Transition metal catalysis carbene reactions

Transition metal-catalysed reactions have emerged as powerful tools for carbon-carbon (C-C) bond formation [1], Cross-coupling reactions (Suzuki-Miyaura, Mizoroki-Heck, Stille, etc.) are recognised to be extremely reliable, robust and versatile. However, some other catalysed arylation reactions have been studied and have been reported to be very efficient [2]. In recent years, A -heterocyclic carbenes (NHC) have been extensively studied and their use as ligands for transition-metal catalysis has allowed for the significant improvement of many reactions [3]. This chapter highlights the use of NHC-bearing complexes in those arylation reactions. 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

Transition-metal catalysis, especially by copper, rhodium, palladium and ruthenium compounds, is another approved method for the decomposition of diazo compounds. It is now generally accepted that short-lived metal-carbene intermediates are or may be involved in many of the associated transformations28. Nevertheless, these catalytic carbene transfer reactions will be fully covered in this chapter because of the close similarity in reaction modes of electrophilic carbenes and the presumed electrophilic metal-carbene complexes. 

Abstract. N-Heterocyclic carbenes (NHC) have become an important class of organocatalysts and class of ligands for transition-metal catalysis. In organocat-alyzed umpolung reactions, thiazolium salt-derived NHC have been used successfully for decades. Even so, during recent years there has been an increased interest in NHC-catalyzed transformations and many new reactions have been developed. This article focuses on the use of NHC in the conjugate umpolung of ,f>-unsaturated aldehydes. 

It is worth mentioning that the synthesis of allenes from propargyl halides in Grignard reactions is favored by transition metal catalysis more than by the participation of allenic carbene intermediates in as much as the level of metal impurities in magnesium turnings is not reduced to a minimum, or metal salts such as ferric chloride and other metal chelates such as nickel acetoacetonate are added to the reaction mixture. ... 

Cyclopropanation reactions with disubstituted diazo esters are given in Table 13. The reactions with ethyl 2-diazopropanoate (entry 1) shows that 1,2-hydrogen shift at the carbenoid stage, a process that occurs readily in free carbenes and can also be effected by transition metal catalysis, is not competitive under these conditions. 

The diazo reactions in this chapter are characterized by processes run either in the gas phase, in relatively inert matrices, or in — typically, but not exclusively — aprotic and comparatively apolar solvents, either thermally or photolytically or with transition metal catalysis of various types. The metastable intermediates are carbenes (RR C ), i. e., neutral, apparently divalent, carbon compounds, or their transition metal complexes (coined carbenoids, see later in this section). It is interesting to recall that the synthesis of a compound that we now call a carbene, namely methylene (H2C ), was already attempted in the early 19th century, i.e., before the tetravalency of carbon was established. Dumas (1835) and Regnault (1839) thought then that it should be possible to obtain a compound consisting of one carbon and two hydrogen atoms by dehydration of methanol (a compound of which only the atomic ratio 1C 4H lO was then known). ... 

ABSTRACT. The applications of transition metal catalysis for controlling the reactions of carbenes (carbenoids and alkylidene complexes) are reviewed. 

Factors governing catalytic activity. Both catalytic activity and selectivity of carbene reactions are determined by the classical faptors governing homogeneous transition metal catalysis, particularly... 

The metallocarbene intermediates are most often formed from thermal, photolytic, or metal-catalyzed deconposition of diazocarbonyl compounds, with concomitant loss of dinitrogen. Under transition metal catalysis, the initially formed species is a metallocarbene rather than a free carbene, and this is usually desirable due to the moderated reactivity (and, hence, fewer undesired side reactions) of the metal-complexed carbene. The two most common methods for introduction of the diazo group are acylation of diazoalkanes with suitably activated carboxylic acid derivatives and diazo transfer reactions in the case of more acidic active methylene substrates fScheme 16.12T... 

The fact that carbenes are characterized by their stereospecific transformations has effectively paved the way for numerous innovative developments. The discovery that rhodium acetate can function as a catalyst for carbene generation from diazoacetates [15, 16] with much higher efficiency than copper salts, which had been traditionally used, provided an opening into the vast possibilities offered by transition metal catalysis. The increasing number of novel, stereoselective reactions of carbenoid species have become an indispensable toolbox for the modern practitioner of chemical synthesis [17-22],... 

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