Acychc carbenes

Underlying the growing interest in acychc carbene hgands are their imique hg-and properties, which differ substantially from those of cyclic carbenes. Data regarding donor ability and structural/steric features are of particular relevance for catalysis. However, these features have to be balanced against potential decomposition routes that do not affect NHCs. 

Bartolome C, Garcia-Cuadrado D, Ramiro Z, Espinet P. Synthesis and catalytic activity of gold chiral nitrogen acycHc carbenes and gold hydrogen bonded heterocyclic carbenes in cyclopropanation of vinyl arenes and in intramolecular hydroalkoxylation of allenes. Inorg Chem. 2010 49 9758-9764. 

AcycHc diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkyhdene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cycHc compounds, while the intermolecular reaction, called acycHc diene metathesis (ADM FT) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Nolan reported that the use of the carbene ligand derived from the deprotonation of 7 results in the formation of an efficient catalyst for the reaction of aryl chlorides and acyclic secondary amines [52]. For example, di- -butylamine efficiently reacts with 4-chloroanisole at 100°C in the presence of this catalyst, Eq. (54). Hartwig disclosed that the saturated carbene Hgand derived from the deprotonation of 16 yields a considerably more reactive catalyst which is capable of coupling acycHc secondary amines and aryl chlorides at room temperature [76]. 

In addition to cationic cyclizations, other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol. For example, treatment of (259) and (260) with Mn(III) and Cu(II) afford the D-homo-5a-androstane-3-ones (261) and (262), respectively, in approximately 30% yield. In this cyclization, seven asymmetric centers are established in one chemical step (226,227). Another intramolecular cyclization reaction of iodo-ene poly-ynes was reported using a carbopaUadation cascade terminated by carbonylation. This carbometalation—carbonylation cascade using CO at 111 kPa (1.1 atm) at 70°C converted an acycHc iodo—tetra-yne (263) to a D-homo-steroid nucleus (264) [162878-44-6] in approximately 80% yield in one chemical step (228). Intramolecular aimulations between two alkynes and a chromium or tungsten carbene complex have been examined for the formation of a variety of different fiised-ring systems. A tandem Diels-Alder—two-alkyne annulation of a triynylcarbene complex demonstrated the feasibiHty of this strategy for the synthesis of steroid nuclei. Complex (265) was prepared in two steps from commercially available materials. Treatment of (265) with Danishefsky s diene in CH CN at room temperature under an atmosphere of carbon monoxide (101.3 kPa = 1 atm), followed by heating the reaction mixture to 110°C, provided (266) in 62% yield (TBS = tert — butyldimethylsilyl). In a second experiment, a sequential Diels-Alder—two-alkyne annulation of triynylcarbene complex (267) afforded a nonaromatic steroid nucleus (269) in approximately 50% overall yield from the acycHc precursors (229). 

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