Cyclization biphasic

Rhodium catalysis in an aqueous-organic biphasic system was highly effective for intramolecular [2+2+2] cyclotrimerization. It has been shown that the use of a biphasic system could control the concentration of an organic hydrophobic substrate in the aqueous phase, thus increasing the reaction selectivity. The intramolecular cyclization for... 

DipolarCycloaddition Reactions. The 1,3-dipolar cyclization of nitrile oxide with dipolarophiles generates structurally important heterocycles. As shown by Lee,139 the reaction can be carried out in an aqueous-organic biphasic system in which the nitrile oxide substrates can be generated from oximes or hydrazones in situ. The method provides a convenient one-pot procedure for generating a variety of heterocyclic products. 

The [2+2+2] cyclotrimerization of triynes has been reported using rhodium catalysts <2003JA12143> under biphasic conditions. Thus, the cyclization of triyne 174 proceeds rapidly in a biphasic system to produce the tricyclic compound in good yield using an in t/r -generated water-soluble rhodium catalyst (Equation 108) <2003JA7784>. 

Arylbenzimidazolinones (46) have been prepared by cyclization of car-banilides, using a biphasic medium.88... 

Gurtler and Jautelat of Bayer AG have protected methods that use chloroalu-minate(iii) ionic liquids as solvents for both cyclization and polymerization reactions of acyclic dienes [50]. They employed the neutral ionic liquid [EMIMjCl-TUQs (X(A1Q3) = 0.5) to immobilize a mthenium carbene complex for biphasic ADMET polymerization of an acyclic diene ester. Fig. 7-2. The reaction is an equilibrium process, therefore, removal of ethylene drives the equilibrium to products. The reaction proceeds readily at ambient temperatures producing mostly polymeric materials but also 10% dimeric material. 

Usually, the reaction is low yielding depending on the substituents either on the aromatic ring or on the unsaturated aldehyde reaction partner. The low yields are also correlated with the general difficulty to work up and isolate the desired products from the reaction mixture. An improved method was reported lately from Matsugi, et al. using a biphasic system as reaction media for the Doebner-von Miller reaction. The method proved to be advantageous in terms of yield and easy work-up process. Other modifications concern the use of different Lewis acids as promoters for the later cyclization process (InCL, lanthanides, etc.). ... 

Cyclization of pseudoionone to P-ionone is an important reaction used in the synthesis of vitamin A. Conventionally, pseudoionone is slowly dosed to a stirred tank reactor containing a biphasic mixture of concentrated sulfuric acid and an organic solvent to control the temperature of the highly exothermic reaction [88]. The reaction takes place in the acid phase, where by-products are formed very quickly. The by-product formation is observed to increase with increasing temperature. The product yield obtained in conventional semibatch reactors is in the range of 70%. 

More recently, alkaloid derivatives have also found application in other transformations such as electrocyclization reactions. For instance. Smith and co-workers have shown that treatment of benzaldimine 101 with 10 mol% of cin-chonidinium chloride 102 in a biphasic system of aqueous potassium carbonate and toluene gives cyclized product 103 in 99% yield and 97% ee. The sense of stereoinduction within this 6n electrocyclization (as represented by 104) was tentatively rationalized using a modification of the... 

Alternatively, cyclic PSTY was also synthesized through a two-step reaction process reported by Ishizu et al. [53]. First, the living dianionic PSTY was capped with an excess of 1,4-dibromobutane to give an a, -dibromo PSTY, and then was reacted with tetrametylenediamine as illustrated in Scheme 7. It is notable that the reaction between bromide PSTY with diamine was carried out in a water/toluene biphase system. The lower concentration of reactants at the interface favored the cyclization reaction, giving a yield of more than 90%. 

More conventional liquid-liquid biphasic conditions have been also conveniently employed to increase the efficiency of the photo-Fries reaction. For instance, p-methoxyphenyl esters of a, 5-unsaturated carboxyhc acids (30) undergo photochemical rearrangement, followed by basic cyclization to give... 

Another group of polycondensations which allows for comparison of one-phasic and biphasic systems are polycondensation of bisphenols or bisthiophenols with a,(o-dibromoalkanes or bis(chloromethyl)aromatics. However, syntheses of polyamides from primary diamines are not suited for such studies, because association and precipitation of polyamide chains (via H-bonds) interferes with chain growth and cyclization. 

---