Unexpected Formation of Dimethylthioketene Cycloadducts in the Reaction of 1,3‐Diphenylaziridine‐2,2‐dicarboxylate with Cyclobutanethione Derivatives

The reaction of 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone ( 1 ) with cis‐1‐alkyl‐2,3‐diphenylaziridines 5 in boiling toluene yielded the expected trans‐configured spirocyclic 1,3‐thiazolidines 6 (Scheme 1). Analogously, dimethyl trans‐1‐(4‐methoxyphenyl)aziridine‐2,3‐dicarboxylate (trans‐ 7 ) reacted with 1 and the corresponding dithione 2 , respectively, to give spirocyclic 1,3‐thiazolidine‐2,4‐dicarboxylates 8 (Scheme 2). However, mixtures of cis‐ and trans‐derivatives were obtained in these cases. Unexpectedly, the reaction of 1 with dimethyl 1,3‐diphenylaziridine‐2,2‐dicarboxylate ( 11 ) led to a mixture of the cycloadduct 13 and 5‐(isopropylidene)‐4‐phenyl‐1,3‐thiazolidine‐2,2‐dicarboxylate ( 14 ), a formal cycloadduct of azomethine ylide 12 with dimethylthioketene (Scheme 3). The regioisomeric adduct 16 was obtained from the reaction between 2 and 11 . The structures of 6b , cis‐ 8a , cis‐ 8b, 10 , and 16 have been established by X‐ray crystallography.     

Cross‐Aldol Reaction of Isatin with Acetone Catalyzed by Leucinol: A Mechanistic Investigation

Comprehensive mechanistic studies on the enantioselective aldol reaction between isatin ( 1 a ) and acetone, catalyzed by L ‐leucinol ( 3 a ), unraveled that isatin, apart from being a substrate, also plays an active catalytic role. Conversion of the intermediate oxazolidine 4 into the reactive syn‐enamine 6 , catalyzed by isatin, was identified as the rate‐determining step by both the calculations (ΔG^≠=26.1 kcal mol^?1 for the analogous L ‐alaninol, 3 b ) and the kinetic isotope effect (k_H/k_D=2.7 observed for the reaction using [D₆]acetone). The subsequent reaction of the syn‐enamine 6 with isatin produces (S)‐ 2 a (calculated ΔG^≠=11.6 kcal mol^?1). The calculations suggest that the overall stereochemistry is controlled by two key events: 1) the isatin‐catalyzed formation of the syn‐enamine 6 , which is thermodynamically favored over its anti‐rotamer 7 by 2.3 kcal mol^?1; and 2) the high preference of the syn‐enamine 6 to produce (S)‐ 2 a on reaction with isatin ( 1 a ) rather than its enantiomer (ΔΔG^≠=2.6 kcal mol^?1).     

Gas permeability and n‐butane solubility of poly(1‐trimethylgermyl‐1‐propyne)

The gas permeability and n‐butane solubility in glassy poly(1‐trimethylgermyl‐1‐propyne) (PTMGP) are reported. As synthesized, the PTMGP product contains two fractions: (1) one that is insoluble in toluene and soluble only in carbon disulfide (the toluene‐insoluble polymer) and (2) one that is soluble in both toluene and carbon disulfide (the toluene‐soluble polymer). In as‐cast films, the gas permeability and n‐butane solubility are higher in films prepared from the toluene‐soluble polymer (particularly in those films cast from toluene) than in films prepared from the toluene‐insoluble polymer and increase to a maximum in both fractions after methanol conditioning. For example, in as‐cast films prepared from carbon disulfide, the oxygen permeability at 35 °C is 330 × 10^?10 cm³ (STP) cm/(cm² s cmHg) for the toluene‐soluble polymer and 73 × 10^?10 cm³ (STP) cm/(cm² s cmHg) for the toluene‐insoluble polymer. After these films are conditioned in methanol, the oxygen permeability increases to 5200 × 10^?10 cm³ (STP) cm/(cm² s cmHg) for the toluene‐soluble polymer and 6200 × 10^?10 cm³ (STP) cm/(cm² s cmHg) for the toluene‐insoluble polymer. The rankings of the fractional free volume and nonequilibrium excess free volume in the various PTMGP films are consistent with the measured gas permeability and n‐butane solubility values. Methanol conditioning increases gas permeability and n‐butane solubility of as‐cast PTMGP films, regardless of the polymer fraction type and casting solvent used, and minimizes the permeability and solubility differences between the various films (i.e., the permeability and solubility values of all conditioned PTMGP films are similar). © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2228–2236, 2002     

