Question

What is the difference in the Wittig reaction outcome if triethylphosphite is used instead of triphenylphosphine?

Answer 1

The difference in the Wittig reaction outcome if triethylphosphite is used instead of triphenylphosphine is that then it is called  Horner–Wadsworth–Emmons reaction (HWE)

The phosphonates used in Horner–Wadsworth–Emmons reaction (HWE) is more reactive than classical Wittig reaction which stabilized ylides when react with ketones and aldehydes.

Another advantage of the Horner–Wadsworth–Emmons reaction (HWE).i.e. when triphenylphosphite is used in the reaction over the classic Wittig reaction which uses triethylphosphine is that the phosphorus byproducts are water-soluble and hence readily separated from the desired product.

Phosphonates which lack a stabilizing a substituent at R² (for example COO^-, COOR, CN, SO₂R, vinyl, aryl P(O)(OR)₂, OR or NR₂) generally result in a low yield of the product alkene.