Telluronium ylide

Optically active telluronium ylides were not obtained for a long time. Optically active diastereomeric telluronium ylides 7 were obtained for the first time in 1995 by fractional recrystallization of the diastereomeric mixture.19 The absolute configurations of the chiral telluronium ylides were determined by comparing their specific rotations and circular dichroism spectra with those of the corresponding selenonium ylide with known absolute configuration. The telluronium ylides were found to be much more stable toward racemization than the sulfonium and selenonium ylides (Scheme 4). 

From telluronium ylides 4.7.3.1 Stabilized telluronium ylides... 

Stabilized telluronium ylides such as dibutyltelluronium carbethoxy, phenacyl/ cyano- and carbamoylmethylide (easily prepared by the reaction of dibutyl teUurides with the appropriate substituted methyl hahdes, followed by treatment with a base), undergo Wittig-type olefmation reactions with a variety of carbonyl compounds, giving the expected olefins in satisfactory yields (method A). This behaviour is in sharp contrast to that of stabilized sulphonium yhdes, which are inert towards carbonyl compounds. 

Reaction of stabilized telluronium ylide with aldehydes. Method A - using telluronium ylide (typical procedure).A solution of carbethoxymethyldibutyltelluronium bromide (1.23 g, 3 mmol) in dry THF (3 mL) is added dropwise to a solution of KOf-Bu (0.337 g, 3 mmol) in THF (4 mL) at -20°C. After a few minutes, fran -cinnamaldehyde (0.264 g, 2 mmol) is added over 1 min. The reaction mixture is then stirred for 1 h at -20°C. After the usual work-up, the product is purified by Si02 column chromatography, to give ethyl 5-phenylpenta-(2 , 4 )-dienoate (0.291 g (72%)). GC and NMR of the product reveals a purity of 98%. 

In sharp contrast to the olefmation reaction employing stabilized telluronium ylides, semi-and non-stabilized ylides react with carbonyl compounds to give epoxides (like non-stabilized sulphonium and selenonium ylides). 

Thus, allyl telluronium ylides generated in situ from the corresponding telluronium salts in the presence of Li salts react with a,)3-unsaturated esters and amides to afford trans-2-vinyl-/rany-3-substituted cyclopropyl compounds in high yieds. 

Propargylic silylated telluronium ylides react analogously. 

The semi-stabilized telluronium ylides, generated in situ from the corresponding telluronium salts (156 R1 = CH=CHSiMe3, CH=CH2, CH=CHMe, CH=CHPh, Ph), have been reported to react with a,/(-unsaturated carbonyl compounds (157 R2 = Ph, OR, NR)) to afford 2-vmylcyclopropyl derivatives (158) with high... 

Vinyl aziridines have been prepared trans- and diastereo-selectively by reaction of N -/ - h u tv I sul 1 i n v limin e s with telluronium ylides.75... 

An account on telluronium and sulfonium ylides has briefly described the development of ylide olefination, cyclopropanation, epoxidation, and aziridination.56 Optically active m-2-substituted vinylaziridines (22) have been synthesized by the reaction of jV-r-butylsulfinylimines with telluronium ylides with excellent diastereoselectivity (g) (de > 98%) in good to excellent yields (56-98%) (Scheme ll).57... 

Telluronium ylides,118 derived from triorganotelluronium salts 63, react with aldehydes, ketones, and a,/3-unsaturated esters to give different products depending on the nature of the substituent at the a-carbon of the telluronium salt 63. In Scheme 31,121-123 examples of such transformations are given. 

Triorganyltelluronium salts, RsTeX, have been known for over a century. In recent years, they have attracted considerable interest because of their role in organic synthetic chemistry For example, it has recently been reported that the telluronium salts are precursors for telluronium ylides that react with the carbonyl compounds, forming a variety of products like oxiranes and secondary alcohols. ... 

The related telluronium ylides also add to a,/3-unsaturated imines through a Michael addition-elimination to the olefin followed by a second equivalent of telluronium ylide addition to the imine, which subsequently eliminates to form aziridines 654 and 655 in a ratio of 13 1 (Scheme 159) <2005JA12222>. 

Catalytic one-pot procedure. Since in the described telluronium ylide olefmation tellurox-ide is formed as a by-product, and the tellnroxide is susceptible to reduction by triphenyl phosphite, a catalytic procedure can be employed, providing a practical one-pot synthesis of a,)3-nnsatnrated esters and ketones (method E).i By this procedure, a catalytic amount of n-dibntyl tellnride reacts with the a-bromoester or a-bromoketone, and the formed tellnroninm salt is converted in situ under phase transfer conditions (solid KjCOj/trace H2O) into the ylide, which reacts in turn with the aldehyde, giving the olefin. Since the reaction is performed in the presence of triphenyl phosphite, the formed dibutyl teUuroxide is reduced back to the dibutyl telluride, which is then recycled. 

We add an investigation of Chinese chemists to this section, although it is not related to carbenoid reagents that we have discussed above. Zhou et al. (1993) studied reactions of dimethyl diazomalonate and ethyl diazoacetate with carbonyl compounds mediated by diorganyl tellurides and catalytic amounts of cuprous iodide (8-69). Dibutyl telluride (8.155) yields the dimethyl l-arylethene-2,2-dicarboxylate 8.157 with 4-chlorobenzaldehyde in 95<7b yield at 100 °C. It is assumed that the reaction passes the telluronium ylide 8.156 as intermediate. If so, the process is clearly different from the carbenoid transformations discussed in this section. The originally diazo-substituted C-atom has nucleophilic character in 8.156 and is not electrophilic, as in 8.104. 

Di-, tri-, and tetra-substituted 2-azadienes (64) carrying electron-withdrawing groups have been synthesised by the aza-Wittig reaction. A one-pot synthesis of olefins from diorganyltelluride, diazo compounds, and carbonyl compounds in the presence of a Cu(I) catalyst is suggested to involve a Wittig-type reaction of an intermediate telluronium ylide. ... 

Related cyclopropanations have also been reported using sulfonium and telluronium ylides as intermediates. In particular, the cyclopropanation of enones has been carried out employing an allyl bromide as the cyclopropa-nating reagent and sulfonium and telluronium salts 134 and 135 as pre-catalysts (Scheme 7.84). These species, in the presence of a base, generated the corresponding ylide which underwent the cascade Michael/intramolecular nucleophilic substitution and it is in this second step that the real catalytically active species is released, able to interact with another molecule of the allyl bromide and thus regenerating the sulfonium or telluronium salts pre-catalysts, which can afterwards continue in the catalytic cycle. The substitution at the... 

Scheme 7.84 Enantioselective cyclopropanation of chalcones via sulfonium and telluronium ylides.

In parallel with this is the ease with which the ylides are proton ated to form heteronium salts. Thus again taking the ylides (XII) as examples, the sulphonium and selenonium ylides do not form salts readily, the phosphonium, arsonium and stibonium ylides are readily protonated with increasing basicity Ptelluronium ylides are destroyed by mineral acids. [149,156]. Protonation may take place at different sites in the molecule depending partly on.the substitution pattern, and the site appears to depend on a balance of electronic and steric factors. [126]. Thus dimethylsulphonium cyclopentadienylide in trifluoroacetic acid is protonated at the 1-, 2- and 3-positions in the ratios of 11 56 33 [157]. 

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