Alkenes carbonyl-alkene couplings

The ketyl radical anion intermediates can be exploited in carbon-carbon bond-forming reactions. Intermolecular and intramolecular pinacol couplings between the carbonyl groups of ketones and aldehydes are well known (Chapter 5, Section 5.1), as are intermolecular and intramolecular carbonyl-alkene couplings (Chapter 5, Section 5.2). 

Essentially all carbonyl-alkene coupling reactions are carried out in the presence of a proton source, typically an alcohol. The use of a protic additive is... 

Alternatively, enantiomerically enriched 3-aryl-y-lactones have been synthesised using a Sml2-mediated carbonyl-alkene coupling between enantiomerically pure arylaldehyde-Cr(CO)3 complexes and ketones (Scheme 5.25). 

Allylic sulfides can also be used as radical acceptors in carbonyl-alkene couplings, as demonstrated by Matsuda in his synthetic approach to graya-notoxin III.23 After cyclisation, the spontaneous expulsion of sulfide installs a new double bond (see Chapter 7, Section 7.4). 

In addition to alkenes, arenes can sometimes be used as radical acceptors in Sml2-mediated carbonyl-alkene couplings. For example, Schmalz reported extensive studies on ketyl additions to arenechromium tricarbonyl complexes 66,67 tetralin-Cr(CO)3 complex 49 underwent reductive carbonyl addition to the aromatic ring upon treatment with Sml2 to furnish the skeleton of the naturally occurring aryl glycoside pseudopterosin G (Scheme 5.37).66,67 Here, the bulky metal tricarbonyl group not only serves to control the... 

Iridium-phosphine complexes were found to be efficient carbonylative alkyne-alkene coupling catalysts [62]. Although frequently applied in other transformations, the dimeric complex [ Ir( x-Cl)(cod) 2] appeared to be a very active catalyst in the coupling of silylated diynes with CO [63], giving bicyclic products with a carbonyl moiety (Scheme 14.12). 

An interesting deoxygenation of ketones takes place on treatment with low valence state titanium. Reagents prepared by treatment of titanium trichloride in tetrahydrofuran with lithium aluminum hydride [205], with potassium [206], with magnesium [207], or in dimethoxyethane with lithium [206] or zinc-copper couple [206,209] convert ketones to alkenes formed by coupling of the ketone carbon skeleton at the carbonyl carbon. Diisopropyl ketone thus gave tetraisopropylethylene (yield 37%) [206], and cyclic and aromatic ketones afforded much better yields of symmetrical or mixed coupled products [206,207,209]. The formation of the alkene may be preceded by pinacol coupling. In some cases a pinacol was actually isolated and reduced by low valence state titanium to the alkene [206] (p. 118). 

Free-radical-mediated four-component coupling reactions are rare. However, when an allyltin-mediated radical carbonylation is conducted in the presence of electron-deficient alkenes, four-component coupling reactions take place efficiently to give good yields of p-functionalized <5,fi-unsaturated ketones [40]. The wide scope of this four-component coupling reaction is noteworthy Primary, secondary, and tertiary alkyl bromides and iodides can be used as well as aromatic and vinylic halides. A variety of electron-deficient alkenes, such as methyl vinyl ketone, ethyl acrylate, acrolein, acrylonitrile, and vinyl sulfone, can be used as the acyl radical trap (Scheme 6.23). Fluorous allyltin compounds can also be used in four-component coupling reactions [41]. 

Alkyne-alkene carbonylative coupling. Intramolecular carbonylative coupling of dialkynes catalyzed by Fe(CO)3 provides a route to cyclopentadienones (equation I). The more difficult carbonylative alkyne-alkene coupling to provide cyclopen-tenones (Pauson-Khand reaction) can also be effected with Fe(CO)s, but in modest yield. In an improved coupling, acetone is treated with Fe2(CO)9 to form Fe-... 

A tetrahydrofuran fused with a seven-membered ring was obtained from an enyne through a [5+2] cycloaddition reaction catalyzed by [(C10H8)Rh(COD)]+ SbF6 complex <02AG(E)4550>. Rhodium-catalyzed carbonylative alkene-alkyne coupling reactions... 

The stereochemistry of the alkene formed by coupling of aliphatic carbonyl compounds is related to the energy difference between the E) and (Z)-alkene. If AE is larger than 4-5 kcal/mol, the ( )-isomer is formed selectively. 

Later the same group reported that a-bromo carbonyl compounds could be added to terminal alkenes [108] or coupled with allylgallium [109] through intermolec-ular radical reactions still initiated with triethylborane. Another remarkable example was reported by Miyabe et al. They described the indium [110] or triethylborane [111]-mediated radical addition of alkyl iodide to electron deficient C=N bond and C=C bond in water (Eq. 10). 

Diiodonickel (1 mol%) accelerated significantly many of the reductions performed by Sml2 in THE For example, some epoxides were deoxygenated in good yields to the corresponding alkenes at room temperature in 20 min, 2-octanone gave the pinacol in 10 min (instead of 24 h without catalyst) at room temperature, and cyclohexane carbonyl chloride coupled quantitatively into 2-hydroxy-1,2-dicyclohexylethanone in 1 min (reaction performed in THP). 

---