Π-allyl Complexes Research Articles

A Double Activation Method for the Conversion of Vinyl Epoxides into vic-Amino Alcohols and Chiral Benzoxazine/Quinoxaline Derivatives.

A new method for the synthesis of 1,2-syn-vic amino alcohols (with double inversion of configuration) from vinyl epoxides, by the amination of a π-allyl palladium-borate complex generated by using Pd(0) and phenyl-o-phenylene borate (a double activation technique), is reported. Further, this new method with broad functional group compatibility was extended to a one-pot/two-step synthesis of chiral benzoxazine and quinoxaline derivatives.

Palladium(II)-Catalysed Oxidation of Alkenes

This review provides a summary of recent developments in the palladium(II)-catalysed oxidation of alkenes, focusing largely on reactions which lead to the formation of new carbon–oxygen or carbon–nitrogen bonds. Three classes of reaction are covered: i) oxidations proceeding via allylic C–H bond cleavage and formation of a π-allyl complex; ii) Wacker-type oxidations proceeding via nucleopalladation followed by β-hydride elimination; and iii) 1,2-difunctionalisation of alkenes proceeding via nucleopalladation followed by functionalisation of the resulting σ-alkylpalladium(II) intermediate. The mechanisms are discussed alongside the scope and limitations of each reaction. 1 Introduction 1.1 Background 1.2 Oxidation Pathways 1.3 Observation of Reaction Intermediates 2 Allylic Oxidation 2.1 Background 2.2 Allylic Oxygenation 2.3 Allylic Amination 2.4 Allylic Functionalisation with Other Nucleophiles 3 The Wacker Oxidation 3.1 Background 3.2 Variation of the Co-Oxidant 3.3 Direct Oxygen-Coupled Wacker Oxidations 3.4 Aldehyde-Selective Wacker Oxidations 3.5 Wacker Oxidation of Internal Alkenes 3.6 Aza-Wacker Oxidations 4 Intermolecular 1,2-Difunctionalisation of Alkenes 4.1 Introduction 4.2 Oxyhalogenation Reactions 4.3 Dioxygenation Reactions 4.4 Oxycarbonylation Reactions 4.5 Aminohalogenation Reactions 4.6 Diamination Reactions 4.7 Aminooxygenation Reactions 4.8 Aminocarbonylation Reactions 5 Summary and Conclusions

N-Heterocyclic Carbene-Copper-Catalyzed Group-, Site-, and Enantioselective Allylic Substitution with a Readily Accessible Propargyl(pinacolato)boron Reagent: Utility in Stereoselective Synthesis and Mechanistic Attributes.

The first instances of catalytic allylic substitution reactions involving a propargylic nucleophilic component are presented; reactions are facilitated by 5.0 mol % of a catalyst derived from a chiral N-heterocyclic carbene (NHC) and a copper chloride salt. A silyl-containing propargylic organoboron compound, easily prepared in multigram quantities, serves as the reagent. Aryl- and heteroaryl-substituted disubstituted alkenes within allylic phosphates and those with an alkyl or a silyl group can be used. Functional groups typically sensitive to hard nucleophilic reagents are tolerated, particularly in the additions to disubstituted alkenes. Reactions may be performed on the corresponding trisubstituted alkenes, affording quaternary carbon stereogenic centers. Incorporation of the propargylic group is generally favored (vs allenyl addition; 89:11 to >98:2 selectivity); 1,5-enynes can be isolated in 75-90% yield, 87:13 to >98:2 SN2'/SN2 (branched/linear) selectivity and 83:17-99:1 enantiomeric ratio. Utility is showcased by conversion of the alkynyl group to other useful functional units (e.g., homoallenyl and Z-homoalkenyl iodide), direct access to which by other enantioselective protocols would otherwise entail longer routes. Application to stereoselective synthesis of the acyclic portion of antifungal agent plakinic acid A, containing two remotely positioned stereogenic centers, by sequential use of two different NHC-Cu-catalyzed enantioselective allylic substitution (EAS) reactions further highlights utility. Mechanistic investigations (density functional theory calculations and deuterium labeling) point to a bridging function for an alkali metal cation connecting the sulfonate anion and a substrate's phosphate group to form the branched propargyl addition products as the dominant isomers via Cu(III) π-allyl intermediate complexes.

