Conjugated systems 2 + 2 cycloadditions

Cyclo ddltion. Ketenes are ideal components ia [2 + 2] cycloadditions for additions to the opposite sides of a TT-system as shown ia the cyclobutane product (2) ia Figure 1. Electron-rich double bonds react readily with ketenes, even at room temperature and without catalysts. In conjugated systems, ketenes add ia a [2 + 2] fashion. This is illustrated ia the reaction foUowiag, where the preferential orientation of L (large substituent) and S (small substituent) is seen (40). This reaction has been used ia the synthesis of tropolone [533-75-5]. 

The quiaones have excellent redox properties and are thus important oxidants ia laboratory and biological synthons. The presence of an extensive array of conjugated systems, especially the a,P-unsaturated ketone arrangement, allows the quiaones to participate ia a variety of reactioas. Characteristics of quiaoae reactioas iaclude nucleophilic substitutioa electrophilic, radical, and cycloaddition reactions photochemistry and normal and unusual carbonyl chemistry. 

We have also used poly(propynoic acid) in our studies of the photochemical interaction of PCSs with dienophiles, such as maleic anhydride, tetracyanoethylene, and styrene. This photochemical reaction of Diels-Alder type is accompanied by the breakdown of the conjugation system and the formation of slightly colored adducts266. Together with the cycloaddition reaction, photodegradation of PPA and its adducts takes place. A cycloaddition reaction is always preceded by the formation of a donor-acceptor complex of a PCS with a dienophile. 

The reaction is carried out simply by heating a diene or another conjugated system of n bonds with a reactive unsaturated compound (dienophile). Usually the reaction is not sensible to catalysts and light does not affect the course. Depending on the specific components, either carboxylic or heterocyclic products can be obtained. The stereospecificity of the reaction was firmly established even before the importance of orbital symmetry was recognized. In terms of orbital symmetry classification, the Diels-Alder reaction is a k4s + n2s cycloaddition, an allowed process. 

A general cheletropic reaction is shown in Figure 12.2. This reaction involves the addition to, or extrusion from, a conjugated system of a group bound through a single atom. The reaction usually involves the elimination of simple stable molecules such as SO2, CO, or N2. The atom to which there were two a bonds carries away a pair of electrons, usually in a spn hybrid orbital. The addition of a carbene to a simple olefin to form a cyclopropane is also a cheletropic reaction which, as discussed in Chapter 14, is not predicted to be concerted. Cheletropic reactions incorporate features of both cycloaddition and electrocyclic reactions. 

Cycloadditions are characterized by two components coming together to form two new o-bonds, at the ends of both components, joining them together to form a ring, with a reduction in the length of the conjugated system of orbitals in each component. Cycloadditions are by far the most abundant, featureful, and useful of all pericyclic reactions. 

Whereas cycloadditions are characterized by two components coming together to form two new a-bonds, electrocyclic reactions are invariably unimolecular. They are characterized by the creation of a ring from an open-chain conjugated system, with a a-bond forming across the ends of the conjugated system, and with the conjugated system becomes shorter by one p-orbital at each end. 

A conjugated system of 8 electrons would normally have the two ends of the conjugated system far apart, but there are a few molecules in which the two ends are held, more or less rigidly, close enough to participate in cycloadditions to a double or triple bond. Thus, the tetraene 2,69 has the two... 

Cycloadditions are a little more common than [8+2] cycloadditions, since it is a little easier to find pairs of conjugated systems that have the ends a suitable distance apart. Thus, in one of the first examples of such a reaction... 

Antarafacial overlap on one component in a cycloaddition would need a most unusually long and flexible conjugated system ... 

The preliminary rules, thermal and photochemical, given on p.16, need now to be qualified—they apply only to cycloadditions that are suprafacial on both components. Nevertheless, almost all pericyclic cycloadditions are suprafacial on both components. It is physically difficult for one conjugated system to suffer antarafacial attack from another, since it implies that one or another of the components can reach round from one surface to the other 2,85. Only if at least one of the components has a long conjugated system can it twist enough to make this even remotely reasonable. Straightforward antarafacial attack in cycloadditions is therefore very rare indeed. Keep in mind, however, that these rules only apply to pericyclic cycloadditions— there are other kinds of cycloaddition, in which the two bonds are formed one at a time, and to which none of these rules applies. 

All the other cycloadditions, such as the [4+2] cycloadditions of allyl cations and anions, and the [8+2] and [6+4] cycloadditions of longer conjugated systems, have also been found to be suprafacial on both components, wherever it has been possible to test them. Thus the trans phenyl groups on the cyclopentene 2.65 show that the two new bonds were formed suprafacially on the rrans-stilbene. The tricyclic adducts 2.61, 2.77, 2.79, and 2.83, and the tetracyclic adduct 2.82, show that both components in each case have reacted suprafacially, although only suprafacial reactions are possible in cases like these, since the products from antarafacial attack on either component would have been prohibitively strained. Nevertheless, the fact that they have undergone cycloaddition is important, for it is the failure of thermal [2+2], [4+4] and [6+6], and photochemical [4+2], [8+2] and [6+4] pericyclic cycloadditions to take place, even when all-suprafacial options are open to them, that is significant. 

There are other stereochemical features which have nothing to do with the symmetry of the orbitals, and are much less powerfully controlled. In many cycloadditions, there are two possible all-suprafacial approaches one having what is called the extended transition structure 2.102, in which the conjugated systems keep well apart, and the other called the compressed 2.103, where they lie one above the other. Both are equally allowed by the rules that we shall see in Chapter 3, but one will usually be faster than the other. This type of stereochemistry applies only when the conjugated systems have at least three atoms in each component it is therefore only rarely a consideration. It shows up in the cycloadditions of allyl cations to dienes, where the two adducts 2.56 and 2.57 on p. 13 are the result of the compressed transition structure 2.104 and the extended 2.105, respectively, with the former evidently lower in energy. 

It is frequent but not invariable that where a longer conjugated system has a geometrically accessible and symmetry-allowed transition structure like that in 5.90, the longer system is used. Thus, the [8+2] and [6+4] cycloadditions on pp. 15 16, and the [14+2] cycloaddition on p. 44 take place rather than perfectly reasonable Diels Alder reactions, and the 8-electron electrocyclic reactions of 4.51 and 4.54 takes place rather than disrotatory hexatriene-to-cyclohexadiene reactions. This kind of selectivity is called periselectivity. 

The novel four-center two-electron delocalized o-bishomoaromatic species 593, 594,599,601a, and 603 are representatives of a new class of 2jt-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations985 (Figure 3.25, 604). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 605 having a conventional p-type delocalized electron structure (Figure 3.25). 

A series of -conjugated poly(dithiafulvene)s (12) have been prepared by cycloaddition polymerization of aldothioketenes and their alkynethiol tautomers, which were derived from aromatic diynes (Scheme 3) [70-73]. Efficient expansions of -conjugation systems in the polymers were evident... 

Cheletropic reactions are cyclizations - or the reverse fragmentations - of conjugated systems in which the two newly made o bonds terminate on the same atom. However, a cheletropic reaction is neither a cycloaddition nor a cycloreversion. The reason is that the chelating atom uses two AOs whereas in cycloadditions, each atom uses one and only one AO. Therefore, Dewar-Zimmerman rules cannot apply to cheletropic reactions. Selection rules must be derived using either FO theory or correlation diagrams 38 The conjugated fragment39 of 4n + 2 electron systems reacts in a disrotarory (conrotarory) mode in linear (nonlinear) reactions. In 4n electron systems, it reacts in a disrotarory (conrotarory) mode in nonlinear (linear) reactions. 

---