O Bond Formation

1) Benzotrifluoride (C HsCFj) or BTF is a partially fluorinated solvent that dissolves both fluorous-and purely organic molecules. 

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Tire first C—O bond formation is probably not influenced strongly by the presence of a templating cation. Since it is not crucial for one end of the chain to meet the other rather than reacting with a different molecule, it is not necessary to superimpose either a template or dilution condition on the reaction to prejudice the statistics. In the second step, however, such a prejudicial condition is required. This is available in the form of an... 

In the case of terminal alkynes having oxygenated functions in the linear chain (Scheme 10, route D), Martin, Padron, and coworkers found that homopropargylic alcohols reacted properly, yielding 2-substituted dihydropyrans as sole products, probably via a Prins-type cyclization. This cyclization provides a new approach toward 2-alkyM-halo-5,6-dihydro-2//-pyrans through a concomitant C-C and C-O bond formation (Scheme 21) [35]. 

The proposed mechanism (Scheme 5-27) suggests that the phosphite tautomer-izes to 25 to allow Ti-O bond formation. 

Muci AR, Buchwald SL (2002) Practical Palladium Catalysts for C-N and C-O Bond Formation. 219 131-209... 

Palladium-catalyzed aromatic C—O bond formation is less developed than palladium-catalyzed aryl amination. Except when the aryl halide is strongly electron deficient,107-110 catalysts ligated by the conventional aryl phosphines such as DPPF and BINAP are ineffective for coupling of... 

Only transformations in the longest linear sequence (LLS) are considered. The term skeleton constructions refers to C-C and C-O bond formations (notwithstanding redox reactions) that directly introduce native structural features of the bryostatins without further modification. The term other functional group manipulations refers to steps that indirectly introduce native structural elements, the interconversion of functional groups (e.g., the introduction and removal of auxiliaries) and miscellaneous transformations that do not involve skeleton construction... 

This process is likely to proceed via a palladacycle intermediate followed by a Pd(ll) to Pd(iv) oxidation. Reductive elimination occurs with C-O bond formation and regeneration of the Pd(ll) catalyst. Evidence for a palladacycle intermediate is supported by the high regioselectivity (8-Me group oxidized) observed for the oxidative functionalization of 5,8-dimethylquinoline, which, in the absence of a possibility of coordination, would otherwise contain two identical methyl groups (Equation (57)). 

C-O Bond Formation through Transition Metal-mediated Etherification... 

Mechanistic studies carried out by the Buchwald group97,98 on the key C-O bond formation step in these reactions have reinforced the importance of the properties of the ligand, and have led to the adoption of a variety of more sterically hindered ligands (e.g., 23, 24, 27, 29, and 30), which have given improved results for the preparation of biaryl ethers (Equation (15)),89 /-butyl aryl ethers,99 and aryl ethers of primary alcohols.100... 

Pd-catalyzed intramolecular etherification reactions of aliphatic alcohols have also been practiced in tandem with other bond-forming processes, such as a Pd-catalyzed allyltin addition to an aldehyde (Equation (32)).160 Similarly, a tandem C-N and G-O bond formation sequence occurs (Equation (33)) during the reactions of /3-amino alcohols with biscarbonates in the presence of the N,O-acetal-derived ligand 43.161-163... 

Another approach toward C-O bond formation using alkynes that has been pursued involves the intermediacy of transition metal vinylidenes that can arise from the corresponding y2-alkyne complexes (Scheme 13). Due to the electrophilicity of the cr-carbon directly bound to the metal center, a nucleophilic addition can readily occur to form a vinyl metal species. Subsequent protonation of the resulting metal-carbon cr-bond yields the product with anti-Markovnikov selectivity and regenerates the catalyst. 

Like alkynes, a variety of mechanistic motifs are available for the transition metal-mediated etherification of alkenes. These reactions are typically initiated by the attack of an oxygen nucleophile onto an 72-metalloalkene that leads to the formation of a metal species. As described in the preceding section, the G-O bond formation event can be accompanied by a wide range of termination processes, such as fl-H elimination, carbonylation, insertion into another 7r-bond, protonolysis, or reductive elimination, thus giving rise to various ether linkages. 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

Nitrosoimines can undergo thermal reaction, a unimolecular, two-step mechanism has been proposed, as shown in Scheme 3.22 [193]. In this mechanism, a concerted electrocyclization is envisioned to form the strained four-membered ring in 41, followed by a presumably forbidden, but highly exothermic, deazetization to give 41. The electrocyclic ring closure is, at first glance, a 4-electron process, analogous to the cyclization of butadiene [194] or acrolein [194, 195]. This would be expected to involve rotation around the C=N bond coupled with C-O bond formation. 

The Buchwald-Hartwig C—N bond and C—O bond formation reactions 1.7.1 Pd-catalyzed C—N bond formation... 

The Pd-catalyzed intermolecular C—O bond formation has also been achieved [105-108]. Novel electron-rich bulky phosphine ligands utilized by Buchwald et al. greatly facilitated the Pd-catalyzed diaryl ether formation [109], When 2-(di-tert-butylphosphino)biphenyl (95) was used as the ligand, the reaction of triflate 93 and phenol 94 elaborated diaryl ether 96 in the presence of Pd(OAc)2 and K3PO4. The methodology also worked for electron-poor, neutral and electron-rich aryl halides. 

The mechanism for Pd-catalyzed C—O bond formation is similar to that of C—N bond formation. Application of this method to heterocyclic chemistry is yet to be seen, partially because the SnAt displacements of many heteroaryl halides with alkoxides are facile without the aid of palladium. 

Keywords Asymmetric Catalysis a Natural Product Synthesis a Chiral Metal-Based Complexes a Enantioselective C-C Bond Formation a Enantioselective C-O Bond Formation... 

These percentages were determined using 29Si and 13C NMR spectroscopies, the highest rates of Si-O bond formation being observed with X = OTf, whereas chlorosilanes rather led to pentacoordinated compounds. 

In silicon derivatives, X-ray studies of compound 27 were consistent with a covalently bonded trigonal bipyramidal molecule <2000CC565>. In addition, nucleophilic substitution at silicon for similar compounds was modeled either by NMR or X-ray techniques and both methods correlate in the calculation of % Si-O bond formation <2003JOM66, 2003JOM154>. 

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