Imine mechanistic aspects

It is very well known that jr-allyl palladium complex 1, which is a key intermediate for the Tsuji-Trost type allylation, has an electrophilic character and reacts with nucleophiles to afford the corresponding allylation products. We discovered that bis 7r-allyl palladium complex 2 is nucleophilic and reacts with electophiles such as aldehydes [27] and imines [28-32] (Scheme 2, Structure 2). We have also shown that bis 7r-allyl palladium complex 2 can act as an amphiphilic catalytic allylating agent it reacts with both nucleophilic and electrophilic carbons at once to produce double allylation products [33]. These complexes incorporate two allyl moieties that can bind with different hapticity to palladium (Scheme 3). The different complexes may interconvert by ligand coordination. The complexes 2a, 2b and 2c are called as r]3,r]3-bisallypalladium complex (also called bis-jr-allylpalladium complex), r)l,r)3-bis(allyl)palladium complex, -bis(allyl)palladium complex, respectively. Bis zr-allyl palladium complex 2 can easily be generated by reaction of mono-allylpalladium complex 1 and allylmetal species 3 (Scheme 4) [33-36]. Because of the unique catalytic activities of the bis zr-allyl palladium complex 2, a number of interesting cascade reactions appeared in the literature. The subject of the present chapter is to review some recent synthetic and mechanistic aspects of the interesting palladium catalyzed cascade reactions which in-... 

Matsuoka, T., Harano, K., Uemura, T., Hisano, T. Hetero Diels-Alder reaction of N-acyl imines. I. The reaction of N -thiobenzoyl-N,N-dimethylformamidine with electron-deficient dienophiles. Stereochemical and mechanistic aspects. Chem. Pharm. Bull. 1993, 41, 50-54. Mikami, K., Motoyama, Y., Terada, M. Asymmetric Catalysis of Diels-Alder Cycloadditions by an MS-Free Binaphthol-Titanium Complex Dramatic Effect of MS, Linear vs Positive Nonlinear Relationship, and Synthetic Applications. J. Am. Chem. Soc. 1994, 116, 2812-2820. 

Plaquevent and coworkers re-examined and sharply improved this method for a rapid access to enantioenriched P-trifluoromethyl-P-amino acid [47]. Nine cinchona-based catalysts were screened, and the best result was obtained using (DHQ)2PHAL 80. The reaction was performed starting with the p-nitrobenzyl enaminoester 81 at 80 °C and afforded the expected imine ester 82 in 90% isolated yield and 71% ee (Scheme 7.36). The authors put a special emphasis on the mechanistic aspect of the reaction using a deuterated substrate. According to the results, the deprotonation is both rate and asymmetric determining step. 

A special attention was focused on the catalyst reuses. Simultaneously we examined the mechanistic aspects of carbonyl activation by the primary amine via activated imine intermediate on the condensation reaction in considering also the activity of a tertiary amine group covalently grafted to the surface. 

Mechanistic aspects of both the Ni and Pd di-imine catalysts systems have been addressed using density functional, molecular orbital, and molecular mechanics calculations in an extensive series of pa-pers. ° - 2 2 It is beyond the scope of this review to present a comprehensive account of this complex body of work, but a few features should be noted. In general, calculations have been consistent with the experimental observations available. Calculated barriers to insertion in the [(diimine)Pd(olefin)R]+ complexes are in reasonable agreement with experiment. For example, the calculated barrier to insertion in [(2,6-i-PrPh)2DABMe2]Pd(C2H4)CH3)]+ is 14.1 kcaF mol, which compares favorably with the experimentally determined barrier of 17.3 kcaFmol. The barrier to insertion in the Ni analogue was calculated to be 13.2 kcal/mol, which is very close to the measured barrier of 13.5 kcaFmol. ° Other calculations significantly underestimate this barrier. ... 

Watkins JD, Taylor JE, Bull SD, Marken F (2012) Mechanistic aspects of aldehyde and imine electro-reduction in a liquid-liquid carhon nanoflher membrane microreactOT. Tetrahedrtm Lett 53 3357-3360... 

In a series of three papers, Noguchi and co-workers have reported their continuing studies on the formation of heterocycle-fused azepine systems <96X13081, 96X13097, 96X13111>. A typical example is the conversion of the aldehyde 15 into the azepines 16 and 17 (Scheme 3). Xhe reaction also proceeds with imines when the dihydroazepine prior to bridging can be isolated. Mechanistic and stereochemical aspects of the reaction have been explored. 

Abstract The main computational studies on the formation of (3-lactams through [2+2] cycloadditions published during 1992-2008 are reported with special emphasis on the mechanistic and selectivity aspects of these reactions. Disconnection of the N1-C2 and C3-C4 bonds of the azetidin-2-one ring leads to the reaction between ketenes and imines. Computational and experimental results point to a stepwise mechanism for this reaction. The first step consists of a nucleophilic attack of the iminic nitrogen on the sp-hybridized carbon atom of the ketene. The zwitterionic intermediate thus formed yields the corresponding (3-1 actant by means of a four-electron conrotatoty electrocyclization. The steroecontrol and the periselectivity of the reaction support this two-step mechanism. The [2+2] cycloaddition between isocyanates and alkenes takes place via a concerted (but asynchronous) mechanism that can be interpreted in terms of a [n2s + (n2s + n2s)] interaction between both reactants. Both the regio and the stereochemistry observed are compatible with this computational model. However, the combination of solvent and substituent effects can result in a stepwise mechanism. 

The Mannich reaction has been reviewed comprehensively by Blicke (1942), Reichert (1959), Hell-mann and Opitz (1960), and Tramontini (1973). These reviews also include synthetic applications of Mannich bases. Mechanistic studies of the Mannich reaction have been reviewed by Thompson (1968). Some variants of the Mannich reaction have been covered as subtopics in other reviews for example. Layer (1963) and Harada (1970) have reviewed general additions of stabilized carbanions to imines, while Bdhme and Haake (1976) have reviewed similar additions to methyleneiminium salts. In more specific reviews, Pai and coworkers (1984) have summarized stabilized carbanion additions to 3,4-dihy-droisoquinolines and 3,4-dihydroisoquinolinium salts in connection with the total synthesis of protober-berines and phthalide isoquinolines, and Evans et al. (1982) " have analyzed the stereochemical aspects of ester enolate and silyl ketene acetal additions to imines. 

The mechanism of this hydrogenation has not been revealed in detail, but several aspects are noteworthy in the context of the mechanistic discussion earlier in this chapter. First, this reaction almost certainly occurs by the insertion of an imine into a metal hydride (see Chapter 9), rather than by the transfer of a hydride and a proton by a metal-ligand bifunctional system. This iridium catalyst does not contain a protic ligand. Second, acid and iodide are needed as promoters to obtain the fast rates. The iodide is thought to help stabilize an iridium(III) species and the acid is thought to help in the release of the amine product from the metal. 

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