With consecutive carbonylation reactions

Radical anions of carbonyl groups and imines also seem to be produced in the presence of titanium (IV) chloride in methanol as solvent. Consecutive oxidation and deprotonation of methanol leads to hydroxymethyl radicals which combine with the carbonyl radical anions to give 1,2-diols and 1,2-aminoalcohols, respectively. The synthesis of the pheromone frontalin has been achieved in a one-pot reaction by hydroxy-methylation of a diketone [127-129]. Likewise triplet sensitizers [130] can be used for direct excitation of the substrate in methanol [131]. Chiral aldimines can be conveniently hydroxymethylated with moderate diastereoselectivity by irradiation of methanolic solutions in the presence of an excess TiCU (Scheme 34) [132]. 

The homologation of acetic acid to carboxylic acids in general and to propionic acid in particular is not a typical carbonylation reaction, in that the substrate is not treated with carbon monoxide gas but with synthesis gas. However, as outlined below, the homologation step itself is, within the sequence of consecutive reactions, a typical carbonylation reaction and, therefore, the reaction sequence will be dealt with here (cf. Sections 2.1.2.4 and 3.2.7). 

The vast majority of transition metal clusters contain carbonyl ligands, which have been shown in many cases to be fluxional on the metal skeleton of the cluster (40,41). Therefore, the most obvious reactions to be catalyzed by such clusters should be those involving carbon monoxide. In fact, catalytic carbonylations are frequently encountered with transition metal carbonyl cluster catalysts, but very often the carbonylation step is followed by a consecutive step, e.g., a hydrogenation step, to give an overall hydroformylation. Simple carbonylation reactions have nevertheless been observed for various structures. 

Any attempt to rationalize the complex stereochemistry of olefin formation will have to be concerned with the mechanism of the Wittg reaction. Unfortunately many mechanistic details of this reaction have not been satisfactorily elucidated. Thus, it is even uncertain whether the reaction system (ylide plus carbonyl compound) passes, on its way to phosphine oxide and olefin, through an open chain zwitterion, i.e. a betaine, through a cyclic oxaphosphetane, or through both of them consecutively. The reaction sequence depicted in Figure I has no other merit than that of being believed to be the most probable one (1). 

It should be mentioned here however that products of this type are seldom reported and therefore seem not to be formed in considerable quantities. The reaction of the halide with the metal and consecutive reaction of the intermediate organometallic species (see Sect. 4.4 for a discussion of the mechanism of the Barbier reaction) with the carbonyl compound seems to be fast enough to prevent this ether formation. 

As the above description indicates, the mechanism for Yang photocychzation reactions involves at least two consecutive intermediates first an excited state and then a biradical. (As with most excited carbonyl reactions, both singlet and triplet states can react.) The quantum efficiency for cycHzation, as described by Eq. (8.1), depends on how many competitive reactions each intermediate undergoes ... 

An alkene activated by an electron-withdrawing group—often an acrylic ester 2 is used—can react with an aldehyde or ketone 1 in the presence of catalytic amounts of a tertiary amine, to yield an a-hydroxyalkylated product. This reaction, known as the Baylis-Hillman reaction, leads to the formation of useful multifunctional products, e.g. o -methylene-/3-hydroxy carbonyl compounds 3 with a chiral carbon center and various options for consecutive reactions. 

The three-component synthesis of benzo and naphthofuran-2(3H)-ones from the corresponding aromatic alcohol (phenols or naphthols) with aldehydes and CO (5 bar) can be performed under palladium catalysis (Scheme 16) [59,60]. The mechanism involves consecutive Friedel-Crafts-type aromatic alkylation and carbonylation of an intermediate benzylpalla-dium species. The presence of acidic cocatalysts such as TFA and electron-donating substituents in ortho-position (no reaction with benzyl alcohol ) proved beneficial for both reaction steps. 

The direct conversion deals with the straight hydrogenation of carbon monoxide to paraffins, olefins and heteroatom (oxygen, nitrogen) containing products. The indirect conversion invokes intermediates such as methanol, methyl formate and formaldehyde. The latter ones in a consecutive reaction can yield a variety of desired chemicals. For instance, acetic acid can be synthesized directly from CO/H2, but for reasons of selectivity the carbonylation of methanol is by far the best commercial process. 

In the presence of proton and/or Lewis acid and strong nucleophiles bicyclo[3.2.0]heptan-6-ones are converted to 3-substituted cycloheptanones (Table 15). Bicyclo[3.2.0]heptan-6-ones rearrange to give 3-iodocycloheptanones on treatment with iodotrimethylsilane. Zinc(II) iodide or mercury(II) halides as catalysts enhance the rate and the selectivity of the reaction.31 If a second, enolizable carbonyl group is present, an intramolecular alkylation may follow the ring enlargement under these reaction conditions.32 Consecutive treatment with tributyltin hydride/ 2,2 -azobisisobutyronitrile affords reduced, iodo-free cycloheptanones, whilst treatment with l,8-diazabicyclo[5.4.0]undecene yields cycloheptenones.33 Similarly, benzenethiol adds to the central bond of bicyclo[3.2.0]heptan-6-ones in the presence of zinc(II) chloride and hydrochloric acid under anhydrous conditions to form 3-(phenylsulfanyl)cycloheptanones.34... 

A number of reactions, principally of olefinic substrates, that can be catalyzed by supported complexes have been studied. These include hydrogenation, hydrosilylation, hydroformylation, polymerization, oxidative hydrolysis, acetoxylation, and carbonylation. Each of these will be considered in turn together with the possibility of carrying out several reactions consecutively using a catalyst containing more than one kind of metal complex. 

Both aromatic and aliphatic fluoroformates 7 can be readily prepared from phenols or alcohols and carbonyl difluoride and treated with sulfur tetrafluoride without isolation. Hydrogen fluoride evolved in the reaction of hydroxy compounds with carbonyl di fluoride serves as a catalyst for the consecutive reaction with sulfur tetrafluoride.15<)-162 This provides a general, convenient, direct synthesis of aryl and alkyl trifluoromethyl ethers 5 from phenols and alcohols. When the intermediate fluoroformate 7 is isolated prior to treatment with sulfur tetrafluoride, at least one mole equivalent of hydrogen fluoride is necessary to promote the fluorination reaction. 159 163 Representative examples of the conversion of hydroxy compounds 6 into trifluoromethyl ethers 5 via intermediate fluoroformates 7 are given (for other examples 7 -> 5, see Houben-Weyl, Vol. E4, pp 628. 629). 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

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