Precursors aprotic

Mohanty et al. were the first to introduce pendent r-butyl groups in die polymer backbones. The resulting material was quite soluble in aprotic dipolar solvents.83 The PEEK precursors were prepared under a mild reaction condition at 170°C. The polymer precursor can be converted to PEEK in die presence of Lewis acid catalyst A1C13 via a retro Friedel-Crafts alkylation. Approximately 50% of die rerr-butyl substitutes were removed due to die insolubility of the product in die solvent used. Later, Risse et al. showed diat complete cleavage of f< rf-butyl substitutes could be achieved using a strong Lewis acid CF3SO3H as both die catalyst and the reaction medium (Scheme 6.15).84... 

Thioethers lack the capacity to neutralize positive charge and display weak donor properties. Consequently, they do not readily displace strong donor solvents (water) or strongly bonding anions (such as halides) from the coordination sphere. As a consequence, many thioether complex syntheses employ aprotic or alcoholic solvents and precursor complexes with weakly bound solvents (such as DMSO or acetone) or anions (such as C+3S03 ). Despite the synthetic challenges, a wide range of complexes has been reported, particularly with the cyclic poly-thioethers, where the macrocyclic effect overcomes many of the above difficulties. 

An electrochemical cell [93,94] was used to obtain an efficient anodic deposition of no carrier added F-fluoride solubilized in the target water. The radioisotope is electrochemically adsorbed on the anode (glassy carbon electrode) and can be easily dried. An opposite electrical field releases the radionuclide directly into a solution of a phase transfer catalyst in dipolar aprotic solvents. The nucleophilic fluorination can be performed simultaneously if the electrochemically and thermally induced desorption of radioactivity is done in the presence of the precursor. However, the yields remain poor (3 % in the electrochemical n.c.a [ F]fluorination of anisole). 

Either protic (alcohols, preferentially methanol) or aprotic solvents (toluene, dichloromethane, THE) can be used, depending on the structure of the metal precursors that can generate the catalysts by a number of pathways. Metals other than palladium, for example nickel [4], can form active catalysts for alkene/CO copolymerisation, yet with largely lower productivities as compared to structurally similar palladium precursors [1]. For this reason, only Pd"-catalysed alkene/CO copolymerisation reactions are reviewed and commented in the present chapter. 

As previously mentioned, the catalyst precursors in aprotic solvents generally contain a Pd-alkyl moiety for immediate insertion of CO without the need of activators. In some cases, bis-halide Pd" precursors have been employed in conjunction with activators such as methyl aluminoxane (MAO), which is able to replace one halide ligand with a methyl group and to create a free coordination site at the metal [17]. 

Thus, N-pyrimidine phthalimide reacted with hexylamine at room temperature to form an amide-amide. The initial amide-amide formation proceeded more rapidly in chloroform as compared to dimethylsulfoxide (DM SO). However, the ring closure reaction to the imide was favored by the more polar, aprotic DMSO solvent, yielding the imide in nearly quantitative yield after 3 hours at 75 °C. The authors were able to utilize this synthetic approach to prepare well-defined segmented poly(imide-siloxane) block copolymers. It appears that transimidi-zation reactions are a viable approach to preparing polyimides, given that the final polyimide has a Tg sufficiently low to allow extended excursions above the Tg to facilitate reaction without thermal decomposition. Additionally, soluble polyimides can be readily prepared by this approach. Ultimately, high Tg, insoluble polyimides are still only accessable via traditional soluble precursor routes. 

These effects are particularly pertinent in type (I) -> (II) reactions, where the high anion activity is required to overcome the lattice energy of the non-molecular precursor (I). Thus many non-molecular metal halides, sulfides and thiolates which are insoluble and unreactive in aqueous media will dissolve with anionic ligands in aprotic solvents, and clusters can be isolated from the resulting solutions. Anion activation by use of cation complexands to solubilize salts in hydrocarbon solvents is advantageous, as is the use of complexands in the isolation of polymetallate anions.342... 

Di-iminoisoindoline was used as a precursor for Pc in different protic and aprotic systems, without catalysts or promoters, to study the solvent effect on the possibility of phthalocyanine formation [32], As can be observed (Example 13), it is possible to carry out the chemical and electrochemical synthesis of metal-free Pc in aprotic solvents, such as DMF or DMSO, in contrast to the results with PN. It is surprising that the yields of Pc in ROH are comparatively small. The N,N-dimethyletanolamine is characterized by the best yields, as in the case when PN was used as precursor. 

Various transition metal-based catalysts not containing preformed metal-carbon bonds have been developed for the polymerisation of conjugated dienes [27-35, 150-158]. These catalysts include monometallic precursors such as Rh, Co and Ni salts and bimetallic precursors such as C0CI2-AICI3. Some of them are soluble in a polymerisation medium, e.g. Rh(N03)3 in protic solvents (ROH, H2O) [27,150-154] and C0CI2—AICI3 in aprotic solvents [155-157], and some others are insoluble in a polymerisation diluent, e.g. TiCL—Ni(PCl3)4 [158]. 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

Since the advantage of using nonaqueous systems in electrochemistry lies in their wide electrochemical windows and low reactivity toward active electrodes, it is crucial to minimize atmospheric contaminants such as 02, H20, N2, C02, as well as possible protic contaminants such as alcoholic and acidic precursors of these solvents. In aprotic media, these contaminants may be electrochemically active on electrode surfaces, even at the ppm level. In particular, when the electrolytes comprise metallic cations (e.g., Li, Mg, Na), the reduction of all the above-mentioned atmospheric contaminants at low potentials may form surface films as the insoluble products precipitate on the electrode surfaces. In such cases, the metal-solution interface becomes much more complicated than their original design. Electron transfer, for instance, takes place through electrode-solution rate limiting interphase. Hence, the commonly distributed solvents and salts for usual R D in chemistry, even in an analytical grade, may not be sufficient for use as received in electrochemical systems. 

A further complicating feature in these reactions is the finding that HETh and its thiazolium and benzothiazolium analogs can, in the presence of a base such as Me3N or DBU, be tautomerized to the rather stable 2-benzoylthiazolines9,10. This reaction apparently requires a aprotic medium. Further, Chen showed55,57 that for a number of aromatic aldehydes, when the reaction is performed in methanol, the principal product is not HBT but rather the dimethoxyacetal of the precursor aldehyde. Thiazolium salts appear to catalyze conversion of some aromatic aldehydes to their acetals in reasonable yields. This appears to be a rare example of acetal formation under alkaline conditions. These various reactions of aldehydes and thiazolium salts, additional to the benzoin condensations, are outlined in Scheme 5. 

However, there are a few true titanyl examples, mostly porphyrins and related complexes that do have Ti=0 bonds. The Ti=0 bond appears to be short 1.62 A while ir bands are in the region 890 to 972 cm 1 (Raman, v = 975 in 2 M HC1) consistent with a Ti=0 double bond. Octahedral titanyl complexes, such as (Me3tacn)Cl2TiO, are readily available from Tim precursors by oxidation with dioxygen in an aprotic solvent.17 A cationic titanyl complex which is stable in aqueous media18 can be made according to... 

---