Easy access to medium rings by entropy/strain reduction,Part 3. Competition between dihydroazepine and enamine formation in reaction of tetrahalosubstituted dibromides with primary amines

The reaction of Z,Z‐2,3,4,5‐tetrahalo‐2,4‐dien‐1,6‐dibromides ( 3 , R¹ ‐ R⁴ = Cl, Br) with primary amines in the presence of potassium carbonate leads to both the dihydroazepines 4 and secondary enamines 5 . The formation of enamine is suppressed with toluene sulfonamide as nitrogen source. (2Z,4Z)‐2,3,4,5‐Tetrabromo‐hexa‐2,4‐diene‐1,6‐diol (2, R¹ ‐ R⁴ = Br) is atropisomeric in solution.     

Solvent- and temperature-dependent functionalisation of enantioenriched aziridines

A highly stereo‐ and regioselective functionalisation of chiral non‐racemic aziridines is reported. By starting from a parent enantioenriched aziridine and finely tuning the reaction conditions, it is possible to address the regio‐ and stereoselectivity of the lithiation/electrophile trapping sequence, thereby allowing the preparation of highly enantioenriched functionalised aziridines. From chiral N‐alkyl trans‐2,3‐diphenylaziridines (S,S)‐ 1 a , b , two differently configured chiral aziridinyllithiums could be generated (trans‐ 1 a , b‐Li in toluene and cis‐ 1 a , b‐Li in THF), thus disclosing a solvent‐dependent reactivity that is useful for the synthesis of chiral tri‐substituted aziridines with different stereochemistry. In contrast, chiral aziridine (S,S)‐ 1 c showed a temperature‐dependent reactivity to give chiral ortho‐lithiated aziridine 1 c‐ ortho ‐Li at ?78 °C and α‐lithiated aziridine 1 c‐α‐Li at 0 °C. Both lithiated intermediates react with electrophiles to give enantioenriched ortho‐ and α‐functionalised aziridines. The reaction of all the lithiated aziridines with carbonyl compounds furnished useful chiral hydroxyalkylated derivatives, the stereochemistry of which was ascertained by X‐ray and NMR spectroscopic analysis. The usefulness of chiral non‐racemic functionalised aziridines has been demonstrated by reductive ring‐opening reactions furnishing chiral amines that bear quaternary stereogenic centres and chiral 1,2‐, 1,3‐ and 1,5‐aminoalcohols. It is remarkable that the solvent‐dependent reactivity observed with (S,S)‐ 1 a , b permits the preparation of both the enantiomers of amines ( 11 and ent‐ 11 ) and 1,2‐aminoalcohols ( 13 and ent‐ 13 ) starting from the same parent aziridine. Interestingly, for the first time, a configurationally stable chiral α‐lithiated aziridine ( 1 c‐α‐Li ) has been generated at 0 °C. In addition, ortho‐hydroxyalkylated aziridines have been easily converted into chiral aminoalkyl phthalans, which are useful building blocks in medicinal chemistry.     

Olefinic reagents tested for peptide derivatization with switchable properties: Stable upon collision induced dissociation and cleavable by in‐source Paternò‐Büchi reactions

