Rearrangement of Tetraaryldisilenes

West and his coworkers [3, 4] have followed the isomerization of the gem tetra-substituted disilene, Ar2Si=SiAr2, in which Ar and Ar are distinguishable aryl groups with similar steric and electronic properties, e.g. Ar = mesityl and Ar 2 = 2,6-xylyl. The two 1,2-isosubstituted isomers Z and E are formed at different rates. Having provisionally identified the first product as the Z isomer, the authors propose that the 1,1-isosubstituted molecule equilibriates with its 1,2-isomers by a stepwise mechanism An initial dyotropic shift via a bicyclobutanelike transition state to the Z isomer is followed by slower conversion to the E isomer, presumably by rotation about the SiSi double bond. 

Tetraaryldisilenes are essentially planar molecules [6, 7] that distort out-of-plane with great ease to a trans conformation [8]. Ignoring the asymmetry imposed by the substituents, the molecule can be regarded as having D2/1 symmetry. When it is placed in the xy plane with the SiSi bond along x, the soft out-of-plane displacement is assigned to b g and desymmetrizes the molecule to a motion that can play no essential role in the isomerization.  

The diagrams in Fig. 8.1 are purely schematic, no attempt being made to reproduce the separation between the various MOs or even their order. Each substituent is represented by its a bond to silicon, on the assumption that the aryl groups remain intact during the isomerization the MOs of the disilene thus have the same irreps as ethylene. The only new element in Fig. 8.1 is the presence in each of the transition structures of two linear combinations of three-center bonds. Both of these are necessarily symmetric with respect to (r yz) in (I) they are labeled ag and 62 (H) Because the diagram is 

The mechanistic conclusion that can be drawn from Fig. 8.1 is that the pathways for rearrangement of the gem molecule to its E and Z isomers are equally consistent with the conservation of orbital symmetry. Since the authors 

Halide ions are closed shell species with the same symmetry properties as H . and are therefore depicted in Fig. 8.2 as bearing just the two valence electrons that bond to carbon to form the new covalent bond, or are detached with the leaving group as the bond is broken.  

---

A similar rearrangement is observed between tetrasilyldisilenes 34 and 35 [Eq. (56)].14 The activation free energy for the rearrangement from 35 to (Z)-34 (or ( )-34) is 17.4kcalmol-1 at 283 K, which is ca. 1.7 kcal mol-1 larger than that for the /i,Z-isomerization and 7.7 kcal mol-1 smaller than those for the dyotropic type rearrangement of tetraaryldisilenes.96... 

---