Pent-4-enals

Claisen-Cope rearrangement of the allyl vinyl ether. The ether (1.1 g) is heated for 30 minutes at 180-185 °C under nitrogen in a fully immersed half-filled sealed Pyrex glass tube (CAUTION). After cooling, the product is distilled under reduced pressure to furnish 4-(4-methylphenyl)pent-4-enal (850 mg, 87%), b.p. 100-102 °C/5-7mmHg i.r. (film) 2700, 1710 (CHO), 3070, 1650, 890 (>C=CH2), and 825cm-1 (para-substituted benzene). 

Considerable information about the course of aldehyde decarbonylations has been gleaned from the decarbonylations of alk-4-enals. Pent-4-enals form cyclopentanones in high yield in decarbonylations catalyzed by [RhCl(PPh3)3], The major product from the decarbonylation of hex-4-enal is 2-methylcyclopentanone. As shown in Scheme 5, the cyclization reaction requires a vacant site on rhodium. The other products result from decarbonylation of the unsaturated acyl before cyclization can take place. In these cases, there is competition between addition of deuterium to C-1 of the alkenyl ligand or its addition to the alkene bond and the formation of an unstable metallocycle. ... 

The cychzation of pent-4-enals and related compounds to cyclopentanones is catalyzed by [RhCl(PPh3)3] in a reaction that involves oxidative addition of the -CHO group to the rhodium(I) catalyst. This reaction has much in common with the decarbonylation that most aldehydes undergo in the presence of this catalyst (see Decarbonylation Catalysis). 

Cyclopropane formation is also observed in transition-metal-mediated conversions of pent-4-enals. This reaction proceeds via oxidative addition of the metal to the aldehyde C —H group, decarbonylation and cyclization of the resulting homoallylmetal complex. 

This reaction competes with intramolecular hydroacylation of pent-4-enals to form cyclopen-tanones. In the case of exo- and ent/o-norborn-5-ene-2-carboxaldehyde (4) if treated with Wilkinson s catalyst [tris(triphenylphosphane)rhodium(I) chloride] only decarbonylation occurs. While the exo-aldehyde exo-4 leads to norbornadiene (5), the e fi o-aldehyde endo-4 reacts to form nortricyclene (6 tricyclo[2.2.1.0 ]heptane). These results support organometallic pathways and exclude radical intermediates, since here identical products should be formed. 

Hoffman and Carreira [31] reported a Rh-catalyzed asymmetric intramolecular hydroacylation of pent-4-enal substrates, providing P-substituted cyclopentanones in good yield and excellent selectivity by using chiral spiro phosphoramidite-alkene ligand (R)-29 (Scheme 29). 

Intramolecular hydroacylation of 4-substituted pent-4-enals is catalyzed by Rh (I) complexes and leads to 3-substituted cyclopentanones. When the ligand of rhodium is (l ,2,S)-3.35 (X = CH2CH2) or even better (Ky or (S)-binap 3.43 (Ar = Ph), cycloalkanones are obtained with an excellent enantiomeric excess... 

The calculated large thermodynamic stability of 11a over 11b by 83.3 kJ/mol in free energy indicates that the awh -Markovnikov insertion path is unlikely. In addition, the Markovnikov path is also more favored kinetically by 7.1 kJ/mol in free energy. The higher stability of 11a derives from the sy -7/ -allyl coordination. Thus, the formation of the linear 1,2-addition product, pent-4-enal, can be ruled out. 

In 2011, the Carreira group developed the Rh-catalyzed asymmetric intramolecular hydroacylation reactions of pent-4-enals 17 for the preparation of cyclopentanones 18 (Scheme 8.7). In this study, the biindane-derived phos-phoramidite/olefin ligand (R)-Ll was found to be highly efficient to promote... 

The phenyl analogue likewise gave a high yield of benzoylethylene. Along the same line, King and Harding have reported a sulpho-Cope rearrangement which appears to involve sulphene formation. Vinyl allyl sulphone, when heated to 170 °C in the liquid phase or to 800 °C in the gas phase, probably was converted into the sulphene (87). In the gas phase, pent-4-enal was isolated, whereas in the liquid phase addition of phenol led to a phenyl sulphonate. 

Scheme 8.6 Side products in the rhodium-catalyzed decarbonylation of pent-4-enal in hs presence of ethylene.

Substituted v-butyrolactones can be prepared by reaction of aldehydes or ketones with tbe dianion (28), and direct condensation of symmetrical ketones with diethyl 2-oxomalonate provides a useful synthetic route to the butenolides (29). A number of initiators have been used previously to promote the free-radical addition of ketones to alkenes now transition-metal oxides have been shown to be effective. Pent-4-enal is cyclized to cyclopentanone by chlorotris(triphenylphos-phine)rhodium(i) through a non-radical pathway. ... 

Under these conditions, several 5-halofuranosides and 6-halopyranosides ring-opened to the corresponding pent-4-enals and hex-5-enals, respectively [16,17]. Mechanistically, the Bernet-Vasella reaction proceeds via oxidative insertion of zinc into the carbon-bromide bond, followed by reductive elimination with the loss of methoxide to give the aldehyde product (Scheme 3.2) [18]. 

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