Allyl Fragment Research Articles

Allylic Amination Using Well‐Defined [(NHC)Pd(η3‐allyl)Cl] Complexes and PPh3

AbstractThe allylic amination reaction catalyzed by [(NHC)Pd(allyl)Cl] complexes has been studied, and the presence of PPh3 found to be essential for the catalytic system to be active.[(1‐Mesityl‐3‐methylimidazol‐2‐ylidene)Pd(η3‐C3H5)Cl] has been used to optimize the conditions of the reaction with (E)‐1,3‐diphenylprop‐3‐enyl acetate and benzylamine under biphasic conditions (aqueous base/CH2Cl2). The best yields of isolated product (98 %) were obtained using aqueous 1 M K2CO3. The influence of the NHC ligand and the allyl fragment on the pre‐catalyst was also examined. Two new neutral [(NHC)Pd(η3‐1‐RC3H4)Cl] complexes [NHC = 1‐mesityl‐3‐methylimidazol‐2‐ylidene or 1‐(2,6‐diisopropylphenyl)‐3‐methylimidazol‐2‐ylidene; R = H or Ph] have been prepared and a decrease of the reaction time observed with the former. NMR studies have shown that this pre‐catalyst is more easily activated than its η3‐allyl analogue and that the predominant activation pathway involves attack of the amine at the allyl fragment. This reaction occurs exclusively in the presence of PPh3, thus suggesting that cationic [(NHC)Pd(allyl)(PPh3)]+ complexes, which are more electrophilic, are formed in situ and allow the amine to react with the allyl fragment. A tetrafluoroborate cationic complex has therefore been prepared from [(NHC)Pd(allyl)Cl], PPh3, and AgBF4 and fully characterized. This complex is an active pre‐catalyst in the allylic amination reaction. The scope of the reaction was examined under the optimized reaction conditions with several different nitrogen nucleophiles and allylic acetates. The amination products were obtained in yields ranging from 73 to >98 %, except for that from cyclohexenyl acetate and dibenzylamine. Finally, the reaction performed directly with the allylic alcohol and benzylamine led to a mixture of allylic amines in 9 % yield. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

On the Claisen rearrangement mechanism

It is believed that the key stage of the reaction is a concerted [3,3] sigmatropic shift accompanied by inversion of the allyl fragment [3]. However, there are data that the thermal ortho rearrangement can produce alkenylphenols whose structure indicates the lack of inversion of the allyl fragment ( α rearrangement) [4]. The reaction products in some instances involve dialkenylphenols [5, 6], and their formation is explained by intermolecular reactions via dissociation of C–O bond. para -Substituted phenols are assumed to result from ortho migration at the first reaction stage and allyl group migration through a new cyclic transition state to the para position at the second stage [3]. A dissociative mechanism for the formation of these products has also been supposed [5]. Thus, there some contradictions in the notion on the Claisen rearrangement mechanism.

Reactions of H2Os3(CO)10 with triallylboranes: formation of novel triosmium boron-containing olefin clusters

H2Os3(CO)10 (1) reacts with triallylborane B(CH2CHCH2)3 at 25 °C to form the cluster [Os3(CO)10(μ-η2:η2-trans-MeHCCH)2BC3H7] (2). Reaction of 1 with a mixture of triallylborane–trimethallylborane (1 : 3) gives a mixture of clusters 2, [Os3(CO)10(μ-η2:η2-Me2CCH)2BC3H7] (3) and {Os3(CO)10[μ-η2:η2-(trans-MeCHCH)(Me2CCH)]BC3H7} (4). The reaction results in the B–C bond cleavage and the isomerization of two allyl fragments into substituted vinyl groups coordinated to the osmium atoms in an η2-fashion, and reduction of the third fragment to a propyl group. The mechanism of the reaction is discussed based on isotope labelling and influence of the boron substituent ligand on the reaction path and product identity. Clusters 2, 3, 4 are characterized by spectroscopic means as well as by X-ray studies.

