Phosphonium ylides cyclopropanation

D.K. Taylor and co-workers investigated thoroughly a new route to diastere-omerically pure functionalized cyclopropanes utilizing stabilized phosphonium ylides and y-hydroxyenones derived from 1,2-dioxines (Scheme 7) [34-38]. 

In comparison to the stabilized phosphonium ylides which fail to react with cv,/3-unsaturated esters or ketones, the corresponding arsonium ylides react smoothly with ,/3-unsaturated esters or ketones (e.g., PhCO—CH=CH—CH3) to form cyclopropanes in fair to good yields. Furthermore, the reaction is stereospecific (45, 45). 

The synthetic utility of a-phosphorus- and a-thio-stabilized carbanions is the subject of numerous reviews.21 Notable are additions of phosphonium ylides (237),183 sulfonium ylides (238),l84 ° oxosulfo-nium ylides (239)184 " and sulfoximine ylides (240)184,1 to electron-deficient alkenes which afford nucleophilic cyclopropanation products. In contrast, with a-(phenylthio)-stabilized carbanions, which are not acyl anion equivalents, either nucleophilic cyclopropanation or retention of the hetero substituent occurs, depending on the acceptor and reaction conditions used. For example, carbanion (241) adds to 1,1-... 

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... 

Ylides of other elements have been used much less commonly than sulfur ylides in cyclopropanations. Rather, other ylides are better known for their uses in other types of reactions, the best example being the use of phosphonium ylides in the Wittig reaction with carbonyl compounds to give alkenes. Nonetheless, some cases of cyclopropanations have been reported with phosphonium ylides and the related arsenic derivatives. Examples are given in Table 9. 

Interaction of 4,5 6,7-di-0-cyclohexylidene-2,3-dideoxy-l-C-phe-nyl-L-arafeino-hept-2-enose (65) with phenylmethylenetriphenylphos-phorane was accompanied9 6 by the formation of triphenylphosphine, instead of the expected triphenylphosphine oxide, thus indicating the abnormal character of this reaction. This result may be interpreted as involving possible addition of the phosphonium ylide to the alkenic bond, with subsequent stabilization of the intermediate betaine 82 through elimination of triphenylphosphine, and closure of the three-membered ring2(f) with formation of the cyclopropane derivative 83, as shown in equation 5. 

Miscellaneous Reactions of Methylenephosphoranes.—The synthesis of three-membered rings by means of phosphorus ylides has been reviewed and there have been several new reports of this type of reaction. Optically active a,p-un-saturated esters have been converted into cyclopropanes highly stereoselectively by reaction with isopropylidenetriphenylphosphorane as the key step in syntheses of both (li ,3 )-chrysanthemic acid methyl ester (53) and the (IR) cis-gem dibromovinyl analogue (54). The addition of reactive phosphonium ylides to functionalized 1,3-butadienylphosphonates provides a new synthesis of 2-cyclopen ten-1 -ylphosphonates (5 5). ... 

Cyclopropanes are formed in the condensation of epoxides with ylides derived from phosphonates. Attempts to carry out similar reactions with phosphonium ylides were generally unsuccessful, an exception being the reaction of the phosphonium halide (25) with oxiran. ... 

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). °... 

Preparation.—A historical account of the development of the synthesis of methylenephosphoranes has been given. Details have appeared - of the use of epoxides in the generation of ylides from phosphonium salts. Weakly acidic salts are deprotonated by the anions such as XCHaCH20 formed by attack of the phosphonium covmter-anion on epoxide, but strongly acidic phosphonium salts are deprotonated more rapidly than these species are produced. The base in these cases must be either epoxide or original anion. Side-reactions in olefin syntheses using epoxides as base include acetal formation from aldehyde and epoxide, cyclopropane formation from ylide and epoxide, and decomposition of quinquecovalent phosphoranes, e.g. (1), formed from phosphonium salt and... 

The unique chemistry exhibited by vinyl phosphonium salts and sulphur ylides is married in a versatile synthesis of disubstituted cyclopropanes (Scheme 1), and... 

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