Mechanistic Suggestions

The same rearrangement can be induced by applying LDA on thienylethinyltrimethylsilane and warming up to RT. At first the influence of temperature on the rearrangement was examined:

Scheme 17

45 min stirring at -70°C	0-1 %
within 15 min to - 40°C	39 %
60 min stirring at -40°C	50 %
60 min stirring at -20°C	76 %
60 min stirring at 10°C	100 %

Scheme 18

The reaction was repeated in DME and THF/TMEDA giving practically the same composition again - no significant influence by solvents could be established. The reaction was run again in THF/TMEDA for 1 h and for 5 hrs and analyzed via nmr quenching experiments:

Scheme 19

• The rearrangement comes to an equilibrium after about 1 hour
• Solvolysis occurs relatively fast
• The mechanism seems to be rather complex

• 0.9 equ. of BuLi were applied and the reaction was stirred at RT for 1 h, providing a constant amount of approximately 10 % (2-TMS-3-Br-5-thienylethinyl-TMS): no lithium/bromo exchange, but lithium/silyl transfer occurred.

  
  

  
  

  Scheme 20

• Crossing experiment of (2-Li-3-Br-5-thienylethinyl-TMS) with 10 equiv.% of (2-TMS-3-Br-5-thienylethinyl-TMS), followed by aqueous work-up: the di(TMS)-compound acts like a trans-silylation catalyst, transforming the starting Li-substrate and generating the final equilibrium composition.

  
  

  
  

  Scheme 21

• Crossing experiment of (2-TMS-3-Br-5-thienylethinyl-TMS) with thienyl lithium:
  Both TMS-groups react with the lithium atom, proving the very similar stabilities of both the lithium intermediates formed.

  
  

  
  

  Scheme 22

All the reactions depicted above - and very clearly the last one - show unambiguously that the whole reaction cascade and the reactivities of the substrates investigated can be clearly correlated with the different acidities of the corresponding thienyl and acetylene C-H-moieties. The bromine contributes due to its electron withdrawing property a stabilizing effect to the lithium-carbon bond compared to the unsubstituted product: this seems mainly to be the reason why the reaction reaches its equilibrium at ca. 60 % of the rearrangement target.

A good proof for this assumption is the successful migration discussed below: no ortho-bromo/lithium pattern is blocking the reactivity of the lithium-carbon bond.

Scheme 23

This example shows again that the ortho-vicinity of bromine and lithium atoms is the reason for the lower conversion rates during the rearrangement reaction.