Ab Initio Computational Studies of Conformationally Restricted Cope Rearrangements. First Examples of Fully Concerted Allenyl Cope Rearrangements

Results of (8,8)CASPT2/6-31G*//(8,8)CASSCF/6-31G* level calculations on the potential surface for the conformationally restricted allenyl Cope rearrangements of <i>syn</i>-5-propadienylbicylco[2.1.0]pent-2-ene (<b>14</b>) and <i>syn</i>-6-propadienylbicyclo[2.1.1]hex-2-ene (<b>15</b>) are reported. Both are found to proceed through concerted pathways. Also included are the results of (6,6)CASPT2/6-31G*//(6,6)CASSCF/6-31G* level calculations on the Cope rearrangements of <i>syn</i>-5-ethenylbicyclo[2.1.0]pent-2-ene (<b>18</b>), <i>syn</i>-6-ethenylbicyclo[2.1.1]hex-2-ene (<b>19</b>), and <i>syn</i>-7-vinylnorborene (<b>20</b>), which are found to involve diallylic diradical intermediates <b>26</b>, <b>30</b>, and <b>36</b>, respectively. Previous studies have shown that the allenyl Cope rearrangement of 1,2,6-heptatriene (<b>1</b>) to 3-methylene-1,5-hexadiene (<b>2</b>) involves a single transition structure that either proceeds to the monoallylic cyclohexane-1,4-diyl derivative <b>3</b> or bypasses <b>3</b> to form <b>2</b> directly. More recently, the conformationally restricted allenyl Cope rearrangement of <i>syn</i>-7-allenylnorbornene (<b>7</b>) has also been found to involve tricyclic monoallylic cyclohexane-1,4-diyl intermediate <b>11</b>. The rearrangements of <b>14</b> and <b>15</b> appear to represent the first reported examples of fully concerted allenyl Cope rearrangements. Concertedness in these cases is ascribed to two parallel factors:  (1) the relative instability of possible tricyclic diradical intermediates <b>16</b> and <b>17</b>, compared to diradical intermediates <b>3</b> and <b>11</b> formed in the rearrangements of <b>1</b> and <b>7</b>, respectively; and (2) the opportunity that exists to form sp-sp² σ bonds in transition structures <b>21</b> and <b>23</b> that lead, respectively, to products <b>22</b> and <b>24</b>. By contrast, only weaker sp²-sp² σ bonds could form in unobserved concerted transition structures leading to products <b>28</b> and <b>32</b>, formed in the nonconcerted rearrangements of <b>18</b> and <b>19</b>.