Nonchelated d0 zirconium-alkoxide-alkene complexes

The reaction of Cp'2Zr(O(t)Bu)Me (Cp' = C5H4Me) and [Ph3C][B(C6F5)4] yields the base-free complex [Cp'2Zr(O(t)Bu)][B(C6F5)4] (6), which exists as Cp'2Zr(O(t)Bu)(ClR)+ halocarbon adducts in CD2Cl2 or C6D5Cl solution. Addition of alkenes to 6 in CD2Cl2 solution at low temperature gives equilibrium mixtures of Cp'2Zr(O(t)Bu)(alkene)+ (12a-l), 6, and free alkene. The NMR data for 12a-l are consistent with unsymmetrical alkene bonding and polarization of the alkene C=C bond with positive charge buildup at C(int) and negative charge buildup at C(term). These features arise due to the lack of d-pi* back-bonding. Equilibrium constants for alkene coordination to 6 in CD2Cl2 at -89 degrees C, K(eq) = [12][6](-1)[alkene](-1), vary in the order: vinylferrocene (4800 M(-1)) > ethylene (7.0) approximately alpha-olefins > cis-2-butene (2.2) > trans-2-butene (<0.1). Alkene coordination is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating groups and the beta-Si effect. Compounds 12a-l undergo two dynamic processes: reversible alkene decomplexation via associative substitution of a CD2Cl2 molecule, and rapid rotation of the alkene around the metal-(alkene centroid) axis.     

Enantioconvergent synthesis by sequential asymmetric Horner-Wadsworth-Emmons and palladium-catalyzed allylic substitution reactions

A new method for enantioconvergent synthesis has been developed. The strategy relies on the combination of an asymmetric Horner-Wadsworth-Emmons (HWE) reaction and a palladium-catalyzed allylic substitution. Different alpha-oxygen-substituted, racemic aldehydes were initially transformed by asymmetric HWE reactions into mixtures of two major alpha,beta-unsaturated esters, possessing opposite configurations at their allylic stereocenters as well as opposite alkene geometry. Subsequently, these isomeric mixtures of alkenes could be subjected to palladium-catalyzed allylic substitution reactions with carbon, nitrogen, and oxygen nucleophiles. In this latter step, the respective (E) and (Z) alkene substrate isomers were observed to react with opposite stereospecificity: the (E) alkene reacted with retention and the (Z) alkene with inversion of stereochemistry with respect to both the allylic stereocenter and the alkene geometry. Thus, a single gamma-substituted ester was obtained as the overall product, in high isomeric purity. The method was applied to a synthesis of a subunit of the iejimalides, a group of cytotoxic macrolides.     

Phosphine‐Catalyzed Remote β‐CH Functionalization of Amines Triggered by Trifluoromethylation of Alkenes: One‐Pot Synthesis of Bistrifluoromethylated Enamides and Oxazoles

An unprecedented phosphine‐catalyzed remote β‐C? H functionalization of amine derivatives triggered by trifluoromethylation of an alkene with Togni’s reagent was disclosed. This reaction proceeded through the highly selective and concomitant activation of an unactivated alkene and the β‐C? H bond of an amine derivative, providing bistrifluoromethylated enamides in excellent yields with good regio‐, chemo‐, and stereoselectivity. Furthermore, the newly developed one‐pot protocol provides a facile and step‐economical access to valuable trisubstituted 5‐(trifluoromethyl)oxazoles. Mechanistic studies showed that this reaction may initiate with a novel phosphine‐catalyzed radical trifluoromethylation of unactivated alkene via a phosphorus radical cation.     

Rücktitelbild: Phosphine‐Catalyzed Remote β‐C?H Functionalization of Amines Triggered by Trifluoromethylation of Alkenes: One‐Pot Synthesis of Bistrifluoromethylated Enamides and Oxazoles (Angew. Chem. 13/2015)

An unprecedented phosphine‐catalyzed remote β‐C H functionalization of amine derivatives triggered by trifluoromethylation of an alkene with Togni’s reagent was disclosed. This reaction proceeded through the highly selective and concomitant activation of an unactivated alkene and the β‐C H bond of an amine derivative, providing bistrifluoromethylated enamides in excellent yields with good regio‐, chemo‐, and stereoselectivity. Furthermore, the newly developed one‐pot protocol provides a facile and step‐economical access to valuable trisubstituted 5‐(trifluoromethyl)oxazoles. Mechanistic studies showed that this reaction may initiate with a novel phosphine‐catalyzed radical trifluoromethylation of unactivated alkene via a phosphorus radical cation.     

