Alkyl ketone, 121 phase

Calvert has studied the competing type I and II processes in various -propyl alkyl ketones in the gas phase, and finds that increases and decreases as the alkyl group is changed from methyl through r-butyl,359 as might be expected. Perhaps the most dramatic illustration of the inherent inefficiency of type II photoelimination is provided by the fact that at 100°, where the quantum yield for type I cleavage of diethyl ketone is 1.0, the total quantum yield for disappearance of di- -propyl ketone is only 0.58.359... 

Compared with boranes, borohydrides are inexpensive and easy to handle. As early as 1978 Colonna and Fornasier reported that aryl alkyl ketones such as acetophenone can be reduced asymmetrically by sodium borohydride by use of an aqueous-organic two-phase system and chiral phase transfer catalysts [20], In this study, the best enantiomeric excess (32%) was achieved when pivalophenone (11) was reduced in the presence of 5 mol% benzylquininium chloride (12) (Scheme 11.4) [20]. Other chiral phase-transfer catalysts, for example ephedrinium salts, proved less effective. 

The asymmetric organocatalytic transformation of a ketone into an alcohol may be realized with the combination achiral silanexhiral phase-transfer catalyst, such a quaternary ammonium salt. The final alcohol is then recovered by an additional hydrolytic step. The asymmetric reduction of aryl alkyl ketones with silanes has been reported (ee-values up to 70%), the catalysts utilized being ammonium fluorides prepared from the quinine/quinidine series (e.g., 18 in Scheme 11.6) [19]. (For experimental details see Chapter 14.21.1). The more appropriated silanes were (Me3SiO)3SiH or (MeO)3SiH (some examples are... 

When borohydride reductions are carried out in the presence of either a chiral phase transfer catalyst or a chiral crown ether, asymmetric reduction of ketones occurs but optical yields are low. In the reduction of acetophenone with NaBH4 aided with a phase transfer catalyst (57), 10% ee was obtained. Similarly, reduction of acetophenone with NaBH4 in the presence of the chiral crown ether (58) was ineffective (6% ee)J Sodium borohydride reduction of aryl alkyl ketones in the presence of a protein, bovine semm albumin, in 0.01 M borax buffer at pH 9.2 affords (R)-carbinols in maximum 78% cc. ... 

The present author has investigated the photochemistry of alkyl ketones adsorbed on porous Vycor glass to examine how the reactivity of the excited states or of the radical species themselves varies when they are formed on the solid surfaces. In those studies, we have found that the photochemical reactivities of adsorbed alkyl ketones are markedly different from those in the gas phase, leading to some general characteristics of the photochemistry in the adsorbed layer (5-13). 

Results of the photolyses of acetone, 2-butanone, and 2-pentanone adsorbed on Vycor glass are shown in Table 2. It is well known that alkyl ketones with / hydrogen atoms, such as 2-pentanone, undergo the Norrish Type II processes (intramolecular elimination) as well as the Norrish Type I processes (C -cleavage into radical pairs), as shown in the following reaction mechanisms. In the gas phase photolysis of 2-pentanone at room temperature, the amount of products derived from the Type I processes is less than 5-15% of that derived from the Type II process (26). As seen in Table 2, the rate of CgHg formation is more than 75% that of C2H formation. 

As shown in Table 2, the efficiencies of the photolyses of these ketones can be compared acetone (1.0), 2-butanone (34.3), and 2-pentanone (202). A marked difference is seen in their efficiencies, in contrast with essentially the same efficiencies observed for the gas phase photolyses of these ketones (6). From these, together with the results mentioned in section 3, the following conclusion emerges. The photochemical reaction efficiencies of alkyl ketones adsorbed on Vycor glass are decreased on adsorption owing to the increase in their efficiencies of radiationless deactivation and also to the decrease in their extinction coefficients. Thus, the more blue shifs, i. e., the more strongly hydrogen bonded a ketone molecule is, the more efficient radiationless deactivation becomes. 

Nevertheless, it must be pointed out that the formation of such transient species has never been spectroscopically observed. Native CDs are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols, epoxydation,or oxidation " of olefins, reduction of a,/ -unsaturated acids,a-keto ester,conjugated dienes,or aryl alkyl ketones.Interestingly, chemically modified CDs like the partially 0-methylated CDs show a better catalytic activity than native CDs in numerous reactions such as the Wacker oxidation,hydrogenation of aldehydes,Suzuki cross-coupling reaction, hydroformylation, " or hydrocarboxylation of olefins. Methylated /3-CDs were also used successfully to perform substrate-selective reactions in a two-phase system. 

Chiral phase-transfer catalysts have been often used as promoters in the conjugate addition of activated malonates to a,p-unsaturated ketones. Phase-transfer catalysts are stronger bases compared to the amine catalysts so their use was initially focused on the conjugate addition of less-acidic nucleophiles where chiral amines had not been successful. Thus, different chiral phase-transfer systems such as iV-alkylated cinchonium derivative 143 (Scheme 2.85) [206, 228] and ephedrinium... 