Highly Active Nickel Catalysts for C-H Functionalization Identified through Analysis of Off-Cycle Intermediates.

An inhibitory role of 1,5-cyclooctadiene (COD) in nickel-catalyzed C-H functionalization processes was identified and studied. The bound COD participates in C-H activation by capturing the hydride, leading to a stable off-cycle π-allyl complex that greatly diminished overall catalytic efficiency. Computational studies elucidated the origin of the effect and enabled identification of a 1,5-hexadiene-derived pre-catalyst that avoids the off-cycle intermediate and provides catalytic efficiencies that are superior to those of catalysts derived from Ni(COD)2.

ChemInform Abstract: Regiodivergent Addition of Phenols to Allylic Oxides: Control of 1,2‐ and 1,4‐Additions for Cyclitol Synthesis.

Control of 1,2- and 1,4-addition of substituted phenols to allylic oxides is achieved by intercepting palladium π-allyl complexes. The interconversion of palladium complexes results in the total synthesis of MK 7607, cyathiformine B type, streptol, and a new cyclitol.

Cobalt-Promoted Tandem C(sp2)–H Activation and C–C Coupling of Vinyldiphenylphosphine: σ- vs π-Coordination Modes within Cyclometalation

We have studied the reactivity of a series of terminal alkenyldiphenylphosphines {Ph2P(CH2)n-CH═CH2), n = 0, 1, 2, 3} under mild conditions (−70 °C) in reaction with univalent Co(I). Methane and ethane elimination occurs when different olefinic P,C-coordination modes are established. Subsequent C–H activation and C–C coupling take place in the reaction of Co(CH3)(PMe3)4 with vinyldiphenylphosphine (1:2 molar ratio) to form a novel anionic P,C,P-type ligand with asymmetric ring sizes attached to the cobalt center of (PMe3)2Co{κ3-(P,C,P)-(Ph2)CH2CH═CH-CH(PPh2)} (1). X-ray study of 1 revealed that the complex has a unique cyclometalated structure of a three-membered ring within the newly assembled bis-phosphine ligand. Cyclometalation via selective vinyl-group C(sp2)–H activation of stoichiometric amounts allyl-diphenylphosphine with Co(CH3)(PMe3)4 afforded for the first time σ-allyl coordination incorporated in a five-membered cobaltacycle of (PMe3)2Co{κ2(P,C)-HC═CHCH2PPh2} (2); subsequent C–C coupling is not observed. By contrast, equimolar mixtures of pentenyl-diphenylphosphine and Co(CH3)(PMe3)4 afforded a η3-coordinated π-allyl complex of composition (PMe3)2Co{η3-κ4-(P,C,C,C)-H2C═CH-CH(CH2)2PPh2} (3). All unprecedented complexes were characterized by multinuclear NMR spectroscopy and X-ray diffraction, and their mechanisms of formation are discussed.

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(sp(3))-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.

Trapping hemiacetals with phosphono substituted palladium π-allyl complexes for the synthesis of substituted cyclic ethers

Oxidation of hydroxy substituted phosphono allylic carbonates gave the aldehyde substituted phosphonates in good yield. Stereospecific palladium (0)-catalyzed cyclization in the presence of methanol or water gave acetal tetrahydrofuran and tetrahydropyran vinyl phosphonate products derived from hemiacetal trapping. The tetrahydrofuran acetals undergo Lewis acid catalyzed addition of nucleophiles to give diastereoisomeric mixtures of substituted tetrahydrofurans.

Theoretical Study on Pd-Catalyzed Acylation of Allylic Esters with Acylsilanes and Acylstannanes

