Vinyl complexes synthesis

The Heck reaction is a powerful process for vinyl substitution by a carbon unit, with no direct analogy with traditional organic synthesis methodology. While it has not seen many applications in complex synthesis, it has been adapted to the preparation of a variety of value added organic intermediates. 

The annulations of alkenyl complexes of the type (233 Scheme 34) have been examined mainly with alkyl substituents and in a few cases with oxygen - ° and silicon - substituents in a variety of cyclic and noncyclic systems. It is notable that the benzannulations of the parent vinyl complex of (233) (42a Scheme 7) fail, which is apparently due to competing polymerization of (42a). ° Annulations of complexes of the type (233) are successful if at least one of the three substituents is nonhydrogen. Benzannulations of alkenyl complexes have been employed in the synthesis of 7-ethoxyprecocene, ° vitamin E - and daunomycinone. " ... 

This reaction is very facile, and the use of 1 mole of acid provides a convenient method for the synthesis of vinyl complexes. The proposed mechanism (287) is shown in Fig. 6 which suggests an oxidative addition of HX followed by insertion into the Pt—H bond to form the intermediate vinyl compound. If excess acid is present, a second oxidative addition of HX occurs to give a hydrovinyl platinum(IV) intermediate. If a second insertion reaction occurs, the olefin is formed along with PtX2(PPh3)2. 

The foregoing analysis can be extended from the chemistry of ammonia to a more complex catalytic system such as vinyl acetate synthesis. Vinyl acetate is produced by the acetoxylation of ethylene in the presence of oxygen over supported Pd/Au particles. While this is a well-established commercial route, the mechanism is still poorly understood. It was postulated that the chemistry could occur in the liqiud layer via homogeneous solublized Pd-acetate complexes. Recent evidence, however, indicate that the chemistry occurs on the Pd metal surface rather than on Pd(2+) particles. While we have explored both homogeneous as well as heterogeneous [36,47, 118] mechaiusms, we discuss only the heterogeneous results here. 

Scheme 12. Vinyl carbyne synthesis from (aryloxy)carbyne complexes Tp W(=COAr)(CO)2 (Ar - Ph,...

The reductive electrochemistry of iron-carbene porphyrins has been investigated in aprotic solvents [115]. With the vinylidene complex, there is a 2e -I- H reduction of the ligand leading to the formation of the corresponding iron(II) vinyl complex. The energies required to reduce by two electrons the other carbene complexes are quite similar [115]. The dichlorocarbene complex is an exception because the reduction is facihtated by the extreme instability of the one-electron intermediate. Formation of cr-alkyl iron(III) porphyrins has been confirmed by independent synthesis [116]. The a-alkyl iron(III) porphyrins can then be obtained by a one-electron reoxidation re-... 

Electropolymerized films of vinyl-substituted polypyridine complexes Synthesis, characterization, and appHcations 13CCR1357. Encapsulation of anions MacrocycHc receptors based on metal coordination and anion—7T interactions 13CCR1716. 

Vinyl complexes can, under favorable circumstances, rearrange by a [1,2]-H shift to car-byne complexes (Scheme 18).l ] The reaction appears to proceed via a q -vinyl complex 46, which has been isolated in some cases, and thus requires an open coordination site on the metal center. The vinyl precursors are often prepared in situ by hydride addition to alltyne complexes, e.g. for the synthesis of 47 and indenyl analogues, by deprotonation of alkene complexes, or by transmetalation, as for the s50ithesis of 48. The vinyl complex intermediate may not be observed in some cases. q -Allyl complexes are possible b5q)roducts of this reaction when the substituents R and R bear a-hydrogen atoms (e.g., 47), in which case the rearrangement to the carbyne product is favored by the presence of free ligands and higher temperatures. 

Thiazole, 4-methyl-5-(2-hydroxyethyl)-in thiamine biosynthesis, 1, 97 Thiazole, 4-methyl-2-methylami nosynthesis, 6, 300 Thiazole, 4-methyl-2-phenyl-alkylation, 6, 256 mercuration, 6, 256 Thiazole, 2-(methylthio)-methylation, 6, 290 thermodynamic values, 6, 291 Thiazole, 2-methylthio-5-phenyl-synthesis, 5, 153 Thiazole, 4-methyl-5-vinyl-occurrence, 6, 327 Thiazole, 2-phenyl-acetylation, 6, 270-271 Conformation, 6, 237 synthesis, 5, 113, 6, 306 Thiazole, 4-phenyl-conformation, 6, 237 2,5-disubstituted synthesis, 6, 304 Thiazole, 5-phenyl-conformation, 6, 237 Thiazole, 2-phenyl-5-triphenylmethyl-synthesis, 6, 265 Thiazole, 2-(2-pyridyl)-metal complexes, 5, 51 6, 253 Thiazole, 4-(2-pyridyl)-metal complexes, S, 51 6, 253 Thiazole, tetrahydro-ring cleavage, 5, 80 Thiazole, 2,4,5-trimethyl-occurrence, 6, 327... 

The original Sonogashira reaction uses copper(l) iodide as a co-catalyst, which converts the alkyne in situ into a copper acetylide. In a subsequent transmeta-lation reaction, the copper is replaced by the palladium complex. The reaction mechanism, with respect to the catalytic cycle, largely corresponds to the Heck reaction.Besides the usual aryl and vinyl halides, i.e. bromides and iodides, trifluoromethanesulfonates (triflates) may be employed. The Sonogashira reaction is well-suited for the synthesis of unsymmetrical bis-2xy ethynes, e.g. 23, which can be prepared as outlined in the following scheme, in a one-pot reaction by applying the so-called sila-Sonogashira reaction ... 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

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