. ▁This ▁result ▁suggests ▁that ▁the ▁exchange ▁of ▁ch lor ine ▁for ▁hydro gen ▁occurs ▁through ▁ σ - b ond ▁met ath esis , ▁without ▁the ▁formation ▁of ▁a ▁plan ar ▁sil ic enium ▁ion . ▁Red u ctions ▁in ▁more ▁polar ▁sol vent s ▁may ▁involve ▁sil ic enium ▁ ions , ▁although ▁the ▁extent ▁of ▁rac em ization ▁at ▁sil icon ▁( expected ▁to ▁be ▁complete ▁if ▁free ▁sil ic enium ▁ ions ▁are ▁involved ) ▁depends ▁on ▁the ▁polar ity ▁of ▁the ▁sol vent . ▁( 2 ) ▁Hyper val ent ▁sil icon ▁species ▁are ▁likely ▁the ▁active ▁reducing ▁agent ▁when ▁nucle oph iles ▁with ▁a ▁high ▁aff inity ▁for ▁sil icon , ▁such ▁as ▁flu or ide , ▁are ▁added ▁to ▁the ▁reaction ▁mixture . ▁Co ord ination ▁of ▁an ▁an ionic ▁nucle oph ile ▁such ▁as ▁flu or ide ▁to ▁sil icon ▁increases ▁its ▁reducing ▁power ▁substantial ly , ▁such ▁that ▁redu ctions ▁of ▁al de hy des ▁and ▁k et ones ▁can ▁occur ▁without ▁the ▁help ▁of ▁a ▁Lewis ▁acid . ▁( 3 ) ▁ ▁S tere ose lect ivity ▁In ▁organ os il ane ▁redu ctions ▁of ▁substr ates ▁bearing ▁prost ere ogen ic ▁groups , ▁di aster e ose lect ivity ▁is ▁often ▁high . ▁Red u ction ▁of ▁either ▁di aster e omer ▁of ▁ 2 - phen yl - 2 - nor born an ol ▁leads ▁exclus ively ▁to ▁ ▁the ▁end o ▁di aster e omer ▁of ▁ 2 - phen