X-ray and conformation analysis of the new trinuclear cluster of osmium Os3(μ,η2-OCC6H5)(η3-C3H5)(CO)9

The crystal structure of the Os3(μ,η2-O=CC6H5)(η3-C3H5)(CO)9 cluster synthesized by the reaction of the (μ-H)Os3(μ-O=CC6H5)(CO)10 complex with allylamine in chloroform was determined by X-ray analysis. Prolonged storage of the reaction mixture led to N-C bond cleavage in allylamine and η3-addition of the allyl fragment at one of the Os atoms (Os-C 2.246 A, 2.248 A, and 2.273 A). The unit cell parameters of the complex are a = 9.494(1) A, b = 10.479(1) A, c = 12.474(2) A, α = 84.55(1)°, β = 70.08(1)°, γ = 70.72(1)°, V = 1255.8(4), A3, space group P\( \bar 1 \), Z = 2; C19H10O10Os3; dcalc = 2.922 g/cm3, 3085 Ihkl > 2σI of 3611 collected reflections; R = 0.0252. The structure of Os3(μ,η2-O=CC6H5)(η3-C3H5)(CO)9 is molecular. The plane of the Os3 triangle and the OsCOOs plane are connected according to the “butterfly” principle with an angle of 103.4° between them. The Os-Os distances in the cluster core vary from 2.836(1) A to 2.844(1) A; the Os-Ccarb distances are 1.88(1)–1.97(1) A; the distances to the atoms of the bridging ligands are Os-C 2.11(1) A, Os-O 2.14(1) A; the O-C bridging bond is 1.24(1) A. of the Os3(μ,η2-O=CC6H5)(η3-C3H5)(CO)9 triosmium cluster were studied theoretically. The potential curve of the internal rotation of the allyl ligand relative to the Os(1)-C(9) bond was determined. The rotation barrier of the allyl ligand in crystal relative to the Os(1)-C(9) bond is 8.38 kJ/mol, and the rotation of the ligand is not hindered. The effects of the intra-and intermolecular interactions on the conformation state of the cluster complex are considered.

Stereoselective Carbonyl Allylation by Umpolung of Allylpalladium(II) Complexes

AbstractThe nucleophilic palladium‐mediated allylation of a carbonyl group requires the inversion of the reactivity (umpolung) of the η3‐allylpalladium moiety. If the reaction is performed with a metal more electropositive than palladium, the stereochemical outcome is influenced by the solvent and by the reagent used for the umpolung. The stereoselectivity is also determined by the rate of η3–η1–η3 equilibration of the η3‐allylpalladium system relative to that of the transmetallation and addition to the aldehyde. When the umpolung is carried out with η1‐ or bis(allylpalladium) complexes, the stereoselectivity mainly depends on the nature of the substituents on the allyl fragment. Asymmetric addition has been achieved by using chiral ligands that promote palladium‐mediated reactions with “hard” nucleophiles. Utilization of selected chiral monophosphanes/phosphoramidite as ligands induces enantioselectivity that depends on the nature of the aldehyde as well as of the umpolung agent. Modest enantiomeric excesses have been obtained by using preformed bis(allylpalladium) systems with one chiral allylic fragment.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Using a Tripod as a Chiral Chelating Ligand: Chemical Exchange Between Equivalent Molecular Structures in Palladium Catalysis with 1,1,1‐Tris(oxazolinyl)ethane (“Trisox”)

