Hydrogen intramolecular elimination

The above reaction is a convincing example of an intermolecular hydrogen abstraction leading essentially to the same result as obtained in the pyrolysis of alkyl-substituted thiirane oxides through an intramolecular /(-elimination of hydrogen. 

The formation of these compounds has been rationalized according to Scheme 6. The reaction of Os (E )-CH=C 11 Ph C1 (C())( P Pr3)2 with n-BuLi involves replacement of the chloride anion by a butyl group to afford the intermediate Os (/i> CH=CHPh ( -Bu)(CO)(P Pr3)2, which by subsequent hydrogen (3 elimination gives OsH ( >CI I=CHPh (CO)( P Pr3)2. The intramolecular reductive elimination of styrene from this compound followed by the C—H activation of the o-aryl proton leads to the hydride-aryl species via the styrene-osmium(O) intermediate Os r 2-CH2=CHPh (CO)(P Pr3)2. In spite of the fact that the hydride-aryl complex is the only species detected in solution, the formation of OsH ( )-CH=CHPh L(CO)(P Pr3)2 and 0s ( )-CH=CHPh (K2-02CH)(C0)(P,Pr3)2 suggests that in solution the hydride-aryl complex is in equilibrium with undetectable concentrations of OsH ( )-CH=CHPh (CO)(P,Pr3)2. This implies that the olehn-osmium(O) intermediate is easily accessible and can give rise to activation reactions at both the olefinic and the ortho phenyl C—H bonds of the... 

Recently, an interesting method involving an intramolecularly assisted dchydrolluorination, mediated by (benzotriazol-l-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), was described.37 The reaction proceeds via a hydrobenzotriazole ester intermediate followed by concerted intramolecular elimination of hydrogen fluoride. 

Vinyl azide intramolecular cycloaddition is further illustrated by the formation of azidotriazoline 32 as a minor product in the thermolysis of the bisvinyl azide 31 (Scheme 41).200 An analogy is provided by the formation of 2,5-diphenylpyrrole from the slow decomposition of a-azidostyrene.202 Pyrrole formation is interpreted in terms of cycloaddition of the azide onto the electron-rich double bond of a second molecule to give a triazoline that loses nitrogen and rearranges to a pyrroline followed by hydrogen azide elimination (Section IV,D).203... 

As another possibility, the ene-type reaction can be explained by involvement of the hydride species H—Pd—OAc 328, generated by the oxidative addition of HOAc to Pd(0), as an initiator [134], Preferencial insertion of the triple bond in 329 gives 330, and subsequent intramolecular insertion of the double bond gives 331. Finally, depending on which /i-hydrogen is eliminated, either 1,4-diene 324 (ene-type), or 1,3-diene 325 is formed with regeneration of H—Pd—OAc (328). BBEDA [N,N bis(benzylidene)ethylenediamine] is used as a ligand [135,136]. 

Control over regioselectivity and stereoselectivity in the formation of new C-C a-bond is required to utilize the Heck reaction in complex molecule synthesis. For the intramolecular Heck reaction, the size of the ring formed in the insertion step controls the regiochemistry, with 5-exo and 6-exo cyclization favoured. A mixture of regioisomers is formed from Heck insertions of acyclic alkenes, whereas cyclic alkenes such as cycloalkenes as a Heck substrate produce a a-arkylpalladium(II) intermediate A, which has only one syn-P-hydrogen. Syn-elimination of the hydrogen provides only product B (Scheme 5.6). 

R = H or alkyl, the thiolate salts 27 are formed but no elimination takes place because of the low acidity of the hydrogen . Dialkylsulphide elimination from dialkylsulphonium methyl sulphates of -oxocarboxylic acids proceeds readily with aqueous alkali at 0 °C to furnish predominantly high yields of alkynoic esters (equation 86)" . Very recently intramolecular thiol eliminations (i.e. ring openings)... 

Toluene carrier technique products in toluene were carbon dioxide and 1,2-diphenylethane with smaller amounts of carbon monoxide, hydrogen and methane intramolecular elimination of water was also thought to occur from phenyl acetic acid to give phenyl ketene and water. 

The most well known and well investigated degradation reaction of 13S-HPOTE is the generation of jasmonic acid. 13S-HPOTE is converted by allene oxide hydroperoxydehydrase by an intramolecular elimination of water to an allene oxide [160]. The latter is cyclized by allene oxide cyclase to 9S,13S,12-oxo-10,15-phytodienoic acid [160] followed by a trifold P-oxidation and a hydrogenation step (Scheme 7) [108]. 

