X=Y-ZH systems as potential 1.3-dipoles : Part 14. bronsted and Lewis acid catalysis of cycloadditions of arylidene imines of α-amino acid esters

Cycloadditions of arylidene imines of α-amino acid esters to a range of dipolarophiles show substantial rate enhancements in the presence of Bronsted and Lewis acids. For Bronsted acids the rate is related to the pK_a of the acid and cycloadditions to reactive dipolarophiles occur at room temperature. For the Lewis acids studied the rate acceleration decreases in the order Zn(OAc)₂ > AgOAc> LiOAc> MgOAc₂ but is also anion dependent with LiBr> LiOAc and AgOAc> AgOTs. The Lewis acid catalysed processes are believed to be examples of cycloadditions of metallo-1,3-dipoles. In both Bronsted and Lewis acid catalysed processes the cycloadditions are regio- and stereo-specific.     

Kinetics and mechanism of the 1,3-dipolar cycloaddition of phenyl azides to methyl 3-pyrrolidinoacrylate

Kinetics of cycloadditions of phenyl azides to methyl 3-pyrrolidinoacrylate (2) produced Hammett ? = 2.2. An E_a of 13.5 kcal mole ^?1 and Δ S^* of ?37.4 cal.K^?l mole^?1 were calculated for the cycloaddition of p-O₂NC₆H₄N₃ to 2. The cycloadditions are concerted, non-synchronous, and are controlled by LUMO (azide) - HOMO (dipolarophile) interactions.     

Lanthanide catalysis of cycloadditions of heterodienes with enol ethers

The cycloadditions of enol ethers with α,β-unsaturated aldehydes is catalyzed by Yb(fod)₃.     

N-Substituted C-diethoxyphosphorylated nitrones as useful synthons for the synthesis of α-aminophosphonates

A convenient method for the synthesis of N-substituted C-phosphorylated nitrones 2 from hydroxymethylphosphonates via successive Swern oxidation and treatment with respective hydroxylamines is reported for the first time. Moderate (20%) to excellent (up to 90%) diastereoselectivities in cycloadditions of nitrone 2a and terminal alkenes were observed with trans C5-substituted isoxazolidines predominating. In ZnCl₂-catalysed cycloadditions, mixtures enriched in cis diastereoisomers were produced.     

Near‐Infrared Photoinduced Coupling Reactions Assisted by Upconversion Nanoparticles

We introduce nitrile imine‐mediated tetrazole–ene cycloadditions (NITEC) in the presence of upconversion nanoparticles (UCNPs) as a powerful covalent coupling tool. When a pyrene aryl tetrazole derivative (λ_{abs, max}=346 nm) and UCNPs are irradiated with near‐infrared light at 974 nm, rapid conversion of the tetrazole into a reactive nitrile imine occurs. In the presence of an electron‐deficient double bond, quantitative conversion into a pyrazoline cycloadduct is observed under ambient conditions. The combination of NITEC and UCNP technology is used for small‐molecule cycloadditions, polymer end‐group modification, and the formation of block copolymers from functional macromolecular precursors, constituting the first example of a NIR‐induced cycloaddition. To show the potential for in vivo applications, through‐tissue experiments with a biologically relevant biotin species were carried out. Quantitative cycloadditions and retention of the biological activity of the biotin units are possible at 974 nm irradiation.     

Rh(I)-catalyzed intramolecular hetero-[4+2] cycloaddition of ω-alkynyl-vinyl oximes

Cationic rhodium(I) catalyst derived from [RhCl(cod)]₂ and AgSbF₆ in hexafluoroisopropanol (HFIP) efficiently catalyzed the formation of bicyclic pyridine derivatives by intramolecular hetero-[4+2] cycloadditions of ω-alkynyl-vinyl oximes.     

Temperature dependence of polar and steric effects in CCl2 cycloadditions

In cycloadditions of CCl₂ activation enthalpies increase with bulky substituents at the alkenes (Table I, Figure III), whereas by polar groups only activation entropies are changed.     

Rh(I)-catalyzed mild intramolecular [4+2] cycloaddition reactions of ester-tethered diene-yne compounds

The cationic rhodium(I) species derived from [Rh(COD)Cl]₂ and AgSbF₆ efficiently catalyze intramolecular [4+2] cycloadditions of ester-tethered 1,3-diene-8-yne derivatives such as 2-propynyl penta-2,4-dienoate and 2,4-pentadienyl propiolate derivatives in fluorinated alcohols.     

One-pot syntheses of 1,2,3-triazoles containing a pentafluorosulfanylalkyl group via click chemistry

1,4-Disubstituted 1,2,3-triazoles containing a pentafluorosulfanylalkyl group were synthesized in good to excellent yields (57-91%) by the click cycloadditions of in situ generated SF₅-alkyl azides with aromatic and aliphatic alkynes. Nucleophilic substitution of the SF₅ group was observed for the first time in a bench-top reaction.     

Facile Synthesis of 3-Aroyl-3-Sulfolenes Through Cycloadditions of Arylnitrile Oxides & 3-Sulfolene

3-Sulfolene undergoes [2+3] cycloadditions with arylnitrile oxides to yield the corresponding 2-isoxazolines which on cleavage in the presence of Mo(CO)₆ followed by dehydration gave 3-aroyl-3-sulfolenes in moderate yields.     

