Phosphine Catalyzed Mechanism

This proposed mechanism has been supported by theoretical studies on the aza-MBH reaction between acrolein and mesyl imine catalyzed by tri-methylamine and trimethylphosphine. The relative energies of the crucial transition states for the PMe3-catalyzed reaction have been found to be lower 

Subseqently, Kwon et al. described the synthesis of stable phosphonium enolate zwitterions 22, which have been proposed as intermediates in MBH 

Mechanistic insights into the MBH/aza-MBH Reaction Using Co-cataiytic Systems or Multi /Bifunctional Catalysts 

Sunoj and coworker have identified the role of protic co-catalysts such as water, methanol, and formic acid in the MBH/aza-MBH reaction by theoretical studies [17bj. They found that the protic co-catalysts have a profound influence in decreasing the activation barriers associated with the key elementary steps due to the improved stabilization of the proton transfer transition state through a relay mechanism. 

The MBH/aza-MBH reaction involving proline as a catalyst with imidazole as a co-catalyst was proposed to proceed through an iminium ion intermediate. 

Recently, Santos and coworkers have studied the mechanism of proline-catalyzed and imidazole-co-catalyzed intramolecular MBH reactions by DFT calculations [24]. They first investigated the catalytic path for the MBH reaction of a, 3-unsaturated dialdehyde catalyzed by L-proline in the absence of imidazole, and found that water acted as an important catalyst when imidazole was not present. When imidazole was used as a co-catalyst, water was still important in the imidazole addition step. Their results rationalized the experimental outcome of the intramolecular MBH reaction, and provided theoretical evidence for some mechanistic proposals. 

B3LYP/6-31G(d,p) (plain text) and PCM B3LYP/6-31++G(d,p)B3LYP/6-31G(d,p) italic) levels of theory. The lowest-energy reaction path is indicated by bold arrows. 

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Extensive studies of kinetics and isotope effects by Hartwig and coworkers support the mechanism shown in Scheme 5 for the lr(I)/dtbpy catalyzed borylation [81]. In particular, these studies indicate that the iridium(III) trisboryl bipyridine complex (10) is the species that activates the arene C-H bond this is in agreement with DFT calculations by Sakaki et al. predicting the key intermediacy of the trisboryl complex and the seven-coordinated Ir(V) species resulting from C-H addition [82]. C-H addition to Ir(III) was also proposed in the (Ind)Ir(COD)/ phosphine-catalyzed borylation by Smith et al. [76]. 

In 2001, Oi et al. [54] reported on the ruthenium(II) phosphine catalyzed re-gioselective arylation of 2-arylpyridines using aryl halides (Eq. 29). C-C bond formation occurs predominantly at the position ortho to the pyridyl group. The same catalyst system is also effective for the arylation of aromatic imines (Eq. 30) [55]. Although the reaction mechanism has not been elucidated, it was proposed that a tetravalent arylruthenium complex,for example,Ru(Ph)(Br)(Cl)2(I) ,reacts electrophilically with the arylimines. Therefore, C-H bond cleavage is believed to proceed via an electrophilic substitution pathway. 

Tetrachloropalladate(II) ion catalyzes the interconversion of 1- and 2-butenes in aqueous solutions containing chloride and hydronium ions. Sodium tetrachloropalladate(II) catalyzes the conversion of allylbenzene to propenyl-benzene in acetic acid solutions. Tetrakis(ethylene))Lt,/x -dichlororhodium(l) catalyzes butene isomerization in methanolic hydrogen chloride solutions . Cyclooctadienes isomerize in benzene-methanol solutions of dichlorobis-(triphenylphosphine)platinum(11) and stannous chloride. Chloroplatinic acid-stannous chloride catalyzes the isomerization of pentenes. Coordination complexes of zero-valent nickel with tris(2-biphenylyl)phosphite or triphenyl-phosphine catalyze the isomerization of cis-1,2-divinylcyclobutane to a mixture of c/5,m-l, 5-cyclooctadiene and 4-vinylcyclohexene . Detailed discussions of reaction kinetics and mechanisms appear in the papers cited. 

Figure 8.6. Proposed Celanese mechanism for the rhodium-phosphine catalyzed hydroformy-lation. L2 = chelating ligand L = 1/2 of bridging ligand R = CH2CH2/ . ...

Blank NF, Moncarz JR, Brunker TJ, Scriban C, Anderson BJ, Amir O, Glueck DS, Zakharov LN, Golen JA, Incarvito CD, Rheingold AL (2007) Palladium-catalyzed asymmetric phosphination. Scope, mechanism, and origin of enantioselectivity. J Am Chem Soc 129 6847-6858... 

Krische and co-workers have investigated the tertiary phosphine-catalyzed regjospecific allylic amination of MBH acetates through a tandem 5n2 -5n2 mechanism by using phthalimide derivatives as nucleophiles. When (1 )-C1-MeO-BIPHEP was used as a catalyst, the MBH adduct obtained from p-nitrobenzaldehyde and methyl acrylate reacted with phthalimide to give the allylic substituted product 297 in 80% yield with 56% ee (Scheme 3.126). ... 

The tertiary phosphine catalyzed [3+2] cycloadditions of allenoates 98 and electron-deficient olefins 99 was first discovered by Zhang and Lu [61] in 1995. Only two years later, the first enantioselective organocatalytic version of this reaction was reported by Chen and co-workers [62]. They showed that chiral phosphabicyclo[2.2.1]heptanes 100 catalyzes this reaction to give synthetically interesting optically active cyclopaitene derivatives 102 (Scheme 6.23). The mechanism for this reaction was later investigated... 

Scheme 4.4 Mechanism of the nickel-phosphine catalyzed asymmetric Grignard crosscoupling [78]...

Unruh, J.D. and Christenson, J.R. (1982) A study of the mechanism of rhodium phosphine catalyzed hydroformylation use of l,l -bis(biarylphosphino) ferrocene ligands. Journal of Molecular Catalysis, 14, 19. 

Scheme 20.25 Proposed mechanism of the phosphine-catalyzed annulation reaction.

Most recently, Tong et al. isolated the stable phosphonium-enamine zwitterion 23, which has long been postulated as one of the key intermediates in the aza-MBH reaction, from the PPhs-catalyzed reaction between propiolate and N-tosylimine (Scheme 31.8), providing experimental evidence to support the postulated reaction mechanism of the phosphine-catalyzed MBH reaction [22]. 

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