Catalytic metal carbene transformations

Heterocycles as ligands in asymmetric catalytic metal carbene transformations 98CRV911. 

M. P. Doyle and D. C. Forbes, Recent Advances in As3fmmetric Catalytic Metal Carbene Transformations, Chem. Rev. 1998, 98, 911. 

Intramolecular cyclopropanation reactions of alkenyl diazo carbonyl compounds are among the most useful catalytic metal carbene transformations, and the diversity of their applications for organic syntheses is substantial [7,10,24,84]. Their catalytic asymmetric reactions, however, have only recently been reported. An early application of the Aratani catalyst 2 (A = PhCH2) to... 

M. P. Doyle, W. R. Winchester, J. A. A. Hoorn, V. Lynch, S. H. Simonsen, and R. Ghosh, ]. Am. Chem. Soc., 115, 9968 (1993). Dirhodiuim(II) Tetrakis(Carboxamidates) with Chiral L igands. Structure and Selectivity in Catalytic Metal-Carbene Transformations. 

The development of catalytic metal carbene transformations for the construction of heterocycles has dramatically increased due to their synthetic advan-... 

Among transition-metal compounds that are effective for metal carbene transformations, those of Cu and Rh have received the most attention [7-10]. Cu catalysis for reactions of diazo compounds with olefins has been known for more than 90 years [11], but the first report of Rh catalysis, in the form of dirhodium(II) tetraacetate, has been recent [12], Although metal carbene intermediates with catalytically active Cu or Rh compounds have not yet been observed, those... 

Diazocarbonyl compounds, especially diazo ketones and diazo esters [19], are the most suitable substrates for metal carbene transformations catalyzed by Cu or Rh compounds. Diazoalkanes are less useful owing to more pronounced carbene dimer formation that competes with, for example, cyclopropanation [7]. This competing reaction occurs by electrophilic addition of the metal-stabilized carbocation to the diazo compound followed by dinitrogen loss and formation of the alkene product that occurs with regeneration of the catalytically active metal complex (Eq. 5.5) [201. 

Scheme I Catalytic Cycle for Metal Carbene Transformations...

M.P.Doyle, Catalytic Methods for Metal Carbene Transformations , Chem.Rev., 1986, 86, 919. 

There has been no comprehensive monograph on carbenes since the books of Kirmse, Jones, and Maas were published in the early 1970 s. Does this development indicate a decrease in interest in carbene chemistry Not at all One might even say that it seems that the enormous increase in the number of investigations on carbene reactions, particularly with the help of complex rhodium and related metal catalysts, makes it difficult to write a book on all aspects of carbene chemistry. The catalytic procedures for metal-carbene transformations from aliphatic diazo compounds are now the most important tool for cyclopropanations and related processes in organic synthesis. We shall mention reviews on that subject in Sections 8.7 and 8.8. 

There is relevant material on carbenes in reviews on catalytic methods for metal carbene transformations formation of metal carbon multiple bonds from alkynes, metathesis catalysis. ... 

Abstract In this chapter, alkene synthesis based on the reactiOTi of //-tosylhydrazones is described. The reactivity of tosylhydrazones is determined by either the acidity of a-proton and hydrazcMie proton or the electropositivity of the carbon of C=N bond. This leads to diverse reactivities and a series of N-tosylhydrazone-based olefination methodologies. Both non-catalytic and transition metal-catalyzed olefinations from Al-tosylhydrazones are introduced in this chapter. Most of the transition metal-catalyzed reactions proceed via metal carbene transformations. The synthesis of alkenes through Pd-catalyzed cross-coupling reactions of Af-tosylhydrazones is particularly attractive and will be discussed in detail. 

Few methodologies have either the diversity of synthetic transformations or the high level of product selectivity as catalytic reactions with the intermediate involvement of metal carbenes [ 1-5]. They provide synthetic opportunities that are clearly demonstrated in the preparation of the antidepressant sertraline (1)... 

