1,3-Stannyl shift

Another nice cascade, one of many based on the Barton-McCombie reaction, is depicted in Scheme 8 [16] it involves addition to an aldehyde, 1,3-stannyl shift from carbon to oxygen, and cyclization to give a diquinane derivative. 

Finally, alkyltins can participate in radical chemistry, especially when the /3-elimination is thermodynamicaly favored, leading, for instance, to carbocyclic ring expansions [313], In a similar way, radical reactions involving a 1,3-stannyl shift could afford 5-exo cyclizations [314]. 

Further evidence for the above-mentioned mechanism of HOMO elevation by group 14 elements is provided by studies of thioethers. The decrease in oxidation potential of silyl ethers as compared to ethers is not realized in the case of a-silylthioethers whereas a-stannyl substituents in thioethers cause a considerable cathodic shift in oxidation potential. Moreover, the effect is geometry-dependent. Values for substituted cyclic dithianes 15 are summarized in Table 21. The difference between Si and Sn in this case is illustrative. The lone nonbonding pair in the 3p orbital of sulfur is much too low in energy compared to... 

Typical NMR data were compiled in CHEC-II(1996) <1996CHEC-II(8)2> for 3.77-pyrrolizine 1, its lithium salt, the pyrrolizin-3-one 2, and its regioisomer (pyrrolizin-2-one). More recently, Kissounko et al. <1998JOM(556)145> reported the H chemical shifts of the parent pyrrolizine anion and anion 21a as well as those of their silylated or stannylated derivatives 22-26. 

Posner and coworkers concluded that the low activity of the stannyl-substituted tri-oxanes indicated that the killing species must be an intermediate somewhere along the route after the 1,5-H shift has generated a C4-centred radical. 

Only the more strongly electrophilic diarylcarbodiimides react with stannyl ynamines to give the transient dipolar adducts 62. They evolve further either by undergoing a 1,3-shift of the organyl group to give ynamines 63 or they cyclise to diaminoquinolins 64(108), 48>153 . 

This vanadium method enables the cross-coupling only in combinations of silyl enol ethers having a large difference in reactivity toward radicals and in their reducing ability. To accomplish the crosscoupling reaction of two carbonyl compounds, we tried the reaction of silyl enol ethers and a-stannyl esters based on the following consideration. a-Stannyl esters (keto form) are known to be in equilibrium with the enol form such as stannyl enol ethers, but the equilibrium is mostly shifted toward the keto form. When a mixture of an a-stannyl ester such as 45 and a silyl enol ether is oxidized, it is very likely that the stannyl enol ether will be oxidized preferentially to the silyl enol ether. The cation radical of 45 apparently cleaves immediately giving an a-keto radical, which reacts with the silyl enol ether selectively because of the low concentration of the stannyl enol... 

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