Stereochemistry of the formation of π-allylpalladium complexes from dialkylcyclohexa-1,3-dienes

1,4-Dimethyl-, 1-isopropyl-4-methyl- and 1-t-butyl-4-methylcyclohexa-1,3-diene reacted with a palladium salt to form, in each case, a single isomer of the corresponding π-allylpalladium chloride complexes, while 2-isopropyl-5-methylcyclohexa-1,3-diene gave two stereoisomeric complexes. An excess of diene (diene/Pd = 2.5–3.0) was required to produce a high yield of the complex. The hydrogen atom, which is incorporated onto the terminal carbon of the diene system, is shown (i) to come from the excess diene, which in turn is converted to an aromatic compound, and (ii) to attack the diene, stereo- and regio-selectively, from the same side as the palladium chloride portion.     

Reversal of facial selectivity in complex Diels-Alder reactions

A complex Diels-Alder reaction between a semi-cyclic diene with allylic silyloxy substituents and a bromo enone presented an unusual diastereoselectivity: attack of the diene occured on its more hindered face, and this reversal of selectivity was shown to be induced by the presence of a bromo substituent in the dienophile.     

Rhodium complexes with the chelating and binucleating ligands P(CH2CH2Py)nPh3-n (Py = 2-pyridyl; n = 1,2): structures and fluxional behavior

Several rhodium(I) complexes of the type [RhX(CO)(PePy2)], [Rh(diene)(PePy)]+, and [Rh(diene)(PePy2)]+ (PePyn = P(CH2CH2Py)nPh3-n; Py = 2-pyridyl; n = 1, 2) have been prepared. The two former are square planar; the latter are pentacoordinated for diene = tetrafluorobenzobarrelene or norbornadiene (confirmed by X-ray diffraction), but an equilibrium of 4- and 5-coordinate isomers exists in solution for diene = 1,5-cyclooctadiene. The fluxional behavior of all these complexes is studied by NMR spectroscopy. The complex [Rh(NBD)(PePy2)]PF6.Cl2CH2 crystallizes in the monoclinic space group P21/n with a = 8.455(1) A, b = 18.068(3) A, c = 19.729(3) A, beta = 99.658(3)degrees, and Z = 4. The complexes [Rh(diene)(PePy2)]+ react with CO to give the dimeric complex [Rh2(CO)2[P(CH2CH2Py)2Ph]2](BF4)2 with the pyridylphosphine acting as P,N-chelating and P,N-bridging.     

Resolution of BIPHEP on Rh with a chiral diene auxiliary

Resolution of the atropisomeric chiral BIPHEP ligand on Rh has been achieved with the aid of 2-(4-tert-butyl-phenyl)-8-methoxy-1,8-dimethyl-bicyclo[2.2.2]octa-2,5-diene, a chiral chelating diene ligand. The diene complex 3 containing an (S)-BIPHEP ligand was found to be configurationally stable in CDCl₃ solution at RT. Conversion of the diene complex 3 to a dicarbonyl Rh complex 4 that had a barrier of 25.0 kcal/mol for atropisomerization of the BIPHEP ligand. Preliminary studies of the use of the resolved complex 3 for catalysis are presented.     

Double cyclization via intramolecular coupling between cyclohexadiene-Fe(CO)3 complexes and pendant conjugated dienes

Intramolecular double cyclization between a cyclohexadiene-Fe(CO)3 complex and a pendant diene yielded a tricyclic molecule containing a spirocenter, with defined relative stereochemistry of four contiguous carbon centers. The nature of the substituent on the pendant diene moiety has a significant effect on the yield: an ester group leads to lower yield. The product has a conjugated cyclohexadiene or methoxycyclohexadiene system; the latter was converted to an enone.     

Palladium‐Catalyzed CH Activation of N‐Allyl Imines: Regioselective Allylic Alkylations to Deliver Substituted Aza‐1,3‐Dienes

A new mode of activation of an imine via a rare aza‐substituted π‐allyl complex is described. Palladium‐catalyzed C(sp³)? H activation of the N‐allyl imine and the subsequent nucleophilic attack by the α‐alkyl cyanoester produced the 1‐aza‐1,3‐diene as the sole regioisomer. In contrast, nucleophilic attack by the α‐aryl cyanoester exclusively delivered the 2‐aza‐1,3‐diene, which was employed in an inverse‐electron‐demand Diels–Alder reaction for heterobiaryl synthesis.     

Palladium‐Catalyzed C?H Activation of N‐Allyl Imines: Regioselective Allylic Alkylations to Deliver Substituted Aza‐1,3‐Dienes

A new mode of activation of an imine via a rare aza‐substituted π‐allyl complex is described. Palladium‐catalyzed C(sp³) H activation of the N‐allyl imine and the subsequent nucleophilic attack by the α‐alkyl cyanoester produced the 1‐aza‐1,3‐diene as the sole regioisomer. In contrast, nucleophilic attack by the α‐aryl cyanoester exclusively delivered the 2‐aza‐1,3‐diene, which was employed in an inverse‐electron‐demand Diels–Alder reaction for heterobiaryl synthesis.     

