Stabilized ylide

With "non-stabilized" ylides the Wittig Reaction gives predominantly Z-olefins. Seebach et al... 

Accumulating evidence makes it increasingly clear that there is no single dominant Wittig transition state geometry and, therefore, no simple scheme to explain cis/trans selec-tivities. The conventional betaine pathway may not occur at all, the stabilized ylides, e,g., PhsP—CH —C02Et, can be ( )- or (Z)-selective, depending on the solvent and substrate (E. Vedejs, 1988 A, B, 1990). 

In addition, NaOMe, and NaNH2, have also been employed. Applieation of phase-transfer conditions with tetra-n-butylammonium iodide showed marked improvement for the epoxide formation. Furthermore, many complex substituted sulfur ylides have been synthesized and utilized. For instance, stabilized ylide 20 was prepared and treated with a-D-a/lo-pyranoside 19 to furnish a-D-cyclopropanyl-pyranoside 21. Other examples of substituted sulfur ylides include 22-25, among which aminosulfoxonium ylide 25, sometimes known as Johnson s ylide, belongs to another category. The aminosulfoxonium ylides possess the configurational stability and thermal stability not enjoyed by the sulfonium and sulfoxonium ylides, thereby are more suitable for asymmetric synthesis. 

As the formation of betaines from amide-stabilized ylides is known to be reversible (in contrast with aryl- or semistabilized ylides, which can exhibit irreversible anti betaine formation see Section 1.2.1.3), the enantiodifferentiating step cannot be the C-C bond-forming step. B3LYP calculations of the individual steps along the reaction pathway have shown that in this instance ring-closure has the highest barrier and is most likely to be the enantiodifferentiating step of the reaction (Scheme 1.16) [25]. 

Recently, several one-pot oxidation-Wittig procedures that circumvent the need to isolate the intermediate aldehydes have been developed. Various oxidants, including Swern [52], Dess-Martin periodinane [53], IBX [54], Mn02 [55], and BaMnCU [56], can be used in the presence of stabilized ylides to generate a,(3-unsaturated esters. 

Because of the strongly basic conditions needed for Wittig reactions with now-stabilized ylides, one-pot reactions have not been carried out [57]. Ley s recent report of a TPAP-Wittig oxidation performed in a sequential one-pot manner is thus a promising alternative [58]. The reaction is fast and straightforward the... 

Photolysis of alkoxycarbene complexes in the presence of stabilized ylides produced allenes having a donating group at one terminus and an accepting group at the other. These were highly reactive and rearranged to 1,3-dienes under mildly acidic conditions and hydrolyzed to y-keto-a,/J-unsaturated esters (Eq.31) [117]. 

Sulfur-stabilized ylides underwent photodriven reaction with chromium alkoxy-carbenes to produce 2-acyl vinyl ethers as E/Z mixtures with the E isomer predominating (Table 22) [ 121-123]. The reaction is thought to proceed by nucleophilic attack of the ylide carbon at the chromium carbene carbon followed by elimination of (CO)5CrSMe2. The same reaction occurred thermally, but at a reduced rate. Sulfilimines underwent a similar addition/elimination process to produce imidates or their hydrolysis products (Table 23) [ 124,125]. Again the reaction also proceeded thermally but much more slowly. Less basic sulfilimines having acyl or sulfonyl groups on nitrogen failed to react. 

Table 22 Photo-driven reaction of sulfur-stabilized ylides with alkoxycarbenes...

The premier example of this process in an ylide transformation designed for [2,3]-sigmatropic rearrangement is reported in Eq. 15 [107]. The threo product 47 is dominant with the use of the chiral Rh2(MEOX)4 catalysts but is the minor product with Rh2(OAc)4. That this process occurs through the metal-stabilized ylide rather than a chiral free ylide was shown from asymmetric induction using allyl iodide and ethyl diazoacetate [107]. Somewhat lower enantioselectivities have been observed in other systems [108]. 

This review concerns in the first part the works published during the last three years on the synthesis and reactivity of stabilized ylides C-substituted by electron-withdrawing groups (COR, CO2R, CN, etc.). The second part deals with the works published in the same period on the chemistry of phosphorus ylides mainly C-substituted by heteroatoms of groups 1-16 (metals, metalloids and nonmetal elements Li, Ba, Ca, Ti, Zr, Nb, Mo, Re, Fe, Ru, Rh, Pd, Pt, Au, Zn, Hg, B, Si, Sn, N, P, As, Sb, O, S, Te). 

Triphenylphosphine gives Michael additions to the activated triple bond of acetylene dicarboxylic esters in presence of acidic compounds HY (Scheme 1). The reactions take place easily at room temperature, even at -10°C [1], through formation of intermediate activated vinylic phosphonium salts, which undergo a subsequent Michael addition of HY. The reactions afford various stabilized ylides which can be isolated in high yields or undergo possibly evolution, for example by intramolecular Wittig reaction [2]. 

Among various modifications of the side chain of stabilized ylides [13,14] can be pointed out the preparation and transformation of the phenyliodonio a-substi-tuted phosphonium yhdes (Scheme 3) [15]. These compounds represent a potentially useful class of reagents, in which the iodonium group can be further substituted by nucleophiles such as PhSLi. 

The thermolysis of various substituted phosphonium ylides between 600 °C and 900 °C can afford either substituted alkynes [16,25,27] or cyclic dienes [20] by extrusion of PhjPO, or new stabilized ylides by cyclization of the functional groups [27,28]. 

