Strained multiple bonds

The successful inner phase stabilization of cyclobutadiene suggests that this approach may allow stabilization of other molecules with highly strained multiple bonds. An interesting class of hydrocarbons with twisted C=C bonds are anti-Bredt bridgehead olefins. These bicycloalkenes have a trani-cycloalkene and are unstable if their olefinic strain (OS) is OS > 21kcalmor. Bicyclo[2.2.2]oct-l-ene 60 and (Z)-bicyclo[3.2.1]oct-l-ene... 

Eliminations Giving Rise to Strained Multiple Bonds... 

As described above, the charge-transfer irradiation of the [HB, TCNE] complex produces the contact ion pair, in which the strained HB+ undergoes multiple bond cleavages to afford three isomeric cation radicals depicted in Scheme 19, which then undergo coupling with TCNE- to form a mixture of isomeric cycloadducts (A, B, and C) in equation (73).208... 

Besides the activation of the olefinic partner by a metal, the unfavorable thermodynamics associated with the addition of an enolate to a carbon—carbon multiple bond could be overwhelmed by using a strained alkene such as a cyclopropene derivative286. Indeed, Nakamura and workers demonstrated that the butylzinc enolate derived from A-methyl-5-valerolactam (447) smoothly reacted with the cyclopropenone ketal 78 and subsequent deuterolysis led to the -substituted cyclopropanone ketal 448, indicating that the carbometallation involved a syn addition process. Moreover, a high level of diastereoselectivity at the newly formed carbon—carbon bond was observed (de = 97%) (equation 191). The butylzinc enolates derived from other amides, lactams, esters and hydrazones also add successfully to the strained cyclopropenone ketal 78. Moreover, the cyclopropylzincs generated are stable and no rearrangements to the more stable zinc enolates occur after the addition. 

Besides alkenes and alkynes, other multiple bonds can be used as acceptors in addition reactions of carbon radicals provided the usual requirements of reactivity and selectivity are met. Other types of carbon-carbon multiple bonds that have been used as acceptors include dienes,162 allenes,61 enolates (and ni-tronates, see below) and quinones.223 Even highly strained cr-bonds have served as acceptors on occasion.224... 

The reaction between stannylenes and carbon-carbon multiple bonds generates unsaturated heterocycles containing tin and between two and four other atoms (Scheme 2). Formation of five-membered rings from [4 + 2] addition reactions are by far the most common (43). With the isolated, strained alkynes 11 and 12, both 1-stannacyclopropene (13) and... 

The simplest member of the class is the [l]-ladderane, a small and familiar strained molecule known as cyclobutane. The strain in a [n]-ladderane increases with the number of fused rings and the introduction of multiple bonds." The [2]-ladderane derivative, Dewar benzene, is highly unstable and converts readily to the conjugation-stabilized counterpart, benzene. Depending on the stereochemistry of bridgehead atoms in the fiised-ring system, [njladderanes... 

Methylene ( CH2) generated photochemically or thermally from diazomethane is highly reactive and is prone to incur side reactions to a substantial extent. In order to avoid these undesirable complexities, the cyclopropanation of multiple bonds with diazomethane has usually been carried out under catalytic conditions The catalysts most frequently employed are copper salts and copper complexes as well as palladium acetate. The intermediate produced in the copper salt-catalyzed reactions behaves as a weak electrophile and exhibits a preference to attack an electron-rich double bond. It is also reactive enough to attack aromatic nuclei. In contrast, the palladium acetate-catalyzed decomposition of diazomethane cyclopropanates a,a- or a,jS-disubstituted a,jS-unsaturated carbonyl compounds in high yields (equation 47). The trisubstituted derivatives, however, do not react. The palladium acetate-catalyzed reaction has been applied also for the cyclopropanations of some strained cyclic alkenesstyrene derivatives and terminal double bondsHowever, the cyclopropanation of non-activated, internal double bonds occurs only with difficulty. The difference, thereby. 

By molecular mechanics we mean a method by which we calculate the total energy of a molecule in a particular geometry with reference to a hypothetical molecule with no bond-angle or bond-length deformations, no torsional strain and no steric repulsion and with a given number of single and multiple bonds. The energy difference is obtained as the sum of six components ... 

Each Ti bonds to 3C via a bonds and each C bonds to 2Ti and one C. The all-carbon analogue, C20, is not expected to be stable because of severe internal strain (it would be the smallest possible fullerene, p. 280). Note, however, that dodecahedrane, C20H20, is known.An alternative description of the structure (Fig. 8.18b) would be as a weakly bonded cube. Tig, each face of which is capped by a C2 unit. The calculated distances are Ti Ti 302 pm, Ti-C 199 pm and C-C 140 pm (implying some multiple bonding cf. 140 pm in benzene). An alternative Th structure for TigC, which is calculated to have a lower energy, has also been proposed.In this, the Tig array is a tetracapped tetrahedron containing six Ti4 faces in butterfly conformation each of these Ti4 faces can then accommodate a C2 unit as shown in Fig. 8.18c. 

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