Aldehydes alkyl vinyl ketones

Recently, Trost et al. reported the vanadium-catalyzed addition reaction of 2,3-allenols [180], Here the oxygen in 401 served as an intramolecular nucleophile to attack the center carbon atom of allene to form a vanadium enolate 402. Aldol condensation of 402 with an aldehyde afforded (2-hydroxy)alkyl vinylic ketones 403. 

Enamines of aldehydes react with alkyl vinyl ketones.212 Substituted cyclohexanones may be obtained after hydrolysis. Application of this reaction to a,j8-unsaturated aldehydes leads to substituted glutardialdehydes.269 The ratio of the products from the addition of methyl vinyl ketone to the pyrrolidine enamine derived from jS-decalone depends on the configuration of the decalone.76... 

Scheme 2.20 Enantioselective intermolecular Michael reaction between aldehydes and alkyl vinyl ketones.

The additions of aldehydes or imines to alkyl vinyl ketones proceed well with... 

Inspired by the use of chiral imidazolidinones as highly enantioselective catalysts for Diels-Alder, 1,3-dipolar cycloaddition and Friedel-Crafts reactions, Tan et have synthesized a series of novel chiral imidazolines and examined their application in MBH reactions. Up to 54% ee and high yields were obtained by using stoichiometric amounts of imidazoline 175 for the MBH reactions of various aromatic aldehydes with unactivated acrylates. Furthermore, the imidazolines were also suitable promoters for reactions between aromatic aldehydes and alkyl vinyl ketone. Using 50 mol.% of imidazoline 176, which bears a chiral methylnaphthyl group, afforded adducts in high yield with up to 78% ee (Scheme 2.86). These chiral imidazolines are readily prepared from commercially available amino alcohols and can be easily recovered for reuse without loss of product enantioselectivity. 

The TiCU-mediated MBH reactions of alkyl vinyl ketones with various electrophiles such as a-keto esters, fluoromethyl ketones and aldehydes have been developed by Basavaiah et al. An interesting reaction trend was found in that a-keto esters and fluoro methyl ketones provided the usual MBH adducts 244 and 245, whereas aldehydes provided the (Z)-allyl chlorides 246 exclusively (Scheme 2.144). Subsequently, this strategy was also extended to aryl 1,2-diones 247 and 248. Instead of the usual MBH adducts, an interesting class of functionalized fused furans 249 and 250 were obtained (Scheme 2.145). ... 

At higher temperatures the mixture of 10 and methyl vinyl ketone yields the 1,4-carbocyclic compound as described previously. Methyl isopropenyl ketone (5), ethyl acetylacrylate (d), 2-cyclohexenone (21), and 1-acetyl-1-cyclohexene (22) also undergo this type of cyclization reaction with enamines at higher temperatures. This cycloalkylation reaction occurs with enamines made of strongly basic amines such as pyrrolidine, but the less reactive morpholine enamine combines with methyl vinyl ketone to give only a simple alkylated product (7). Chlorovinyl ketones yield pyrans when allowed to react with the enamines of either alicyclic ketones or aldehydes (23). 

The definition of what constitutes reactive or unstable functionality naturally varies somewhat, but there is also a good deal of consensus (aldehydes, alkyl halides, vinyl ketones, and so forth) [14,16-18]. 

By far the most important compounds of this type are the a,/ -unsaturated aldehydes and ketones, of which the simplest general representation is (8) and (9) respectively. Branching may be present at either or both of the a- or p- positions as for example in 2-ethylhex-2-enal (10) and 4-methylpent-3-en-2-one (11). Compounds where the carbon-carbon double bond is terminal are acraldehyde [(8), R=H], and the alkyl (or aryl) vinyl ketones [(9), R = H, R = alkyl, aryl etc.]. 

Lewis acid complexes of -substituted a, 3-unsaturated ketones and aldehydes are unreactive toward alkenes. Crotonaldehyde and 3-penten-2-one cannot be induced to undergo ene reactions like acrolein and methyl vinyl ketone. The presence of a substituent on the -carbon stabilizes the enal- or enone-Lewis acid complex and stericdly retards the approach of an alkene to the -carbon. However, Snider et al. have found that a complex of these ketones and aldehydes with 2 equiv. of EtAlCk reacts reversibly with alkenes to give a zwitterion (22). This zwitterion, which is formed in the absence of a nucleophile, reacts reversibly to give a cyclobutane (23) or undergoes two 1,2-hydride or alkyl shifts to generate irreversibly a p, -disubstituted-a,P-unsaturated carbon compound (24). 

Just as trialkylboranes add to simple alkenes (15-16), they rapidly add to the double bonds of acrolein, methyl vinyl ketone, and certain of their derivatives in THF at 25°C to give enol borinates (also see, p. 631), which can be hydrolyzed to aldehydes or ketones." If water is present in the reaction medium from the beginning, the reaction can be run in one laboratory step. Since the boranes can be prepared from alkenes (15-16), this reaction provides a means of lengthening a carbon chain by three or four carbons, respectively. Compounds containing a terminal alkyl group, such as crotonaldehyde (CH3CH=CHCHO) and 3-penten-2-one, fail to react under these conditions, as does acrylonitrile, but these compounds can be induced to react by the slow and controlled addition of O2 or by... 

This chapter focuses the attention on the reactions of nonstabilized carbanionic compounds such as alkyl, vinyl, aryl, alkynyl metals, etc., and the chemistry of the stabilized system, i.e. allylic, propargylic or oxaallylic carbanions is presented in Volume 2 of this series. Electrophiles with C=X bonite which are discussed include aldehydes, ketones, epoxides, aziridines, acetals, orthoesters and imines, all of which turn into highly reactive electrophiles in the presence of Lewis acids. 

For a useful synthetic sequence involving reaction of an aldehyde diethylacetal with an alkyl vinyl ether, see Ethyl vinyl ketone. 

---