Acylation of allylic esters with acylsilanes and acylstannanes in the presence of a palladium complex was investigated theoretically using the DFT (B3PW91) method. We examined along the reaction that was reported by Tsuji’s. In this mechanism for generating active species, a Pd dinuclear complex 5 (the reaction of Pd and 2) was produced. Then, 5 is decomposed to two mononuclear complex 6. The reaction of 6 and 1 forms an intermediate 7, which is active species. In catalytic cycle from 7, the O (1) atom of 7 attacks the Si or Sn atom in TS7-8 to produce 8. Then, the C(1)-C(2) reductive elimination from 8 occurs through the TS8-9 to yield 9. Therefore, 9 decomposed to Pd(0), 3 and 4. However, reaction mechanism from 9 to 6 should be considered because Pd(0) + 3 + 4 are less stable than 9 by 29.2 kcal/mol, 9 does not decompose. We proposed the reaction mechanism from 9, as shown below: 1) 2 attacks 9 to form 10. 2) 10 released 4 to produce a five coordinated intermediate 11. 3) 11 changes its structure to another π-allyl complex 12. 4) The product 3 was released from 12 and 6 formed again for a next catalytic cycle. The rate-determining step of these reaction is nucleophilic attack of carbonyl oxygen to RA (7 → 8), and the △G≠ for I, II and III was calculated to be 27.1, 39.1, and 30.9 kcal/mol, respectively. As a result, we elucudated the reaction mechanism of acylation of allylic esters with acylsilanes and acylstannanes in the presence of a palladium complex.

Regiodivergent addition of phenols to allylic oxides: control of 1,2- and 1,4-additions for cyclitol synthesis.

Control of 1,2- and 1,4-addition of substituted phenols to allylic oxides is achieved by intercepting palladium π-allyl complexes. The interconversion of palladium complexes results in the total synthesis of MK 7607, cyathiformine B type, streptol, and a new cyclitol.

Catalytic diamination of olefins via N-N bond activation.

ConspectusVicinal diamines are important structural motifs present in various biologically and chemically significant molecules. Direct diamination of olefins provides an effective approach to this class of compounds. Unlike well-established oxidation processes such as epoxidation, dihydroxylation, and aminohydroxylation, direct diamination of olefins had remained a long-standing challenge and had been less well developed.In this Account, we summarize our recent studies on Pd(0)- and Cu(I)-catalyzed diaminations of olefins using di-tert-butyldiaziridinone and its related analogues as nitrogen sources via N–N bond activation. A wide variety of imidazolidinones, cyclic sulfamides, indolines, imidazolinones, and cyclic guanidines can be obtained from conjugated dienes and terminal olefins. For conjugated dienes, the diamination proceeds regioselectively at the internal double bond with the Pd(0) catalyst. Mechanistic studies show that the diamination likely involves a four-membered Pd(II) species resulting from the insertion of Pd(0) into the N–N bond of di-tert-butyldiaziridinone. Interestingly, the Cu(I)-catalyzed process occurs regioselectively at either the terminal or internal double bond depending on the reaction conditions via two mechanistically distinct pathways. The Cu(I) catalyst cleaves the N–N bond of di-tert-butyldiaziridinone to form a Cu(II) nitrogen radical and a four-membered Cu(III) species, which are likely in rapid equilibrium. The Cu(II) nitrogen radical and the four-membered Cu(III) species lead to the terminal and internal diamination, respectively.Terminal olefins are effectively C–H diaminated at the allylic and homoallylic carbons with Pd(0) as catalyst and di-tert-butyldiaziridinone as nitrogen source, likely involving a diene intermediate generated in situ from the terminal olefin via formation of a π-allyl Pd complex and subsequent β-hydride elimination. When di-tert-butylthiadiaziridine 1,1-dioxide is used as nitrogen source, cyclic sulfamides are installed at the terminal carbons via a dehydrogenative diamination process. When α-methylstyrenes (lacking homoallylic hydrogens) react with Pd(0) and di-tert-butyldiaziridinone, spirocyclic indolines are formed with generation of four C–N bonds and one spiro quaternary carbon via allylic and aromatic C–H amination.With Cu(I) catalysts, various terminal olefins can be effectively diaminated at the double bonds using di-tert-butyldiaziridinone, di-tert-butylthiadiaziridine 1,1-dioxide, and 1,2-di-tert-butyl-3-(cyanimino)-diaziridine as nitrogen sources, giving a variety of imidazolidinones, cyclic sulfamides, and cyclic guanidines in good yields, respectively. In the case of monosubstituted olefins using di-tert-butyldiaziridinone as nitrogen source, the resulting diamination products (imidazolidinones) are readily dehydrogenated under the reaction conditions, leading to the corresponding imidazolinones in good yields. Esters can also be diaminated to form the corresponding hydantoins with di-tert-butyldiaziridinone in the presence of a Cu(I) catalyst. A radical mechanism is likely to be operating in these Cu(I)-catalyzed reaction processes.Asymmetric processes have also been developed for the Pd(0)- and Cu(I)-catalyzed diamination reactions. Biologically active compounds such as (+)-CP-99,994 and Sch 425078 have been synthesized via the diamination processes. The diamination reactions described herein provide efficient methods to access a wide variety of vicinal diamines from readily available olefins and show great potential for synthetic applications.

