BINAPHANE-Catalyzed Asymmetric Synthesis of trans-β-Lactams from Disubstituted Ketenes and N-Tosyl Arylimines

Electronic origins of the stereochemistry in β-lactam formed through the Staudinger reaction catalyzed by a nucleophile

This paper evaluates the electronic effects of molecular substituents on the stereoselectivity of the umpolung Staudinger catalytic reaction. This is especially important because experimental studies on constructing the β-lactam ring, a core structure of most antibiotics, through catalyzed Staudinger reactions have been massively progressing over the last century. Yet, there is a necessity for an in-depth understanding of the reaction mechanisms to help chemists, working on the well-established discoveries, improve these to optimize the stereoselectivity and yield of synthetic methods. Access to practical and effective advancements in forming optically pure β-lactam is paramount in the field of medical chemistry. This paper specifically investigates how changing the N-protecting group in the imine fragment can switch the stereoselectivity of the PPY-catalyzed Staudinger reaction. To do so, we employed the density functional theory (DFT) for geometry optimization and electronic analysis at the B3LYP/6-31G(d) level of theory to examine and compare the role of N-tosyl (N-Ts) and N-triflyl (N-Tf) imine on the mechanism pathways, i.e., imine-first or ketene-first, and stereochemistry of the reaction, i.e., cis or trans β-lactam. Our results show that the reaction mechanism pathway cannot be simply switched from ketene-first to imine-first by changing the substituent on the imine nitrogen atom, which is contrary to the reported experimental results, and both imines go through the ketene-first mechanism with different stereochemistries, which is cis selective for imine-Ts and trans selective for imine-Tf. Based on electronic analyses, the reversal in diastereoselectivity in the N-triflyl imine system could be attributed to the charge transfers and electron-density distribution over the transition states. Therefore, the cis/trans selectivity of the PPY-catalyzed Staudinger reaction could be effectively controlled by the electronic characteristics of the molecular substituents in the reactants. A N-protecting group in imine with a more electron-withdrawing nature seems to accelerate the stereo-determining step, ring closure, and increase the stabilization charge transfers in the transition state, leading to a preference for trans β-lactam formation. It seems that using a N-substituent with a higher electron-withdrawing nature can initially activate the imine by the nucleophilic catalyst in competition with ketene, i.e., imine-first versus ketene-first. These results can provide an insight into select proper substituents for the fragments to synthesis β-lactam with a desired stereochemistry. Also, a comprehensive comparison was performed between calculations with and without dispersion.

Reactions of 1,3-Diphenyl Cyclopentadiene with α-Aryldiazo Ketones to Enable C-H Insertions versus [4 + 2]-Cycloadditions via Au Catalyst and P(C6F5)3 Additive, Respectively

Two distinct reaction chemoselectivities were reported for the reactions of α-aryldiazo ketone with 1,3-diphenylcyclopentadiene using gold catalyst and phosphine additives, respectively. In the presence of gold catalyst, α-aryldiazo ketone forms gold carbenes initially that are trapped with this 1,3-disubstituted cyclopentadiene to afford C-H insertion products. In the presence of P(C6F5)3 additive, α-aryldiazo ketone forms diarylketenes initially at elevated temperature, which are further stabilized by P(C6F5)3 to secure their entity before proceeding to unprecedented [4C + 2C] cycloadditions.

Synthesis of α-Quaternary β-Lactams via Copper-Catalyzed Enantioconvergent Radical C(sp3 )-C(sp2 ) Cross-Coupling with Organoboronate Esters

The copper-catalyzed enantioconvergent radical C(sp³ )-C(sp² ) cross-coupling of tertiary α-bromo-β-lactams with organoboronate esters could provide the synthetically valuable α-quaternary β-lactams. The challenge arises mainly from the construction of sterically congested quaternary stereocenters between the tertiary alkyl radicals and chiral copper(II) species. Herein, we describe our success in achieving such transformations through the utilization of a copper/hemilabile N,N,N-ligand catalyst to forge the sterically congested chiral C(sp³ )-C(sp² ) bond via a single-electron reduction/transmetalation/bond formation catalytic cycle. The synthetic potential of this approach is shown in the straightforward conversion of the corresponding products into many valuable building blocks. We hope that the developed catalytic cycle would open up new vistas for more enantioconvergent cross-coupling reactions.

