Ester stability

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

Enolate Reactivity at the Anomeric Site Based on Ester Stabilization... 

It is also appropriate to recognize the role of enolates stabilized by an exocyclic carbonyl function in C-glycoside synthesis. The use of LN to achieve reductive cleavage of anomeric sulfones to provide access to an ester-stabilized enolate and, ultimately, 2-deoxy- -C-glycosides has already been illustrated in Scheme 11 (Sect. 2.1.1). 

One interesting aspect of the polymerization of styrene is its possible pseudoca-tionic propagation. It was observed, for example, that in the presence of HC104 styrene forms only oligomers and no real cationic species could be found.160 161 This led to the suggestion that the initiating species is a perchlorate ester stabilized by monomer molecules ... 

A. Guerrero, P. Partal, and C. Gallegos, Linear viscoelastic properties of sucrose ester-stabilized oil-in-water emulsions, J. Rheol., 42 (1998) 1375-1388. 

The formation of ester-stabilized organozinc reagents and their addition to carbonyl compounds... 

An ester-stabilized organozinc reagent RING CLOSING METATHESIS (RCM)... 

Stabilized ylides react with aldehydes in water to give Wittig products, sometimes with remarkable acceleration.260 For example, pentafluorobenzaldehyde reacts with ester-stabilized ylide, Ph3P=CHC02Me, at 20 °C in 5 min in 86% yield, with 99 1 E Z-selectivity. Water s ability to stabilize the polar transition state of the reaction, and its participation in the reaction (as determined by deuterium exchange), are discussed. 

To explain the enantioselectivity obtained with semi-stabilized ylides (e.g., benzyl-substituted ylides), the same factors as for the epoxidation reactions discussed earlier should be considered (see Section 10.2.1.10). The enantioselectivity is controlled in the initial, non-reversible, betaine formation step. As before, controlling which lone pair reacts with the metallocarbene and which conformer of the ylide forms are the first two requirements. The transition state for antibetaine formation arises via a head-on or cisoid approach and, as in epoxidation, face selectivity is well controlled. The syn-betaine is predicted to be formed via a head-to-tail or transoid approach in which Coulombic interactions play no part. Enantioselectivity in cis-aziridine formation was more varied. Formation of the minor enantiomer in both cases is attributed to a lack of complete control of the conformation of the ylide rather than to poor facial control for imine approach. For stabilized ylides (e.g., ester-stabilized ylides), the enantioselectivity is controlled in the ring-closure step and moderate enantioselectivities have been achieved thus far. Due to differences in the stereocontrolling step for different types of ylides, it is likely that different sulfides will need to be designed to achieve high stereocontrol for the different types of ylides. 

As indicated in Scheme VII/32, cyclononanone (VII/165) is transformed into hydroperoxide hemiacetal, VII/167, which is isolated as a mixture of stereoisomers. The addition of Fe(II)S04 to a solution of VII/167 in methanol saturated with Cu(OAc)2 gave ( )-recifeiolide (VII/171) in quantitative yield. No isomeric olefins were detected. In the first step of the proposed mechanism, an electron from Fe2+ is transferred to the peroxide to form the oxy radical VII/168. The central C,C-bond is weakened by antiperiplanar overlap with the lone pair on the ether oxygen. Cleavage of this bond leads to the secondary carbon radical VII/169, which yields, by an oxidative coupling with Cu(OAc)2, the alkyl copper intermediate VII/170. If we assume that the alkyl copper intermediate, VII/170, exists (a) as a (Z)-ester, stabilized by n (ether O) —> <7 (C=0) overlap (anomeric effect), and (b) is internally coordinated by the ester to form a pseudo-six-membered ring, then only one of the four -hydrogens is available for a syn-//-elimination. [111]. This reaction principle has been used in other macrolide syntheses, too [112] [113]. 

Evidence of complexing was obtained with the anomeric methyl 3,6-anhydro-D-glucopyranosides (see Table II), which suggests that the oxygen atoms on the pyranoid ring can play a role in ester stabilization (45) but, with l,6-anhydro-/3-D-glucopyranose (46), no complex was... 

The substituent effect data indicates that the charge polarity in the vinyl ether transition states is less than in the corresponding ester elimination transition states. The activation energies (corrected) naturally parallel the rate coefficient variations and show a systematic decrease of 2.3 to 2.4 kcal.mole for successive methyl substitutions for hydrogen at the a-carbon. Corrected for gauche destabilizations of the ether ground states, this indicates a transition state charge stabilization by methyl of about 1.6 kcal per CH3. Ester stabilizations have previously been estimated to be about 3.3 kcal per CH3. 

According to Eq. (11), Confalone has presented a new method of generation of azomethine ylides through the condensation of N-substituted ot-amino esters with aldehydes (83JOC2994 84JA7175). Thus, 5-formylmethyldibenzo-[a.d]tropylidene or o-(allyloxy)benzaldehyde is heated with ethyl sarcosinate or methyl prolinate under reflux in toluene. The water formed is continuously driven off with the aid of a Dean-Stark trap. The ester-stabilized azomethine ylide 77 or 78 quantitatively generated is trapped in an intramolecular fashion. 

N-Metallated azomethine ylides 140 of ester-stabilized types are tautomeric to the metal ester enolates (141) of chelate-stabilized types. The only structural difference is which heteroatom between the imine nitrogen and the ester carbonyl oxygen is connected with the metal (M) by a covalent bond. The difference in chemical properties expected for the ylidic forms 140 and enolate forms 141 is not yet clear. 

Ester-stabilized azomethine ylides 76 (R = MeO, EWG = COOR ) show a high reactivity to imines such as 2-phenyl-1-azirine and Af-benzylidene-methylamine, cycloadduct 220 being obtained in the former case (82LA2146). 

Azomethine ylides 221 of the ester-stabilized type can be generated by ring opening of the corresponding ring-fused aziridines and can cycloadd to 2-aryl-1-azirines in a stereoselective manner to give 222 (86JCS(P1)1119). 

The N-metallated azomethine ylides having a wider synthetic potential are N-lithiated ylides 141, derived from the imines of a-amino esters, lithium bromide, and triethylamine, and 144 from the imines of a-aminonitriles and LDA (Section II,G). Ester-stabilized ylides 144 undergo regio- and endo-selective cycloadditions, at room temperature, to a wide variety of unsym-metrically substituted olefins bearing a carbonyl-activating substituent, such as methyl acrylate, crotonate, cinnamate, methacrylate, 3-buten-2-one, ( )-3-penten-2-one, ( )-4-phenyl-3-buten-2-one, and ( )-l-(p-tolyl)-3-phenyl-propenone, to give excellent yields of cycloadducts 142 (88JOC1384). 

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