[36] Heteroacene synthesis through C–S cross coupling / 5-endo-dig cyclization

P. Oechsle, U. Flörke, H. Egold, J. Paradies*
Chem. Eur. J. 2016, accepted.

The highly efficient, one step of sulfur-containing heteroacenes was achieved through palladium-catalyzed C–S cross coupling of bisalkynes with thioacetate as hydrogen sulfide surrogate. The heteroacenes consisting of three, five and seven fused aromatic rings were obtained in one single catalytic step by four-, six- and eight-fold C–S bond formation.

[35] Concise synthesis of dithiophene derivatives by palladium-catalyzed multiple C–S cross coupling/cyclization sequence

P. Oechsle, P. Hou, U. Flörke; J. Paradies*
Adv. Synth. Catal. 2016, accepted.

The facile synthesis of new sulfur-containing fused heterocycles was achieved by a twofold domino reaction consisting of a carbon-sulfur cross coupling, followed by a 5-endo-dig cyclization. Using this strategy a series of benzo, thiopheno, pyridino and pyrazino dithienoacenes with electron-neutral (-C6H4-nHex), electron-rich (-C6H4-NPh2) and electron-deficient (-C4H3N2) substituents were synthesized in high yields. The developed method was applied in the efficient synthesis of a complex donor-acceptor molecule. The photophysical and electrochemical properties of the products were analyzed by UV-VIS/luminescence spectroscopy and cyclic voltammetry.

[34] Frustrated Lewis Pair-catalyzed dehydrogenative oxidation of indolines and other heterocycles

A. F. G. Maier, S. Tussing, T. Schneider, U. Flörke, Z.-W. Qu, S. Grimme,* J. Paradies*
Angew. Chem. 2016, 128, 12407–12411.

Angew. Chem. Int. Ed. 2016, 55, 12219–12223.

An acceptorless dehydrogenation of heterocycles catalyzed by frustrated Lewis pairs (FLPs) was developed. Oxidation with concomitant liberation of molecular hydrogen proceeded in high to excellent yields for N-protected indolines as well as four other substrate classes. The mechanism of this unprecedented FLP-catalyzed reaction was investigated by mechanistic studies, characterization of reaction intermediates by NMR spectroscopy and X-ray crystal analysis, and by quantum-mechanical calculations. Hydrogen liberation from the ammonium hydridoborate intermediate is the rate-determining step of the oxidation. The addition of a weaker Lewis acid as a hydride shuttle increased the reaction rate by a factor of 2.28 through a second catalytic cycle.

[33] Structure–Reactivity Relationship in the Frustrated Lewis Pair (FLP)-Catalyzed Hydrogenation of Imines

Sebastian Tussing, Karl Kaupmees and Jan Paradies*
Chem. Eur. J. 2016, 22, 7422–7426.

The autoinduced, frustrated Lewis pair (FLP)-catalyzed hydrogenation of 16 benzene ring substituted N-benzylidene-tertbutylamines with B(2,6-F2C6H3)3 and molecular hydrogen was investigated by kinetic analysis. The pKa values for imines and for the corresponding amines were determined by quantum-mechanical methods and provided a direct proportional relationship. The correlation of the two rate constants k1 (simple catalytic cycle) and k2 (autoinduced catalytic cycle) with pKa difference between imine and amine pairs (ΔpKa) or Hammett’s σ parameter served as useful parameters to establish a structure-reactivity relationship for the FLP-catalyzed hydrogenation of imines.

[32] Frustrated Lewis Pair-Catalyzed Cycloisomerization of 1,5-Enynes via a 5-endo-dig Cyclization/Protodeborylation Sequence

Sergej Tamke, Zheng-Wang Qu, Nikolai A. Sitte, Ulrich Flörke, Stefan Grimme* and Jan Paradies*
Angew. Chem. 2016, 55, 4336-4339.

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh3 was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

[31] Microwave-assisted FLP-catalyzed hydrogenations

S. Tussing, J. Paradies*
Dalton Trans. 2016, 45, 6124-6128.

FLP-catalyzed hydrogenations of 15 substrates were compared using microwave irradiation and conventional heating. The direct comparison revealed that a rate acceleration of up to 2.5 was achieved in the presence of microwaves. This heating method is particularly promising for the hydrogenation of nitrogen-containing heterocycles. Acridine, quinines and especially 1-methyl indole were reduced very efficiently under mild conditions and only 4 bar hydrogen pressure in high yields.

