Diastereoselectivity in the zinc-mediated Barbier allylation

Penumati & Kommu (2014) report the stereoselective synthesis of (+)-$\alpha$-conhydrine from R-(+)-glyceraldehyde. In their synthesis, the imine **6** underwent a zinc-mediated Barbier allylation to produce secondary amine **7**. 


[![enter image description here](https://i.stack.imgur.com/bxTx8.png)](https://i.stack.imgur.com/bxTx8.png)


They explained the preferential formation of the major product **7a** over **7b** via the following Felkin-Ahn model, claiming that the anionic allyl nucleophile approaches the less hindered face. However, it appears to me that there is no significant difference in the steric hindrance posed by each face of the imine group. Hence, I am skeptical if application of the Felkin-Ahn model here is appropriate and whether it does give the correct predicted major product. I thought of using the chelation model, since certainly there is a possibility $\ce {Zn^2+}$ can be involved, but I realised that it doesn't seem to be appropriate as well, or at least that is what appears to be the case for me. 


[![enter image description here](https://i.stack.imgur.com/tKJPo.png)](https://i.stack.imgur.com/tKJPo.png)


**Reference**


Penumati, N. R.; Kommu, N. Stereoselective Synthesis of (+)-$\alpha$-Conhydrine from R-(+)-Glyceraldehyde. *Organic Chemistry International*, **2014**. doi: [10.1155/2014/982716](https://www.hindawi.com/journals/oci/2014/982716/)


