Elimination reactions decarbonylation

The photodecarbonylation of a series of dibenzyl ketones was studied by Robbins and Eastman/63 The results of this study are presented in Table 4.5. The data in Table 4.5 indicate that the presence of a p-methyl or a p-methoxy group has little effect on the quantum yield for this reaction. p-Cyano groups, on the other hand, essentially totally eliminated the decarbonylation. Since the reaction could also be quenched (inefficiently) by benzonitrile or biphenyl, it was concluded that the decarbonylation occurs from a short-lived triplet state. The effect of the p-cyano groups then could result from internal triplet quenching. 

This reaction occurs thermally in toluene at 30°C with an equilibrium constant (K) equal to 1.5 (13). Both bis(phosphine) and (carbonyl)phosphine Soret bands are present in the active catalyst solutions (see Decarbonylation Procedure), together with the unassigned, and likely critical, band at 420 nm. This could be due to some species giving rise to, or resulting from, a RuII + RCO reaction this is equivalent, of course, to a (Ru H-COR) acyl or a RuIII(C0)R (carbonyl)alkyl species, and the final elimination reaction after loss of CO could be written as ... 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

Decarboalkoxylation Palladium(II) acetate, 232 Tetrabutylphosphonium bromide, 288 Decarbonylation (see also Elimination reactions)... 

Theoretical evidence for the thermal decomposition mechanism for ethyl oxam-ate, ethyl oxanilate, and ethyl A,A-dimethyl oxamate have been provided.27 Ethyl oxamate and ethyl oxanilate undergo rapid decarbonylation to give the corresponding carbamates. Ethyl A,lV-dimethyloxamate elimination reaction yields in one step, through a six-membered cyclic transition state, dimethyloxamic acid and ethylene gas. 

Stable mononuclear transition-metal germylene complexes with a formal triple bond between the Ge and metal atoms (Cr, Mo, W, 56-62) have been synthesized by a combination of salt-elimination and decarbonylation reaction (equation 44)150,151. The decarbonylation process for the Cr and W intermediates 61 and 62 can involve either a thermal or a photochemical reaction the molybdenum germylyne complexes 56 and 57 were obtained directly, even at low temperatures. 

First, following the results of the 1,6-dioxa-spiro[2.5]octane rearrangement (5,19), continuous gas phase conditions were applied in a fixed bed reactor and secondly under liquid phase conditions in a slurry reactor. The catalytic experiments carried out showed that two main reactions took place rearrangement of 18 to the aldehyde 19 and a oxidative decarbonylation reaction to the olefine 1,3,3,4-tetramethyl-cyclohex-l-ene 20, which is assumed to be caused by a formaldehyde elimination reaction. Also observed was a deoxygenation reaction to the alkane 1,1,2,5-tetra-methylcyclohexane 21 (Eq. 15.2.7), explained by elimination of CO. There are several other side-products such as 2,2,3,6-tetramethylcyclohex-l-enyl-methanol, ringcontracting compounds and double bond isomers of dimethyl-isopropylene-cyclopentene. 

This is the second chapter of a two-part review concerned with insertion reactions of transition metal-carbon a-bonded compounds. The first chapter, which appeared in Volume 11 of this series (137), provided a broad introduction to the subject of insertion reactions in general and a detailed treatment of the carbon monoxide insertion and decarbonylation. Presented herein are the insertion and elimination reactions of sulfur dioxide and of a few other unsaturated molecules. The reactions of sulfur dioxide are accorded a complete literature coverage, whereas those of the other inserting species are treated selectively. Metal-carbon a-bonded compounds of the main group elements are discussed only in the context of comparisons with their transition metal analogs. 

The decarbonylation is completed by a reductive elimination reaction. The exact form of this reaction is determined... 

Although cyclopropylidenes have been generated by a number of independent methods which include cycloaddition of atomic carbon to alkenes, decarbonylation of cyclo-propylideneketenes, decarboxylation of oxaspiropentanes, from a preparative viewpoint, the a-elimination reaction of cyclopropane derivatives is most practical because of the mild reaction conditions and the availability of the starting materials. These methods are illustrated by the formation of allenes from atomic carbon/alkene reactions in Table 1 and in the preparation of 3/4, and from cyclopropanes in the preparation of 5/6. Examples showing allenes from decarbonylation of cyclopropylideneketenes are shown in the preparations of compounds 7/8, 9, 10, " and 11. ... 

On the other hand, the oxidative addition of aliphatic acid chlorides occurs in the absence of alkyne, but the oxidative addition complex could not be isolated due to fast decarbonylation followed by facile /1-hydrogen elimination. The decarbonylation of carboxylic acid was reported with palladium catalysts as well [47-56], In general, the reactions to acid anhydride as the intermediate need relatively high temperatures. 

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). 

In both this and the previous method of preparation, low yields are often found, due to competitive coupling reactions or to further reactions such as the decomposition of the M— R], products. Also tire metal anions are strong nucleophiles and they may cause the or mic halides to undergo elimination reactions. Metal-acyl complexes are often by-products of the reaction of organic halides with the metal carbonyl anions they are formed by insertion reactions (see p. 258). Indeed, acyl halides may be used to give metal-acyl complexes which are dien decarbonylated either thermally or by photochemi techniques [. Some metal-aryl complexes which are... 

Diphenylketene (253) reacts with allyl carbonate or acetate to give the a-allylated ester 255 at 0 °C in DMF, The reaction proceeds via the intermediate 254 formed by the insertion of the C = C bond of the ketene into 7r-allylpalla-dium, followed by reductive elimination. Depending on the reaction conditions, the decarbonylation and elimination of h-hydrogen take place in benzene at 25 °C to afford the conjugated diene 256(155]. 

Wilkinson s catalyst has also been reported to decarbonylate aromatic acyl halides at 180°C (ArCOX ArX). This reaction has been carried out with acyl iodides, bromides, and chlorides. Aliphatic acyl halides that lack an a hydrogen also give this reaction, but if an a hydrogen is present, elimination takes place instead (17-16). Aromatic acyl cyanides give aryl cyanides (ArCOCN—> ArCN). Aromatic acyl chlorides and cyanides can also be decarbonylated with palladium catalysts. °... 

Whereas some acyl products, especially RCOMn(CO)5 and RCOCo-(CO)4, easily eliminate carbon monoxide via the reverse of the insertion, others decarbonylate through a different route. For example, the reaction... 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

Processes such as decarbonylation, decarboxylation, elimination of water, and several other reactions may also occur prior to ionization, i.e., as non-mass spectral reactions, typically as a result of thermal degradation upon heating of the sample to enforce evaporation. In such a case, the mass spectrum obtained is not that of the analyte itself, but of its decomposition product(s). Sometimes, those thermal reactions are difficult to recognize, because the same neutral loss may also occur by a true mass spectral fragmentation of the corresponding molecular ion. 

---