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Carbethoxyallylstannane was employed along with the bis-pi-allylpalladium complex to achieve a useful conversion of prochiral imines to chiral 2-(2-aryl-2-aminoethyl)acrylates which are important building blocks for further asymmetric synthesis of a wide range of compounds.
Elimination reactions of N-alkyl-N-chlorothenylamines 1-4 with MeONa-MeOH and Et(2)NH-MeCN have been studied kinetically. The elimination reactions are regiospecific, producing only the conjugated imines. The reactions are second order and exhibit substantial values of Hammett rho and k(H)/k(D), and an E2 mechanism is evident. The relative rates of elimination for Me/Et/i-Pr/t-Bu substituents are 1/0.5/0.2/0.02 with MeONa-MeOH and 1/0.4/0.2/0.06 with Et(2)NH-MeCN. The transition state structure changes toward more product-like as the base is changed from MeONa-MeOH to Et(2)NH-MeCN. Comparison with existing data reveals that the structure of the transition state is relatively insensitive to the beta-aryl group variation.
The silylcupration of allenes and the subsequent capture of the intermediate cuprate with alpha,beta-unsaturated nitriles is reported. The influence of the substitution of the nitrile, the nature of the silylcopper species, and the temperature on the selectivity of the reaction is studied. An interesting diaddition process was observed (1,2-addition and 1,4-addition), leading to oxo compounds which simultaneously have an allylsilane and a vinylsilane group. The different reactivity of these two units has been employed in the intramolecular allylsilane-terminated cyclization of these adducts, where the vinylsilane moiety remains unchanged. To shed some light on the reaction pathway, the behavior of alpha,beta-unsaturatedd imines was also checked and a new and convenient way for cycloheptane annulation presented. In light of the former results a feasible mechanism is proposed.
[reaction: see text] The enantioselective catalytic 1,4-addition to alpha,beta-unsaturated ketimines is an unprecedented process. Herein, we document the copper-catalyzed addition of dialkylzinc reagents to (2-pyridylsulfonyl)imines of chalcones. This process occurs rapidly in the presence of a chiral phosphoramidite ligand to afford exclusively the 1,4-addition product. In the case of addition of dimethylzinc, enantioselectivities in the range 70-80% ee are obtained. The presence of the metal-coordinating 2-pyridylsulfonyl group proved to be essential for this reaction to proceed.
The treatment of various aromatic and aliphatic aldimines with a mixture of a terminal alkyne and a commercially available dimethylzinc solution in toluene yields the corresponding protected propargylic amines in moderate to excellent yields. The reaction proceeds in the absence of any activator. These observations led to the development of a three-component synthesis of propargylic amines in which the product was obtained upon mixing an aldehyde with ortho-methoxyaniline and phenylacetylene in the presence of dimethylzinc, through in situ formation of the corresponding imine.
This is the first report of the use of triallylaluminum as a reagent for the allylation of carbonyl compounds and imines. The allylation of ketimines without additional metal catalyst is known so far only in the case of the Grignard reagent. Triallylaluminum is a useful alternative to provide the homoallylic amines in excellent yield upon addition to aldimines and ketimines. The significant reactivity of this reagent was confirmed by its reaction with a sterically rigid ketone such as adamantanone to provide 1-adamantyl-3-buten-1-ol in 98% yield. The chemoselectivity of triallylaluminum was demonstrated by using different ketoesters. It is noteworthy that triallylaluminum is prepared from allyl bromide and aluminum metal, and not from a Grignard reagent, and that the procedure is operationally simple, leading to good to excellent product yields.
The reaction that occurs when vapors of 2,2-dimethylpropanal and methanamine are allowed to mix in an infrared gas cell has been examined. The disappearance of starting materials and formation of E-imine product, monitored simultaneously, is best fit by a process involving wall-associated water. The same or closely related processes have been successfully modeled; such processes may also be common to pyridoxal-catalyzed transamination and related reactions in biological systems.
Using a procedure analogous to that developed for nucleophilic trifluoromethylation, the perfluoroalkyl anion reagents created by mixing C2F5I and n-C4F9I with tetrakis(dimethylamino)ethylene (TDAE) were effective in their nucleophilic reactions with aldehydes, ketones, imines, disulfides, and diselenides. Irradiation proved beneficial in the aldehyde and ketone reactions.
