Surprising exocyclic regioselectivity in electrophilic additions to alkylidenecyclobutenes
作者:Hillel Pizem、Ofer Sharon、Aryeh A. Frimer
DOI:10.1016/s0040-4020(02)00267-3
日期:2002.4
alkylidenecyclobutenes shows a strong regioselectivity—in some cases regiospecificity—for the exocyclicdoublebond. Depending on the reagent, the endocyclic doublebond will only undergo reaction upon the addition of a second equivalent of reagent. In the case of the first three reagents, this exocyclic regioselectivity is rationalized by invoking the formation of the more stabilized carbocation intermediate
向亚烷基环丁烯中添加过酸,溴,TCNE和单线态氧显示出对环外双键的强烈区域选择性(在某些情况下为区域特异性)。取决于试剂,内环双键将仅在添加第二当量的试剂时发生反应。在前三种试剂的情况下,通过调用更稳定的碳正离子中间体或过渡态的形成来合理化这种环外区域选择性。关于1 O 2,其中极性机制很少,我们将此环外区域选择性归因于不适当的排列,因此缺乏环内烯丙基氢的反应性。
Study of the substituted vinylallene-methylenecyclobutene electrocyclic equilibria. Comparison with the butadiene-cyclobutene and bisallene-bismethylenecyclobutene electrocyclic equilibria
作者:Daniel J. Pasto、Wei Kong
DOI:10.1021/jo00278a010
日期:1989.8
Preparation and Photosensitized Oxidation of Isopropylidenecyclobutanes and -cyclobutenes
作者:Aryeh A. Frimer、Joseph Weiss、Hugo E. Gottlieb、Joel L. Wolk
DOI:10.1021/jo00083a019
日期:1994.2
Isopropylidenecyclobutanes 2-5 underwent facile ene reaction with singlet dioxygen, yielding (upon Ph(3)P reduction) the corresponding pairs of epimeric allylic alcohols 9 and 10, 11 and 12, 13 and 14, and 15 and 16, respectively. A combination of spectral evidence and molecular modeling studies were utilized in the structural assignment of the epimers. The data clearly indicate that steric considerations play an important role in determining the face of the ring which O-1(2) approaches. Isopropylidenecyclobutenes 6 and 7 reacted with singlet oxygen more slowly than their monoolefinic analogs, yielding upon reduction allylic alcohols 21b and 22, respectively. Benzo analog 7 also generated a small and solvent-dependent amount of isomeric aldehydes 23 and 24, presumably via a free-radical mechanism. n-Butyl diene 8 underwent rapid photosensitized oxygenation producing allylic alcohol 35 (as the O-1(2) ene product) and dione 37 (the Hock-cleavage product of allylic hydroperoxide 39, formed in turn via a free-radical route) in a 1:9 ratio. Ah initio (STO-3G) calculations confirm that, in their lowest energy conformations, compounds 2-8 are planar with the methylene ring hydrogens displaced ca. 36 degrees from the perpendicular. As a result, only exocyclic ene product is formed, since O-1(2) strongly prefers axial or pseudoaxial allylic hydrogens. These calculations combined with the relative rate data suggest that the initial interaction between the electrophilic O-1(2) and alkylidenecyclobutenes involves both ends of the singlet dioxygen molecule, in which the ''front'' end attacks the reactive exocyclic double bond while the ''back'' end obtains stabilization by interacting with the more electron rich but unreactive endocyclic olefin linkage. Because of this added, and presumably substantial, stabilization, the relative rates within this system are determined in part by the orbital coefficients at the latter olefinic center.
Cycloaddition reactions of 1,1-dimethylallene with substituted alkynes
作者:Daniel J. Pasto、Wei Kong
DOI:10.1021/jo00255a027
日期:1988.9
PASTO, DANIEL J.;KONG, WEI, J. ORG. CHEM., 54,(1989) N3, C. 3215-3216