Reactions of thioalkynes with diarylketenes via [3+2]-annulation versus benzannulation using Au and P(C6F5)3 catalysts

Two catalytic annulations of non-symmetric diarylketenes with thioalkynes are described using gold and phosphine catalysts respectively. We employed α-aryldiazo ketones to generate gold-π-ketenes, ultimately yielding azulen-1-one derivatives. With the same reactants, we utilized P(C₆F₅)₃ to increase the yields of 1-naphthols, notably with a complete regioselectivity.

Lewis Base Catalyzed Synthesis of Sulfur Heterocycles via the C1‐Pyridinium Enolate

While the addition of C1‐Lewis base enolates to carbonyls and related structures are well established, the related addition to thiocarbonyls compounds are unknown. Herein, we report a reaction cascade in which a C1‐pyridinium enolate undergos addition to dithioesters, trithiocarbonates and xanthates. The reaction provides access to a range of dihydrothiophenes and dihydrothiopyrans (28‐examples). Mechanistic investigations, including isolation of intermediates, electronic correlation, and kinetic isotope effect studies support the viability of an activated acid intermediate giving rise to the C1‐pyridinium enolate which undergoes turnover limiting cyclization. Subsequent formation of a β‐thiolactone regenerates the catalyst with loss of carbon oxysulfide providing the observed products.

Modular Synthesis of α-Quaternary Chiral β-Lactams by a Synergistic Copper/Palladium-Catalyzed Multicomponent Reaction

An asymmetric multicomponent, interrupted Kinugasa allylic alkylation (IKAA) reaction has been developed with a synergistic Cu-catalyzed Kinugasa system and a Pd-catalyzed allylic alkylation reaction. This unprecedented reaction provides in high yields and with high stereoselectivity a synthesis of α-quaternary chiral β-lactams, which cannot be produced with existing synthetic methods. Stereoselective coupling of two catalytic amounts of transient organometallic intermediates formed in situ is an important feature of this reaction.

Visible-Light-Induced Cycloaddition of α-Ketoacylsilanes with Imines: Facile Access to β-Lactams

We report the synthesis of β-lactams from α-ketoacylsilanes and imines, which proceeds via a formal [2+2] photochemical cycloaddition with in situ generation of siloxyketene. This mild and operationally simple reaction proceeds in an atom-economic fashion with broad substrate scope, including aldimines, ketimines, hydrazones, and fused nitrogen heterocycles, affording a variety of important β-lactams with satisfactory diastereoselectivities in most cases. This reaction also features good functional-group tolerance, facile scalability and product diversification. Experimental and computational studies suggest that α-ketoacylsilanes can serve as photochemical precursors by engaging in a 1,3 silicon shift to the distal carbonyl group.

Synthesis of heterocyclic compounds through nucleophilic phosphine catalysis

Nucleophilic phosphine catalysis is a practical and powerful tool for the synthesis of various heterocyclic compounds with the advantages of environmentally friendly, metal-free, and mild reaction conditions. The present report summarizes the construction of four to eight-membered heterocyclic compounds containing nitrogen, oxygen and sulphur atoms through phosphine-catalyzed intramolecular annulations and intermolecular [2+2], [3+2], [4+1], [3+1+1], [5+1], [4+2], [2+2+2], [3+3], [4+3] and [3+2+3] annulations of electron-deficient alkenes, allenes, alkynes and Morita-Baylis-Hillman carbonates.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Lewis base-catalyzed diastereoselective [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes: an access to isoxazolidine derivatives

A Lewis base-catalyzed [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes has been achieved under mild reaction conditions, affording functionalized isoxazolidines as single diastereomers in moderate to excellent yields.

Catalytic Asymmetric Synthesis of Ketene Heterodimer β-Lactones: Scope and Limitations

In this article we describe extensive studies of the catalytic asymmetric heterodimerization of ketenes to give ketene heterodimer β-lactones. The optimal catalytic system was determined to be a cinchona alkaloid derivative (TMS-quinine or Me-quinidine). The desired ketene heterodimer β-lactones were obtained in good to excellent yields (up to 90%), with excellent levels of enantioselectivity (≥90% ee for 33 Z and E isomer examples), good to excellent (Z)-olefin isomer selectivity (≥90:10 for 20 examples), and excellent regioselectivity (only one regioisomer formed). Full details of catalyst development studies, catalyst loading investigations, substrate scope exploration, protocol innovations (including double in situ ketene generation for 7 examples), and an application to a cinnabaramide A intermediate are described. The addition of lithium perchlorate (1-2 equiv) as an additive to the alkaloid catalyst system was found to favor formation of the E isomer of the ketene heterodimer. Ten examples were formed with moderate to excellent (E)-olefin isomer selectivity (74:25 to 97:3) and with excellent enantioselectivity (84-98% ee).

