Acyl from metallates

The mechanism of the condensation in Part D probably involves thioformylation of the metallated isocyanoacetate followed by intramolecular 1,1-addition of the tautomeric enethiol to the isonitrile. This thi2izole synthesis is analogous to the formation of oxazoles from acylation of metallated isonitriles with acid chlorides or anhydrides. " Interestingly, ethyl formate does not react with isocyanoacetate under the conditions of this procedure. Ethyl and methyl isocyanoacetate have been prepared in a similar manner by dehydration of the corresponding N-formylglycine esters with phosgene and trichloromethyl chloroformate, respectively. The phosphoryl chloride method described here was provided to the submitters by Professor U. Schollkopf and is based on the procedure of Bohme and Fuchs. The preparation of O-ethyl thioformate in Part C was developed from a report by Ohno, Koi/.uma, and Tsuchihaski. " ... 

From Acyl Complexes Generated from Metallates... 

Fig. 2.4. Generation and O-alkylation of nucleophilic acyl complexes from metallates and acyl halides [101].

The main steps in the catalytic MeOH carbonylation cyde which were proposed for the Co catalysed process [2] have served, with some modification perhaps in the carbonylation of MeOAc to AC2O, to the present day and are familiar as a classic example of a metal catalysed reaction. These steps are shown in Eigure 5.1. They are of course, (i) the oxidative addition of Mel to a metal center to form a metal methyl species, (ii) the migratory insertion reaction which generates a metal acyl from the metal methyl and coordinated CO and (iii) reductive elimination or other evolution of the metal acyl spedes to products. Broadly, as will be discussed in more detail later, the other ligands in the metal environment are CO and iodide. To balance the overall chemistry a molecule of CO must also enter the cycle. 

By far the most generally useful synthetic application of allyltributyltin is in the complementary set of transition metal- and radical-mediated substitution reactions. When the halide substrates are benzylic, allylic, aromatic or acyl, transition metal catalysis is usually the method of choice for allyl transfer from tin to carbon. When the halide (or halide equivalent) substrate is aliphatic or alicyclic, radical chain conditions are appropriate, as g-hydrogen elimination is generally not a problem in these cases. 

The transfer of carbamate groups from metal carbamates to acylating agents, such as acyl halides and chloroformates, has been extensively studied by Calderazzo and coworkers. 

Calderazzo has reported the synthesis of isocyanates from metal N-alkyl carbamates and acyl or aroyl chlorides (Scheme 6.27), used in this context as oxygen... 

The product isolated from the reaction of a 1 1 stoichiometric amount of CO with the lutetium complex was characterized as a monoinsertion product, an acyl. From spectroscopic data, a dihaptoacyl structure involving significant Lu-0 interaction was postulated. With excess CO, a dimeric complex was isolated in which four CO molecules were coupled via one C=C double bond and two C—C single bonds to form an enedionediolate moiety. The condensation of these four CO molecules can be rationalized based on the carbenoid character of the dihaptoacyl carbon atom. This multiple coupling of CO has precedent in early transition metal... 

S.8.2.8.4. rC o-Acyls from Alkall-Motal Carbanions and Metal Carbonyls. 

Metal triflates can be easily prepared from metal halides and triflic acid at -78 C. They show several unique properties compared with the corresponding metal halides. In an early study, Olah reported the use of boron-, aluminum-, and gallium triflates [M(OTf)J as effective Friedel-Crafts catalysts. In the benzoylation and acetylation of toluene and benzene with acyl chlorides, the relative reactivity is boron triflate > gallium triflate > aluminum triflate, in agreement with the relative acidity strength. 

G.4. t] ff-Acyls from Aikali-Metal Carbanions and Metal Garbonyls. 

Metal triflates that can be easily prepared from metal halides and triflic acid at -78°C [14] show several unique properties compared with the corresponding metal halides. The use of bismuth(III) triflate allows for acylation of both activated and deactivated aromatic compounds with anhydrides and acyl chlorides [15]. Thus, the acylation of deactivated aromatics such as trifluoromethoxyben-zene, fluorobenzene, and chlorobenzene can be achieved in high yields with benzoyl chloride in the presence of bismuth(III) triflate (10% mol) without solvent. The />ara-acylation product is the most abundant in all cases (trifluoromethoxybenzene 87% yield, ortho.para 4 96 fluorobenzene 86% yield, orthoipara 0 100 chlorobenzene 89% yield, ortho para 13 87). 

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