Phenylselenyl bromide

Alkylthiocyanates and alkylselenocyanates are obtained by treatment of trialkylboranes with potassium thiocycanate (260) and sodium selenoisocyanate (261), in the presence of iron(III) compounds, respectively. Unsymmetrical trialkylboranes react preferentially at the more highly branched alkyl group. Alkenylphenyl selenides are obtained by the reaction of alkenylboronic acids with phenylselenyl bromide (262). 

This strategy is applied to a general method for the preparation of 2,2-disubstituted 1-ni-troalkenes. Conjugate 1,4-addition of complex zinc cuprates to l-nitroalkenes, followed by trapping with phenylselenyl bromide and subsequent oxidative elimination, affords the corresponding 2,2-disubstituted l-nitroalkenes in good yields (Eq. 5.82).132... 

Interestingly, bis(methylthio)-l-nitroethylene (380) reacts with dimetallic zinc-copper species leading to the corresponding exo-methylene cycloalkenes, such as 381 (Scheme 100) . / -Disubstitutcd nitroolefins are especially difficult to prepare by nitroaldol condensation. The addition of zinc-copper reagents to nitroolefins followed by a reaction with phenylselenyl bromide produces, after IFO, oxidation, EtZ mixtures of -disubstituted nitroalkenes, such as 382 (Scheme 100) . 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

The formation of bicyclic nitrones of the 2-azetidinone A-oxide type, 32 and (33), has been achieved in a two-step route, through condensation of the corresponding 2-azetidinone tethered-alkenyl(alkynyl)aldehyde with hydroxylamine followed by phenylselenyl bromide treatment <02JOC7004>. 

Phenylselenyl chloride, C HjSeCI, and phenylselenyl bromide, C Hs eBr, in connection with oxidizing agents such as hydrogen peroxide or sodium periodate, are used for the conversion of aldehydes, ketones, and esters into their a,p-unsaturated analogues. The key intermediate is alkyl phenyl selenoxide, which decomposes via a five-membered transition state [167] (equation 27). 

From enolates via selenoxidesA Lithium enolates derived from ketones, lactones, and esters react with PhSe-SePh or with phenylselenyl bromide or chloride (PhSeX) to form a-(phenylseleno)carbonyl compounds. These can be oxidized to the corresponding selenoxides with subsequent 5yn-elimination of benzeneselenic acid to form enones. 

The transylidation reaction of phenylselenyl bromide with two moles of alkylidenetriphenylphos-phorane yields 1-phenylselenoalkylidenetriphenylphosphoranes (equation 28). ... 

Alkylidenephosphoranes react with phenylselenyl bromide to yield selenophos-phoranes (59), which react normally with aldehydes. However, with ketones, the corresponding 2-phenylseleno-derivatives, e.g. (60), are formed. ... 

Dihydrooxazoles are also accessible by phenylselenyl bromide <9lJOC3425> or phenylselenyl sulfate <93J0C1349> induced cyclization of Y-allylamides. Oxidation of the 5-phenylselenylraethyl... 

The treatment of a,p-unsaturated ketones with organocopper reagents provides another method to access specific enolates of unsymmetrical ketones. Lithium dialkylcuprates (see Section 1.2.1) are used most commonly and the resulting enolate species can be trapped with different electrophiles to give a,p-dialkylated ketones (1.27). Some problems with this approach include the potential for the intermediate enolate to isomerize and the formation of mixtures of stereoisomers of the dialkylated product. The intermediate enolate can be trapped as the silyl enol ether and then regenerated under conditions suitable for the subsequent alkylation. Reaction of the enolate with phenylselenyl bromide gives the a-phenylseleno-ketone 12, from which the p-alkyl-a,p-unsaturated ketone can be obtained by oxidation and selenoxide elimination (1.28). 

The sequence provides a method for converting a, -unsaturated ketones into (3-alkyl derivatives by alkylation with an organocuprate (see Section 1.2.1) and reaction of the intermediate copper enolate with phenylselenyl bromide, followed by oxidative elimination (6.52). 

A new convenient method of synthesis of a-phenylseleno-aldehydes and -ketones involves the reaction of phenylselenyl bromide and enol silyl ethers. ... 

Phenylselenyl bromide is an alternative reagent for allylic oxidation of alkenes. Addition takes place, and reaction of the adduct with acetic acid, followed by oxidation, generates the allylic acetate. 

Selenides eliminate readily without a base. They are generally prepared from enolate anions by reaction with diphenyldiselenide or phenylselenyl bromide to give phenylselenides. The phenylselenides are oxidized with sodium periodate, hydrogen peroxide, or peracids to the selenoxides, which eliminate even at room temperature to afford the a,p-unsamrated ketones and esters [107]. 

Biochemical.—In the course of wider studies connected with the biosynthesis of 3-lactam antibiotics, the oxidation of a number of cysteinylpep-tides has been found to yield products incorporating the isothiazolidinone residue (71). Cysteinyldehydrovaline (72 R = H), for example, reacts with phenylselenyl bromide, yielding the thioselenide (72 R = SePh), which is cyclized oxidatively to the corresponding isothiazolidinone (73 R = H,... 

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