1,3-dithiolanes from cycloadditions of alicyclic and aliphatic thiocarbonyl ylides with thiones: regioselectivity

The regiochemistry of 1,3-dithiolanes obtained from thiocarbonyl ylides 9 and thiones 10 shows a striking dependence on substituents. Previously and newly performed experiments indicate that sterically hindered cycloalkanethione S-methylides and dialkylthioketone S-methylides react with alicyclic and aliphatic thiones to give the 2,2,4,4-tetrasubstituted 1,3-dithiolanes 11 exclusively. Aryl groups in one or both reactants lead to a preference for, or even complete formation of, 4,4,5,5-tetrasubstituted 1,3-dithiolanes 12. Several mechanisms appear to be involved, but the paucity of experimental criteria is troubling. Quantum-chemical calculations (see preceding paper) on the cycloaddition between thioacetone S-methylide and thioacetone furnish lower activation energies for the concerted process than for the two-step pathways via C,S- or C,C-biradicals; the favoring of the 2,4-substituted 1,3-dithiolanes over the 4,5-substituted type would be expected to increase with growing bulk of substituents. Aryl groups stabilize intermediate biradicals. Experimental criteria for the differentiation of regioisomeric dithiolanes are discussed. Thiocarbonyl ylides 9 are prepared by 1,3-cycloadditions between diazomethane and thioketones and subsequent N(2) elimination from the usually isolable 2,5-dihydro-1,3,4-thiadiazoles 17; different ratios of the two rate constants lead to divergent product formation scenarios.     

Thioformaldehyde S-methylide and thioacetone S-methylide: an ab initio MO study of structure and cycloaddition reactivity

The mechanisms of cycloaddition of thioformaldehyde S-methylide and thioacetone S-methylide, as models for an alkyl-substituted ylide, to thioformaldehyde and thioacetone, as well as to ethene as a model for a C=C double bond have been studied by ab initio calculations. Restricted and unrestricted B3LYP/6-31G* calculations were performed for the geometries of ground states, transition structures, and intermediates. Although basis sets with more polarization functions were tested, the 6-31G* basis set was applied throughout. Single-point CASPT2 calculations are reported for analysis of the unsubstituted system. The stabilities of structures with high biradical character seem to be overestimated by DFT methods in comparison to CASPT2. The general trends of the results are independent of the level of theory. Thioformaldehyde adds to thioformaldehyde S-methylide without activation energy, and the activation energies for two-step biradical pathways to 1,3-dithiolane are low. C,S biradicals are more stable than C,C biradicals. The two-step cycloaddition is not competitive with the concerted cycloaddition. Methyl substitution in the 1,3-dipole and the dipolarophile does not change the mechanistic relationships. TSs for the concerted formation of the regioisomeric cycloadducts of thioacetone Smethylide and thioacetone were located. Concerted addition remains the preferred reaction. The reactivity of the C=S double bond is high relative to that of the C=C double bond.     

A computational study of the cycloaddition of thiobenzophenone S-methylide to thiobenzophenone

The cycloaddition of thiobenzophenone S-methylide to thiobenzophenone, an experimentally well-known reaction, was studied, using (U)HF/3-21G* for finding stationary points and (U)B3LYP/6-31G*//(U)HF/3-21G* single-point calculations for energies. Some optimizations were performed by (U)B3LYP/ 6-31G* to check the reliability of the calculations. The comparison of the concerted pathways and stepwise reactions via C,C-biradicals and C,S-zwitterions showed that the formation of a tetraphenyl-substituted C,C-biradical and its ring closure to 4,4,5,5-tetraphenyl-1,3-dithiolane constitutes the energetically most probable pathway of product formation, despite the fact that the regioisomeric 2,2,4,4-tetraphenyl-substituted product is more favorable by 17 kcal mol(-1). Model calculations on bond dissociation energies showed that (U)B3LYP with various basis sets overestimates radical stabilization, whereas CBS-QB3 closely reproduced experimental values. Results with the BLYP functional are similar to those with B3LYP. The consequences of the overestimation of radical stability for the cycloaddition mechanism involving biradicals are discussed. Thiobenzophenone S-methylide, if not captured by a dipolarophile, dimerizes to 2,2,3,3-tetraphenyl-1,4-dithiane. Calculation disclosed likewise a tetraphenyl-substituted C,C-biradical as intermediate.     

