Allyl alcohols, isomerization with

The observed ees are in the range 64-86%. Z-allylic alcohols isomerize with higher ees than the E-allylic counterparts. This process has been applied to the synthesis of two naturally-occurring sesquiterpenes. It is interesting to note here that the highest ee previously reported for such an isomerization (53%) employed an Rh+/BINAP catalyst where a modest yield (47%) was also observed [76]. 

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

Allyl alcohol isomerization is typically conducted as a single-phase reaction, needing efforts for separation of the catalyst [110, 113], One driver was to exploit a catalyzed liquid/liquid route with aqueous (catalytic) and organic phases as commonly employed in the chemical industry. 

Olefins that lack an aromatic substituent can also be isomerized by Rh(l)/PF-P(o-To1)2 with good enantioselectivity (Eq. 8). Interestingly, for this class of substrates the reactions of E- and Z-allylic alcohols proceed with similar enantioselection. 

Methoxycyclopropanemcthanols, as potential precursors of cyclobutanones. are also obtained by addition of 1-methoxy-l-vinyllithium to carbonyl compounds followed by cyclo-propanation of the resulting allylic alcohol. Starting with cyclohexanone the final product was spiro[3.5]nonan-l-one (3).154 Cyclopropanation and rearrangement of an isomeric allylic alcohol 4 yielded spiro[3.5]nonan-2-one (5).15S... 

Figure 7.11 presents the mechanism of propylene oxide synthesis by allyl alcohol isomerization. This assumption correlates well with experimental data the kinetic curve shape... 

Bicyetic acetals.5 Cyclic allylic alcohols couple with ethyl vinyl ether when treated with Pd(OAc)2. Only a catalytic amount of Pd(II) is required if Cu(OAc)2 is present as a reoxidant. The absence of double-bond isomerization is a useful feature of this coupling. 

Heterogeneous copper catalysts prepared with the chemisorption-hydrolysis technique are effective systems for hydrogen transfer reactions, namely carbonyl reduction, alcohol dehydrogenation and racemization, and allylic alcohol isomerization. Practical concerns argue for the use of these catalysts for synthetic purposes because of their remarkable performance in terms of selectivity and productivity, which are basic features for the application of heterogeneous catalysts to fine chemicals synthesis. Moreover, in all these reactions the use of these materials allows a simple, safe, and clean protocol. 

The other major [2,3]-rearrangement, the Evans rearrangement,463 can be extended to synthesize functionalized allyl alcohols.464-465 The Evans rearrangement involves the rearrangement of an allylic sulfoxide to an allyl alcohol.463 The reverse reaction is also possible the conversion of an allyl alcohol to the analogous sulfoxide. The push-pull nature of the two rearrangements (Scheme 26.19), coupled with the transfer of chirality, provides a method to invert an allyl alcohol together with the isomerization of the alkene (Scheme 26.20)438,466... 

The absence of a substituent a to the carbonyl group in such systems does not necessarily preclude a successful Claisen-Cope reaction sequence. Thus, reaction of allylic alcohol 50 with acetal 51 produces the Claisen product 53. Double-bond isomerization of 53 to give a,/J-unsaturated aldehyde 54 is not observed. Instead, 53 is transformed to Cope product 57, probably via a conjugation-deconjugation-Cope rearrangement sequence1162. 

Observed Arrhenius activation parameters for allylic alcohol isomerizations are also consistent with a unimolecular rate-limiting step. In aqueous ethanol and aqueous dioxane, differences in reactivity due to structural changes in the allylic system are reflected primarily in the energy of activation. Values of entropy of activation are generally near Harris... 

Related to the alkene-to-allylic alcohol conversion is the oxiran to allylic alcohol isomerization. Two recent reports describe the use of trimethylsilyl iodide, either ready-made or prepared in situ from hexamethyldisilane and iodine, with DBU (Scheme 12) the initially formed 2-iodo-alkoxysilane (31) eliminates HI to give an allylic silyl ether that is cleaved during hydrolytic work-up. An alternative procedure (also Scheme 12) employs t-butyldimethyl-silyl iodide, generated in situ according to equation (14), and DBN. In this... 

The caged (CH3)3PAu cation also catalyzes with high turnovers of the allyl alcohol isomerization in aqueous solutions by Scheme 5.26, performing more than 1000 catalytic cycles [24]. 

The commonest of these for oxirane opening are amines and azide ion [amide ions promote isomerization to allylic alcohols (Section 5.05.3.2.2)]. Reaction with azide can be used in a sequence for converting oxiranes into aziridines (Scheme 49) and this has been employed in the synthesis of the heteroannulenes (57) and (58) (80CB3127, 79AG(E)962). 

Isomerization of the double bond in allylic alcohols may result in aldehydes or ketones (I07a). The reaction can have synthetic value (8bJ3c). If isomerization is desired, palladium is probably the preferred catalyst, operated best under hydrogen-poor conditions (/47fl). Allylic ethers can be converted to alcohols by isomerization with (Ph3P)3RhCl at pH 2 to the vinyl ether, which undergoes hydrolysis (36a). 

Allyl alcohol is produced by the catalytic isomerization of propylene oxide at approximately 280°C. The reaction is catalyzed with lithium phosphate. A selectivity around 98% could be obtained at a propylene oxide conversion around 25% ... 

Owing to the reversible nature of the allylic sulfenate/allylic sulfoxide interconversion, the stereochemical outcome of both processes is treated below in an integrated manner. However, before beginning the discussion of this subject it is important to point out that although the allylic sulfoxide-sulfenate rearrangement is reversible, and although the sulfenate ester is usually in low equilibrium concentration with the isomeric sulfoxide, desulfurization of the sulfenate by thiophilic interception using various nucleophiles, such as thiophenoxide or secondary amines, removes it from equilibrium, and provides a useful route to allylic alcohols (equation 11). 

The metal catalysis method has been used for the preparation of simple enols, for example, by isomerization of allylic alcohols. These enols are stable enough for isolation (see p. 75), but slowly tautomerize to the aldehyde or ketone, with half-lives ranging from 40-50 min to several days. ... 

In Section 3.5 on alkene isomerization, it was mentioned that Li and co-workers reported a RuCl2(PPh3)3-catalyzed shuffling of functional groups of allylic alcohols in water (Eq. 3.35).140 Since the reaction proceeds through an enol intermediate, allyl alcohols can thus be considered as enol equivalents.203 This has been developed into an aldol-type reaction by reacting allyl alcohols with aldehyde (Scheme 3.11).204 The presence of In(OAc)3 promoted the aldol reaction with a-vinylbenzyl alcohol and aldehyde.205... 

An example of this type of reaction that does not produce a byproduct is isomerization (the reaction of a feed to a product with the same chemical formula but a different molecular structure). For example, allyl alcohol can be produced from propylene oxide5 ... 

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