Substituted benzophenone , substituent

The reduction is general for a variety of substituted benzophenones Such substituents as CH3 OH, OCH3, F, Br. N(CH3)2, NO2. COOH, COOCH3, NHCOC Hreaction conditions and do not alter the course of the reduction Diarylmethanols are reduced to diarylmethanes under the same conditions and probably are the intermediates in the reduction of ketones [26] Triethylsilane also can be used as a reducing agent in trifluoroacetic acid medium [27J This reagent is used for the reduction of benzoic acid and some other carboxylic acids under mild condiUons (equation 14) Some acids (phthalic, sue cinic, and 4-nitrobenzoic) are not reduced under these conditions [27]... 

The enthalpy changes associated with proton transfer in the various 4, -substituted benzophenone contact radical ion pairs as a function of solvent have been estimated by employing a variety of thermochemical data [20]. The effect of substituents upon the stability of the radical IP were derived from the study of Arnold and co-workers [55] of the reduction potentials for a variety of 4,4 -substituted benzophenones. The effect of substituents upon the stability of the ketyl radical were estimated from the kinetic data obtained by Creary for the thermal rearrangement of 2-aryl-3,3-dimethylmethylenecyclopropanes, where the mechanism for the isomerization assumes a biradical intermediate [56]. The solvent dependence for the energetics of proton transfer were based upon the studies of Gould et al. [38]. The details of the analysis can be found in the original literature [20] and only the results are herein given in Table 2.2. 

The FAB plasma provides conditions that allow to ionize molecules by either loss or addition of an electron to form positive molecular ions, M" , [52,89] or negative molecular ions, M, respectively. Alternatively, protonation or deprotonation may result in [Mh-H]" or [M-H] quasimolecular ions. Their occurrence is determined by the respective basicity or acidity of analyte and matrix. Cationization, preferably with alkali metal ions, is also frequently observed. Often, [Mh-H]" ions are accompanied by [MH-Na]" and [Mh-K]" ions as already noted with FD-MS (Chap. 8.5.7). Furthermore, it is not unusual to observe and [Mh-H]" ions in the same FAB spectmm. [52] In case of simple aromatic amines, for example, the peak intensity ratio M 7[Mh-H] increases as the ionization energy of the substrate decreases, whereas 4-substituted benzophenones show preferential formation of [Mh-H]" ions, regardless of the nature of the substituents. [90] It can be assumed that protonation is initiated when the benzophenone carbonyl groups form hydrogen bonds with the matrix. 

Fig. 2. Substituent effects on the triplet energy (0-0 band) of substituted benzophenones in methylcyclohexane isopentane (4 1).

A rather more complex scheme is required for the preparation of a derivative that bears a trifluoromethyl substituent on one of the benzene rings. The scheme starts with the condensation of the nitrile group in (32-1) with phenyhnagnesium bromide to give the corresponding imine treatment with aqueous acid leads to the substituted benzophenone (32-2). The future methyl on one of the bridges is introduced in a sequence involving the addition of a trimethylsulfonium yhde. 

Triplet state hydrogen abstraction by substituted benzophenones, all of which have n,n lowest triplets, reveals that ring substituents exert very little effect on reactivity. A Hammett a— q plot indicates a q value of only 0.8 — i.e., a twofold rate reduction for a -methoxy, a three-fold increase for p—CN 66>. The direction of this inductive effect is easily understandable in terms of the simplest model for an n n excitation 2>. Loss of an n electron makes the oxygen somewhat electron deficient. 

Solid phase synthesis of organic molecules is exemplified by the pioneering work of Bunin and Ellman on solid phase synthesis of a small benzodiazepine library (29). As shown in Fig. 1, a substituted benzophenone was tethered to a solid support via a substituent (Ri). The solid supported benzophenone was then elaborated through five synthetic steps to the target benzodiazepines with four points of diversity. Cleavage from the solid support re-... 

Comparison of Figures 5.21(b) and (d) reveals that the compound at an Rf of 0.6 is identical or very similar to UV 531. The light stabiliser in the polymer extract is certainly a substituted benzophenone, although it may differ from UV 531 in the length of the alkoxy substituent, which is known to have little or no influence on the IR spectrum of... 

Fig. 72. Dependence of half-wave potentials of the reduction of substituted benzophenones on the Hammett total polar substituent constant a. Full point deviates.

The carbonyls on naphthoquinones and anthraquinones also can abstract hydrogens from ortho substituents. One such example is a 1,2-di-r-butylanthraquinone, which undergoes the same photocy-chzation as the similarly substituted benzophenones and acetophenones as well as some butyl rearrangement. Interestingly, the ratio of cyclization to rearrangement and disproportionation is 2 1, with neither reaction of the biradical intermediate dominating, unhke the simple monoketone cases. 

Fig. 13. Dependence of half-wave potentials for the reduction of substituted benzenes on the total polar substituent constant, ax- Examples shown benzophenones [pH 0], benzophenone-oximes [pH 0], thiobenzophenones [pH 0], nitrobenzenes [pH 2-0], azo derivatives [pH 2-6]. (Taken from Zuman, 1969.)...

The reactions of complex 2a with ketones and aldehydes show a strong dependence on the substituents. With benzophenone, substitution of the silyl-substituted acetylene leads to the r]2-complex 58, which is additionally stabilized by a THF ligand. This complex can serve as an interesting starting material for other reactions. With benzaldehyde and acetophenone, the typical zirconadihydrofuran 59, akin to 2c, is obtained from a coupling reaction. This complex is unstable in the case of benzaldehyde and dimerizes, after elimination of bis(trimethylsilyl)acetylene, to yield 60. In this respect, it is similar to the above discussed complex 2c, since both of them show a tendency to eliminate the bis(trimethyl-silyl)acetylene. The reaction of methacrolein with complex 2a depends strongly on the solvent used [40]. 

Investigations have also examined the photochemical outcome of the inclusion of other aromatic substituents onto the norbornadienes. Examples of this are the direct and sensitized irradiation of the naphthyl-substituted derivatives 249 that brings about cyclization to 250. Sensitization of the cyclization with ketones such as benzophenone leads to a much cleaner reaction. Biacetyl has also been used as the sensitizer130,131. Cyclization also occurs with the norbornadiene 251132. 

Tseng and Ullman demonstrated in 1976 that substituting the p-alkyl position of the o-alkyl benzophenone with good leaving groups leads to the elimination of the p-substituent on irradiation, presumably via formation of photoenols (Scheme 13). ... 

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