This contribution is part of our ongoing efforts to develop innovative cross‐linking (XL) reagents and protocols for facilitated peptide mixture analysis and efficient assignment of cross‐linked peptide products. In this report, we combine in‐source Paternò‐Büchi (PB) photo‐chemistry with a tandem mass spectrometry approach to selectively address the fragmentation of a tailor‐made cross‐linking reagent. The PB photochemistry, so far exclusively used for the identification of unsaturation sites in lipids and in lipidomics, is now introduced to the field of chemical cross‐linking. Based on trans‐3‐hexenedioic acid, an olefinic homo bifunctional amine reactive XL reagent was designed and synthesized for this proof‐of‐principle study. Condensation products of the olefinic reagent with a set of exemplary peptides are used to test the feasibility of the concept. Benzophenone is photochemically reacted in the nano‐electrospray ion source and forms oxetane PB reaction products. Subsequent CID‐MS triggered retro‐PB reaction of the respective isobaric oxetane molecular ions and delivers reliably and predictably two sets of characteristic fragment ions of the cross‐linker. Based on these signature ion sets, a straightforward identification of covalently interconnected peptides in complex digests is proposed. Furthermore, CID‐MSⁿ experiments of the retro‐PB reaction products deliver peptide backbone characteristic fragment ions. Additionally, the olefinic XL reagents exhibit a pronounced robustness upon CID‐activation, without previous UV‐excitation. These experiments document that a complete backbone fragmentation is possible, while the linker‐moiety remains intact. This feature renders the new olefinic linkers switchable between a stable, noncleavable cross‐linking mode and an in‐source PB cleavable mode.     

One‐Pot Synthesis of 4‐(Alkylamino)‐1‐(arylsulfonyl)‐3‐benzoyl‐1,5‐ dihydro‐5‐hydroxy‐5‐phenyl‐2H‐pyrrol‐2‐ones via a Multicomponent Reaction

An effective route to novel 4‐(alkylamino)‐1‐(arylsulfonyl)‐3‐benzoyl‐1,5‐dihydro‐5‐hydroxy‐5‐phenyl‐2H‐pyrrol‐2‐ones 10 is described (Scheme 2). This involves the reaction of an enamine, derived from the addition of a primary amine 5 to 1,4‐diphenylbut‐2‐yne‐1,4‐dione, with an arenesulfonyl isocyanate 7 . Some of these pyrrolones 10 exhibit a dynamic NMR behavior in solution because of restricted rotation around the C? N bond resulting from conjugation of the side‐chain N‐atom with the adjacent α,β‐unsaturated ketone group, and two rotamers are in equilibrium with each other in solution ( 10 ? 11 ; Scheme 3). The structures of the highly functionalized compounds 10 were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS), by elemental analyses, and, in the case of 10a , by X‐ray crystallography. A plausible mechanism for the reaction is proposed (Scheme 4).     

Intermolecular insertion of an N,N-heterocyclic carbene into a nonacidic C-H bond: Kinetics, mechanism and catalysis by (K-HMDS)2 (HMDS = Hexamethyldisilazide)

