Tetrahydrofuran , basicity species

Table 1) are converted to solvated tetramers in basic solvents such as ethers. Species which are dimeric in hydrocarbon solution, such as benzyllithium and poly (styryl)lithium, are converted into the unassociated species in tetrahydrofuran solution+2,56). The claim61 that poly(styryl)lithium active centers can exist in an associated form in tetrahydrofuran is known 62) to be incorrect. 

As noted above, the first monomeric pyrazol-l-ylborane was isolated as its trimethyl-amine adduct The compound (CH3)3N—BH2[pz-3,5-(Cp3)2] (Hpz = pyrazoie) was obtained as a distillable material on reaction of (CH3)3N—BH3 with 3,5-bis(tri-fluoromethyl)pyrazole = H[pz-3,5-(Cp3)2] = Hpz. Surprisingly, when THP—BH3 (THF = tetrahydrofuran) was employed as reagent, the dimeric species H2B(p-pz )2BH2, a pyrazabole (see Sect. IV.), was obtained without difficulty Apparently, the two-coordinate nitrogen of the pyrazolyl group in THP—BH2[pz-3,5-(Cp3)2] is sufficiently basic to displace THP but this base displacement cannot occur in the corresponding (CH3)3N—BH2[pz-3,5-(Cp3)j]. 

The formulation of two types of ion-pair is an attractive hypothesis which has been used for other systems [130] to explain differences in reactivity. The polymerization of styrene-type monomers in ether solvents, all of which solvate small cations efficiently, seems to be a particularly favourable case for the formation of thermodynamically distinct species. Situations can be visualized, however, in which two distinct species do not exist but only a more gradual change in properties of the ion-pair occurs as the solvent properties are changed. These possibilities, together with the factors influencing solvent-separated ion-pair formation, are discussed elsewhere [131, 132]. In the present case some of the temperature variation of rate coefficient could be explained in terms of better solvation of the transition state by the more basic ethers, a factor which will increase at lower temperatures [111]. This could produce a decrease in activation energy, particularly at low temperatures. It would, however, be difficult to explain the whole of the fep versus 1/T curve in tetrahydrofuran with its double inflection by this hypothesis and the independent spectroscopic and conductimetric evidence lends confidence to the whole scheme. 

Finally, regioselectivities are almost identical in tetrahydrofuran (THF) solution and onto alumina. Consequently, the proposed reactive species can be deducted by analogy (Scheme 2), with assumed rather identical donor-ability (basicity) of oxygen atoms for THF and Alumina. 

With regards to oxidative reactivity, it seems that dicopper(II) end-on peroxo complexes are basic, or nucleophilic, in nature, without oxidizing substrates such as PPha or 2,4-di-tert-butylphenol. Instead, O2 is released and PPhs binds, or deprotonation of the phenol occurs leading to a Cu -hydroperoxide. By contrast, the side-on bound dicopper(II) peroxide and its isoelectronic isomer, the dicopper(III) bis(p-oxo) species, both are electrophilic and have been shown to react with a wide array of substrates, in intramolecular (ligand) oxidations, and intermolecular organic transformations. Our group has extended the oxidative capability of CU2O2 complexes to novel (for copper chemistry) substrates such as tetrahydrofuran and dimethylaniline, which are oxidized to 2-hydroxy-tetrahydrofuran, and methylaniline plus formaldehyde, respectively. 

Acetoxylation of poly(vinyl chloride) can be carried out under homogeneous conditions. Crown ethers, like 18-crown-6, solubilize potassium acetate in mixtures of benzene, tetrahydrofuran, and methyl alcohol to generate unsolvated, strongly nucleophilic naked acetate anions. These react readily with the polymer under mild conditions. Substitutions of the chlorine atoms on the polymeric backbones by anionic species take place by a Sn2 mechanism. The reactions can also proceed by a Sivl mechanism. That, however, requires formations of cationic centers on the backbones in the rate-determining step and substitutions are in competition with elimination reactions. It is conceivable that anionic species may (depending upon basicity) also facilitate... 

Total syntheses of diterpenoid hydrokempenones have been accomplished by Paquette et al.,f using the Pd-catalyzed [3 + 2] cycloaddition methodology. One example is outlined on Scheme 43 and describes the synthesis of an isomeric compound 208 of 3/3-hydroxy-7/3-kemp-8(9)-en-6-one, a defense secretion agent of the neotropical species Nasutitermes octopilis. 3-AUcoxy-2-cyclohexenone 204 was efficiently functionalized and transformed to bicylic adduct 205 via a Robinson annulation reaction. Reduction of the double bond followed by condensation of dimethyl carbonate and oxidation gave the keto ester 206, which was treated with [2-(acetoxymethyl)-3-allyl]trimethylsilane, palladium acetate, and triisopropyl phosphite in refluxing tetrahydrofuran to afford a 98% yield of 207. Substituted methylenecyclopentane 207 was then functionalized by stereoselective reduction and protections, and final closure was done under basic conditions after an ozonolysis step. A modified Barton-McCombie reaction produced the desired tetracyclic adduct 208. 

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