Aldehyde-alkene => allyl vinyl ethers

It is possible to treat ketones with allyl alcohol and an acid catalyst to give y,8-unsaturated ketones directly, presumably by initial formation of the vinylic ethers, and then Claisen rearrangement. In an analogous procedure, the enolates (134) of allylic esters [formed by treatment of the esters with lithium isopropylcyclohexyla-mide (LICA)] rearrange to y,5-unsaturated acids. Allylic alcohols can be treated with a catalytic amount of mercuric acetate, and in the presence of an excess of allyl vinyl ethers give an alkene-aldehyde via a Claisen rearrangement. 

This chapter will discuss 1,3-dienes in a reaction with alkenes to give cyclohexene derivatives. This is a thermal reaction driven by interactions of molecular orbitals rather than ionic or polarized intermediates. In addition to the reaction of 1,3-dienes, 1,5-dienes undergo a rearrangement to a different 1,5-diene in what is known as a sigmatropic rearrangement. Similarly, allyl vinyl ethers rearrange to form alkenyl aldehydes or ketones. Both of these reactions tend to give difunctional molecules as products. 

The Diels-Alder disconnection is actually a two-carbon disconnection. If a molecule has cyclohexene unit, the Diels-Alder disconnection leads to a diene and alkene for precursors as shown. A Cope disconnection has a 1,5-diene target with a 1,5-diene precursor. The oxy-Cope disconnection has an alkene aldehyde or an alkene-ketone target with a diene-alcohol precursor. The Claisen disconnection also has an alkene-aldehyde target, but the precursor is an allyl vinyl ether. The Ireland-Claisen disconnection is included, with an alkene-acid target and an alkene-ester precursor. 

Blaser and Spencer used aroyl halides in place of aryl halides, with aroyl chlorides being of specific interest as ubiquitous, relatively cheap compounds ( Blaser reaction ) [24], This latter reaction is normally conducted in aromatic solvents phosphines are not used here as catalyst ligands since they fully inhibit the reaction. In the same way, benzoic acid anhydrides can be used as the aryl source in combination with PdCl2 and catalytic amounts of NaBr [79]. In this reaction, one of the arenes is used in the coupling reaction by elimination of CO, whereas the other benzoate serves as the base. The benzoic acid thus formed can easily be recycled into the anhydride. The use of aryl and vinyl triflates according to Cacchi [25] and Stille [26] extends the scope of the Heck coupling to carbonyl compounds phenol derivatives act via triflate functionalization as synthetic equivalents of the aryl halides. The arylation of cyclic alkenes [27], electron-rich vinyl ethers [28], and allylic alcohols [29] is accessible through Heck reactions. Allylic alcohols yield C-C-saturated carbonyl compounds (aldehydes) for mechanistic reasons (y9-H elimination), as exemplified in eq. (6). 

Very similar amounts of branched and linear aldehydes are obtained for aU the linear 1-alkenes (i.e. b l = 48/52 for 1-hexene at 20 C and 100 bar CO/H2, 1 1) [22], while an oxygen in b position to the vinyl group favors the formation of the branched isomers as observed for (allyl)ethyl ether (b l = 70/30) and similar substrates [lid]. It should be noted that the linear isomer largely predominate over the branched ones in the hydroformylation of 3-alkyl substituted allyl alkenes (i.e. 3-methylbut-l-ene) [5a]. 

A comparison between experimental and MO data on regioselectivity concerning the hydroformylation of several vinyl substrates (propene, 2-methylpropene, 1-hexene, 3,3-dimethylbutene, fluoroethene, 3,3,3-trifluoropropene, vinyl methyl ether, allyl methyl ether, styrene) with unmodified rhodium catalysts was reported. The activation energies for the alkyl rhodium intermediate formation, computed at either level along the pathways to branched or linear aldehydes, allowed one to predict the regioselectivity ratios. Steric effects may be less important for the unmodified carbonyl complexes and electronic factors dominate, assuming that alkene insertion in Rh-H is irreversible. 

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