Two-ring Flip Research Articles

Chirality in the Absence of Rigid Stereogenic Elements: Steric and Electronic Effects on the Configurational Stability of C3 Symmetric Residual Tris‐Aryl Phosphanes

Residual stereoisomers result whenever closed subsets of appropriately substituted interconverting isomers (the residual stereoisomers) are generated from a full set of stereoisomers under the operation of a favored stereomerization mechanism. In the case of the three-bladed propellers, differentiation of the edges of the blades and strict correlation in the motion of the rings are the prerequisites for the existence of residual stereoisomers. In these systems, the two-ring flip mechanism is the lowest energy process. It does not interconvert all possible conformational stereoisomers generated by helicity and the three-blade-hub rotors. In the case of C3 symmetric systems, two noninterconverting subgroups (the residual stereoisomers) are generated, each one constituted of quickly interconverting diastereoisomers. A series of tris-aryl phosphanes, structurally designed for existing as residual enantiomers or diastereoisomers, bearing substituents differing in size and electronic properties on the aryl rings, were synthesized and characterized. The configurational stability of residual phosphanes, evaluated by dynamic (1) H- and (31) P-NMR analysis and by dynamic enantioselective high-performance liquid chromatography (HPLC), was found 10 kcal mol(-1) lower than that shown by the corresponding phosphane-oxides. In accordance with the calculations, an unexpectedly low barrier for phosphorus pyramidal inversion was invoked as responsible for the scarce configurational stability of the residual tris-arylphosphanes.

Triarylmethanols with Bulky Aryl Groups and the NOESY/EXSY Experimental Observation of a Two‐Ring‐Flip Mechanism for the Helicity Reversal of Molecular Propellers

Triarylmethanols - the direct precursors of persistent trityl radicals - are racemic mixtures of chiral three-bladed molecular propellers. Depending on bulkiness of aryl groups they exhibit various liabilities to interconversion, the half- life time of room temperature racemization varying in a range between 8.4 hours and 1.32 years. NOESY/EXSY experiment performed on two representative models strongly supports the two-ring flip mechanism for the configurational interchange.

Conformational studies by dynamic NMR. 79. Dimesityl sulfine revisited: detection of the helical antipodes and determination of their enantiomerization pathways.

By means of low-temperature NMR spectra, it is demonstrated that dimesityl sulfine (Mes2C=SO) adopts in solution the same chiral propeller conformation (C1 symmetry) determined by X-ray diffraction in the crystalline state. With the help of MM calculations, it has been also shown that a correlated rotation (cog wheel effect) of the two mesityl rings reverses the molecular helicity according to an enantiomerization process entailing a one-ring flip pathway with delta G++ = 5.9 kcal mol-1 and a two-ring flip pathway with delta G++ = 13.8 kcal mol-1. On the contrary the Z- and E-isomers of mesityl phenyl sulfine (MesPhC=SO) adopt essentially achiral conformations (Cs symmetry), having the Ph-CSO rotation barriers equal to 5.2 and 5.8 kcal mol-1, respectively, and the mesityl-CSO rotation barriers equal to 21.3 and 15.1 kcal mol-1, respectively.

Conformational studies by dynamic NMR. 78. Stereomutation of the helical enantiomers of trigonal carbon diaryl-substituted compounds: dimesitylketone, dimesitylthioketone, and dimesitylethylene.

The free energies of activation for the enantiomerization of the title compounds (Mes2C = X, Mes = 2,4,6-trimethylphenyl) were determined by dynamic NMR to be 4.6, 6.5, and 9.2 kcal mol-1 for X = O, S, and CH2, respectively. Single-crystal X-ray diffraction showed that the structure of dimesitylketone is that of a propeller (C2 symmetry) with the mesityl rings twisted by 50 degrees with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agreement between computed and experimental barriers for a dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Solid-state NMR spectra indicated that the enantiomerization barrier in the crystal must be much higher (at least 19 kcal mol-1) than that in solution. Contrary to the case of dimesitylketone, the calculated barrier of dimesitylethylene agrees better with the experimental value if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.

Conformations and Threshold Rotational Mechanisms of the (Z)-1,2-Diarylethene Moiety: A Structural Correlation Study.

