Geometry cisoid

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction ... 

In a synchronous Diels-Alder reaction, the following facts have come to light about dienes 1. Simple dienes react readily with good dienophiles and they must adopt a cisoid geometry about the bond (s-cis). 

As the inherent chain stiffness of the system, opposing rotation out of the molecular plane, tends to retain the cisoid conformer geometry at the affected ring, the probability of bond reformation by recombination ( healing ) is h, and restoration of the original ring structure may occur. For detailed treatments of the thermostability problem in pcdymers, the reader is referred to the more specialized literature " ... 

Fignre 14. Geometries for the attack of the (n, t ) state of a ketone to an ethylene, yielding either a cisoid singlet biradical, A, or a transoid one, B. The attack by the corresponding (n, it ) state may polarize and twist the ethylene n system, yielding a twisted cisoid biradical, C the (n, it ) state may also lead to a transoid triplet biradical, D. From Ref. 79 with permission of Elsevier Sequoia S.A. 

In 1962, Fischer and co-workers reported that ultraviolet irradiation of spiropyrans results in formation of an unstable isomer, X, of the colored modification and proposed the existence of an open cis-cisoid isomer.16 The geometry of this isomer resembles very closely that of the parent spiropyran, i.e the central C3=C4 bond has a cis-cisoid conformation ... 

The cisoid conformation is only found in compounds that feature particular strains [13] or contain unusually long S-S bonds such as (TMS)3-S-S-(TMS)3 [14] and when the barrier to rotation is relatively low, in particular with 4,4 -dithiopyr-idine [15,16], a motif currently used for the assembly of bimetallic supramolecular structures [17]. In contrast, bis(diphenylthiophosphoryl)disulfide showed a cisoid geometry whereas bis(diphenoxythiophosphoryl)disulfide was in a transoid conformation, all of which suggests a pretty high barrier to interconversion. As anticipated, the S2-S2 bond in 6 (2.027 A) was shorter than the S2-S3 bond in 7 (2.070 A) [18]. 

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