Scheme 18 illustrates the proposed stages in 6-MSA biosynthesis in which the first and second condensation steps proceed with inversion to give the triketide (63). Ketoreduction gives the alcohol (64) and then elimination followed by a final malonyl condensation generates the tetraketide (65) which cyclises via an intramolecular condensation and enolises to give the aromatic nucleus of (66). In the first set of experiments (J )- and (S)-[l- C, H]nialonales were incubated separately with 6-MSA synthase purified from Penicillium patulum [56]. Isotope incorporations were determined by mass spectrometry. All the possible isotope patterns for retention or loss of the pro-J or pro-S hydrogens from C-3 and C-5 were permutated. Comparison with the actual spectra obtained demonstrated that opposite prochiral hydrogens were eliminated. The absolute stereochemistry was established in an analogous experiment [57] where the chiral malonates were incubated with acetoacetyl CoA rather than acetyl CoA. Subsequent mass spectral analysis showed that it is the Hr proton that is retained at C-3 of 6-MSA and so it can be deduced that the hydrogen at C-5 must be derived from the opposite prochiral hydrogen, Hg. The overall result is summarised in Scheme 18. In a recent collaborative study we have synthesised the triketide alcohol (64) as its NAC thioester and shown that it is indeed a precursor as, on incubation with 6-MSA synthase and malonyl CoA, 6-MSA production is observed [unpublished results]. Current work is aimed at synthesis of both enantiomers of (64) to study the overall stereochemistry of the ketoreduction and elimination reactions.

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. 

Some intramolecular eliminations that occur thermally are known as pyrolytic eliminations, and many of these reactions result in syn elimination. Often these reactions are carried out in the gas phase, where they are not affected by solvent, counterions, or other species that can affect reactions in solution. One of the most-studied pyrolytic eliminations is the Chugaev reaction (equation 10.70). Reaction of an alcohol having a j8-hydrogen atom with sodium or potassium metal or with a strong base... 

The sulphurdi-imines are hydrolysed in acidic media to sulphur dioxide and two equivalents of amine. Oxidation with ozone afforded a nitrocompound and sulphinylamine. Pyrolysis of the diphenyl derivative (100 R = Ph) gave azobenzene, but pyrolysis of the di-t-butyl derivative (100 R = t-butyl) resulted in an intramolecular elimination to isobutylene, hydrogen sulphide, and ammonia. The bis-tosyl imine (100 R = toluene-p-sulphonyl) reacted with diamines (1,2----------1,6-) in an... 

The methyl-ethyl interaction in 48B makes the steric hindrance higher, and that rotamer is higher in energy when compared to the interactions in 48A. If 48A is the lower energy eclipsed rotamer for removal of P-hydrogen H, then 48A will lead to the major product, 1-butene (43). Alkene 43 is the less stable alkene. An E2 reaction always gives the more substituted (more stable) alkene product in an acyclic system via an anti rotamer (which implies an anti transition state), but this intramolecular elimination generates the less substituted (less... 

Normally intramolecular elimination of alkane from alkyl(hydride) complexes occurs readily and is favoured thermodynamically. There is interest, however, in the possibility of carrying out the reverse reaction, the addition of a C—H bond to an unsaturated transition metal centre. Alkanes are susceptible to electrophilic attack, for example by Lewis or Br nsted acids which convert linear alkanes into their branched isomers via carbonium ion intermediates. Linear and cyclic alkanes can be converted into aromatic hydrocarbons and hydrogen over metal surfaces such as platinum. These reactions are carried out on a large scale industrially in the reforming of petroleum. 

FIGURE 18.61 In these intramolecular elimination reactions, only a cis hydrogen is removed. Because the pheuyl groups are required to stay as far from each other as possible for steric reasous, syu elimiuatiou demauds that H he lost from stereoisomer A and D lost from stereoisomer B. 

On the other hand, condensation of methyl 3-methoxy-2-(trifluoromethyl) acrylate 9 with arylhydrazines under weakly basic conditions allowed the synthesis of methyl l-aryl-5-fluoropyrazole-4-carboxylates 11 (Scheme 4) the lower yields were obtained with arylhydrazines bearing electron-withdrawing substituents [7]. The process involves hydrogen fluoride elimination followed by intramolecular nucleophilic addition from the 3-hydrazinoacrylates 10 previously formed. When alkylhydrazines were employed, no heterocyclic compounds were isolated. 

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