Syn-anti interconversion

The syn-anti interconversions and facial rearrangements necessary to achieve this preferred configuration are generally believed to be accomplished via -q1 -intermediates (equation 147). 

Dimethoxybenzhydrylamine (DMB) exhibited loss of stereospecificity in its Pd-catalyzed addition to acyclic allylic acetates. Pure (Z)-allylic acetates yielded only ( >products in contrast to the complete stereospecificity in the addition of malonate anions to the same substrates, presumably due to the enhanced rate of syn-anti interconversion caused by the ability of the amine to coordinate to Pd (equation 189).200... 

Scheme 5 syn-anti interconversion of Schrock-type catalysts... 

Secondary amines are especially good nucleophiles for allylic substitutions. Alkylation-amination of the (Z)-bis-allylic compound shown below in the presence of Pd(PPh3)4 with one equivalent of malonate furnishes the bis-substitution product with ( )-selectivity. Alkene isomerization due to syn-anti interconversion in the 7t-allyl intermediate is often observed. 

Figure 5. Possible pathways for the syn-anti interconversion in oximes...

Fig. 5.4 Syn-anti interconversion in Mo-based Schrock carbenes. R =CMe3, CMejCFj, CMe(CF,)2, C(CF3)2, etc.

Activation Parameters Associated with syn-anti Interconversion Processes... 

As shown in Scheme 2, ti-o-ji isomerization can result in syn-anti interconversion by rotation around the o-(C-C) bond in the r -intermediate syn and anti refer to the positions syn and anti to the substituent at C2). The syn position is sterically favored and, consequently, in 1 -monosubstituted allyl systems the syn-isomer is more stable than the anfi-isomer.In general, the thermodynamic equilibrium lies far on the side of the syn-isomer and only if a substituent is sufficiently small (e.g., R=Me) is the anfi-isomer present in notable amounts. However, certain ligands, that exert strong steric hindrance in the coordination plane toward the [Pg.792]

Rotation of the allyl group, in which syn-anti interconversion does not occur, is now known in many instances for complexes with coordination numbers greater than four (see Table III). NMR spectra consistent with allyl rotation in most square palladium complexes have been shown to result from ligand exchange (47). 

The rates of interconversion of the syn and anti isomers were found to be one to two orders of magnitude faster for W MAP complexes than for Mo MAP complexes. Little is known about the rates of interconversion of syn-and anti-protons for MAP species [2c], although the rates of syn/anti interconversions in bisalkoxide Mo imido alkylidene complexes have been found to vary over approximately six orders of magnitude [57]. It is not surprising that the sterically demanding NArj jg52 ligand would destabilize the syn isomer for steric reasons and lead to mixtures that contain both syn and anti species, as observed. 

Magnetic Resonance.—An n.m.r. method has been used to determine the energy barrier to syn-anti interconversion in a number of /9-imino-oxetans (1). The n.m.r. spectra of L-azetidine-2-carboxyIic acid and its N-acetylated derivative have been exhaustively analysed. The data provide... 

Another cation that, hke bridgehead cations, is difficult to form by conventional methods is the 7-norbornyl cation. The problem in this case is angle strain. Thus, extended treatment of 7-iodonorbornane (52) with methanohc AgN03 afforded only a trace of ether 53 (R = CH,). By contrast, irradiation in CH3OH readily afforded ether 53 (R = CHj), along with smaher amounts of the reduction product norbornane (9), the unprecedented 1,3-ehmination product 54 and the rearrangement products 55-57. The 2-chloro derivatives 58 underwent syn,anti interconversion in competition with formation of the reduction product 60, the ethers 61 and 63, and the 1,3-elimination product 62. By contrast, treatment... 

---