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Molecular Physics
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before connecting to each other. Even in this case, an equivalent transition state, in
which one more phosphine is on the metal, has been located: this structure is 4.1 kcal
mol -1 less stable than TS19. Once again, even if bonded to the metal, an additional
phosphine does not afford an energy stabilization. After the discussed C-C bond
formation, the system evolves to an intermediate, M25, in which the palladium is
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bearing three ligands only: one phosphine, one chlorine atom and the allyl carbonyl
chain. The CO unit has inserted into the carbon-palladium bond and now the metal is
linked to a 4-carbon chain. In spite of the unsaturated metal coordination sphere, this
complex is more stable than all the other structures determined so far, because of the
formation of a new carbon-carbon bond (M25 lies 0.4 kcal mol -1 below the AL and
5.1 lower than the previous intermediate). At this point, there is space for an easy
approach of a triphenylphosphine to the unsaturated palladium (TS20, activation
energy= 0.4 kcal mol -1 ) that brings the system to the already discussed M18. The
subsequent steps are obviously those already described in the section 3.B.1 and
terminates the catalytic cycle. It is worth mentioning that a transition state for an
approach of the chloroallyl unit to the palladium in the initial Pd(PPh 3 ) 3 complex that
would eventually lead directly to its oxidation has been found. This transition state
would lead to M24 (see Scheme 8), after the concerted breaking of the allylic carbonchlorine
bond and the formation of the relative carbon-palladium and chlorinepalladium
bonds. Additionally, one phosphine leaves the metal. Although this
transition state exists, it is located very high in energy compared to any other points
discussed so far: 39.6 kcal mol -1 . It is evident that there is a high cost of energy for the
system when a single carbon atom is involved in the breaking of a bond and the
simultaneous forming of an interaction with the palladium. In all the cases of carbonmetal
σ-bond formation described above, the carbon atom was previously involved in
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