The reaction of 2‐[¹³C]‐1‐ethyl‐3‐isopropyl‐3,4,5,6‐tetrahydropyrimidin‐1‐ium hexafluorophosphate ([¹³C₁]‐ 1 ‐PF₆) with a slight excess (1.03 equiv) of dimeric potassium hexamethyldisilazide (“(K‐HMDS)₂”) in toluene generates 2‐[¹³C]‐3‐ethyl‐1‐isopropyl‐3,4,5,6‐tetrahydropyrimid‐2‐ylidene ([¹³C₁]‐ 2 ). The hindered meta‐stable N,N‐heterocyclic carbene [¹³C₁]‐ 2 thus generated undergoes a slow but quantitative reaction with toluene (the solvent) to generate the aminal 2‐[¹³C]‐2‐benzyl‐3‐ethyl‐1‐isopropylhexahydropyrimidine ([¹³C₁]‐ 14 ) through formal C? H insertion of C(2) (the “carbene carbon”) at the toluene methyl group. Despite a significant pK_a mismatch (ΔpK_a 1 ⁺ and toluene estimated to be ca. 16 in DMSO) the reaction shows all the characteristics of a deprotonation mechanism, the reaction rate being strongly dependent on the toluene para substituent (ρ=4.8(±0.3)), and displaying substantial and rate‐limiting primary (k_H/k_D=4.2(±0.6)) and secondary (k_H/k_D=1.18(±0.08)) kinetic isotope effects on the deuteration of the toluene methyl group. The reaction is catalysed by K‐HMDS, but proceeds without cross over between toluene methyl protons and does not involve an HMDS anion acting as base to generate a benzyl anion. Detailed analysis of the reaction kinetics/kinetic isotope effects demonstrates that a pseudo‐first‐order decay in 2 arises from a first‐order dependence on 2 , a first‐order dependence on toluene (in large excess) and, in the catalytic manifold, a complex noninteger dependence on the K‐HMDS dimer. The rate is not satisfactorily predicted by equations based on the Brønsted salt‐effect catalysis law. However, the rate can be satisfactorily predicted by a mole‐fraction‐weighted net rate constant: ?d[ 2 ]/dt=({x ₂ k_uncat}+{(1?x ₂ ) k_cat})[ 2 ]¹[toluene]¹, in which x ₂ is determined by a standard bimolecular complexation equilibrium term. The association constant (K_a) for rapid equilibrium–complexation of 2 with (K‐HMDS)₂ to form [ 2 (K‐HMDS)₂] is extracted by nonlinear regression of the ¹³C NMR shift of C(2) in [¹³C₁]‐ 2 versus [(K‐HMDS)₂] yielding: K_a=62(±7) M ^?1; δ_C(2) in 2 =237.0 ppm; δ_C(2) in [ 2 (K‐HMDS)₂]=226.8 ppm. It is thus concluded that there is discrete, albeit inefficient, molecular catalysis through the 1:1 carbene/(K‐HMDS)₂ complex [ 2 (K‐HMDS)₂], which is found to react with toluene more rapidly than free 2 by a factor of 3.4 (=k_cat/k_uncat). The greater reactivity of the complex [ 2 (K‐HMDS)₂] over the free carbene ( 2 ) may arise from local Brønsted salt‐effect catalysis by the (K‐HMDS)₂ liberated in the solvent cage upon reaction with toluene.     

Structural and Magnetic Insights into the Trinuclear Ferrocenophane and Unexpected Hydrido Inverse Crown Products of Alkali‐Metal‐Mediated Manganation(II) of Ferrocene

With the aim of introducing the diisopropylamide [NiPr₂] ^? ligand to alkali‐metal‐mediated manganation (AMMMn) chemistry, the temperature‐dependent reactions of a 1:1:3 mixture of butylsodium, bis(trimethylsilylmethyl)manganese(II), and diisopropylamine with ferrocene in hexane/toluene have been investigated. Performed at reflux temperature, the reaction affords the surprising, ferrocene‐free, hydrido product [Na₂Mn₂ (μ‐H)₂{N(iPr)₂}₄]?2 toluene ( 1 ), the first Mn hydrido inverse crown complex. Repeating the reaction rationally, excluding ferrocene, produces 1 in an isolated crystalline yield of 62 %. At lower temperatures, the same bimetallic amide mixture leads to the manganation of ferrocene to generate the first trimanganese, trinuclear ferrocenophane, [{Fe(C₅H₄)₂}₃{Mn₃Na₂(NiPr₂)₂ (HNiPr₂)₂}] ( 2 ) in an isolated crystalline yield of 81 %. Both 1 and 2 have been characterised by X‐ray crystallographic studies. The magnetic properties of paramagnetic 1 and 2 have also been examined by variable‐temperature magnetisation measurements on powdered samples. For 1 , the room‐temperature value for χT is 3.45 cm³ K mol^?1, and on lowering the temperature a strong antiferromagnetic coupling between the two Mn ions is observed. For 2 , the room‐temperature value for χT is 4.06 cm³ K mol^?1, which is significantly lower than the expected value for three isolated paramagnetic Mn^II ions.     

Synthesis and antimicrobial activity of a new class of methyleneamine‐linked bis‐heterocycles

A new class of methyleneamine‐linked bis‐heterocycles that exhibit antimicrobial activity was synthesized. Bromination of 1 followed by condensation with thiourea gave 3 . The reaction of 3 with propargyl bromide in dry toluene under inert atmosphere led to the formation of 4 . Its subsequent reaction with different aromatic azides 5 using CuSO₄.5H₂O‐sodiumascorbate system in a 2:1 mixture of water and tert‐butylalcohol yielded the title compounds 6a , 6b , 6c , 6d , 6e , 6f , 6g , 6h , 6i , 6j in good yields. The identities of these compounds were confirmed following elemental analysis, IR, ¹H, ¹³C NMR, and mass spectral studies. All the title compounds exhibited pronounced in vitro antibacterial and antifungal activities. J. Heterocyclic Chem., (2011).     

