Amine Catalyzed Mechanism

Although the elementary steps of the MBH reaction have been described in the earliest publications [la], the exact reaction mechanism, in particular that controlling the asymmetric induction, has been debated frequently, and remains as the core of the mechanistic discussion. The commonly accepted mechanism for the MBH reaction was first proposed by Hoffmann [3b] and refined from kinetic studies by Hill and Isaacs [6] in the late 1980s and others [7j. Their proposed 

McQuade pathways) are competing mechanisms, and depending on the amount of protic species and the reaction progress (early or late stage) either of the two mechanisms is favored. 

Miller and coworkers have performed kinetic studies on a pyridylalanine-peptide catalyzed enantioselective coupling of allenoates 16 and N-acyl imines 17 to investigate the mechanism of the aza-MBH reaction [20]. In the catalytic cycle of a typical MBH/aza-MBH reaction, the proton transfer step or C-C bond formation is often considered as the rate-determining step. However, through mechanistic 

Catalyst Addition to Allenoate (k ) Possible Rate-Determining Step, OBn OBn 

In addition, the Aggarwal proposed model shed light on the asymmetric catalysis of the MBH reaction. It suggested that all four diastereomers of the 

To investigate the MBH mechanism, Coelho and Eberlin et al. have also used electrospray ionization mass spectrometry [ESI-MS/MS)] to characterize key MBH reaction intermediates. Using ESI-MS-(/MS), they also shed light on the co-catalytic role of ionic liquids in MBH reactions. Stimulated by the 

---

V,/V dipheny1ethy1enediamine. The cure mechanism probably involves an amine-catalyzed decomposition of the sulfonyl chloride group or a path of radical anions. The cross-link probably involves the HVA-2. Calcium hydroxide or other SO2 absorbers must be included for development of good mechanical properties. 

Scheme 4.17 Proposed mechanisms for amine catalyzation of the NCO/OH reaction.

Melt reaction mechanisms of tertiary aliphatic amine catalyzed phenolic-epoxy reactions were proposed to begin with a trialkylamine abstracting a phenolic hydroxyl proton to form an ion pair (Fig. 7.36). The ion pair was suggested to complex with an epoxy ring, which then dissociated to form a /1-hydroxycther and a regenerated trialkylamine.87... 

Figure 7.36 Proposed mechanism for tertiary amine-catalyzed phenol-epoxy reaction.

The mechanism of the polymerization of NCA with tertiary amine is still controversial. Mori and Iwatsuki claim that the true initiator is the primary amino group formed by hydrolysis of the NCA with contaminated water and that tertiary amine forms a complex with the NCA and accelerates the addition reaction37 . Harwood et al. confirmed the propagating carbamate by NMR in polymerization initiated with a strong base37 . The successive addition of NCA to the polymer end catalyzed with a strong base affords an alternative procedure for the synthesis of block copolypeptides. Block copolypeptides of poly(oxyethylene) were prepared by triethyl amine catalyzed polymerization of NCA in the presence of poly(oxyethylene)bis-eMoroformate38 . 

There is an extensive literature on the use of 2,1,3-benzoxadiazole 1-oxide [often called benzofuroxanie) (BFO) (462)] as a substrate for the primary synthesis of quinoxaline 1,4-dioxides and occasionally quinoxaline mono-A -oxides or even simple quinoxalines. Very few substituted derivatives of the parent substrate (462) have been employed in recent years. The general mechanism clearly involves a fission (usually amine-catalyzed) of the oxadiazole ring followed by reaction with an ancillary synthon. The following examples are divided according to the type of synthon employed. 

Thalji NK, Crowe WE, Waldrop GL (2009) Kinetic mechanism and structural requirements of the amine-catalyzed decarboxylation of oxaloacetic acid. J Org Chem 74(1) 144—152... 

Scheme 1 SSAO/VAP-1 TPQ-catalyzed amine oxidase mechanism.

Armstrong, A. Amine-catalyzed Epoxidation of Alkenes A New Mechanism for the Activation of Oxone. Angew. Chem. Int Ed. 2004, 43, 1460-1462. 

Platinum-catalyzed hydrolytic amidation of unactivated nitriles was reported by Cobley and coauthors. The platinum(ii) complex, [(Me2PO- H- PMe2)PtH (PMe2OH)], efficiently catalyzes the direct conversion of unactivated nitriles into N-substituted amides with both primary and secondary amines. Possible mechanisms for this reaction are discussed and evidence for initial amidine formation is reported. Isolated yields vary from 51 to 89% [25]. 

Aliphatic amine transalkylation has been observed on alumina by Derrien and Jungers (10) and by Gatry and Jungers (11) at a higher temperature. Under the experimental conditions used here, an acid-catalyzed mechanism seems to be acceptable. An explanation that would lead to... 

Tertiary amine catalyzed reactions proceeded by a general base catalyzed mechanism. 

Brill et al.16 confirmed earlier observations that for the substitution on the C2 of pyrimidines the less nucleophilic anilines are actually more reactive than aliphatic amines, presumably because of an auto-acid-catalyzed mechanism. In turn, less reactive centers like the C6 chloro group of pyrimidines could be displaced with anilines in the presence of tm-butoxide as a base (see the experimental section). 

The known properties of proteins and the known mechanism of amine-catalyzed decarboxylation of /3-oxo acids (Laursen, 1966) were combined to generate a synthetic decarboxylase in the form of two tetradecameric peptides, termed Oxaldie 1 and 2, with leucine and lysine as the main amino acids (Johnsson, 1993). 

Whereas the examples discussed so far proceed according to the iminium ion mechanism (A), amine-catalyzed additions of, e.g., ketones to nitroolefins are effected by intermediate enamine formation (B). List et al. were the first to report that L-proline catalyzes the addition of several ketones to nitroolefins (Scheme 4.23). Whereas both the yields and diastereoselectivity were high in DMSO as solvent, the ee did not exceed 23% [38]. A related study of this process by Enders and Seki resulted in identification of methanol as a superior solvent, and enantioselec-tivity up to 76% was achieved (Scheme 4.23) [39]. 

The supposed mechanism for Al(salen)Cl/amine catalyzed 1,4-addition involves a crucial stereo controlled formation of an intermediate octahedral Schiff base-aluminum enolate 132 as depicted in Fig. 2. 

Mechanism for Amination Catalyzed by P(o-C6H4Me)3 Palladium complexes... 

Scheme 9. Overall mechanism for aryl halide amination catalyzed by P(o-C6H4Me)3-ligated palladium complexes.

Mechanism for Amination Catalyzed by Palladium Complexes with Chelating Ligands... 

---