Catalytic reactions of disubstituted styrenyl substrates diminish oligomeric product formation because of the presence of ethylene. That is, if the initial transformation of the Ru-carbene occurs with the undesired regiochemis-try (e.g., 49->52 in contrast to 49->51, Scheme 13), dimerization and oligomerization may predominate, particularly in situations where reclosure of the carbocyclic ring is relatively slow (e.g., cycloheptenyl substrates). In contrast, as illustrated in Scheme 14, in the presence of ethylene atmosphere, the unwanted metal-carbene isomer 52 may rapidly be converted to triene 53. The resulting triene might then react with LnRu=CH2 to afford metal-carbene 51 and eventually chromene 41. 

Metal allenylidene complexes (M=C=C=CR2) are organometallic species having a double bond betv een a metal and a carbon, such as metal carbenes (M=CR2), metal vinylidenes (M=C=CR2), and other metal cumulenylidenes like M=C=C= C=CR2 [1]. These metal-carbon double bonds are reactive enough to be employed for many organic transformations, both catalytically and stoichiometrically [1, 2]. Especially, the metathesis of alkenes via metal carbenes may be one ofthe most useful reactions in the field of recent organic synthesis [3], vhile metal vinylidenes are also revealed to be the important species in many organic syntheses such as alkyne polymerization and cycloaromatization [4, 5]. 

Lewis bases (B ) that can occupy the open coordination site inhibit catalytic activity. The electrophilic nature of the metal carbene is seen in its subsequent reactions with nucleophiles (S ), which occur with the transfer of the carbene entity from the metal to a nucleophile without ever having generated or transformed an actual free carbene. 

A select number of transition metal compounds are effective as catalysts for carbenoid reactions of diazo compounds (1-3). Their catalytic activity depends on coordination unsaturation at their metal center which allows them to react as electrophiles with diazo compounds. Electrophilic addition to diazo compounds, which is the rate limiting step, causes the loss of dinitrogen and production of a metal stabilized carbene. Transfer of the electrophilic carbene to an electron rich substrate (S ) in a subsequent fast step completes the catalytic cycle (Scheme I). Lewis bases (B ) such as nitriles compete with the diazo compound for the coordinatively unsaturated metal center and are effective inhibitors of catalytic activity. Although carbene complexes with catalytically active transition metal compounds have not been observed as yet, sufficient indirect evidence from reactivity and selectivity correlations with stable metal carbenes (4,5) exist to justify their involvement in catalytic transformations. 

Up to this time, because complex 13 was not a highly active catalyst, there was concern that it may not be the true catalytic species in Katz s CM and ROMP experiments. It was suspected that 13 could first transform to a carbene complex with W in a high oxidation state before metathesis occurred. Further advancement in the field occurred a few years later, with Schrock s report in 1980 of alkylidenes 14 (M = Nb and Ta), which were active catalysts that led to productive metathesis.23 It was Schrock s opinion that true metathesis catalysts consisted of alkylidenes with metals in high oxidation states and that the discovery of complex 14 was significant because it was the first carbene complex to catalyze productive metathesis. To this day, there is disagreement about who can claim to have first discovered a metal-carbene complex that was a true metathesis catalyst.24... 

With respect to the ionic hquid s cation the situation is quite different, since catalytic reactions with anionic transition metal complexes are not yet very common in ionic liquids. However, the 1,3-dialkyhmidazolium cation can act as a hgand precursor for the dissolved transition metal. Its transformation under the reaction conditions into a ligand has been observed in three different ways (i) formation of metal carbene complexes by oxidative addition of the imidazolium cation (ii) formation of metal-carbene complexes by deprotonation followed by coordination of the imidazolylidene on the metal center (iii) dealkylation of the imidazolium cation and formation of a metal imidazole complex. These different ways are displayed in a general form in Scheme 5.3-2. 

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