Nucleophilic Addition to CC Bonds,Part X

A mechanistic model is presented for the base‐catalyzed intramolecular cyclization of polycyclic unsaturated alcohols of type A to ethers D (Scheme 1). The alkoxide anion B is formed first in a fast acid‐base equilibrium. For the subsequent reaction to D , a carbanion‐like transition state C is proposed. This mechanism is in full agreement with our results regarding the influence of substituents on the regioselectivity and the rate of cyclization. We studied the effect of alkyl substituents in allylic position (alkylated endocylic olefinic alcohols 1 – 3 ) and, especially, at the exocyclic double bond ( 12 – 15 ). The fastest cyclization (k_rel=1) is 12 → 16 , which proceeds via a primary carbanion‐like transition state ( E : R¹=R²=H). The corresponding processes 13 → 17 and 14 → 17 are characterized by a less‐stable secondary carbanion‐like transtition state ( E : R¹=Me, R²=H, or vice versa) and are slower by a factor of 10⁴. The slowest reaction (k_rel ca. 10⁻⁶) is the cyclization 15 → 18 via a tertiary carbanion‐like transition state ( E : R¹=R²=Me).     

Synthesis,Characterization, and In Vitro Antimicrobial Activity of Methyleneamine‐Linked Bis‐heterocycles

A new class of methyleneamine‐linked bis‐heterocycles that exhibit antimicrobial activity was synthesized. Bromination of 1 followed by condensation with thiourea gave 3 . The reaction of 3 with propargyl bromide in dry toluene under inert atmosphere led to the formation of 4 . Its subsequent reaction with different nitrile oxides using CuSO₄.5H₂O–sodiumascorbate system in a 2:1 mixture of water and tert‐butyl alcohol yielded the title compounds 6a , 6b , 6c , 6d , 6e , 6f , 6g , 6h , 6i , 6j , 6k , 6l in good yields. The identities of these compounds were confirmed following elemental analysis, IR, ¹H, ¹³C NMR, and mass spectral studies. All the title compounds exhibited pronounced in vitro antibacterial and antifungal activities.     

Friedel crafts reactions of three-member ileterocycles II: Alkylation of aromatic compounds with aziridines

The Friedel Crafts reaction of propylenimine with symmetrical arenes in the presence of aluminum chloride was investigated. Electron donating substituents increase the α-methyl-β-phenethylamine/ β-methyl-β-phenethylaimine ratio, while increasing the temperature has the opposite effect. In the reaction of chlorobenzene or toluene with aziridine, the nature of the substituent has little effect on the ortho/para ratio.     

A novel tridentate nitrogen donor as ligand in copper catalyzed ATRP of methyl methacrylate

A tridentate ligand, BPIEP: 2,6‐bis[1‐(2,6‐diisopropyl phenylimino) ethyl] pyridine, having central pyridine unit and two peripheral imine coordination sites was effectively employed in controlled/“living” radical polymerization of MMA at 90°C in toluene as solvent, Cu^IBr as catalyst, and ethyl‐2‐bromoisobutyrate (EBiB) as initiator resulting in well‐defined polymers with polydispersities M_w/M_n ≤ 1.23. The rate of polymerization follows first‐order kinetics, k_app = 3.4 × 10^?5 s^?1, indicating the presence of low radical concentration ([P*] ≤ 10^?8) throughout the reaction. The polymerization rate attains a maximum at a ligand‐to‐metal ratio of 2:1 in toluene at 90°C. The solvent concentration (v/v, with respect to monomer) has a significant effect on the polymerization kinetics. The polymerization is faster in polar solvents like, diphenylether, and anisole, as compared to toluene. Increasing the monomer concentration in toluene resulted in a better control of polymerization. The molecular weights (M_n,SEC) increased linearly with conversion and were found to be higher than predicted molecular (M_n,Cal). However, the polydispersity remained narrow, i.e., ≤1.23. The initiator efficiency at lower monomer concentration approaches a value of 0.7 in 110 min as compared to 0.5 in 330 min at higher monomer concentration. The aging of the copper salt complexed with BPIEP had a beneficial effect and resulted in polymers with narrow polydispersitities and higher conversion. PMMA obtained at room temperature in toluene (33%, v/v) gave PDI of 1.22 (M_n = 8500) in 48 h whereas, at 50°C the PDI is 1.18 (M_n = 10,300), which is achieved in 23 h. The plot of lnk_app versus 1/T gave an apparent activation energy of polymerization as (ΔE^≠_app) 58.29 KJ/mol and enthalpy of equilibrium (ΔH⁰_eq) to 28.8 KJ/mol. Reverse ATRP of MMA was successfully performed using AIBN in bulk as well as solution. The controlled nature of the polymerization reaction was established through kinetic studies and chain extension experiments. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 4996–5008, 2005     

