122.6(2),
Ack n ow led gm en t. Grants from the National Sci-
ence Foundation supported this work (Grant No. CHE-
0101348) and provided a portion of the funds for the
purchase of the NMR (Grant No. CHE 9221835) equip-
ment. We thank a reviewer for pointing out the diaza
diene view of 7.
Su p p or tin g In for m a tion Ava ila ble: Text and tables
giving characterization data for 1-8, including X-ray crystal-
lographic data for 2, 3, 6 (R ) t-Bu), and 7; X-ray data are
also available as CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
C7-C8-C1
) 144.5(2), C3-C8-C1 ) 92.89(18), N1-C9-C1 )
118.22(19).
(16) X-ray data for 7 (173 K): C28H34N3Zr, a ) 8.8770(13) Å, b )
27.103(4) Å, c ) 30.421(4) Å, orthorhombic, P212121, Z ) 12. Data were
collected using a Siemens SMART CCD diffractometer. The data were
refined on F2 (SHELXL) to R1 ) 0.0413. Selected distances (Å) and
angles (deg): Zr1-N1 ) 2.130(6), Zr1-N2 ) 2.152(7), Zr1-C9 )
2.470(7), Zr1-C10 ) 2.479(8), N1-C9 ) 1.375(10), N1-C15 ) 1.497(9),
N2-C10 ) 1.338(10), N2-C11 ) 1.498(10), C1-C9 ) 1.529(10), C1-
C8 ) 1.534(12), C1-C2 ) 1.555(15), C2-C3 ) 1.568(13), C2-C10 )
1.592(11), C3-C8 ) 1.327(14), C3-C4 ) 1.409(12), C4-C5 ) 1.394(14),
C5-C6 ) 1.396(17), C6-C7 ) 1.326(14), C7-C8 ) 1.395(12), C9-C10
) 1.402(11); N1-Zr1-N2 ) 89.1(3), N1-Zr1-C9 ) 33.8(2), N2-Zr1-
C9 ) 63.4(2), N1-Zr1-C10 ) 64.4(3), N2-Zr1-C10 ) 32.6(3), C9-
Zr1-C10 ) 32.9(3), C9-N1-C15 ) 120.8(6), C9-N1-Zr1 ) 86.8(4),
C15-N1-Zr1 ) 148.7(5), C10-N2-C11 ) 117.9(8), C10-N2-Zr1 )
87.2(5), C11-N2-Zr1 ) 150.7(7), C9-C1-C8 ) 115.0(7), C9-C1-C2
) 86.4(6), C8-C1-C2 ) 89.2(8), C1-C2-C3 ) 82.4(8), C1-C2-C10
) 87.9(6), C3-C2-C10 ) 115.7(7), C8-C3-C4 ) 122.9(10), C8-C3-
C2 ) 96.7(8), C4-C3-C2 ) 140.3(11), C5-C4-C3 ) 112.8(12), C4-
C5-C6 ) 123.4(11), C7-C6-C5 ) 121.3(12), C6-C7-C8 ) 116.6(12),
C3-C8-C7 ) 122.9(10), C3-C8-C1 ) 91.6(8), C7-C8-C1 ) 145.5(12),
N1-C9-C10 ) 126.0(6), N1-C9-C1 ) 136.2(7), C10-C9-C1 )
96.2(7), N1-C9-Zr1 ) 59.4(4), C10-C9-Zr1 ) 73.9(4), C1-C9-Zr1
) 137.3(5), N2-C10-C9 ) 126.4(7), N2-C10-C2 ) 142.6(8), C9-
C10-C2 ) 89.5(7), N2-C10-Zr1 ) 60.2(4), C9-C10-Zr1 ) 73.2(4),
C2-C10-Zr1 ) 136.0(5).
OM020829H
(17) Fisher and Buchwald obtained a double t-BuNC insertion
product from the cyclobutene complex that may have
a similar
structure, but in the absence of an X-ray crystal structure determi-
nation the complex was formulated as a simple diiminoacyl insertion
product.9
(18) Titanacyclobutane complexes add isocyanides in
a similar
fashion to produce five-membered rings: Greidanus-Strom, G.; Carter,
C. A. G.; Stryker, J . M. Organometallics 2002, 21, 1011-1013.
(19) Isocyanide coupling reviews and recent references: (a) Carna-
han, E. M.; Protasiewicz, J . D.; Lippard, S. J . Acc. Chem. Res. 1993,
26, 90-97. (b) Durfee, L. D.; Rothwell, I. P. Chem. Rev. 1988, 88, 1059-
1079. (c) Ong, T.-G.; Wood, D.; Yap, G. P. A.; Richeson, D. S.
Organometallics 2002, 21, 1-3. (d) Thorn, M. G.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1999, 18, 4442-4447. (e) Tomaszewski,
R.; Lam, K.-C.; Rheingold, A. L.; Ernst, R. D. Organometallics 1999,
18, 4174-4182.
(20) (a) Scholz, J .; Dlikan, M.; Stroehl, D.; Dietrich, A.; Schumann,
H.; Thiele, K. H. Chem. Ber. 1990, 123, 2279-2285. (b) Latesky, S. L.;
McMullen, A. K.; Niccolai, G. P.; Rothwell, I. P.; Huffman, J . C.
Organometallics 1985, 4, 1896-1898.