Aryltrimethylstannane Cation Radical Fragmentation Selectivities That Depend on Codonor: Evidence for Reactions from Heterodimer Cation Radicals

Abstract

The aryl/methyl fragmentation selectivities for the photooxidations of phenyltrimethylstannane and (4-methylphenyl)trimethylstannane by 1,2,4,5-tetracyanobenzene in acetonitrile were found to depend on the codonor used to generate the stannane cation radical intermediates. The aryl/methyl fragmentation selectivities for phenyltrimethylstannane and (4-methylphenyl)trimethylstannane varied by factors of 26 and 5.6, respectively, depending on the structures of the codonors. The fragmentation selectivities could be correlated with the oxidation potentials of the codonors and their steric bulk. The results can be interpreted by the intermediacy of heterodimer cation radicals formed between the stannane cation radicals and the neutral codonors, which thereby affect the fragmentation selectivities.

Mechanism and Selectivity of Aryltrimethylgermane Cation Radical Fragmentations

“…The reaction workup was designed to convert these adducts to germyl chlorides by treatment with LiCl, following the analogous procedure previously used for aryltrialkylstannane photo-oxidations (Scheme 1). 1,2 The two expected products are (4-methoxyphenyl)dimethylgermyl chloride (3), indicative of methyl radical loss, and trimethylgermyl chloride (4), indicative of aryl radical loss. In practice, these were the only products formed as revealed by 1 H NMR spectroscopy.…”

“…Remarkably, the selectivity for the aryl/Me loss ratio, indicative of aryl versus methyl radical loss, was ∼55:1 for the reaction with HOMe and ∼365:1 for the reaction with HOBu t significantly greater selectivities than those observed in analogous stannane cation radical fragmentations (see Table 1). 1,2 Addition of acetonitrile resulted in more competitive aryl and methyl radical cleavages but still showed a small preference for aryl radical loss. To test whether aryl loss could, in part, be due to protodegermylation, the photo-oxidations were also conducted in the presence of anhydrous K 2 CO 3 as an acid scavenger.…”

“…In analogy to the previously studied aryltrimethylstannane cation radicals, , we first chose to attempt the generation of the (4-methoxyphenyl)­trimethylgermane cation radical ( 1 +• ). Nanosecond transient absorption spectroscopy (NTAS) using a cosensitized photo-oxidation method described previously was used to generate 1 +• .…”

“…The combined NTAS and product studies provide strong support for the hypothesis that the C–Ge bond fragmentations of 1 +• proceed by a nucleophile-assisted mechanism that shows a higher preference for substitution to form the less stable aryl radical than aryltrimethylstannane cation radials. , A Bent’s rule explanation has previously been proposed to explain the fragmentation selectivities for the reaction of aryltrimethylstannane cation radicals with nucleophiles, where the more electronegative aryl substituent prefers to be in an apical, leaving group position of a trigonal bipyramidal transition state for nucleophilic substitution . If the same effect applies to the fragmentation of germane cation radicals, the lower reactivity of 1 +• versus its stannane cation radical analogue could result in a more advanced (later) transition state for the reaction of 1 +• with nucleophiles, which could lead to the difference in electronegativities of the departing radicals playing a greater role in determining the fragmentation selectivities for 1 +• .…”

“…Aryltrialkylstannane cation radicals generated in solution have recently been shown to undergo C–Sn bond fragmentation by a nucleophile-assisted mechanism (S N 2). , An interesting feature of these reactions is that the stannane cation radicals show a remarkable preference for loss of the (less stable) aryl radicals over methyl radicals, a result hitherto unprecedented for cation radical bond fragmentations. These curious results prompted us to study the fragmentations of analogous germane cation radicals to determine if they showed fragmentation selectivities similar to their stannane cation radical analogues.…”