A Rapid and Efficient Stereoselective Synthesis of (Z)‐ and (E)‐Allyl Bromides from Baylis–Hillman Adducts Using Bromo(dimethyl)sulfonium Bromide

Treatment of Baylis–Hillman adducts 1 with bromo(dimethyl)sulfonium bromide, Br(Me₂)S⁺Br^?, in MeCN was found to stereoselectively afford (Z)‐ and (E)‐allyl bromides 2 . The reaction is rapid at room temperature, high‐yielding, and highly stereoselective.     

Studies on the Stereochemistry of 2-(Nitromethylidene)-Heterocycles

The ¹H-NMR spectra of 2-(nitromethylidene)pyrrolidine ( 7 ), 1-methyl-2-(nitromethylidene)imidazolidind ( 10 ) and 3-(nitromethylidene)tetrahydrothiazine ( 11 ) in CDCl₃ and (CD₃)₂SO indicate that these compounds have the intramolecularly H-bonded structures (Z)- 7 , (E)- 10 and (Z)- 11 while the N-methyl derivative 8 of 7 is (E)-configurated in both solvents. 1-Benzylamino-1-(methyltio)-2-nitroehtylene ( 13 ), an acylic model, has the H-bonded configuration (E)- 13 in CDCl₃ and in (CD₃)₂SO. 2-(Nitromethylidene)thiazolidine ( 3 ) has the (E)-configuration in CDCl₃ but exists in (CD₃)₂SO as a mixture of (Z)- and (E)-isomers with the former predominating. Both species are detected to varying proportions in a mixture of the two solvents. ¹⁵N-NMR spectroscopy of 3 ruled out unambiguously the nitronic acid structure 6 and the nitromethyleimine structure 5 . The N-methyl derivative 4 of 3 is (Z)-configurated in (CD₃)₂SO. Comparison of the olefinic proton shifts of (Z)- 3 and (Z)- 4 with those of analogues and also of 1,1-bis(methylti)-2-nitroethylene ( 12 ) shows decreased conjugation of the lone pair of electrons of the ring N-atom in (Z)- 3 and (Z)- 4 . This is also supported by ¹³C-NMR studies. Plausible explanations for the phenomenon are offered by postulating that the ring N-atoms are pyramidal in (Z)- 3 and (Z)- 4 and planar in other cases or, alternatively, that the conjugated nitroenamine system gets twisted due to steric interaction between the NO₂-group and the ring S-atom. Single-crystal X-ray studies of 3 and 8 show that the former exists in the (Z)-configuration and the latter in (E)-configuration; the ring N-atom in the former has slightly more pyramidal character than in the latter.     

Field‐Induced Slow Magnetic Relaxation in a Mononuclear Manganese(III)–Porphyrin Complex

We report on a novel manganese(III)–porphyrin complex with the formula [Mn^III(TPP)(3,5‐Me₂pyNO)₂]ClO₄?CH₃CN ( 2 ; 3,5‐Me₂pyNO=3,5‐dimethylpyridine N‐oxide, H₂TPP=5,10,15,20‐tetraphenylporphyrin), in which the Mn^III ion is six‐coordinate with two monodentate 3,5‐Me₂pyNO molecules and a tetradentate TPP ligand to build a tetragonally elongated octahedral geometry. The environment in 2 is responsible for the large and negative axial zero‐field splitting (D=?3.8 cm^?1), low rhombicity (E/|D|=0.04) of the high‐spin Mn^III ion, and, ultimately, for the observation of slow magnetic‐relaxation effects (E_a=15.5 cm^?1 at H=1000 G) in this rare example of a manganese‐based single‐ion magnet (SIM). Structural, magnetic, and electronic characterizations were carried out by means of single‐crystal diffraction studies, variable‐temperature direct‐ and alternating‐current measurements and high‐frequency and ‐field EPR spectroscopic analysis followed by quantum‐chemical calculations. Slow magnetic‐relaxation effects were also observed in the already known analogous compound [Mn^III(TPP)Cl] ( 1 ; E_a=10.5 cm^?1 at H=1000 G). The results obtained for 1 and 2 are compared and discussed herein.     