[30] Determination of the H2-activation parameters by kinetic analysis of the autoinduced FLP-catalyzed imine hydrogenation

S. Tussing, L. Greb, S. Tamke, B. Schirmer, C. Muhle-Goll, B. Luy, J. Paradies*
Chem. Eur. J. 2015, 21, 8056–8059.

The frustrated Lewis pair (FLP)-catalyzed hydrogenation and deuteration of N-benzylidene-tert-butylamine (2) was kinetically investigated by using the three boranes B(C6F5)3 (1), B(2,4,6-F3-C6H2)3 (4), and B(2,6-F2-C6H3)3 (5) and the free activation energies for the H2 activation by FLP were determined. Reactions catalyzed by the weaker Lewis acids 4 and 5 displayed autoinductive catalysis arising from a higher free activation energy (2 kcal mol−1) for the H2 activation by the imine compared to the amine. Surprisingly, the imine reduction using D2 proceeded with higher rates. This phenomenon is unprecedented for FLP and resulted from a primary inverse equilibrium isotope effect.

[29] Synthesis and Photophysical Properties of GemPhos Noble Metal Complexes

C. Sarcher, S. Bestgen, F. C. Falk, S. Lebedkin, J. Paradies, M. M. Kappes, P. W. Roesky*
J. Organomet. Chem. 2015, 797, 11–17.

GemPhos, a diphosphine ligand with a rigid paracyclophane scaffold, was used to prepare complexes of the noble metals gold, palladium, and platinum. The reaction of GemPhos with [Au(tht)2][ClO4] (tht = tetrahydrothiophene) surprisingly yielded a mononuclear charge separated gold compound [(GemPhos)Au][ClO4], in which the gold atom exhibits an uncommon trigonal planar coordination geometry. Furthermore, similar palladium [(GemPhos) (PdCl2)] and platinum [(GemPhos)(PtCl2)] complexes were obtained in very good yields by the reaction of GemPhos with [MCl2(COD)] (M = Pd, Pt; COD = 1,5-cyclooctadiene) in hot DMSO. All compounds were fully characterized by analytical and spectroscopic techniques and their solid-state structures were established by single X-ray crystallography. Their photoluminescent properties were measured at low and ambient temperatures, revealing different behavior depending on the metal and coordination mode.

[28] Frustrated Lewis pair catalyzed hydrosilylation and hydrosilane mediated hydrogenation of fulvenes

S. Tamke, J. Paradies*
Org. Biomol. Chem. 2014, 12, 9139–9144.

The frustrated Lewis pair (FLP) mediated hydrosilylation of pentafulvenes is described yielding allyl silanes with high regioselectivity in excellent yields. While phenyl substituted allyl silanes undergo B(C6F5)3-mediated rearrangement to vinyl silanes dimethyl derivatives experience FLP-catalyzed hydrogenantion followed by an unprecedented protodesilylation. This observation allowed the metal-free hydrogenation of 6,6-dimethylfulvene to iso-propyl cyclopentene according to a FLP-catalyzed triple domino reaction consisting of hydrosilylation, hydrogenation and protodesilylation. The mechanisms were investigated by deuteration experiments.

[27] Ambidextrous catalytic access to dithieno[3,2-b:2',3'-d]thiophene (DTT) derivatives by both palladium-catalyzed C–S and oxidative dehydro C–H coupling

P. Oechsle, J. Paradies*
Org. Lett. 2014, 16, 4086-4089.

A modular two-step synthesis of dithieno[3,2-b:2′,3′-d]thiophene (DTT) derivatives by C−S cross-coupling and oxidative dehydro C−H coupling is herein described. Dibenzo[d,d′]thieno[3,2-b;4,5-b′]dithiophene (DBTDT) and associated two donor (anisyl) and acceptor (acetyl) substituted DTT derivatives were synthesized by palladium-catalyzed cross-coupling sequences in 17% to 71% yield over two steps. The 5,5′-disubstituted DTT derivatives were characterized in terms of their photophysical (UV and fluorescence spectroscopy) and electrophysical (cyclovoltammography) properties.

