Promotion, isomerization

Opening by trimethylsilyl trifluoromethanesulfonate yields an adduct (54) from which trifluoromethanesulfonic acid can be eliminated to give an allylic alcohol (Scheme 47) (79JA2738) [cf. base-promoted isomerization to allylic alcohols (Section 5.05.3.2.2)]. 

L. Ploense, M. Salazar, B. Gurau, and E.S. Smotkin, Proton Spillover Promoted Isomerization of n-Butylenes on Pt-black Cathodes/Nafion 117, JACS119, 11550-11551(1997). 

Ligand screening experiments were conducted on the alkenes 1-pentene and pent-4-en-l-ol, because such substrates were inert to 3a-3c (15). Pentene lacks any polar or protic group and pentenol contains the alkene and OH separated by 3 carbons. The preliminary studies involved phosphines with both imidazol-2-yl and pyrid-2-yl substituents on P as well as t-Bu, i-Pr, Ph, and Me groups (16). From the screening, complex 1 derived from the phosphine ligand 4 (17) was identified as the most capable (in terms of both reaction rate and final yield) of promoting isomerization of both 1-pentene and pent-4-en-l-ol. 

A careful investigation of the extent of isomeric products formed by reaction of several alcohols with thionyl chloride has been reported. The product compositions for several of the alcohols are given below. Identify the structural features that promote isomerization and show how each of the rearranged products is formed. 

Carbamates (R2NCOOR ) may be O or N bonded. Base-promoted isomerization of carbamates from the O- to the deprotonated A--bound form in pentaamminecobalt(III) complexes have been defined.986... 

Figure 7.13f). Although addition of Cu initially promotes isomerization of 1-alk-ene (initially higher l-alkene/2-alkene ratio for 100Fe/4.6Si/5.0K/2.0Cu in comparison with 100Fe/5.1Si/5.0K catalyst Figure 7.13e), at steady conditions the effect is marginal.

Intermediates occurring in the acid-promoted isomerization of an alkoxyacetyl complex, CpFe(C0)2C0CH20Me (5a) to its carboalkoxymethyl compound CpFe(C0)2CH2C02Me (11a) have been observed by IR. Upon addition of acid to 5a, its IR terminal v (2025,... 

BOP, PyBOP, HBTU, HATU, and so forth with an additive It has been considered essential to use an additive with these reagents because the tertiary amine required to effect the coupling promotes isomerization. Diisopropylethylamine or possibly trimethylpyridine are the bases of choice to minimize the side reaction, but the additive may increase isomerization (see Section 7.18). 

Because a benzilic acid rearrangement is the last step in the formation of saccharinic acids the difference between these results could be due to complexation of hydroxyl groups and alkoxide anions by the divalent calcium ions. Such a complexation apparently promotes isomerization and, hence, a fragmentation-recombination to the xylosaccharinic acid. This route has been questioned, and yet it provides a rational explanation of the results. 

Base-promoted isomerization of 2,3-epoxy alcohols. Also known as epoxide migration. 

Scheme 37. Lithium iodide-promoted isomerization of Michael adducts 94 into cyclobutene-carboxylates 109,110 [8]...

Nevertheless, whereas the base-promoted isomerization of simple linear oxiranes and cyclohexene oxide occurs via a -deprotonation mechanism, recent denterinm-labeUng experiments demonstrate that the LDA-mediated rearrangement of cyclopentene oxide in nonpolar solvents furnishes the corresponding cyclopentenol through an a-deprotonation route (Scheme 7) . 

Based on this feature, aggregation states of transition-state structures for base-promoted isomerization of oxiranes have been established by kinetic studies of LDA-mediated isomerizations of selected oxiranes in nonpolar media in the presence of variable concentrations of coordinating solvents (ligands). Results reported provide the idealized rate law V = [ligand]" [base] [oxtrane] for a-deprotonation and v = fc[ligand]°[base] / [oxirane]... 

The RLi homochiral ligand complexes are seldom used for the base-promoted isomerization of oxiranes into allylic alcohols because their poor chemoselectivity lead to complex mixtures of products. As examples, the treatment of cyclohexene oxide by a 1 1 i-BuLi/(—)-sparteine mixture in ether at low temperature provides a mixture of three different products arising respectively from -deprotonation (75), a-deprotonation (76) and nucleophilic addition (77) (Scheme 32) . When exposed to similar conditions, the disubstituted cyclooctene oxide 78 affords a nearly 1 1 mixture of a- and -deprotonation products (79 and 80) with moderate ee (Scheme 32, entry 1). Further studies have demonstrated that the a//3 ratio depends strongly on the type of ligand used (Scheme 32, entry 1 vs. entry 2) . ... 

Both dienic enamide 220a and its homolog 220b (obtained by base-promoted isomerization of the respective propargylamides 219a and b) afford in a tandem RCM mixtures of 1,3-dienic lactams 221 and 222, though in different proportions (02OL2417). 

Hydroprocessing and special absorption techniques are utilized to remove sulfur and nitrogen from the reformer. If not removed through hydroprocessing, feedstock sulfur will be converted to H2S in the reformer. The H2S will then serve as a poison to the platinum reformer catalyst and diminish the dehydrogenation and dehydrocyclization reactions. When present, H2S can neutralize the acid sites on the catalyst diminishing the ability of the catalyst to promote isomerization, dehydrocyclization, and hydrocracking reactions. 

Enolates, e.g., 11, can be prepared by conversion of an allylic alcohol via rhodium-promoted isomerization of the corresponding lithium alkoxide29. 

Other evidence advanced in favor of alkyl nitrenium ions came from Ag+ promoted isomerization and solvolysis of various cyclic and bicyclic A-chloroamines (e.g., A representative example is shown in Figure 13.12. It was argued... 

Scheme 10 Base-promoted isomerization of propargyi amide foiiowed by RCM.

Carbon deposition blocks the valleys rather than the summits, which relatively enhances formation of 3C complexes and relatively suppresses formation of 2C complex. Moreover, carbon -covered surface has a lower concentration of hydrogen and this relatively promotes isomerization and, with higher hydrocarbons—dehydrocyclization. 

The ability of a catalyst to promote isomerization plays two roles in reforming it increases the amount of branched chain paraffins in the product and it converts naphthene hydrocarbons with cyclopentane rings into cyclohexane ring naphthenes which are necessary for the formation of aromatics by dehydrogenation. 

In order to interpret the remarkably high activity of platinum to promote isomerization of neopentane to isopentane, the direct formation and involvement of metalla-cyclobutane intermediate 18 was suggested.152,158 This a,y diadsorbed species bonded to a single platinum atom is in accordance with the existence of platinum complexes and the ability of platinum to catalyze a-y exchange.156,159... 

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