Nonlinear Brønsted and Hammett Correlations Associated with Reactions of 4‐Chloro‐7‐nitrobenzofurazan with Anilines in Dimethyl Sulfoxide Solution

The kinetics and mechanism of nucleophilic aromatic substitution reactions of 4‐chloro‐7‐nitrobenzofurazan 1 with 4‐X‐substituted anilines 2a–g (X = OH, OCH₃, CH₃, H, I, Cl, and CN) are investigated in a dimethyl sulfoxide (Me₂SO) solution at 25°C. The Hammett plot of log k₁ versus σ is nonlinear for all the anilines studied due to positive deviations of the electron‐donating substituents. However, the corresponding Yukawa–Tsuno plot resulted in a good linear correlation with σ+r (σ⁺?σ). The corresponding Brønsted‐type plot is also nonlinear, i.e., the slope (β_nuc) changes from 1.60 to 0.56 as the basicity of anilines decreases. These results indicate a change in a mechanism from a polar S_NAr process for less basic nucleophiles (X = I, Cl, and CN) to a single electron transfer for more basic nucleophiles (X = OH, OCH₃, and CH₃). The satisfactory log k₁ versus E^o correlation obtained for the reactions of 1 with anilines 2a–d in the present system is consistent with the proposed mechanism. Interestingly, the β_nuc = 1.60 value measured for 1 in Me₂SO reflects one of the highest coefficients Brønsted ever observed for S_NAr reactions. © 2013 Wiley Periodicals, Inc. Int J Chem Kinet 45: 152–160, 2013     

EQUILIBRIUM STUDIES OF THE REACTION BETWEEN TETRACHLOROPALLADIUM(II) ANION AND SULFOXIDES. THE FORMATION OF THE MONO-S-SULFOXIDE COMPLEXES

Abstract

The equilibrium coefficient, K₁, for the reaction [PdCl₄]^2- + RR′ SO ? [Pd(RR′ SO)Cl]^? + Cl^?, has been determined for dimethylsulfoxide, tetramethylensulfoxide, and phenylmethylsulfoxide and found to be 67, 46 and 8.8 respectively at 25°C, ü= 1.0 in 95:5 methanol-water. Values for the equilibrium constants for the dimethylsulfoxide complex are also reported at other ionic strengths. The equilibrium constants for the second stage, [Pd(Me₂SO)Cl₃]- + Me₂SO)?-[Pd(Me₂SO)₂Cl₂] + Cl^?, has been determined for dimethylsulfoxide only, K₂=2.5 × 10^?2 at 25°C (μ not controlled). The causes of the mutual destabilisation of two dimethylsulfoxides are discussed.     

Unprecedented Bonding Situation in Viable E2(NHBMe)2 (E=Be,Mg; NHBMe=(HCNMe)2B) Complexes: Neutral E2 Forms a Single E−E Covalent Bond

Is it possible to facilitate the formation of a genuine Be?Be or Mg?Mg single bond for the E₂ species while it is in its neutral state? So far, (NHC^R)Be?Be(NHC^R) (R=H, Me, Ph) have been reported where Be₂ is in ¹Δ_g excited state imposing a formal Be?Be bond order of two. Herein, we present the formation of a single E?E (E=Be, Mg) covalent bond in E₂(NHB^Me)₂ (E=Be, Mg; NHB^Me=(HCN^Me)₂B) complexes where E₂ is in ³∑_u⁺ excited state having (nσ_g⁺)²(nσ_u⁺)¹((n+1)σ_g⁺)¹ (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron‐shared bonding with two NHB^Me radicals. The effects of bonding with nσ_u⁺ and (n+1)σ_g⁺ orbitals will cancel each other, providing the former E?E bond order as one. Be₂(NHB^Me)₂ complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg₂(NHB^Me)₂ → Mg + Mg(NHB^Me)₂ and Mg₂(NHB^Me)₂ → Mg₂ + (NHB^Me)₂, are kinetically inhibited by a free energy barrier of 15.7 and 18.7 kcal mol^?1, respectively, which would likely to be further enhanced in cases of bulkier substituents attached to the NHB ligands. Therefore, the title complexes are first viable systems which feature a neutral E₂ moiety with a single E?E covalent bond.     

