Deoxygenation of allyl alcohols

Examples of direct C-0 bond cleavage of allylic alcohols are relatively rare compared to other allylic compounds. Deoxygenation of allylic alcohols by WCl2(PMePh2)4 has been reported [ 17]. Several other examples of the C-O bond cleavage in allylic alcohols have been reviewed previously [1]. 

A rare example of C—O bond scission in aqueous solution is the deoxygenation of allylic alcohols catalyzed by [HCo(CN)5]3 [137]. When the two-phase reaction was carried out in the presence of /3-cyclodextrin it yielded trans-alkenes selectively whereas, with no cyclodextrin added, a mixture of cis- and trans-alkenes was formed. In a typical example, reaction of 2-hexen-l-ol yielded 91% trans-2-hexene and 4% 1-hexene. 

Deoxygenation of allylic alcohols without affecting isolated tertiary alcohols, double bonds, and acetals is accomplished by reaction with EtsSiH in the presence of LiC104. A route to /3-C-glycosyl aldehydes involves reaction of sugar lactones with 2-lithio-l,3-dithiane, reduction with EtsSiH, and hydrolysis of the dithiane. ... 

Nevertheless, it must be pointed out that the formation of such transient species has never been spectroscopically observed. Native CDs are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols, epoxydation,or oxidation " of olefins, reduction of a,/ -unsaturated acids,a-keto ester,conjugated dienes,or aryl alkyl ketones.Interestingly, chemically modified CDs like the partially 0-methylated CDs show a better catalytic activity than native CDs in numerous reactions such as the Wacker oxidation,hydrogenation of aldehydes,Suzuki cross-coupling reaction, hydroformylation, " or hydrocarboxylation of olefins. Methylated /3-CDs were also used successfully to perform substrate-selective reactions in a two-phase system. 

Transpositional deoxygenation. o-Nitrobenzenesulfonylhydrazine supplies a hydride source to effect the transpositional deoxygenation of allylic alcohols via the V-sulfonyl-V-allylhydrazines, which are formed by an S 2 reaction. This reaction has been used to synthesize allenes from propargylic alcohols in one step. ... 

Another interesting application of the deoxygenation reaction is shown in Scheme 12.6. Sharpless epoxides are transformed to enantiomerically pure allylic alcohols [14]. It should be noted that the disadvantage of the loss of one-half of the allylic alcohol, as in the case of kinetic resolutions of allylic alcohols, is not a problem when this protocol is employed. 

Hydroalumination of allylic alcohols or ethers. llydroalumination of these substrates under usual conditions (9. 276) proceeds poorly because of deoxygenation associated with titanium catalysts.3 The desired reaction, however, can be effected in 50 80" yield with ZrC l4 in the case of alcohols and Cp.ZrCL in the case of cllieis (equations 1 and II). 

Deoxygenation of allyttc alcohols.3 A method for conversion of allylic alcohols to l-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an Oallyl xanthate to 2. The second step is an allyl transfer from sulfur to tin with tri-n-butyltin hydride to give the allylic stannane (3). The last step, destannylation, is a well-known mule to terminal alkcnes.4... 

Deoxygenation of aUyBc and benzyUc alcohols. Allylic and benzylic alcohols are conveniently reduced to the corresponding hydrocarbons in two steps. The alcohol is first converted into a pyridinium alkyl sulfate by reaction with sulfur trioxide-pyridine in THF at 0-3° (3-20 hours). Then lithium aluminum hydride (or LiAIH -AlClj, 3 1) in THF is added and the mixture stirred for I hour at 0° and then at 25° for 3-5 hours. Yields are high. In the case of allylic alcohols... 

Hydrogenation and Deoxygenation. Nickel boride, produced in situ by the action of sodium borohydride on nickel(II) chloride, has been used to carry out a two-step hydrogenolysis of allylic alcohols via reductive cleavage of the trimethylsilyl ethers. Tertiary alcohols may be deoxygenated in reasonable yields by means... 

An effective deoxygenation using enantiomerically pure epoxides from primary allylic alcohols ( Sharpless epoxides ) [44] to give enantiomerically pure secondary allylic alcohols was described by Yadav [45]. This approach circumvented a kinetic resolution of secondary allylic alcohols that implies a maximum yield of 50% ( Scheme 5). 

Allyl Alcohols. Secondary cyclic allylic alcohols are reduced with the combination of Et3SiH and ethereal LiC104, even in the presence of a tertiary alcohol (Eq. 35) or ketal function.173 Primary allylic alcohols do not undergo deoxygenation under similar conditions.173... 

The next step was reduction of the ester and O-benzyl ether groups in 6 with lithium in liquid ammonia. Note how the allylic alcohol survived these reduction conditions. Sometimes, allylic deoxygenation can... 

When 28 was subjected to Cp2TiCl in dry THF, the allylic alcohol 29, together with the cyclization product 30 and a minor quantity of the reduction product 31, was isolated. The expected deoxygenation product 32 was not formed at all (Scheme 22) [73, 74],... 

Phenols are transformed into aryl chlorides by PhPCl4779. Aromatic and aliphatic aldehydes give, in high yields, gem dibromides with combination of triphenylphosphite and bromine780. Brominative deoxygenation of ketones is achieved by 2,2,2-tribromo-l,2,3-benzodioxaphosphone781. Allylic alcohols are transformed into allylic iodides by reaction with P2I4782. 

Deoxygenation. Greene has applied Kabalka s method for reduction of an enone (6, 98 7, 54) to the tosylhydrazone of a cross-conjugated ketone, 6-epi-a-santonin (2). Unexpectedly only one olefin (3) was obtained in 50% yield. This product was converted into the diene 4 by allylic oxidation, reduction to an allylic alcohol, and dehydration. The product was converted into (—)-dictyolene (5) by a method developed previously. ... 

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