Palladium-catalyzed synthesis of 2,1'-disubstituted tetrahydrofurans from gamma-hydroxy internal alkenes. Evidence for alkene insertion into a Pd-O bond and stereochemical scrambling via beta-hydride elimination

Palladium-catalyzed reactions of gamma-hydroxy internal acyclic alkenes with aryl bromides afford 2,1'-disubstituted tetrahydrofurans in good yields with diastereoselectivities of 3-5:1. The analogous transformations of substrates bearing internal cyclic alkenes afford fused bicyclic and spirocyclic tetrahydrofuran derivatives in good yields with excellent diastereoselectivities (>20:1). A series of deuterium labeling experiments indicate that the origin of the modest diastereoselectivity in reactions of acyclic internal alkene substrates likely derives from a series of reversible beta-hydride elimination and sigma-bond rotation processes that occur following a rare intramolecular alkene syn-insertion into an intermediate Pd(Ar)(OR) complex. In addition, these studies shed light on the chemoselectivity of insertion, suggesting that the alkene inserts into the Pd-O bond in preference to the Pd-C bond.     

Cyclization by intramolecular carbolithiation of alkyl- and vinyllithiums prepared by the action of aromatic radical anions on phenyl thioethers. High stereoselectivity in the cyclization accelerated by an allylic lithium oxyanion

The reductive lithiation of alkyl and vinyl phenyl thioethers by aromatic radical anions is shown to be the most general method yet known for preparing organolithiums capable of intramolecular carbometalation of unactivated alkenes to produce five-membered rings and in one case a four-membered ring (in a far higher yield than known cases). The relative rates of cyclization for alkyllithiums are secondary > tertiary > primary, and the yields are very high. In the secondary case, the stereoselectivity is extremely high, producing a cyclopentylmethyllithium with a trans-2-alkyl substituent. A remarkable finding is that for all of the organolithiums a lithium oxyanionic group in the proximal allylic position to the alkene greatly accelerates the cyclization and leads almost exclusively to a trans relationship between the CH(2)Li group and the OLi group, the opposite relationship from that observed in intramolecular carbolithiations by allyllithiums. A mechanistic rationale for this divergence is discussed. One of the two types of proximal homoallylic lithium oxyanions exerts an analogous effect. An intriguing limitation, even occurring with the highly reactive secondary organolithium and in the presence of an allylic oxyanionic group, is the failure of intramolecular carbolithiation when a methyl group is at the terminus of the alkene.     

Diastereoselective formation of indanes from arylboronate esters catalyzed by rhodium(I) in aqueous media

[reaction: see text] Arylboronate esters bearing a pendant Michael-acceptor alkene can add to norbornene and cyclize to give indane systems in yields ranging from 62% to 95% with high diastereomeric excess (>20:1). The reaction is performed in an organic/aqueous emulsion and catalyzed using [Rh(COD)Cl]2 with t-Bu-amphos chloride, a sterically bulky, electron-rich, water-soluble phosphine ligand.     

Axially Chiral Aryl‐Alkene‐Indole Framework: A Nascent Member of the Atropisomeric Family and Its Catalytic Asymmetric Construction

A new class of axially chiral aryl‐alkene‐indole frameworks have been designed, and the first catalytic asymmetric construction of such scaffolds has been established by the strategy of organocatalytic (Z/E)‐selective and enantioselective (4+3) cyclization of 3‐alkynyl‐2‐indolylmethanols with 2‐naphthols or phenols (all >95 : 5 E/Z, up to 98% yield, 97% ee). This reaction also represents the first catalytic asymmetric construction of axially chiral alkene‐heteroaryl scaffolds, which will add a new member to the atropisomeric family. This approach has not only confronted the great challenges in constructing axially chiral alkene‐heteroaryl scaffolds but also provided a powerful strategy for the enantioselective construction of axially chiral aryl‐alkene‐indole frameworks.     

