1,5-Hexadiene derivatives

The thermolysis of ladderanes has been studied in detail (Scheme 1). On heating, bicyclo[2.2.0]hexane and its derivatives exhibit skeletal inversion and cleavage to 1,5-hexadiene derivatives.26 The thermolysis of anti- and yyft-tricyclo[4.2.0.02,5]octanes and their derivatives gives cis,cis- and cis, trans-1,5-cyclooctadienes, cis- and trans-1,2-divinylcy clobutanes, and 4-vinylcyclohexene as ring-opening products.27-29 Furthermore, syn-tricyclo-[4.2.0.02,5]octane isomerizes to aw//-tricyclo[4.2.0.02,5]octane.29c,d The thermodynamic parameters and the reaction mechanisms for these thermal reactions have been discussed. 

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

Cope himself formulated this transformation as what would now be called a synchronous pericyclic reaction . This interpretation was supported by Woodward-Hoffmann s analysis of pericyclic processes. The Cope rearrangement of 1,5-hexadiene derivatives was regarded therefore for a long time as a classical example of an allowed pericyclic reaction... 

The Cope rearrangement is the conversion of a 1,5-hexadiene derivative to an isomeric 1,5-hexadiene by the [3,3] sigmatropic mechanism. The reaction is both stereospecific and stereoselective. It is stereospecific in that a Z or E configurational relationship at either double bond is maintained in the transition state and governs the stereochemical relationship at the newly formed single bond in the product.137 However, the relationship depends upon the conformation of the transition state. When a chair transition state is favored, the EyE- and Z,Z-dienes lead to anli-3,4-diastereomcrs whereas the E,Z and Z,/i-isomcrs give the 3,4-syn product. Transition-state conformation also... 

The difficulty in completing the full Cope reaction can, however, be circumvented by installing cation radical stabilizing groups at the 3 and 4 positions of a 1,5-hexadiene derivative [94] or by providing for the delocalization of the cation radical... 

Kawashima, T., Kihara, T., Inamoto, N. The azaphospha-Cope rearrangement of 2-aza-3-phospha-1,5-hexadiene derivatives. Chem. Lett. 1988,577-580. 

This problem was intensively studied both experimentally and theoretically. The quantum chemical calculations were carried out using various methods at different levels. The earlier calculations for the Cope reanangement based on a CASSCF wave function for six electrons in the bonds rearranged were found to overestimate the diradical character of the wave funclion- --. More recently, MP2 methods for the multireference wave function have been developed whose application to an estimate of the energy of the chair transition state has been described. AMI calculations of altemative transition states for the Cope rearrangement of 1,5-hexadiene derivatives have been discussed by Dewar and colleagues i -217... 

The Cope rearrangement is a term applied to conversion of a 1,5-hexadiene derivative into an isomer that also has the 1,5-hexadiene structure a bond is formed between C-l and C-6, the bond between C-3 and C-4 is broken, and the double bonds alter their positions, e.g. ... 

The coordinated alkene or alkyne ligand can be attacked by other alkene or alkyne molecule to accomplish some metal-catalyzed synthetically useful transformations. Typical examples include dimerization and polymerization of alkenes catalyzed by highly electrophilic cations [PdL2(MeCN)2] + (L = MeCN, PR3) (e.g. Scheme 8.35) [57], and Cope rearrangement of 1,5-hexadiene derivatives catalyzed by PdCl2 (Scheme 8.36) [58], It was proposed that the key step in these reactions was the C-C bond formation via attack of the external alkene at the alkene carbon which was made highly electron-dehcientby coordination to Pd(II) ion. 

The MBH adduct shown in Scheme 3.162, prepared from aromatic aldehydes and methyl acrylate, can directly undergo smooth dehydroxylation with concomitant olefin isomerization with low-valent titanium (LVT, prepared from TiCls-LAH-THF) reagent to afford the trisubstituted alkenes 364 with high ( )-selectivity. However, it is unsatisfactory in view of the low yield and the purity of products.More recently, Zhang et al. developed the samarium diiodide-promoted hydroxyl elimination of MBH adducts to form trisubstituted alkenes 364 with total ( )-stereoselectivity in good to excellent yields. This method also provided a new route to synthesizing a class of 1,5-hexadiene derivatives 365 by temperature tuning (Scheme 3.162). ... 

Allyl acetates undergo homocoupling to provide 1,5-hexadiene derivatives in good yield in the presence of 1.8 equiv of zinc powder and 5 mol % of Pd(PPh3)4 in THF (Scheme 3). ... 

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