Cyclobutenes, cyclizations

For the solution of this problem we proposed a simple numerical method [144] on the basis of which the above model was applied to several selected pericyclic reactions. In order to maintain the close correspondence with alrea presented example of the butadiene to cyclobutene cyclization discussed in coimection with the topological criterion of concertedness, the proposed formalism will be demonstrated first just for this specific pericyclic process. On the basis of the idealized reaction scheme the whole procedure is quite straightforward and consists in the conversion of the corresponding structural formulae into the approximate wave functions. 

Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

SUBSTITUTED BUTADIENES. The consequences of p-type orbitals rotations, become apparent when substituents are added. Many structural isomers of butadiene can be foiined (Structures VIII-XI), and the electrocylic ring-closure reaction to form cyclobutene can be phase inverting or preserving if the motion is conrotatory or disrotatory, respectively. The four cyclobutene structures XII-XV of cyclobutene may be formed by cyclization. Table I shows the different possibilities for the cyclization of the four isomers VIII-XI. These structmes are shown in Figure 35. 

Dimethyl acetylenedicarboxylate (80) undergoes initial 1,2 cycloaddition with acyclic enamines to form cyclobutene intermediates which immediately decompose into acyclic dienaminodiesters (94,95). When an acyclic n/c-enediamine is used instead of a simple acyclic enamine, a dienediamino-diester is produced via a cyclobutene intermediate (95a). A cyclization reaction of dimethyl acetylenedicarboxylate with an acyclic enaminoketone... 

The reaction mechanism was considered to be oxidative cyclization, and pal-ladacyclopentene 32 was formed. Reductive elimination then occurs to give cyclobutene 33, whose bond isomerization occurs to give diene 28. The insertion of alkyne (DMAD) into the carbon palladium bond of 32 followed by reductive elimination occurs to give [2+2+2] cocyclization product 27. Although the results of the reactions of E- and Z-isomers of 29 with palladium catalyst 26a were accommodated by this pathway, Trost considered the possibility of migration of substituents. Therefore, 13C-labeled substrate 25 13C was used for this reaction. 

Cyclobutenes have been applied as latent functionality of 1,4-dicarbonyl systems. Photolysis of (466) gave a 1.5 1 mixture (60%) of (467a) and (467b), which were cleaved by ozone and subsequent reduction of the ozonides yielded the epimeric (468). Cyclization and dehydration process converted (468) to the furan (469) 160). Furan (469) was converted to hibiscone C in a few steps, 60). 

The ozonolysis of cyclobutene derivatives in the preparation of 1,4-diketones was also applied to the total synthesis of eyclopentanoid antibiotics 161 162k The oxidative cleavage of (470) by ozone and reductive work-up yielded the diketone (471) in 73 % yield. Diketone (471) underwent intramolecular aldol cyclization to give the key intermediate (472), which was used to synthesize ( )-xanthocidin161,162), (+)-epi-xanthocidin 162), ( )-p-isoxanthocidin161,162) as well as ( )-desdihydroxy-4,5-didehydroxanthocidin162). 

CYCLOBUTANONE, 51, 76 CYCLOBUTANONE VIA SOLVOLYTIC CYCLIZATION, 54, 84 Cyclobutanones, 54, 32 CYCLOBUTENE, cis-3,4-DICHLORO-,... 

These processes may be designated conrotatory (16) and disrotatory (17). In practice the isomerization of the appropriately substituted cyclobutenes follow a conrotatory pathway. Thus ci5-3,4-dimethylcyclobutene yields only cia-irans-2,4 hexadiene, and iraws-3,4-dimethyleyclobutene yields only transition state suggested previously, the conrotatory process is in fact the one to be expected. However, the situation is not quite as simple as here implied. By similar arguments the thermal cyclization of hexatrienes would also be expected to be conrotatory, whereas in fact it is disrotatory, viz. ... 

For an open chain system with (4rH- 2) 7T-electrons, the reverse is true and hence cyclization and ring rupture are in this case disrotatory. This description of the process shows why the conrotatory process is favoured in the cyclobutene isomerization. It does not rule out the reverse process where the conrotatory process is energetically very unfavourable. 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

Similarly, electrocyclic reactions7 follow the Woodward-Hoffmann rules and proceed in either a con- or disrotatory manner. For example, the diene 5 cyclizes to the cyclobutene 6 or 7, which are enantiomers. No diastereomers with R1/Ri trams geometry are formed. 

Trost and Tanoury found an interesting skeletal reorganization of enynes using a palladium catalyst.In this reaction, the second product is derived from a metathesis reaction (Equation (5)). It was speculated that the reaction would proceed by oxidative cyclization of enynes with the palladium complex followed by reductive elimination and then ring opening. To confirm this reaction mechanism, they obtained a compound having a cyclobutene ring, which was considered to be formed by the reductive elimination (Equation (6)). 

Benzodioxocin (m.p. 5 °C) (245), prepared from the dihydro compound (244) via either the mono- or di-bromide, shows no aromatic character <67AG(E)697). The H NMR spectrum contains signals in the alkenic region at 8 5.35 and 6.95 p.p.m. for the dioxocin protons. 1,6-Benzodioxocin gives a Diels-Alder adduct (247) and on irradiation undergoes cyclization to the cyclobutene (246). 

Ring expansion of vinylcyclopro-penes and cyclobutenes 8-34 Ring expansion of vinylcycloal-kanes cyclization of diynes 8-39 Metathesis of dienes 8-40 Metal-ion-catalyzed o-bond rearrangements... 

For butadiene, two photochemical cyclization products are cyclobutene or bfcyclobutane. 

Heating with antimony(V) fluoride can result in cyclization to perfluoro(1.2-dialkylcy-clopentenes) (excess SbF5) or to perfluoro(l,2-dialkylcyclobutenes) (catalytic SbF5), as has been reviewed by Krespan and Petrov.21 Thus, heating perfluoroocta-2,6-diene (28) in the presence of a threefold excess of antimony(V) fluoride gives perfluoro(l-ethyl-2-methylcy-clopentene) (29) in 90% yield.22,23 In the presence of a catalytic amount of antimony(V) fluoride, 28 is converted into isomeric cyclobutenes 30 and 31 together with cyclopentene 29. 

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