Cyclobutene ring opening

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. 

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

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... 

The reaction of cyclobutene ring opening leading to 1,3-cyclobutadiene has been extensively studied by experimental and theoretical methods.64,66- 71... 

This electrocyclic cyclobutene ring opening also gives the product as a single stereoisomer. 

Figure 7-20. The molecular orbitals participating in the cyclobutene ring opening.

Murakami, M. Usui, 1. Hasegawa, M. Matsuda, T. Contrasteric sto-eochemical dictation of the cyclobutene ring-opening reaction by a vacant boron p orbital, 7. Am. Chem. Soc. 2005,127,1366-1367. 

A consideration of the cyclobutene ring-opening reactions [34, 35] (Table 7.4) reveals that the changes in hybridisation of carbon bonded to fluorine are the same for both compounds T7.4A and T7.4B, and so any changes in carbon-fluorine bond energies must also be the same. Consequently, as A// for T.7.4A is more endothermic than for T7.4B,... 

RULES PERTAINING TO THE THERMAL CYCLOBUTENE RING OPENINGS... 

J.I Substituent Effects on the Cyclobutene Ring Opening Reaction... 

Two-carbon Ring Ejqtansion via Cyclobutene Ring Openings... 

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