The highly enantioselective bifunctional organocatalysts for the Michael addition of сyclohexanone to titroolefins

A new family of organocatalyst derived from proline has been developed and shown to be an efficient catalyst for asymmetric Michael addition of cyclohexanone to nitroolefins with high diastereo- and enanthio -selectivities. (syn: anti ratio up to 99:1, ee. up to 95%.). The result of computational studies at the B3LYP/6-31G* level indicate that both the hydrogen bonding and the stereo hindrance play the crucial role in the activation of the nitro alkene and help to discriminate between the two diastereofacial approaches.     

Stereospecific Synthesis of Alkenes by Eliminative Cross‐Coupling of Enantioenriched sp3‐Hybridized Carbenoids

1‐Aryl‐1,2‐dialkylethenes were generated by a sequence of electrophilic substitution, 1,2‐metalate rearrangement, and β‐elimination initiated by the addition of enantioenriched α‐(carbamoyloxy)alkylboronates to enantioenriched lithiated carbamates. The carbenoid stereochemical pairing [i.e., “like”=(S)+(S) or “unlike”=(S)+(R)] and the elimination mechanism (syn or anti), not substituent effects, determined the configuration of the trisubstituted alkene target. For example, (Z)‐2,5‐diphenyl‐2‐pentene was produced in 70 % yield with E/Z=5:95 by a like combination of Li and B carbenoids and syn (thermal) elimination whereas the E isomer was obtained in the same yield with E/Z>98:2 by an otherwise identical process involving an unlike stereochemical pairing. The concept elaborated overcomes an intrinsic limitation of traditional strategies for direct connective alkene synthesis, which cannot realize meaningful stereochemical bias unless the alkene substituents are strongly differentiated.     

Asymmetric Palladium‐Catalyzed Alkene Carboamination Reactions for the Synthesis of Cyclic Sulfamides

The synthesis of cyclic sulfamides by enantioselective Pd‐catalyzed alkene carboamination reactions between N‐allylsulfamides and aryl or alkenyl bromides is described. High levels of asymmetric induction (up to 95:5 e.r.) are achieved using a catalyst composed of [Pd₂(dba)₃] and (S)‐Siphos‐PE. Deuterium‐labelling studies indicate the reactions proceed through syn‐aminopalladation of the alkene and suggest that the control of syn‐ versus anti‐aminopalladation pathways is important for asymmetric induction.     

Selective synthesis of 5- or 6-aryl octahydrocyclopenta[b]pyrroles from a common precursor through control of competing pathways in a Pd-catalyzed reaction

The Pd/phosphine-catalyzed reaction of 1 with aryl bromides leads to the selective synthesis of either 6-aryl octahydrocyclopenta[b]pyrroles (3), the corresponding 5-aryl isomers 5, diarylamine 2, or hexahydrocyclopenta[b]pyrrole 4 depending on the structure of the phosphine ligand. These transformations are effective with a variety of different aryl bromides and provide 3-5 with excellent levels of diastereoselectivity (dr > or = 20:1). The changes in product distribution are believed to derive from the influence of Pd-catalyst structure on the relative rates of reductive elimination, beta-hydride elimination, alkene insertion, and alkene displacement processes in a mechanistically complex reaction. The effect of phosphine ligand structure on product distribution is described in detail, along with analysis of a proposed mechanism for these transformations.     

Oxidation of organic films by beams of hydroxyl radicals

We have studied the oxidation of self-assembled monolayers (SAMs) of alkanes and alkenes with a thermal beam of OH radicals. The target films were produced by bonding alkane thiols and alkene thiols to a gold surface and the SAMs are mounted in a vacuum chamber at a base pressure of 10-9 Torr. Hydroxyl radicals were produced by a corona discharge in an Ar/H2O2/water mixture. The resultant molecular beam was scanned by an electrostatic hexapole and the OH radicals [4 (+/- 1) x 1011 OH radicals cm-2 sec-1] were focused onto the target SAM. All of the hydroxyl radicals impinging on the SAM surface are rotationally (J' ' 相似文献   

Five- and six-membered ring formation from the intramolecular reaction of a protonated oxirane and alkene

Computational studies of competing five- and six-membered cyclisation of alkenyloxiranes 1a-d show that intramolecular reaction of a protonated oxirane and alkene is a concerted, single-step, exothermic process. The reactions proceed via reactant-like transition states, but where the oxirane C-O bond is considerably stretched. Two factors are seen to affect the regiochemistry: (1) stabilisation of the transitory positive charge in the transition state favours cyclisation to the more highly substituted oxirane carbon; and (2) there is an inherent stereoelectronic preference for six-membered cyclisation over five-membered cyclisation. The inherent preference for six-membered cyclisation has a parallel in Baldwin's rules for six-membered ring closure of a carbocation with an alkene, rather than Baldwin's rule for intramolecular nucleophilic reaction of three-membered rings, suggesting that the protonated oxirane mimics a carbocation. The electronic and stereoelectronic effects for cyclisation are modified by steric interactions of axial methyl groups. These systems provide a model for the A-ring cyclisation of oxidosqualene.     

