P-hydrogen atom

The Somme let-Hauser rearrangement competes when Z is an aryl group (see 13-22). Hofmann elimination competes when one of the R groups contains a p hydrogen atom (17-6 and 17-7). 

It is well known that elimination of a sulfonyl group from a system bearing acidic P-hydrogen atoms proceeds under relatively mild conditions to give olefins (equation... 

Powdered aluminium had been added to oleic acid. The mixture detonated after being prepared. Such an accident could not be repeated and it was thought that it was caused by the presence of a peroxide formed by the effect of air on oleic acid. In fact, the acid functional group has obviously nothing to do with the peroxidation. It is more likely that the chain s double bond that activates p hydrogen atoms (ally position) was involved in it. This is a well-known phenomenon since it is responsible for the rancidity of some oils and greases. 

The Mannich reaction consists in the condensation of formaldehyde with ammonia or a primary or a secondary amine and a compound containing at least one hydrogen atom of pronounced reactivity the active hydrogen atom may be derived from a methylene group activated by a neighbouring keto group, or from a nitroparaffin, or it may be the o- or p-hydrogen atoms in phenols. Thus when acetophenone is boiled in alcoholic solution with formaldehyde and dimethylamine hydrochloride, the Mannich base P-dimethylamino-propiopbenone hydrochloride (I) is readily formed ... 

We have shown that a number of aliphatic ethers, containing steric-ally accessible p-hydrogen atoms, are cleaved in good yield when the tetrahalogenoanthranilic acids are diazotised in ethers 128,59). Diethyl ether is cleaved to the tetrahalogenophenetole (90) and methyl-cyclohexyl ether affords the tetrahalogenoanisole. In this latter reaction we were able to detect cyclohexene and therefore a plausible, but as yet unproved, mechanism is as shown. 

Reaction Modes of Titanium Alkyl Derivatives Possessing P-Hydrogen Atoms... 

Droege, A.T. andTully, F.P. Hydrogen-atom abstraction from alkanes by hydroxyl radical. 6. Cyclopentane and cyclohexane,/ Phys. Chem., 91(5) 1222-1225, 1987. 

Other pathways in the reaction between nitrate esters and alkalis are simple hydrolysis and P-eHmination — the abstraction of a P-hydrogen atom simple hydrolysis (Eq. (5)) ... 

When a haloalkane with p-hydrogen atom Is heated with alcoholic solution of potassium hydroxide, there Is elimination of hydrogen atom from p-carbon and a halogen atom from the a-carbon atom. As a result, an alkene is formed as a product. Since p-hydrogen atom is involved in elimination. It Is often called p-elimination. 

A chemical reaction Is the result of competition It Is a race that Is won by the fastest runner. A collection of molecules tend to do, by and large, what Is easiest for them. An alkyl halide with p-hydrogen atoms when reacted with a base or a nucleophile has two competing routes substitution (Sj,jl and Sj,j2) and elimination. Which route will be taken up depends upon the nature of alkyl halide, strength and size of base/nucleophile and reaction conditions. Thus, a bulkier nucleophile will prefer to act as a base and abstracts a proton rather than approach a tetravalent carbon atom (steric reasons) and vice versa. 

Sulfoxides containing P-hydrogen atoms, eg, di-Abutylsulfoxide [2211 -92-9], react with strongly basic systems, eg, potassium /-butoxide, in DMSO by sulfenic acid elimination to produce olefins (eq.12) (44) ... 

Addition of H-MgX to the double bond takes place according to the catalytic cycle shown below. A chloride ion from the titanocene dichloridc lirst undergoes substitution bv the Griguard reagent. Since the alkyl residue contains p hydrogen atoms, propenc (50) is then eliminated Alkene 52 coordinates with 51. whereupon insertion into the Ti-H bond occurs A subsequent expulsion regenerates catalyst 49 and releases product 45.13... 

It has been supposed that the reason for the high reducive activity ofjust the isopropoxogroup lies in the possibility of p-hydrogen atom transfer. 

A remaining question about the elimination process on a solid surface is the geometric structure of the adsorbed molecule during reaction. If the elimination is a concerted reaction, the acidic function of the catalyst surface (which binds the amine part of the reactant) and the basic function (which abstracts the p -hydrogen atom) should act simultaneously. In that case, it seems as if the reacting amine can assume only a configuration close to the syn configuration shown in Fig. 7. However, syn eliminations are less common than trans eliminations, and we have shown that the elimination of ammonia from amines occurs primarily via trans elimination [F. Rota, V. Ranade, and R. Prins, J. Catal. 200 (2001), to be published]. This problem was realized even... 

Ariel, S., Askari, S.H., Scheffer, J.R., and Trotter, J. (1986) Cyclopropanol formation via p-hydrogen atom abstraction the five membered transition state analog of the Norrish type II reaction. Tetrahedron Letters, 27 (7), 783—786. 

Homologs of <5, e.g., ethyl and n-butyl derivatives 9 and 10 are more sensitive, but can be handled in solution21 300. However, this does not apply to branched derivatives such as isopropyltitanium triisopropoxide, which appear to be unstable even in solution due to P-Hydride elimination. Moderate to pronounced degrees of thermal stability are characteristic of compounds in which p-hydrogens are not easily abstracted due to strain in the corresponding olefin (e.g., 1135) or 12 36) or in which they are lacking (e.g., 13 37)). Tetramethyltitanium 14 has no P-hydrogen atoms but is... 

An extension of the synthetic use of vinylidene lithium halocarbenoids is the method of Zweifel60) who succesfully generated a vinylidenecarbenoid bearing a p-hydrogen atom, as outlined in Eq. (55). Usually, attempts to prepare these intermediates resulted in the elimination of hydrogen halide with the formation of the corresponding acetylene 3). 

The hyperfine data presented in Table 7.2 are in favorable agreement with their calculated counterparts. Particularly, the dominant H hfc values attributed to the p-hydrogen atoms serve as well-suited reporters of the bonding situation within the inner cyclobutanoid fragment. 

Neither corrsponent needs to be aromatic and. although not appropriate to this book, these processes are widely applicable to C-C bond-forming reactions. However, the components should not contain p-hydrogen atoms bound to sp carbons, since these can undergo [1-hydrrde elimination. 

This reaction may also involve intramolecular p-hydrogen atom transfer... 

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