Growth of nano hexagon-like flake arrays cerium carbonate created with PAH as the substrate

Petals-like Ce2(CO3)3 on Ce2(CO3)3 nano hexagon-like flake arrays have been precipitatingly fabricated using PAH substrates. By changing the way of feeding, PAH concentration and aging time, petals-like Ce2(CO3)3 was created best when adding PAH into the Ce(NO3)3 solution, joined (NH4)2CO3 solution along with mixing, PAH concentration is 0.9g/L, aging time is 4h. A growth mechanism was proposed to account for the growth of the petals-like Ce2(CO3)3 with PAH as the substrate. Poly allylamine hydrochloride (PAH) is as template agent which forms π-allyl complex with Ce3+ and controls the morphology of Ce2(CO3)3 particle. PAH and Ce3+ form π-allyl complex, and then induce the formation of Ce2(CO3)3 crystal nucleus. And infrared spectrum analysis verified. XRD show that after adding PAH which is adsorbed on the crystal plane, the growth of Ce2(CO3)3 crystal is inhibited on (242), the growth is promoted on (202) which is differentiated into the new (151), (222) is unchanged, Ce2(CO3)3 crystal is accumulated petals shape by hexagon-like flake. UV absorption spectra show that CeO2 as prepared precursor Ce2(CO3)3 after calcinations in air at high temperatures, the petal-like CeO2 has strong UV absorption and reflection effects, and absorption interval changed significantly by the move to UVA from UVB.

A New Concise Synthesis of (+)-Ipomeamarone, (−)-Ngaione, and Their Stereoisomers

Abstract A new concise and enantiocontrolled total synthesis of (+)-ipomeamarone (1), a well-known phytoalexin, is described. The key step involved a tetrahydrofuran ring construction from optically active furyl alcohol 5 via the π-allyl palladium complex with a chiral phosphine ligand. This procedure was also applicable to the synthesis of (−)-ngaione (2) and its stereoisomers.

1,4- and 1,3-Metal Migration in a Cp*IrIII Complex: Experimental Evidence of Direct 1,3-Metal Migration

Reactions of the phenyliridium(III) complex [Cp*IrCl(Ph)(PMe3)] with internal alkynes (RC≡CR, R = Ph or Me) in the presence of NaBArF4 give rise to the formation of the (o-vinylaryl)iridium complexes [Cp*Ir{o-C6H4C(R)═CHR}(PMe3)][BArF4] (2a, R = Ph; 2b, R = Me) via the alkyne insertion and the subsequent vinyl-to-aryl 1,4-miration of the Ir(III) center, while a similar reaction of PhC≡CMe affords the π-allyl complex [Cp*Ir{η3-CH(Ph)C(Ph)CH2}(PMe3)][BArF4] (4) in high yield. The latter is the formal 1,3-Ir migration product formed from the vinyliridium complex [Cp*Ir{C(Ph)═C(Me)Ph}(PMe3)][BArF4] (3c). Deuterium labeling experiments have revealed that this π-allyl complex is produced by two distinct mechanisms, (1) C═C bond rotation in 3c, followed by direct 1,3-Ir migration, and (2) successive vinyl-to-aryl and aryl-to-allyl 1,4-Ir migrations by way of the o-vinylaryl complex [Cp*Ir{o-C6H4C(Me)═CHPh}(PMe3)][BArF4] (2c). This reaction provides the first experimental evidence of the direct 1,3-metal migratio...

