Barium-catalysed reactions

Another highly efficient DYKAT was reported in 2009 by Shibasaki s group based on a barium-catalysed aldol/retro-aldol reaction.Thus, a-alkylidene-j9-hydroxy esters were obtained by reaction of aryl, heteroaryl, alkenyl and alkyl aldehydes with )S,y-unsaturated esters in both excellent enantio- and... 

Barium sulphide has the usual dangerous reactions of sulphides (detonations with potassium chlorate and nitrate, combustion with phosphorus pentoxide). It catalyses the explosive combustion of dichlorine oxide. 

Examples are the formation of diacetone alcohol from acetone [reaction type (A)] catalysed by barium or strontium hydroxide at 20—30°C [368] or by anion exchange resin at 12.5—37.5°C [387], condensation of benzaldehyde with acetophenone [type (C)] catalysed by anion exchangers at 25—-45°C [370] and condensation of furfural with nitromethane [type (D)] over the same type of catalyst [384]. The vapour phase self-condensation of acetaldehyde over sodium carbonate or acetate at 50°C [388], however, was found to be first order with respect to the reactant. 

In addition to the reactions described above which relate to the internal combustion engine emissions questions, the catalysed low temperature oxidative coupling of methane, the water gas shift reaction and many other catalytic reactions are also promoted by ceria [10-12]. A study of dkali and alkaline earth metal doped ceria catalysts has shown that barium or calcium doped ceria were the most active catalyst for the oxidative coupling of CH4 [ 13]. Zhang and Baems explained the observed dependence of selectivity on the Ca content in terms of oxygen-ion conductivity... 

Cuprous oxide accelerates the substitution reaction, and, for instance, chlorobenzene under these conditions affords a 92% yield of phenol in 1 h at 316 °C. Copper and barium chlorides catalyse also the steam hydrolysis of chlorobenzene over silica gel . 

A review in Russian on the synthesis of carbohydrates from formaldehyde has appeared. An investigation of the formose reaction by g.c.-n.m.r. has shown that intermediate glycoaldehyde, glyceraldehyde, and dihydroxyacetone are present as mixtures of monomers, e.g. hydroxy carbonyl compounds, epoxides and hydrates, and dimers such as half and full acetals. Further study of the barium chloride-catalysed formose reaction at pH 12 has shown that the main product forming 33% of the total sugars is the branched pentulose (1). The sugar yield reached a constant value at 70% completion of the reaction, i.e., within... 

The Cannizarro reaction under heterogenous conditions catalysed by barium hydroxide is considerably accelerated (Scheme 28) by sonication. The yields are 100% after 10 min, whereas no reaction is observed during this period with the use of ultrasound/ ... 

Due to the electronegativity of the carboxylic function, it is possible to isomerise aldonolactones at C-2 in the presence of a base. The reaction, which has been known for more than a century, can be carried out in water at 100-140°C with calcium or barium hydroxide or with pyridine as the base [8]. With hydroxide it is possible to isomerise the aldonate while the weaker base pyridine can isomerise the lactone form. The reaction is slow to reach equiUbrium and then typically gives an epimeric ratio of 3 1 in favour of the 2,3-t/ireo-epimer [8]. After separation of the two epimers by crystallisation, the starting material can be recycled. However, due to the harsh conditions, the isolated yields of the two epimers are often moderate to low. As a result, the base-catalysed C-2 isomerisation is most useM in cases where the starting aldonolactone is very cheap and the C-2 epimer very expensive, as is the case for the isomerisation of galactonolactone... 

Further work from Vollhardt s group on cobalt-catalysed cycloadditions of olefins and acetylenes has led to a synthesis of cyclohexadiene derivatives (188 n = 0—2) by addition of trimethyl silylacetylenes the reactions can also be carried out in an intramolecular sense, leading to tricyclic compounds. The novel tricyclic compound (189) has been obtained from naphthoquinone by a [2 + 2]addition of l,l-bis(silylmethyl)ethylene followed by treatment with a Lewis acid. A key step in the first total synthesis of a decipiene diterpene is the formation of tricyclic ketone (190) by an intramolecular aldol condensation, which required the use of methanolic barium oxide. ... 

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