Transition metal catalysis cycloisomerizations

Transition-metal-catalysed cycloisomerizations relying on carbenoid chemistry and avoiding diazocompounds as precursors have been reported twice. First, the syntheses of functionalized carbo- or hetero-cycles (67) have been achieved via the cycloisomerization of cyclopropene-ene substrates (68) upon Rh-catalysis. Rhodium carbenoid... 

The cycloisomerization of various skeletons via carbenoid intermediates has been achieved under transition metal-catalysis. Bis-nucleophilic propargyl ethers (88), ° as well as indole-based allenols (91), ° have been shown to be appropriate substrates for cycloisomerization under Pt(II)-catalysis, respectively furnishing bis-heterocyclic systems (89) and carbazole derivatives (92). While the formation of bis-heterocycles (89) mainly relies on the reactivity of the a, -unsaturated carbene (90) as key intermediate, the cyclization of (91) to carbazoles (92) has been suggested to proceed via (93) as the key carbene intermediate. Similarly to substrates (91), allenols (94) have been successfully cycloisomerized under Au(I)-catalysis, thus resulting in the formation of... 

The elimination of a hydrogen atom positioned on a carbon to the central metal constimtes an important reaction in transition metal catalysis. In the classical example, an alkylmetal intermediate is reversibly converted to an alkene and a metaUiydride (scheme 1.12). Despite the fact, that the resulting hydridometal complex can be exploited in various catalytic processes including polymerization reactions, [57] cycloisomerizations, [58] annulations, [59] etc., the ]S-hydride elimination is often considered undesired in transition metal catalyzed cross couplings. Thus, efforts have often been concentrated towards the prohibition of this fundamental reaction [60]. Nevertheless, the ]S-hydride elimination is a vital transformation in a number of catalytic processes including the ene-yne coupling reported by Trost [61] and Skrydstrup, [62] oxidation of alcohols, [63] the Heck reaction etc [64]. 

The 5-endo and 6-endo cyclizations of a,a>-alkynols leading to dihydrofurans and dihydropyrans have been achieved with molybdenum and tungsten catalysis [6]. Transition-metal vinylidene intermediates have been claimed to be involved in these cycloisomerizations [7]. Related cyclizations of bis-homo-propargyl alcohols were recently developed using ruthenium catalysis as shown in Eq. (3) [8]. In the presence of the sodium salt of N-hydroxysuccinimide 9,... 

Sequential transition-metal-catalyzed processes based upon cycloisomerization and cyclization are receiving increasing attention because they combine the increase of structural complexity (cyclizing reactions) with an atom economically benign use of the employed catalyst sources (sequential catalysis). This reactivity-based concept has been predominantly illustrated for sequentially Pd-, Rh-, and Ru-catalyzed processes, and currently the literature is dominated by the three transition metals. However, many other metal complexes can be envisioned to be equally fruitful in further developments and, without any doubt, even more unusual sequences are waiting to be discovered. Inevitably, only the opening chapter of this never-ending story has been told. 

Phosphorus is a key element in catalysis, and the last two Nobel prizes in molecular chemistry were awarded to Noyori, Sharpless and Knowles (2001) for their work on enantioselective catalysis and to Grubbs, Schrock and Chauvin (2005) for their work on the chemistry of transition metal carbene complexes and their applications in metathesis. In both cases the development of highly efficient, specifically tailored phosphorus based ligands are of paramount importance The book opens with an account of the recent studies on a new family of air-stable chiral primary phosphines based on the binaphthyl backbone and their applications in asymmetric hydrosilylations (Chap. 1). The concept of applying phosphorus ligands to enantioselective catalysis is also the main subject of Chaps. 5 and 10, dealing with P-based planar chiral ferrocenes and chiral phosphorus ligands for enantioselective enyne cycloisomerizations, respectively. 

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