Dienes and Neutral Molecules

The cation-radical version of diene synthesis, in which the diene is in a strongly electron-deficient state, is characterized by an unusual high endoselectivity. In this case, endoselectivity is sig-nihcantly higher than that of thermal or photochemical initiation of a neutral molecule (cf. Mlcoch and Steckhan 1987). As follows from the charge diagram depicted in Scheme 7.21, when a cation-radical and neutral molecule approach each other, not only the C(l)-C(6) and C(4)-C(5) interactions are bonding (indeed, these interactions result in cyclization), but C(2)-C(7) and C(3)-C(8) interactions are also bonding. As a result, the endo product is formed. 

Bimolecular ion/molecule reactions of dienes and polyenes have been extensively studied for several reasons. Some of them have been mentioned implicitly in the previous sections, that is, in order to structurally characterize the gaseous cations derived from these compounds. In this section, bimolecular reactivity of cationic dienes, in particular, with various neutral partners will be discussed, and some anion/molecule reactions will be mentioned also (cf Section IV). In addition, the reactions of neutral dienes with several ionic partners will also be discussed. Of this latter category, however, the vast chemistry of reactions of neutral dienes with metal cations and metal-centred cations will not be treated here. Several reviews on this topic have been published in the last decade178. 

The publications of Bauld et al., which were reviewed earlier in Section 7.4.1, deal only with reactions in which a cation-radical (readymade) acted as a reactant. However, there can be cases where a cation-radical is formed in the course of donor-acceptor interaction between initially neutral molecules. Then the rigid or sharply enhanced selectivity of the reaction acquires diagnostic significance. For such cases, there is a general rule—condensation is permissible only for the (dienophile cation-radical -l- diene) pair and forbidden for the (dienophile -l- diene cation-radical) pair. 

In general, radical cations of alkenes or cyclopropanes produce nonconjugated radicals, while those of dienes give rise to allyl radicals, and those of vinylcyclopropane or vinylcyclobutane systems generate either allylic or nonconjugated radicals with an additional element of unsaturation. In contrast to the most thoroughly characterized nucleophilic substitution of appropriate neutral molecules. 

We are not restricted to the reactions of neutral molecules. Allyl anions can react with olefins, allyl cations with dienes, and pentadienyl cations with olefins, as we can see from the examples in Fig. 4-6, in which we can also see why the reactions of allyl cations with olefins, of allyl anions with dienes, and of pentadienyl cations with dienes are not observed. 

The rational synthesis of anionic, neutral, and cationic dinuclear palladium complexes containing bridging conjugated dienes is shown in Scheme 32 for butadiene, and consists of the formal redox condensation of Pd(0) and Pd(ii) complexes in the presence of the conjugated diene to give Pd(l) dimers. In addition, some complexes with isoprene and with 1,3-cyclohexadiene were made. The isoprene complexes consisted of the expected pair of regioisomers in the case of the less symmetric neutral molecules. The crystal structures of one neutral complex... 

X = Br diene = butadiene) and one anionic complex (X = Cl diene = isoprene) were determined. The diene had a zigzag structure in both cases and two enantiomers were found in the complexes, corresponding to the two arrangements of the zigzag chain. The coordination of the diene in the compounds was described as rf-s-trans. Some structural parameters are Pd-Pd = 2.662, Pd-C = 2.12-2.25 for the neutral molecule, Pd-C = 2.332-2.389A for the anionic molecule. 

The main features of metal complex catalyst are caused by a set of orbitals (s-, p-, d-, and /orbitals) in a transition metal ion (Fig. 17.1-3). They interact with orbitals of ligands and substrates. The catalytic properties of complexes depend strongly on the properties of partially occupied neutral molecules, namely, substrates, which can donate electrons (CO, olefins, dienes, etc.). The substrate is activated during its coordination. 

We are being somewhat disingenuous here. If performed and interpreted correctly and with the appropriate ancillary phase-change enthalpy information, the enthalpy of formation of an arbitrary species by ion-molecule reaction chemistry and by combustion calorimetry must be the same. That the ionization potential of quinuclidine is higher than l,4-diazabicyclo[2.2.2]octane simply says that there is a stabilizing effect in the radical cation of the latter not found in the former. This information does not say that there is a stabilizing effect in the neutral molecular form of the latter not found in the former. After all, we trust the reader is not bothered by the fact that the ionization potential order of the cyclohexenes increases in the order 1,3-diene < 1,4-diene < 1-ene < 1,3,5-triene (benzene). 

Neutral dienes have been reacted with a large variety of ions in the gas phase. Besides the cases concerning the same reactants discussed above but with reversed charge distribution, e.g. those of neutral 1,3-butadiene with ionized alkenes, there are interesting studies of reactions of 1,3-dienes with even-electron cations and studies on ion/molecule... 

Unsaturated even-electron cations have been used in the gas phase to react with olefins, including dienes, in a way that characterizes their structure. In most cases, these ion/molecule reactions take place by [4 + 2] cycloadditions followed by specific elimination of even-electron neutrals. A most suitable instrumental setup for these studies are triple-quadrupole and pentaquadrupole mass spectrometers in which the ion/molecule addition reactions take place subsequent to the selection of the reagent ion. In most... 

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