The outcome of the cycloaddition between activated ketenes and alpha,beta-unsaturated imines has been investigated both experimentally and theoretically. Our results indicate that activated monosubstituted ketenes yield exclusively [2 + 2] cycloadducts. Disubstituted activated ketenes yield [2 + 2] and/or [4 + 2] cycloadducts. In one case, an unexpected piperidin-2-one has been obtained, although its relative abundance with respect to the corresponding [2 + 2] or [4 + 2] cycloadducts can be minimized by the proper choice of experimental conditions. The ability of different ab initio and semiempirical methods to account for these results has been tested. The best agreement between theory and experiment is achieved at the MP2/6-31G level of theory, with solvent effects taken into account. The semiempirical hamiltonian AM1, at the RHF level, tends to overestimate the stability of the transition structures leading to six-membered cycloadducts, whereas 3 x 3CI-HE/AM1 and CASSCF(2,2)/6-31G methods tend to overestimate the stability and the biradical character of the transition structures leading to [2 + 2] cycloadducts.
Azametallacyclobutene Cp(2)ZrN-t-BuCEt=CEt (1) underwent an insertion reaction with CO to form the acyl complex 2 (Cp(2)Zr(N-t-BuCEtCEtCO), 67% yield). The addition of acetone to azametallacyclobutene 3 (Cp(2)Zr(NArCMeCPh), Ar = 2,6-dimethylphenyl) yielded the N-bonded enamine and O-bonded enolate complex of zirconocene 4 (Cp(2)Zr(NArCMeCPhH)(OCMeCH(2)), 76% yield). The addition of aldehydes RCOH to metallacycle 3 resulted in the insertion of the aldehyde into the Zr-C bond to form complexes Cp(2)Zr(NArCMeCPhCRHO) (8a) and Cp(2)Zr(NArCMeCPhC(i-Pr)HO (9) in 85% (R = Ph) and 73% yields, respectively. Similarly, treatment of metallacycle 10 (Cp(2)Zr(NArCEtCEt)) with benzaldehyde yielded the insertion product 11 (Cp(2)Zr(NArCEtCEtCPhHO)) in 56% isolated yield. The structure of complex 11 was confirmed by an X-ray crystallographic study. Heating the insertion products 8a and 9 led to elimination of the alpha,beta-unsaturated imines 13 and 14a (ArN=CMeCPh=CRH) in 53% and 72% yields, respectively, and the formation of oxozirconocene oligomer (Cp(2)ZrO)(n)(). The oxozirconocene monomer was trapped by dimethylzirconocene, preventing the formation of oligomer and resulting in the isolation of product 15. A kinetic study of this retrocycloaddition produced the following activation parameters: DeltaH() = 26.5 kcal/mol, DeltaS() = 3.48 eu. A Hammett sigma/rho study showed that electron-donating groups alpha to the metallacycle oxygen accelerate the retrocycloaddition (rho = -0.8).
A set of chiral aziridino alcohols 2-5 has been synthesized starting from either readily available amino acids (L-serine, L-threonine, and allo-L-threonine) or simple olefins (using Sharpless asymmetric aminohydroxylation and dihydroxylation reactions). Chiral ligands 2-5 have been tested as promoters for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl) imines 1. The influence of the substituents on the aziridine ring and the alcohol moiety on the selectivity has been studied, and in the best case, an enantiomeric excess of up to 94% could be obtained. Acidic hydrolysis of the initially formed N-protected amines 6 led to the corresponding free amines 7 without racemization. Although a stoichiometric amount of the ligand was used, about 90% of it could be recovered during the workup and reused without significant loss of chiral induction. The utility of the aziridino alcohols 2-5 as catalysts for the same reaction has also been evaluated and enantiomeric excesses of up to 76% were achieved using 0.25 equiv of the chiral ligand. A possible transition state for the addition reaction is also proposed.
