Ylide

Summary

An ylide or ylid (ˈɪlɪd) is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons. The result can be viewed as a structure in which two adjacent atoms are connected by both a covalent and an ionic bond; normally written X+–Y−. Ylides are thus 1,2-dipolar compounds, and a subclass of zwitterions. They appear in organic chemistry as reagents or reactive intermediates. The class name "ylide" for the compound should not be confused with the suffix "-ylide". Resonance structures Many ylides may be depicted by a multiple bond form in a resonance structure, known as the ylene form, while the actual structure lies in between both forms: : The actual bonding picture of these types of ylides is strictly zwitterionic (the structure on the right) with the strong Coulombic attraction betw

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https://en.wikipedia.org/wiki/Ylide

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Mixts. prepd. from alkyltriphenylphosphonium bromides and powd. NaNH2 were stored indefinitely. Upon addn. of an ethereal solid, however, they immediately generated the ylide, which then reacted with aldehydes, ketones, and formates. The use of such instant ylide mixts. offered distinct advantages over conventional Wittig procedures: optimum yields (even with enolizable ketones), highest cis/trans ratios, largest range of applicability, shortest reaction times, and easiest handling. Thus, condensation of Ph3:CHEt with BzH gave 94% (Z)-PhCH:CHEt and 6% (E)-PhCH:CHEt. Among the approx. 30 compds. prepd. were cis- and trans-stilbenes, methylenecyclohexane, and cis- and trans-BuOCH:CHEt. [on SciFinder (R)]

1982

Using 13C-chem. shifts as a probe for the electronic environment of carbon centers, triphenylphosphoniomethanide, the model case of a reactive phosphorus ylide, have much more zwitterion than PC double bond character. Triphenylphosphoniopropenide (triphenylphosphonium-allyide), a moderated ylide, accumulates electron excess mainly at the a-carbon atom, whereas triphenylphosphonioethenoate (formyl-methylenetriphenylphosphorane) and other stabilized ylides carry roughly equal fractions of neg. charge at the a-carbon and the oxygen atom. The one-bond C,H coupling const. of triphenylphosphoniomethanide argues against a perfectly planar ylide center. The three-bond P,C coupling consts. permit the assignment of endo- or exo-configurations to ylides having an allyl-type side-chain. [on SciFinder (R)]

1990

Unstabilized P ylides and their betaines added Li salts, losing most of their reactivity, and were reactivated with tert-BuOK, making thus possible the retardation or the acceleration of the Wittig reaction. The C:O olefination often yielded an isomer mixt. contg. .apprx.1:1 cis- and trans-olefins, due to 2 opposite effects. P ylides were condensed with aldehydes, under kinetic control, preferentially into erythro-betaines, and yielded, when the betaine formation was rendered irreversible, mostly cis-olefins. On the other hand, the threo-betaine was thermodynamically more stable, so that the rapid attainment of diastereomeric equil. between erythro- and threo-betaine led mostly to trans-olefins. Preparative methods of the cis or transselective C:O olefination are described. [on SciFinder (R)]

1967