[26] Mono- versus Dinuclear Gold-Catalyzed Intermolecular Hydroamidation

J. M. Serrano-Becerra, A. F. G. Maier, S. González-Gallardo, E. Moos, C. Kaub, M. Gaffga, G. Niedner-Schatteburg, P. W. Roesky, F. Breher, J. Paradies*
Eur. J. Org. Chem. 2014, 21, 4515-4522.

Mono- and dinuclear gold catalysts were investigated in the intermolecular hydroamidation of olefins. Upon activation of [Ph3PAuCl] and [xantphos(AuCl)2] with various silver salts (AgOTf, Ag[BF4] and Ag[SbF6]), diverging reactivity of the resulting cationic gold-complexes was observed. It was found that both the binding ability of the counterion and the solvent have significant impact on the reactivity of the mono- and dinuclear complexes.

[25] Unsymmetrical Bisphosphines for the Amidation of Aryl Chlorides: A Kinetic Study

F. C. Falk, P. Oechsle, W. R. Thiel, C.-G. Danilluc, J. Paradies *
Eur. J. Org. Chem. 2014, 21, 3637–3645.

The rate-determining step of the palladium-catalyzed amidation of 4-chlorotoluene with benzamide in the presence of unsymmetrical bisphosphines was investigated. Isostructural [2.2]paracyclophane-derived bisphosphines bearing dicyclohexylphosphino and diarylphosphino moieties were investigated as ligands in the oxidative addition and reductive elimination by kinetic studies. The reductive elimination was accelerated when a bisphosphine ligand was applied that contained an electron-rich phosphine group and a less electron-releasing phosphino moiety. Apart from the reductive elimination, the transmetallation had the largest impact on the palladium-catalyzed amidation of aryl halides.

[24] Metal-free dehydro Si-N cross-coupling

S. Tamke, L. Greb, J. Paradies*
Chem. Commun. 2014, 50, 2318-2320.

The metal-free B(C6F5)3 catalyzed dehydrocoupling of hydrosilanes with anilines, carbazoles and indoles is reported. For anilines and carbazoles the reaction proceeds by the liberation of H2 as the sole Si–N coupling byproduct. Indoles react with diphenyl(methyl) hydrosilane to give N-silyl indolines with high diastereoselectivity (d.r. 10 : 1) in excellent yields. A mechanism for this Si–N coupling/hydrogenation sequence is proposed.

[23] Metal-free Hydrogenation of Unsaturated Hydrocarbons Employing Molecular Hydrogen

J. Paradies
Angew. Chem. Int. Ed. 2014, 53, 3552-3557.

The metal-free activation of hydrogen by frustrated Lewis pairs (FLPs) is a valuable method for the hydrogenation of polarized unsaturated molecules ranging from imines, enamines, and silyl enol ethers to heterocycles. However, one of the most important applications of hydrogenation technology is the conversion of unsaturated hydrocarbons into alkanes or alkenes. Despite the fast development of the FLP chemistry, such reactions proved as highly challenging. This Minireview provides an overview of the basic concepts of FLP chemistry, the challenge in the hydrogenation of unsaturated hydrocarbons, and first solutions to this central transformation.

[22] Desymmetrization of 4,6-diprotected myo-inositol

M. B. Lauber, C.-G. Daniliuc, J. Paradies*
Chem. Commun. 2013, 49, 7409-7411.
DOI:10.1039/C3CC43663B .

The metal-free activation of hydrogen by frustrated Lewis pairs (FLPs) is a valuable method for the hydrogenation of polarized unsaturated molecules ranging from imines, enamines, and silyl enol ethers to heterocycles. However, one of the most important applications of hydrogenation technology is the conversion of unsaturated hydrocarbons into alkanes or alkenes. Despite the fast development of the FLP chemistry, such reactions proved as highly challenging. This Minireview provides an overview of the basic concepts of FLP chemistry, the challenge in the hydrogenation of unsaturated hydrocarbons, and first solutions to this central transformation.