The crystal structures of substituted (Z)-1,2-diarylethenes and (Z)-1,2-diarylcyclopropenes, -cyclobutenes, -cyclopentenes, and -cyclohexenes that were reported in the literature were retrieved from the Cambridge Structural Database, and the dihedral angles of the aryl rings with the double bond were plotted in a conformational map. Analysis of the data by the Structural Correlation method together with molecular mechanics calculations suggest that the conformation and the threshold rotational mechanism of the (Z)-1,2-diarylvinyl fragment strongly depend on its steric environment. In 1,2-diarylcyclopropenes the aryl rings are nearly coplanar with the double bond, whereas in 1,2-diarylcycloalkenes with larger rings a propeller conformation is adopted. The threshold rotational mechanism is the one-ring flip for (Z)-1,2-diphenylethene and 1,2-diphenylcyclobutene, and is the two-ring flip for 1,2-diarylcyclopentene and 1,2-diarylcyclohexene. The calculated rotational barriers of the aryl rings in the threshold mechanism for all systems were very low (0.2-2.4 kcal mol(-)(1)). The different conformations and rotational behaviors are dictated by an interplay of conjugation and steric effects.

Conformational Studies of Phenyl- and (1-Pyrenyl)triarylmethylcarbenium Ions: Semiempirical Calculations and NMR Investigations under Stable Ion Conditions

Highly stable crowded carbenium ions such as (1-pyrenyl)diphenylmethylcarbenium ion (2) and 1,6- (3) and 1,8-bis(diphenylmethylenium)pyrene [dication] (4) and their dibrominated analogues (3Br and 4Br) have been studied at low and at ambient temperatures. 2 shows a conventional two-ring flip (ph,ph), whereas the disubstituted pyrene derivatives show one-ring flips (py) and two-ring flips (ph,ph) with a higher rotational barrier in agreement with AM1 calculations. A series of calculations show that the AM1 method gives the transition-state energies in best agreement with experiment. The propeller-shaped geometry of these molecules is reflected in characteristic low-frequency resonances of the phenyl rings. At low temperature, 3 and 4 exist as rotational isomers with C2 and Cs (or Ci) symmetry. In 4, steric interaction makes the two rotamers slightly different in energy (1 kcal). For 4 or 4Br, the 13C chemical shift differences between the C2 and the Cs species of 4 or 4Br correlate roughly with the calculated charge differences between the C2 and the Cs species. The charge at the C+ carbon is most extensively delocalized in 2, whereas in 3 and 4 with two C+ groups the pyrene moieties are less effective in charge delocalization.

Static and Dynamic Stereochemistry and Solvation of 2,2-Ditipylethenols(1).

The effect of increased bulk of the beta-aryl group in enols Ar(2)C=C(OH)R from Ar = mesityl = Mes (1) to Ar = 2,4,6-i-Pr(3)C(6)H(2) = Tip (2) was investigated. The solid-state structure when R = H (a) does not significantly differ for 1a and 2a. The dynamic behavior of 2a resembles that of 1a, but the rotational barriers for both the threshold one-ring flip process and the two-ring flip process are higher for 2a. The one-ring flip barrier for 2a is solvent-dependent. The threshold two-ring flip barriers when R = Me (2b) and t-Bu (2c) are higher than for the mesityl analogues, but that for 2c is higher than predicted. Solvations of 2a and 1a and their associations with DMSO are similar. The C=COH conformation is syn-planar with an OH.pi(Tip) association in non-hydrogen-bond-accepting solvents and is anti-clinal with OH.solvent association in hydrogen-bond-accepting solvents. In summary, the increased bulk associated with the change Mes --> Tip changes the structure and behavior in the expected direction but, except for the DeltaG(c)() values, not to a large extent.

Synthesis and Dynamic Stereochemistry of Tris(2-methyl-1-azulenyl)methyl Cation and the Corresponding Methane Derivative. Evidence That the Conjugative Effect Largely Contributes to the Transition State of the Ring Flipping

Abstract The stable carbocation, tris(2-methyl-1-azulenyl)methyl hexafluorophosphate (4·PF6−) was prepared by a hydride-abstraction reaction of the corresponding methane derivative, tris(2-methyl-1-azulenyl)methane (5). The dynamic stereochemistry of 4 and 5 was studied based on the temperature-dependent 1H NMR spectra, which were analyzed by a flip mechanism; the steric effect of the three 2-methyl groups was also investigated by comparing it with that of the 3,3′,3″-trimethyl analogue, tris(3-methyl-1-azulenyl)methyl hexafluorophosphate (1b·PF6−). The threshold rotation mechanism for 4 was a two-ring flip, in contrast to a one-ring flip for 1b, and the activation energies for 4 (78.0 and 73.4 kJ mol−1) were higher than those for 1b due to increased crowding in the transition state for the rotation. Although the activation energies for 5 (49.9 and 43.2 kJ mol−1) were lower than those for 1b, the mechanism for 5 was also a two-ring flip. The mechanism was variable between one- and two-ring flip processes due to a steric effect of the three 2-methyl groups. These results indicate that a conjugative interaction between the central cation and the three azulene rings largely contribute to the transition state of the ring flipping as well as to the ground state.