1,2-Selective Hydrosilylation of Conjugated Dienes

Selective 1,2-hydrosilylation of 1,3-dienes is a challenging problem in transition metal catalysis. Butadiene, specifically, would be a useful substrate because 3-butenylsilane products have promise as superior coupling reagents for hybrid organic/inorganic materials synthesis. We report the first selective 1,2-hydrosilylation of conjugated dienes including butadiene. Hydrosilylation proceeds through a Pt(II/IV) cycle, and selectivity is generated at a hexacoordinate Pt(IV) complex that favors η(2)-diene coordination and prevents π-allyl complex formation.

A highly efficient dual catalysis approach for C-glycosylation: addition of (o-azaaryl)carboxaldehyde to glycals.

A novel and efficient dual catalysis approach by concurrent activation of glycals and (o-azaaryl)-carboxaldehydes using palladium and N-heterocyclic carbene has been developed. The two electrophiles could react after activation through formation of the Breslow intermediate and a π-allyl Pd complex, widening the scope of reacting glycosylation partners and opening up possibilities for future glycosylation.

A silicon-position dependent 6-endo-trig cyclization during Tsuji–Trost alkylation

Two silylated cyclohexenes products have been prepared by using a Tsuji–Trost palladium-catalyzed cyclization. It involves the generation of a cationic π-allylic palladium complex bearing a triethylsilyl group on C-3, which cyclizes via a 6-endo-trig process to afford the cyclohexene derivatives. It is also demonstrated that the position of the silyl group on the starting allylic substrate strongly influenced the reaction. It could favor either the production of the expected cyclohexenyl ring or a diene by an elimination that occurs on the silyl-substituted C-2 π-allylic palladium complex.

Palladium-Catalyzed Allylic Alkylations as Versatile Tool for Amino Acid and Peptide Modifications

Palladium-catalyzed allylic alkylations are especially suitable for the introduction of γ,δ-unsaturated side chains into amino acids and even peptides. Glycine ester enolates are generally used as nucleophiles in these reactions, they react at a very low temperature (–78 °C) to give the products of isomerization-free allylation. In reactions of cis-configured allylic substrates, the olefin geometry can be transferred to the product. Because the syn position of the corresponding syn/anti π-allyl complex formed in this case is more reactive, this isomerization-free protocol also allows regioselective and stereoselective allylations. Using stannylated allylic substrates gives metalated amino acid derivatives that are ideal substrates for subsequent Stille couplings or tin–iodine exchange reactions. If peptides are deprotonated with excess­ strong base, the corresponding ester or amide enolates formed can also be subjected to allylation; in this case the stereochemical outcome can be controlled by the peptide chain.

N-Heterocyclic Carbene C,S Palladium(II) π-Allyl Complexes: Synthesis, Characterization, and Catalytic Application In Allylic Amination Reactions

A series of five-membered N-heterocyclic carbene C,S palladium(II) π-allyl complexes were successfully developed and characterized. Structural analyses of these complexes revealed that the organopalladium chelates adopt a skew-envelope conformation with a trans disposition of the substituents on the metal chelate rings. Utilizing these C,S palladium(II) π-allyl complexes as catalysts, a catalytic system for the allylic amination reaction has been developed. A series of C–N bond formations between amines and unsymmetrically substituted allylic carbonates could be catalyzed efficiently by complex (±)-9 in a regioselective manner.

Development of Unprotected Syntheses in Aqueous Media

Introduction of carbon side chain at C(3)-position of indole ring was accomplished by using Pd-catalyzed allylation and vinylation. The selective vinylation at C(3)-position of 4-bromoindole was applied to the synthesis of optically active 4-bromotryptophan derivatives, which was used as a starting material for the synthesis of several optically active ergot alkaloids, which were clavicipitic acids, chanoclavine-I, costacalvine, and 1,1-dimethylallyltryptophan (DMAT). The three-step synthesis of optically active clavicipitic acids were accomplished without using a protecting group starting from 4-bromoindole and dl-serine. Some new synthetic reactions using unprotected amino acids were developed. Those were the biomimetic synthesis of tryptophan, the bromination of free aromatic amino acids, and the Pd-catalyzed N-allylation of free amino acids with allylic alcohol in aqueous media. Unique reactivity of π-allyl palladium complex or η3-(benzyl)palladium complex in aqueous media was found through Pd-catalyzed reaction of anthranilic acid, 2-aminobenzamide, and indole with allylic alcohols or benzyl alcohols, respectively.