[21] Electronic Factors for low Temperature Metal-Free H2-Activation: A Kinetic and Computational Study

L. Greb, S. Tussing, B. Schirmer, I. Leito, S. Grimme, J. Paradies*
Chem. Sci. 2014, 4, 2788-2796.

The frustrated Lewis pair-mediated reversible hydrogen activation is studied as a function of the electron-donor quality of a series of phosphines. The increasing acidity of the generated phosphonium species leads to a stepwise lowering of the temperature for the highly reversible H2-activation and permits concrete classification for the first time. The influence of the acid strength on the metal-free hydrogenation of selected olefins is investigated by kinetic experiments and quantum chemical calculations. Detailed information for the rate-determining steps fully support our mechanistic model of a protonation step prior to hydride transfer. The rate of hydrogenation is strongly dependent on the electronic nature of the phosphine and of the acidity of the corresponding phosphonium cation. A careful balance of these two factors provides highly efficient metal-free hydrogenation catalysts. The provided findings are used to revise the reactivity of Lewis bases in the hydrogenation of imines, one of the most recognized applications of FLPs.

[20] Towards Functional Group Tolerance in Frustrated Lewis Pair Chemistry: Hydrogenation of Nitroolefins and Acrylates

L. Greb, C.-G. Daniliuc, K. Bergander, J. Paradies*
Angew. Chem. Int. Ed. 2013, 52, 5876-5879.

Hydridoborate derived from B(2,6-F2C6H3)3 shows significant hydride character. Solid-state and solution structure analysis revealed a dihydrogen-bonded aggregate. The new frustrated Lewis pair was applied in the hydrogenation of nitroolefins and acrylates (see scheme; EWG=electron-withdrawing group). The decreased Lewis acidity provides higher reactivity and functional-group tolerance.

[19] Frustrated Lewis pair catalyzed hydrogenations

J. Paradies
Synlett 2013, 4, 777-780.

The frustrated Lewis pair (FLP) catalyzed hydrogenation of organic molecules is discussed. The saturation of polarized double bonds by FLP can be described as the nucleophilic addition of hydrides to the polar double bond prior to proton transfer. In contrast, the hydrogenation of olefins proceeds first by protonation forming a transient carbocation, which is subsequently attacked by the hydride. Both processes give rise for efficient conversion of unsaturated organic compounds by a metal-free methodology employing molecular hydrogen.

[18] Metal-free Catalytic Olefin Hydrogenation: Low-Temperature H2 Activation by Frustrated Lewis Pairs

L. Greb, P. Oña-Burgos, B. Schirmer, D. W. Stephan, S. Grimme,* J. Paradies*
Angew. Chem. Int. Ed. 2012, 51, 10164-10168.

Weak nucleophiles for strong activation: The reversible activation of dihydrogen by an electron-deficient phosphine, (C6F5)PPh2, in combination with the Lewis acid B(C6F5)3 at −80 °C was accomplished. The catalytic hydrogenation of olefins proceeds through protonation and subsequent hydride attack. Electron-deficient phosphines and diarlyamines were demonstrated to be viable Lewis bases for the reaction, thus allowing catalyst loadings of 10 to 5 mol %.

[17] [2.2]Paracyclophane-derived Dinuclear Gold Complexes

C. Sarcher, A. Lühl, F. C. Falk, S. Lebedkin, M. Kühn, J. Paradies,* M. M. Kappes,* W. Klopper,* P. W. Roesky*
Eur. J. Inorg. Chem. 2012, 51, 5033-5042.

We have prepared digold(I) complexes with the rigid-backbone diphosphane ligands PhanePhos, xyl-PhanePhos, and Ph2-GemPhos. All complexes were characterized by single-crystal X-ray diffraction, NMR, IR, Raman, and photoluminescence (PL) spectroscopy. [PhanePhos(AuCl)2] and [GemPhos(AuCl)2] show a very similar ligand scaffold, but different (aurophilic vs. nonaurophilic) intramolecular Au–Au distances. Absorption and PL spectra of both compounds are quite similar. Theoretical investigations reveal that the excited states are of different character (i.e., influenced by the Au–Au contacts). The respective transition energies, however, lie close to each other, thus resulting in similar experimental spectra. The gold atoms appear to produce a significant heavy-atom effect in the electronic relaxation dynamics. Thus, [PhanePhos(AuCl)2] and [GemPhos(AuCl)2] both show bright-green millisecond-long phosphorescence at low temperatures, although the latter is quenched at ambient temperature. [PhanePhos(AuCl)2] and [GemPhos(AuCl)2] were also used as catalysts in the intra- and intermolecular hydroamination of alkynes. Good to quantitative conversions on a reasonable time scale were observed. The overall performance of these catalysts in the studied reactions was similar, showing that Au–Au contacts do not have a major influence on the catalytic performance.

