Wittig reaction transition state

Several structures of the transition state have been proposed (I. D. Williams, 1984 K. A. Jorgensen, 1987 E.J. Corey, 1990 C S. Takano, 1991). They are compatible with most data, such as the observed stereoselectivity, NMR measuiements (M.O. Finn, 1983), and X-ray structures of titanium complexes with tartaric acid derivatives (I.D. Williams, 1984). The models, e. g., Jorgensen s and Corey s, are, however, not compatible with each other. One may predict that there is no single dominant Sharpless transition state (as has been found in the similar case of the Wittig reaction see p. 29f.). 

As can be seen from the data presented, the high energies of complex formation decrease sharply the endothermicity of the retro-Wittig type decomposition and, moreover, fundamentally change the reaction mechanism. As has been shown for betaines (")X-E14Me2-CH2-E15( + )Me3 (X = S, Se E14 = Si, Ge E14 = P, As), the reaction occurs as bimolecular nucleophilic substitution at the E14 atom. For silicon betaines, the transition states TS-b-pyr with pentacoordinate silicon and nearby them no deep local minima corresponding to the C-b complexes can be localized in the reaction coordinate. 

Camphor sultam derivatives have proved to be effective chiral auxiliaries in many different types of asymmetric reactions. As shown in Scheme 5-44, chiral camphor sulfam can be applied in the synthesis of (—)-pulo upone precursor 151 using an intramolecular Diels-Alder reaction. A Wittig reaction of 148 with 147 connects the chiral auxiliary to the substrate, and subsequent intramolecular Diels-Alder reaction via transition state 150 affords product 151. Compound 151 already has the stereochemistry of (—)-pulo upone 153.72... 

Subsequent monosilylation and Wittig reaction furnished unsymmetrical double diene 170. The synthesis of the other Diels-Alder partner started from bromophenol 173 (prepared in three steps from dimethoxytoluene), which was doubly metalated and reacted with (S,S)-menthylp-toluenesulfinate 173. CAN oxidation delivered quinone 171, which underwent a Diels-Alder reaction with double diene 170 to give compound 175 possessing the correct regio- and stereochemistry. Upon heating in toluene the desired elimination occurred followed by IMDA reaction to adduct 176, which was obtained in an excellent yield and enantioselectivity. Both Diels-Alder reactions proceeded through an endo transition state the enantioselectivity of the first cyclization is due to the chiral auxiliary, which favors an endo approach of 170 to the sterically less congested face (top face) (Scheme 27). 

The only example known for the formation of azetidine 82 by direct intramolecular aza-Wittig reaction is the reaction of the /3-azidoketone 81 with triphenylphosphane (Scheme 41). Attempts to transfer this reaction to 83 and 84 were not successful (87NKK1250). This failure can be attributed to the formation of intermediates with highly energetic transition states, where the rate of intramolecular attack on the carbonyl function is so slow that oligo- and polymeric compounds are preferentially formed. 

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 ... 

Stabilized ylides react with aldehydes in water to give Wittig products, sometimes with remarkable acceleration.260 For example, pentafluorobenzaldehyde reacts with ester-stabilized ylide, Ph3P=CHC02Me, at 20 °C in 5 min in 86% yield, with 99 1 E Z-selectivity. Water s ability to stabilize the polar transition state of the reaction, and its participation in the reaction (as determined by deuterium exchange), are discussed. 

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... 

In Chapter 19 you saw that anti-periplanar transition states are usually preferred for elimination reactions because this alignment provides the best opportunity for good overlap between the orbitals involved. Syn-periplanar transition states can, however, also lead to elimination—and this particular case should remind you of the Wittig reaction (Chapter 14) with a four-membered cyclic intermediate. 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

Zn(Cu)] and the Wittig-Schwarzenbach reagent [CH2N2 4- Znl2] give rise to the formation of one and the same carbenoid. Zinc chloride catalyzes the reaction (5) and an explanation in terms of ZnClg-assisted elimination of chloride ion via a transition state (X) was suggested to be most probable (546). The possibility of a two-step (addition and elimination) mechanism [Eq. (6)], 200) which was proposed for the reaction of... 

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