P- enones

Further insight into the P-borylation reaction of a,P-enones (Scheme 2.32) showed that the reaction can be carried out in THF, and the catalyst generated in situ from CuCl (5 mol%), the imidazolium salt (5 mol%), and NaO Bu (10 mol%), to form the [Cu(O Bu) (NHC)] as the catalysis initiating species. In this case, stable boron enolate products are formed which need to be hydrolysed by methanol to the ketone products [114]. 

As discussed before, a,P-unsaturated ketones are usually reduced to saturated ketones in isopropyl alcohol as solvent, the reaction proceeding via the enol after H-abstraction by the carbonyl group. When some selected a,p-enones are irradiated in their n-n absorption band (i.e. excitation... 

Cyclic a,P-enones photodimerize via the Tt-state, as in (4.23)426) and (4.24)427). By chosing the appropriate substituents the reactions proceed regio- and stereoselectively. In contrast, pyrimidine bases dimerize from the first excited singlet state to afford head to tail products (4.25) 428). 

E. J. Corey, F.-Y. Zhang, Mechanism and Conditions for Highly Enantioselective Epoxidation of a,p-Enones Using Charge-Accelerated Catalysis by a Rigid Quaternary Ammonium Salt , Org. Lett. 1999,1,1287-1290. 

Michael reactions of silyl enol ethers.1 2 3 The silyl enol ether of 1-acetylcy-clohexene (1) undergoes two consecutive Michael reactions with an a,p-enone or -enal in the presence of this Lewis acid to form 1-decalones. 

Silyl enol ethers.1 Aldehydes, ketones, ot,p-enals, and a,p-enones are converted into the silyl enol ethers in moderate to high yield by reaction with iodo-trimethylsilane, generated in situ, and N(C2H5)3 at 25° in acetonitrile. In some cases intermediate stable 1,2- or 1,4- adducts can be isolated. Thus the 1,2-adduct a hag been isolated as an intermediate in the reaction of some aldehydes and shown to decompose to the silyl enol ether. 

Vinylcyclopropanation cyclopentenes. The lithium dienolate of 1 adds to a,p-enones to form an a-keto vinylcyclopropane, which on pyrolysis (550°) provides a bicyclic keto acrylate (equation I).1... 

Manganese-copper-catalyzed conjugate addition of organomagnesium reagents to a,p-enones is a general synthetic method. Table I shows that the reaction can be successfully performed even with enones of low reactivity. 30% Manganese chloride is necessary to obtain good yields. In most cases, however, 0.1% cuprous chloride is sufficient.3... 

Figure 3.15. Scope of Rh/(S)-binap-catalyzed asymmetric 1,4-addition of organoboronic acids to a,P-enones.

Following the initial work by Hayashi and Miyaura using (5)-binap, several other chiral ligands were reported to achieve high enantioselectivity in the rhodium-catalyzed asymmetric 1,4-addition of arylboronic acids to ot,p-enones (Figure 3.18). A polymer-supported (X)-binap analog 42 was also synthesized and it was successfully utilized in the rhodium-catalyzed asymmetric 1,4-addition reactions in water (Figure 3.19)." The stereoselectivities observed in this system are comparable to those obtained in the unsupported Rh/(5)-binap system. It was also... 

Figure 3.17. Scope of [Rh(OH)((5)-binap)]2-catalyzed as5dnmetric 1,4-addition of aryl-boroxines to a,P-enones.

By tethering a,p-enones with another carbonyl-containing moiety, Rrische developed rhodium-catalyzed asymmetric 1,4-addition/aldol cyclization reactions." These reactions proceed with high diastereo- and enantioselectivity, furnishing structurally complex cyclic compounds in a single step [Eqs. (3.11-3.13)]. 

Figure 3.18. Chiral ligands that exhibit high enantioselectivity in the rhodium-catalyzed 1,4-addition of arylboronic acids to a,P-enones.

Not only organoboron reagents but also organosilicon reagents are suitable nucleophiles in the rhodium-catalyzed asymmetric 1,4-additions to a,p-enones in... 

A rhodium-catalyzed asymmetric synthesis of chiral titanium enolates by 1,4-addition of aryltitanium reagents to a,p-enones under aprotic conditions has... 

Figure 3.28. Scope of Rh/(5)-binap-catalyzed as5mmetric 1,4-addition of ArTi(Ot-Pr)4Li to a,P-enones in the presence of Me3SiCl.

The cationic chiral Lewis acids 10, generated from the corresponding oxazaboroli-dines by protonation by trifluoromethanesulfonic acid, are excellent catalysts for the enantioselective reaction of 2-substituted acroleins, a-unsaturated a,p-enones, a-unsaturated acrylic acid esters, and a-unsaturated acrylic acids with a variety... 

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