Cyclobutene ring

Steric factors would ordinarily be expected to induce a preference for the larger group to move outward and thus generate the -isomer. It was observed, however, that in the case of l,2,3,4-tetrafluoro-tran5-3,4-bis(trifluoromethyl)cyclobutene, ring opening occurred with an inward rotation of the trifluoromethyl groups. ... 

Table 11.1 Directive Effect of Substituents on Inward/Outward Conrotation in Cyclobutene Ring Opening"...

Scheme 1. Schreiber s oxy-Cope rearrangement/cyclobutene ring opening strategy for the synthesis of cyclodecadienone 8.

The reaction of ethyl 2,2-diethoxyacrylate with alkynylalkoxycarbene complexes affords 6-ethoxy-2H-2-pyranylidene metal complexes [92] (Scheme 48). The mechanism that explains this process is initiated by a [2+2] cycloaddition reaction (see Sect. 2.3), followed by a cyclobutene ring opening to generate a tetracarbonylcarbene complex. This complex can be isolated and on standing for one day at room temperature renders the final 6-ethoxy-2Ff-pyranylidene pentacarbonyl complex. This last transformation requires the formal transfer of one carbonyl group and one proton from the diethoxy methylene moiety to the metal and to the C3 2H-pyranylidene ring, respectively, with concomitant cyclisation. Further studies on this unusual transformation have been extensively performed by Moreto et al. [93]. 

In a similar process, tertiary enaminones react with alkynylcarbene complexes to give the corresponding pyranylidene complexes following a reaction pathway analogous to that described above. First, a [2+2] cycloaddition reaction between the alkynyl moiety of the carbene complex and the C=C double bond of the enamine generates a cyclobutene intermediate, which evolves by a conrotatory cyclobutene ring opening followed by a cyclisation process [94] (Scheme 49). 

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

Aumann et al. have observed an unusual formal [6S+2C] cycloaddition reaction when they performed the reaction between an alkynylcarbene complex and 1-aminobenzocyclohexenes. The solvent used in this reaction exerts a crucial influence on the reaction course and products of different nature are obtained depending on the solvent chosen. However, in pentane this process leads to cyclooctadienylcarbene complexes in a reaction which can be formally seen as a [6S+2C] cycloaddition [117] (Scheme 71). The formation of these compounds is explained by an initial [2+2] cycloaddition reaction which leads to a cy-clobutenylcarbene derivative which, under the reaction conditions, undergoes a cyclobutene ring opening to furnish the final products. 

The nucleophilic attack with 2-700 and 2-701 can proceed in a syn or anti manner to provide either 2-702 or 2-703, or both [357]. If2-703 is formed, it follows a charge-driven conrotatory opening of the cyclobutene ring with generation of the coiled 1,3,5,7-octatetraene 2-704. This intermediate is capable of a rapid helical equilibration [357] and a regioselective 8jt electrocyclic ring closure to give 2-705 [358]. 

This process is quite unexpected for another reason. The cyclobutene ring is highly strained, making this monomer one of the most easily polymerized of all the cycloolefins. Thus, the variety of catalysts effective for cyclobutene polymerization is much broader than that effective for metathesis of low-strained cycloolefins and acyclic olefins (73). Therefore, the recovery of monomeric cyclobutene rather than its respective polymer is remarkable and indicates the lack of substantial metathesis activity in the above retrocarbenation system. 

TABLE 11. Substituted butadienes through cyclobutene ring-opening reactions... 

The simple picture given above agrees remarkably well with the observed experimental facts but it has raised certain problems. For example, let us consider butadiene-cyclobutene ring closure... 

We break the C4-C6 bond, and we form C3-C8 and C4-C9. The formation of the latter two bonds and the fact that we re forming a cyclobutanone suggests a [2+2] cycloaddition between a ketene at C3=C4=0 and the C8=C9 n bond. We can generate the requisite C3=C4 n bond by electrocyclic ring opening of the cyclobutene ring in the S.M. 

Other disulfones like (32) and (33) were studied (see Ref [6]). They exhibit surprisingly stable anion radicals (and even a stable dianion with (32)). Such charged species could be used as redox mediators, tiowever, reduction of alkyl halides RX by (33) did not permit the formation of alkylated products presumably because of the strong steric strain induced by alkylation of the corresponding carbon atoms in the cyclobutene ring. 

A transition complex which is consistent with the low energy of activation of the isomerization involves the simultaneous deformation (twisting) of the cyclobutene ring with the stretching of the carbon-carbon bond opposite the double bond ... 

As in the case of the bicyclo-octane the isomerizations must occur by a disrotatory process. It is clear that, owing to the rigid nature of these bicyclobutenes, considerable stretching of the bridgehead bond is necessary before appreciable twisting of the cyclobutene ring can... 

The first example illustrates how a 1,4-dehydroaromatic system with cyclohexane ring having two double bonds may be also disconnected according to a retro-Diels-Alder to give a diene and an acetylene as the dienophile [25]. The second example makes clear that even an aromatic double bond may be -in some instances-involved in a retrosynthetic pericyclic disconnection [26]. In the synthetic direction, the polycyclisation involves a conrotatory electrocyclic cyclobutene ring opening, (16 15) followed by an intramolecular Diels-Alder addition (see Scheme... 

Discussion. The combination of BCB with BMI significantly improves the properties of the BMI, apparently by tying up the maleimide in a Diels-Alder reaction with the transient diene formed from the opening of the cyclobutene ring. Both the thermal and the thermomechanical properties are improved almost to the level of the BCB... 

Because 1,2-diphenylcyclo-l-butene (DPCB) has a rigid planar structure with the c-St-chromophore structurally constrained by the cyclobutene ring, and because it is stable for geometrical isomerizations, TPCB" is expected to have the same rigid planar structure. Pulse radiolysis of DPCB in DCE produces DPCB, which shows bands at... 

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)). 

If quinoline derivatives are expected to be synthesized by this procedure, the starting cycloalkene should contain a cyclobutene ring. When cyclobutene derivative 65c is reacted with Ig under ethylene gas, isoquinoline derivative 66c is obtained in 60% yield. Furthermore, cyclic amino acid ester 66d is obtained from glycine derivative 65d in 75% yield by a one-step reaction. Various isoquinolone derivatives 66e-66i are also synthesized from cyclobutenyl amides... 

Dienes may be involved in electrocyclization reactions as well. Two well-documented examples are the cyclobutene ring opening244 and the 1,3-cyclohexadiene formation245 reactions. Predictions regarding the stereochemical outcome of these rearrangements can be made applying the orbital symmetry rules. The thermally... 

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