Reagents for elimination

Treatment of the isomeric dihydroselenines 93 containing electron-withdrawing groups in both the 2- and 6-positions with w-chloroperbenzoic acid also leads to formation of Pummerer-type oxidation products, the 4H-selenines 94 and the corresponding w-chlorobenzoate ester by-products (Equation 39) <1999J(P1)1155>. The esters could be converted to the corresponding 477-selenines by treatment with polyphosphoric acid trimethylsilyl ester (PPSE), a useful reagent for eliminations under neutral conditions. These 4/7-selenines were key intermediates in the synthesis of selenabenzenes. 

Cyclopropanone dithioketals. Methyllithium (1 equiv.) in combination with TMEDA (2 equiv.) is an excellent reagent for elimination of thiophenoxide from a l,l,3-tris(phenylthio)alkane to form cyclopropanone dithioketals (equation I). 

The competitive elimination (ET) and substitution (iSn2) reactions of cyclohexyl tosylate with triphenylphosphine have been examined. Triphenylphosphine is considered to be representative of neutral weak bases which have good nucleophilic afiinity for carbon, but it is a poor reagent for elimination when compared with anionic weak bases that are also good carbon nucleophiles. The reaction of triphenylphosphine with cyclohexyl bromide occurs with almost complete substitution. Tertiary phosphines react with fluorosulphonyl isocyanate and with isothiocyanates to form the zwitterionic adducts (56) and (57). 

Table 6 Chemical interferences in the determination of some elements, and reagents for eliminating the interferences (R = releasing, S Shielding) (Source T.C. Rains, Chemical Interferences in Condensed Phase, in Flame Emission and Atomic Absorption Spectrometry, VoL V, ed.s J.A. Dean and T.C. Rains, Marcel Dekker, New York, 1%9)...

Mono-substitution products of primary amines cannot easUy be prepared by direct action of the appropriate reagent for example, bromination of aniline yields largely the 2 4 6-tribomo derivative and nitration results in much oxidation. If, however, the amino group is protected as in acetanilide, smooth substitution occurs. Thus with bromine, />-bromoacetanilide is the main product the small quantity of the ortlio isomeride simultaneously formed can be easily eliminated by crystallisation. Hydrolysis of p-bromoacetanilide gives/ -bromoaniline ... 

Hydrazoyl halides are useful reagents for the synthesis of pyrazolines and pyrazoles (80JHC833). The elimination of HX, usually with triethylamine, is now the preferred method for the generation of the nitrilimine (621) in situ. Although in some cases it is not clear if the mechanism involves a nitrilimine (621) (as for example in the Fusco method in which sodium salts of /3-diketones are used), in other reactions it is the most reasonable possibility. For example, the synthesis of pyrazolobenzoxazine (633) from the hydrazoyl halide (631) probably occurs via the nitrilimine (632). Trifluoromethylpyrazoles (634) have been prepared by the reaction of a hydrazoyl halide and an alkynic compound in the presence of triethylamine (82H(19)179). 

Numerous reactions have been described in which the oxygen of the oxepin system is removed to give benzene derivatives. The formation of the aromatic products can be rationalized by an arene oxide as intermediate. A suitable reagent for the elimination of an oxygen atom from this heterocycle is triphenylphosphane, e.g. formation of l,24 2a,12 and 2b.1,9... 

Besides simple alkyl-substituted sulfoxides, (a-chloroalkyl)sulfoxides have been used as reagents for diastereoselective addition reactions. Thus, a synthesis of enantiomerically pure 2-hydroxy carboxylates is based on the addition of (-)-l-[(l-chlorobutyl)sulfinyl]-4-methyl-benzene (10) to aldehydes433. The sulfoxide, optically pure with respect to the sulfoxide chirality but a mixture of diastereomers with respect to the a-sulfinyl carbon, can be readily deprotonated at — 55 °C. Subsequent addition to aldehydes afforded a mixture of the diastereomers 11A and 11B. Although the diastereoselectivity of the addition reaction is very low, the diastereomers are easily separated by flash chromatography. Thermal elimination of the sulfinyl group in refluxing xylene cleanly afforded the vinyl chlorides 12 A/12B in high chemical yield as a mixture of E- and Z-isomers. After ozonolysis in ethanol, followed by reductive workup, enantiomerically pure ethyl a-hydroxycarboxylates were obtained. 

Although this material contains a small amount of the symmetrical dihydrazide, which is not easily eliminated on crystallization, it is entirely satisfactory for use as a reagent for the isolation of ketones. A purer product, m. p. 192°, with decomposition, can be obtained by adding the solution prepared from ethyl chloroacetate and trimethylamine to an alcoholic solution containing a considerable excess of the hydrazine hydrate. 

The presence of the catalyst can also favor multiple Diels-Alder reactions of cycloalkenones. Two typical examples are reported in Schemes 3.6 and 3.7. When (E)-l-methoxy-1,3-butadiene (14) interacted with 2-cyclohexenone in the presence of Yb(fod)3 catalyst, a multiple Diels-Alder reaction occurred [21] and afforded a 1 1.5 mixture of the two tricyclic ketones 15 and 16 (Scheme 3.6). The sequence of events leading to the products includes the elimination of methanol from the primary cycloadduct to afford a bicyclic dienone that underwent a second cycloaddition. Similarly, 4-acetoxy-2-cyclopenten-l-one (17) (Scheme 3.7) has been shown to behave as a conjunctive reagent for a one-pot multiple Diels-Alder reaction with a variety of dienes under AICI3 catalysis, providing a mild and convenient methodology to synthesize hydrofluorenones [22]. The role of the Lewis acid is crucial to facilitate the elimination of acetic acid from the cycloadducts. The results of the reaction of 17 with diene... 

Another useful reagent for reduction of a-acetoxyketones and similar compounds is samarium diiodide.233 Sml2 is a strong one-electron reducing agent, and it is believed that the reductive elimination occurs after a net two-electron reduction of the carbonyl group. 

Deliberate production of (vinyl)polystyrene from (toluenesul-foxyethyl)polystyrene or (haloethyl)polystyrenes was best accomplished by quaternization with N,N-dimethylaminoethanol, followed by treatment with base beta-deprotonation is encouraged in the cyclic zwitterionic intermediate. Reaction was faster and cleaner than with other reagents recommended (64, 76, 77) for eliminations, such as alkoxide, diazabicycloundecene or quaternary ammonium hydroxide this new and efficient procedure may find application elsewhere. Hydrometallation or other additions to polymer-bound olefin may prove useful steps in future syntheses by polymer modification. 

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