Molybdenum amides bonding

Our group has prepared a number of [Leu Enkephalin derivatives with organometallic compounds attached to the N terminus via amide bond formation. In addition to the ferrocenoyl derivative 27 and the related cobaltocenium derivative 28 [61], we have also published the synthesis and characterization of two molybdenum derivatives 29 and 30 (see Schemes 5.13 and 5.14) [62]. In all cases, the enkephalin was prepared by standard solid phase peptide synthesis (SPPS) techniques the by Fmoc strategy. For compounds 27-29, the metal complex was attached to the deprotected N terminus of the peptide as the final step on the solid support. Subsequent cleavage from the resin was achieved by cone. NHj solution in MeOH (27 and 29) or cone. TFA (28), respectively, in accordance with... 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

The total syntheses of these pepper alkaloids are not those of pyrrolidines but rather syntheses of their acid parts. Thus dihydrowisanidine (137) has been prepared by a series of reactions, the key step of which is the formation of the carbon-carbon double bond by a Wittig-Homer reaction (217, 218). Schemes 41 and 42 summarize two syntheses of okolasine from sesamolmethyl ether (279) of course, routes to okolasine also yield the corresponding piperidine alkaloid wisanine. Molybdenum-catalyzed elimination of allylic acetate (149) yielded (E,E)-diene ester 150 en route to trichonine (220) worthy of note is the use of an aluminum amide in the preparation of amide 143 from ester 150 (Scheme 43). 

The simplest networks are one-dimensional a-networks which may be composed of secondary amides, primary amide dimers or nucleophospholipids. In chapter 5, such structures were discussed as micellar rods and tubules in bulk aqueous solutions. Two-dimensional materials such as copper oxide superconductors, molybdenum sulfide lubricants and intercalated graphites are mostly inorganic. The anisotropic properties are a result of covalent bonds in two dimensions and weak interactions in the third dimension. One may, however, also envision strong hydrogen-bond interactions within an organic layer, whereas adjacent layers are held together only by van de Waals interactions. The two-dimensional, or p-network may form spontaneously from an... 

Enones can be reduced to the saturated ketones with triethylsilane and Wilkinson s catalysis62 (equation 54). Interestingly, the same product was prepared via a Tiffeneau-Demjanov ring expansion wherein trimethylsilyl cyanide was used (equation 55). The selective reduction of the double bond of enones can also be carried out with diphenylsilane in the presence of palladium(O) or palladium(II) and zinc chloride63 (equation 56), or more effectively with phenylsilane and molybdenum hexacarbonyl64 (equation 57). This latter reagent was also used to reduce the double bond of a,/ -unsaturated esters, amides and nitriles. 

The formation of bridging nitrogen-hydride ligands as intermediates on route to the reduction of dinitrogen to ammonia and hydrazine has been suggested by many authors. The unique feature of this molybdenum system is the pathway by which the bridge is constructed. Loss of dinitrogen from an intermediate such as D followed by N-N bond homolysis leads eventually to a molybdenum(III) amide F that should be readily converted to ammonia in the presence of acid. In no step is intermolecular electron transfer a requirement. 

Titanium- and tungsten-based catalysts tend to react with olefinated ketones and esters, whereas molybdenum-based counterparts are more reactive toward olefins (though they can also react with aldehydes and other polar or protic groups). Ruthenium-based catalysts react preferentially with C=C double bonds over most other moieties, a feature that makes them unusually stable toward alcohols, amides, aldehydes, and carboxylic acids. Because of this trend, the functional-group tolerance of an olefin metathesis catalyst can be increased by focusing on a later class of transition metal such as ruthenium [8]. 

The remarkable utility of the amide route has been established by the synthesis of alkoxides of molybdenum and tungsten (M) in their lower oxidation states (+3 or +4), e.g. [M(0R)3] and [M(OR)4] . Derivatives of the empirical formula M(OR)3 are of special interest as, in these, association occurs primarily through the formation of metal-metal triple (M=M) bonds rather than through the normal alkoxo bridges (Eq. 2.113)... 

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