Dynamic stereochemistry of tri(2-methyl-1-azulenyl)methyl cation; steric effect of 2-methyl groups on rotational barriers and mechanism

The stable carbocation, tri(2-methyl-1-azulenyl)methyl hexafluorophosphate ( 2) was prepared. Steric effect of three 2-methyl groups was investigated by comparison with the 3,3′,3″-trimethyl analogue, tri(3-methyl-1-azulenyl)methyl cation ( 1). Dynamic stereochemistry of 2 was studied by temperature dependent 1H NMR spectra, which were analyzed by flip mechanism. The threshold rotational mechanism for 2 was a two-ring flip in contrast to a one-ring flip for 1, and the activation energies for 2 (18.6 and 17.5 kcal/mol) were higher than for 1 due to increased crowding in the transition state for the rotation.

The effective ‘size’ of the tris(trimethylsilyl)silyl group in several molecular environments

The effective size of the tris(trimethylsilyl)silyl group in several molecular environments has been estimated. 2,2-Dimesityl-1-tris(trimethylsilyl)silylethanol 1g has been prepared and its structure determined by X-ray crystallography. The Mes–CC torsional angles are 59.6 (φ2) and 63.3°(φ2) and the CC–Si bond angle α4 is 133.8°. The two-ring flip barrier for the correlated rotation of the two mesityl rings around the Mes–C bonds is ΔGc‡= 10.2 kcal mol–1. The structures of enols Mes2CC(OH)R, R = H, Me, Et, Pri, But(1a–1e), Me3Si (1f), (Me3Si)3Si (1g) and (Me3Si)3C (1h) and the two-ring flip barriers have been calculated by the MM2* force-field. The calculated and the experimental values are in good agreement, except for somewhat lower calculated α4 for 1b–1e and a shorter C–Si distance in 1g. From the linear correlations between the observed cos φ2 or ΔGc‡ values and Es values for the enols 1a–1e, and the values observed for 1g an average Es value of –1.46 has been calculated for (Me3Si)3Si. MM2* calculations gave an A value for (Me3Si)3Si of 4.89 kcal mol–1. These steric parameters resemble those for the But group (Es=–1.54; A= 4.9 kcal mol–1) indicating a similar effective size for the But and (Me3Si)3Si groups in these specific environments. (Me3Si)3C is significantly larger (A= 13.3 kcal mol–1; estimated Es=–3.7).

Structural systematics. Part 5. Conformation and bonding in the chiral metal complexes [M(η5-C5R5)(XO)Z(PPh3)

Date were retrieved from the Cambridge Structural Database for 67 crystal structures containing suitable geometric data for 74 molecular fragments of the form [M(η5-C5R5)(XO)Z(PPh3)]1(X = C or N; Z = any ligand). These data were analysed to examine the conformational preferences for the triphenylphosphine ligand in respect of orientations about the M–P bond and about the P–C bonds of the attached phenyl groups, and the mechanisms by which the preferred conformers interconvert. There are two main diastereomeric conformers: group I which has S configuration at the metal and anticlockwise propeller (left-hand helix) conformation at PPh3; and group II which has S configuration at the metal and clockwise propeller (right-hand helix) PPh3. Group I structures predominate in the available dataset (56 members to 18 in group II). In group I structures the phenyl group closest to the Z and XO ligands (ring B) of the PPh3 ligand is on average rotated ca. 35° away from Z towards the XO ligand and is face-on to Z. In group II structures phenyl B is edge-on to Z and face-on to XO, on average being rotated a further 36° from Z. Analysis of the conformational data suggests the following order for the energy barrier of rotational processes involving the PPh3 ligand: rotation about the M–P bond > PPh3 helicity inversion by one-ring flip > PPh3 helicity inversion by asynchronous two-ring flip. Helicity inversion can occur without full rotation about the M–P bond and is accompanied by a rocking about the M–P bond. Full rotation about the M–P bond takes place with concomitant PPh3 helicity inversion. This analysis of experimental data provides a detailed test of a previous conformational model. The PPh3 groups show distortions in M–P and P–C distances and C–P–C angles consistent with significant and asymmetric M–P π bonding in those complexes for which Z is a poor π acceptor and provide evidence for a model of PPh3π back bonding which involves participation of P–C σ* orbitals.

