Thermal Isomerization of the Buchwald Seven-Membered
Zirconacyclocumulene and Its Interaction with Acetylenes. Synthesis
and Structures of Novel Seven-Membered Zirconacyclocumulene Complexes
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Abstract
On
prolonged heating of the Buchwald seven-membered zirconacyclocumulene Cp<sub>2</sub>Zr[η<sup>4</sup>-Me<sub>3</sub>SiC<sub>4</sub>(SiMe<sub>3</sub>)–C(C<sub>2</sub>SiMe<sub>3</sub>)CSiMe<sub>3</sub>] (<b>1</b>) in toluene
at 100 °C, the isomerization of <b>1</b> takes
place with the formation of a seven-membered zirconacyclocumulene
complex, Cp<sub>2</sub>Zr[η<sup>4</sup>-Me<sub>3</sub>SiC<sub>4</sub>(SiMe<sub>3</sub>)–C(SiMe<sub>3</sub>)C(C<sub>2</sub>SiMe<sub>3</sub>)] (<b>8</b>), containing the Me<sub>3</sub>SiCC group in α-position and the SiMe<sub>3</sub> group in β-position with respect to the Zr atom. A remarkable
peculiarity of complex <b>8</b> is its ability to undergo a
rapid degenerate rearrangement at room temperature in toluene solution,
as a result of which only two rather than four singlets of the Me<sub>3</sub>Si groups are observed in the <sup>1</sup>H NMR spectrum of <b>8</b> under such conditions. At temperatures below 200 K, the
rate of this rearrangement decreases and, as a consequence, the <sup>1</sup>H NMR spectrum of <b>8</b> acquires the normal view.
If <b>1</b> is heated at 100 °C in the presence of acetylenes
such as 1,4-bis(<i>tert</i>-butyl)butadiyne (<sup><i>t</i></sup>BuCC–CC<sup><i>t</i></sup>Bu) and tolane, the corresponding seven-membered zirconacyclocumulenes,
viz., Cp<sub>2</sub>Zr[η<sup>4</sup>-<sup><i>t</i></sup>BuC<sub>4</sub>(<sup><i>t</i></sup>Bu)–C(C<sub>2</sub>SiMe<sub>3</sub>)CSiMe<sub>3</sub>] (<b>7</b>) and Cp<sub>2</sub>Zr[η<sup>4</sup>-Me<sub>3</sub>SiC<sub>4</sub>(SiMe<sub>3</sub>)–C(Ph)CPh] (<b>14</b>), are formed along with free Me<sub>3</sub>SiCC–CCSiMe<sub>3</sub>. The interaction of <b>1</b> with 1,4-diphenylbutadiyne
(PhCC–CCPh) at 100 °C leads to the formation
of a mixture of products, from which a zirconacyclopentadiene metallacycle,
Cp<sub>2</sub>Zr[η<sup>2</sup>-PhCC(C<sub>2</sub>Ph)–C(C<sub>2</sub>Ph)CPh] (<b>10</b>), as well as seven-membered
zirconacyclocumulenes Cp<sub>2</sub>Zr[η<sup>4</sup>-PhC<sub>4</sub>(Ph)–C(C<sub>2</sub>SiMe<sub>3</sub>)CSiMe<sub>3</sub>] (<b>11</b>), Cp<sub>2</sub>Zr[η<sup>4</sup>-Me<sub>3</sub>SiC<sub>4</sub>(SiMe<sub>3</sub>)–C(C<sub>2</sub>Ph)CPh]
(<b>12</b>), and Cp<sub>2</sub>Zr[η<sup>4</sup>-PhC<sub>4</sub>(Ph)–C(C<sub>2</sub>Ph)CPh] (<b>13</b>) were isolated. The structures of <b>8</b>, <b>9</b>, <b>10</b>, <b>12</b>, and <b>14</b> have been
established by X-ray crystallography. The mechanism of the reactions
found is discussed
