Decarbonylation synthetic usefulness

A number of additional experiments have been carried out in an attempt to determine the overall mechanism for the catalytic decarbonylation reaction using [Rh(dppp)2]+. Since this reaction is a useful synthetic method and is a model for the discreet processes in other organometallic reactions, a detailed knowledge of the mechanisms and... 

CO. The increased lability of CO compared with that of the neutral complex rra 5-RhCl(CO)(PPh3)2 is presumably due in part to the lowered effective basicity of rhodium. [Rh(PPh3)2] was shown to decarbonylate benzal-dehyde at 100°C catalytically and with a rate considerably faster than that obtained using ra 5-RhCl(CO)(PPh3)2/ Unfortunately, this catalyst system decomposed readily at 100°C and therefore the reaction was not synthetically useful. 

Uses. Furfural is primarily a chemical feedstock for a number of monomeric compounds and resins. One route produces furan by decarbonylation. Tetrahydrofuran is derived from furan by hydrogenation. Polytetramethylene ether glycol [25190-06-1] is manufactured from tetrahydrofuran by a ring opening polymeri2ation reaction. Another route (hydrogenation) produces furfuryl alcohol, tetrahydrofurfuryl alcohol, 2-methylfuran, and 2-methyltetrahydrofuran. A variety of proprietary synthetic resins are manufactured from furfural and/or furfuryl alcohol. Other... 

An alternative synthetic route to platinum(II) thiolates is by the oxidative addition of the S—S bond to platinum(O). When the sulfur atom has phenyl or electron-withdrawing substituents such as CF3, this reaction is a useful one to synthesize the thiolato platinum(II) complexes (equation 503).1703-1705 Simple alkyl disulfides such as Me2S2 and Et2S2 do not form stable dithiolato complexes of platinum(II) by S—S addition to Pt(PPh3)3, but if chelation can occur, chelate-assisted oxidative addition can induce S—S cleavage (equation 504).30 An unusual cyclic thiolato complex is obtained by the decarbonylative cleavage of a C—S bond (equation 505).1707... 

Decarbonylation of aldehydes and acid halides is an important synthetic reaction (i, 2) and using various transition-metal complexes as stoichiometric or catalytic reagents for this process has... 

However, in the final analysis, the fundamental limitation of the technique described above is that it is yet to be demonstrated that more than two CF3 groups can be affixed to any transition metal center by means of thermal decarbonylation-methyl migration reactions. Thus this procedure is extremely useful for the synthesis of certain categories of trifluoromethyl transition metal compounds, provided that only one or at most two CF3 groups are desired, but it cannot be regarded as a generally applicable synthetic method. 

In a straightforward refinement of their concept Rubin et al. have turned to cyclophyne 14 [32] in which the vinylic hydrogens of 13 are replaced by 1,2-dioxocyclobuteno groups. This cyclic diketone moiety has been used previously [18, 33, 34] as a synthetic equivalent for alkynyl groups which can be generated from the dione by thermally or photochemically induced CO expulsion. Successive decarbonylation of 14 should ultimately lead to cyclophyne 15 with the composition Q0H6. 

The major drawback in the development of efficient catalytic PK protocols is the use of carbon monoxide. Many groups probably refuse to use this reaction in their synthetic plans in order to avoid the manipulation of such a highly toxic gas. Carbonylation reactions without the use of carbon monoxide would make them more desirable and would lead to further advances in those areas. Once the use of rhodium complexes was introduced in catalytic PKR, two independent groups realized these species were known for effecting decarbonylation reactions in aldehydes, which is a way to synthesize metal carbonyls. Thus, aldehydes could be used as a source of CO for the PKR. This elegant approach begins with decarbonylation of an aldehyde and transfer of the CO to the enyne catalyzed by rhodium, ruthenium or iridium complexes under argon atmosphere (Scheme 36). 

