Butyllithium preparing ylides with

Carbodiphosphoranes (R3P = C = PR3) are known,79 but ylides with a P-H bond are rare.80 Therefore, the spectroscopic characterization of 77 was unexpected. Even more surprising was the characterization of the carbodiphosphorane 79 featuring two P-H bonds.31 This compound, prepared by treatment of 2d with tert-butyllithium, rearranged in solution at room temperature over a period of 16 h to afford the phosphorus ylide 80 with one remaining P-H bond. This compound was also unstable and transformed completely into the diphosphinomethane 81 overnight. Note that calculations for the model compounds where R = NH2 predicted 79 to be 28 kcal/mol less stable than 80, which is also 34 kcal/mol above 81.16 The surprising stability of 79 and 80 is probably due to the presence of bulky substituents, since tetracoordinate phosphorus atoms can more readily accommodate the increased steric constraints than can their tricoordinate counterparts. 

Complexes containing the ylide ligand can be also obtained by reaction of the yellow solution of ylides (prepared by addition of butyllithium to tetrahydrofurane or diethylether solutions of the phosphonium salt) with [AulCfiFs) (tht)]. The weak ligand tht is replaced and the ylide complex is obtained (Equation 3.5) [70]. 

A series of conjugated polyenes capped with chromophores and containing an androstane spacer were synthesized by Wittig or Wittig-type olefinations from epi-androsterone 5150. For example, vinyl carboxaldehyde 52, prepared from 51 in 60% yield as shown in equation 32, was treated with 9-anthrylmethylphosphonium bromide and n-butyllithium to give diene 53. Exocyclic diene 53 was subsequently oxidized to vinyl carboxaldehyde 54. The androsterone vinyl aldehyde intermediate could either be treated with a tetraphenylporphyrinpolyenyl phosphonium ylide, or, as shown below, the phosphonium salt of the androsterone (55) could be reacted with TPP polyeneal 56. The desired all-(E) isomer, 57, was obtained from the ( )/(Z)-isomeric mixture by chromatographic purification. 

The overall sequence of three steps may be called the Wittig reaction, or only the final step. Phosphonium salts are also prepared by addition of phosphines to Michael olefins (like 5-7) and in other ways. The phosphonium salts are most often converted to the ylides by treatment with a strong base such as butyllithium, sodium amide,640 sodium hydride, or a sodium alkoxide, though weaker bases can be used if the salt is acidic enough. For (PhjP CHj, sodium carbonate is a strong enough base.641 When the base used does not contain lithium, the ylide is said to be prepared under "salt-free conditions.642... 

Fluonnated ylides have also been prepared in such a way that fluorine is incorporated at the carbon p to the carbamomc carbon Various fluoroalkyl iodides were heated with tnphenylphosphine in the absence of solvent to form the necessary phosphonium salts Direct deprotonation with butyllithium or lithium dusopropy-lamide did not lead to y hde formation, rather, deprotonation was accompanied by loss of fluonde ion However deprotonation with hydrated potassium carbonate in dioxane was successful and resulted in fluoroolefin yields of45-80% [59] (equation 54) p-Fluorinated ylides may also be prepared by the reaction of an isopropyli-denetnphenylphosphine yhde with a perfluoroalkanoyl anhydride The intermediate acyl phosphonium salt can undergo further reaction with methylene tnphenylphosphorane and phenyllithium to form a new ylide, which can then be used in a Wittig olefination procedure [60] (equation 55) or can react with a nucleophile [61] such as an acetylide to form a fluonnated enyne [62] (equation 56)... 

The ylide is prepared by deprotonating a triphenylalkylphosphonium salt with a strong base, commonly an organometallic base such as butyllithium or phenyllithium. The hydrogens on the carbon that is bonded to the phosphorus of the salt are somewhat acidic because the carbanion of the conjugate base (the ylide) is stabilized by the inductive effect of the positive phosphorus atom. In addition, a resonance structure with five bonds to phosphorus makes a minor contribution to the structure and provides some additional stabilization. The triphenylalkylphosphonium salt can be prepared by an SN2 reaction of triphenylphosphine with the appropriate alkyl halide (see Section 10.9). 