[16] Development of Tartaric Acid Derived Hydrogen-Bond Catalysts

C. Sarcher, A. Lühl, F. C. Falk, S. Lebedkin, M. Kühn, J. Paradies,* M. M. Kappes,* W. Klopper,* P. W. Roesky*
Synthesis 2012, 44, 3209-3215.

A flexible synthesis of bifunctional thiourea derivatives based on the TADDOL framework is described. Hydroxy as well as primary, secondary, and tertiary amino-substituted bifunctional hydrogen­-bond donors were synthesized.

[15] [2.2]Paracyclophane derived Bisphosphines for the Activation of Hydrogen by FLPs: Application in Domino Hydrosilylation/Hydrogenation of Enones

L. Greb, P. Oña-Burgos, A. Kubas, F. C. Falk, F. Breher, K. Fink, J. Paradies*
Dalton Trans. 2012, 40, 9056-9060.

The heterolytic splitting of hydrogen by two types of [2.2]paracyclophane derived bisphosphines (1, 2a and 2b) in combination with tris(pentafluorophenyl)borane (3) at room temperature is described. The corresponding frustrated Lewis pairs (FLPs) exhibit different behavior in the activation of hydrogen. This results from diverse steric and electronic properties of the bisphosphines. The reactivity of the frustrated Lewis pairs was exploited in the first diastereoselective domino hydrosilylation/hydrogenation reaction catalyzed by FLPs.

[14] Development of [2.2]Paracyclophane derived Hydrogen-Bond Receptors

J. F. Schneider, J. Paradies*
Isr. J. Chem. 2012, 52, 76-91.

The development of planar-chiral hydrogen-bond donors based on the [2.2]paracyclophane scaffold is discussed. General strategies to access functionalized enantiopure [2.2]paracyclophane derivatives are briefly reviewed, with the focus on suitable precursors for the synthesis of planar-chiral thiourea derivatives. The synthesis of fourteen hydrogen-bond donors is described. The interaction of four thiourea derivatives with hydrogen-bond acceptors (DMSO and tetramethylammonium chloride) was investigated by 1H NMR spectroscopy and X-ray crystallography. A selection of enantiomerically pure planar-chiral derivatives was applied in asymmetric hydrogen-bond catalysis.

[13] [2.2]Paracyclophane derivatives: Synthesis and Application in Catalysis

J. F. Schneider, J. Paradies*
Synthesis 2011, 3749-3766.

Advances in the field of [2.2]paracyclophane chemistry are reviewed including syntheses, resolution of enantiomers and application in organic synthesis. Transition-metal catalyzed as well as organocatalytic transformations are presented focusing on the development of [2.2]paracyclophane derived ligands and catalysts.

[12] A Bidentate Phosphine for the efficient Amidation of Arylchlorides

F. C. Falk, R. Fröhlich, J. Paradies*
Chem. Commun. 2011, 47, 11095-11097.

Voluminous amides were coupled with deactivated, sterically hindered aryl chlorides in excellent yields providing products, which have not been efficiently accessible by transition metal catalysis so far. Application of an unsymmetric bisphosphine ligand was critical for the high catalytic activity.

[11] Thiourea as Sulfur Surrogate in the Palladium catalyzed C-S Bond Formation

M. Kuhn, J. Paradies*
Org. Lett. 2011, 13, 4100-4103.

The first C–S bond formation/cross-coupling/cyclization domino reaction using thiourea as a cheap and easy to handle dihydrosulfide surrogate has been developed. Structurally important biarylthioether, benzo[b]thiophenes, and thieno[3,2-b]thiophene scaffolds are provided in high yield.

[10] Readily Available Hydrogen-Bond Catalysts for the Asymmetric Transfer Hydrogenation of Nitroolefins

J. F. Schneider, M. B. Lauber, V. Muhr, D. Kratzer, J. Paradies*
Org. Lett. 2011, 9, 4323-4327.

This paper focuses on readily accessible thiourea hydrogen bond catalysts derived from amino acids, whose steric and electronic features are modulated by their degree of substitution at the carbinol carbon center. These catalysts were applied in the asymmetric transfer hydrogenation of nitroolefins furnishing the chiral products in up to 99% yield and 86% enantiomeric excess. The proposed catalyst's mode of action is supported by mechanistic investigations.

