Selenium complexes nucleophiles

The homologation of selenoesters 379 with diazomethane in the presence of Cu or Cul to give a-selenoketones is thought not to involve a carbenoid pathway and an Se-ylide intermediate but rather a tetrahedral species resulting from nucleophilic attack of CH2N2 at the carbonyl carbon atom. The role of the catalyst is seen in facilitating nucleophilic attack at C=0 by complexation at the selenium atom. 

NLO properties, 12, 128 organometallic complexes, 3, 318 as reaction materials, 3, 368 and selenium nucleophiles, 9, 471 surface chemistry on oxides, 12, 502 in triruthenium cyclopentadienyl clusters, 6, 799 trivalent halides, with metal carbonyl monoanions, 3, 347 Group 14 elements... 

Well before the wide use of organoselenium compounds in chemistry, it was discovered that electrophilic selenium compounds of the type RSeX add stereospecifically to alkenes.45 Since that time this reaction has been an important tool in the portfolio of organic chemists and has been used even for the construction of complex molecules. Comprehensive reviews on this chemistry have appeared46-49 and in recent times the synthesis of chiral selenium electrophiles and their application in asymmetric synthesis has emerged. As shown in Scheme 1, the addition reactions of selenium electrophiles to alkenes are stereospecific anti additions. They involve the initial formation of seleniranium ion intermediates 1 which are immediately opened in the presence of nucleophiles. External nucleophiles lead to the formation of addition products 2. The addition to unsymmetrically substituted alkenes follows the thermodynamically favored Markovnikov orientation. The seleniranium ion intermediates of alkenes with internal nucleophiles such as 3 will be attacked intramolecularly to yield cyclic products 4 and 5 via either an endo or an exo pathway. Depending on the reaction conditions, the formation of the seleniranium ions can be reversible. 

One of the main properties of selenides is their aptitude for attack by various nucleophilic reagents. Alkyllithiums, which are not reactive with sulfides, have a marked selenophilic character. They form an ate complex on the selenium atom, which immediately decomposes with cleavage of one C-Se bond leading to the more stable carbanionic species. 

By further evaluation of the redox system SeBr4/SeBr2/Se/Br in aprotic solvents (Section III,A), the first examples of bromoselenates-(II,IV) containing selenium in both oxidation states +2 and +4 were prepared. Three dinuclear, trinuclear, and tetranuclear types of anions are reported in this novel class of mixed-valence compounds Se2Brg, SeaBrio , and Se4Bri2 . They are shown in Fig. 31. Similar to the bromoselenates(II), they were obtained by nucleophilic addition reactions of bromide ions within the complex reaction mixtures of selenium bromides. Evidently, the formation of Bra" is important for the stabilization of Se(II) in addition to Se(IV), shifting the equilibrium... 

Synthesis of selenocyclopropanes has rarely been carried out by routes that result in formation of a bond between selenium and a cyclopropyl carbon atom. The few exceptions that exist involve polycyclic systems containing a cyclopropane moiety. When (l ,la 8,9b)8)-l-chloro-la-trimethylsilyl-1a,9b-dihydrocyclopropa[/lphenanthrene was stirred in a tetrahydrofuran solution of potassium /crr-butoxide and potassium benzeneselenolate a complex reaction mixture was obtained, from which la-phenylseleno-1a,9b-dihydrocyclopropa[/]phenanthrene (1) and (la,laa,9ba)-l-phenylseleno-1a,9b-dihydrocyclopropa[/]phenanthrene (2) were isolated in 10 and 15% yield, respectively, by preparative TLC. Both selenocyclopropanes arise from the same intermediate, la//-cyclopropa[/]phenanthrene, which means that the addition of benzeneselenolate to this alkene is nonregioselective. In contrast, the nucleophilic addition of meth-... 

Phenol and (thiophenol)FeCp complexes were prepared by substitution for Cl by OH- and SH- nucleophiles. Alcohols are initially converted to alkoxides using a base such as KOt-Bu or NaH and then give smooth substitution [63,64]. Both aryl and alkyl thiols react smoothly using K2CO3 as a base [65]. Substitution with selenium nucleophiles is also known [66]. Primary and secondary amines are sufficiently reactive without added base if used in excess, although a base (e.g., Et3N, pyridine or K2CO3) can be used to drive the reaction [67-69]. Mild reaction conditions allow arylation of amino acids with (PhF)FeCp (Eq. 17) [69]. The process is of limited value, however, since decomplexation of the arene led to decomposition. 

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