Transition state 2,3 -Wittig rearrangement

Several mechanisms had been proposed for the [1,2]-Wittig rearrangement, including (i) a concerted process taking place through a pentacoordinate transition state, (ii) a... 

On the other hand, recent ab initio calculations have shown that the [1,2]-Wittig rearrangement preferentially proceeds via the heterolytic dissociation in gas phase. One might speculate that the solvent could generate a situation different from the transition state obtained computationally. 

The stereochemistry of the Wittig rearrangement can be predicted in terms of a cyclic flve-membered transition state in which the a substituent prefers an equatorial orienta-... 

Wittig rearranged products derived from (Z)-substrates. It is possible that this rearrangement proceeded via a six-membered transition state in which a lithium atom co-ordinates to an oxygen atom of a methoxy group to afford (Z)-ketene silyl acetal preferentially [38]. (Fig. 3)... 

Diethyl-substituted ethers 23i-k are less reactive. Ether 23k cannot be induced to rearrange, even after 72 hours. The authors speculate that this substrate may experience excessive steric compression in the envelope transition state of the [2.3] Wittig rearrangement. At higher temperatures (0°C) decomposition of the substrate and formation of multiple products is observed. 

The [2,3]-Wittig Rearrangement is a [2,3]-sigmatropic reaction, a thermal isomerization that proceeds through a six-electron, five-membered cyclic transition state. A general scheme for [2,3]-sigmatropic reactions is given here ... 

The Wittig rearrangement is primarily used in the transformation of an allylic ether to an a-allyl alcohol (Scheme 26.18).444-445 The transition-state geometry plays an important role to determine the reaction outcome that, in turn, is dependent on the stereochemistry of the double bond (Figure... 

Aza-Wittig rearrangement in the acyclic series are harder to control.141 The most reliable turn out to be those of allyl amides such as 192 in which the allyl group bears a (3-silyl substituent, whose function is to stabilise the anionic transition state of the reaction.142 143... 

Alkenic stereoselection in the Still-Wittig rearrangement results presumably from conformational control in a very early transition state (Scheme 2). There, the substituent R prefers equatorial (15) over axial (16) orientation, if the 1,2-allylic strain is smaller than the 1,3-allylic strain, and vice versa. This implies trans products in the former vide supra) and (Z)-products in the latter case vide infra). 

Scheme 3 Transition states rationalizing cis and syn selectivity of Wittig rearrangements...

Scheme 4 Transition states rationalizing the cis and trans dichotomy of Katsuki s Wittig rearrangements...

Another example of remote control of cis/trans selectivity was provided by Kallmerten. He studied Wittig rearrangements of the tertiary oxazolinyl ethers (40 equation 14) (Table 3). ° By changing (off the pericyclic transition state) a meAyl group into MOMO the previous 72 28 (Z)-selectivity was completely inverted into >100 1 ( )-selectivity. Remarkably, the MOMO effect depends heavily on which of... 

Scheme 11 Transition states rationalizing the diastereoselectivity of Wittig rearrangements with EWG = acyl...

The [1,2]-Wittig rearrangement proceeds via a radical-pair dissociation-recombination mechanism, while the [2,3]-Wittig rearrangement is a concerted, thermally allowed sigmatropic process proceeding via an envelope-like transition state in which the substituents are pseudo-equatorial. 

Mikami, K., Nakai, T. Transition-state model for diastereoselection in [2,3]Wittig sigmatropic rearrangement. Stud. Org. Chem. (Amsterdam) 1987, 31, 153-160. 

Still-Wittig rearrangement of the nonracemic (/ )-propargylic ethers (100a) and (100b) has been found to give the (/ )-allenes (100c) and (lOOd) with complete asymmetric transfer (equation 25a). The stereochemistry is consistent with a five-membered cyclic transition state. 

Scheme 6 Envelope transition states for 2,3-Wittig rearrangements of propargyl allyl ethers...

---