Mercury triflate

In the total synthesis of (-l-)-citreofuran, the transannular cyclization of a 5-oxoalkene was brought about by acid catalysis (Equation 9) <2003JOC1521>. ra-Toluenesulfonic acid turned out to be optimal for this particular substrate. Mercury triflate is also an effective catalyst for the cyclization of 5-oxoalkynes. Through a protodemercura-tion, 2-methyl-5-substituted- and 2,4,5-trisubstituted furans can be prepared in high yields <2004OL3679>. 

Mercury triflate was an effective catalyst for the transformation of l-alkyne-5-ones to 2-methyl-5-substituted furans. The reaction involves a protodemercuration of a vinylmercury intermediate generated in situ. When other substituents are present at the a-position of the carbonyl group, corresponding 2-methyl-4,5-disubstituted furans were provided. A plausible reaction path was provided <04OL367. ... 

Samarium(ll) triflate, a halogen-free samarium(ll), can also be prepared by disproprotionation of samarium(iii) triflate and samarium(O) in DMF in the presence of a catalytic amount of mercury. 

Paulsen and co-workers [156] have also prepared the blood group B (type 2) tetrasaccharide (324) from the lactosamine derivative (318). They attached the 2 -0-l-fucosyl residue using the fucosyl bromide (322) in the presence of mercury(II) bromide and molecular sieves and obtained (319) in 80% yield. Removal of the benzoyl group and glycosidation with 2,3,4,6-tetra-O-benzyl-oc-gaIactopyranosyl bromide in the presence of mercury(II) bromide and molecular sieves at 20 °C or with silver triflate — silver carbonate at —25 °C gave the 3 -0-a-D-galactosyl derivative (323) in 80% yield. In this paper Paulsen discusses in detail the conditions (reactivity of alcohol,... 

With the bromide (412) and the glucoside (409) in the presence of silver triflate in benzene at 20 °C both the a-linked (28 %) and the P-linked (53 %) disaccharides were obtained. In toluene at —10 °C more of the a- (64%) and less of the p- (15%) linked disaccharides were formed. The major product was the P-linked disaccharide (32%) using mercury(II) cyanide and mercury(II) bromide as catalysts. 

Thio- and selenoacetals and esters are excellent substrates for mild Friedel-Crafts reactions, because of the affinity of sulfur and selenium for copper (Sch. 23). Anisole was readily acylated with methylselenoesters 94 at room temperature with activation by CuOTf to affordpnra-substituted (> 95 %) derivatives 95 [50,51]. Mercury(II) and copper(II) salts, which were effective for the activation of selenyl esters for reaction with alcohols, amines, and water, were not effective for the Friedel-Crafts reaction. Aromatic heterocycles 96 could be acylated in high yields, and the alkylation product 100 was obtained from dibutylthioacetal 99 and anisole. Vedejs has utilized this methodology in the cyclization of 101 to afford 102 in 77 % yield [52]. This intramolecular variant did not require the use of the more reactive bis copper triflate-benzene complex. 

Dondoni and coworkers [63] have shown that homologation of a-hydroxycarbaldehydes can be achieved with high antiselectivity by addition of 2-(trimethylsilyl)thiazole (42) (Scheme 13.25). For instance, D-glyceraldehyde acetonide (R)-24 reacts with 42 giving 43 in 96% yields with the anti vs. syn diastereoselectivity better than 95 5. Release of the aldehyde requires protection of the alcohol as a benzyl ether, methylation of the thiazole generates intermediate 43 Me that is not isolated but reduced in situ with NaBH4 to give thiazoline 43 H. Mercury(II)-catalyzed hydrolysis liberate the semiprotected D-erythrose derivative d-45 in 62% overall yield [64]. Methylation of the thiazole moiety can also be achieved with methyl triflate instead of Mel, and copper(II)chloride can be used instead of mercury(II)chloride [65]. 

Experimental details 11 A diethyl ether solution of the triene 115 (10.8 him) and copper(I) triflate (0.94 him) was irradiated with a medium-pressure mercury lamp... 

Periana et al. have reported a mercury system that catalyzes the partial oxidation of methane to methanol.81 Hg(II) is typically considered to be a soft electrophile and is known to initiate electrophilic substitution of protons from aromatic substrates. The catalytic reaction employs mercuric triflate in sulfuric acid, and a key step in the catalytic cycle is Hg(II)-mediated methane C—H activation. For methane C—H activation by Hg(II), an oxidative addition reaction pathway via the formation of Hg(IV) is unlikely. Thus, an electrophilic substitution pathway has been proposed, although differentiation between proton transfer to an uncoordinated anion versus intramolecular proton transfer to a coordinated anion (i.e., o-bond metathesis) has not been established. Hg(II)-based methane C H activation was confirmed by the observation of H/D exchange between CH4 and D2S04 (Equation 11.9). 

Intramolecnlar alkenylation at a furan a- or P-position by an alkyne occurs, with the formation of bicycUc derivatives, when promoted by mercury(II) acetate (or Hg(OAc)(OTf), generated in situ from mercuric acetate and scandinm triflate). In the case of closure onto a p-position, a spirocyclic intermediate from preferred attack at the a-position, may be involved, as shown. 

These observations subsequently led to studies on Koenigs-Knorr-type glycosylations employing promotion by mercury salts and silver triflate. The studies were based on product composition rather than on kinetics.69... 

---