Ylide Substituents Research Articles

Inverting the Electronic Structure of Diylidylgermylenes by Backbone Modification.

Owing to the strong electron-donating ability of ylide substituents, diylidyltetrylenes are usually highly nucleophilic species with strong donor capacities. Here, we demonstrate that their electronic properties are in fact highly flexible and can be effectively tuned through variation of the substituent in the ylide backbone. Initial density functional theory studies showed that cyano groups are particularly capable in lowering the LUMO energy of diylidyl germylenes thus turning these usually highly nucleophilic species into electrophilic compounds. This was confirmed by experimental studies. Attempts to synthesize the germylene (YCN)2Ge [with YCN = Ph3P-(C)-CN] from the corresponding metalated ylides YCNK selectively led to germanide [(YCN)3Ge)K]2 thus reflecting the electrophilic nature of the intermediate formed germylene. XRD analysis of single crystals of (YCN)2Ge - serendipitously obtained through protonative cleavage of one ylide from the germanide - revealed a monomeric structure with rather long Ge-ylide linkages, which corroborates well with a pure single bond and no stabilization of the empty pπ orbital at germanium through pi bonding. The germanide exhibits methanide-like reactivity towards chalcogens but a likewise weak Ge-C bond as demonstrated by the insertion of carbon dioxide.

Thiophosphinoyl-Tethered Ylide-Substituted Heavier Carbenes: Synthesis, Structures and Stabilities.

The synthesis and structure analysis of a series of mono and diylide-substituted tetrylenes of type YEX and Y2 E (E=Ge, Sn, Pb; X=Cl or Br) using a thiophosphinoyl-tethered metallated ylide (Y=Ph2 P(S)-C-P(pip)Ph2 with pip=piperidyl) is reported, amongst the first ylide-substituted plumbylenes. The tetrylenes feature distinct trends in the spectroscopic and structural properties of the ylide ligand with increasing atomic number of the tetrel element. For instance, an increasingly high-field shifted signal for the thiophosphinoyl group is observed in the 31 P{1 H} NMR spectrum as a consequence of the increasing polarity of the element-carbon bond, which likewise results in a shortening of the ylidic C-P bond in the solid-state structure. The diylidyltetrylenes are unstable towards transylidation forming the mono(ylide)tetrylenes when treated with the tetrel dihalides according to the stability trend: Y2 Pb<Y2 Sn<Y2 Ge<YPbBr<YSnCl<YGeCl. Starting from the monoylide-substituted chlorotertrylenes the first unsymmetrical diylidyltetrylenes of type YEY' can be accessed, whose solid-state structures revealed that not the C-E but the S-E bond to the thiophosphinoyl group is most affected by the second ylide substituent.

Phosphorus-ylides: powerful substituents for the stabilization of reactive main group compounds.

Phosphorus ylides are 1,2-dipolar compounds with a negative charge on the carbon atom. This charge is stabilized by the neighbouring onium moiety, but can also be shifted towards other substituents thus making ylides strong π donor ligands and hence ideal substituents to stabilize reactive compounds such as cations and low-valent main group species. Furthermore, the donor strength and the steric properties can easily be tuned to meet different requirements for stabilizing reactive compounds and for tailoring the properties and reactivities of the main group element. Although the use of ylide substituents in main group chemistry is still in its infancy, the first examples of isolated compounds impressively demonstrate the potential of these ligands. This review summarizes the most important discoveries also in comparison to other substituents, thus outlining avenues for future research directions.

Ylide-Functionalization via Metalated Ylides: Synthesis and Structural Properties.

The α-metallated ylides [Ph3P-C-Z]-M+ (with Z=SO2Tol or CN and M=Na or K) were used as versatile nucleophiles for the facile access to ylide-substituted compounds. Halogenations, alkylations, carbonylations and functionalization reactions with main group element halides were easily accomplished by simple trapping reactions with the appropriate electrophiles. X-ray crystallographic studies of all compounds - including the first structures of α-fluorinated P-ylides - showed remarkable differences in the ylide backbone depending on the substituents. In the fluorinated compounds, a change from a fully planar to a pyramidalized ylidic carbon centre was observed despite the strongly anion-stabilizing ability of the yldiide substituent. π-Donation from the ylide substituent also resulted in geometric restrictions depending on the steric and electronic properties of the introduced substituents.

Ylide-Functionalized Phosphines: Strong Donor Ligands for Homogeneous Catalysis.

Phosphines are important ligands in homogenous catalysis and have been crucial for many advances, such as in cross‐coupling, hydrofunctionalization, or hydrogenation reactions. Herein we report the synthesis and application of a novel class of phosphines bearing ylide substituents. These phosphines are easily accessible via different synthetic routes from commercially available starting materials. Owing to the extra donation from the ylide group to the phosphorus center the ligands are unusually electron‐rich and can thus function as strong electron donors. The donor capacity surpasses that of commonly used phosphines and carbenes and can easily be tuned by changing the substitution pattern at the ylidic carbon atom. The huge potential of ylide‐functionalized phosphines in catalysis is demonstrated by their use in gold catalysis. Excellent performance at low catalyst loadings under mild reaction conditions is thus seen in different types of transformations.