[9] Synthesis of Planar-Chiral Thioureas

J. F. Schneider, R. Fröhlich, J. Paradies*
Synthesis 2010, 3486-3492.

An efficient access to enantiopure pseudo-geminally substituted 4-amino-13-bromo[2.2]paracyclophane is described. The aminobromide was employed in cross-coupling reactions to yield arylated 4-amino[2.2]paracyclophanes. The amines were converted into enantiopure planar-chiral thioureas, which are potential hydrogen-bond donors for enantioselective organocatalysis.

[8] Planar-Chiral Thioureas as Hydrogen-Bond Catalysts

J. F. Schneider, F. C. Falk, R. Fröhlich, J. Paradies*
Eur. J. Org. Chem. 2010, 12, 2265-2269.

The synthesis of the first enantiopure planar-chiral thiourea catalysts is herewith described. New catalysts 1–3 were applied in asymmetric transformations, such as the Friedel–Crafts alkylation of indole, as well as in the transfer hydrogenation of nitroolefins. Bifunctional catalysts 2 and 3 exhibit enhanced activity in the investigated reactions, demonstrating their potential for organic synthesis.

[7] Synthesis of Divinylsulfides

J. Paradies*
Synthesis 2010, 947-942.

Arylacetylenes react with sodium sulfide in the presence of water to yield divinylsulfides. The reaction proceeds in good to excellent yield for both electron-neutral and electron-deficient aromatic systems; for electron-rich aryls, longer reaction times are necessary. The sulfides represent useful substrates for further transformations, for example, oxidation to the corresponding divinylsulfoxides and divinylsulfones. Three selected divinylsulfide derivatives were oxidized selectively to the corresponding sulfoxides or sulfones.

[4] Ansa-metallocene polymerization catalysts derived from [2+2]cycloaddition reactions of bis(1-methylethenyl-cyclopentadienyl)zirconium systems

J. Paradies*
Proceedings of the National Academy of Sciences of the United States of America 2006, 1103, 15333-15337.

Bis(1-methylethenyl-cyclopentadienyl)zirconium dichloride (7a) was prepared by a fulvene route. Photolysis at 0°C with Pyrex-filtered UV light resulted in a rapid and complete intramolecular [2+2]cycloaddition reaction to yield the corresponding cyclobutylene-bridged ansa-zirconocene dichloride isomer (8a). This is one of the rare examples of an organic functional group chemistry that leads to carbon–carbon coupling at the framework of an intact sensitive group 4 bent metallocene complex. More sterically hindered open metallocenes that bear bulky isopropyl or tert-butyl substituents at their Cp rings in addition to the active 1-methylethenyl functional group undergo the photochemical ansa-metallocene ring closure reaction equally facile. The metallocene systems used and obtained in this study have served as transition metal components for the generation of active metallocene propene polymerization catalysts.

[5] Formation of an Organometallic Ladderane Derivative by Dynamic Topochemical Reaction Control

J. Paradies, I. Greger, G. Kehr, G. Erker,* K. Bergander, R. Fröhlich
Angew. Chem. Int. Ed. 2006, 45, 7630-7633.

Bis(butadienylcyclopentadienyl) dichlorozirconium (1) undergoes dynamic topochemical reactions upon photolysis that eventually lead to the formation of ladderane derivative 2 and the organometallic cyclooctadiene derivative 3.

[4] Observation of Two Consecutive Photoreactions Upon UV-Irradiation of a Bis(1,3-dialkenyl-Cp)zirconium Complex

