Attack by Ylides Wittig Reaction

A carbanion C can form a p-d n bond (Section 3.11) with an adjacent P or S. The resulting charge delocalization is especially effective if P or S, furnishing the empty d orbital, also has a + charge. Carbanions with these characteristics are called ylides, e.g. 

The Wittig reaction uses P ylides to change O of the carbonyl group to 

Sulfur ylides react with aldehydes and ketones to form epoxides (oxiranes)  

The sulfur ylide is formed from the sulfonium salt, 

The boxed portions below come from the ylide. 

Problem 15.41 Which alkenes are formed from the following ylide-carbonyl compound pairs (a) 2-butanone and CH,CH2CH2CH=P(C H,)j. (b) acetophenone and (CftH,)3P==CH2, (c) benzaldehyde and 

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There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

If compound II is electrophilic, the involvement of the 7 carbon of VI now becomes clear. Thus, the sodium hydride generated anion V may be imagined as attacking nucleophilically carbon C-2 of activated cyclopropane II. The resulting anion is precisely the proposed phosphorous ylide postulated as structure VII by our previous fragmentation analysis. What follows then is an intramolecular Wittig reaction with the departure of triphenylphosphine oxide that was predicted by the atom budget procedure (see Scheme 17.2). 

The process involves Wittig reaction type attack by the ylide at the carbonyl group followed by preferential elimination of triphenylphosphine oxide instead of triphenylar-sine oxide. 

Triphenylarsine oxide reacts with a number of electrophilic acetylenes having electron-withdrawing substituents in what are, in effect, reverse-Wittig reactions, thereby providing stable arsonium ylides (equation 31) Reaction is presumably initiated by Michael-type reaction of the oxide with the alkyne, as exemplified in equation 32. As would be expected from such a mechanism, use of an unsymmetric alkyne, as in the foregoing example, results in virtually regiospecific attack by the oxide to give the product shown. 

Phosphorus ylides are very important because of their use in the well-known Wittig reaction (1954) for the synthesis of alkenes. In the Wittig reaction, a phosphorus ylide (1) reacts with an aldehyde or ketone to yield the corresponding alkene (16) (Scheme 7). The reaction involves nucleophilic attack by the ylide (1) on the electrophilic carbonyl carbon atom to yield the betaine intermediate, which then collapses with elimination of the phosphine oxide and formation of the alkene (16). The driving force of the Wittig reaction is the production of the very strong phosphorus-oxygen double bond in the phosphine oxide (Scheme 7). 

The Wittig reaction has been used to construct the side-chain in a synthesis of (-)-sirenin (208), a water mold sperm-attacking hormone.ii The intermediate (206) was generated, without competing formation of the structural isomer (207), by reaction of the ylide (205) with (204) under salt-free conditions in DME. Standard Wittig methods have been used to construct the side-chain in an enantioselective synthesis of (+)-(7 , 9Z)-methyl trisporate (209) and its (9E)-isomer.l 12 The reaction of triphenyl-(vinylimino)phosphorane derivatives (210) with tropones provides a convenient synthesis of 1-aza-azulenes (211) and (212) in either one or two steps (Scheme 28).H The mechanism of the one-step reaction was investigated using deuterium-labelled tropane derivatives. 

The resonance form 5a accounts for the nucleophilic character of the ylide carbon atom. The reaction between ylide and carbonyl component can be regarded as nucleophilic attack of the ylide carbon atom on the carbonyl group (see Section D for a discussion of the reaction mechanism of the Wittig reaction). The reactivity, Le. the nucleophilicity of the ylide, on the one hand is influenced to a considerable extent by the character of the substituents and R, but, on the other hand, it is also influenced by the so-called stationary substituents on the phosphorus. 

Another useful reagent in nucleophilic additions contains a carbanion that is stabilized by an adjacent, positively charged phosphorus group. Such a species is called a phosphorus ylide, and its attack on aldehydes and ketones is called the Wittig reaction. The Wittig reaction is a powerful method for the selective synthesis of alkenes from aldehydes and ketones. 

Diazomethylenephosphorane 21 can also be used as a synthetic equivalent of "naked carbon". Indeed, when a toluene solution of 21 was heated (70°C) in the presence of excess elemental sulfur, the chlorophosphine was obtained along with CS2 (88 % yield). Carbon disulfide formally results from the trapping of naked carbon by sulfur. However, it is quite clear that the first step of the reaction is the nucleophilic attack of the ylidic carbon at sulfur, as observed in the reaction of elemental sulfur with Wittig ylides. The transient adduct 22 could then eliminate the chlorophosphine, giving the thioxodiazomethane 23 which finally reacts with sulfur with loss of dinitrogen (Scheme 9). 

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