Unprecedented Bonding Situation in Viable E2(NHBMe)2 (E=Be,Mg; NHBMe=(HCNMe)2B) Complexes: Neutral E2 Forms a Single E−E Covalent Bond

Is it possible to facilitate the formation of a genuine Be?Be or Mg?Mg single bond for the E₂ species while it is in its neutral state? So far, (NHC^R)Be?Be(NHC^R) (R=H, Me, Ph) have been reported where Be₂ is in ¹Δ_g excited state imposing a formal Be?Be bond order of two. Herein, we present the formation of a single E?E (E=Be, Mg) covalent bond in E₂(NHB^Me)₂ (E=Be, Mg; NHB^Me=(HCN^Me)₂B) complexes where E₂ is in ³∑_u⁺ excited state having (nσ_g⁺)²(nσ_u⁺)¹((n+1)σ_g⁺)¹ (n=2 for Be and n=4 for Mg) valence electron configuration and it forms electron‐shared bonding with two NHB^Me radicals. The effects of bonding with nσ_u⁺ and (n+1)σ_g⁺ orbitals will cancel each other, providing the former E?E bond order as one. Be₂(NHB^Me)₂ complex is thermochemically stable with respect to possible dissociation channels at room temperature, whereas the two exergonic channels, Mg₂(NHB^Me)₂ → Mg + Mg(NHB^Me)₂ and Mg₂(NHB^Me)₂ → Mg₂ + (NHB^Me)₂, are kinetically inhibited by a free energy barrier of 15.7 and 18.7 kcal mol^?1, respectively, which would likely to be further enhanced in cases of bulkier substituents attached to the NHB ligands. Therefore, the title complexes are first viable systems which feature a neutral E₂ moiety with a single E?E covalent bond.     

“Instant Methylide” Modification of the Corey–Chaykovsky Epoxide Synthesis

Abstract

We report that Me₃S(O)⁺I^? (1) and Me₃S⁺I^? (2) form stable, dry mixtures with KOt-Bu and NaH, respectively, which remain stable upon prolonged storage (>1 year). The corresponding methylides (Me₂SO=CH₂ and Me₂S=CH₂) are generated upon addition of DMSO or DMSO/THF solutions of carbonyl compounds, cleanly affording epoxides via the Corey–Chaykovsky reaction in good yields and short reaction times (as short as 20 min when 1–2 mmol of various ketones and aldehydes were treated with a mixture of 1 and KOt-Bu at 50–60°C).     

N,N,N′,N′-Tetramethylethylendiamin-di(tert-butyl)aluminium-Kationen — Molekülstruktur des [(Me3C)2Al · TMEDA]⊕[(Me3C)2AlBr2]⊖

(N,N,N′,N′ -tetramethylethylendiamine) di(tert-butyl)aluminium Cations — Molecular Structure of [(Me₃C)₂Al(TMEDA)]^⊕[(Me₃C)₂AlBr₂]^? Dimeric di(tert-butyl)aluminium halides (Me₃C)₂AlX (X = Cl, Br) react with N,N,N′,N′ -tetramethylethylendiamine (TMEDA) to give three compounds: the salt-like [(Me₃C)₂Al(TMEDA)]^⊕[(Me₃C)₂AlX₂]^? 1 , characterized by crystal structure determination, and [(Me₃C)₂Al(TMEDA)]^⊕X^? 3 both with chelating amine, and the more covalent, pentane soluble (Me₃C)₂AlX(TMEDA) 2 with TMEDA bound by only one nitrogen atom. The reaction resembles the symmetrical and unsymmetrical cleavage of diborane(6). 3 (X = Cl) is also formed by treatment of 1 with boiling n-hexane in the presence of TMEDA over a period of 24 hours, while for X = Br the more covalent 2 is the main product under similar conditions. In solution 2 decomposes slowly yielding different products in dependency of the solvent: in benzene 3 and in n-pentane 1 are formed.     

