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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: maverick1996 on April 18, 2016, 06:56:36 AM

Title: Wittig reagent - cis or trans?
Post by: maverick1996 on April 18, 2016, 06:56:36 AM

Hey guy!

When I react a ylid with an aldehyde, the product is an alkene and I just assumed that a trans alkene would be more favourable due to less steric repulsion etc.

But I am doing some questions, the specific question is EtCHO with Ph3P=CHCH2CH2CH3 (sorry about not formatting this properly) and again I assumed the trans alkene is the majority product but the answers say 'EtCH=CHCH2CH2CH3, 90% cis-isomer'

Can somebody help me out and explain why the cis alkene is the majority product?

Thank you

Title: Re: Wittig reagent - cis or trans?
Post by: Alwin Kristen on April 18, 2016, 10:42:51 AM

Stereoselectivity of Wittig-reaction depends of the ylid. With stabilized ylid (delocalized negative charge) reaction is E (trans) selective and with unstabilized ylid its Z (cis) selective. I have attached a picture from J. Claydens book; Organic Chemistry, to clarify Z selectivity.

Title: Re: Wittig reagent - cis or trans?
Post by: maverick1996 on April 18, 2016, 12:59:03 PM

Quote from: Alwin Kristen on April 18, 2016, 10:42:51 AM

Stereoselectivity of Wittig-reaction depends of the ylid. With stabilized ylid (delocalized negative charge) reaction is E (trans) selective and with unstabilized ylid its Z (cis) selective. I have attached a picture from J. Claydens book; Organic Chemistry, to clarify Z selectivity.

Thank you