Structural systematics. Part 4. Conformations of the diphosphine ligands in M2(µ-Ph2PCH2PPh2) and M(Ph2PCH2CH2PPh2) complexes

Data were retrieved from the Cambridge Structural Database for 405 crystal structures containing suitable geometric data for either M2(µ-dppm)1(dppm = Ph2PCH2PPh2) fragments (409 located) or M(dppe)2(M = transitional metal, dppe = Ph2PCH2CH2PPh2) fragments (274 such located). These data were analysed to examine the conformational preferences for the five-membered MP2C2(or M2P2C) rings and the attached phenyl groups. The technique of principal component analysis was used to reduce the dimensionality of the torsion angle data set and to aid in identification of favoured conformations. The M(dppe) fragments show a preference for a twist (C2) conformation of the MP2C2 five-membered ring, in which the P–C–C–P torsion angle is far from zero (typically ca. ±50°). In contrast the M2P2C five-membered rings of the M2(µ-dppm) fragments are predominantly of the envelope (Cs) conformation, with the methylene carbon out of the plane of the near-planar M2P2 unit, i.e. having the P–M–M–P torsion angle close to zero and the M–P–C–P torsion angles ca. ±45°. The distribution of structures indicates that in both cases the ring conformations interconvert by a pseudo-rotation pathway, similar to those observed for other five-membered ring systems, in which planar intermediates are avoided. In both 1 and 2 the two phenyl groups on each phosphorus adopt conformations typical of one- and two-ring flip mechanisms of rotation. For M2(µ-dppm) there is considerable constraint of phenyl group orientation due to clashes between phenyl groups in axial sites on the envelope M2P2C ring. In contrast, although weak preferences may be seen in the phenyl group conformations for the M(dppe) fragments, these arise primarily because of contacts between the phenyl groups and the ethylene hydrogens of the MP2C2 ring and not due to transannular Ph ⋯ Ph contacts. The implications of these observations for the design of new diphosphine ligands are discussed.

Experimental and theoretical studies of correlated rotation in tris(ortho-tolyl) derivatives of P, As and Si

Variable temperature NMR studies have established equilibrium constants and/or activation parameters for the exo2↔exo3 equilibrium in the series M(o-tolyl)3(M = P, As), XM (o-tolyl)3(M = P, X = O, S, Se; M = As, X = O, S) and [MeM(o-tolyl)3]n+(n= 0, M = Si; n= 1, M = P, As). The crystal structure of OAs(o-tolyl)3H2O is reported. Molecular mechanics studies of P(o-tolyl)3 reproduce correctly the ground state exo3 conformation and provide an analysis of the lowest energy two-ring flip exchange mechanism.

Static and dynamic stereochemistry of ortho-bridged diarylsulphones

Ortho-bridged diarylsulphones I and II have been synthesized and their static and dynamic stereochemistry has been investigated by variable temperature 1H NMR spectroscopy. The results indicate that both compounds adopt a C 2 propeller conformation in solution and the enantiomerization process occurs with Δ G ≠ = 20.0 kcal mol −1 for I and Δ G ≠ = 18.15 kcal mol −1 for II. The mechanism leading to the stereoisomerization process is believed to occur through a two-ring flip transition state.

On the possible isomerisation pathway for triarylmethanes Ar3CX, including propeller crowns

The crystal structures of two propeller crown ethers are presented. The properties of the seven propeller crown structures examined to date are compared critically with geometric information retrieved from the Cambridge Crystallographic Database for other Ar3CX systems. Following the method of Bye, Schweizer, and Dunitz (BSD) each conformation was regarded as a sample point defined by the torsion angles of its three aryl groups, and possible features of the potential energy surface for the interconversion of the propeller isomers were considered graphically. Low-energy stereoisomerisation paths were apparent; the two-ring and one-ring flip transition states of earlier hypotheses did not lie on these paths.