Carbon-13 n.m.r. study of 31P?13C spin–spin coupling constants in dichloro- and monochloro-vinyl phosphate derivatives

Carbon-13 chemical shifts and J(PC) coupling constants of 29 vinyl phosphate derivatives are presented. In the series of compounds (R₁O)₂P(O)OC¹(R)?C²X₂ (where 3 in R indicates the first carbon of the R₂ substituent) large differences were found between the ³J(P, O, C-1, C-3) and ³J(P, O, C-1, C-2) coupling constants of the chlorinated (X?CI) and the unsubstituted (X?H) derivatives. A possible explanation of this phenomenon is given on the basis of Jameson's s bond character theory. Strong stereospecificity of ³J(P, O, C-1, C-3) coupling constants was observed in the series of compounds (R₁O)₂ P(O)OC¹(R)?C²HR₃. Coupling constants varied between 3.2–4.9 Hz in the E isomers, while peaks could not be resolved in the Z isomers. The ³J(P, O, C-1, C-2) coupling constants were regularly 20–30% greater in the Z than in the E isomers.     

Difluorenyl carbo‐Benzenes: Synthesis,Electronic Structure,and Two‐Photon Absorption Properties of Hydrocarbon Quadrupolar Chromophores

The synthesis, crystal and electronic structures, and one‐ and two‐photon absorption properties of two quadrupolar fluorenyl‐substituted tetraphenyl carbo‐benzenes are described. These all‐hydrocarbon chromophores, differing in the nature of the linkers between the fluorenyl substituents and the carbo‐benzene core (C?C bonds for 3 a , C?C?C?C expanders for 3 b ), exhibit quasi–superimposable one‐photon absorption (1PA) spectra but different two‐photon absorption (2PA) cross‐sections σ_2PA. Z‐scan measurements (under NIR femtosecond excitation) indeed showed that the C?C expansion results in an approximately twofold increase in the σ_2PA value, from 336 to 656 GM (1 GM=10^?50 cm⁴ s molecule^?1 photon^?1) at λ=800 nm. The first excited states of A_u and A_g symmetry accounting for 1PA and 2PA, respectively, were calculated at the TDDFT level of theory and used for sum‐over‐state estimations of σ_2PA(λ_i), in which λ_i=2 hc/E_i, h is Planck’s constant, c is the speed of light, and E_i is the energy of the 2PA‐allowed transition. The calculated σ_2PA values of 227 GM at 687 nm for 3 a and 349 GM at 708 nm for 3 b are in agreement with the Z‐scan results.     

Large-Z and -N dependence of atomic energies from renormalization of the large-dimension limit

By combining Hartree–Fock results for nonrelativistic ground-state energies of N-electron atoms with analytic expressions for the large-dimension limit, we have obtained a simple renormalization procedure. For neutral atoms, this yields energies typically threefold more accurate than the Hartree–Fock approximation. Here, we examine the dependence on Z and N of the renormalized energies E(N, Z) for atoms and cations over the range Z, N = 2 → 290. We find that this gives for large Z = N an expansion of the same form as the Thomas–Fermi statistical model, E → Z^7/2(C₀ + C₁Z^?1/3 + C₂Z^?2/3 + C₃Z^?3/3 + ?), with similar values of the coefficients for the three leading terms. Use of the renormalized large-D limit enables us to derive three further terms. This provides an analogous expansion for the correlation energy of the form δE δZ^4/3(δC₃ + δC₅Z^?2/3 + δC₆Z^?3/3 + ?); comparison with accurate values of δE available for the range Z ? 36 indicates the mean error is only about 10%. Oscillatory terms in E and δE are also evaluated. © 1994 John Wiley & Sons, Inc.     

