Selenolate examples

Like the three-coordinate aluminum and gallium alkoxides, the corresponding thiolates, selenolates, and tellurolates require very bulky substituents at the chalcogen atom to ensure a low metal coordination number. In addition, the steric requirements associated with the preservation of the low coordination number at the metal are increased by the longer M—S and M—Se bond lengths. The ten examples of low-coordinate,... 

Nickel-selenium coordination compounds have received attention in recent years, because a unique Ni-selenocysteine interaction was revealed in the active site of [FeNiSe]-hydrogenases.1083,1084 Of particular interest in this regard are mixed CO/selenolate complexes. Distorted square planar (393) was prepared from [CpNi(CO)]2, PhSeSePh, and [Fe(CO)3(SePh)3] and provides the first example of CO bound to a square planar Ni11 center in thiolate/selenolate environment.1085,1086 Upon addition of RSSR, species of the series [Ni(CO)(SR) (SePh)3 ] are formed. //(CO) ranges from 2,023 cm-1 to 2,043 cm-1 and is regarded as a spectroscopic reference for the CO binding site in [NiFeSe] hydrogenases. 

The aryl-substituted derivatives of [Bi(SeC6H2R3-2,4,6)3] (R = Me, Pr, Bu) are the only isolated examples of selenolate complexes. They have been characterized by mp, elemental analysis, NMR spectroscopy, and thermal gravimetric analysis, and the solid-state structure of the isopropyl-substituted derivative reveals a tricoordinate environment for bismuth [Bi-Se 2.630(8)-2.711(8) A Se-Bi-Se 92.3(2)-103.3(2)°], imposed by the steric bulk of the ligands (38). 

Unfortunately, the appeal of solid phase extractions on small scale fades as the scale increases due to the cost and inconvenience of using large amounts of fluorous silica gel. Here, modified techniques to reduce the tedium of repeated extractions are attractive. For example, Crich has recently introduced the minimally fluorous selenide C6Fi3CH2CH2C6H4SeH[171. This selenol is added in catalytic quantities to tin hydride reductions of reactive aryl and vinyl radicals. The high reducing capacity of the aryl selenide suppresses undesired reactions of product radicals without suppressing the reactions of the aryl and vinyl radicals themselves. After the reaction is complete, the selenol can be recovered by a modified continuous extraction procedure. 

Example 2, in addition to oxyselenide formation, carbo- and heteroseleno cycliza-tion, A -PSP can be used to generate selenides from alcohols and selenol esters from carboxylic acids, respectively, in the presence of a stoichiometric amount of n-Bu,P. ... 

An example of adding selenols to triple bonds in a Markovnikov fashion is the reaction of selenophenol and 1-hexyne (equation 310).562... 

Early syntheses of SeMet were tedious, non-stereospecific or limited to small-scale preparations. Klosterman and Painter (1947), for example, first reacted 5-((3-bromoethyI)- hydantoin with benzyl selenol to yield y-benzylselenoho-mocysteine. The latter was converted to the sodium salt of DL-selenohomo-cysteine with sodium in liquid ammonia, and reacted with methyl iodide to yield DL-SeMet. Plieninger (1950) obtained DL-SeMet by the reaction of sodium selenomethyl mercaptide with a-amino-y-butyrolactone in an inert solvent at 170°C. A synthesis of DL-SeMet from acrolein was also described (Zdansky, 1968). The first stereospecific synthesis of L-SeMet via esters of tosylated homoserine was reported by Pande et al. (1970). 

The rotation in toluene derivatives will be highly dependent on the size and electronic properties of the substituents. An interesting example is provided by 3,5-dichlorobenzyl alcohol, thiol and selenol (LXXI, X = O, S, and Se, respectively),... 

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. 

This naked selenolate is more reactive than the one complexed with borane 1 [1]. For example, in the presence of HMPA, 4 undergoes an SN2-type ester cleavage to produce the corresponding acids and alkyl phenyl selenides (Sect. 3.3) [6 a]. Uncomplexed selenolate 4 can also be prepared by the reduction of benzeneselenol (PhSeH) with sodium hydride (NaH) (Scheme 4b) [6aj. 

The analogy between phenyl thiolate (Ph-S ) and ferrocenyl thiolate (Fc-S ) anions leads to the expectation that many compounds containing ferrocenyl thiolate units as either substituents or ligands should exist. This is indeed the case and, although with fewer examples, is also true for ferrocenyl selenolate and tellurolate. Similarly, l,T-ferrocenylene dithiolate compounds (which correspond to o-phenylene dithio-lates) are known, together with the analogous l,r-ferrocenylene diselenolates and ditellurolates. 

Thiols or selenols react with alkoxy compounds, halides and acetates to give R M(ER )3 (E = S, Se). Several examples are shown in equations 94" , 95 ° , 96 °... 

It has been reported that some selenols are directly acylated by carboxylic acids to give selenol esters. " For example, heating of 4-hydroselenobutyric acid at about 160 C resulted in ring closure to give the corresponding y-lactone (equation 1). ... 

Areneselenyl magnesium bromide has been reported to react with /7-substituted benzoyl halides, giving aromatic selenol esters " as the first examples exhibiting liquid crystal properties (equation 2). ... 

As well as acylation of selenols, alkylation of selenocarboxylates with alkyl halides is one of the most straightforward methods for synthesis of selenol esters. However, known examples of selenol esters synthesized using this strategy have been limited to a few cases, due to the difficulty of the preparation of selenocarboxylates. 

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