Palladium bond angles

Detailed structural studies of bicyclic azetines have been difficult to carry out because of the instability inherent in this type of ring system. The presence of fluorine-containing substituents though can stabilize some bicyclic azetines sufficiently to obtain physical data in support of these structures. X-ray crystallographic examination of palladium complex (117) showed that each l-azabicyclo[2.2.0]hexadiene ring is planar. Selected bond angles and distances for (117) are listed in Table 9 (76CPB2219). 

Reference to the bond angles and interatomic distances for these molecules shows that as far as the carbon atoms are concerned their values are little different from those of the olefin molecule. It would be expected, therefore, that catalysts which were effective for hydrogenation of the latter would also function with the heterocyclic molecules. This is found to be so, in that nickel catalysts are known to give tetrahydrofuran and pyrrolidine by the hydrogenation of furan and pyrrole at 180° (Padoa, 25). Tetrahydrofuran is also formed by the use of platinum (Starr and Hixon, 26), osmium, or palladium (Shuikin, Nikiforov, and Stolyarova, 27) as the catalyst, and pyrrolidine is similarly produced by palladium or rhodium catalysts (Zelinskii and Yurev, 28). In all these metals there are spacings of the atoms very similar to those in metallic nickel, the hexagonal osmium lattice having a equal to 2.71 A. 

The four androstanes isomeric at C-5 and C-14 have been equilibrated over palladium at elevated temperatures. The free energies of the isomers relative to the most stable (5a,14j5) are 5a,14a-, -1-1.8 5j5,14a-, -1-2.7 and 5) ,14) -, -1-1.5 kcal moP Molecular force-field calculations gave somewhat lower free energy values, but placed the isomers in the correct order of stability. The major contribution to instability comes from the 14a-configuration c/o-trans), as inferred from X-ray data. Calculated torsion and bond angles agree very well with X-ray data, where these are available for comparison. 

Two types of reactions are summarized in this section (i) the intermolecular carbopalladation leads to a Pd functionality such as alkyl-, alkenyl-, or allylpalladium complexes, which is intramolecularly trapped by a heteroatom (again Wacker-type processes are mechanistic alternatives) (ii) the palladium catalyst is not directly involved in the hetero-cyclization step, but the carbopalladation builds up a suitable functionality or changes bond angles so that the heterocyclization can take place. 

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