Substituent

About: Substituent is a research topic. Over the lifetime, 42877 publications have been published within this topic receiving 516716 citations. The topic is also known as: side chain & side group.

Total Synthesis of Roseophilin

Abstract: The first total synthesis of the antitumor agent roseophilin 1 is reported. Its intricate macrotricyclic core 2 is obtained by means of a new palladium-catalyzed manifold for the formation of ansa-bridged pyrroles which proceeds via vinyl oxirane 8 and allyl lactone 11 as key intermediates. After conversion of the latter into pyrrolophane 14, a base-induced elimination of the sulfone group followed by the Michael addition of a zincate onto the resulting enone 18 installs the isopropyl substituent in a stereoselective manner. The pyrrolylfuran side chain 3 of roseophilin is prepared from 4-methoxy-2(5H)-furanone (24) and methyl 4-chloropyrrole-2-carboxylate (26) as the starting materials. The appropriate building blocks 25a and 28 derived thereof are combined via a sequence comprising a directed metal−halogen exchange reaction, transmetalation of the resulting lithiopyrrole to the corresponding organozinc compound, a palladium-catalyzed cross coupling of the latter, and a subsequent acid-catalyzed closure ...

Iridium-Catalyzed Allylic Substitution

Abstract: Iridium-catalyzed asymmetric allylic substitution has become a valuable method to prepare products from the addition of nucleophiles at the more substi- tuted carbon of an allyl unit. The most active and selective catalysts contain a phosphoramidite ligand possessing at least one arylethyl substituent on the nitrogen atom of the ligand. In these systems, the active catalyst is generated by a base- induced cyclometalation at the methyl group of this substituent to generate an iridium metalacycle bound by the COD ligand of the (Ir(COD)Cl)2 precursor and one additional labile dative ligand. Such complexes catalyze the reactions of linear allylic esters with alkylamines, arylamines, phenols, alcohols, imides, carbamates, ammonia, enolates and enolate equivalents, as well as typical stabilized carbon nucleophiles generated from malonates and cyanoesters. Iridium catalysts for enantioselective allylic substitution have also been generated from phosphorus ligands with substituents bound by heteroatoms, and an account of the studies of such systems, along with a description of the development of iridium catalysts is included.

Carbene-Pnictinidene Adducts

Abstract: The syntheses, characterizations, and X-ray crystallographic structure determinations are described for adducts of stable nucleophilic carbenes with pnictinidenes. The adducts between 1,3-dimesitylimidazol-2-ylidene and phenylphosphinidene, phenylarsinidene, (trifluoromethyl)phosphinidene, and (pentafluorophenyl)arsinidene are reported. These carbene-pnictinidene adducts are formed by the direct reaction of a stable nucleophilic carbene with the corresponding pnictinidene cyclic oligomers. The synthesis and structure of the adduct between 1,3-dimesitylimidazolin-2-ylidene and phenylphosphinidene from the reaction of 1,3-dimesitylimidazolin-2-ylidene with phenylphosphorus dichloride are also reported. These carbene-pnictinidene adducts possess strongly polarized pnictinidene-carbene bonds. The C-Pn-C angles are all typically small at 97-102 degrees, and there is only a 4% shortening of the nominal Pn=C double bond compared to the Pn-C single bond to the second substituent on the pnictogen. The (31)P NMR shifts of the phosphorus adducts suggest strongly shielded phosphorus centers in accord with the polarized structures.

Conjugated 1,10-Phenanthrolines as Tunable Fluorophores.

Abstract: Multiple emission colors can be generated with the same compound from a novel family of highly emissive and visibly fluorescent 1,10-phenanthrolines 1. The emission wavelength of any derivative is dictated by the nature of its substituent and can be further modulated by exogenous additives such as protons or metal ions. R=H, Me, OMe, NMe(2).

On the Way to Carbene and Carbyne Complexes

Abstract: Publisher Summary The chapter focuses on carbene andcarbyne complexes. If one of the hydrogen atoms in an alkane hydrocarbon, such as ethane, is formally replaced by a metal atom, an organometallic compound in which the organic entity is bonded to the metal by a σ-bond is obtained. If a system with 2 carbon atoms bonded to each other by a double bond, i.e., an alkene molecule, is considered, a number of separate paths leading to organometallic derivatives are recognized. One path involves the replacement of a substituent by a metal atom and leads to σ compounds that are exemplified by vinyl-lithium derivatives. If molecules with a carbon-carbon triple bond of the type that exists in alkynes are considered, then it is realized that there also are three possible paths to metal-containing derivatives. The chapter discusses various properties of transition metal-carbene complexes such as preparation of the first carbene complexes, bonding concepts, and spectroscopic findings. It also presents various properties of transition metal-carbyne complexes such as preparation of the first carbyne complexes and x-ray structural analyses.