Geometry transoid

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 ... 

FIGURE 13. Extended octant rale applied to enantiomeric dissymmetric orientations of /S,y-unsaturated ketone chromophores. When the absolute of the dihedral angle 0 is >90°, a positive Cotton effect is predicted for the geometry of (a) and a negative for (b). In a transoid arrangement of /S,y-unsaturated ketone (c) the As values are often quite small (References 200 and 201)... 

In8R6 which consists of a In6R.4 octahedron (bare In atoms trans) capped by two RIn fragments in a transoid geometry. Formally the RIn fragments contribute two electrons each and the In contribute one if a lone pair is assumed, yielding sep = 7. 

There is considerable evidence that at high acid concentrations these ligands may pick up a second proton (448, 492, 730) and the agreement between values of the second stability constant for [bipyH2] is good. This species has also been shown by NMR spectroscopy to occur in solution (143). The mono- and diprotonated species both have transoid skew conformations. The enforced cis geometry of the phenanthrolinium ions may account for the difficulty, relative to the bipyridylium ions, of adding a second proton, as shown in Table II. 

The crystal structure of yellow metallacyclic-osmium(iv) complex 9 was also reported <20030M414>. The distribution of ligands around the osmium atom can be described as a four-legged piano-stool geometry with the carbon atom C-5 of the metallated phenyl group disposed transoid to the hydride ligand. The bidentate carbon donor... 

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. 

---