Allylic phosphonium ylides

Axelrod et alP have describe the synthesis of 1,5-dienes by alkylation of an allylic phosphonium ylide with an allylic bromide. The method was used to synthesize pure all-( )-squalene from ( , )-fame-syl bromide and its tributylphosphonium salt. No isomerization occurred during the reaction and reduction of the resulting phosphonium salt with lithium in ethylamine gave squalene in 65% yield. 

The enantioselective 3 + 3-cycloaddition reaction of aromatic azomethine imines with donor-acceptor cyclopropane diesters produced 6,6,6-tricyclic dihydroquinolines in high yields (99%) and up to 98% ee. The side-arm-modified In-TOX/Ni(II) complex was shown to be an effective stereoselective catalyst for this reaction. The formal 3 + 3-cycloaddition reaction of allylic phosphonium ylides (127) to a, -unsaturated carbonyls (128) formed multi-substituted benzenes (129) in high yields (36-93%) (Scheme 38). °... 

A few functionalized allylic phosphonium ylides are generated by the nucleophilic attack of phosphines on certain allenoates followed by a set of proton transfers. Taking advantage of this chemistry, He et al. have developed a triarylphosphine mediated olefination of aldehydes with y-substituted allenoates to afford trisubstituted conjugated dienes in moderate to excellent yields and with high E selectivity (Scheme 25) [119]. Similarly, the reaction of aldehydes with a-substimted allenoates has been developed in the presence of tributylphosphine to afford polysubstimted conjugated dienes (Scheme 26) [120, 121]. 

Allylic phosphonium ylides can also be generated in situ by the nucleophilic attack of phosphines on aziridines (or epoxides) followed by a set of proton transfers. On the basis of these reaction pathways, Hou et al. have realized a slightly E selective synthesis of conjugated dienes from aldehydes (or ketones) and aziridines (or epoxides) in the presence of tributylphosphine (Scheme 27) [122]. 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

Allylic alkylation of 3-acetoxy-l,3-diphenylpropene by sodium dimethytmalonate, catalysed by the Pd-allyl complex 115, bearing the non-symmetric phosphonium ylide NHC ligand (5 mol%), proceeds to completion with 100% regioselectivity. 

A limited number of other anionic species have been employed as Michael donors in tandem vicinal difunctionalizations. In a manner similar to sulfur ylides described above, phosphonium ylides can be used as cyclopropanating reagents by means of a conjugate addition-a-intramolecular alkylation sequence. Phosphonium ylides have been used with greater frequency261-263 than sulfur ylides and display little steric sensitivity.264 Phosphorus-stabilized allylic anions can display regiospecific 7-1,4-addition when used as Michael donors.265... 

As a general rule, ylides with allylic or benzylic functionality do not proceed with a high degree of stereoselectivity. There have been recent examples of arachidonic acid derivatives in which the coupling of an allylic phosphonium salt with an unsaturated aldehyde resulted in (Z)-selective alkenation. As in the case of nonstabilized ylides, replacing aromatic phosphorus substituents with allylic (113 equation 25) or alkyl (115 equation 26) groups dramaticaUy increases the production of the (, -al-kene. ... 

Treatment of P-oxido ylides with electrophiles other than proton donors provides a route to stereospecific trisubstituted alkenes. For example, trapping the P-oxido phosphonium ylide B with formaldehyde (generated from paraformaldehyde) leads to dioxido phosphonium derivative D to yield, after elimination of triphenylphosphine oxide, the trisubstituted allylic alcohol... 

Sato has reported a new route to allylic sulphides through the reaction of a-sulphenyl aldehydes (35) with phosphonium ylides or phosphonate carbanions.25 Similar reactions have been used to achieve a two-carbon... 

Phosphonium ylide, generated from allyl triphenylphosphonium chloride, is capable of addition to 1-nitronaphthalene or 5-nitro-8-methoxyquinoline in the presence of DBU and titanium tetraisopropoxide to form unstable A -hydroxyindole derivative, which is transformed by action of ethyl bromoacetate into benzo- or pyridoindoles (Scheme 80) [200]. 

---