Nd-AI双金属活性体的活性部位及其对共轭双烯烃聚合机理的研究

<正> 稀土催化剂活性体对双烯烃配位聚合的研究已有很多报道,使用催化剂活性体对双烯烃聚合机理的研究比以前所用的多组分混合物的催化体系显然具有很大的优越性。本文利用NdCl_3·3TBP和Ndel_3·3P_(350)配合物与Al(i-Bu)_3体系所得活性体合成了共轭双烯烃低聚物,用IR、~(13)C-NMR光谱研究了这些低聚物分子的链端结构,从而推测活性体的活性部位组成和聚合机理。     

Synthesis of (+)‐Discodermolide by Catalytic Stereoselective Borylation Reactions

The marine natural product (+)‐discodermolide was first isolated in 1990 and, to this day, remains a compelling synthesis target. Not only does the compound possess fascinating biological activity, but it also presents an opportunity to test current methods for chemical synthesis and provides an inspiration for new reaction development. A new synthesis of discodermolide employs a previously undisclosed stereoselective catalytic diene hydroboration and also establishes a strategy for the alkylation of chiral enolates. Furthermore, this synthesis of discodermolide provides the first examples of the asymmetric 1,4‐diboration of dienes and borylative diene–aldehyde couplings in complex‐molecule synthesis.     

Consecutive Rh(I)-catalyzed Alder-ene/Diels-Alder/Diels-Alder reaction sequence affording rapid entry to polycyclic compounds

Conversion of acyclic allenynes to polycyclic compounds using consecutive transition metal catalyzed carbon-carbon bond forming reactions in a single chemical operation is described. Reaction of an allenyne with a Rh(I) catalyst affords a cross-conjugated triene via a formal Alder-ene reaction. The triene then participates in a Rh(I)-catalyzed intramolecular [4+2] cycloaddition reaction to generate a new conjugated diene. An external dienophile is added to this diene which then undergoes a second [4+2] cycloaddition reaction to afford a complex polycyclic ring system. This reaction sequence demonstrates the synthetic potential of allenynes and cross conjugated trienes and highlights the rapid increases in molecular complexity that are possible by one-pot sequential transition metal catalyzed carbon-carbon bond forming reactions.     

Theoretical Investigations on the Mechanism of Hetero‐Diels–Alder Reactions of Brassard’s Diene and 1, 3‐Butadiene Catalyzed by a Tridentate Schiff Base Titanium(IV) Complex

The mechanism of the hetero‐Diels–Alder reactions of Brassard’s diene and 1,3‐butadiene catalyzed by a titanium(IV) complex of a tridentate Schiff base was investigated by DFT and ONIOM methods. The calculations indicate that the mechanism of the reaction is closely related to the nucleophilicity–electrophilicity between diene and carbonyl substrates. A stepwise pathway is adopted for Brassard’s diene, and the step corresponding to the formation of the C? C bond is predicted to be the rate‐determining step with a free‐energy barrier of 8.4 kcal mol^?1. For 1,3‐butadiene, the reaction takes place along a one‐step, two‐stage pathway with a free‐energy barrier of 14.9 kcal mol^?1. For Brassard’s diene as substrate, the OCH₃ and OSi(CH₃)₃ substituents may play a key role in the formation of the transition state and zwitterionic intermediate by participating in charge transfer from Brassard’s diene to formaldehyde. The combination of the phenyl groups at the amino alcohol moiety and the ortho‐tert‐butyl group of the salicylaldehyde moiety in the chiral tridentate Schiff base ligand plays an important role in the control of the stereoselectivity, which is in agreement with experimental observations.     

Iridium‐Catalyzed Enantioselective C?H Alkylation of Ferrocenes with Alkenes Using Chiral Diene Ligands

The first catalytic and enantioselective C H alkylation of ferrocene derivatives with various alkenes was achieved. A cationic iridium complex, having a chiral diene ligand, and an isoquinolyl moiety as a directing group are essential for regioselective and enantioselective C H bond activation.     

Inter- and intramolecular carbene reactions of diazoketones tethered to tricarbonyliron coordinated acyclic dienes. New tricarbonyliron complexes of cyclohexa-2,4-dienone and cyclopent-2-enone

A series of diazoketones tethered to tricarbonyliron coordinated dienes at the terminal position have been synthesized, and their decomposition reactions as carbene precursors studied. Intermolecular reactions with nucleophilic olefins were observed with Cu(acac)₂ as catalyst, the tricarbonyliron playing the role of an efficient protecting group of the diene fragment. With rhodium^II acetate, intramolecular reactions predominate, with formation of five-membered rings, in the side chain, or less commonly, as a cyclopentenone joined side by side to a rearranged diene ligand. Unusually, due to cationic stabilization by the iron, six-membered rings in the side chain are also formed here. A still different reaction was observed during the thermal decomposition of a diazoketone tethered to a coordinated diene at position 2: the formation of a stable complex of a cyclohexa-2-4-dienone (iron assisted carbene reaction and multiple hydrogen migrations).     