Scandium triflate and secondary amine promoted AA′B 2:1 coupling and formal inverse electron demand Diels-Alder reactions of dienals

Reaction prospecting studies resulted in the discovery of a Sc(OTf)₃ promoted novel AA′B 2:1 coupling of imidazolone or benzofuran substituted enals with morpholine. A related Sc(OTf)₃/morpholine promoted reaction was demonstrated for the coupling of a benzofuran substituted enal with dihydrofuran. The formation of these adducts is consistent with formal inverse electron demand Diels-Alder cycloadditions, most likely in a stepwise manner via a domino Michael-Mannich annulation process, involving iminium ion activation of the diene. A purely amine promoted formal inverse electron demand Diels-Alder cycloaddition approach was also demonstrated in the 2:1 coupling of 2,4-hexadienal with methanol. These reactions demonstrate the concepts of dual metal/amine catalysis and amine promoted formal inverse electron demand Diels-Alder cycloadditions. They differ from known examples of organocatalyzed Diels-Alder reactions, in which iminium ion activation of the dienophiles or enamine activation of the dienes occur.     

Diversity in Gold‐Catalyzed Formal Cycloadditions of Ynamides with Azidoalkenes or 2H‐Azirines: [3+2] versus [4+3] Cycloadditions

Gold‐catalyzed cycloadditions of ynamides with azidoalkenes or 2H‐azirines give [3+2] or [4+3] formal cycloadducts of three classes. Cycloadditions of ynamides with 2H‐azirine species afford pyrrole products with two regioselectivities when the C_β‐substituted 2H‐azirine is replaced from an alkyl (or hydrogen) with an ester group. For ynamides substituted with an electron‐rich phenyl group, their reactions with azidoalkenes proceed through novel [4+3] cycloadditions to deliver 1H‐benzo[d]azepine products instead.     

Novel synthesis of heterocyclic spirans by tandem cycloaddition to sulfur-containing heterocumulenes

Novel Ni(C0)₄-promoted tandem cycloadditions of diphenylcyclo- propenone to isothiocyanates and to CS₂ were found to provide new heterocyclic spirans, pyrroline-2-one-5-spiro-5′-thiolene-4′-ones and a thiolene-2-one-5-spiro-5′-thiolene-4′-one, respectively, in moderate yields.     

A regio and diastereoselective transformation of 3-dienyl-2-azetidinones to novel pyrroloxazine

The regioselective nitroso Diels-Alder (NDA) cycloadditions of 3-dienyl-2-azetidinones with nitrosobenzene to generate oxazine-substituted β-lactams in excellent yields are reported. The amidiolytic ring opening of the cycloadducts with sodium methoxide followed by iodocyclization using I₂/K₂CO₃ etiquette to capitulate previously unknown, multisubstituted pyrroloxazine in outstanding yields is also accounted.     

Regioselectivity in nickel(0)/phosphine catalyzed cycloadditions of alkynes and isocyanates

The regioselectivity of Ni(0)-catalyzed cycloadditions of various isocyanates and asymmetrical alkynes to afford pyridones was explored. The use of PEt₃ provided, in most cases, two of the four possible pyridone regioisomers in high overall yields. Mechanistic rationale for the product distribution is provided.     

The Low Energy of Concert in Many Symmetry‐Allowed Cycloadditions Supports a Stepwise‐Diradical Mechanism

The thesis of this paper is that orbital symmetry allowedness does not guarantee concert. The activation energy (E_a) for concerted Diels–Alder and 1,3‐dipolar cycloadditions must be substantially lower than that for a stepwise‐diradical pathway because the diradical has two fewer bonding electrons than anything on the concerted reaction coordinate, including the transition state. Two bonding electrons provide tens of kcal/mol of stabilization. The difference between the experimental E_a and that for diradicals is called the energy of concert (E_con). If experiment represents concert, E_con must be large, and if it represents diradicals E_con will be very small. In this paper, 42 examples are adduced in which firm experimental data show that E_con = 0. These cycloadditions cannot be concerted. While concert remains possible for all cycioadditions not proven stepwise, there is none with compelling evidence for concert.     

Mise en evidence de la formation de R2MMR2 a partir de disila-1,2 et digerma-1,2 cyclohexenes-4

Intermediates with π-bonded Si or Ge are generated during gas phase pyrolysis of 1,2-disila- or 1,2-digerma-cyclohex-4-enes.Compounds such as R₂MMR₂ are successfully trapped by addition or [2 + 2] and [4 + 2] cycloadditions. These results were proved by mass spectrometry.     

1,3-Cycloaddition of nitrones in ionic liquids catalyzed by Er(III): an easy access to isoxazolidines

1,3-Dipolar cycloadditions of nitrones with alkenes afforded the corresponding isoxazolidines in ionic liquids in the presence of Er(OTf)₃. The ionic liquid and the catalyst are recycled up to five times without any specific treatment or loss of activity. Extension of the procedure to the synthesis of isoxazolidinyl nucleosides has been investigated.     

N-Functionalized 1-Alkynylamides: New Building Blocks for Transition Metal Mediated Inter- and Intramolecular [2+2+1] Cycloadditions

A conceptually new, stereoselective approach to compounds such as 1 and 2 is expected from the so far unknown class of N-functionalized 1-alkynylamides. Building blocks of the type I and II can be applied to inter- and intramolecular, regio- and stereoselective [Co₂(CO)₈]-mediated [2+2+1] cycloadditions.     

A sustainable procedure for highly enantioselective organocatalyzed Diels-Alder cycloadditions in homogeneous ionic liquid/water phase

The MacMillan iminium catalyst was investigated for asymmetric Diels-Alder cycloadditions in ionic liquid/H₂O homogeneous phase. Superior selectivity, product yield, and shorter reaction times were observed in comparison with classical organic solvents. Additional advantages are the easy synthetic procedure, the excellent recovery of products, and the recyclability of the whole system.