Geminal elimination

C). While H3CS1H3 decomposes under these conditions primarily by geminal elimination of molecular H2 from the silicon moiety and secondarily by a slow surface-catalysed reaction yielding H-and CH3SiH2 radicals [27], the significantly lower decomposition temperatures of (fluoromethyl)silanes require different low energy decomposition pathways. 

Polynuclear aromatics react with fluoroxy reagents to give high yields of ortho substitution products accompanied by varying yields of geminal difluoro products Thegeminal difluonnation occurs presumably by an addition-elimination mechanism [27 28, 29, 30, 31, 32] Unactivated aromatic systems are fluorinated in lower yield to give monofluonnated products (Table 1, entries 6 and 7) Examples of fluonnation of polynuclear systems [/5, 21, 25, 30, 32, 7 ] are shown m equations 7-10... 

Double dehydrohalogenation of geminal dihalides (Section 9.7) An E2 elimination reaction of a geminal dihalide yields an alkenyl halide. If a strong enough base is used, sodium amide, for example, a second elimination step follows the first and the alkenyl halide is converted to an alkyne. 

Double dehydrohalogenation of vicinal dihalides (Section 9.7) Dihalides in which the halogens are on adjacent carbons undergo two elimination processes analogous to those of geminal dihalides. 

Aldehydes and ketones react with water to yield 1,1-diols, or geminal (gem) diols. The hydration reaction is reversible, and a gem diol can eliminate water to regenerate an aldehyde or ketone. 

COSY and HETCOR experiments are extremely useful in the structure elucidation of complex organic molecules. The geminal and vicinal protons and their one-bond C-H connectivities are first identified from the HETCOR spectrum, and then the geminal couplings are eliminated from the COSYspectrum, leaving vicinal connectivities. By careful interpretation of the COSY and the one-bond HETCOR spectra, it is then possible to obtain information about the carbon-carbon connectivities of the protonated carbons ( pseudo-INADEQUATE information). In this way the carbon-carbon connectivity information of protonated carbons is obtainable through a combination of COSY and HETCOR experiments. 

SRN I reactions using related p-nitrophenyl or p-nitrocumyl systems41 as reductive alkylating agents have been studied by Komblum and co-workers these are well summarized in the reviews.39 At the same time, Russell discovered the S l reaction of geminal halonitroalkanes with stabilized carbanions (see Eq. 5.25).42 The products are readily converted into alkenes via elimination of nitro groups (see Section 7.3). 

In dichloromethane, the acidic ESE cation radical undergoes a rapid proton transfer (k = 1.9 x 109 s ) to the CA anion radical within the contact ion pair (CIP) to generate the uncharged radical pair (siloxycyclohexenyl radical and hydrochloranil radical) in Scheme 6. Based on the quantum yields of hydro-chloranil radical (HCA ), we conclude that the oxidative elimination occurs by geminate combination of the radical pair within the cage as well as by diffusive separation and combination of the freely diffusing radicals to yield enone and hydrochloranil trimethylsilyl ether, as summarized in Scheme 6. 

The most important example to be discussed here is that of the drug chloramphenicol (11.39, R = 2-hydroxy-l-(hydroxymethyl)-2-(4-nitrophen-yl)ethyl, Fig. 11.7), the many metabolic pathways of which have yielded a wealth of information [75], The dichloroacetyl moiety is especially of interest in that dechlorination proceeds by three proven routes glutathione-dependent dechlorination, cytochrome P450 catalyzed oxidation, and hydrolysis. Of particular value is that the oxidative and hydrolytic routes can be unambiguously distinguished by at least one product, as shown in Fig. 11.7. Oxidation at the geminal H-atom produces an unstable (dichloro)hydroxyacet-amido intermediate that spontaneously eliminates HC1 to yield the oxamoyl... 

The 1,4-addition of sp3-geminated organodimetal compounds, prepared by the Gaude-mar/Normant coupling reaction, to alkylidenemalonate gives Z-alkene selectively through elimination of the malonate moiety (equation 33)56,57. 

The high specificity of elimination of alkyl side branches from poly (olefin) s is attributed to (1) enhanced scission of C-C bonds at tertiary carbon atoms, reflecting their lower bond energies, and (2) a cage effect which produces geminate recombination of many of the main-chain cleavages, whereas the mobility of fragments of low molar mass enables them to escape. 

The isomerization reactions. At least 10 reactions of the type described by Eq. 16-34 are known397 (Table 16-1). They can be subdivided into three groups. First, X = OH or NH2 in Eq. 16-34 isomerization gives a geminal-diol or aminoalcohol that can eliminate H20 or NH3 to give an aldehyde. All of these enzymes, which are called hydro-lyases or ammonia-lyases, specifically require K+ as well as the vitamin B12 coenzyme. 

Fluorinated alkenes with one fluorine atom attached to the C = C bond are converted into a-hydroxy ketones by potassium permanganate.92 In this case, the geminal fluorohydrin grouping in 52 eliminates hydrogen fluoride to give the end product 53.92... 

Unsaturatcd carboxylic acids arc selectively hydroxylated on the double bonds. Pentafluoro-prop-1-enyl)malonate is selectively dihydroxylated by potassium permanganate on the fluorinated carbons. The intermediate geminal fluorohydrins 3 immediately eliminate hydrogen fluoride to give dioxo esters, such as 4, as the final product (vide supra).90... 

In the reaction of /h/f-difluoro ketones with amines, a,/I-unsaturated / -amino ketones are formed, either by inline formation and imine enaminc tautomerization or via elimination of hydrogen fluoride and subsequent substitution of the other fluorine.22 Geminal sp2-bonded difluoridcs arc converted into ketenimines.12-49,89,90... 

A related approach is the treatment of geminal dihalides with metal reagents to give an intermediate a-haloalkylmetal species, from which halide may in principle be lost in a net overall a-elimination of the two halide substituents (see Section 4.6.4.2). 

A procedure which combines characteristics of the Simmons-Smith reaction with the previously discussed a-eliminations (see Section 4.6.3.1) is the reaction of a geminal dihalide with an alkyllithium re-... 

The cyclopropanation reaction originally developed by Simmons and Smith utilized for eliminating iodine from a geminal diodide the reducing agent zinc, which had previously been activated by copper.23... 

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