Sulfonyl azides: diazo or azide transfer?

Sulfonyl azides, $\ce{RSO2N3}$, are capable of acting as sources of electrophilic azide ("$\ce{N+3}$"), but are also capable of effecting diazo transfer, as shown in the scheme below, taken from ref 1:


[![ArSO2N3 + enolate with Evans auxiliary](https://i.stack.imgur.com/ZLrrS.png)](https://i.stack.imgur.com/ZLrrS.png)


In 1990, Evans and coworkers1 showed that the aryl group on the reagent was important in controlling the outcome of the reaction:


$$\begin{array}{ccc}
\ce{Ar} & \text{yield of }\textbf{3a-S}\text{ / %} & \text{yield of }\textbf{5a}\text{ / %} \\ \hline
\ce{(4-NO2)C6H4} & 15 & 70 \\
\ce{(4-Me)C6H4} & 51 & 26 \\
\ce{(2,4,6-^iPr3)C6H2} & 91 & 0 \\
\end{array}$$


which suggests that $\ce{TrisN3}$ (the third entry) is the best azide transfer reagent.


However, confusingly, it has also been reported that under phase transfer conditions, $\ce{TrisN3}$ outperforms $\ce{TsN3}$ (the second entry) in diazo transfer to ketones.2


With this in mind, Evans concluded that



> 
> *Collectively, these independent observations underscore the lack of mechanistic sophistication which currently surrounds these reactions.*
> 
> 
> 


Since then, have there been any efforts to rationalise the effect of the aryl group on the product distribution? And if so, how can the results above be explained?




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**References**


1. Evans, D. A.; Britton, T. C.; Ellman, J. A.; Dorow, R. L. The asymmetric synthesis of α-amino acids. Electrophilic azidation of chiral imide enolates, a practical approach to the synthesis of (*R*)- and (*S*)-α-azido carboxylic acids. *J. Am. Chem. Soc.* **1990,** *112* (10), 4011–4030. [DOI: 10.1021/ja00166a045](https://doi.org/10.1021/ja00166a045). Non-paywall version available from the [Evans group website](http://evans.rc.fas.harvard.edu/pdf/evans126.pdf).
2. Lombardo, L.; Mander, L. N. A One-Step Synthesis of Cyclic α-Diazoketones. *Synthesis* **1980,** Vol. 5, 368–369. [DOI: 10.1055/s-1980-29022](https://doi.org/10.1055/s-1980-29022).


