Quadricyclanes cycloaddition

When additional substituents are introduced in the 2- and/or 4-position, the thermal rearrangement gives products in which the substituents that were originally located in the 1- and 5-position of the quadricyclane are then located at C4 and C5 of the oxepin 7.30,123 In order to trap intermediates of this rearrangement reaction by intramolecular cycloaddition vinyl and acetylene groups were linked with different spacer groups to C2 of quadricyclane.123 In this manner two different intramolecular cycloadducts were isolated in addition to oxepin derivatives.123... 

There are several known examples of silenes participating in [2 + 2 + 2] cycloadditions, including reactions with norbornadiene and with quadricyclane. Examples of these reactions with norbornadiene are shown in Eq. (30), where the silene bridges the 2,6-positions.38 5, 53 ,7U85 ,88 This reaction has only been investigated using some of the Auner-type silenes. In many cases mixtures of stereoisomers were obtained, and in some cases products of an ene reaction were also observed. 

The reactions with quadricyclane, shown in Eq. (31), gave products identical to those formed by the same silene reacting in a [2 + 2] manner with norbornene. Mixtures of exo endo isomers were frequently observed. Again, only silenes of the Auner type have been studied with this reagent,51-53,185,188 so it is not known whether the Wiberg- or Brook-type silenes will undergo this mode of cycloaddition. 

Like the cyclization of the parent system, the photocyclization of hetero analogues of norbornadiene was reported many years ago. Recent interest in this area will serve to illustrate the potential of the conversions. Thus the irradiation of the oxanorbomadiene system 259 brings about conversion to the oxepine 260138. The formation of 260 presumably arises by quadricyclane 261 formation, followed by secondary photolytic ring-opening. Other research has shown that irradiation of the oxanorbomadiene 262 follows the same reaction mode and it undergoes (2 + 2)-cycloaddition to the quadricyclane derivative 263. Apparently, in this instance, cycloreversion affords the ylide 264 that can be trapped by suitable addends, giving the adducts illustrated in Scheme 3139. 

The forbidden retro-[ls -I- 2s]-cycloaddition can now be treated using a simple curve-crossing model analagous to the Marcus-Hush theory of electron-transfer [11]. The ground state at the quadricyclane-like geometry is the... 

Quadricyclane is a highly strained and reactive compound. It reacts readily with acetic acid to give a mixture of nortricyclyl acetate and exo-norbornyl acetate and with bromine to yield a mixture of 2,6-dibromonortricyclene and exo-5-anti-l-dihvo-monorbornene.3 Quadricyclane undergoes cycloaddition reactions with a variety of dienophiles to give 1 1 adducts.15 1 2 3 4 5 6 7... 

A thermal [2+2] cycloaddition of quadricyclane can be achieved by allowing a 10-fold excess to react with in toluene at 80 °C (Scheme 4.66) [357]. 

The reaction proceeds smoothly and gives 326 in 43% yield. The stereochemistry of the addition is exo with respect to the norbomene moiety and in line with the usual cycloaddition behavior of quadricyclane [358]. The norbomene double bond in 326 is easily accessible by electrophiles, and, for example, the anti-addition of benzenesulfenyl chloride proceeds quantitatively at room temperature (Scheme 4.66). 

A unique example is the production of a polymer that contains Cgo- The fuller-ene monomer was prepared by cycloaddition of quadricyclane followed by polymerization in norbomene at room temperature.235 The product has cis trans ratios between 3 1 and 6 1 depending on the concentration of the monomer. A processable film containing 1% C60 exhibits electronic and electrochemical properties that are typical of the carbon cluster. 

It appears likely that transient metallacyclobutanes are involved in a variety of organic reactions which are catalyzed by transition metal complexes. Thus, cycloadditions of activated alkenes to strained hydrocarbons such as quadricyclane and bicyclo[2.1.0]pentane are catalyzed by complexes such as Ni(CH2=CHCN)2 and probably involve initial formation of a nickelacyclobutane (Scheme 2) (79MI12200). The nature of the organometallic intermediates in related metal-catalyzed rearrangements (72JA7757) and retro-cyclo-addition reactions (76JA6057) of cyclopropanoid hydrocarbons, e.g. bicyclo[n.l.O]alkanes, has been discussed. 

Photolysis (> 290 nm) induced quantitative conversion of (20c and d) to the crystalline 3-azaquadricyclanes (22c and d) that isomerize in solution at 20° to the azepines (23c and d) at measurable rates. The azepines rapidly dimerize. The thermally forbidden 47 reversal of 22 to 20 was not observed.80 Prinzbach and Eberbach81 have carried out numerous cycloadditions and acid-catalyzed rearrangements of 22c and d and of 23c and d. The 3-azaquadricyclanes 22 have diene properties but, as valence isomerization to azepines is so easy (above), cycloadditions were achieved only with the very reactive dienophiles DMAD and EP. Reactions with 22c and d at -15° to - 30° over 24 to 48 hours using a large excess of ester gave 24 (R = H or E). If equimolecular proportions of the quadricyclane and ester are used, the products (24) compete as dienophile for example, 24d (R = H) added to 20d to give 25.81... 

The fullerene monomer (200), made by the cycloaddition of quadricyclane (199) to Cgo (equation 56), can be copolymerized with an excess of norbomene in the presence of 8 (R = Me) to yield a high-molecular-weight, soluble, film-forming copolymer (86% cis), containing 1% of C60 derivative and exhibiting electronic and electrochemical properties which are typical of the carbon cluster536. 

