Cationic complexes, synthetic methods

In a systematic study, it was demonstrated that, using a specially designed bulky benzamidinate ligand, it is possible to isolate mono(amidinato) dialkyl complexes over the full size range of the Group 3 and lanthanide metals, i.e., from scandium to lanthanum. The synthetic methods leading to the neutral and cationic bis(alkyls) are summarized in Scheme 56. Figure 18 displays the molecular structures of the cations obtained with Sc, Gd, and La. ... 

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

Almost all stable carbenes behave as 2-electron-CT-donating ligands with a few exceptions. In particular, in almost all cases corresponding Rh(I) complexes were targeted due to the easy synthetic method. An exception is the cyclopropenylidene carbene, with an extremely acute carbene angle. In this case, a second equivalent of carbene squeezes into the rhodium center, eliminating a chloride anion, giving the cationic dicarbenic rhodium(l) complex [51] (Scheme 5). 

This important synthetic problem has been satisfactorily solved with the introduction of lithium dialkylamide bases. Lithium diisopropylamide (LDA, Creger s base ) has already been mentioned for the a-alkylation of acids by means of their dianions1. This method has been further improved through the use of hexamethylphosphoric triamide (HMPA)2 and then extended to the a-alkylation of esters3. Generally, LDA became the most widely used base for the preparation of lactone enolates. In some cases lithium amides of other secondary amines like cyclo-hexylisopropylamine, diethylamine or hexamethyldisilazane have been used. The sodium or potassium salts of the latter have also been used but only as exceptions (vide infra). Other methods for the preparation of y-Iactone enolates. e.g., in a tetrahydrofuran solution of potassium, containing K anions and K+ cations complexed by 18-crown-6, and their alkylation have been successfully demonstrated (yields 80 95 %)4 but they probably cannot compete with the simplicity and proven reliability of the lithium amide method. 

Homogeneous asymmetric hydrogenation is a practical synthetic method (27). The DIPAMP-Rh-catalyzed reaction has been used for the commercial production of (S)-DOPA [(5)-3-(3,4-dihydroxy-phenyl) alanine] used to treat Parkinson s disease (Monsanto Co. and VES Isis-Chemie) (Scheme 12) (27, 28). (S)-Phenylalanine, a component of the nonnutritive sweetener aspartame, is also prepared by en-antioselective hydrogenation (Anic S.p.A. and Enichem Synthesis) (29). A cationic PNNP-Rh(nbd) complex appears to be the best catalyst for this purpose (15c) (see Scheme 5 in Chapter 1). 

Early in the development of the field, compounds were often prepared to define the limits of both the synthetic methods and the stable products that could be formed. To date, many thousands of heteromacrocycles have been prepared. The dominant application of the vast family of host or receptor molecules has been to bind or complex a guest structure. The guests can be metal cations, organic cations, neutral substrates, anions, or complementary molecules. The complexation process can be understood from the simple example of 18-crown-6 complexing K+Cl- in solution. Ignoring structural and solvation/desolvation issues, the process can be described simply as... 

Dendrimeric macromolecules containing germanium have also been made via hydrogermylation. The cycliza-tion/hydrogermylation of functionalized 1,6-dienes catalyzed by cationic palladium complexes has also been carried out. Hydrogermylation is also an effective synthetic method for the synthesis of germafranes. ... 

Although many cationic secondary carbene complexes are known, deprotonation of these carbene ligands to afford carbyne complexes is not a common synthetic method. Conversion of a Ta-neopentylidene complex to a Ta-neopentylidyne complex by using Ph3P=CH2 as an external base occurs by deprotonation of a cationic Ta-neopentyli-dene intermediate ... 

Recently we set out to synthesize AuFg" and obtained our first salt of this anion in the form of the complex cation salt Xe2Fii AuF(,. Since both ions were novel and of structural interest, we were fortunate that our synthetic method yielded suitable single crystals for an X-ray structural analysis. 

The trityl cation (EbjC" ") is a commonly used reagent to effect the abstraction of a hydride from eoordinated ligands. For example, the preparation of metal alkylidene and alkene complexes by a- and jS-hydride abstraction from metal alkyls using trityl salts is a well-established synthetic method, Eqs 35 and 36. 

Recently, Stryker et al. have developed a new synthetic method that provides 3-sUbstituted metallacyclobutanes. Alkylation of the cationic allyl complex 23 with nucleophiles [74] and thermal reaction of the bis(allyl) complex 25 [75] result in the formation of metallacyclobutanes 24 and 26, respectively. Radical addition of RX to Ti(III)-ally complex 23 takes place exclusively at the center carbon atom, providing a metallacyclobutane [76]. 

Nickel. The synthesis of unusual metal complexes by our new synthetic method is shown by the formation of the square-planar cation in [Ph3PI][Ni(PPh3)l3], whereas, according to conventional wisdom, large ligands such as PPh3 and iodide should force tetrahedral geometry around nickel(ii). 

Based on the above analysis, the development of metal oxides of nanometric dimensions can result in devices and materials with superior performance. However, these developments are directly related to the development of synthetic methods that allow for controlled particle size, particle morphology, and deposition. Once again, the bottom-up methods of wet chemical nanocrystal synthesis are apparently the most viable approach to achieve such control. Compared with the control attained in the synthesis of metal and 11-lV semiconductor nanocrystals, the control of metal oxide nanocrystals is still incipient, particularly insofar as the synthesis of complex metal oxide nanocrystals (oxides formed of more than one cation) is concerned. 

The data on cycloadditions of alkene radical cations indicate that dimerization will usually compete efficiently with cross additions and demonstrate the necessity for obtaining detailed kinetic data in order to design appropriate synthetic methods based on radical cation chemistry. The mechanistic data obtained from both time-resolved and steady-state experiments demonstrate the complexity of cycloaddition chemistry. This may be a particular limitation in the use of cycloaddition reactions in the design of mechanistic probes for assessing whether a particular reaction involves radical cation intermediates. The results also highlight the importance of using both product studies and the kinetic and mechanistic data obtained from time-resolved methods to develop a detailed understanding of the reactions of radical cations. 

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