A’,p-eliminations

Step 2 The oxidation step can be viewed as a p elimination Water acts as a base to remove a proton from carbon while the O—Cr bond breaks... 

This is an extrusion reaction (see Chapter 17). In another example of a process in category 2, aldehydes and ketones with a 7 hydrogen can cleave in still another way (a P elimination, see Chapter 17) ... 

An a hydrogen is obviously necessary in order for the ylide to be formed. This type of mechanism is called a, p elimination, since a P hydrogen is removed by the a carbon. The mechanism has been confirmed by labeling experiments similar to those described at and by isolation of the intermediate ylids. An important... 

Other possible mechanisms, such as ElcB (p. 1308) or a, p elimination (p. 1333), were ruled out in all these cases by other evidence. 

Yoshida Y, Y Nakano, A Amano, M Yoshimura, H Fukamachi, T Oho, Y Koga (2002) led from Streptoccus anginosus encodes a C-S lyase with a,P-elimination activity that degrades L-cysteine. Microbiology (UK) 148 3961-3970. 

If a hydrogen atom is present in the p position of the halogen, a p-elimination reaction becomes possible. If the unsaturated compound obtained is particularly unstable, the reaction will be dangerous. 

It is quite easy to eliminate two hydrogen atoms via a-elimination from a silicon atom (preferred to a p-elimination with the second hydrogen bonded to a carbon or nitrogen atom). 

Scheme 11 Catalyst deactivation routes in copolymerizations with polar olefins a P-elimination of a leaving group to afford allylic or dimeric species, b poisoning by N-complexation in the attempted copolymerization of acrylonitrile...

FIGURE 7.5 Preparation of a protected dipeptide by the mixed-anhydride method, employing a chloroformate that generates a cleavable urethane.13 The urethane impurity is destroyed by a P-elimination reaction. NMM = A-methylmorpholine, Msc = methane-sulfonylethoxycarbonyl. 

Rajagopal and Burris (1999) studied the degradation reaction of ethylene dibromide in water with zero-valent iron. Ethylene dibromide degraded rapidly following pseudo-first order kinetics. The observed end products were ethylene and bromide ions and were probably formed via a P-elimination pathway. Bromoethane and vinyl bromide were not observed as possible intermediate products. 

In the M-R bonded intermediate an a-elimination is not possible, whereas a P-elimination produces a metal hydride. This is energetically unfavorable compared to the oxy-bonded intermediate partly because of the lower M-H bond strength compared to MO-H (see section on thermodynamics). Thus, because of kinetics (lack of decomposition pathways) and thermodynamics (energetics), the metal bound M-R intermediate is less reactive on the surface than die M-O-R intermediate. 

In addition to decomposition by loss of free radicals, Ga(C2H5)3 and Ga compounds with larger alkyl groups can undergo decomposition through a P-elimination reaction (118, 130). 

Bayer, Pan, and Kazmaier (7) proposed that labile sulfur originates from cysteinyl residues by a p-elimination reaction in the presence of... 

Losse et al. have demonstrated that the difficulty in the hydrogenolytic debenzylation of S-protected cysteine derivatives is due to the strong poisoning action of thiols that may be formed probably by a P elimination of the cysteine to give the dehydroalanine derivative.83 The overall reaction in the case of Z-Cys(Bzl)-OBzl may be formulated as in eq. 13.43. 

A radical addition reaction has been used to functionalize cyclopentane. The chain carrier, C1-, was generated and regenerated by a P-elimination (wocess (equation 33). Other activated alkenes. such as maleic anhydride, furanone and acrylonitrile, have also b added to cyclohexane in 15-45% yield in a reaction initialed by Bu OOH or light. ... 

To be transformed in aroma, the precursor must enter into the yeast, then be cleaved by a-p elimination of the cysteinylated precursor and finally excreted in the medium where it is more or less stable according its reactivity with other compounds. Obviously, the yield of this transformation can be influenced by several... 

The dehydrohalogenation of alkyl halides, discussed in Chapter 8, is one way to introduce a n bond into a molecule. Another way is to eliminate water from an alcohol in a dehydration reaction. Dehydration, like dehydrohalogenation, is a P elimination reaction in which the elements of OH and H are removed from the a and P carbon atoms, respectively. 

Amines, like alcohols, contain a poor leaving group. To undergo a P elimination reaction, for example, a 1 ° amine would need to lose the elements of NH3 across two adjacent atoms. The leaving group, NH2, is such a strong base, however, that this reaction does not occur. 

Zaitsev rule (Section 8.5) In a P elimination reaction, a rule that states that the major product is the alkene with the most substituted double bond. 

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