Olefins reagents

Triethylphosphonoacetic acid [867-13-0] (PEPA) is a useful olefination reagent for Homer-Emmons reactions in organic synthesis. 

Using allyl alcohol as an olefinic reagent, the allenyl aldehyde 37 was formed by the elimination of a /3-hydrogen atom which is adjacent to the -OH group in the intermediate 36 (Scheme 3.20) [54]. 

Carbonyl Olefination Reagents with Heavy Organoelement Groups as... 

New Carbonyl Olefination Reagents 5.1 General Remarks about Carbonyl Olefination... 

Four lithium derivatives which are accessible by element/lithium exchange (Chap. 4) triphenylgermyl- 26, triphenylstaimyl- 27, triphenylplumbyl- 28 and di-phenylstibanyl-methyllithium 25a, proved to be carbonyl olefination reagents. In the right hand side column of Table 11 the yield is given, related to compound 29. 

Bis(cyclopropyl)titanocene is prepared from cyclopropyllithium and dichloroti-tanocene in 95% yield. The complex has been utilized as an olefination reagent in reactions with a wide variety of carbonyl compounds, including esters, furnishing the corresponding methylenecyclopropane derivatives (equation 13). ... 

Apart from the Takai method and titanium reagents such as 15, silyl reagents 16 and 17 frequently find application in the synthesis of vinylic silanes from carbonyl compounds. Reagent 16 can be utilized with aldehydes and non-enolizable ketones in a reaction analogous to the Peterson olefination Reagent 17 also reacts successfully with enolizable ketones.6... 

With nitrile as one of the electron-withdrawing groups, the triazoline decomposes in situ to an aziridine that exists in equilibrium with the corresponding azomethine ylide the latter reacts with a molecule of the olefinic reagent and gives rise to a pyrrolidine (Scheme 188).319,322... 

An alternative approach that we examined involved the conceptual joining of the terminal ends of both the 2 and 6 substituents of requisite cyclohexanone 12 (Eq. 5).27 For example, known bicyclo[3.2.1]nonenone 1358 is incapable of deprotonation by the olefination reagent and indeed bis(trimethylsilyl)methyllithium55 provided pivotal vinyl silane 14 (Eq. 6). 

Scheme IS. Domino hydroformylation-Wittig olefination. Reagents and conditions 1.1 Equiv.

Coupling of Substituted Aromatic and Olefinic Reagents with Silylalkynes. . 222... 

The first progress was made by Takai and Lombardo, who developed an in situ entry to titanium-alkylidene chemistry starting from the reagent combinations 5 and 6 (Scheme 4) [9]. These reactions proceed via a gem-dizinc compound 7 (its formation is catalyzed by traces of lead or lead(II) salts), which is subsequently transmetalated with TiCl4 to the titanium-alkylidene species 8, the actual olefination reagent. To date, 8 has not been characterized in detail [10]. These in situ reagents exhibit chemoselectivities similar to those of the structurally defined methylenation reagents 1-3. 

Quaternary phosphonium salts are organophosphorous compounds used as Wittig olefination reagents, phase transfer catalysts, electrolytes, ionic liquids, and as surface active reagents. Their preparation involves the C-P bond formation in tertiary phosphines. We envisaged that addition of phosphines to unsaturated compounds should be preferable as compared to the conventional method using a substitution reaction of organohalogen compounds (Scheme 1). In this chapter, we describe our recent study on this subject. 

Yields are highest in the case of aromatic aldehydes and ketones application of the reaction to butanone-2, for example, gives only a 10 %, yield of olefin. Reagent I is more effective than the other tungsten reagents (II-VI) for the coupling reaction. 

The new tungsten reagents (1-Vl) are also effective for deoxygenation of epoxides to the parent olefin. Reagents V and Vl are the cheapest and most convenient reagents for this purpose however, reagents l-lll serve well for small- and medium-scale reactions. In a typical experiment WCl (0.15 mole) is added to THF (420 ml.) cooled to —62° n-butyllithium (0..30 mole) is added slowly at this temperature. The mixture is allowed to warm slowly to room temperature. The oxide (0.08 mole) is added. A rapid exothermic reduction takes place. After 0.5 hr. the product is poured into an aqueous solution containing sodium tartrate and NaOH. and the olefin extracted with hexane. 

An interesting application of this method is the preparation of (0,P)- and N,P)-acetals, not otherwise an easy process. Treatment of an aldehyde with diethyl hydrogen phosphite in the presence of TiCU affords the (a-hydroxy) phosphonate in good yield (Eq. 235) [539,540]. Alternatively, the methoxy group of an (N,0)-acetal was replaced with a phosphonate group via the reaction of triethyl phosphite and TiCU (Eq. 236) [541], In the latter reaction the resultant phosphonate was subsequently used as an olefination reagent. 

Dendralene-type TTF vinylogues 837-842 containing tellurium or selenium atoms were obtained by condensation of the 2-diformylmethylene-l,3-diselenole or -1,3-ditellurole with phosphorus olefination reagents 834-836 under basic conditions (Scheme 126) <2000CEJ1955, 2001JOC7757, 20020L2581>. 

The photolysis and pyrolysis of difluorodiazirine has been shown to involve difluoromethylene and stereospecific addition to 2-butene indicates that it is probably in a singlet state. In argon or nitrogen matrices, Cp2 has been detected by spectroscopic measurements . Products arising from the thermolysis or photolysis of fluoromethoxydiazirine, cyanofluorodiazirine, difluoroaminodiazirine and chlorofluorodiazirine have been reported the main feature of these reactions is that the intermediate carbene in all cases can be trapped by olefin reagents, whereas fluorocarbenes from other sources are quite unreactive. 

After addition of HF to the chlorinated olefinic reagent, the reaction can proceed through successive Cl/F halogen exchanges or by formation of olefinic intermediate by elimination of HCI and successive hydrofluorination studies carried out with use of DF have allowed to discover that the first pathway is prevalent [6]. The outline of reaction (1) is very simplified a series of isomerization and disproportionation reactions are known [7] but these can be limited using appropriate operating conditions and adapted catalysts. 

Arsonium bromides are useful olefination reagents employed for condensation with a,p-unsaturated aldehydes, using potassium carbonate... 

Various photochemical (2 + 2)-cycloadditions of heteroaromatic compounds have been reported in which an enone moiety is incorporated either into the olefinic reagent or into the heteroaromatic compound. Both furan and thiophene have been found to give cycloaddition reactions with maleic anhydride derivatives in the presence of a sensitizer.202 213 The cycloadducts (185 and 186) were formed in high yield, but in the case of 2,5-dimethylthiophene, cyclobutane formation was the minor pathway, as oxetane formation predominated.210 Cyclic enones, such as 2-cyclopenten-l-one and 2-cyclohexen-l-one reacted with furan to afford mixtures of (2 + 2)-cycloadducts (187a, R = H) and (188),... 

Cyclic phosphonamide a-carbanions are excellent olefination reagents particularly useful for enolizable carbonyl compounds. P-stabilized allyl anions have found application as Michael donors. " ... 

Other titanium-based olefination reagents have been developed. Eisch used a zinc analog of the Tebbe reagent (688) in a reaction with benzophenone to give 1,1-diphenylethene in 78% yield. Similarly, Clawson et al. used 689 in olefination reactions with ketones and aldehydes. Alkoxytitanium reagents such as 690 have been employed, as in the conversion of cyclohexane carboxaldehyde to 1-cyclohexyl-1,3-butadiene (691), in 86% yield.In this olefination reaction, the (Z)-isomer predominated over the ( ) (96 4 Z/E). 

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