Speciation of volatile antimony compounds in culture headspace gases of Cryptococcus humicolus using solid phase microextraction and gas chromatography–mass spectrometry

Direct analysis of the volatile antimony compounds stibine (SbH₃), monomethylantimony, dimethylantimony (Me₂Sb) and trimethylantimony (Me₃Sb) using solid phase microextraction (SPME) with polydimethylsiloxane fibres and gas chromatography–mass spectrometry (GC–MS) is described. The best analyte to background signal ratio was achieved using a 20 min extraction time. Antimony species were separated using a 3% phenylmethylsilicone capillary column operated at a column pressure of 70 kPa, a flow rate of 1.4 ml min^?1 and temperature ramping from 30 to 36 °C at 0.1 °C min^?1. Cryogenic focusing of desorbed species was required to achieve resolution of antimony species. The optimized SPME–GC–MS method was applied to the analysis of headspace gases from cultures of Cryptococcus humicolus incubated with inorganic antimony(III) and (V) substrates. The headspace gases from biphasic (aerobic–anaerobic) biomass‐concentrated culture incubations revealed the presence of SbH₃, Me₂Sb and Me₃Sb. Stibine was the major antimony species detected in cultures amended with inorganic antimony(V). Me₃Sb was the sole volatile antimony species detected when cultures were amended with antimony(III). Copyright © 2002 John Wiley & Sons, Ltd.     

Reactions of Methyl Diazoacetate with (E)‐ and (Z)‐1,2‐Bis(trifluoromethyl)ethene‐1,2‐dicarbonitrile: Novel and Unanticipated Pathways

The cycloadditions of methyl diazoacetate to 2,3‐bis(trifluoromethyl)fumaronitrile ((E)‐ BTE ) and 2,3‐bis(trifluoromethyl)maleonitrile ((Z)‐ BTE ) furnish the 4,5‐dihydro‐1H‐pyrazoles 13 . The retention of dipolarophile configuration proceeds for (E)‐ BTE with > 99.93% and for (Z)‐ BTE with > 99.8% (CDCl₃, 25°), suggesting concertedness. Base catalysis (1,4‐diazabicyclo[2.2.2]octane (DABCO), proton sponge) converts the cycloadducts, trans‐ 13 and cis‐ 13 , to a 94 : 6 equilibrium mixture (CDCl₃, r.t.); the first step is N‐deprotonation, since reaction with methyl fluorosulfonate affords the 4,5‐dihydro‐1‐methyl‐1H‐pyrazoles. Competing with the cis/trans isomerization of 13 is the formation of a bis(dehydrofluoro) dimer (two diastereoisomers), the structure of which was elucidated by IR, ¹⁹F‐NMR, and ¹³C‐NMR spectroscopy. The reaction slows when DABCO is bound by HF, but F^? as base keeps the conversion to 22 going and binds HF. The diazo group in 22 suggests a common intermediate for cis/trans isomerization of 13 and conversion to 22 : reversible ring opening of N‐deprotonated 13 provides 18 , a derivative of methyl diazoacetate with a carbanionic substituent. Mechanistic comparison with the reaction of diazomethane and dimethyl 2,3‐dicyanofumarate, a related tetra‐acceptor‐ethylene, brings to light unanticipated divergencies.     

NMR of enaminones. Part 7 1H-, 13C-, and 17O-NMR and X-ray structure determinations of 1,2-disubstituted conjugated 3-[(tert-Butyl)amino]enones

¹H-, ¹³C-, and ¹⁷O-NMR spectra for the 2-substituted enaminones MeC(O)C(Me)?CHNH(t-Bu) ( 1 ), EtC(O)C(Me)?CHNH(t-Bu) ( 2 ), PhC(O)C(Me)?CHNH(t-Bu) ( 3 ), and MeC(O)C(Me)?CHNH(t-Bu) ( 4 ) are reported. These data show that 3 exists mainly in the (E)-form, 4 in (Z)-form, and 1 and 2 as mixtures of both forms. Polar solvents favour the (E)-form. The (Z)- and (E)-forms exist in the 1,2-syn,3,N-anti and 1,2-anti,1,N-anti conformations A and B , respectively. The structures of the (E)- and (Z)-form are confirmed by X-ray crystal-structure determinations of 3 and 4. The shielding of the carbonyl O-atom in the ¹⁷O-NMR spectrum by intramolecular H-bonding (Δλ_HB) ranging from ?28 to ?41 ppm, depends on the substituents at C(l) and C(2). Crystals of 3 at 90 K are monoclinic. with a = 9.618(2) Å, b = 15.792(3) Å, c = 16.705(3) Å, and β = 94.44(3)°, and the space group is P2₁/c with Z = 8 (refinement to R = 0.0701 on 3387 independent reflections). Crystals of 4 at 101 K are monoclinic, with a = 16.625(8) Å, b = 8.637(6) Å, c = 11.024(7) Å, and β = 101.60(5)°, and the space group is Cc with Z = 4 (refinement to R = 0.0595 on 2106 independent reflections).     