Ylide‐Functionalized Phosphines: Strong Donor Ligands for Homogeneous Catalysis

AbstractPhosphines are important ligands in homogenous catalysis and have been crucial for many advances, such as in cross‐coupling, hydrofunctionalization, or hydrogenation reactions. Herein we report the synthesis and application of a novel class of phosphines bearing ylide substituents. These phosphines are easily accessible via different synthetic routes from commercially available starting materials. Owing to the extra donation from the ylide group to the phosphorus center the ligands are unusually electron‐rich and can thus function as strong electron donors. The donor capacity surpasses that of commonly used phosphines and carbenes and can easily be tuned by changing the substitution pattern at the ylidic carbon atom. The huge potential of ylide‐functionalized phosphines in catalysis is demonstrated by their use in gold catalysis. Excellent performance at low catalyst loadings under mild reaction conditions is thus seen in different types of transformations.

A Fairly Stable Crystalline Silanone

AbstractSilanones 2 substituted by bulky amino‐ and phosphonium ylide substituents have been synthesized and isolated in crystalline form. Thanks to the steric protection and the strong electron‐donating ability of the substituents, silanones 2 are persistent and only slowly dimerizes at room temperature (t1/2=0.5 or 5 h). Structural and theoretical analysis of 2 indicate a short Si=O bond (1.533 Å) and an enhanced polarization toward the O atom compared to Me2Si=O owing to the strong π‐electron donation from the phosphonium ylide substituent.

A Fairly Stable Crystalline Silanone.

Silanones 2 substituted by bulky amino- and phosphonium ylide substituents have been synthesized and isolated in crystalline form. Thanks to the steric protection and the strong electron-donating ability of the substituents, silanones 2 are persistent and only slowly dimerizes at room temperature (t1/2 =0.5 or 5 h). Structural and theoretical analysis of 2 indicate a short Si=O bond (1.533 Å) and an enhanced polarization toward the O atom compared to Me2 Si=O owing to the strong π-electron donation from the phosphonium ylide substituent.

Conjugation, Resonance, and Stability in N-Heterocyclic Silylenes and in Phosphorus Ylide Substituted Silylenes

The role of π conjugation in the thermodynamic stabilization of N-heterocyclic silylenes and phosphorus ylide substituted silylenes is analyzed using the block localized wave function (BLW) method. This method enables the direct calculation of the resonance stabilization energies caused by π-electron delocalization and/or by cyclic 6-π-electron delocalization (aromaticity). The major advantage of the BLW method is that there is no need for reference compounds, as the fully conjugated molecule itself serves as the reference compound. The observed high stability of the C═C unsaturated N-heterocyclic silylenes 1(E = Si; silaimidazol-2-ylidene) and the C–C saturated 2 (E = Si; silaimidazolin-2-ylidene) is rationalized by their high resonance energies (REs) of 79.4 and 53.4 kcal/mol, respectively. The nuclear independent chemical shift (NICSzz(1.0)) value of −22.0 for 1 (E = Si) indicates significant diatropic ring current, implying the existence of aromaticity. The additional stabilization of 1 due to aromaticity (6-π-electron delocalization) is 14 kcal/mol, which is only 18% of the total RE of 1 but is 55% of the aromatic stabilization of benzene. Thus, aromaticity contributes to the stability of 1, but it is not a prerequisite for the isolation of N-heterocyclic silylenes. Cyclic unsaturated silylenes substituted by exocyclic phosphorus ylide substituents, i.e., 12, have calculated cyclic resonance energies and thermodynamic stabilization energies similar to those of 1 (E = Si), and they are significantly larger than those of saturated 2 (E = Si); i.e., the REs of 12 (R″ = CH3, SiH3) are 73.4 and 75.6 kcal/mol, respectively. These data suggest that 12 is sufficiently stable to be isolated, as indeed was recently reported. The energy of the lone pair orbital on Si in 12 (R″ = Ph) is higher by 1.9 eV than that of 1 (E = Si, R = t-Bu), suggesting that 12 are better σ donors and thus may exhibit higher activity in transition-metal catalysis.

Synthesis of quinine and quinidine using sulfur ylide-mediated asymmetric epoxidation as a key step

The epoxidation of meroquinene aldehyde with a chiral sulfur ylide as the key step in the synthesis of quinine and quinidine is described. The epoxidation reactions proceed under reagent control with high selectivity and good yield. The effect of sulfide and ylide substituents on the stereochemical outcome of the reaction is discussed.