A two-ring flip as the threshold enantiomerization route for a triarylvinyl propeller. 1D and 2D NMR and static and dynamic stereochemistry of trimesitylethylene

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTA two-ring flip as the threshold enantiomerization route for a triarylvinyl propeller. 1D and 2D NMR and static and dynamic stereochemistry of trimesitylethyleneSilvio E. Biali and Zvi RappoportCite this: J. Org. Chem. 1986, 51, 12, 2245–2250Publication Date (Print):June 1, 1986Publication History Published online1 May 2002Published inissue 1 June 1986https://doi.org/10.1021/jo00362a016RIGHTS & PERMISSIONSArticle Views94Altmetric-Citations15LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (731 KB) Get e-Alerts Get e-Alerts

Comparison of the steric barriers in three- and two-bladed propeller crowns

The propeller crowns (2) and (4)(dinaphthopolyoxacycloalkanins) have been characterised by single crystal X-ray structure determinations. Only one enantiomeric pair of isomers is present in each crystal structure. The presence of the trimethoxyphenyl substituent in (4) has remarkably little effect on the conformation of the ether ring. The propeller skeleton of (4) is different from that observed previously in the o-methoxyphenyl propeller; these two skeletal isomers are related by a two-ring flip of one naphthyl and the phenyl ring. Room-temperature 1H n.m.r. spectra of (4) and of the 6-t-butyldinaphthylpolycycloalkanin correspond to one ‘averaged’ species with a rapid averaging of naphthyl rings via a two-ring flip, slowed at 220 K. The relative steric demands of the propeller and the ether moieties are discussed.

ChemInform Abstract: STABLE SIMPLE ENOLS. 6. A SHIFT IN THE THRESHOLD MECHANISM OF CORRELATED ROTATION IN 2,2‐DIMESITYLETHENOLS FROM A ONE‐ TO A TWO‐RING FLIP

Chemischer InformationsdienstVolume 15, Issue 36 Preparative Organic Chemistry ChemInform Abstract: STABLE SIMPLE ENOLS. 6. A SHIFT IN THE THRESHOLD MECHANISM OF CORRELATED ROTATION IN 2,2-DIMESITYLETHENOLS FROM A ONE- TO A TWO-RING FLIP D. A. NUGIEL, D. A. NUGIELSearch for more papers by this authorS. E. BIALI, S. E. BIALISearch for more papers by this authorZ. RAPPOPORT, Z. RAPPOPORTSearch for more papers by this author D. A. NUGIEL, D. A. NUGIELSearch for more papers by this authorS. E. BIALI, S. E. BIALISearch for more papers by this authorZ. RAPPOPORT, Z. RAPPOPORTSearch for more papers by this author First published: September 4, 1984 https://doi.org/10.1002/chin.198436072AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume15, Issue36September 4, 1984 RelatedInformation

Stable simple enols. 6. A shift in the threshold mechanism of correlated rotation in 2,2-dimesitylethenols from a one- to a two-ring flip

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTStable simple enols. 6. A shift in the threshold mechanism of correlated rotation in 2,2-dimesitylethenols from a one- to a two-ring flipDavid A. Nugiel, Silvio E. Biali, and Zvi RappoportCite this: J. Am. Chem. Soc. 1984, 106, 11, 3357–3359Publication Date (Print):May 1, 1984Publication History Published online1 May 2002Published inissue 1 May 1984https://doi.org/10.1021/ja00323a052RIGHTS & PERMISSIONSArticle Views34Altmetric-Citations13LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (393 KB) Get e-Alerts Get e-Alerts

Restricted rotation in methyl- and cyanomethyl-tris-(2,4,6-trimethylphenyl)phosphonium compounds. A 1H nuclear magnetic resonance spectroscopic study. The crystal structure of cyanomethyltris-(2,4,6-trimethylphenyl)phosphonium tetraiodomercurate(II)

The crystal structure of cyanomethyltris-(2,4,6-trimethylphenyl)phosphonium tetraiodomercurate(II), [Mes3-PH2CN]2˙Hgl42–, is reported, and the bonding and conformational characteristics of the cation are compared with those related structures which also exist in a propeller conformation. The dynamic stereochemistry of Mes3PH2CN, Mes3PH3, and Mes3P, as studies by 1H n.m.r. spectroscopy, is described. The calculated ΔG‡values for enantiomerisation of the propeller configuration, assumed to occur via the two-ring flip mechanism, are discussed in the context of related studies. For Mes3PH2CN, the enantiomerisation mechanism is related to the conformation in the crystalline state.