Oxazole metal complexes

Ru, Os, and Ir carbene complexes have been prepared from reactions of anionic or low-valent metal complexes with some organic salts or neutral compounds with highly ionic bonds. Oxidative addition of halothiazole and -oxazole species to IrCl(CO)(PMe2Ph)2 affords Ir(III) complexes which on protonation yield cationic carbenes (69), e.g.,... 

Deprotonation of oxazole-borane complex 59 with -BuLi or r-BuLi generates an organolithium species which reacts with electrophiles, such as an aldehyde, at the C-2 position (Scheme 8) <1996JOC5192>. This oxazole-borane complex can also be generated in situ prior to deprotonation without loss in yields. It should be noted that only one equivalent of the metallating agent is needed, and the deprotonation occurs exclusively at the more acidic C-2 position in preference to C-5 or C-4. 

Moreover, aryl-oxazoles, -imidazoles [17], or-thiazoles [18], anhydrides [19], and imides [20] are accessible via intramolecular Heck-type carbonylations. In addition to typical acid derivatives, aldehydes [21], ketones [22], aroyl cyanides, aroyl acetylenes, and their derivatives [23] could be synthesized via nucleophilic attack of the acyl metal complex with the corresponding hydrogen or carbon nucleophiles. Even anionic metal complexes like [Co(CO)4] can act as nucleophiles and lead to aroylcobalt complexes as products [24]. 

Similarly, ra 5-cyclopropanes were obtained from alkenes, such as styrene and 2,5-dimethyl-hexa-2,4-diene, with relative yields > 90% when a diazoacetate bearing a bulky ester group was decomposed by a copper catalyst with bulky salicylaldimato ligands. Several metal complexes with bulky Cj-symmetrlc chiral chelating ligands are also suitable for this purpose, e.g. (metal/ligand type) copper/bis(4,5-dihydro-l,3-oxazol-2-yl)methane copper/ethyl-enediamine ruthenium(II)/l,6-bis(4,5-dihydro-l, 3-oxazol-2-yl)pyridine cobalt(III)/ salen. The same catalysts are also suited for enantioselective reactions vide infra). For the anti selectivity obtained with an osmium-porphyrin complex, see Section 1.2.1.2.4.2.6.3.1. 

The recent developments on the metallation chemistry of oxazoles and benzoxazoles, isoxazoles and benzisoxazoles, pyrazoles and indazoles, thiazoles and benzo-thiazoles, and isothiazoles, benzo[c]isothiazoles, and benzoMisothiazoles have been reviewed. The two-decade history of catalytic carbon-carbon bond formation via direct borylation of alkane C-H bonds catalysed by transition metal complexes has been reported. The alkane functionalization via electrophilic activation has been underlined. " Recent advances of transition-metal-catalysed addition reactions of C-H bonds to polar C-X (X=N, O) multiple bonds have been highlighted and their mechanisms have been discussed. The development and applications of the transition metal-catalysed coupling reactions have been also reviewed. - ... 

Addition to Aldehydes. The Vedejs oxazole metalation has often been applied in the addition of oxazoles to aldehydes. In order to avoid ring opening to the isonitrile, the oxazole is first complexed with borane, then reacted with BuLi to generate the 2-metalo nucleophile (eq 15). This has been applied in the synthesis of a-ketooxazole inhibitors, and angiotensin II (AT2)... 

Reactivity of Nickel Pincer Complexes with CO2. There is great interest involving the activation of CO2 and its conversion into value-added chemicals. One strategy utilizes late transition-metal complexes where the key step involves metal insertion to form metal carboxylates. Nickel pincer complexes were prepared and the subsequent reactivities of these complexes with CO2 were studied. The complex [(PCP)Ni(Cl)] was refluxed in THF with excess NaNH2 for 24 h to form [(PCP)Ni(NH2)] (eq 38). Further refluxing with 1 equiv of oxazole for 3 day via protonolysis, liberated ammonia to form [(PCP)Ni(oxazole)]. It is worth noting that the protonolysis is air and moisture sensitive and that if rigorously dry conditions are not employed, [(PCP)Ni(OH)] will form instead. 

Oxygen-containing azoles are readily reduced, usually with ring scission. Only acyclic products have been reported from the reductions with complex metal hydrides of oxazoles (e.g. 209 210), isoxazoles (e.g. 211 212), benzoxazoles (e.g. 213 214) and benzoxazolinones (e.g. 215, 216->214). Reductions of 1,2,4-oxadiazoles always involve ring scission. Lithium aluminum hydride breaks the C—O bond in the ring Scheme 19) 76AHC(20)65>. 

