Alcohols, allylic from alkenes

The l,5-hexadien-3-ol derivatives 792 and 794 are cycli2ed to form the cyclo-pentadiene derivatives 793 and 795 by insertion of an alkene into -allylpalla-dium formed from allylic alcohols in the presence of trifluoroacetic acid (lO mol%) in AcOH[490],... 

The high selectivity of the catalyst in forming ( )-alkenes can be used in interesting ways (eq. 1). For example, in acetone-iie solution, within 15 min at room temperature allyl alcohol is converted to nearly pure enol (E)-26. Under these mild conditions, the product slowly isomerizes to the more stable aldehyde tautomer. We know of one other report of rapid enol formation from allyl alcohol, using a Rh... 

The configuration of the product in diastereoselective hydrogenation -whether 1,2-syn or 1,2-anti - is related to the substitution pattern of the starting alkene. The allyl alcohol with a 1,1-disubstituted olefin unit affords the antiproduct, while the syn-product is formed from the allyl alcohol with a trisubsti-tuted olefmic bond (Table 21.8, entries 6-9). The complementarity in diastereoselective hydrogenation of di- and tri-substituted olefins may be rationalized based on the conformation analysis of the intermediary complex (Scheme 21.1)... 

The hydroxymercuration-demercuration of conjugated dienes generally does not afford monohydration products selectivity, but diols can sometimes be obtained in reasonable yield. The direction of addition of H—OH is that expected by extrapolation from simple alkenes and allylic alcohols. 

Z)-Trisubstitutedalkenes. Still and Mitra have described an efficient synthesis of alkenes of this type from allylic alcohols by a [2.3] sigmatropic Wittig rearrangement. The alcohol 2 is converted into the allyl stannylmethyl ether (3), which can be isolated if desired. Treatment with n-butyllithium results in tin-lithium exchange and rearrangement to the homoallylic alcohol 4 in 95% overall yield. When 3 is transmetalated and immediately quenched with cyclohexanone, 5 is obtained in 73% yield. 

The Simmons-Smith reaction is an efficient and powerful method for synthesizing cyclopropanes from alkenes [43]. Allylic alcohols are reactive and widely used as substrates, whereas a,j8-unsaturated carbonyl compounds are unreactive. In 1988, Ambler and Davies [44] reported the electrophilic addition of methylene to a,/3-unsaturated acyl ligands attached to the chiral-at-metal iron complex. The reaction of the racemic iron complex 60 with diethylzinc and diiodomethane in the presence of ZnCl2 afforded the c/s-cyclopropane derivatives 61a and 61b in 93 % yield in 24 1 ratio (Sch. 24). 

This process occurs for X = O only in two special cases, which have been developed into synthetic methods for alkenes and allylic alcohols. Reaction of BusSnH with 1,2-bis-MDC derivatives gave al-kenes from diols in a process which was independent of the geometry of the starting diol. Examples are the conversions of (84) to (85) and of (86) to (87). 

Very efficient asymmetric hydroformylation of alkenes, dienes, and heterocycloalkenes is achieved with a complex formed by mixing RhCacacXCO) and 52. The diphosphines 53 and 54 have found utility in the preparation of y-lactones" from allylic alcohols and 3-substituted cyclopentanones " from 4-substituted 4-pentenals, respectively, and in chiral forms. 

The formation of stabilized imino-iodanes as nitrene precursors has been exploited widely in aziridination of alkenes. Traditionally, isolated iminoiodanes have been the preferred route toward transition metal mediated or catalyzed aziridination or C-H amination chemistry [33]. Despite the great success in this field, some reports have become available on the corresponding transition metal-free variants. For example, Padwa reported an iodine(lll)-mediated aziridination of some carbamates 43 from allylic alcohols that in situ formed the corresponding imino-iodanes. Unexpectedly, these compounds underwent direct aziridination to the putative tricyclic compound 44, which was subsequently opened by addition of suitable nucleophiles to arrive at the 1,2-difunctionalized product 45 in a completely stereoselective manner (Scheme 11) [34]. This transformation constituted the proof of concept that iodine(lll)-mediated aziridination does not necessarily require metal promoters. 

Exciplex formation initially prevented application of the method to compounds such as amines or alkenes which fail to undergo simple C-H homolysis with Hg. The method was therefore extended [22a] by running the reaction under H2 in which case H atoms are formed and either abstract H atoms directly from the substrate or add to C=C double bonds. The first makes it possible to dehydrodimerize amines, the second allows hydrodimerization of alkenes. In the alkene case, we do not get C-C bond formation at the weakest C-H bond as usual, but at the most substituted of the two vinyl carbons of the double bond. For example, n-alcohols tend to give 1,2-diols because the weakest CH bond is a to O (eq. 37), but unsaturated alcohols give other isomers, for example the 1,4-diol tend to be formed from allyl alcohols (eq. 38). 

Allylic anda-Allenic Alcohols. The electrochemical generation of a selenenylat-ing reagent, presumably PhSeOR, from a catalytic amount of diphenyl diselenide in the presence of water or an alcohol (ROH) is the basis of a new method for direct conversion of alkenes to allylic alcohols or ethers (Scheme 13). Regio-specific Markovnikov oxyselenenylation followed by electrochemical oxidation and selenoxide elimination, regenerating a selenenylating species, accounts for a sequence that is obviously closely related to the bromide-mediated method reported last year (5,158). 

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 group of Nicewicz reported the development of a new organocatalytic synthetic method to directly synthesize highly substituted tetrahydrofurans 127 from allylic alcohols 125 and alkenes 126 by a photochemical-induced polar-radical-crossover cycloaddition sequence involving the photoredox catalytic system 128/129 (Scheme 2.33) [49]. This method demonstrated a broad range of functional group compatibility and could find unique applications in complex molecule synthesis. 

Another option to gain access to T -allyliron complexes is the removal of a leaving group from substituted neutral (T] -alkene)iron complexes. This procedure starts from allylic alcohols, allylic ethers, or carboxylates, which are complexed with nonacarbonyldiiron and subsequently treated with tetrafluoroboric acid to aflbrd the cationic Ti -allyl(tetracarbonyI)iron complexes (Scheme 4-78). The reaction is stereoselective with respect to the geometry of the starting allyl alcohols. ( )-AlIyl alcohols give anti products, whereas syn complexes are formed from (Z)-allyl alcohols. The alkene complexes can also be generated in situ to afford the q -allyliron complexes after treatment with acid. ... 

Recently, Szabo and co-workers synthesized allyl silanes either from alkenes by C-H silylation or from allylic alcohols. ... 

When allylic alcohols are used as an alkene component in the reaction with aryl halides, elimination of /3-hydrogen takes place from the oxygen-bearing carbon, and aldehydes or ketones are obtained, rather than y-arylated allylic alcohoIs[87,88]. The reaction of allyl alcohol with bromobenzene affords dihydrocinnamaldehyde. The reaction of methallyl alcohol (96) with aryl halides is a good synthetic method for dihydro-2-methylcinnamaldehyde (97). 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

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