Synthesis and characterization of bromonium ylides and their unusual ligand transfer reactions with N-heterocycles

Stable aliphatic bromonium ylides (RfSO2)2C--Br+C6H4-p-CF3 (Rf = CF3, CF3(CF2)3) have been synthesized and structurally characterized for the first time. X-ray crystallographic analyses indicated a ylide structure with an sp2 hybridization of the ylide carbanions and with little double-bond character for the ylidic bond. The bromonium ylides selectively undergo transfer of the aryl group to nitrogen heterocycles, such as pyridines, yielding N-arylpyridinium salts. This is in a marked contrast to the reaction of the iodonium ylides, which produces pyridinium ylides through transylidations.     

Synthesis of Amino Acid Derived Cyclic Phosphorus Ylides

Abstract

We recently reported the thermal elimination of Ph₃PO from suitably protected aminoacyl ylides 1 as a route to acetylenic amino acid analogues 2. Pyrolysis of ylides such as 3 with a free amino group takes a different course. Ethanol is eliminated to give the c h i d cyclic ylides 4 which can be viewed as 3-triphenylphosphoranylidene tetramic acids. Specific examples prepared include 4 (R = Me, Pri), the parent compound 5 (from glycine) and the six-membered ring compound 6 (from p-alanine). Using a similar approach, bicyclic ylides such as 7 (from proline) can be prepared. In the case of the glutamate derived ylide 8, thermolysis initially gives a mixturc of 9 and 10 but these both cyclise to the bicyclic product 11 with time. The structure and reactivity of these interesting cyclic ylides are now being examined.     

Generation of thiocarbonyl ylides with release of disiloxane from bis(trimethylsilylmethyl) sulfoxides

A novel and general method for generation of thiocarbonyl ylide by release of disiloxane from bis(trimethylsilylmethyl) sulfoxide (1) has been found. This method was also demonstrated for generation of aliphatic and aromatic thioaldehyde S-methylide.     

含氟膦、胂叶立德的质谱研究

本文报道37个含氟膦.胂羰基的叶立德衍生物的电子轰击(EI)和8个含氟胂羟基叶立德的甲烷化学电离(Cl)正.负离子质谱. 研究其断裂规律,氧和氟原子重排以及不同取代基对一些特征离子强度的影响.     

Triphenylphosphine-Catalyzed Simple Synthesis of Vinyl-Substituted Saccharins

Saccharin (1,1-dioxo-1,2-dihydro-1 u 6 -benzo[ d ]-isothiazol-3-one) undergoes a smooth reaction with dialkyl acetylenedicarboxylates in the presence of triphenylphosphine to produce highly-functionalized salt-free sulfur-containing ylides in nearly quantitative yields. These stabilized phosphorus ylides exist as a mixture of two geometrical isomers as a result of restricted rotation around the carbon-carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. These ylides are converted to dialkyl 2-(1,1-dioxo-1 H -1 u 6 -benzo[ d ]-isothiazol-3-yl)-but-2-enedioates in boiling toluene.     

Reactivity and selectivity in the Wittig reaction: a computational study

The salt-free Wittig reaction of non-, semi-, and stabilized ylides has been investigated on realistic systems using density functional theory (DFT) calculations, including continuum solvation. Our results provide unequivocal support for the generally accepted mechanism and are in very good agreement with experimental selectivities. This study shows that E/Z selectivity of non- and semi-stabilized ylides cannot be fully understood without considering the energy of the elimination TS. The influence of ylide stabilization and the nature of phosphorus substituents on reversibility of oxaphosphetane formation is clarified. Unexpectedly, the puckering ability of addition TSs is shown not to depend on ylide stabilization, but the geometry of the TS is decided by an interplay of 1,2; 1,3; and C-H...O interactions in the case of non- and semi-stabilized ylides, whereas a dipole-dipole interaction governs the addition TS structures for stabilized ylides. The well-known influence of ylide stabilization on selectivity of PPh(3) derivatives is explained as follows: in non- and semi-stabilized ylides reactions, cis and trans addition TSs have, respectively, puckered and planar geometries, and selectivity is governed by an interplay of 1,2 and 1,3 interactions. For stabilized ylides, the high E selectivity is due to a strong dipole-dipole interaction at the addition TS. The influence of the nature of phosphorus substituents on selectivity is also detailed, the different behavior of (MeO)(3)PCHCO(2)Me ylides being explained by their lower dipole. This novel picture of the factors determining TS structures and selectivity provides a sound basis for the design of new ylides.     