Threefold symmetrical chiral podands may simplify the stereochemistry of key catalytic intermediates for cases in which they only act as bidentate ligands. This applies to systems in which chemical exchange between the different kappa2-coordinated forms takes place and in which the non-coordinated sidearm may play a direct or indirect role at some earlier or later stage in the catalytic cycle. Palladium(II)-catalysed allylic substitutions provide appropriate test reactions along these lines. A series of neutral dichloropalladium(II) complexes, [PdCl2(iPr-trisox)] (1a), [PdCl2(Ph-trisox)] (1b), [PdCl2(Bn-trisox)] (1c) and [PdCl2(Ind-trisox)] (1d) (trisox=1,1,1-tris(oxazolinyl)ethane) were synthesised by reaction of the respective trisox derivative with [PdCl2(PhCN)2] and characterised inter alia by 15N NMR spectroscopy. Direct detection of the heteronuclei without isotope enrichment and with "normal" sample concentrations was achieved with the aid of a cryogenically cooled NMR probe on a 600 MHz NMR spectrometer. Whereas the 15N nuclei of the coordinated oxazoline rings resonate at delta=160-167 ppm and appear as two singlets due to their diastereotopicity, the signal assigned to the dangling oxazoline "arm" is observed at delta=238-240 ppm. Variable-temperature NMR studies along with a systematic series of magnetisation transfer experiments established exchange between ligating and non-ligating oxazoline rings. Reaction of [Pd(allyl)(cod)]BF4 (cod=cyclooctadiene) with Ph-trisox in CH(2)Cl(2) gave the corresponding allyl complex 2, for which fast exchange between the three oxazoline heterocycles as well as between the exo and endo diastereomers was observed along with a very slow eta3-eta1-eta3 process of the allyl fragment (magnetisation transfer). Palladium(0) complexes were prepared by reaction of trisox derivatives or sidearm-functionalised BOX (BOX=bis(oxazolinyl)dimethylmethane) ligands with [Pd(nbd)(alkene)] (nbd=norbornadiene, alkene=maleic anhydride or tetracyanoethylene). X-ray diffraction studies of the iPr-trisox and Ph-trisox complexes (3a and 3b) established Y-shaped trigonal planar coordination geometries with the trisox ligand coordinated in a bidentate fashion, whilst the pi-coordinated maleic anhydride ligand adopts one of the two possible diastereotopic orientations. As the catalytic test reaction, the allylic alkylation of 1,3-diphenylprop-2-enyl acetate substrate with dimethyl malonate as nucleophile (in the presence of N,O-bis(trimethylsilyl)acetamide) was investigated for the trisox derivatives, their BOX analogues, and a series of less symmetric "sidearm" functionalised bisoxazolines. The trisoxazoline-based catalysts generally induce a better enantioselectivity compared to their bisoxazoline analogues and display significant reduction of the induction period as well as rate enhancement.

Palladium-Catalyzed Diastereoselective Allylation/Aza-Cope Rearrangement

Decarboxylative dearomatization-rearomatization has made possible this report of a tandem Pd-catalyzed allylation/aza-Cope rearrangement which generates 2-allyl-substituted heterocycles with tertiary or quaternary carbon centers. The regioselectivities are unusual given that Pd allylations normally occur with attack at the least-hindered end of the allyl fragment. Furthermore, the reaction occurs with high diastereoselectivities, which is rare with acyclic nucleophiles. A variety of heteroaromatic amines provide the same regioselecitivity and high dr values (except for N-methyl benzimidazole), though usually with only moderate yields. Elimination products were commonly seen but less prevalent at higher temperatures. Prenyl and crotyl esters can also be used in the reaction with the less basic heterocyles (benzoxazole and benzothiazole) with benzothiazole substrates generally giving higher yields and dr values.

Synthesis and Characterization of Palladium(II) π-Allyl Complexes with Chiral Phosphinocarbene Ligands. Kinetics and Mechanism of Allylic Amination

New palladium π-allyl complexes with chelating phosphinocarbene ligands (P−C) have been prepared and fully characterized. The adopted synthetic strategy is based on the reaction between the appropriate silver precursors and the labile [Pd(η3-allyl)OCOCF3]2. These complexes react with amines in the presence of dimethyl fumarate, yielding the new palladium(0) complexes [Pd(η2-dmfu)(P−C)] and allylamine. Under pseudo-first-order conditions, the amination rates obey the simple law kobs = k2[NHR2], and this observation seems to rule out the possibility of a simultaneous amine attack at the central atom with displacement of the bidentate ancillary ligand (or of one of its donor groups). The k2 values increase with increasing basicity of the amine, with decreasing steric hindrance at the allyl fragment, and with increasing bulkiness of the P−C heterocyclic nitrogen substituent. Moreover a remarkable decrease of the amination rate is observed for the phosphinocarbene complexes as compared to the “isostructural” p...

