Butadiene system

The classic example is the butadiene system, which can rearrange photochemi-cally to either cyclobutene or bicyclobutane. The spin pairing diagrams are shown in Figure 13. The stereochemical properties of this reaction were discussed in Section III (see Fig. 8). A related reaction is the addition of two ethylene derivatives to form cyclobutanes. In this system, there are also three possible spin pairing options. 

Figure 13. Anchors, coordinates, and loop for the butadiene system.

For the butadiene-cyclobutene interconversion, the transition states for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, i.e., the total set of 2p orbitals involved in the reaction process, not the individual MOs. Each of the orbitals is tc in character, and the phase difference is represented by shading. The tilt at C-1 and C-4 as the butadiene system rotates toward the transition state is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

The cyclohexadiene-hexatriene system seems to be less complicated than the cyclobutene-butadiene system. Cyclohexadiene undergoes photochemical electrocyclic ring opening ... 

Use of an ethylene atmosphere can be beneficial to increase catalyst selectivity and reactivity [176] but it can also lead to the unwanted production of the corresponding 1,3-butadiene due to competition of ethylene with the desired cross-partner [177]. On the other hand ECM with ethylene as the cross-partner can be used as a very convenient method for the production of 2,3-disubstituted butadiene systems. The successful application of this methodology has for example been achieved by Mori et al. in the total synthesis of anolignan A (Scheme 3.20) [178]. 

In the butadiene system of a cyclopentadienylstannane, the LUMO has the same phase at the 1- and 4-positions 25, and it is argued that this results in enhanced hyperconjugation through the Whiffen effect,307 which was first... 

The same arguments can be applied to construct the correlation diagram for the conrotatory process in the transformation of cyclobutene v " butadiene system. In such a case, as we have seen, a C2 symmetry is maintained throughout. 

Fig. 5.8 Correlation diagram between cyclobutene-butadiene system in a conrotatory interconversion...

The linear dependence of the sum of the four substituents Taft constants suggests that the electron density due to the substituents is delocalized along the entire butadiene system. 

A highly active zinc powder can be generated in bulk by electrolysis of a tetraalkylammonium salt in dimethylformamide between a zinc anode and a platinum cathode [183]. This reagent has been used in stoichiometric amounts for Re-formatsky-type reactions where the tribromide 61 is used as a precursor for the isoprenyl group [184]. Zinc promoted 1,4-elimination of bromine to give the butadiene system is followed by a Reformatsky reaction of the remaining allylic bromine substituent. 

In a fused system there are not six electrons for each ring.8" In naphthalene, if one ring is to have six, the other must have only four. One way to explain the greater reactivity of the ring system of naphthalene compared with benzene is to regard one of the naphthalene rings as aromatic and the other as a butadiene system.81 This effect can become extreme, as in the case of triphenylene.82 For this compound, there are eight canonical forms like A. 

Stereoselective l.4-reduction oi the 1.3-butadiene system to olefin 57 lakes place tinder the conditions of the Birch reduction. Intramolecular protonation of the intermediate carbanion at the 18-position to give 57 occurs with high selectivity syn to the hydroxymethy-iene group Conversion into phosphoric acid derivative 58 and cleavage of the phosphoric acid amide group under (he conditions of the Bcnkeser reduction provides compound 5921 Fluonde ion causes the release of free p-amyrin (1) in a final step I Li, NH3(iyTllF (1/1.75), -78 C 93%. 

Bis(aryloxy)Ti(butadiene) systems react with a variety of a-olefins to form linear C6-coupling products.95... 

The constrained geometry (Cp /amido)zirconocene butadiene system exhibits a pair of equilibrating [(j,-czj,-C4H6)Zr(C5Me4SiMe2NCMe3)] isomers (140) that add B(C6F5)3 to yield a pair of Z-[ B(C6F5)3BCH2-7i-allyl Zr]... 

Just as the four p orbitals of buta-1,3-diene overlap to form four molecular orbitals, the three atomic p orbitals of the allyl system overlap to form three molecular orbitals, shown in Figure 15-11. These three MOs share several important features with the MOs of the butadiene system. The first MO is entirely bonding, the second has one node, and the third has two nodes and (because it is the highest-energy MO) is entirely antibonding. 

A simple theoretical model of the butadiene system predicts the energy of the bonding 1 orbital to be [a + 1.62PJ and that of bonding orbital to be [ex + 0.62f)]. With both of these orbitals fully occupied, the total energy of the electrons is[4a + 4.48 j. Remember that the energy of the bonding n molecular orbital for ethene was [a +13] (p. 152) so, if we were to have two localized 71... 

We have previously emphasized the activation of organic molecule via coordination. The converse may also be true, such that transition metals may be used as protecting groups for reactive unsaturated functional groups. Thus t 4-metal coordination of a butadiene system deactivates it pjgure 6.27 Butadiene towards Diels-Alder reactions. Friedel-Crafts acylation of free polyenes coordination modes... 

The energies for the molecular orbitals for these two extremes are shown in Fig. 2.3. The true orbital energy for the orbitals of acrolein must be in between those of the corresponding orbitals of the allyl cation and butadiene. We can perhaps expect the true structure to be more like the butadiene system than the allyl cation system (for the same reason that we prefer to draw it as 2.1 rather than 2.2). We can see that the effect of having a Z-substituent conjugated with the double bond of... 

The N atoms in (133) can be replaced by C atoms to give 1,4-diaza-1,3-butadiene systems (134) that have a close skeletal relationship to bipyridyl ligands (96) (Section 3.1.3). 

If we examine the classification of orbitals of starting material and product with respect to each of the two symmetry elements in turn, we see that the bonding orbitals, and for starting material and product share symmetry only with respect to the C2 axis. (With respect to the a plane, both bonding orbitals of the cyclobutene system are symmetric, but in the butadiene system, only is symmetric with respect to the a plane.)... 

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