Molecular eliminations ketones

One of the most interesting molecular elimination reactions was first discovered by Norrish and Appleyard65 in 1934 and studied further by Bamford and Nor-rish66-69 in papers appearing in 1935 and 1938. These authors found that, on photolysis, aliphatic ketones with hydrogen atoms on carbons in the gamma position to the carbonyl yielded olefins and a methyl ketone. An early example was found in 2-hexanone, viz. 

Eschenmoser and collaborators (28) have reported the base induced fragmentation of a.B-epoxy-tosylhydrazones to produce acetylenic ketones (e.g. 69 71). Interestingly, in this fragmentation, both the triple-bond and molecular nitrogen are produced from the key intermediate 70 via an anti mode. The decarboxylation of nitrobenzisoxazole carboxylate (72) into 2-cyano-5-nitrophenol (73) can also be viewed as a trans-elimination (29, 30). 

The spectra of various disubstituted ferrocenes were examined by Clancy and Spilners (44) at low ionizing voltage, and they observed only the molecular ions. They have also used mass spectrometry to identify products from the reaction of lithioferrocene with benzyl chloride one of the products was dibenzylferrocene (189). The mass spectra of [5]-ferroceno-phanes show very intense molecular ion peaks and fragmentation is limited (9, 10). The ketone derivatives (LXXXT) and (LXXXII) fragment by elimination of carbonyl groups first. 

Collection of the GC effluent and subsequent MS analysis allowed assignment of a possible molecular formula as Ci9H280. Since boar taint can be eliminated by castration of male pigs, attention was focused on the testosterone and androsterone family of compounds as possible candidates. When the crude volatiles were treated with 2,4-dinitrophenylhydrazine, the boar taint odor was completely removed, implicating a ketone functionality for the lone oxygen atom. Anecdotal information implicated several androstene derivatives, including 47, which was described as having an intense, urine-like odor. 143 An authentic sample of 47 was prepared, and comparison of the GC and MS properties allowed the definitive structural identification of the boar taint compound. 

The evidence for the mechanisms of the mass-spectrometric and photochemical reactions leading to the eliminations of an olefin from a ketone [equation (120)] have been summarized (Section VIIDl). If it is accepted that the structure of the fragment ion from this process has an enolic structure, it is possible to discuss the mechanism of the reaction theoretically. The reaction appears to consist of two parts, first the transfer of hydrogen and second, the elimination of olefin. There has been considerable conjecture as to whether these parts of the mass-spectrometric McLafferty rearrangement are stepwise or concerted. Prom their self-consistent field calculations, Boer et al. (1968) have concluded the reaction is step-wise. From perturbation and valence-electron molecular orbital calculations, Dougherty (1968b) has concluded the reaction is concerted. The above results depend on the adjustable parameters fed into the equations one set of parameters may eventually prove to be better. 

The heterobimetallic complexes [N(n-Bu)4] [Os(N)R2(/u.-0)2Cr02] catalyze the selective oxidation of alcohols with molecular oxygen. A mechanism in which alcohol coordinates to the osmium center and is oxidized by B-hydrogen elimination (see -Hydride Elimination) is consistent with the data. The hydroxide adduct of OSO4, [0s(0H)204], with ferric cyanide and other co-oxidants catalyzes the oxidative dehydrogenation of primary aromatic and aliphatic amines to nitriles, the oxidation of primary alcohols to carboxylic acids, and of secondary alcohols to ketones. Osmium derivatives such as OsCb catalyze the effective oxidation of saturated hydrocarbons in acetonitrile through a radical mechanism. ... 

At elevated temperatures of 60 100 °C, n-dSb/X complexes will react with water to give aldehydes or ketones (equation 58). The mechanism probably involves attack of OH on the coordinated tt-allyl, and elimination of Pd H to generate the aldehyde group. Molecular bromine cleaves TT-allyl-palladium bonds to give aUyhc bromides. Peracids have been reported to convert 7r-allyl complexes into aUyhc alcohols. Photolysis of itt-allyl complexes in the presence of oxygen results in the conversion of an aUylic group into an a, jS-unsaturated ketone. 

Step (a) could be the analog of the (n, n ) rearrangements of cyclopropyl ketones , and step (b) is that of the molecular ethane elimination from azomethane and probably occurs from the cw-isomer of the parent compound with yields of 15-25 %. Further studies on this and similar molecules would be of great value. 

The mechanistic outline of carbenoid/carbonyl reactivity follows the paradigm illustrated at the outset of this chapter (Scheme 1 X = halogen). The nucleophilic lithium species adds to the carbonyl compound and suffers elimination to provide the epoxide. Competition from molecular rearrangements emanating from the intermediate halohydrin or the product epoxides is sometimes a problem, particularly with cyclic ketones. Also, the initial adduct frequently fails to cyclize when the reaction is quenched at low temperature, but it is usually a simple matter to effect ring closure by treatment of the halohydrin with mild base in a separate step. 

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