A Facile and Efficient Synthesis of Arylsulfonamido‐Substituted 1,5‐Benzodiazepines and N‐[2‐(3‐Benzoylthioureido)aryl]‐3‐oxobutanamide Derivatives

A facile and efficient synthesis of 1,5‐benzodiazepines with an arylsulfonamido substituent at C(3) is described. 1,5‐Benzodiazepine, derived from the condensation of benzene‐1,2‐diamine and diketene, reacts with an arylsulfonyl isocyanate via an enamine intermediate to produce the title compounds of potential synthetic and pharmacological interest in good yields (Scheme 1). In addition, reaction of benzene‐1,2‐diamine and diketene in the presence of benzoyl isothiocyanate leads to N‐[2‐(3‐benzoylthioureido)aryl]‐3‐oxobutanamide derivatives (Scheme 2). This reaction proceeds via an imine intermediate and ring opening of diazepine. The structures were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses. A plausible mechanism for this type of cyclization is proposed (Scheme 3).     

Addition of diphenylphosphinoyl azide to [60]fullerene

The reaction of [60]fullerene with diphenylphosphinoyl azide in toluene or ino-dichlorobenzene in the presence of traces of water affords 2-[N-(diphenylphosphoryl)amino]-1-hydroxy[60]fullerene This reaction in THF gives a mixture of (N-diphenylphosphoryl)[60]fullerenol[1,2-b]aziridine and a product of partial hydrolysis of the bisadduct of phosphorylated azide and fullerene. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2168–2172, November, 1999.     

Synthesis of O6‐Corona[3]arene[3]pyridazines and Their Molecular Recognition Property in Organic and Aqueous Media

O₆‐Corona[3]arene[3]pyridazines were synthesized from the one‐pot macrocyclic condensation reaction of 3,6‐dichlorotetrazine with 1,4‐dihydroquinone derivatives followed by the inverse electron demand Diels‐Alder reaction of the tetrazine rings with a cyclopentanone‐derived enamine. Conversion of six ester groups within macrocycle into all sodium acetate moieties afforded a water soluble O₆‐corona[3]arene[3]pyridazine. The coronary macrocycle host formed complexes selectively with organic ammoniums and dinitrile guests in a 1: 1 stoichiometric ratio in organic solvents with association constants ranging from (2.96 ± 0.10) × 10¹ to (2.53 ± 0.33) × 10⁵ L·mol⁻¹. Water soluble O₆‐corona[3]arene[3]pyridazine was also able to complex strongly with organic ammoniums in water to give an association constant up to (2.67 ± 0.21) × 10⁴ L·mol⁻¹. The pseudo‐rotaxane and inclusion structures of the host‐guest complexes were revealed by the X‐ray crystallography.     

