Imines geometry

Reformatsky reaction has covered aspects of this topic (20b). If analogous pericyclic transition states are involved in these condensations, the added stereochemical control element imposed on the condensation by the imine geometry should provide a more well-defined set of transition states than for the analogous aldehyde condensations. The four diastereomeric chair and boat transition states for ( )- and (Z)-enolates with ( )-imines are illustrated in Scheme 15. 

An analogous set of four transition states for the (Z)-imine geometry could also be constructed. The transition state descriptors, such as B(Z,E) and C(Z,E), may be employed to denote boat (B) or chair (C) transition states respectively, possessing (Z)-enolate and ( )-imine geometries. The interrelationships between the stereochemical features of reactants, transition states, and products are summarized in Table 28. Unfortunately, the kinetic diastereoselection in such condensations as a function of either enolate or imine geometry has not been systematically studied. In all the early work in this field... 

Enolate Geometry Imine Geometry Transition State Product Stereochemistry... 

Is the second step of the overall reaction for R=Me (N-methylphthalimide + hydrazine —> phthalimide hydrazide + methylamine) exothermic or endothermic Will higher temperatures accelerate or inhibit the reaction Is the structure drawn above for phthalimide hydrazide its lowest-energy form or are either the imine or diimine tautomers preferred Compare energies for the hydrazide and imine and diimine tautomers. Examine the geometry of phthalimide hydrazide and any low energy tautomer, and draw the Lewis structure(s) that best describes it. Can your Lewis structures account for the energy differences Examine electrostatic potential maps for all three molecules. Which molecule(s) are stablized by favorable electrostatic interactions Which are destabilized Can this help explain the energy differences Elaborate. 

Cyclic and open transition state models have been used to explain syn/anti stereoselectivity in these reactions1. The possible transition states (including boat B and chair C transition states) can be deduced from the E/Z geometry of the crotyl reagent and the imine. The postulated cyclic transition states for the preferred E geometry of the imine arc shown below. 

According to these transition state models2,. (y -products are formed via a chair (C) conformation where both the enolate and the imine are in E geometry (E,E) or via a boat (B) transition state where the enolate is in Z and the imine in Econfiguration C(E.E) or B(Z,E). antt-Products are formed via B(E,E) and C(Z,E) transition states. The transition states leading to 1 and 2 are based upon the more stable E geometry of the imine. For cyclic imines a complementary set of transition states can be applied based on the Z geometry of the imine. 

In pyridine- X-C interactions, the C - X moiety is roughly coplanar with the pyridine and the two C-N- X angles are approximately 120° [129,143, 144]. The same holds for other nitrogen heteroaromatics (e.g. pyrazine, quinoline, etc.) [145-147]. A carbonyl group pins the donors after a trigonal planar geometry and works either as a mono- [148,149] or bidentate XB acceptor [150]. Sulfoxides behave similarly [151,152] and imines form XB along the expected axis of the lone pair [153]. 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

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