Electron-poor strained olefins

Living copolymerization of the electron-poor strained olefins diethyl bicyclo [2.2.1]hepta-2,5-diene-2,3-dicarboxylate and diethyl 7-oxabicyclo[2.2.1]hepta-2,5-di-ene-2,3-dicarboxylate was achieved using palladium(II) neutral initiators such as iodo(endo-6-phenyl-2-norbornene- ndo-5s,2p)(triphenylphosphine)palladium(II)... 

Whereas C-H bond cleavage has been used only rarely, e.g. in adamantane [103], C-C bond rupture is more common, but only in strained carbocycles [104-106]. C-N bond cleavage is observed in azoalkane + decomposition [106d], although little synthetic use has been elaborated [107]. In contrast, C-Sn bond cleavage in alkyl-staimanes has been developed into a novel alkylation of a, -unsaturated ketones [108] and a versatile alkylation of electron-poor olefins [109]. Oxidative Si-Si [110] and Ge Ge bond cleavage [102] have also found use. 

Best efficiencies under the standard quasi-catalytic PK conditions are gained with electron-poor alkynes, strained olefins (e. g., norbomene), and open-chain olefins with low substitution (e. g., ethylene). Ethylene has the advantage that it can be applied in large excess by pressurizing the reaction system. Equations (13) and (14) are standard examples. 

As outlined earlier, the reaction is currently hmited to heterodimerization of ethylene with vinylarenes, cychc dienes, or strained olefins such as norbornene. Among other a-olefins, propene (yields >90%) alone has been shown to participate in the reaction under conditions described in Eq. (2) [24]. Methyl substitution at the a- or P-carbons of the styrene also leads to poor yields (21 and 49%, respectively) under these conditions. No enantioselective reactions of these substrates have been carried out. Preliminary experiments indicate that vinylarenes with strongly electron-withdrawing groups on the aromatic nucleus [for example, 3,5-bis(trifluromethyl)styrene or 2-vinylpyridine] are poor substrates for this reaction [22]. 

Simple a,/3-unsaturated aldehydes, ketones, and esters participate preferentially in inverse electron demand (LUMOdlcne controlled) Diels-Alder reactions with electron-rich, strained, or simple olefinic and acetylenic dienophiles.3 5 The thermal reaction conditions for promoting the [4 + 2] cycloadditions of simple 1-oxabutadienes (R = H > alkyl, aryl > OR), cf. Eq. (1), are relatively harsh (150-250°C), and the reactions are characterized by competitive a,/3-unsaturated carbonyl compound dimerization or polymerization. Usual experimental techniques employed to compensate for poor conversions include the addition of radical inhibitors to the reaction mixture and the use of excess 1-oxabutadiene for promoting the [4 + 2] cycloaddition. Recent efforts have demonstrated that Lewis acid catalysis and pressure-promoted reaction conditions28-30 may be used successfully to conduct the [4 + 2] cycloaddition under mild thermal conditions (25-100°C). 

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