Delineation of the factors governing reactivity and selectivity in epoxide formation from ammonium ylides and aldehydes

Diastereoselectivity in reactions of aryl-stabilised ammonium ylides are highly sensitive to the nature of the amine and the ylide substituent. DFT calculations are consistent with a mechanism in which reversibility in betaine formation [despite the high energy (and therefore instability) of ammonium ylides] is finely balanced due to the high barrier to ring closure.

Ylidyl‐substituted Phosphonio‐benzophospholides as Chelating Ligands

AbstractPhosphonio‐benzo[c]phospholides with an additional phosphonium ylide substituent in 3‐position were synthesized by deprotonation of appropriately substituted 1, 3‐bis‐phosphonio benzophospholide cations and characterized by spectroscopic and analytical data. The ability of these molecules to act as bidentate P, C‐chelating ligands to transition metal atoms was demonstrated in the reactions with [W(CO)4(norbornadiene)] and [MCl2(cyclooctadiene)] (M = Pd, Pt). The PdII and PtII complexes are distinguished by a strong inclination towards addition of H2O to the 10π‐electron system of the ligand. The molecular structures of a W0 complex with a P, C‐chelating ylidyl‐phosphonio‐benzophospholide ligand and of the product resulting from H2O‐addition to a corresponding PtII complex were determined. The structural parameters of the W0 complex provide evidence for the presence of substantial steric strain around the metal atom.

THE 31P-CHEMICAL SHIFT OF DIPHOSPHENES

Including recent examples the phosphorus-31 chemical shift range of (E)-diphosphenes extends from δ = 192 to 816. For a given substituent at PA the chemical shift δA decreases when the substituent at PB changes from electron accepting to electron donating. The smallest δA values (and also the smallest coupling constants 1JAB) are found for diphosphenes with an ylide substituent at PB, the largest δA value is found for a disilyl diphosphene.

1,3-Bisylidylsubstituted Cyclotetraphosphines

Abstract Ylide substituents effectively stabilize low coordination of phosphorus as long as it is bonded to electronegative partners1,2. Thus ylidyl-thioxophoqhines and -selenoxo-phosphines are monomeric in solution and crystal3. In contrast ylidsubstituted dis-phosphines are unstable towards dimerimtion in accord with the anticipated polarity of the PP-bond: Consequently, the reaction of ylidyldichlorophosphnes and bis(trimethylsily1)phenylphosphine yields cyclotetraphosphmes:

Thermolysis of 2-acyloxy-Δ3-1,3,4-oxadiazolines. Evidence for a preferred sense of cycloreversion to carbonyl ylides and for fast 1,4-sigmatropic ylide rearrangement

Thermolysis of 2-acyloxy-2,5,5-trialkyl-Δ3-1,3,4-oxadiazolines in benzene solution at 80 °C furnishes acyloxy-substituted enol ethers (hemiacylals) in high yield. Mixtures of cis:trans isomers of such oxadiazolines afford mixtures of isomeric hemiacylals in nearly the same ratio. Those and other results are rationalized in terms of cycloreversion of the oxadiazolines to carbonyl ylides that are not equilibrated during their lifetimes and undergo primarily 1,4-sigmatropic H-migration. Some fragmentation of the ylides to anhydrides and carbenes was also observed. A consistent mechanistic account includes concerted suprafacial (4π + 2π) cycloreversion in the sense that places a large ylide substituent at C-1 or at C-3, preferentially exo. A smaller preference for the cycloreversion that places the acetoxy group at C-1 in the endo position, when the steric effect of the alkyl group at C-1 is small, can be inferred. The possibility that the overall 1,4-H shift is the result of sequential 1,7-antarafacial and 1,4-suprafacial shifts, in some cases, is considered. Keywords: carbonyl ylide; 1,4-sigmatropic rearrangement; cycloreversion, thermal, of oxadiazolines; ylide, carbonyl; oxadiazoline, thermolysis of.

ChemInform Abstract: PHOTOREACTIONS OF IMINOPHOSPHORANES

AbstractDie Bestrahlung von N‐Aryltriphenyliminophosphoranen wie (I) in inerten Lösungsmitteln führt zu Triphenylphosphin (III) und den Diarylazoverbindungen (II).

Photoreactions of iminophosphoranes

The irradiation of N-aryltriphenyliminophosphoranes in inert solvents leads to efficient production of triphenylphosphine and the diaryl azo compounds derived from the N-aryl ylide substituent. The irradiation of N-alkyltriphenyliminophosphoranes leads to rearrangements of the N-alkyl substituent analogous to those observed during the irradiation of the corresponding azides. Triphenyliminophosphoranes which possess N-substituents capable of strong delocalization of the nitrogen lone pair (e.g. benzene sulfonyl, benzoyl) are photostable. Since all attempts to trap nitrene intermediates suspected to be formed during the photoreactions of the photolabile iminophosphoranes were unsuccessful, the reactions are considered to proceed via excited ylide intermediates.