Titanium catalysts bicyclization

TL4419). The exo adduct 16 is formed in 95% ee from diphenylnitrone and 3-crotonyl-oxozolidin-2-one in the presence of titanium catalysts generated in situ from diisopropyl-titanium dichloride and chiral diols (94JOC5687). Intramolecular cycloaddition of the nitrone 17 yields the bicyclic isoxazoli-dine 18 stereoselectively (94JCS1661). The oxadiazabicyclo[3.3.0]octane 21 is formed when the oxime 19 is heated. The reaction proceeds by an intramolecular 1,3-dipolar... 

A practical titanium-catalyzed synthesis of bicyclic cyclopentenones and allylic amines is described. The process converts enyne substrates to iminocyclopentenes using 10 mol% of the air- and moisture-stable pre-catalyst Cp2TiCl2 in the presence of LiBu" and triethylsilyl cyanide.1... 

The catalytic [2 + 2 + 1]-cycloaddition reaction of two carbon—carbon multiple bonds with carbon monoxide has become a general synthetic method for five-membered cyclic carbonyl compounds. In particular, the Pauson-Khand reaction has been widely investigated and established as a powerful tool to synthesize cyclopentenone derivatives.110 Various kinds of transition metals, such as cobalt, titanium, ruthenium, rhodium, and iridium, are used as a catalyst for the Pauson-Khand reaction. The intramolecular Pauson-Khand reaction of the allyl propargyl ether and amine 91 produces the bicyclic ketones 93, which bear a heterocyclic ring as shown in Scheme 31. The reaction proceeds through formation of the bicyclic metallacyclopentene intermediate 92, which subsequently undergoes insertion of CO to give 93. 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

Some indispensable transition elements are iron (steel), copper (wiring), chromium (plumbing fixtures), gold and silver (jewelry and electronics), platinum (catalytic converters), titanium (bicycle and aircraft parts), nickel (coins and catalysts), and zinc (batteries), to mention a few of the better known ones. There are also the lesser known zirconium (nuclear-reactor liners), vanadium (axles and crankshafts), molybdenum (boiler plates), tantalum (organ-replacement parts), palladium (telephone-relay contacts)—the list goes on and on. As ions, quite a few of these elements also play vital roles in organisms. 

Bicyclic meso-N-ary aziridines are ring-opened by anilines, giving trans-1,2-diamines in the presence of a Ti(0-Bu-t)4/(l )-BlN0L catalyst in CH2Cl2." The reactions occur in yields between 65 and 96% with 57-99% ee. Lower enantioselectivities were found when electron-withdrawing and orifto-substituents were on the aniline. Alkyl amines failed to react. ESI-MS experiments indicate that the catalyst contains two titanium and two linked bis-BINOL ligands. 

Remarkably, very active catalysts for the copolymerization of bicyclic and polycyclic olefins with a-olefins are derived from transition metal metallocenes and cyclic or linear almninoxanes [31]. Cycloolefins (41 5) are reported to undergo high yield polymerization with linear olefins using metallocenes of titanium, zirconium, vanadium, or chromium associated with almninoxanes [31a]. 

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