Ab initio and density functional calculations on the pericyclic vs pseudopericyclic mode of conjugated nitrile ylide 1, 5-electrocyclizations

Ab initio (MP2/6-311+G and MP4(SDTQ)/6-311+G//MP2/6-311+G) and density functional (B3LYP/6-311+G) calculations on the ring closure reactions of conjugated nitrile ylides 1a-e, 3, and 6 to the corresponding oxazoles 2a, 5, 7, and 8; thiazoles 2b and 4; imidazole 2c; and pyrroles 2d and 2e, respectively, are reported. Vinyl nitrile ylides 1d and 1e cyclize with a substantially higher barrier than nitrile ylides containing a heteroatom. Geometric features as well as electronic structures as obtained by NBO analysis are indicative of a pericyclic, monorotatory 1, 5-electrocyclization of 1d and 1e. For nitrile ylides where X = heteroatom, a pseudopericyclic heteroelectrocyclization pathway, characterized by in-plane attack of the heteroatom's lone pair at the nitrile ylide group, is found. For 3 and 6, where two different cyclization products are possible, the calculated barriers and reaction energies are in line with the experimentally observed direction of reaction. Vinyl nitrile ylides 1d and 1e are characterized by an allene, acyl substituted derivatives 1a, 1b, 3, and 6 by a propargyl type structure. The nitrogen derivative 1c represents an intermediate case.     

Density functional theory investigations on sulfur ylide promoted cyclopropanation reactions: insights on mechanism and diastereoselection issues

The mechanism and diastereoselectivity of synthetically useful sulfur ylide promoted cyclopropanation reactions have been studied using the density functional theory method. Addition of different substituted ylides (Me2S+CH-R) to enone ((E)-pent-3-en-2-one, MeHC=CH-COMe) has been investigated. The nature of the substituent on the ylidic carbon brings about subtle changes in the reaction profile. The stabilized (R=COMe) and semistabilized (R=Ph) ylides follow a cisoid addition mode, leading to 1,2-trans and 1,2-cis cyclopropanes, respectively, via syn and anti betaine intermediates. The simplest and highly reactive model ylide (R=H) prefers a transoid addition mode. Diastereoselectivity is controlled by the barrier for cisoid-transoid rotation in the case of stabilized ylides, whereas the initial electrophilic addition is found to be the diastereoselectivity-determining step for semistabilized ylides. High selectivity toward trans cyclopropanes with stabilized ylides are predicted on the basis of the relative activation energies of diastereomeric torsional transition states. The energy differences between these transition states could be rationalized with the help of weak intramolecular as well as other stereoelectronic interactions.     

Kinetic and mechanistic study on the thermal reactivity of stabilized phosphorus ylides,part 3: [(Acetyl)(arylcarbamoyl)methylene]triphenylphosphoranes and [(alkoxycarbonyl)(arylcarbamoyl)methylene]triphenylphosphoranes and their thiocarbamoyl analogues

A series of five [(acetyl)(arylcarbabmoyl)methylene]triphenyl‐phosphoranes 1a–e and their thiocarbamoyl analogues 2a–e , [(alkoxycarbonyl)(arylcarbamoyl)methylene]triphenylphosphoranes 3a–e and their thiocarbamoyl analogues 4a–e were prepared and fully characterized. All ylides are found under conditions of flash vacuum pyrolysis to fragment giving arylisocyanate or isothiocyanate and acetyl ylides or alkoxy ylides which undergo thermal extrusion of Ph₃PO. A kinetic study shows that these reactions are unimolecular and are of first‐order nature with no significant substituent effect. The thiocarbamoyl ylides 2 react from 4.6 to 42 times faster than their carbamoyl ylides 1 , while the thiocarbamoyl ylides 4 react from 6.6 to 20.9 times faster than their carbamoyl ylides 3 . © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 39: 6–16, 2007     

An ab initio and density functional theory study of the structure and bonding of sulfur ylides

Sulfur ylides are useful synthetic intermediates that are formed from the interaction between singlet carbenes and sulfur-containing molecules. Partial double-bond character frequently has been proposed as a key contributor to the stability of sulfur ylides. Calculations at the B3LYP, MP2, and CCSD(T) levels of theory employing various basis sets have been performed on the sulfur ylides H(2)S-CH(2) and (CH(3))(2)S-CH(2) in order to investigate the structure and bonding of these systems. The following general properties of sulfur ylides were observed from the computational studies: C-S bond distances that are close in length to that of a typical C-S double bond, high charge transfer from the sulfide to the carbene, and large torsional rotation barriers. Analysis of the sulfur ylide charge distribution indicates that the unusually short C-S bond distance can be attributed in part to the electrostatic attraction between highly oppositely charged carbon and sulfur atoms. Furthermore, n --> sigma* stabilization arising from donation of electron density from the carbon lone pair orbital into S-H or S-C antibonding orbitals leads to larger than expected torsional barriers. Finally, natural resonance theory analysis indicates that the bond order of the sulfur ylides H(2)S-CH(2) and (CH(3))(2)S-CH(2) is 1.4-1.5, intermediate between a single and double bond.     