A Diosphenol-Based Strategy for the Total Synthesis of (−)-Terpestacin

A novel diosphenol-based strategy has been developed for the enantioselective synthesis of (−)-terpestacin by multiple usage of the α-diketone functionality, first in the “Pd AAA−Claisen rearrangement” protocol, and second by the employment of its oxidized form, the ene-1,2-dione, as an excellent Michael acceptor. This synthesis demonstrates that the sequence of O-allylation−Claisen rearrangement provides a chemo- and regioselective enolate allylation, which can be performed asymmetrically with respect to the enolate or allyl fragment or both. In addition, many interesting chemoselectivity issues, including a highly selective RCM and a dihydroxylation, have been addressed. Overall, this synthesis was accomplished in 20 longest linear steps (24 total steps) from the inexpensive and commercially available 3-methyl-1,2-cyclopentanedione.

Catalytic hydroallylation of norbornadiene with allyl formate

Norbornadiene (NBD) reacts with allyl esters All—OC(O)R (R = Me, But, Ph, CCl3, CF3) in acetonitrile solutions of palladium(0) complexes to give a mixture of four isomeric nontraditional allylation products and the corresponding carboxylic acids. Under similar conditions, the reaction of NBD with allyl formate in solutions of Pd0 and PdII complexes occurs selectively, resulting in the product of addition of the allyl fragment and the H atom to an NBD double bond, 5-allylbicyclo[2.2.1]hept-2-ene, and CO2. The hydroallylation of NBD is accompanied by catalytic addition of formic and acetic acids to one double bond of the diene to give bicyclo[2.2.1]hept-2-en-5-ol and nortricyclan-3-ol acetates and formates. Unlike most known palladium-based catalyst systems, these complexes exhibit catalytic activity also in the absence of phosphines.

Unusual reactivity of a sterically hindered diphosphazane ligand, EtN{P(OR)2}2, (R = C6H3(Pri)2-2,6) towards (η3-allyl)palladium precursors

The reactivity of (eta(3)-allyl)palladium chloro dimers [(1-R-eta(3)-C(3)H(4))PdCl](2) (R = H or Me) towards a sterically hindered diphosphazane ligand [EtN{P(OR)(2)}(2)] (R = C(6)H(3)(Pr(i))(2)-2,6), has been investigated under different reaction conditions. When the reaction is carried out using NH(4)PF(6) as the halide scavenger, the cationic complex [(1-R-eta(3)-C(3)H(4))Pd{EtN(P(OR)(2))(2)}]PF(6) (R = H or Me) is formed as the sole product. In the absence of NH(4)PF(6), the initially formed cationic complex, [(eta(3)-C(3)H(5))Pd{EtN(P(OR)(2))(2)}]Cl, is transformed into a mixture of chloro bridged complexes over a period of 4 days. The dinuclear complexes, [(eta(3)-C(3)H(5))Pd(2)(mu-Cl)(2){P(O)(OR)(2)}{P(OR)(2)(NHEt)}] and [Pd(mu-Cl){P(O)(OR)(2)}{P(OR)(2)(NHEt)}](2) are formed by P-N bond hydrolysis, whereas the octa-palladium complex [(eta(3)-C(3)H(5))(2-Cl-eta(3)-C(3)H(4))Pd(4)(mu-Cl)(4)(mu-EtN{P(OR)(2)}(2))](2), is formed as a result of nucleophilic substitution by a chloride ligand at the central carbon of an allyl fragment. The reaction of [EtN{P(OR)(2)}(2)] with [(eta(3)-C(3)H(5))PdCl](2) in the presence of K(2)CO(3) yields a stable dinuclear (eta(3)-allyl)palladium(I) diphosphazane complex, [(eta(3)-C(3)H(5))[mu-EtN{P(OR)(2)}(2)Pd(2)Cl] which contains a coordinatively unsaturated T-shaped palladium center. This complex exhibits high catalytic activity and high TON's in the catalytic hydrophenylation of norbornene.

