Catalytic system imine

Interestingly, Hong and co-workers evidenced that the selectivity of the coupling between aldehydes and amines can be modified by adding small amounts of an alcohol to the reaction medium (Scheme 29) [124]. Thus, with the same catalytic system, imine formation was favored in absence of alcohol, whereas amide was... 

Cyclopropyl ketones 32 and cyclopropyl imines 33 can also undergo [3+2] cycloaddition reactions with enones 34 in presence of NHC-Ni complexes to afford the corresponding cyclopentane compounds 35 (Scheme 5.9) [11]. The catalytic system is prepared in situ from the use of [Ni(COD),], SIPr HCl salt and KOBu, the reaction also required the use of Ti(O Bu) as an additive to improve yields and increase reactions rates. In most of the cases, th products 35 were obtained in good to excellent diastereoselectivities. 

The first examples utilising A-heterocyclic carbenes as ligands in the Buchwald-Hartwig amination involved the in situ formation of the catalyst from the corresponding imidazolium salt and a Pd(0) source. Nolan reported IPr-HCl/PdjCdbalj as a catalytic system for the amination of aryl chlorides in excellent yields, using different types of amines, anilines, and also imines or indoles [142,143] (Scheme 6.46). Hartwig showed later that in some cases the reactions could be performed at room temperature and without anhydrous conditions even for aryl chlorides [ 144]. This was later shown for the less challenging bromides and iodides [145,146]. 

The Cp2VCl2/R3SiCl/Zn catalytic system can be used for the reductive coupling of the imines 29 (Scheme 18) [55]. These components of the ternary catalyst are essential and, interestingly, meso-diastereoselectivity is observed in contrast to the coupling with cat. Cp2TiCl2/Sm system. The selectivity de-... 

The regioselective arylation occurs when the reaction of 2-arylpyridines with aryl halides is conducted with the aid of the ruthenium(ii)-phosphine complex as catalyst (Equation (66)).81 The ortho-position to the 2-pyridyl group is arylated predominantly. The aromatic imines are also arylated with the same catalytic system.82... 

This catalytic system can be used for the kinetic resolution of di-substituted 1-pyrrolines, for which high ee-values are achieved for both the amine and the recovered materials, especially when they are substituted in positions 2 and 5 (Table 6.6) [111]. Moreover, it should be noted that acyclic enamines are converted with high ee-values into their corresponding amines (89-98% ee Table 6.7), which is in sharp contrast to what is obtained for acyclic imines (vide supra) [112]. 

A trons-[RuCl2(diphosphine)(l,2-diamine)] complex with (R,R)-Et-DuPhos and (R,R)-l,2-diaminocyclohexane as the ligand combination has been found to be effective for the hydrogenation of imine 143, with up to 94% ee being obtained under the standard basic conditions employed for this catalytic system [198]. Unfortunately, the optimum combination of chiral diphosphine and diamine was found to be substrate-dependent, with only 40% ee being obtained for 2-methylquinoxaline 144 with Et-DuPhos. 

Table 34.2 Selected results for the enantioselective hydrogenation of N-aryl imines 1 and 2 (for structures, see Fig. 34.4) Catalytic system, reaction conditions, enantioselectivity, productivity and activity.

Treatment of this precatalyst with phenylsilane yields a very active catalytic system that can be used for the hydrosilylation of imines. This discovery was initiated by the observed hydrosilylation of imines when phenylsilane was added to Cp2TiF2. It was presumed that it might be possible to break the strong... 

Kobayashi and his team have utilized a catalytic system similar to that used in their development of a Zr-catalyzed Mannich reaction (Schemes 6.27—6.29) to develop a related cycloaddition process involving the same imine substrates as used previously (Scheme 6.35) [105]. As the representative examples in Scheme 6.35 demonstrate, good yields and enantioselectivities (up to 90% ee) are achieved. Both a less substituted version of the Danishefsky diene (—> 110) and those that bear an additional Me group (e. g.— 111) can be utilized. Also as before, these workers propose complex 89, bearing two binol units, to be the active catalytic species. 

Interestingly, fundamentally different stereoinduction mechanisms have been proposed for the activation of a number of related imine substrates, studies that resulted in the development of simple and highly effective new catalytic systems (27) for the addition of silyl ketene acetals to Al-Boc-protected aldimines (Mannich reaction) (Scheme 11.12c). ... 