Highly enantioselective Ag(i)-catalyzed [3 + 2] cycloaddition of azomethine ylides

A highly reactive Ag(I)-catalyzed [3 + 2] cycloaddition of azomethine ylides is founded using AgOAc as the catalytic precursor and phosphines as ligands. Using a new bis-ferrocenyl amide phosphine (FAP) as the ligand, we found that high enantioselectivities (up to 97% ee) have been achieved in the [3 + 2] cycloaddition of azomethine ylides. Up to four stereogenic centers can be established in this multicomponent coupling reaction from readily available materials such as aldehydes, aminoesters, and dipolarophiles.     

Preparation of Novel Polyoxo Ylides and Diylides and Their Behaviour Towards Pyrolysis and Oxidation

Abstract

We recently reported the preparation of the new trioxoylides 1 and their pyrolysis to give symmetrical diacylalkynes.¹ Reaction of acyl ylides with oxalyl chloride gkes the tetraoxo diylides 2 as shown. The corresponding reactions starting from the α-oxoacyl ylides have been used to obtain examples of tetraoxo ylides 3 and hexaoxo diylides 4.     

2-quinoxalinylnitrenes and 4-quinazolinylnitrenes: rearrangement to cyclic and acyclic carbodiimides and ring-opening to nitrile ylides

This work was undertaken with the aim to obtain direct evidence for the interrelationships between hetarylnitrenes, their ring-expanded cyclic carbodiimide isomers, and ring-opened nitrile ylides. Tetrazolo[1,5-a]quinoxaline 11T and tetrazolo[5.1-c]quinazoline 13T undergo valence tautomerization to the corresponding azides 11A and 13A on mild flash vacuum thermolysis (FVT). Photolysis in Ar matrixes at ca. 15 K affords the triplet nitrenes 12 and 14, identified by ESR, UV, and IR spectroscopy. The nitrenes are converted photochemically to the seven-membered ring carbodiimide 15 followed by the open-chain carbodiimide 22. The 3-methoxy- and 3-chloro-2-quinoxalinylnitrenes 24 yield the ring-expanded carbodiimides 26 very cleanly on matrix photolysis, whereas FVT affords N-cyanobenzimidazoles 28. The ring-opened nitrile ylides 36 and 49 are identified as intermediates in the photolyses of 2-phenyl-4-quinazolinylnitrene 32 and 7-nitro-2-phenyl-4- quinazolinylnitrene 47. In these systems, a photochemically reversible interconversion of the seven-membered ring carbodiimides 35 and 48 and the nitrile ylides 36 and 49 is established. Recyclization of open-chain nitrile ylides is identified as an important mechanism of formation of ring contraction products (N-cyanobenzimidazoles).     

SYNTHESIS OF DIALKYL 2-(1-CYANO-2-OXO-1-PHENYL-ALKYL)-3-(TRIPHENYL-λ5-PHOSPHANYLIDENE)-SUCCINATES

3-Oxo-2-phenyl-butanenitrile or 3-oxo-2-phenyl-pentanenitrile undergo a smooth reaction with dialkyl acetylenedicarboxylates in the presence of triphenylphosphine to produce highly-functionalized salt-free ylides in nearly quantitative yields. These stabilized phosphorus ylides exist as a mixture of two geometrical isomers as a result of restricted rotation around the carbon-carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group.     

Sulfur and selenium ylide bond enthalpies

The bond dissociation enthalpies (BDEs) of sulfur and selenium ylides have been estimated by applying MP2/6-311++G(3df,2p)//MP2/6-31G(d,p), G3, and other computational methods. Computed sulfoxide bond enthalpies were compared to experimental results to ensure the reliability of the computational methods before extending to related compounds. The examined ylides include the following: sulfoxides, sulfilimines, S,C-sulfonium ylides, and selenoxides. Selenoxides have BDEs about 10 kcal/mol smaller than the corresponding sulfoxides. N-H sulfilimines and CH2-S,C-sulfonium ylides have low BDEs, unless the sulfilimine or S,C-sulfonium ylide is stabilized by an electronegative substituent on N or C, respectively. Incorporation of the S or Se into a thiophene or selenophene-type ring lowers the BDE for the ylide.     

Rhodium(II)‐Catalyzed Enantioselective Synthesis of Troponoids

We report a rhodium(II)‐catalyzed highly enantioselective 1,3‐dipolar cycloaddition reaction between the carbonyl moiety of tropone and carbonyl ylides to afford troponoids in good to high yields with excellent enantioselectivity. We demonstrate that α‐diazoketone‐derived carbonyl ylides, in contrast to carbonyl ylides derived from diazodiketoesters, undergo [6+3] cycloaddition reactions with tropone to yield the corresponding bridged heterocycles with excellent stereoselectivity.