Highly Efficient Access to Both Geometric Isomers of Silyl Enol Ethers: Sequential 1,2-Brook/Wittig Reactions

Novel sequential 1,2-Brook/Wittig reactions were developed for the preparation of silyl enol ethers. This method enables highly selective preparation of both geometric isomers of glyoxylate silyl enol ethers, using aldehydes (E-selective) and tosylimines (Z-selective) as a Wittig electrophile. The salt-free conditions of this reaction system are likely to be advantageous for switching the selectivity. The optimal reaction conditions and generality of the reaction were investigated, and plausible explanations for the observed selectivity were also discussed.

Alkaloid-Catalyzed Enantioselective [3 + 2] Cycloaddition of Ketenes and Azomethine Imines

A new asymmetric synthesis of bicyclic pyrazolidinones through an alkaloid-catalyzed formal [3 + 2] cycloaddition of in situ generated ketenes and azomethine imines is described. The products were formed in good to excellent yields (52-99% for 17 examples), with good to excellent diastereoselectivity (dr 5:1 to 27:1 for 11 examples), and with excellent enantioselectivity in all cases (≥96% ee). This method represents the first unambiguous example of an enantioselective reaction between ketenes and a 1,3-dipole.

Phosphine-catalyzed [3 + 2] cycloaddition of phthalazinium dicyanomethanides with allenoates: highly efficient synthesis of 1,2,3,10b-tetrahydropyrrolo[2,1-a]phthalazine derivatives

The phosphine-catalyzed [3 + 2] cycloaddition has been achieved at room temperature, providing novel heterocyclic compounds, 1,2,3,10b-tetrahydropyrrolo[2,1-a]phthalazine derivatives, as single (Z)-isomers in excellent yields (88–99% yield).

Phosphine-catalyzed highly enantioselective [3 + 3] cycloaddition of Morita-Baylis-Hillman carbonates with C,N-cyclic azomethine imines

The first phosphine-catalyzed highly enantioselective [3 + 3] cycloaddition of Morita-Baylis-Hillman carbonates with C,N-cyclic azomethine imines is described. Using a spirocyclic chiral phosphine as the catalyst, a novel class of pharmaceutically interesting 4,6,7,11b-tetrahydro-1H-pyridazino[6,1-a]iso-quinoline derivatives were obtained in high yields with good to excellent diastereoselectivities and extremely excellent enantioselectivities (98->99% ee).

Enantioselective cis-β-lactam synthesis by intramolecular C-H functionalization from enoldiazoacetamides and derivative donor-acceptor cyclopropenes

β-Lactam derivatives are produced through donor–acceptor cyclopropene intermediates in high yield with exclusive cis-diastereoselectivity, and high enantiocontrol.

β-Lactam derivatives are produced through intermediate donor–acceptor cyclopropene intermediates in high yield, exclusive cis-diastereoselectivity, and high enantiocontrol in a chiral dirhodium carboxylate catalyzed intramolecular C–H functionalization reaction of enoldiazoacetamides.

Chiral phosphines in nucleophilic organocatalysis

This review discusses the tertiary phosphines possessing various chiral skeletons that have been used in asymmetric nucleophilic organocatalytic reactions, including annulations of allenes, alkynes, and Morita–Baylis–Hillman (MBH) acetates, carbonates, and ketenes with activated alkenes and imines, allylic substitutions of MBH acetates and carbonates, Michael additions, γ-umpolung additions, and acylations of alcohols.

Advances in nucleophilic phosphine catalysis of alkenes, allenes, alkynes, and MBHADs

In nucleophilic phosphine catalysis, tertiary phosphines undergo conjugate additions to activated carbon-carbon multiple bonds to form β-phosphonium enolates, β-phosphonium dienolates, β-phosphonium enoates, and vinyl phosphonium ylides as intermediates. When these reactive zwitterionic species react with nucleophiles and electrophiles, they may generate carbo- and heterocycles with multifarious molecular architectures. This article describes the reactivities of these phosphonium zwitterions, the applications of phosphine catalysis in the syntheses of biologically active compounds and natural products, and recent developments in the enantioselective phosphine catalysis.