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Why are phosphonate-stabilised ylides used in HWE reaction more nucleophilic than phosphonium ylids used in Wittig reaction? Explanations without rationalisations using d orbitals would be welcome.

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    $\begingroup$ Which ylides are you comparing? Can you give us generic structures? ‘unstabilised ylides’ could be a lot of things … $\endgroup$ – Jan Oct 29 '17 at 2:07
  • $\begingroup$ As per @Jan's comment, it might be helpful if you expanded your question to back up the statement with structures or data. As it stands, I don't think it really holds true. $\endgroup$ – NotEvans. Apr 20 '18 at 23:10
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    $\begingroup$ I will try to find in what textbook I've seen the statement. Anyway, the same is stated on wikkipedia en.wikipedia.org/wiki/… $\endgroup$ – RBW Apr 21 '18 at 11:45
  • $\begingroup$ I have found in Clayden et al. that the phosphonates being anionic are more nucleophilic than the neutral phosphonium ylids. On the other hand, they are less basic than the phosphonium ylids $\endgroup$ – RBW Apr 25 '18 at 13:26
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It might not be the only factor, but I think that the steric effect of the huge phenyl groups in Wittig ylides might contribute in making them less nucleophile. If you give a fast look at a "space filling" model of the two molecules, you see that the negatively charged carbon in a Wittig Ylide is much more masked by the phenyl groupsenter image description here

Note that in order for a nucleophile-electrophile interaction to occur, an electrophile should approach to the carbanion's HOMO orbital (approximatively orthogonal to the screen in both the pictures).

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