2065-43-2Relevant articles and documents
Origins of selectivity for the [2+2] cycloaddition of α,β- unsaturated ketones within a porous self-assembled organic framework
Yang, Jun,Dewal, Mahender B.,Profeta Jr., Salvatore,Smith, Mark D.,Li, Youyong,Shimizu, Linda S.
, p. 612 - 621 (2008)
This article studies the origins of selectivity for the [2+2] cycloadditions of α,β-unsaturated ketones within a porous crystalline host. The host, formed by the self-assembly of a bis-urea macrocycle, contains accessible channels of ～6 A diameter and forms stable inclusion complexes with a variety of cyclic and acyclic α,β-unsaturated ketone derivatives. Host 1 crystals provide a robust confined reaction environment for the highly selective [2+2] cycloaddition of 3-methyl-2-cyclopentenone, 2-cyclohexenone, and 2-methyl-2-cyclopentenone, forming their respective exo head-to-tail dimers in high conversion. The products are readily extracted from the self-assembled host and the crystalline host can be efficiently recovered and reused. Molecular modeling studies indicate that the origin of the observed selectivity is due to the excellent match between the size and shape of these guests to dimensions of the host channel and to the preorganization of neighboring enones into favorable reaction geometries. Small substrates, such as acrylic acid and methylvinylketone, were bound by the host and were protected from photoreactions. Larger substrates, such as 4,4-dimethyl-2-cyclohexenone and mesityl oxide, do not undergo selective [2+2] cycloaddition reactions. In an effort to understand these differences in reactivity, we examined these host-guest complexes by thermogravimetric analysis (TGA), NMR, powder X-ray diffraction (PXRD) and molecular modeling.
Influences of Lewis Acids on the Photochemical Cyclodimerization of Cyclopentenone
Ogawa, Takuji,Masui, Yoshiyuki,Ojima, Shoshi,Suzuki, Hitomi
, p. 423 - 425 (1987)
In the presence of tin(IV) chloride, photochemical dimerization of cyclopentenone gave a cis-transoid-cis head-to-head dimer as predominant product.Other Lewis acids such as zinc(II) bromide, boron trifluoride etherate, tin(II) chloride, and silver(I) trifluoromethylsulfonate, however showed little influence.Tris(2,2,6,6-tetramethyl-3,5-heptanedionato)lanthanum(III) affected regiochemistry of the reaction in a way to favor a head-to-tail dimer.
The mechanism of the photochemical cycloaddition reaction between 2-cyclopentenone and polar alkenes: Trapping of triplet 1,4-biradical intermediates with hydrogen selenide
Andrew, David,Hastings, David J.,Weedon, Alan C.
, p. 10870 - 10882 (2007/10/02)
The regiochemistry of the photochemical cycloaddition reaction between 2-cyclopentenone and the polar alkenes 1,1-difluoroethene and methyl acrylate has been investigated. With 1,1-difluoroethene the major cycloaddition product is a cyclobutane adduct possessing head-to-tail regiochemistry; with methyl acrylate the reaction is non-regioselective and a 46.5:53.5 ratio of head-to-head and head-to-tail cyclobutane regioisomers is obtained. These regiochemical outcomes are not those predicted by the Corey - de Mayo mechanism conventionally used to describe the enone - alkene photocycloaddition reaction. The triplet 1,4-biradicals implicated as intermediates in the photocycloaddition reaction between 2-cyclopentenone and the polar alkenes methyl acrylate and ethyl vinyl ether have been trapped quantitatively using hydrogen selenide as a hydrogen atom donor. The structures of the trapped products have been determined and hence the structures of their biradical precursors have been inferred. The yields of the trapped biradicals indicate that in both reactions the product distribution is controlled by the extent to which each of the isomeric biradical intermediates closes to products in competition with fragmentation to its ground state precursors. The product distribution does not reflect the relative rates of formation (and hence the relative amounts) of the isomeric biradical intermediates formed in each reaction. This conclusion is inconsistent with the long held idea that an oriented exciplex intermediate controls the reaction regiochemistry by favoring the formation of some biradicals over others. The structures of the biradical intermediates also reveal that both the 2-position and the 3-position of the enone are involved in forming the first bond to the alkene; with ethyl vinyl ether this bond is to the less substituted end of the alkene exclusively, while with methyl acrylate no selectivity in the site of initial bonding is observed.
Radical clocks as probes of 1,4-biradical intermediates in the photochemical cycloaddition reactions of 2-cyclopentenone with alkenes
Rudolph, Andreas,Weedon, Alan C.
, p. 1590 - 1597 (2007/10/02)
The products of the photochemical reaction of 2-cyclopentenone with 1,6-heptadiene and with vinylcyclopropane have been examined.With 1,6-heptadiene the products were cyclobutanes, which arise from 2+2 photocycloaddition between the cyclopentenone carbon-carbon double bond and one of the two terminal double bonds of the heptadiene.The 1,4-biradical that is an intermediate in this reaction contains a derivative of a 1-hexenyl radical; no products derived from cyclization of this intermediate to a cyclopentenylmethyl radical were observed.With vinylcyclopropane some of the products isolated also arose from 2+2 photocycloaddition; in addition, comparable quantities of products resulting from rearrangement of the intermediate 1,4-biradical were obtained.In this case the initially produced intermediate 1,4-biradical contains a cyclopropylmethyl radical that can rearrange to a homoallylic radical; the products isolated were derived from intramolecular disproportionation in the new 1,7-biradical containing the homoallylic radical.The rate constant for the rearrangement of the cyclopropylmethyl radical to the homoallylic radical is known and was used as a clock to estimate the lifetime of the initially produced 1,4-biradical.The value estimated is of the order of 50 ns.The structures of the products indicate that the initial bonding between the excited cyclopentenone and vinylcyclopropane takes place at both the α-position and the β-position of the enone and not exclusively at the α-position as a recent report claims.
