Telluronium ylides, reactions

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). 

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... 

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. 

The synthesis of aziridines from imines and sulfur ylides has been reviewed in previous editions of PHC and is a well-known reaction. A current study reveals that telluronium ylides add to a,p-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 137 and 138 in a ratio of 13 1 <05JA12222>. 

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. ... 

Preparation of Carbenoid Species. Copper-catalyzed enan-tioselective cyclopropanation through decomposion of diazo compounds using an optically active tetrakispyrazolylborate has been reported. Telluronium ylides are also prepared by reaction of diazo conqiounds with diorganyl tellurides in the presence of a catalytic amount of CuCl. Amide oxidation has been achieved using diazotization of 0-aminobenzamides with CuCl in a synthetic route toward the antibiotic anisomycin (eq 38). ... 

When compared with phosphonium and sulfonium ylides, the corresponding selenonium and telluronium ylides frequently exhibit better reactivity and nucle-ophilicity, while the use of selenides and tellurides in catalytic ylide reactions is far less reported. In 2001, Metzner and coworkers reported an asymmetric epoxi-dation reaction of aldehydes using a C2-symmetric selenide 42 resembling their sulfide catalyst (Scheme 20.32). High yields (65-97%) and enantioselecti vity (76-94% ee) were obtained with a range of aromatic aldehydes and dnnamalde-hydes. However, no diastereoselectivity (trans cis = 1 1) was observed in these reactions, while the corresponding sulfide gave around 80% diastereoselectivities, which could be rationalized as the formation of a less diastereoselective early transition state with the more reactive selenonium ylides. The reactions of more electron-deficient aryl aldehydes (p-Cl and p-CFs) were less enantioselective (76% ee and 83% ee, respectively) [58]. 

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]. 

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. ... 

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 . 

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. 

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