Enantioselective Synthesis of 2‐(2‐Arylcyclopropyl)glycines: Conformationally Restricted Homophenylalanine Analogs

Starting from simple aromatic aldehydes and acetylfuran, (E)‐1‐(furan‐2‐yl)‐3‐arylprop‐2‐en‐1‐ones ( 2 ) were synthesized in high yields. Cyclopropanation of the C?C bond with trimethylsulfoxonium iodide (Me₃SO⁺I^?) furnished (furan‐2‐yl)(2‐arylcyclopropyl)methanones 3 in 90–97% yields. Selective conversion of cyclopropyl ketones to their (E)‐ and (Z)‐oxime ethers 5 and oxazaborolidine‐catalyzed stereoselective reduction of the C?N bond followed by separation of the formed diastereoisomers, furnished (2‐arylcyclopropyl)(furan‐2‐yl)methanamines 6 in optically pure form and high yield. Oxidation of the furan ring of (S,S,S)‐, (S,R,R)‐, (R,S,S)‐, and (R,R,R)‐ 6a afforded the four stereoisomers of α‐(2‐phenylcyclopropyl) glycine ( 1a ).     

Sels de Pyrylium. XVIII. Etude par RMN du Carbone-13 de Sels d'Aminopyrylium et des Barrières de Rotation dans quelques Cations N,N-Diméthylaminopyrylium

The C-2—N bond of 2-N,N-dimethylaminopyrylium cations has a partial π character due to the conjugation of the nitrogen lone-pair with the ring. The values of ΔG^≠, ΔH^≠, ΔS^≠ parameters related to the corresponding hindered rotation have been determined by ¹³C NMR total bandshape analysis. This conjugation decreases the electrophilic character of carbon C-4 so that the displacement of the alkoxy group is no longer possible. Such a hindered rotation also exists in 4-N,N-dimethylaminopyrylium cations and the corresponding ΔG^≠ parameters have been evaluated. Comparison of these two cationic species shows that hindered rotation around the C—N bond is larger in position 4 than in position 2. Furthermore, the barrier to internal rotation around the C-2? N bond decreases with increasing electron donating power of the substituent at position 4. ΔG^≠ values decreases from 19.1 kcal mol^?1 (79.9 kJ mol^?1) to 12.6 kcal mol^?1 (52.7 kJ mol^?1) according to the following sequence for the R-4 substituents: -C₆H₅, -CH₃, -OCH₃, -N(CH₃)₂.     