Fused 1,2-thiazetidine A-oxides 22 are produced in one step from readily available aryl sulfonamides by cycloaddition with quadricyclane (Equation 13) <1997SL167, 1997SL634>. 

The complex Mo(tfd)3 was reported by King and co-workers (86) to catalyze the conversion of quadricyclane to norbornadiene (Eq. 9), polymerization of quadricyclane or norbornadiene, as well as the addition of water to quadricyclane to give nortricyclanol (Eq. 15). The reactions appear to involve pseudo-[4 + 2] cycloaddition to the sulfur atoms in the dithioketone form of the metal... 

The opposite substrate-induced diastereoselectivity is observed in the cycloadditions to norbor-nenc and norbornadiene, since the adducts exo-9 and endo-10 are obtained, respectively6. The e.w-adduct 10, prepared from norbornene, can also be obtained, together with a rearrangement product, by cycloaddition to quadricyclane followed by double-bond hydrogenation8. 

Treatment of P3C3Bu 3 with [(CO)sW <—PMe], generated / si/u by thermal decomposition of 56b (R = Ph) at 110°C, furnished a 2 1 mixture of quadricyclane 58b and the tricyclic compound 59, which were separated by fractional crystallization as colorless or yellow crystals in 39% and 25% yield, respectively. The presence of a three-membered ring in 59 agrees with an initial attack of the phosphinidene complex at a P=C bond of the triphosphinine to give transient 60. An intramolecular electrophilic aromatic substitution furnished product 59, whereas rearrangement of 60 to tetraphosphanorbornadiene 57a and its intramolecular [2+2] cycloaddition would rationalize the formation of 58b (Scheme 22) <2001CEJ3545>. 

Nonphotochemical cycloadditions of hexafluorothioacetone to alkenes (vinyl ethers, vinyl sulfides, " cyclohexene, and dimethyl maleate " ) have been observed, as illustrated for methyl vinyl ether. The formal addition of thiocarbonyl fluoride to tetrafluorethylene to give hexafluorothietane occurs on thermolysis at 600-700° (lO " mm) of a copolymer of the two components. " QO-Dimethyldithiooxalate undergoes a thermal cycloaddition to quadricyclane to give thietane 51a. ... 

The irradiation of the norbornadiene derivatives (148) yields the quadricyclanes (149). These quadricyclanes can be reconverted to the starting material by treatment with silver ion. The norbornadienes (150) photochemically ring close to the quadricyclanes (151). Both direct and sensitized irradiation were used but the direct irradiation afforded a mixture of products. The reactions were much cleaner using benzophenone as the sensitizer. Irradiation of the norbornadiene derivative (152) affords the quadricyclane (153). Paquette and Racherla - have synthesized the norbornadiene (154) in which it is claimed that homoconjugation is high. This is borne out on acetophenone-sensitized irradiation in pentane when it is converted to the quadricyclane (155). Prinzbach and his coworkers have reported on the photochemical (2+2)-cycloadditive behaviour of adducts of the type represented by (156). The acetone-sensitized irradiation of this compound brings about (2 + 2)-cycloaddition to afford a 2 1 mixture of the adducts (157) and (158). Direct irradiation, however, follows a different path to yield a mixture of the (2+2)-adduct (157) as... 

Other cycloadditions proceed in systems where the reaction partners are preoriented with a geometry ideally suited for the cycloaddition and the system is essentially forced towards the desired transition state. Bridged cyclic dimers or oligomers of norbornadiene and quadricyclane (6) serve as molecular frameworks. 

Almost exclusive intercyclic [(2cr + 2a) + 2n cycloaddition was observed in substrate 10 (prepared from the bis-quadricyclane isomer of the j9-lactam by [(2cr-l-2ff)-l-27t] addition with dimethyl butynedioate). ... 

Domino cycloadditions were observed in cyclic trimers of quadricyclane (e.g. 17 and 19). 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

One typical example is given by G. Kaupp with the [2+2] cycloaddition of diphenyl-acetylene to 1,4-dioxene leading to a bicyclic cyclobutene in good yield (cf. iv of this chapter). Another example is presented by T. Miyashi starting from naphthoquinonor-bomadiene. In an intramolecular reaction a quadricyclane derivative is quantitatively formed upon irradiation at X > 400 nm. It is assumed that this process involves the diketo-norbomadiene in its triplet state. Although the spin multiplicity of intermediates have not... 

Figure 2.59 collects several [2t-2]-cycloadditions that can be carried out with C )-The reaction with dehydrobenzene is instructive as it reveals electronic properties of the fullerene The dehydrobenzene generated in situ from anthranilic acid reacts with Cso exclusively in a [2-i-2]-cycloaddition, although in principle a [4-f2]-reaction would also be possible, and dehydrobenzene usually enters into the latter when adding to electron-rich dienes. The nonoccurrence of this reaction clearly shows the electron-deficient character of C,so. For the same reason it never constitutes the diene part in a Diels-Alder reaction. Furthermore, the [24-2]-cycloaddition may be thermally effected, for example, the addition of long-chain cumulenes, allene amides or quadricyclan. The addition of ketenes as well occurs without irradiating the reaction mixture. Normally a reduced reactivity toward C,so should be expected for the electrophilic ketenes, but in reality the products of a [2-1-2]-cycloaddition are even found to be the major product. 

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