Enantioselective homoallyl-cyclopropanation of dibenzylideneacetone by modified allylindium halide reagents—rapid access to enantioenriched 1-styryl-norcarene

Dibenzylideneacetone ( 8) reacts with in situ-generated allylindium halide reagents to yield the product of a homoallyl-cyclopropanation reaction: 2-(3″-butenyl)-1,1-bis[( E)-2′-phenylethenyl]cyclopropane ( 9), which proceeds via step-wise cleavage of the C O bond and delivery of two allyl fragments from the reagent. A range of enantiomerically enriched ligands have been tested as stoichiometric asymmetric modifiers for this process. Enantiopure compounds such as cinchona alkaloids, ephedra, aminoalcohols and tartaric acid derivatives, which have proven of utility as asymmetric modifiers for the indium-mediated allylation of aldehydes and ketones, were very inefficient in the process 8→ 9. However, mandelic acid derivatives, in particular mandelates, were found to be of significant potential. The absolute stereochemistry of the cyclopropane 9 has been determined by degradation to 1,1-dicarboxymethyl-2-butylcyclopropane, converging with an independent enantioselective synthesis starting from hexene. Under optimised conditions, viz. using allylindium iodide reagents and working-up with aqueous Na 2SO 3 to avoid iodine-mediated polymerisation, ( S)- 9 can be generated in 86% yield and with ( S)-methyl mandelate as modifier useful enantiopurity (94/6 er) was observed. The cyclopropane product (( S)- 9) undergoes RCM using standard conditions to afford a norcarene unit ((1 S,6 S)-1-( E)-2′-(phenylethenyl)-bicyclo[4.1.0]hept-2-ene) without loss of enantiopurity.

Reaction of arenediazonium tetrafluoroborates with allyl methacrylate in the presence of the thiocyanate anion

Allyl 2-thiocyanato-2-methyl-3-arylpropionates were obtained by the reaction of arenediazonium tetrafluoroborates with allyl methacrylate in a water-acetone (1: 5) medium, involving thiocyanatoarylation by the C=C bond of the methacrylic fragment. Under the examined conditions, the allylic fragment of the biunsaturated compound does not react.

Interactions of Cationic Palladium(II)‐ and Platinum(II)‐η3‐Allyl Complexes with Fluoride: Is Asymmetric Allylic Fluorination a Viable Reaction?

AbstractThe complex cations [M(η3‐R2All)(PPFPz{3‐tBu})]+ (M = PdII, R2All = 1,3‐diphenylallyl, 1,3‐dicyclohexylallyl, indenyl; M = PtII, R2All = 1,3‐diphenylallyl; PPFPz‐{3‐tBu} = 3‐tert‐butyl‐1‐{1‐[2‐diphenylphosphanyl‐ferrocenyl]ethyl}‐1H‐pyrazole)have been prepared as salts with PF6– or SbF6–. They have been characterized by NMR spectroscopy in solution and by X‐ray crystallography in the solid state. Their reactions with sources of nucleophilic and “naked” fluoride have been investigated by multinuclear NMR spectroscopy. The PdII complexes did not undergo any nucleophilic substitution with concomitant release of allyl fluorides. The dicyclohexylallyl fragment was released as a 1,3‐diene by elimination, but with other allyl complexes nonspecific decomposition reactions predominated. The complex [Pt(η3‐1,3‐Ph2C3H3)(PPFPz{3‐tBu})]PF6 underwent an anion exchange with Me4NF to give [Pt(1,3‐Ph2C3H3)(PPFPz{3‐tBu})]F which existed as a mixture of interconverting allyl isomers in solution at ambient temperature. For the bromide salt, [Pt(η3‐1,3‐Ph2C3H3)(PPFPz{3‐tBu})]Br, allyl isomerization was slow at ambient temperature. Precursors of Pt0 reacted with bromo‐1,3‐diphenylprop‐2‐ene to give [Pt2(μ‐Br)2(η3‐1,3‐Ph2All)2] and precursors of Pd0 underwent oxidative additions with bromo‐ and fluoro‐1,3‐diphenyl‐2‐propene to give 1,3‐diphenylallyl complexes of PdII. Therefore, the nucleophilic attack of fluoride on the allyl fragment of PdII complexes is endergonic, and the high energy barrier of this step is difficult to overcome in a catalytic allylic fluorination reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Structural and Spectroscopic Demonstration of Agostic C−C Interactions in Electron-Deficient Metallacyclobutanes and Related Cage Complexes: Possible Implications for Olefin Polymerizations and Metatheses

