Furan orbital structure

Let us now direct our attention to the P=C bond in phosphaalkene ion-radicals. The literature contains data on two such anion-radicals in which a furan and a thiophene ring are bound to the carbon atom, and the 2,4,6-tri(tert-butyl)phenyl group is bound to the phosphorus atom. According to the ESR spectra of anion-radicals, an unpaired electron is delocalized on a n orbital built from the five-membered ring (furanyl or thienyl) and the P=C bond. The participation of the phosphaalkene moiety in this MO was estimated at about 60% and some moderate (but sufficient) transmission of the spin density occurs through the P=C bridge (Jouaiti et al. 1997). Scheme 1.6 depicts the structures under discussion. 

Density functional theory computational studies have been used to determine die importance of secondary orbital interactions for the stability of transition-state structures for die 4 + 2-cycloaddition of furan with cyclopropene.175 Kinetic studies of die 2 + 4-cycloaddition of 2-cyclopropylidene acetates with furan and dimethylful-vene suggest a mechanism involving diradicals or zwitterions as intermediates.176 Cyclopropene, produced by die reaction of allyl chloride with sodium bis(bimediyl-silyl)amide, reacts with 1,3-diphenylisobenzofuran to produce both endo- and exo-Diels-Alder cycloadducts isolated for the first tune.177... 

The Patemo-Buchi reaction on furan has been studied from a theoretical point of view [42]. The HOMO of furan has an energy of 0.32 eV, and the atomic coefficients of this orbital (Fig. 3.3) are in agreement with an attack of the carbonyl oxygen on the a carbon. The structures of the possible adducts deriving from the attack of benzaldehyde in the a position of furan are depicted in Figure 3.4. The exo isomer is more stable (0.5 kcal mol 1) than the other isomer, in agreement with the experimental results. 

Structure IVa may also be drawn as IVb (Scheme 48.2) with the O-B unit sitting just above the empty orbital that constitutes the carbenium ion. The interaction of these two sites would afford the curious ortho ester VI, which, having three grossly equivalent C-0 bonds present, gives ample room to operate with it. Only two C-O bond ruptures represent forward reactions, however. In turn, while route C leads to a structure several parsecs removed from 11, the bond separation included in route D affords a second dioxolenium ion VII where trans-H, namely. Villa, results after inversion of configuration of C-4. (This atom becomes C-2 in the furan nomenclature.)... 

As expected from simple MO considerations, the radical cations of five-membered heterocycles, e.g. the blue species formed from furan, pyrrole, thiophene, and their alkyl derivatives, are n ions. The semi-occupied orbital is the n orbital with the heteroatom in the nodal plane (ai), see Scheme 2, structure 1. In radical cations of a,ca-bis-(l-pyrrolyl) alkanes the charge remains localized on a single ring, rather than being delocalized over both units [5, 10, 11]. 

Furan and thiophene have structures that are analogous to the structure of pyrrole. Where nitrogen in pyrrole carries a hydrogen atom, the oxygen or sulfur carries an unshared pair of electrons in an sp orbital. Like nitrogen, the oxygen or... 

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