HX elimination

This route was limited to R = Me or Et, since the use of higher alkyl groups resulted in HX elimination and olefin formation ... 

Molecular reactions are generally more difficult to treat because of the complexity of the possible transition states. The most widely studied complex molecular reaction class is HX elimination from halogenated hydrocarbons. These reactions proceed primarily via the formation of polar, four-centered tight transition states, and examples include... 

Since polarizabilities of groups or atoms are also manifested in bond strengths, some form of Hirschfelder s rule can also be developed to treat these types of reactions (Laidler, 1987). For example, the activation energies for many molecular elimination reactions seem to be correlated with one-third of the sum of the dissociation energies of the bonds that are being broken. This empirical relationship seem to hold well for a large variety of HX elimination reactions (see Table XI). 

An interesting aspect of these substitutions is that the HX-elimination demands a contrapolarization at the nuclear carbon that acts as an acceptor dnring the addition. 

Bis-iV-alkylated complexes of Me2-9 and Me2-ll, as well as the tetra-methylated Ni(II)cyclam (NinTMC) derivatives, have been synthesized by the deprotonation of secondary amines followed by alkylation (34, 47,48). When EtI or other alkyl halide with /3-hydrogen was added to the deprotonated Ni(II) complex of cyclam or 11, HX elimination occurred instead of SN2 reaction. Therefore, ethylene gas was produced instead of -ethylated complex formation when EtI was added to the deprotonated complex of cyclam or 11. However, in the case of 8, bis- -ethylated Ni(II) complex was isolated. This may be because HX elimination is slower than SN2 reaction. The - -alkylated Ni(II) complexes of 9 (Me2-9 and Et2 9) and Me2-ll were stable against ligand dissociation in acidic aqueous solutions. The -alkylated complexes were dealkylated when the complexes were heated in aqueous solutions (34, 47). 

Our recent syntheses of CH2=CH-C=P and HC=C-C=P using the HX elimination approach has indicated that many more RC=P compounds will be soon available. Our structural data indicate that the C=P bond length 1.545 8 is relatively insensitive to the nature of the R substituent, whereas other properties, e.g. NMR parameters and I.P. data, are significantly affected by R. 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

The elimination of HX which occurs upon UY irradiation of <5-haloketones, such as the <5-X-valerophenones, is the result of a photoreaction characteristic of the 0=0 group the reaction proceeds via loss of X from the biradical formed by y-hydrogen abstraction256. A comparable HX elimination does not occur with y-haloketones, but does to some extent with an -iodoketone. 

In presence of dienes (Scheme 1), heterodienes, and heterotrienes (Scheme 2) rapid cycloadditions take place which prevent other reaction modes of the silylenes (insertions, polymerization, or HX-elimination-polymerization). Although a concerted [l+4]-cycloaddition is symmetry-allowed a stepwise mechanism (Scheme 3) via a three-membered intermediate is prefered or at least partly be competing to account for the formation of double-bond isomers. Ususally the formal [4+l]-cycloadducts (allylsilane-type) are the main products while the isomers with vinylsilane-units are side-products (< 30 %). Exceptions are... 

Phosphido-bridged dimers [Pd2(/tr-PR2)2X2(PR2H)2] can be obtained by reaction of secondary phosphine complexes [PdX2(PR2H)2] with a base. HX elimination is easier in the order PPh2H > PPhEtH > PEt2H. 

When any of the above C(10)-disubstituted homoporphyrin derivatives is treated with acid (HX), elimination to the conjugated, cationic complexes 4.31a-c occurs (Scheme 4.1.13). Methanolysis of 4.31 results in the exclusive formation of 4.30a (X = OCH3). Reduction of cation 4.31 with zinc in acetic acid affords derivative 4.32a as the major product. Heating this isomer (4.32a) in o-dichlorobenzene at reflux then affords 4.32b. In the presence of base (e.g., triethylamine), 4.32a rapidly isomerized to the original endo homoporphyrin 4.16a (Scheme 4.1.14). By contrast, under identical conditions 4.32b did not appear to isomerize. ... 

As most recently discussed by Zipse [78], 1,2-shifts and HX eliminations of / -acetoxy and / -phosphatoxyl radicals have often been formulated as proceeding through CO bond heterolysis to give alkene radical cations and delocalized acetoxy or phophatoxy anions that bond in a second step. Calculations suggest that reactions through the radical cations are not the lowest-energy pathways (in the gas phase, for the stripped-down cases calculated, see also the discussion of Ref. [25] in Section 10.2). 

With respect to the mechanistic pathway, as depicted in Scheme 36, after coordination of the allylic halide to Ru-complex 227, giving rise to Ru-complex 229, the C-S bond formation occurs via activation of the allylic C-H bond and subesquent C-H cleavage (step 229—>230). The rotation of the organic moiety around the C-S bond (step 231 —> 232) precedes HX elimination, which is the final step leading to Ru-complex 228. 

With fluoro-compounds, there has been work on the radiolysis of fluorinated methanes , hexafluoroethane , tetrafluoroethylene - , fluorobenzene , cyclic fluorocarbons , trifluoroiodomethane - - , and perfluorocar-bons . Most studies have involved the use of y-radiation, but Yi-Noo Tang and Rowland have discussed the recoil tritium-excitation of ethyl fluoride (and ethyl chloride), providing evidence for 1,2-HX elimination. 

Alternatively, Figure 4 lists energies for Hg atoms and several vinyl halides, showing the corresponding halovinyl Hg compounds, while Figure 5 stresses the pathways leading to HX elimination for the case of Hg + vinyl chloride. 

CH and R = COR or PO(OMe)2 were never isolated. They rapidly go on to alkylcobalt(in) porphyrins (413) after rearrangement and HX elimination 393.a ) Haloalkylcobalt(III) porphyrins result from diazomethane. 9-Diazofluorene is catalytically decomposed by bromocobaltflll) porphyrins to give the azine in high yield... 

To see qualitatively why the vibrationally adiabatic picture breaks down, consider HX elimination from haloethane, H3C—CH2X H2C=CH2 + HX. Here... 

Eliminations, C = C double bond formation by of alcohols, 138,140-141, 276-277, 283 exocyciic methylene with ArjSfOR) 282 undesired side-reaction, 274, 317 of ammonium salts and P-amino ketones, 57, 72-73,140-141, 331 of epoxides to allyiic alcohols, 27 of glycosyl halides (HX elimination), 268 of halides, 52, 123-124,138, 140, 283, 286,... 

As a rule, the initial hetero-DiELS-ALDER adduct 2 cannot be isolated. It eliminates N2 in a retro-DiELS-Alder reaction and is converted into a 4,5-dihydropyridazine 3. This can be stabilized as a 1,4-dihydropyridazine 7 (especially if X = H) by a 1,5 hydrogen shift or (if X = OR and NR2) as the pyridazines 5 and 6 by dehydrogenation or HX elimination. As a diazadiene, it can also engage in a further Diels-Alder reaction with excess of alkene 3delding the stable 2,3-diazabicyclo[2.2.2]oct-2-ene 4. The initial Diels-Alder product tetraazabicyclo[2.2.2]octatriene 8, which arises from the reaction between alkynes and 1,2,4,5-tetrazines, undergoes a cycloreversion with N2 elimination affording the pyridazine 6. With nitriles, 1,2,4-triazines 9 are obtained. 

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