Carbonyl compounds formation, enolate allylation

Another approach of regioselective Mizoroki-Heck reactions in ionic liquids is the synthesis of carbonyl compounds [51, 52]. Allylic alcohols (e.g. 12) react with iodoben-zene (5, X = I) via an enolic intermediate (e.g. 13) to the arylated carbonyl product 14 (Scheme 15.4 [52]), which was separated from the reaction mixture by extraction with diethyl ether. However, when choosing the appropriate reaction conditions (solvent and base), the corresponding allylic alcohols were obtained. If Pd(OAc)2 was applied in tetra-butylammonium acetate (as solvent and base) in the coupling of l-octen-3-ol with bromo-or iodobenzene, then selective formation (96 4) of allylic alcohols was achieved in 94% yield in 30 min reaction time at 70 °C [53]. 

Besides the allylation reactions, imines can also undergo enol silyl ether addition as with carbonyl compounds. Carbon-carbon bond formation involving the addition of resonance-stabilized nucleophiles such as enols and enolates or enol ethers to iminium salt or imine can be referred to as a Mannich reaction, and this is one of the most important classes of reactions in organic synthesis.104... 

More complex products are obtained from cyclizations in which the oxidizable functionality and the alkene are present in the same molecule. y9-Keto esters have been used extensively for Mn(III)-based oxidative cyclizations and react with Mn(OAc)3 at room temperature or slightly above [4, 10, 11, 15], They may be cyclic or acyclic and may be a-unsubstituted or may contain an a-alkyl or chloro substituent. Cycloalkanones are formed if the unsaturated chain is attached to the ketone. y-Lactones are formed from allylic acetoacetates [10, 11]. Less acidic /3-keto amides have recently been used for the formation of lactams or cycloalkanones [37]. Malonic esters have also been widely used and form radicals at 60-80 °C. Cycloalkanes are formed if an unsaturated chain is attached to the a-position. y-Lactones are formed from allylic malonates [10, 11]. yff-Diketones have been used with some success for cyclizations to both alkenes and aromatic rings [10, 11]. Other acidic carbonyl compounds such as fi-keto acids, /3-keto sulfoxides, j8-keto sulfones, and P-nitro ketones have seen limited use [10, 11]. We have recently found that oxidative cyclizations of unsaturated ketones can be carried out in high yield in acetic acid at 80 °C if the ketone selectively enolizes to one side and the product cannot enolize... 

Six-centre transition states for the reactions of a carbonyl compound with a Grignard reagent arc very efficient if they allow the formation of a Mg-O bond in concert with the shifts of two pairs of rr or or electrons. Five examples arc shown in Scheine 1.9 (a)-(e). Concerted reduction iind enolization are mentioned above. The allylic addition. Scheme 1.9 (b), is efficient because of the absence of steric hindrance. The carbonyl carbon of an aldehyde may attack ortho to the side chain in benzylmagnesium chloride forming a reactive dihydrobenzene compound, which may-add a second molecule of aldehyde. Scheme 1.9... 

Cross-coupling of silyl enol ethers and allylic silanes. The formation of y,S-unsaturated carbonyl compounds involves one-electron oxidation. 

The Claisen rearrangement [5], the intramolecular reaction of allyl enol ethers 1 to y,<5-unsaturated carbonyl compounds 3, has become a valuable tool for organic synthesis [6]. The sigmatropic process allows a significant alteration of the molecular framework within a single step. The concerted mechanism involves a highly organized transition state 2 that often directs the stereochemical course in the reaction of substituted derivatives and enables the simultaneous formation of two asymmetric centers (Scheme 1). 

The use of palladium(II) 7i-allyl complexes in organic chemistry has a rich history. These complexes were the first examples of a C-M bond to be used as an electrophile [1-3]. At the dawn of the era of asymmetric catalysis, the use of chiral phosphines in palladium-catalyzed allylic alkylation reactions provided key early successes in asymmetric C-C bond formation that were an important validation of the usefulness of the field [4]. No researchers were more important to these innovations than Prof. B.M. Trost and Prof. J. Tsuji [5-10]. While most of the early discoveries in this field provided access to tertiary (3°) stereocenters formed on a prochiral electrophile [Eq. (1)] (Scheme 1), our interest focused on making quaternary (4°) stereocenters on prochiral enolates [Eq. (2)]. Recently, we have described decarboxylative asymmetric allylic alkylation reactions involving prochiral enolates that provide access to enantioenriched ot-quatemary carbonyl compounds [11-13]. We found that a range of substrates (e.g., allyl enol carbonates,... 

In effect, a rearranged allyl group becomes attached to the carbon alpha to a carbonyl group. The formation of the enol ether requires a dehydrating reagent or a derivative of the carbonyl compound into which the allyl alcohol can be exchanged. Examples are shown in Equations 8.33-8.35 [47-49]. 

Another intermediate for which Die Is-Alder trapping provided convincing evidence is the oxy-allyl cation. This compound can be made from a,oc -dibromoketones on treatment with zinc metal. The first step is the formation of a zinc enolate (compare the Reformatsky reaction), which can be drawn in terms of the attack of zinc on oxygen or bromine. Now the other bromine can leave as an anion. It could not do so before because it was next to an electron-withdrawing carbonyl group. Now it is next to an electron-rich enolate so the cation is stabilized by conjugation. 

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