In the laboratory of F.E. Ziegler, the synthesis of the core nucleus of FR-900482 was accomplished. In the final stages of the synthetic effort, the removal of the formyl group from the C7 quaternary center was necessary. The authors chose the Tsuji-Wilkinson decarbonylation protocol to effect the transformation. The 1,3-diol functionality was protected as the acetonide prior to the decarbonylation. Usually the rate of decarbonylation is slowest for aldehydes that have the formyl group attached to a quaternary carbon, so it was necessary to use more than two equivalents of the catalyst to effect the decarbonylation at the reflux temperature of xylene. 

The reaction of diethyl oxalate with ketones in the presence of sodium ethoxide, or other bases, has been used extensively examples are given in Scheme 66. Reaction may occur with esterketone ratios of 1 1, 2 1, or 1 2, but only the 1 1 case finds substantial use in modem synthetic practice. Frequently the a-oxalyl ketone is thermally decarbonylated to give the 3-heto ester.An early example of this was provided by Bachmann s synthesis of equilenin. The mechanism of this reaction has teen examined labeling studies showed that it was the ester carbonyl that was eliminated. The intact oxalyl group has teen used as a directing group in steroid methylation while, more recently, 2-oxalylcyclohexanone has provided a route to (R)-(-)-hexahydromandelic acid (Scheme 67). The products of acylation of suitable acyclic ketones can cyclize to form (enolic) cyclopentane-1,2,4-triones (equation 39). ... 

Our standard synthetic method for these chelates is the elevated temperature decarbonylation of W(CO)e, either by melt reactions containing excess ligand (2-6,14) or by the use of high boiling solvents, usually either 2,4,6-trimethylpyridine (2-6,14) or mesitylene (3). Evidence for l,2,3,4-tetrahydro-8-quinolinol has been obtained (14) therefore, the reaction has been formulated as ... 

A general synthesis of protoberberines has been achieved from amino-acids of the type (77) by regiospecific decarbonylation, using phosphorus oxychloride, to iminium salts of the type (79), followed by acid cyclization. This method of preparation of the iminium salts has certain advantages over other routes. Amino-acids with the orientation of substituents shown in (77) are relatively easy to obtain from dihydroxyphenylalanine (80), but the more common orientation of substituents found in the protoberberines necessitates more lengthy methods of synthesis, which have been developed. This synthetic route has been further... 

Purpose. This reaction completes the Sequence A experiments. The Diels-Alder reaction is used to form six-membered aromatic rings. You will carry out the decarbonylation and aromatization of an intermediate bicyclic Diels-Alder adduct.You will examine the properties of our synthetic target molecule, hexaphenylbenzene. 

The synthetic utility of the elimination reaction is enhanced by its usefulness in preparing a number of compounds difficult or impossible to synthesize by other methods. For example, in the preparation of RMn(CO)s compounds by the reaction of XMn(CO)s with anionic alkylating agents, reduction of XMn(CO)s to Mn2(CO)io lowers the yields (63). The alternative route of reacting the metal carbonyl anion with an acid chloride and then decarbonylating the resulting acyl complex is often more desirable (55, 64). 

Decarbonylation of aldehydes is frequently used in synthetic organic chemistry [19]. Aromatic aldehydes and enals, but also saturated aldehydes, have been shortened by one C atom with this transformation. In most cases, rhodium complexes were used in a stoichiometric reaction, but also catalytic transformations have been described [20]. It was found that RhCl3-3H20 modified with dppp was less air-sensitive than [Rh(COD)Cl]2 (COD = 1,5-cyclooctadiene) modified with dppp or the Wilkinson complex and therefore better suited for lab-scale experiments [21]. While using the Wilkinson complex, strictly oxygen-free conditions were essential for the success. Besides homogeneous rhodium catalysts, also supported complexes were suggested recently [22]. The reaction in ionic liquids is a possibility to recycle the precious rhodium complex [23]. 

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