A set of new beta-amino alcohols 2, with the 2-azanorbornyl framework, has been prepared and evaluated as promoters for the enantioselective addition of dialkylzinc reagents to N-(diphenylphosphinoyl) imines 1. By variation of the substitution pattern in the ligand, ee's up to 92% could be obtained. Although a stoichiometric amount of the ligand was used, about 90% of it could be recovered during the workup. Amino alcohol 2b, that gave the best enantioselectivities in the stoichiometric reaction, was also applied in a catalytic process, and ee's up to 85% were achieved using 0.25 equiv of the ligand, which is the highest ee obtained so far using that catalytic amount of the ligand. Addition products 3 could be converted into the free amines 4 without racemization by acidic hydrolysis. The utility of ligands 2 as catalysts in the addition of diethylzinc to benzaldehyde has also been investigated, and ee's up to 75% were achieved.
The enantioselective syntheses of (S)-coniine and (2R,6R)-trans-solenopsin A are reported. The key step in both syntheses is the catalytic asymmetric hydrosilylation of a cyclic imine.
The different reaction paths associated with the formal [2 + 2] thermal cycloaddition between several keteniminium cations and imines has been studied computationally. It is found that the reaction takes place via stepwise mechanisms that involve the sequential formation of the N1-C2 and the C3-C4 bonds. In some cases, the second step of the reaction is subjected to torquoelectronic effects and determines its stereochemical outcome. Under these conditions, preferential or exclusive formation of cis cycloadducts is predicted, in good agreement with part of the experimental evidence available. When chloroenamines are used as precursors of the keteniminium ions, the step in which the C3-C4 bond is formed can consist of an intramolecular S(N)2 reaction. Under these conditions, the stereocontrol of the reaction is significantly lower and the trans isomer is preferentially formed. According to our results, generation of keteniminium cations from enamines having good leaving groups is recommended to improve the stereocontrol of the reaction.
The experimental data of pi-facial stereoselection of the imines and the iminium ions of cyclohexanone, tropinone, and adamantan-2-ones have been explained by the exterior frontier orbital extension model (EFOE model) previously proposed. In all cases, facial difference in the pi-plane-divided accessible space (PDAS), which represents simple summation of the pi-plane-divided exterior three-dimensional space nearest to the reaction center outside the van der Waals surface, significantly depends on the structure of the imino moieties. In particular the formation of iminium salt significantly affects the magnitude of both the EFOE density and the PDAS values.
The intramolecular Lewis acid mediated cyclization of gamma-alkoxyallylstannanes 1, 2, and 14, bearing a hydrazone group at the terminus of the carbon chain, afforded exclusively the corresponding trans beta-amino cyclic ethers 3a, 4a, and 15, respectively. The Lewis acid mediated cyclization of gamma-alkoxyallylstannane 5, having (R)-(+)-1-phenylethylamine as a chiral auxiliary, afforded exclusively trans beta-amino cyclic ether 6a with very high diastereomeric excess (de) in very high chemical yields. The asymmetric cyclization of gamma-alkoxyallylstannane with imine 7 in the presence of chiral titanium-BINOL complex 9, afforded predominantly cis beta-amino cyclic ether 8b with high enantiomeric excess (ee). The chiral Lewis acid mediated cyclization of racemic compound 38 containing phenyl as a substituent afforded cis isomer 39 with very high enantiomeric excess (ee).
The diastereoselective nucleophilic addition of alkyllithium to N-alkylidene-alpha-naphthylethylamine was carried out. In the presence of Lewis acids or Lewis bases, organolithiums reacted smoothly with imines to give the corresponding amines in high stereoselectivity (up to 100% de). Furthermore, the resulting optically active amines were found to be useful for asymmetric reactions as chiral ligands.
Chiral and achiral 3-methoxynaphthalen-2-yl oxazolines 4a,b failed to undergo an aromatic nucleophilic displacement of the 3-methoxy group with organolithium reagents and instead afforded dihydronaphthalenes 9 and 14 in 30-95% yield. Dihydronaphthalenes 9 (racemic) and 14 (nonracemic) were easily converted into the corresponding aldehydes 15. Alternatively, aldehydes 15 were prepared via tandem addition of Grignard reagents to imines 17 in 50-65% yield. Aldehydes 15 served as precursors to 3,3, 4-trisubstituted 2-tetralones 16. Use of methyl chloroformate to trap the azaenolate derived from 17f and i-PrMgCl afforded, in 65% yield, a versatile synthetic intermediate 23 which may serve to access 4-alkyl-, 3,4-dialkyl-, 3,4-disubstituted and 3,3, 4-trisubstituted 2-tetralones with diverse substitution patterns.