Organocatalyzed Michael Addition of Aldehydes to Nitro Alkenes – Generally Accepted Mechanism Revisited and Revised

The amine‐catalyzed enantioselective Michael addition of aldehydes to nitro alkenes (Scheme 1) is known to be acid‐catalyzed (Fig. 1). A mechanistic investigation of this reaction, catalyzed by diphenylprolinol trimethylsilyl ether is described. Of the 13 acids tested, 4‐NO₂? C₆H₄OH turned out to be the most effective additive, with which the amount of catalyst could be reduced to 1 mol‐% (Tables 2–5). Fast formation of an amino‐nitro‐cyclobutane 12 was discovered by in situ NMR analysis of a reaction mixture. Enamines, preformed from the prolinol ether and aldehydes (benzene/molecular sieves), and nitroolefins underwent a stoichiometric reaction to give single all‐trans‐isomers of cyclobutanes (Fig. 3) in a [2+2] cycloaddition. This reaction was shown, in one case, to be acid‐catalyzed (Fig. 4) and, in another case, to be thermally reversible (Fig. 5). Treatment of benzene solutions of the isolated amino‐nitro‐cyclobutanes with H₂O led to mixtures of 4‐nitro aldehydes (the products 7 of overall Michael addition) and enamines 13 derived thereof (Figs. 6–9). From the results obtained with specific examples, the following tentative, general conclusions are drawn for the mechanism of the reaction (Schemes 2 and 3): enamine and cyclobutane formation are fast, as compared to product formation; the zwitterionic primary product 5 of C,C‐bond formation is in equilibrium with the product of its collapse (the cyclobutane) and with its precursors (enamine and nitro alkene); when protonated at its nitronate anion moiety the zwitterion gives rise to an iminium ion 6 , which is hydrolyzed to the desired nitro aldehyde 7 or deprotonated to an enamine 13 . While the enantioselectivity of the reaction is generally very high (>97% ee), the diastereoselectivity depends upon the conditions, under which the reaction is carried out (Fig. 10 and Tables 1–5). Various acid‐catalyzed steps have been identified. The cyclobutanes 12 may be considered an off‐cycle ‘reservoir’ of catalyst, and the zwitterions 5 the ‘key players’ of the process (bottom part of Scheme 2 and Scheme 3).     

Synthesis and Crystal Structures of Antimony(III) Complexes With a Bis(amino)silane Ligand

Bis(amino)silane bearing bulky substituents on nitrogen, LH₂ [L = Me₂Si(NDipp)₂, Dipp = 2, 6‐diisopropylphenyl] was reacted with nBuLi (ratio 1:1 and 1:2) in toluene. The Me₂Si(LiNDipp)₂ was treated with SbCl₃ in a 1:1 ratio to yield the four‐membered SiN₂Sb ring compound of composition [η²(N,N)‐Me₂Si(NDipp)₂SbCl] ( 1 ). The mono lithiated bis(amino)silane was used to synthesize the monodentate heterotetraatomic complex [(η¹‐Me₂SiNDipp)NHDippSbCl₂] ( 2 ) by the reaction with SbCl₃. The complexes were characterized by ¹H and ¹³C NMR, elemental analysis, IR, and single‐crystal X‐ray structural analysis.     

Free‐radical copolymerization of styrene and m‐isopropenyl‐α,α′‐dimethylbenzyl isocyanate studied by 1H NMR kinetic experiments

The free‐radical copolymerization of m‐isopropenyl‐α,α′‐dimethylbenzyl isocyanate (TMI) and styrene was studied with ¹H NMR kinetic experiments at 70 °C. Monomer conversion vs time data were used to determine the ratio k_p × k_t^?0.5 for various comonomer mixture compositions (where k_p is the propagation rate coefficient and k_t is the termination rate coefficient). The ratio k_p × k_t^?0.5 varied from 25.9 × 10^?3 L^0.5 mol^?0.5 s^?0.5 for pure styrene to 2.03 × 10^?3 L^0.5 mol^?0.5 s^?0.5 for 73 mol % TMI, indicating a significant decrease in the rate of polymerization with increasing TMI content in the reaction mixture. Traces of the individual monomer conversion versus time were used to map out the comonomer mixture composition drift up to overall monomer conversions of 35%. Within this conversion range, a slight but significant depletion of styrene in the monomer feed was observed. This depletion became more pronounced at higher levels of TMI in the initial comonomer mixture. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1064–1074, 2002