The paramount significance of chiral amines in pharmaceutical and agrochemical substances drives the development of efficient catalytic asymmetric methods for their formation. In contrast to the high enantioselectivities observed in asymmetric reduction of both alkenes and ketones, only limited success has been achieved in the enantiose-lective hydrogenation of imines [118]. Currently, there are few efficient chiral catalytic systems available for the asymmetric hydrogenation of imines. 

On the basis of encouraging work in the development of L-proline-DMSO and L-proline-ionic liquid systems for practical asymmetric aldol reactions, an aldolase antibody 38C2 was evaluated in the ionic liquid [BMIM]PF6 as a reusable aldolase-ionic liquid catalytic system for the aldol synthesis of oc-chloro- 3-hydroxy compounds (288). The biocatalytic process was followed by chemical catalysis using Et3N in the ionic liquid [BMIM]TfO at room temperature, which transformed the oc-chloro-(3-hydroxy compounds to the optically active (70% ee) oc, (3-epoxy carbonyl compounds. The aldolase antibody 38C2-ionic liquid system was also shown to be reusable for Michael additions and the reaction of fluoromethylated imines. 

An even more active related catalytic system has recently been reported by Lee s group. This (salen)cobalt(III) catalyst containing the salen ligand depicted in Fig. 12 exhibits a highly unusual coordination mode for the normally tetradentate salen ligand [35]. That is, whereas the f-butyl-substituted phenolate analog displays conventional imine coordination, the salen ligand in Fig. 12 is proposed to be bound... 

To solve this problem, Pericas and co-workers have introduced a dual catalytic system consisting of a chiral amino alcohol 2, to control the enantioselectivity of the addition process, and a bulky silylating agent, to further activate the inline substrate (Scheme 1) [7]. When the 2/TIPSC1 system was used to promote the addition to imines derived from aromatic aldehydes, the addition reactions took place with good yield (63-75%) and high enantioselectivities (72-91%). Even in this case, a substoichiometric amount of chiral amino alcohol is required for a satisfactory result. 

The nucleophilic addition of organometallic reagents to imines provides an attractive route to amines [4]. Recendy, however, some completely different approaches to the synthesis of a-aryl amine were reported. Hayashi and Ishigeda-ni found a new catalytic system for the asymmetric addition of arylstannanes to imines derived from aromatic aldehydes (Scheme 11) [20]. 

The cyanation of imines, generally known as the Strecker reaction, has been one of the most aggressively studied transformations of asymmetric catalysis over the past several years. Very recent efforts in this area have resulted in the discovery of several highly efficient catalytic systems capable of providing a-ami-... 

The imination of sulfides with this catalytic system has been proved to proceed more readily than for the corresponding sulfoxides [169]. Thus, sulfilimines can be obtained in moderate to good yields, especially when acetylacetone or DMF is added (Table 3.14). 

This catalytic system proved to be highly efficient. Thus, a low catalytic amount of Fe(acac)3 (5-10 mol%) permits the successful imination of a broad variety of substituted sulfides and sulfoxides. The best results were obtained with nosyl sulfonamide as nitrogen source (Figure 3.11). Moreover, as previously observed with FeCl2, this catalyst also iminates sulfides more readily than sulfoxides. 

The C2-symmetric diphosphinite and CVsymmetric phosphinite-phosphate ligands, based on a carbohydrate scaffold, and iridium complexes give catalyst precursors that are active in the hydrogenation of imines. Cationic iridium complexes gave rise to catalytic systems that were more active than the neutral iridium complexes. Enantioselectivities up to 76% were obtained.347... 

Anisidine imines of aldehydes, p-McOCfJ I4-N=CI IR (R = Ar, alkyl) can be ally-lated with anti-selectivity, using triethylborane and a Pd(II)-phosphine catalytic system, avoiding metallic or metalloid allylating agents 54 The imines can be conveniently formed in situ. 

Along these lines and as a proof of principle, reversible imine formation was implemented in 1997 for the generation of enzyme (carbonic anhydrase) inhibitors from a dynamic covalent library [43] and reversible covalent selection approaches to catalytic systems were presented [44]. 

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