Analysis of intermolecular interactions in the crystal structures of oxime molecules has been used to answer that question. In all available complex structures with one central metal ion we found no coordinative bonds from the oxime oxygen to the metal, but exclusively coordination between the nitrogen atom and the metal ion (data were retrieved from the Cambridge Crystallographic Database [14]). In a comprehensive study Bohm et al. investigated complexes of oxazoles, methoxypyridines, and oxime ethers with water [15]. On the basis of interaction energies obtained... 

Transition metal-catalyzed reactions of ct-diazocarbonyl compounds proceed via electrophilic Fischer-type carbene complexes. Consequently, when cr-diazoketone 341 was treated, at room temperature, with catalytic amounts of [ RhiOAcbh, it gave the formation of a single NH insertion product, which was assigned to the enol stmcture 342. At room temperature, in both solid state and in solution, 342 tautomerizes to give the expected 1-oxoperhydropyr-rolo[l,2-c]oxazole derivative 343 (Scheme 50) <1997TA2001>. 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

The reactions of complex metal hydrides occur by an attack of the nucleophilic hydride ion on an electrophilic center.1 Aromatic nitrogen heterocycles in which the nitrogen has contributed only one electron to the -system (1) are electrophilic as compared with benzene, and have been shown to undergo reduction by the active reducing agent, lithium aluminum hydride. The nitrogen heterocycles in which the heteroatom has contributed two electrons to the 77-system (2) are electron-rich as compared with benzene and usually do not undergo reaction by reduction with complex metal hydrides.2 A combination of these two structural features, as in oxazoles (3), usually induces sufficient electrophilicity to allow attack by the hydride ion and reduction. 

The tetrahydro-derivatives of the oxazole and isoxazole system are unstable. As a consequence, only acyclic products have been reported from the reductions with complex metal hydrides. 2,5-Diphenyl-oxazole (119) gave 2-benzylamino-l-phenylethanol (120),141 and 3,5-diphenyl-2-isoxazoline (121) was converted to 3-amino-l,3-diphenylpropanol (122)142 on reduction with lithium aluminum hydride. 3-Phenylbenzisoxazole was resistant to reduction with lithium aluminum hydride and sodium borohydride,143 but benz-oxazole (123), benzoxazol-2-one (124), and benzoxazol-2-thione (125) have been reported 141 to yield 2-methylaminophenol (126) on reduction with lithium aluminum hydride. 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

Imidazoles are amphoteric compounds with a basic, pyridine-type nitrogen (they are about 106 times more basic than oxazoles and 104 times more basic than thiazoles173), and (where the NH is unsubstituted) a weakly acidic, pyrrole-type amino nitrogen in the ring. In consequence, imidazoles readily form salts with acids and often form salts (or complexes) with metals. The sparingly soluble silver salts formed by imidazoles have been used by Giesemann et al.174 as intermediates in the synthesis of 1-triphenylmethylimidazoles. Normally, however, the salts formed with acids are more important in isolation and purification procedures. 

The autoassociation phenomena of oxazole and 2,4-dimethyloxazole have been studied by Meyer et al.sot A cryometric analysis of the inert solvent-oxazole binary system reveals the existence of intermolecular oxazole-oxazole type association. Data obtained with 2,4-dimethyloxazole indicate the importance of steric hindrance in this type of association.306 Oxazoles readily form stable complexes with metal salts of the general formula BMXs (where M is a bivalent metal cation).196 196 These are very useful in isolation and characterization of oxazoles. 

Secheresse reported size-controlled formation of silver nanparticles (43) by direct bonding of ruthenium complex 42 and silver nanoparticles. Oxazole 42 was formed by the reaction of diketone 40 and aldehydes 41 under the influence of NH4OAC. These metallic nanoparticles may find applications in DNA sequencing, catalysis, optics, nanoscale electronics and antimicrocrobials. ... 

The synthesis of 2,5-disubstituted oxazoles from methyl ketones and benzylamines in a metal and peroxide-free environment is achieved using l2-promoted domino oxidative cyclization involving C-H bond cleavage and the formation of C-N and C-0 bonds. Slow oxidation of A-acetyl homocysteine thiolactone by iodate to A-acetyl homocysteine thiolacone sulfoxide occurs in a reaction S1 having 1 3 stoichiometry (oxidant reductant). The stoichiometric ratio in excess of 103 (reaction S2) is 2 5 because excess 103 oxidizes the r ion, generated in reaction SI, to I2. The stoichiometry ratio for the I2 oxidation (reaction S3) is 1 1. Complex kinetics are observed because reactions SI, S2 and S3 occur simultaneously with comparable rates. 

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