The reaction of trialkyl- and triaryl-phosphines with 1,3-benzodithiolylium salts leads to formation of phosphonium salts which are deprotonated by treatment with n-butyllithium to produce (69). The similar reaction of trialkyl phosphites in the presence of sodium iodide yields dialkyl phosphonates which can be deprotonated to (70). Both (69) and (70) can react further with ketones to give the 2-alkylidene-l,3-dithiole derivatives (71) (80AHC(27)151>. The ylide (72) has also been prepared (80H(l4)27l>. 

The phosphorus-stabilized carbanion is an ylide (pronounced ilL-id )—a molecule that bears no overall charge but has a negatively charged carbon atom bonded to a positively charged heteroatom. Phosphorus ylides are prepared from tri-phenylphosphine and alkyl halides in a two-step process. The first step is nucleophilic attack by triphenylphosphine on an unhindered (usually primary) alkyl halide. The product is an alkyltriphenylphosphonium salt. The phosphonium salt is treated with a strong base (usually butyllithium) to abstract a proton from the carbon atom bonded to phosphorus. 

CuCl-catalyzed decomposition of iodonium ylides prepared from -keto esters and diacetoxyiodobenzene, has been developed (equation 151). 1-Methylbenzvalene is obtained in a good yield by treating a mixture of lithium cyclopentadienide and 1,1-dichloroethane with butyllithium l The tandem cyclization substitution in l-selenyl-5-hexenyllithiums derived from corresponding selenacetals via selenium/lithium exchange produces bicyclo[3.1.0]hexane derivatives ... 

Phosphorus ylides are prepared from phosphonium salts by deprotonating them with a strong base. The method consists of the alkylation of triphenylphosphine with alkyl halide. The resulting phosphonium salt is treated with a strong base (phenyUithium or n-butyllithium) to give a phosphorus ylide. The simplest ylide is methylenetriphenylphos-phorane (3.50), which is prepared by the abstraction of a proton from methyltriphenylphos-phonium iodide. 

Ylide 33 has been prepared by deprotonation of dibenzyldiphenylarsonium bromide with butyllithium ". ... 

TTF can also be prepared by the elimination of a proton and of triphenylphosphine in the final step. Thus, 1,3-dithiolylium salts (402) react with 2-triphenylphosphino-l,3-dithioles to afford intermediates which give TTF (403) upon treatment with base at low temperatures (Scheme 76). This method also permits the selective preparation of unsymmetrical TTF <78JOC369,82TLl8l3,83Mi 312-02). The phosphorus ylides used in these reactions are easily accessible by the reaction at — 78°C between -butyllithium and the corresponding phosphonium salt which is obtained by the reaction of triphenylphosphane with 1,3-dithiolylium salts <78JOC369>. 

The carbanion [MejSi—CH ] is isoelectronic with the phosphorus ylide MejP—CHj. The lithium derivative of the anion MOjSiCH can be prepared by treating tetramethylsilane with butyllithium/TMEDA at — 78°C in tetrahydrofuran. 

The customary generalization is that unstabilized ylides react with aldehydes to give predominantly Z-alkenes (cis), while stabilized ylides give predominantly E-alkenes trans) Unstabilized ylides include the typical alkylidenetriphenylphos-phoranes shown in entries 1-5 of Scheme 2.8. Their tendency to form Z-alkenes can be very high, especially when conditions are chosen so that lithium salts are not present in the reaction medium. Sodium amide (entry 3) and the sodium salt of hexamethyldisilazane (entry 5) are convenient strong bases for the preparation of salt-free solutions of ylides. When ethylidenetriphenylphosphorane is prepared from ethyltriphenylphosphonium iodide and n-butyllithium (entry 4), the high... 

---