The reaction of the half-open titanocene, Ti(C5H5)(c-C8H11)(PMe3) (c-C8H11 = cyclooctadienyl), with two equivalents of PhC2SiMe3 leads to their incorporation and coupling to the dienyl fragment. One alkyne inserts into a C-H bond of the central CH2 group of the c-C8H11 ligand's edge-bridge, while the second undergoes a 5+2 coupling with the dienyl fragment, yielding coordinated sigma-allyl and olefin fragments, as demonstrated by X-ray diffraction. Together with the C5H5 and PMe3 coordinations, this leads to a 14-electron count. While the very electron-deficient titanium center passes up potential pi coordination of the allyl fragment, it instead engages in interactions with one or two C-C bonds, and perhaps a C-H bond, as revealed from the structural and spectroscopic data. Similar interactions have been found in electron-deficient metallacyclobutane complexes of titanium and zirconium, but not in the 18-electron molybdenum and tungsten analogues. These and other observations may have implications relating to metatheses and polymerizations of olefins.

Transformations of β-Hydroxo-Substituted η3-Allyl Pd Complexes in Neutral and Weakly Acidic Solutions

The oxidation of some β-hydroxo-substituted η3-allyl Pd complexes based on the simplest 1,3-dienes is studied by the 1H and 13C NMR methods in neutral and weakly acidic methods. The composition of the reaction products is determined by the nature of the oxidizing agent and the structure of allyl fragment. The method of selective oxidation of the β-carbon atom of the allyl ligand with the allyl-metal bond remaining unchanged is suggested.

Cyclopropyl‐ and Allyl‐Substituted Arenes in Reaction with Dinitrogen Tetroxide. Effect of Substrate Oxidation Potential on Reaction Direction.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Cover Picture: Bulky Monodentate Phosphoramidites in Palladium‐Catalyzed Allylic Alkylation Reactions: Aspects of Regioselectivity and Enantioselectivity (Chem. Eur. J. 24/2004)

The cover picture shows a modeled structure (PM3(tm)‐level) of a TADDOL derived monoligated bulky phosphorus amidite palladium–allyl complex. The allyl fragment, shown in green, has the syn,syn conformation and is clearly blocked by the aryl fragment of the S,S ligand upon counterclockwise rotation. Upon nucleophilic attack only clockwise rotation can occur, which explains the formation of the experimentally observed formation of the R product. A scheme of the reaction is shown in the background. For more information see the article by P. W. N. M. van Leeuwen et al. on p. 6232 ff.

Reaction of arenediazonium tetrafluoroborates with 3-(allyloxy)propane-1,2-diol and 2,2-bis(allyloxymethyl)butan-1-ol in the presence of thiocyanates

The reactions of arenediazonium tetrafluoroborates with 3-(allyloxy)propane-1,2-diol and 2,2-bis(allyloxymethyl)butan-1-ol in the presence of the thiocyanate nucleophile were used to obtain 3-(3-aryl-2-thiocyanatopropoxy)propane-1,2-diols and 2-[(allyloxy)methyl]-2-[(3-aryl-2-thiocyanatopropoxy)methyl]-butan-1-ols. Irrespective of reagent ratio, the second allyl fragment of 2,2-bis(allyloxymethyl)butan-1-ol fails to enter thiocyanatarylation. The presence of hydroxy groups in the unsaturated compounds studied render the latter less reactive than allyl derivatives containing no such groups.

Coordination of the Trost Modular Ligand to Palladium Allyl Fragments: Oligomers, Monomers, and Memory Effects in Catalysis

AbstractFor Abstract see ChemInform Abstract in Full Text.