Dienes electronic configuration

Another question that has come up is why the addition of proton to carbanion occurs only at position 6 to give 1,4 diene and not at 2 or 4 to give a 1,3 diene This has been answered on the principle of least motion advanced by Hine according to which those reactions are favoured which involve the least change in atomic position and electronic configuration. 

By sharing the four --electrons of the diene and the six --electrons of the cyclopentadienyl anion, the cobalt(I) atom acquires the electronic configuration of krypton. The formulation of the complex as (XXIV) is supported... 

The reaction is prevented in the case of low-lying rot excited states, which are responsible for the lack of photocyclization of azobenzenes and benzylidene anilines. However, when the electronic configuration switches to a low-lying mt excited state upon complexation with Lewis acids, photocyclization takes place under these conditions [30]. Similarly, the presence of perchloric acid allows that, upon irradiation, (E)-4-benzyloxy-l,l,3,4-tetraphenyl-2-azabuta-l,3-diene is transformed in an isoquinoline derivative (Scheme 9.21) [31, 32]. 

The electronic configurations of ethylene and buta-1,3-diene. In both buta-1,3-diene and ethylene, the bonding MOs are filled and the antibonding MOs are vacant. 

This is a 4-77 electron process. The bonds involved are the tt bonds of the conjugated diene system and the tt bond and the ring-forming electronic configurations and symmetry characteristics of the orbitals involved are as follows ... 

It was suggestedthat bicyclobutane formation from conjugated dienes occurs in a concerted fashion from vibrationally relaxed singlet having an allyl anion-methyl cation electronic configuration. 

On this basis, let us examine the [4 + 2] cycloaddition of 1,3-butadiene and ethylene, the simplest example of the Diels-Alder reaction. The electronic configurations of these compounds—and of dienes and alkenes in general—have been given in Fig. 29.5 (p. 931) and Fig. 29.6 (p. 932). There are two combinations overlap of the HOMO of butadiene ( 2) with the LUMO of ethylene (tt ) and overlap of the HOMO of ethylene (tt) with the LUMO of butadiene ( 3). In either case, as Fig. 29.20 shows, overlap brings together lobes of the same phase. There is a flow of electrons from HOMO to LUMO, and bonding occurs. 

Ground-State and excited-state electronic configurations of confugated dienes and trienes. 

Ground-state and excited-state electronic configuration of conjugated dienes and trienes. ... 

Figure 6. Schematic of the state correlation diagram for the concerted addition of molecular oxygen to a diene. The electronic configurations for states of the (O2 + diene) complex are indicated on the left (state of O2 in the complex given in brackets). States of the cyclic peroxides with their appropriate electronic configurations are on the right in order of increasing energy. Dashed curves indicate the primitive correlations obtained by straightforward application of symmetry and spin restrictions. Solid curves indicate final correlations obtained by using the additional information contained in the orbital correlation...

Figure 8. Schematic of the state correlation diagram for the (diene + O2) peroxide diradical inter conversions. States of the diene-02 complex are on the left [state of molecular oxygen is specifically noted for each state. States of the peroxide are depicted in the middle. The nearly degenerate set of low lying singlet and triplet states of the diradical are shown on the right. The states are arranged in order of increasing energy, and the electronic configuration for each state is indicated...

Low-valent cobalt [Co, Co° and Co" ] are known to complex with unconjugated dienes (e.g., 1,5-COD, NBD, etc.) and acetylenes. The Co or Co " precatalyst is presumably reduced by EtjAlCl to either Co° or Co. Co, which has the electronic configuration [Ar]3d and commonly forms five-coordinate complexes in a trigonal bipyramidal form. Thus, although very little is known about the structure of the catalytic intermediates, an intermediate with structures Ij and/or seems reasonable (Fig. 2). 

The two lowest orbitals are occupied in the 1,3-diene molecule, therefore the electron configuration is ij/lij/l. Cyclobutadiene and trimethylenemethane have electron configuration and thus are biradicals.Table 8.1 gives symmetries of orbitals... 

Usually, 1,3-diene compounds constitute 18 complexes. However, exceptions are known these comprise square-planar 16e complexes possessing electronic configuration of the central atom. 

Several cationic q -alkenyl-complexes obtained by protonation or deuteria-tion of tris(phosphite)iron q -diene complexes have been studied by nmr spectroscopy (Ittel et ai, 1979). The structures are octahedral, coordinated with phosphite ligands at three of the sites. The q -alkenyl group occupies two sites and a hydrogen atom which is adjacent to the alkenyl-group fills the sixth site giving a noble gas (18-electron) configuration at the iron centre. 

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