Ion pairing of quaternary salts in solvent mixtures

Conductances at 25.00°C are reported for the following systems: tetrabutylammonium bromide in dimethyl sulfoxide-acetone mixtures (Bu₄NBr in Me₂SO–Me₂CO); tetraphenylphosphonium bromide (Ph₄PBr) in water Me₂SO, Me₂CO, and in the mixtures H₂O–Me₂SO, Me₂SO–Me₂CO and Me₂CO–H₂O; Ph₄PCl in Me₂SO, Me₂CO, H₂O–Me₂SO, and Me₂SO–Me₂CO; and tetrapropylammonium bromide (Pr₄NBr) in Me₂SO and Me₂CO. The data were analyzed using the Fuoss 1978 equation which is based on the coupled equilibria: (unpaired ions)(solvent-separated pairs)(contact pairs). The conductimetric pairing constantK _A =K _R(l+K _s) is the product of two factors:K _R, which describes the first (diffusion controlled) equilibrium andK _s=exp(–E _s/kT), which describes the second (system-specific) equilibrium. Ions with overlapping cospheres (of diameterR) are defined as paired: their center-to-center distancer lies in the rangearR; contact pairs (r=a) are ions which have one ion of opposite charge as a nearest neighbor, all other nearest and next nearest neighbors being solvent molecules. The quantityE _s is the difference in free energy between the states defined byr=R andr=a. For the Me₂SO–Me₂CO systems,E _s is positive for solutions in Me₂SO and decreases through zero to negative values as the fraction of the less polarizable acetone increases. For solutions in waterE _s is also positive. On addition of Me₂SO or Me₂CO,E _s initially increases, goes through a maximum, and then decreases to negative values as the fraction of the less polarizable component increases. The decrease is an electrostatic effect, common to all the systems. The initial increase inE _s appears when the small water molecules surrounding solvent-separated pairs are replaced by organic molecules which have greater volumes than water.     

Thermolyse sterisch überladener Alanate; Synthese zweier neuer 1-Sila-3-alanata-cyclobutan-Derivate mit AlC2Si-Heterozyklus

Thermolysis of Sterically Stressed Alanates; Synthesis of Two New 1-Sila-3-alanata-cyclobutane Derivatives with Four-membered AlC₂Si-Heterocycles The reaction of high shielded alkyl or aryl alanes with LiCH(SiMe₃)₂ in the presence of the chelating N,N′,N″-trimethyl-triazinane yields the sterically stressed alanates [(Me₃C)₂Al{CH(SiMe₃)₂}₂]^? 12 and [R? Al{CH(SiMe₃)₂}₃]^? (R = Me₃SiCH₂ 13 , Et 14 , Me 15 , C₆H₅ 16 ) each with a Li(triazinane)₂ counter ion. On thermolysis of the sterically most shielded derivatives 12 and 13 at 130 to 150°C one equivalent of bis(trimethylsilyl)methane is liberated, and by deprotonation of methyl groups carbanionic species are formed, which are stabilized by intramolecular coordination to the unsaturated aluminium atoms under formation of AlC₂Si heterocycles ( 19 and 20 ). 20 was characterized by a single crystal structure determination. The remaining alanates give under similar conditions either under dismutation the recently published heterocycle 1 with two intact CH(SiMe₃)₂ groups ( 14 and 15 ) or a methyl alanate by the replacement of a elementorganic substituent ( 16 ).     

Pentiptycene‐Derived Light‐Driven Molecular Brakes: Substituent Effects of the Brake Component

Five pentiptycene‐derived stilbene systems ( 1 R ; R =H, OM, NO, Pr, and Bu) have been prepared and investigated as light‐driven molecular brakes that have different‐sized brake components ( 1 H < 1 OM < 1 NO < 1 Pr < 1 Bu ). At room temperature (298 K), rotation of the pentiptycene rotor is fast (k_rot=10⁸–10⁹ s^?1) with little interaction with the brake component in the trans form ((E)‐ 1 R ), which corresponds to the brake‐off state. When the brake is turned on by photoisomerization to the cis form ((Z)‐ 1 R ), the pentiptycene rotation can be arrested on the NMR spectroscopic timescale at temperatures that depend on the brake component. In the cases of (Z)‐ 1 NO , (Z)‐ 1 Pr , and (Z)‐ 1 Bu , the rotation is nearly blocked (k_rot=2–6 s^?1) at 298 K. It is also demonstrated that the rotation is slower in [D₆]DMSO than in CD₂Cl₂. A linear relationship between the free energies of the rotational barrier and the steric parameter A values is present only for (Z)‐ 1 H , (Z)‐ 1 OM , and (Z)‐ 1 NO , and it levels off on going from (Z)‐ 1 NO to (Z)‐ 1 Pr and (Z)‐ 1 Bu . DFT calculations provide insights into the substituent effects in the rotational ground and transition states. The molar reversibility of the E–Z photoswitching is up to 46 %, and both the E and Z isomers are stable under the irradiation conditions.