Zirconocene catalysis in the synthesis of 1,4-dimagnesium reagents: using competitive inhibition to suppress β-hydrogen abstraction

Substituted 1,4-dimagnesium reagents were synthesized by the zirconocene-catalyzed reaction of alkenes with ethylmagnesium reagents in the presence of a methylmagnesium containing additive. Improved selectivity for formation of dimagnesium reagents over monomagnesium reagents was obtained in the presence of the methylmagnesium containing additive. The ratio of mono- to dimagnesiated products was extrapolated from the ratio of alkene to diene in the products formed when the reaction was quenched with allyl bromide. The extent of the increase in the alkene/diene ratio was dependent on the type of organomagnesium halide, with greatest increases (59%) for the alkylmagnesium chlorides. A mechanism for improved selectivity by suppression of β-hydrogen abstraction in the catalytic cycle is presented. Quenching the 1,4-dimagnesium reagents with allyl bromide yielded decadienes.     

Folding Construction of a Pentacyclic Quadruply fused Polymer Topology with Tailored kyklo‐Telechelic Precursors

A pentacyclic quadruply fused polymer topology has been constructed for the first time through alkyne–azide addition (click) and olefin metathesis (clip) reactions in conjunction with an electrostatic self‐assembly and covalent fixation (ESA‐CF) process. Thus, a spiro‐type, tandem tetracyclic poly(tetrahydrofuran), poly(THF), precursor having two allyloxy groups at the opposite positions of the four ring units was prepared by the click‐linking of one unit of an eight‐shaped precursor having alkyne groups at the opposite positions with two units of a single‐cyclic counterpart having an azide and an alkene group at the opposite positions. Both are obtainable through ESA‐CF. The subsequent metathesis clip‐folding of the tetracyclic precursor could afford a pentacyclic quadruply fused polymer product, of “shippo” form, in 19 % yield.     

Structures and Properties of Nonchelated,d0 Alkyl Alkene Complexes of the Type [Cp2ZrMe(alkene)]+: Elusive Intermediates during Ziegler–Natta Polymerizations of Alkenes

Caught in the act : An alkyl alkene Zr^IV complex (see picture; Cp=C₅H₅) has been synthesized and characterized for the first time. The alkene bonding mode is highly asymmetric, and C2 is quite carbocationic. There is also evidence for rotation about the C1? C2 bond. This extremely unusual complex provides an exemplar of previously unknown intermediates in Ziegler–Natta and carbocationic polymerization reactions of alkenes.

     

Synthesis and Phosphorylation of 5-Bromoalkyl Derivatives of 2-Furancarboxylic Acid

Esters and nitriles of 5-propyl- and 5-isobutylfuran-2-carboxylic acids are brominated with N-bromosuccinimide in the α position of the side chain. The resulting bromides react with triethyl phosphite to give two products: an alkene and a phosphonate. The alkene fraction increases in going from nitrile to ester and with enhancing branching of the side chain.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 258–261.Original Russian Text Copyright © 2005 by Pevzner.     

Synthesis and regiochemistry of spiro indane-1,3-dione compounds

Spiro indane-1,3-dione compounds have been synthesized by 1,3-dipolar cycloaddition of ninhydrin, l-proline, and an alkene (either a chalcone or an (E)-??-arylnitrostyrene). All these reactions proceed with good yield and with high regioselectivity and stereoselectivity. The structures were studied by NMR spectroscopy, MS, and X-ray diffraction analysis. It was found that these two kinds of alkene lead to different regioselectivity. This study has provided information about the regioselectivity of 1,3-dipolar cycloaddition reactions: regioselectivity may be controlled by ?ШC?? stacking state; the order of stability of stacking of the Ar and EWG part with the indane-1,3-dione part is: benzoyl group?>?phenyl group?>?nitro group.