Selenols alkenes

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

A highly regioselective hydroselenation of terminal alkynes RC=CII with benzene-selenol (PhSeH) can be achieved in the presence of palladium acetate as catalyst in pyridine, giving rise to the corresponding terminal alkenes R(PhSe)CH=CH2 as the sole products. Here, the pyridine is believed to serve as a ligand for active palladium intermediates.67... 

Selenocyanates produce selenols or diselenides upon either reduction (e g. with sodium borohydride) or hydrolysis (see Scheme 1). They undergo displacement of the cyanide ion by various nucleophiles and add to alkenes in a maimer similar to selenenyl halides (see equation 14), except that catalysis with Lewis acids is required in the case of unactivated alkenes. The selenocyanates are also popular reagents for the preparation of selenides from alcohols, and (8) from carboxylic acids, as indicated in Scheme 3. 

A number of useful enantioselective syntheses can be performed by attaching a chiral auxihary group to the selenium atom of an appropriate reagent. Examples of such chiral auxiliaries include (49-53). Most of the asymmetric selenium reactions reported to date have involved inter- or intramolecular electrophilic additions to alkenes (i.e. enantioselective variations of processes such as shown in equations (23) and (15), respectively) but others include the desymmefrization of epoxides by ringopening with chiral selenolates, asymmetric selenoxide eliminations to afford chiral allenes or cyclohexenes, and the enantioselective formation of allylic alcohols by [2,3]sigmafropic rearrangement of allylic selenoxides or related species. 

From aziridines. N-Tosylaziridines, easily obtained by aziridination of the corresponding alkenes, can be opened by selenolate reagents and furnish, after straightforward N-alkylation, radical precursors suitable for the preparation of pyrrolidine derivatives [12]. The preparation of octahydro-lff-indole 24 is shown in Eq. (4). A competing process involving radical azi-doselenenylation of alkenes has also been developed and will be discussed later (Sect. 5.2). 

Scheme 28, by application of the known thiol ester-thiopyrone phototransformation to selenium-containing systems. 5/f-[l]Benzoselenino[2,3-6]-pyridine, 4.ff-selenolo[2,3-6][l]benzoselenine, and 9/f-seleno[3,2-6][l]benzo-selenine have similarly been obtained by the corresponding selenol ester-seleninone conversion. Phenyl areneselenosulphonates undergo facile photo-induced homolysis of the selenium-sulphur bond in the presence of alkenes, a free-radical chain reaction leads to the formation of -phenylselenosulphones. ... 

Another group of cinchona alkaloids lacks the 6 -mclhoxy group. Cinchonine (7) and its diastereomer cinchonidine (5) are commercially available and have been used as catalysts in the addition of zinc alkyls to aldehydes (Section D. 1.3.1.4.). Cinchonidine and dihydrocin-chonidine (6) were used to modify the surface of platinum catalysts used in the enantioselective reduction of z-oxo esters to a-hydroxy esters (see Section D.2.3.1. for such applications). Dihydrocinchonidine may conveniently be obtained by catalytic reduction of the double bond of cinchonidine, e.g., with nickel and hydrogen7. Cinchonidine also acts as a catalyst in the enantioselective formation of C-S and C-Se bonds by the addition of thiols and selenols to activated alkenes, such as 1-nitroalkenes (Sections D.5. and D.6.). Another application is the enantioselective protonation of kelenes (SectionD.2.I.). 

They also revealed that primary alkyl-, vinyl-, and aryl-substituted acy radicals generated by treating selenol esters with Bu3SnH can be utilized in intermole-cular alkene addition reactions (Eq. 48) [99,98b]. Acyl radicals exhibit nucleophilic character and react efficiently with alkenes bearing electron-withdrawing or radical-stabilizing groups. 

Addition Reactions. Treating terminal alkynes with benzene-selenol in the presence of Pd(OAc)2 and pyridine results in highly regioselective hydroselenation of the triple bond and provides the corresponding 2-phenylselenyl-substituted alkene as the exclusive product of reaction (eq 79). ... 

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