Particularly interesting are the results obtained with the phosphonium ylides including an acyl rest derived from aminoacid if the N-H bond reactivity is blocked by an amide protection, the alkyne formation takes place [25,27], but if the N-H bond is not deactivated, an intramolecular cyclization occurs to give a new stabilized ylide [27,28]. 

In the case of alkenes simply substituted by an electron-withdrawing group (without a y-hydroxy group), the stabilized ylides give first a Michael addition and most often a subsequent prototropic shift resulting in new functionalized ylides (Scheme 8). Then a possible evolution of the resulting ylides can occur to give the final products [40-44]. 

A review concerning the coordination and organometallic chemistry of P- and As-carbonyl stabilized ylides was published in1998. The references quoted inside, being previous to the period concerned by our article, are not detailed here but they represent a very interesting basis to the results described below [66]. 

The diamagnetic ylide complexes 34 have been obtained from the reaction of electron-deficient complexes [MoH(SR)3(PMePh2)] and alkynes (HC=CTol for the scheme), via the formal insertion of the latter into the Mo-P bond. The structural data show that 34 corresponds to two different resonance-stabilized ylides forms 34a (a-vinyl form) and 34b (carbene ylide form) (Scheme 17) [73]. Concerning the group 7 recent examples of cis ylide rhenium complexes 36 cis-Me-Re-Me) have been reported from the reaction of the corresponding trans cationic alkyne derivatives 35 with PR" via a nucleophilic attack of this phosphine at the alkyne carbon. 

Carbonyl-stabilized ylides and the cyano-stabiHzed ylides Ar3P=CHCN in spite of their low nucleophilicity are able to give stable complexes. By comparison, very few complexes involving the second category of ylides are known. However a new example has recently been described by reaction of the cyano-ylide 60 with palladium(II) to give the complex tra s-[PdCl2 CH(PTol3)CN 2] 61 (Scheme 24) [93]. 

Works developed by Navarro and coworkers for the palladium and stabilized ylides complexes have been extended to the platinum chemistry (Scheme 25) [97,98]. 

Related platinum compounds such as 69 have thus been synthesized from a-keto or cyano-stabilized ylides. A selective C-coordination trans to the P phosphorus atom of the ligand is observed, probably resulting from steric interactions. 

The compound 70 has also been reported showing the ambident character (both C- and N-coordination) of the cyano-stabilized ylide as ligand. The authors have also transposed their work concerning the keto-bis-ylide and palladium, with the synthesis of the C-bonded complex 71 or the new cycloplatinated or-thometallated compound 72. The latter by various treatments allows one to obtain other ylidic cationic complexes of platinum such as 73. A C,C,C-terdentate coordination of the keto bis-ylide, already observed with the palladium is also obtained from the reaction of 73 with gold derivatives. 

In Sect. 2.2.1 we described the thermolysis of various /1-oxophosphorus yUdes affording either substituted alkynes or cyclic dienes by extrusion of PhjPO, or new stabilized ylides. 

Starting from sulfmyl stabilized ylides 126, a loss of PhjP was also observed to give the intermediate formation of the corresponding sulfinyl carbene 128. Via a 1,2-oxygen transfer in the latter, thioesters 130 were this time obtained [130]. 

The cycloaddition of alkynes with the tributylphosphine-carbondisulfide adduct 131 results in the in situ formation of the ylides 132 which react with aldehydes to give the novel 2-arylidene or 2-alkylidene-l,3-dithioles 133 (Scheme 36) [132]. Concerning ylides C-substituted by sulfur we can also mention a publication on the behavior of various keto-stabilized ylides towards acyclic and cyclic a s-disulfides allowing the synthesis of substituted thiazoles, thiols, and dithiols [133]. 

The reaction of ylides with phosphorus(iii) halides has been extended to the ylides (Me2N) Me3 P CH2, = 1, 2, or 3. Alkylation of the resulting stabilized ylides (20) with methyl iodide took place on the tervalent phosphorus, e.g. 

D. Restricted Rotation.—A study on solvent and stereochemical effects on the restricted rotation of the stabilized ylides (35) has shown that although the cisoid (Z) conformation (35a) is generally predominant there is an increase in the amount of the transoid ( ) conformation (35b) as the size... 

Alkyltriphenylphosphonium halides are only weakly acidic, and a strong base must be used for deprotonation. Possibilities include organolithium reagents, the anion of dimethyl sulfoxide, and amide ion or substituted amide anions, such as LDA or NaHMDS. The ylides are not normally isolated, so the reaction is carried out either with the carbonyl compound present or with it added immediately after ylide formation. Ylides with nonpolar substituents, e.g., R = H, alkyl, aryl, are quite reactive toward both ketones and aldehydes. Ylides having an a-EWG substituent, such as alkoxycarbonyl or acyl, are less reactive and are called stabilized ylides. 

Ketophosphonium salts are considerably more acidic than alkylphosphonium salts and can be converted to ylides by relatively weak bases. The resulting ylides, which are stabilized by the carbonyl group, are substantially less reactive than unfunctionalized ylides. More vigorous conditions are required to bring about reactions with ketones. Stabilized ylides such as (carboethoxymethylidene)triphenylphosphorane (Entries 8 and 9) react with aldehydes to give exclusively trans double bonds. 

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