Iridium-mediated asymmetric hydrogenation of 2,3-diphenylbutadiene: a revealing kinetic study

Hydrogenation of 2,3-diphenylbutadiene mediated by a chiral iridium carbene oxazoline complex was studied. The kinetics of the reaction showed that it occurred in two distinct stages. The first corresponded to slow consumption of the diene to form half-hydrogenated products, i.e., monoenes. After all the diene was consumed, however, the reaction was much faster and more stereoselective. These data were interpreted in terms of possible catalytic intermediates.     

Iron tricarbonyl stabilized pentadienyl cation as initiator for cascade polycyclizations: a diastereoselective entry into octahydrophenanthrenes

A new example is provided of completely diastereoselective polycyclization, affording the octahydrophenanthrene framework. Generation of an iron tricarbonyl stabilized pentadienyl carbocation is the triggering event of the cascade reaction. The carbocation is generated by anchimerically assisted regiospecific protonation of a double bond adjacent to the iron tricarbonyl diene moiety. Tetrafluoroboric acid ether complex appears to be the optimum reagent, affording good yields, even under catalytic conditions.     

Iridium‐Catalyzed Enantioselective CH Alkylation of Ferrocenes with Alkenes Using Chiral Diene Ligands

The first catalytic and enantioselective C? H alkylation of ferrocene derivatives with various alkenes was achieved. A cationic iridium complex, having a chiral diene ligand, and an isoquinolyl moiety as a directing group are essential for regioselective and enantioselective C? H bond activation.     

Preparation and Diels-Alder Reactivity of Tricarbonyliron Complexes of [2.2.2]Hericene

Treatment of [2.2.2]hericene 10 with Fe₂(CO)₉ in hexane gave a mixture of monometallic complex 14 , two isomeric dimetallic complexes 15 and 16 , and a trimetallic complex 19 in which all the three diene moieties of 10 , are coordinated. The rate constants of the Diels-Alder additions of tetracyanoethylene (TCE) and dimethyl acetylenedicarboxylate (DMAD) to the uncomplexed diene moieties of 14–16 have been evaluated and compared with those measured for the uncomplexed 10 and its monoadducts 11A, 11B and bis-adducts 12A, 12B. The tricarbonyldieneiron function retards the cycloaddition of an homoconjugated, exocyclic s-cis-butadiene. The effect is significantly larger for TCE- than for DMAD-additions. The origin of this effect is discussed briefly in terms of the valence-bond model which is usually assumed to describe the properties of a tricarbonyldieneiron complex, and in terms of the inductive and steric factors of the Fe(CO)₃-group.     

Selective oxygenation of amphiphilic thiacalix[3]pyridine Rh(I) diene complexes in both water and organic solvents

Thiacalix[3]pyridine (Py3S3) reacted with [Rh(diene)(mu-Cl)]2(diene = 1,5-cyclooctadiene (cod), 2,5-norbornadiene (nbd)) to give amphiphilic trigonal bipyramidal complexes, [Rh(Py3S3)(diene)]Cl. Sulfur bridges of the Py3S3 ligand in these complexes were selectively oxygenated by m-chloroperoxybenzoic acid in dichloromethane to give sulfinylcalix[3]pyridine complexes, [Rh(Py3(SO)3)(diene)]+, in which all three oxygen atoms of the SO groups occupy the equatorial positions. Structures of the complexes were analysed by X-ray crystallography and the oxidation reaction was investigated using 1H NMR spectroscopy and electrospray ionisation mass spectrometry showing that the oxygenation of the sulfur atoms in the ligand proceeded stepwise and further oxygenation of the SO moiety occurred only for the nbd complex having the smaller diene ligand resulting in [Rh(Py3(SO)2(SO2))(nbd)]+. On the other hand, the oxidation of [Rh(Py3S3)(cod)]+ by H2O2 in water did not result in oxygenation of the sulfur bridges but the cod ligand is hydroxygenated to give 1,4,5,6-eta4-2-hydroxycycloocta-4-ene-1,6-di-yl.     

Asymmetric Synthesis of Axially Chiral 2‐Aminobiaryls by Rhodium‐Catalyzed Benzannulation of 1‐Arylalkynes with 2‐(Cyanomethyl)phenylboronates

Asymmetric benzannulation of 1‐arylalkynes, where the aryl group is an ortho‐substituted aromatic group, with 2‐(cyanomethyl)phenylboronate was catalyzed by a rhodium complex coordinated with a chiral diene ligand to give high yields of axially chiral 2‐aminobiaryls with greater than 90 % ee.     

The mechanism of photoinsertion of haloalkenes into Fe---C bonds of dieneiron tricarbonyls

1,1-Dichloro-2,2-difluoroethylene (like other fluoroalkenes) and dieneiron tricarbonyls react photochemically to give products in which the haloalkene has inserted into the Fe---C(1) bond of the diene complex. The reaction is regiospecific with respect to both diene and haloalkene. The reaction is shown to occur by (a) photodissociation of a CO ligand from iron; (b) π-complexation of haloalkene; (c) π → σ ligand rearrangements accompanied by CO reattachment. The adducts undergo hydrolysis on silica gel chromatography, to give substituted 2-chlorocyclohexadienoneiron tricarbonyls.