Alkenes orbitals

An electrophilic addition generally proceeds in two steps.126 In the first, the most important interaction occurs between the LUMO of the electrophile and the HOMO of the alkene. Orbital overlap is largest when the electrophile attacks at the center of the n bond ... 

There are only a few examples of singlet photoreaction between alkenes that are not tethered or constrained in close proximity. In these cases, calculations have suggested the presence of exciplex [6] and/or diradical intermediates [7]. The short lifetime of the typical alkene singlet excited state (on the order of 10-20 ns) [8] limits the chance for productive collisions. On the other hand, photochemistry between electron rich and electron poor alkenes such as tetracyanoethylene and methoxy substituted alkenes, provides evidence for an electron transfer process [9]. These matched pairs benefit from ground state attraction and a resulting preorientation that enhances the alkene orbital overlap. Alternatively, electron transfer pathways have been accessed by employing electron transfer sensitizers (see Sch. 2), DCA is dicyanoanthracene) [10]. 

The first step in the reaction is adsorption of H2 onto the catalyst siur-face. Complexation between catalyst and alkene then occurs as a vacant orbital on the metal interacts with the filled alkene orbital. In the final steps, hydrogen is inserted into the double bond, and the saturated product diffuses away from the catalyst (F re 7.10). The stereochemistry of hydrogenation is syn because both hydrogens add to the double bond from the same catalyst surface. 

The orbital interaction leading to the fission of the M-R bond and the formation of the new bonds is the typical synergetic interaction of transition metals with n-acceptor ligands. The a(M-R) and 7t(alkene) orbitals, which are occupied, interact in a four-centered transition state with the a (M-R) and 7t (alkene) orbitals, which are vacant. This interaction produces a mixing of the rr(alkene) and 7t (alkene) orbitals that polarizes the alkene Compared to the simple alkene coordination (Fig. 6.1, left), where the donor and acceptor alkene orbitals interact only with the metal orbitals (this is the classical Chatt-Dewar model), the interaction with o(M-R) and o (M-R) leading to insertion polarizes the donor alkene orbital towards C ... 

When the metal center lacks filled d orbitals (d metal ions), there is no possibility of M L back donation to the alkene, and the 7t (alkene) orbital is fully available to interact with the a(M-R) electrons, thus this looks an ideal interaction to make a C/3-R bond (Fig. 6.2). If one looks at the whole process, starting from the free alkene, the metal center acts as acceptor of an electron pair from the alkene. If one looks at the R migration step alone, the metal releases the electron pair of the M-R bond, and a vacant coordination site is produced compared to the alkene complex. Theoretical calculations support that this unoccupied d orbital is used to stabilize the new alkyl system by agostic interaction with the occupied a(C -R) orbital. 

To account for this transalkylidenation process a concerted pairwise exchange of alkylidene moieties was initially proposed, a quasi cyclobutane transition state being envisaged. According to Woodward—Hoffman rules, the synchronous formation and breaking of double bonds in this manner is thermally forbidden. However, the interaction of the metal d orbitals with the alkene orbitals in the transition state, was once thought to reduce the symmetry requirements of such a transformation. 

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