J. Paradies, R. Fröhlich, G. Kehr, G. Erker*
Organometallics 2006, 25, 3920-3925.

The cyclobutylene−bis(2-indenyl)zirconium dichloride complex (4), derived from an intramolecular photochemical [2+2] cycloaddition reaction of bis[2-(methylethenyl)indenyl]zirconium dichloride (3), was reacted with methyllithium or phenyllithium to yield the corresponding cyclobutylene−bis(2-indenyl)zirconium dimethyl (12) or diphenyl (13) complex, respectively. Insertion of carbon monoxide into the Zr−phenyl linkage of 13 yielded the “O-inside” η2-benzoyl ansa-zirconocene complexes 18-syn/anti, which were characterized by X-ray diffraction. tert-Butyl isocyanide insertion into the Zr−CH3 bond of 12 gave the “N-inside” η2-iminoacyl metallocene isomers 17-syn and 17-anti; the latter was characterized by X-ray diffraction. Under kinetic control tert-butyl isonitrile insertion into 12 gave the “N-outside” η2-iminoacyl metallocene isomers 25-syn/25-anti, which rearranged to 17-syn/anti at 243 K with Gibbs activation energies of ΔG⧧rearr = 18.0/17.8 ± 0.3 kcal·mol-1, respectively. The analogous rearrangement of the “N-outside” to “N-inside” isomers of the corresponding cyclobutylene−bis(cyclopentadienyl)Zr(η2-iminoacyl) reference systems (23-syn/anti to 24-syn/anti) is much faster (203 K: ΔG⧧rearr = 14.7/14.9 ± 0.3 kcal·mol-1). Similar differences in the kinetic “N-outside” to thermodynamic “N-inside” η2-iminobenzoyl metallocene isomerization were observed for the tert-butyl isonitrile insertion products of the cyclobutylene−bis(2-indenyl)ZrPh2 (13) and cyclobutylene−bis(C5H4)ZrPh2 (15) systems, which indicates a pronounced influence of the covering phenylene groups on the σ-ligand chemistry in these rigid ansa-metallocene systems.

[3] [2+2]-Cycloaddition and Subsequent meso/rac ansa-Metallocene Interconversion by Photolysis of a Bis(1,3-dialkenylcyclopentadienyl)zirconium Complex

J. Paradies, R. Fröhlich, G. Kehr, G. Erker*
Organometallics 2006, 25, 3920-3925.

6,6-Dimethylfulvene (5) was treated with acetone under basic conditions to yield 3-(1-methylethenyl)-6,6-dimethylfulvene (7). Deprotonation with LDA followed by transmetalation with 0.5 molar equiv of zirconium tetrachloride gave bis[1,3-di(1-methylethenyl)C5H3]zirconium dichloride (3a). Photolysis of 3a with Pyrex-filtered UV/vis light at ambient temperature led to a rapid intramolecular [2+2]-cycloaddition reaction to yield a 1:1 mixture of the meso- and rac-isomers of the corresponding singly cyclobutylene-bridged ansa-metallocene system 4a. Photolysis of this mixture with quartz-filtered UV light at −80 °C very slowly converted meso-4a to rac-4a. Over 2 days a 12:1 ratio of rac/meso-4a was achieved under these conditions.

[2] Developing a Functional Group Chemistry of Organolithium Compounds: [2+2] Cycloaddition of Alkenyl-substituted Li-Cyclopentadienides

J. Paradies, G. Erker,* R. Fröhlich
Angew. Chem. Int. Ed. 2006, 45, 3079-3082.

Li and light: Alkenyl-substituted lithium cyclopentadienides, which are in equilibrium with the substituted lithocene anion structure 1, undergo a photochemical [2+2] cycloaddition to yield selectively the carbon–carbon coupling product 2. This is a rare case of organic functional-group chemistry for a reactive organolithium compound.

[1] Photogeneration of Titanium(III) from Titanium(IV) Citrate in Aqueous Solution

J. Paradies, J. Crudass, F. MacKay, L. J. Yellowlees, J. Montgomery, S. Parsons, L. Oswald, N. Robertson, P. J. Sadler*
J. Inorg. Biochem. 2006, 100, 1260-1264.

Current interest in the biochemistry of Ti(IV) arises from its widespread use in white pigments and its potential in therapeutic agents. Citrate is known to form strong complexes with Ti(IV). We show here that Ti(III) citrate is generated in a facile manner and in good yield by the action of UV radiation on Ti(IV) citrate in aqueous solution. The Ti(III)-citrate species formed was isolated and characterised by UV–Visible spectroscopy, showing an absorption at 547 nm (ε = 100 M−1 cm−1), and by electron paramagnetic resonance (EPR) spectroscopy giving a resonance at g = 1.949 (linewidth = 60 G) . An X-ray structure of the parent Ti(IV) complex in the form [TiNa3(C6H6O7)2(C6H5O7)(H2O)6.8] · 2H2O is reported along with a study of the reaction of Ti(IV)-citrate with N,N-ethylenebis(o-hydroxytoluene)glycine (EHTG), which was more rapid than those of other related Ti(IV) complexes.