Of cyclobutenes

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. 

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

There are several general classes of pericyclic reactions for which orbital symmetry factors determine both the stereochemistry and relative reactivity. The first class that we will consider are electrocyclic reactions. An electrocyclic reaction is defined as the formation of a single bond between the ends of a linear conjugated system of n electrons and the reverse process. An example is the thermal ring opening of cyclobutenes to butadienes ... 

The cyclobutene-butadiene interconversion can serve as an example of the reasoning employed in construction of an orbital correlation diagram. For this reaction, the four n orbitals of butadiene are converted smoothly into the two n and two a orbitals of the ground state of cyclobutene. The analysis is done as shown in Fig. 11.3. The n orbitals of butadiene are ip2, 3, and ij/. For cyclobutene, the four orbitals are a, iz, a, and n. Each of the orbitals is classified with respect to the symmetiy elements that are maintained in the course of the transformation. The relevant symmetry features depend on the structure of the reacting system. The most common elements of symmetiy to be considered are planes of symmetiy and rotation axes. An orbital is classified as symmetric (5) if it is unchanged by reflection in a plane of symmetiy or by rotation about an axis of symmetiy. If the orbital changes sign (phase) at each lobe as a result of the symmetry operation, it is called antisymmetric (A). Proper MOs must be either symmetric or antisymmetric. If an orbital is not sufficiently symmetric to be either S or A, it must be adapted by eombination with other orbitals to meet this requirement. 

Fig. 11.3. Symmetry properties of cyclobutene (top) and butadiene (bottom) orbitals.

Fluoroalkyl acetylenes are powerful enophiles and Diels-Alder dienophiles but also can give good yields of cyclobutenes in their cycloadditions [115,116,117] (equations 48 and 49)... 

Fluorinated bicyclo[2 2 0]hex 5-en 2 ones prepared from Dewar benzene derivatives serve as synthons of cyclobutenes [269] Photochemical cleavage of fluonnated azacyclohexadiene in the gas phase also gives fluorinated cyclobutene [270] (equation 41)... 

Compound 6 is a pivotal intermediate in Schreiber s synthesis. It was hoped that the conspicuous and strained bridgehead cyclobutene substructure in 6 would undergo a conrotatory electrocyclic ring opening upon thermolysis to give an isomeric 1,3-diene (8, Scheme 1). In the event, when a solution of cyclobutene 6 in toluene is confined to a sealed tube and heated to 180°C for 12 h, a stereoisomeric mixture of 1,3-dienes 7 and 8 is produced in an excellent yield of 95% (7 8 ca. 5 1). Finally, irradiation of the 5 1 mixture of cis-7 and trans-8, or of each independently, establishes a photostationary state in which the desired trans isomer 8 predominates (8 7 ca. 10 1). 

Since 3-methylenecyclobutane-l,2-dicarboxylic anhydride is easily converted to 3-methyl-2-cydobutene-l,2-dicarboxylic acid, it is an intermediate to a variety of cyclobutenes. The dimethyl ester of 3-methylenecyclobutane-l,2-dicarboxylic acid is also a versatile compound on pyrolysis it gives the substituted allene, methyl butadienoate, and on treatment with amines it gives a cyclobutene, dimethyl 3-methyl-2-cyclobutene-l,2-di-carboxylate. ... 

Scheme 80 Use of ROCM of cyclobutene 406 in synthetic work directed to sporochnol (410) [170]...

The cationic pathway allows the conversion of carboxylic acids into ethers, acetals or amides. From a-aminoacids versatile chiral building blocks are accessible. The eliminative decarboxylation of vicinal diacids or P-silyl carboxylic acids, combined with cycloaddition reactions, allows the efficient construction of cyclobutenes or cyclohexadienes. The induction of cationic rearrangements or fragmentations is a potent way to specifically substituted cyclopentanoids and ring extensions by one-or four carbons. In view of these favorable qualities of Kolbe electrolysis, numerous useful applications of this old reaction can be expected in the future. 

Table 13-1. Computed reaction barriers and isomer stabilities [kcal/mol] for the electrocyclic ring opening of cyclobutene (relative to cyclobutene 1, including zero-point vibrational contributions). Except for G2, the results were obtained using the 6-311+G(d,p) basis set.

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