Isomerization, allyls

The pivaloylamidobenzyl group was stable to acetic acid-water-90°, MeOH-NaOMe, iridium-induced allyl isomerization, and to many of the Lewis acids used in glycosylation. ... 

The Paloc group was developed as an amino acid protective group that is introduced with the p-nitrophenyl carbonate (H2O, dioxane, 68-89% yield). It is exceptionally stable to TFA and to rhodium-catalyzed allyl isomerization, but it is conveniently cleaved with Pd(Ph3P)4 (methylaniline, THF, 20°, 10 h, 74-89% yield). ... 

In contrast to the allylic sulfenates mentioned so far, cinnamyl trichloromethanesulfenate (9), prepared by the usual method, can be isolated and is relatively stable. Furthermore, its rearrangement to cinnamyl trichloromethyl sulfoxide (i.e., without allylic isomerization, equation 7), proceeds at a relatively slow rate (in CC14 at 80.0 °C, k = 3.90 x 10 5s 1). This result also contrasts with the observation mentioned earlier that cinnamyl arenesulfinate rearranges to a-phenylallyl aryl sulfone33,34. Similar behavior has been detected for y, y-dimethylallyl ester 11 which undergoes thermal isomerization to sulfoxide 12 (equation 8)36-38. 

We shall elaborate further, in subsequent sections, on the role played by the bis(r(1) species in the reaction course as possible reactive intermediates involved in allylic isomerization and/or reductive elimination. 

Fig. 4. Allylic enantioface conversion in riVl C1) species (top), and allylic isomerization taking place via an r(3,ri1(C3)-allylic intermediate (below).

Fig. 5. Selected geometric parameters (A) of the optimized rotational transition-state structures for allylic isomerization via the r(3-1s y ,ri1(C3)-octadienediyl-Ni11 TSiSo[3a] and TSiSo[3b],...

The conversion processes of the terminal allylic groups of the octadienediyl-Ni11 complex show very similar characteristics for the two reaction channels. The influence of electronic and steric factors on the allylic isomerization will be scrutinized in Section 5.3 and the overall role played by allylic conversion will be elucidated in the context of the entire reaction course (see Sections 6.1 and 6.2). 

Overall, allylic isomerization in the dodecatrienediyl-Ni11 complex is predicted to require a distinctly lower barrier than for reductive elimination (AAG > 5.5kcalmol 1, see Section 4.6). This leads to the conclusion, that isomerization should be significantly more facile than the subsequent reductive elimination, which is confirmed by NMR investigations of the stoichiometric reaction.22 Consequently, the several configurations and stereoisomers of the bis(allyl),A-/restablished equilibrium, with 7b as the prevalent form. The various bis(r 3-allyl),A-/n2H.v stereoisomers of 7b are found to be close in energy, while bis(allyl), A-cf.v forms are shown to be negligibly populated (cf. Section 4.4) and therefore play no role within the catalytic reaction course. 

Experiments have demonstrated that the stoichiometric cyclotrimeriza-tion becomes accelerated by the presence of donor phosphines (i.e., PMe3, PEt3, PPh3) and also by excess butadiene.93 However, the rotational transition-state structure TS SO[6b] is found to be not stabilized in enthalpy by coordination of butadiene. Therefore, incoming butadiene does not serve to facilitate allylic isomerization and will not assist this process. Accordingly, reductive elimination is indicated to be accelerated by excess butadiene, which will be examined in the next section. 

Overall, steric and electronic factors, which are seen to be small, are found to work in opposite directions and, to some degree, cancel each other out. Consequently, the intrinsic free activation barriers and reaction free energies (AG nt, AG nt), respectively, span a small range for catalysts I-IV and differ by less than l.Okcalmol-1. Thus, oxidative coupling represents the one process (beside allylic isomerization, cf. Section 5.3) among all the critical elementary steps of the C8-cyclodimer channel, that is least influenced by electronic and steric factors. 

Among the several configurations of the crucial [Nin(octadienediyl)L] complex, all of which are in equilibrium, the p3, 1 1) species 2a and the bis(p3) species 4a are predicted to be prevalent. The odonor/71-acceptor ability of the ancillary ligand is shown to predominantly determine the position of the kinetically mobile 2a 4a equilibrium. The conversion of the terminal allylic groups via allylic isomerization and/or allylic enantioface conversion are indicated to be the most facile of all the elementary processes that involve the [NiII(octadienediyl)L] complex. Consequently, the several octadienediyl-Ni11 configurations and their stereoisomers are likely to be in a dynamic pre-established equilibrium, that can be assumed to be always present. 

Bis(p -octadienediyl-Ni11 species are shown (i) to be thermodynamically highly unfavorable, thus indicating them to be sparsely populated, and (ii) not to be involved as reactive intermediates along any viable path either for allylic isomerization or for reductive elimination. This leads to the conclusion, that bis(p ) species play no role within the catalytic reaction cycle. 

B2 is an antibody that catalyzes the allylic isomerization of / ,7-unsaturated ketones (Figure 16) as well as the Kemp reaction. ... 

According to experimental evidence, the allyl ligands may be eliminated from the surface as either 1,5-hexadiene (by reductive elimination of two allyl ligands) or propene. Two tris(trimethylophosphine) complexes of rhodium are formed, only one of which retains the allyl ligand. This is in fact the 7i-o-allyl isomerization in a well-defined surface organometallic complex. 

The ready deprotonation of the vinyl aminosulfoxonium salts 46 with a strong base at the a-position prompted a study of their reactions with weaker bases. It was speculated that in this case a vinyl-allyl isomerization of 46 might occur, followed by an intramolecular substitution of the allyl aminosulfoxonium salt... 

Allylic isomerization of carbon-heteroatom double bonds can be accompanied by the 1,3-shift of an atom or a substituent. An enol-kctone isomerization takes place with cnol ethers 1 to give the acid fluorides 2 when heated in the presence of a catalytic amount of triethylaminc,1,2 in the presence of dimethylformamidc or hexamethylphosphoric triamidc.3 or with triethyl phosphite.4 The authors propose that the enolate (CF3)2C = CF —O- is formed in the first step.2,5 This enolate can attack ether 1 in an SN2 reaction to give product 2 and regenerate the enolate. Ethyl ether 1 b requires more severe conditions than the corresponding methyl derivative la, presumably due to steric hindrance.2,5... 

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