Ylides, telluronium

It is well known that aziridination with allylic ylides is difficult, due to the low reactivity of imines - relative to carbonyl compounds - towards ylide attack, although imines do react with highly reactive sulfur ylides such as Me2S+-CH2-. Dai and coworkers found aziridination with allylic ylides to be possible when the activated imines 22 were treated with allylic sulfonium salts 23 under phase-transfer conditions (Scheme 2.8) [15]. Although the stereoselectivities of the reaction were low, this was the first example of efficient preparation of vinylaziridines by an ylide route. Similar results were obtained with use of arsonium or telluronium salts [16]. The stereoselectivity of aziridination was improved by use of imines activated by a phosphinoyl group [17]. The same group also reported a catalytic sulfonium ylide-mediated aziridination to produce (2-phenylvinyl)aziridines, by treatment of arylsulfonylimines with cinnamyl bromide in the presence of solid K2C03 and catalytic dimethyl sulfide in MeCN [18]. Recently, the synthesis of 3-alkyl-2-vinyl-aziridines by extension of Dai s work was reported [19]. 

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. 

Dibutyltelluronium benzylide, generated by treatment of the telluronium salt with potassium f-butoxide, behaves similarly to the above-stabihzed ylides, undergoing Wittig-type olefmations with aromatic aldehydes. 

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). 

This was the first-described, non-stabilized ylide, obtained by treatment of the corresponding telluronium tetrafluoroborate with hthium 2,2,6,6-tetramethylpiperidide (LiTMP). Epoxides are obtained by reaction with both aldehydes and ketones. ... 

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. 

Dialkyl allyl telluronium bromides and dibutyl ethoxycarbonylmethyl (cyanomethyl, benzoylmethyl) telluronium bromide generated dialkyl alkylidene telluriums on treatment with potassium ferf-butoxide in tetrahydrofuran at low temperatures (s. p.837). The tellurium ylides were not isolated but were trapped with aldehydes and ketones4-6. 

Diisobutyl 2-Propen-l,l-ylideoe TeBurium4 0.87 g (2.4 mmol) of allyl diisobutyl telluronium bromide and 0.27 g (2.4 mmol) of potassium fert-butoxide are placed in a nitrogen-filled vessel. The vessel and 4 ml of dry tetrahydrofuran are cooled to — 78°. With stirring, the tetrahydrofuran is added dropwise to the mixture, stirring is continued for a few minutes, and then a solution of 0.216 g (2 mmol) of bcnzaldehydc in 2 ml of tetrahydrofuran is added dropwise to the ylide. The mixture is allowed to warm slowly to 20°. Work up and column chromatography on silica gel gives diisobutyl telluride and 2-phenyl-3-vinyIoxirane yield 82%. 

Dimethyl and methyl phenyl 4,4-dimethyl-2,6-dioxocyclohexylidene tellurium compounds transferred a methyl group to triethylamine, triphenylphosphane, tris[di-methylamino]phosphane, and triphenylarsane. The methyl onium cations were isolated as lelraphenylborates. The alkylidene organo telluronium intermediate was identified by 31P-NMR spectroscopy in the reaction of the ylide with tris[dimethylamino]phosphanc . 

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>. 

A few examples are known in which a telluronium prop-2-enide, e.g. formation of 20, pyridinium phenacylides, cyanotrimethylammonium ylides, or an iodonium ylide have been used as the nucleophilic species in the MIRC reaction. 

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. 

---