The first real-time study of Norrish type II reaction dynamics was reported recently by Zewail and co-workers for a series of methyl alkyl ketones. Their excited singlet states undergo intramolecular hydrogen transfer in 70 to 90 fs to form biradical intermediates with 400- to 700-fs Hfetimes. It is interesting to compare this gas-phase study with the data Hsted in Table 52.1 for the solution photochemistry of simple alkanones singlet state rate constants for hydrogen abstraction are around 10 s" , values that reflect the equihbrium conformational distribution of the ketones. " ... 

Tautomerism may be of many kinds as illustrated in this book and in Ref. [1]. However, tautomerism may be less obvious depending on the rates of interconversion or the equilibrium constant may be very much in favor of one form. A typical example of the latter is acetone and other ketones. Nevertheless, tautomerism plays a major role in the chemistry of alkyl ketones. Tautomerism may occur both in the gas phase, namely the liquid and condensed phases, and in the solid state, namely in the ground state as well as in the excited state. As tautomerization often involves the movement of light atoms, proton transfer is the usual case and hydrogen bonding will often be involved. Tautomerization can of course be both intra- and intermolecular, and in the latter case a solvent molecule may also be involved. For some characteristic cases, see Ref. [1, Chapter 3]. 

Dicyclohexylarnine may be selectively generated by reductive alkylation of cyclohexylamine by cyclohexanone (15). Stated batch reaction conditions are specifically 0.05—2.0% Pd or Pt catalyst, which is reusable, pressures of 400—700 kPa (55—100 psi), and temperatures of 75—100°C to give complete reduction in 4 h. Continuous vapor-phase amination selective to dicyclohexylarnine is claimed for cyclohexanone (16) or mixed cyclohexanone plus cyclohexanol (17) feeds. Conditions are 5—15 s contact time of <1 1 ammonia ketone, - 3 1 hydrogen ketone at 260°C over nickel on kieselguhr. With mixed feed the preferred conditions over a mixed copper chromite plus nickel catalyst are 18-s contact time at 250 °C with ammonia alkyl = 0.6 1 and hydrogen alkyl = 1 1. 

By-Products. Almost all commercial manufacture of pyridine compounds involves the concomitant manufacture of various side products. Liquid- and vapor-phase synthesis of pyridines from ammonia and aldehydes or ketones produces pyridine or an alkylated pyridine as a primary product, as well as isomeric aLkylpyridines and higher substituted aLkylpyridines, along with their isomers. Furthermore, self-condensation of aldehydes and ketones can produce substituted ben2enes. Condensation of ammonia with the aldehydes can produce certain alkyl or unsaturated nitrile side products. Lasdy, self-condensation of the aldehydes and ketones, perhaps with reduction, can lead to alkanes and alkenes. 

In the petroleum (qv) industry hydrogen bromide can serve as an alkylation catalyst. It is claimed as a catalyst in the controlled oxidation of aHphatic and ahcycHc hydrocarbons to ketones, acids, and peroxides (7,8). AppHcations of HBr with NH Br (9) or with H2S and HCl (10) as promoters for the dehydrogenation of butene to butadiene have been described, and either HBr or HCl can be used in the vapor-phase ortho methylation of phenol with methanol over alumina (11). Various patents dealing with catalytic activity of HCl also cover the use of HBr. An important reaction of HBr in organic syntheses is the replacement of aHphatic chlorine by bromine in the presence of an aluminum catalyst (12). Small quantities of hydrobromic acid are employed in analytical chemistry. 

Class (2) reactions are performed in the presence of dilute to concentrated aqueous sodium hydroxide, powdered potassium hydroxide, or, at elevated temperatures, soHd potassium carbonate, depending on the acidity of the substrate. Alkylations are possible in the presence of concentrated NaOH and a PT catalyst for substrates with conventional pX values up to - 23. This includes many C—H acidic compounds such as fiuorene, phenylacetylene, simple ketones, phenylacetonittile. Furthermore, alkylations of N—H, O—H, S—H, and P—H bonds, and ambident anions are weU known. Other basic phase-transfer reactions are hydrolyses, saponifications, isomerizations, H/D exchange, Michael-type additions, aldol, Darzens, and similar... 

The dominant photochemical reaction of ketones in the gas phase is cleavage of one of the carbonyl substituents, which is followed by decaibonylation and sidsoetprait reactions of the alkyl free radicals that result ... 

Arai and co-workers have used chiral ammonium salts 89 and 90 (Scheme 1.25) derived from cinchona alkaloids as phase-transfer catalysts for asymmetric Dar-zens reactions (Table 1.12). They obtained moderate enantioselectivities for the addition of cyclic 92 (Entries 4—6) [43] and acyclic 91 (Entries 1-3) chloroketones [44] to a range of alkyl and aromatic aldehydes [45] and also obtained moderate selectivities on treatment of chlorosulfone 93 with aromatic aldehydes (Entries 7-9) [46, 47]. Treatment of chlorosulfone 93 with ketones resulted in low enantioselectivities. 

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