ISODICYCLOPENTADIENES AND RELATED MOLECULES .49. SYNTHESIS, STATIC STRUCTURE, AND KINETIC STABILITY OF A SYN-SESQUINORBORNATRIENE

The bisspiro ryn-sesquinorbornatriene 8 has been synthesized and shown to be recalcitrant to attack by several reactive reagents, in contrast to the parent system 4. The spectral properties of 8 compare closely to those of 4, especially the pronounced downfield chemical shift of the internal pyramidalized olefinic carbons (172 ppm). On this basis, the two molecules appear to share a close conformational relationship. The greatly diminished reactivity of 8 has permitted X-ray crystallographic analysis. These data show the central flap angle to be 157.3 (4)° and the relevant pyramidalization angle to be 32.4°. This overall geometric profile constitutes an extreme level of distortion for an isolable pyramidalized alkene. For comparison purposes, several monospirocyclopentyl syn-sesquinorbornatrienes were also prepared. These were found to be uniformly air-sensitive, signaling that both apical carbons need to be suitably substituted to achieve kinetic stability. © 1990, American Chemical Society. All rights reserved.