Ketones, methyl vinyl water promoted

It is believed that clay minerals promote organic reactions via an acid catalysis [2a]. They are often activated by doping with transition metals to enrich the number of Lewis-acid sites by cationic exchange [4]. Alternative radical pathways have also been proposed [5] in agreement with the observation that clay-catalyzed Diels-Alder reactions are accelerated in the presence of radical sources [6], Montmorillonite K-10 doped with Fe(III) efficiently catalyzes the Diels-Alder reaction of cyclopentadiene (1) with methyl vinyl ketone at room temperature [7] (Table 4.1). In water the diastereoselectivity is higher than in organic media in the absence of clay the cycloaddition proceeds at a much slower rate. 

While alcohol oxidations have been the most common metal promoted reactions involving molecular oxygen, a number of other metal catalyzed oxidations of potential synthetic interest have been reported. Supported palladium catalysts are comparable to many soluble palladium catalysts in promoting the selective oxidations of alkenes and aromatics. 2-Butene was oxidized primarily to crotonic acid over Pd/C in water but methyl vinyl ketone and crotonaldehyde were also formed in significant amounts. When this oxidation was run in acetic acid the allyl acetates were the major products, particularly when a Pd/Al203 catalyst... 

On the basis of Monte Carlo simulations [40] and molecular orbital calculations [26a], hydrogen bonding was proposed as the key factor controlling the variation of the acceleration for Diels-Alder reactions in water. Experimental differences of rate acceleration in water-promoted cycloadditions were recently observed [41]. Cycloadditions of cyclopentadiene with acridizinium bromide, acrylonitrile and methyl vinyl ketone were investigated in water and in ethanol for comparison (Scheme 3). Only a modest rate acceleration of 5.3 was found with acridizinium bromide, which was attributed to the absence of hydrogenbonding groups in the reactants. The acceleration factor reaches about 14 with acrylonitrile and 60 with methyl vinyl ketone, which is the best hydrogen-bond acceptor [41]. 

A huge acceleration of the Michael reaction of nitroalkanes with methyl vinyl ketone was mentioned when going from non-polar organic solvents to water. The hydrophobic effect could be at least to some extent involved, since additives, such as glucose or saccharose, accelerate the reaction even more [72]. Cetyltrimethylam-monium chloride as an amphiphilic species which can influence the hydrophobic interactions was found to promote the Michael reaction of various nitroalkanes with conjugated enones in dilute aqueous solutions of sodium hydroxide [73],... 

Diels-Alder reactions may be accelerated and the selectivities enhanced if the cycloaddition reactions are conducted in water or under high pressure or in the presence of a Lewis acid (see Section 3.1.3). In water at room temperature, cyclopentadi-ene reacts with methyl vinyl ketone 700 times faster than in 2,2,4-trimethylpentane and the endo exo selectivity rises from about 4 1 to more than 20 1. This can be ascribed to hydrophobic effects, which promote aggregation of non-polar species. Diels-Alder reactions in water are normally carried out with (at least partially) water-soluble dienes such as sodium salts of dienoic acids. Thus, a key step in a formal synthesis of vitamin D3 involved the cycloaddition of the sodium salt 46 with methacrolein in water, to give the adducts 47 and 48 in high yield and a ratio of 4.7 1 after 16 h (3.43). In contrast, the corresponding methyl ester of the diene in excess neat methacrolein at 55 °C gave only a 10% yield of a 1 1 mixture of isomers after 63... 

Microwave promoted, palladium-catalyzed, DPPP-controlled arylation of butyl vinyl ether with 4-tert-butylphenyl triflate afforded the branched arylation product and the corresponding methyl ketone, indicating the occurrence of selective internal a-arylation. Addition of water to the reaction mixture and microwave-heating for 2.8 min at 55 W smoothly produced the hydrolyzed product, 4-tert-butylacetophe-none, with an isolated yield of 77% (Eq. 11.2) [17]. 

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