Catalysts amidine-based

In the same eontext, Birman s group has recently reported the organoeata-lysed reactions of azlactones with di(l-naphthyl)methanol as the nueleophile. A series of chiral amidine-based catalysts were investigated for these reactions, resulting in the selection of the chiral benzotetramizole depicted in Scheme 2.98 as the most efficient one, which provided the corresponding di(l-naphthyl)-methyl esters of a-amino acids in excellent yields and enantioselectivities of up to... 

Figure 41.3 Structure of amidine-based catalysts developed by Birman et al.

Subsequently, Birman et al. tested various amidine-based catalysts, including commercially available tetramisole (45) and its benzanneUated analog benzote-tramisole (BTM) (23) in the KR of secondary aryl aUcyl alcohols and propargyUc alcohols (Eigure 41.3). Both catalysts displayed far superior enantioselectivity although BTM proved to be more reactive [27c, 50, 51[. 

Propargylic alcohols could also be resolved using amidine-based catalysts such as CF3-PIP (20), Cl-PIQ (22), and BTM (23) [50]. However, the latter proved to be much more efficient, affording selectivity factors ranging from s = 6.8 to 32... 

The reaction proceeds via the formation of a first chiral zwitterionic intermediate from pivaUc anhydride and the amidine-based catalyst A mixed anhydride is then generated in the presence of the racemic carboxylic acid and activated by the chiral acyl-transfer catalyst to form the second zwitterionic intermediate. The latter species selectively reacts with a nucleophilic alcohol to afford the desired enantioenriched carboxyUc ester (Scheme 41.10). 

Weinreb 172 amidine-based catalysts 1230 amidinium ion 269 a-amido sulfones 853, 892 aminals 753, 1039 aminations 892... 

The most widely applied catalyst systems in organocatalytic ROP include N-heterocyclic carbenes [62, 64, 67], guanidine and amidine bases [51,65] and phosphorane-based compounds [68,69]. Strong organic acids are excluded from this section as their mechanism follows the one described for cationic ring-opening polymerisations [70, 71]. 

Epoxy/carboxy/acid anhydride systems are also important in OEM clear coats and powder coatings. Catalysts such as tertiary amines, amidines and imidazoles, 1,4-diazabicyclooctane and some alkaline metal-based catalysts such as zinc-based catalysts are commonly used. 

Structurally related complexes are also active initiators for the living polymerization of carbo-diimides (which are isoelectronic to isocyanates).1003 The proposed intermediate for this polymerization process is a metal amidinate (Scheme 29), and the model complex (349) has been reported to be a highly efficient catalyst, polymerizing 500 equivalents of di- -hexylcarbodiimide in less than 10 s. A more hydrolytically robust series of initiators has also been developed, based upon copper(I) and copper(II) amidinates.1004... 

Two technical applications of C = N-X substrates have been reported. Noyori s Ru-PP-NN catalyst system was successfully applied in a feasibility study by Dow Chirotech for the hydrogenation of a sulfonyl amidine [77], while Avecia showed the commercial viability of its CATHy catalyst based on a pentamethyl cyclopentadienyl Rh complex for the reduction of phosphinyl imines [78] (Fig. 34.11). 

Heteropolyacids Hi4[NaPsW29MoOno] and H3PMO12O4 have been shown to be efficient catalysts for consecutive condensation of aldehydes with 5-aminopyrazole -carboxamide and cyclization into pyrazolo[3,4-t4pyrimidines <2007MI1467>. The reaction of ethyl 5-acylaminopyrazoles with hexachloroethane and triphenylphosphine in the presence of a base has been recently reported to afford an imidoyl chloride that reacted in situ with ethylamine yielding an amidine in 71% yield that cyclized readily in DMF in the presence of potassium carbonate to yield pyrazolopyrimidines in 65% yield <2007TL3983>. 

Where R4 is a hydrogen or carbon atom, 10.15 is simply an amidine. However, urea 10.16, thiourea 10.17, or guanidine 10.18 and their derivatives may be used. These nucleophiles may be condensed with ester and nitrile functionalities as well as with aldehydes and ketones. Such condensations to afford pyridimidine derivatives are usually facilitated by acid or base catalysis, although certain combinations of reactive electrophilic and nucleophilic compounds require no catalyst at all. Some examples are shown below. 

The titanium complexes are sensitive to high temperatures and the presence of oxygen or water. A more robust catalyst system is based on Cu(I) and Cu(II) amidinate complexes. Also, dinuclear copper amidinate complexes are used in the polymerization of carbodiimides. 

Cyclic amidines, among others DBU, and Group V Lewis base-epoxide mixtures, have been applied as transesterification catalysts (84EUP110629). 

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