Chiral imidazoles

The enantioselective P-borylation of a,P-unsaturated esters with (Bpin) was studied in the presence of various [CuCl(NHC)] or [Cu(MeCN)(NHC)] (NHC = chiral imidazol-2-ylidene or imidazolidin-2-ylidene) complexes. The reaction proceeds by heterolytic cleavage of the B-B bond of the (Bpin), followed by formation of Cu-boryl complexes which insert across the C=C bond of the unsaturated ester. Best yields and ee were observed with complex 144, featuring a non-C2 symmetric NHC ligand (Scheme 2.31) [114]. 

Chiral imidazole-functionalized micelles. -(4) Comparison of rates and enantioselectivi-... 

Scheme 2.6 Preparation of chiral imidazoles under Mitsunobu conditions...

N-heterocyclic salt such as chiral imidazolium salt 2.87 is the precursor of N-heterocyclic carbene, chiral imidazol-2-ylidene ° 2.88. 

Enanliopure tetrahydrobenzo-l,4-diazepin-3-(3//)-one 42 was obtained by intramolecular amination of the aryl iodide 41 using BINAP as a ligand without racemization [36], The chiral imidazole 47 was prepared by successive Pd-catalyzed amination of the chiral amine 43 with 1,2-dibromobenzene to give 44, which was iminated with benzophenone imine (45). Then deprotection gave 46, and acid-catalyzed ring closure yielded 47 [37], The reactions have been applied to the synthesis of various chiral imidazolium salts as precursors of chiral carbene ligands [38]. Amination of aryl bromides proceeds very rapidly by temperature-controlled microwave heating [39]. 

Hoveyda and Snapper developed an excellent method for asymmetric silylation by employing a newly developed chiral imidazole catalyst, 45 (Scheme 22.10). By virtue of catalyst 45, silylative asymmetric desymmetrisation of meso-1,l-dioX 46 (Scheme 22.10A) and a-symmetric triol 48... 

Scheme 22.10 Asymmetric silylation promoted by Hoveyda-Snapper s chiral imidazole catalyst 45...

Apart from 37, other carboxamidate catalysts are also efficient intramolecular cyclopropanation catalysts. Rh2(4S-MPPIM)4 38, derived from chiral imidazole, is particularly effective in the cyclopropanation of diazoacetates, which contain rans-disubstituted or n-butyl allyl alkenes, with high enantioselectivities (Scheme 25) (117). In addition, this catalyst also proved to be efficient for the substrates of A -methyl substituted A-allyldiazoacetate, with high yields (88-93%) and high percentage of ee values (>92%) of the corresponding cyclopropanes (118). Catalyst Rh2(4S-MEOX)4, based on chiral oxazoline, is another efficient... 

A chloro complex 599 with an optically active NGN pincer-type ligand, having two chiral imidazole groups ((, )-Phebox), has been prepared via the reaction of the stannyl derivative of the ligand with Zeise s complex (Equation (131))V ... 

Several related compounds, in which either the isopropyl group of the oxazoline moiety has been modified or this latter fragment was replaced by chiral imidazole units, have been obtained following a similar procedure. These and related compounds were found to be the most active catalysts for the rearrangement of prochiral allylic imidates to allylic amides with high enantioselectivities. " ... 

Various chiral imidazoles and triazoles were therefore tested in the KR of 2-methylpiperidine however, Uttle or no enantioselectivity was observed. Fortunately, after screening various co-catalysts, chiral hydroxamic acids [134], in particular the as-(lR,2S)-aminoindanol-derived hydroxamic acid 145, were found to give excellent results at room temperature. Finally, further optimization of the catalytic resolution conditions, including the use of mesityl-substituted a-hydroxyenone 144 to deter N-heterocychc carbene (NHC)-catalyzed side reactions [135], led to the effective KR of various 2-substituted piperidines with selectivity factors ranging from s = 12 to 23 (Scheme 41.59). 

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