Reaction of Simple Aldehydes or Ketones with Alkynes

Reaction of Jt-allylnickel, generated in situ from methyl vinyl ketone, MejSiCl and Nal, with a polarized alkyne in the presence of CO gives the corresponding cyclopentenone in moderate yield [185], When CO is introduced prior to the addition of the alkyne and with 2 lketone/alkyne stoichiometry, cyclopentenone is obtained regioselectively in high yield. In the absence of CO, cyclohexadiene derivative is obtained. When an excess amount of vinyl ketone is used, acyclic product is isolated predominantly. 

Intramolecular alkylative cyclizations of allenes with enones have been developed for the preparation of cyclic unsaturated carbonyl compounds These processes have been used for the synthesis of (-)-a-kainic acid [175]. 

Reaction of Simple Aldehydes or Ketones with Alkynes 

Nickel-catalyzed reactions of aldehydes with alkynes and MejZn afford the corresponding allylic alcohols in good yields [186,187]. The reaction can also proceed intramolecularly [188]. 

The reductive couplings of aldehydes with unsymmetrical internal alkynes are catalyzed by Ni(COD)2 and an imidazolidene ligand in the presence of 2 equivalents of triethylsilane to give regioselectively the corresponding allyl silyl ethers [189]. 

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Reaction of Simple Aldehydes or Ketones with Alkynes 431 R2... 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

Simple a,3-unsaturated aldehydes, ketones, and esters (R = C02Me H > alkyl, aryl OR equation l)i, 60 preferentially participate in LUMOdiene-controlled Diels-Alder reactions with electron-rich, strained, and selected simple alkene and alkyne dienophiles, - although the thermal reaction conditions required are relatively harsh (150-250 C) and the reactions are characterized by the competitive dimerization and polymerization of the 1-oxa-1,3-butadiene. Typical dienophiles have included enol ethers, thioenol ethers, alkynyl ethers, ketene acetals, enamines, ynamines, ketene aminals, and selected simple alkenes representative examples are detailed in Table 2. - The most extensively studied reaction in the series is the [4 + 2] cycloaddition reaction of a,3-unsaturated ketones with enol ethers and E)esimoni,... 

The classical version of the Diels-Alder reaction uses a 1,3-diene as the 4it component and an alkene or alkyne containing electron-withdrawing groups (conjugated aldehydes, ketones, acids and their derivatives, nitroalkenes, etc.) as the 2n reactant. With appropriately chosen dienes and/or dienophiles, a wide array of purely carbocyclic or heterocyclic compounds can be easily synthesized. A representative set of simple examples is given in Scheme 2.118. 

Keto-enol tautomerism was discussed in Chapter 10 (Section 10.6) in connection with the hydration reaction of alkynes, including oxymercuration or hydroboration. The carbonyl form is more stable, and for most aldehydes and ketones only a tiny amount of enol is present. Acetone (2), for example, exists primarily as the ketone, and experiments show only 1.5 x 10 % enol. This experiment titrated acetone with diatomic bromine and measured the extent of reaction, which converted 2 to l-bromo-2-propanone (a-bromoacetone, 4). The second product in this reaction is HBr. By this experiment, the enol content of acetone—and presumably of other simple ketones—is remarkably small. 

Condensation is a class of oiganic reactions, where two molecules combine, usually in the presence of a catalyst, with elimination of water or some other simple molecule. The combination of two identical molecules is known as self-condensation. Aldehydes, ketones, esters, alkynes (acetylenes) and amines are among several organic compounds that combine with each other and except for amines, among themselves to form larger molecules, many of which are useftil intermediate compounds in organic syntheses. Catalysts conunonly used in condensation reactions include acids, bases, cyanide ion, and complex metal ions. 

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