Homoallylic alcohols, transition metal

Direct carbonylation of organomercurials is a low yielding process that reqnires high temperatures and pressures. However, it can be performed efficiently under milder conditions in the presence of transition metal catalysts, particnlarly, rhodium and palladium. Two important applications of this protocol have recently been reported. The Rh -catalyzed formylation of organomercurials has been applied to the synthesis of a polyol-derived natural product. The organomercury chloride substrate is synthesized by oxymercuration of the corresponding homoallylic alcohol with Hg(OAc)Cl (Scheme 5). 

Copper(I) triflate was used as a co-catalyst in a palladium-catalyzed carbonylation reaction (Sch. 27). The copper Lewis acid was required for the transformation of homoallylic alcohol 118 to lactone 119. It was suggested that the CuOTf removes chloride from the organopalladium intermediate to effect olefin complexation and subsequent migratory insertion [60]. Copper(I) and copper(II) chlorides activate ruthenium alkylidene complexes for olefin metathesis by facilitating decomplexation of phosphines from the transition metal [61]. 

Some cationic Rh- and Ir-phosphine complexes have been used for diastereoselective hydrogenation of chiral allylic and homoallylic alcohols. Here, the preexisting chirality of the 5p -hybridized carbons induces new asymmetry on the neighboring alkenic diastereofaces through coordination of the hydroxy group to the transition metals (see Section 3.2.1). 

Other promising methods for the stereoselective introduction of the vinyl moiety into functionalized organic substrates are the reactions of such adducts with epoxides to produce homoallylic alcohols (equation 70) ° and with conjugated enones to yield ketones (equation 71). 58.i59 gQtj, reactions have major limitations, and their application in a special instance may require a study of the effect of Lewis acids or transition metals as promoters or selectivity agents. 

Lewis acids, such as SnCU, also catalyze the reaction, in which case the species that adds to the alkenes is H2C O SnCl4. " Montmorillonite KIO clay containing zinc (IV) has been used to promote the reaction. " The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form homoallylic alcohols. A typical example is the reaction of methylenecyclohexane with an aryl aldehyde to give 89. 

The transmetallation or the metal-metal exchange reaction of an allylic tin species with an electrophile was first observed in 1970 [77]. The possibility that transmetallation may play a role in the Lewis acid-promoted reaction of allylstannanes with aldehydes was initially discussed by Tagliavini [78], Keck [79], Yamamoto [71b, 80], and Maruyama [81]. It is believed that upon transmetallation with either SnCU or TiCl4, the addition of an allylstannane and aldehyde will occur via a cyclic, six-membered transition structure. The reaction occurs by coordination of the aldehyde carbonyl with the Lewis acidic trichlorotin or trichlorotitanium reagent, thus affording the anti homoallylic alcohol (Scheme 10-42). 

Hartung, J. and Greh, M. (2002) Transition metal-catalyzed oxidations of bishomoallyhc alcohols. J. Organomet. Chem., 661,67-84 (b) Hartung, J., Drees, S., Greh, M., Schmidt, P., Svoboda, 1., Fuess, H., Murso, A., and Stalke, D. (2003) (SchifF-base)vanadium(V) complex-catalyzed oxidations of substituted bis(homoallylic) alcohols -Stereoselective synthesis of functionalized tetrahydrofurans. Eur. J. Org. Chem., 2388-2408. 

This method has not yet found widespread use for the preparation of allylboronates. In fact, uncatalyzed hydroborations of dienes tend to provide the undesired regioiso-mer with the boron atom on a terminal carbon, i.e., homoallylic boranes. By making use of certain transition metal catalysts, however, Suzuki and co-workers found that (Z)-allylic catecholboronates such as 22 can be obtained in high yield from various substituted butadienes (e.g., isoprene. Equation 11) [44]. Whereas a palladium catalyst is the preferred choice for acyclic dienes, a rhodium catalyst (Rh4(CO)i2) was best for the hydroboration of cyclohexadiene. A suitable mechanism was proposed to explain the high regioselectivity of this process. In all cases, a reaction quench with benzaldehyde afforded the expected homoallylic alcohol product from a tandem hy-droboration/allylation (Section 6.4.1.4). 

Cyclizations. In addition to the use of SEM-Cl as a means of one-carbon homologation, it has also been employed in the generation of acetals capable of undergoing a variety of cy-clization events. Alkylation of allylic alcohols with SEM-Cl permits haloacetalization to afford di- and trisubstituted 1,2-dioxanes (eq 28). Alternatively, homoallylic alcohols permit Prins-type cyclizations (eq 29) to furnish 2,4,5-trisubstituted tetrahydro-pyrans. Transition metals can also be employed to promote the formation of substituted benzofurans (eq 30). ... 

In addition to stoichiometric reactions, transition metals catalyze retroally-lation reactions of homoallyl alcohols [64]. Homopropargyl [65], allenylmethyl [64f], and allenylethyl ones [66] also undergo analogous C-C cleavage. See Chapter 5 for catalytic reactions. 

In 1996, when transition-metal-catalyzed carbon-carbon bond cleavage was just emerging as a current topic in organic chemistry, Kondo and Mitsudo reported the first transition-metal-catalyzed retro-allylation of homoallylic alcohol to simply cleave off the allylic moiety with a ruthenium complex [7]. After 10 years of silence, transition-metal-catalyzed retro-allylation has been rapidly developing since 2006. Nowadays, catalytic retro-allylation can be regio- and stereoselective, hence being a useful tool for modern organic synthesis [8]. 

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