Provide an explanation on the transformation of a secondary hydroxy group to the corresponding halide with an inversion of stereochemistry.

    In the reaction, the product is formed by using Sn2 substitution. However, many of the reagents attempted to be used were unsuccessful, such as SOCl2/base, PPh3/CCl4, PPh3/ZnCl3/DEAD, MsCl/Pyr., PPh3/Cl2, DMAP/CS(imid)2, (COCl)2, TCT, TMSCl/BiCl3 . One successful reaction was performed using NCS/ PPh3. The reason that this was successful was because the alcohol was more accessible to the electrophile due to a slight bond-lengthening effect, such as retro-aldol ability.  This allows the obstacle of the neopentyl hindrance to be overcome.