Propeller Conformation Research Articles

Bis(1,2‐dimethyl‐3‐indolizinyl)methane: Structural Studies of a Two‐Bladed Molecular Propeller

AbstractAn X‐ray structural investigation on bis(1,2‐dimethyl‐3‐indolizinyl)methane (1) reveals that it occupies a two‐bladed propeller conformation with an anti‐relation between the central methylene protons and the four methyl groups. In the crystal, the molecule lacks a C2 axis and it is asymmetric rather than helical. Since the two indolizinyl rings carry no substituents which can conjugate with their π systems the structural results supply, for the first time, accurate information about the geometry of the important heterocycle indolizine.

The phenomenon of conglomerate crystallization. VIII. Spontaneous resolution in coordination compounds. VI. The chiral behaviour of [ trans-( P, or M)-Co(NH 3) 2 (NO 2) 4] − anions, dissymmetric by virtue of their propeller conformation

Octahedral coordination compounds chiral by virtue of having fixed propeller conformations are, apparently, quite rare. Despite the fluxionality of the anion [ trans-( P, or M)-Co(NH 3) 2(NO 2) 4] −, it is possible to establish its chiroptical properties in the solid. Our studies of its salts suggest that, conformationally, it is relatively impervious to the nature of the compensating cation and to variations in packing forces. Consequently, a useful method of determining the chiral symbol of this anion is proposed based on that of stereochemically rigid cations with which it forms diastereoisomeric salts. In the case of solutions of the ions [ cis-Co(en) 2(NO 2) 2] + and [ trans-Co(NH 3) 2(NO 2) 4] −, the crystals obtained from a conglomerate are expected to contain Λ(δλ)-(−) 589-[ cis-Co(en) 2(NO 2) 2][ trans-( P)- Co(NH 3) 2(NO 2) 4] and Λ(δλ)-(+) 589-[ cis-Co(en) 2(NO 2) 2][ trans-( M)-Co(NH 3) 2(NO 2) 4].

Structural studies of steric effects in phosphine complexes. Part XI. Synthesis, characterization, crystal and molecular structure of the dinitrato(trimesitylarsine)mercury(II) dimer

The synthesis and crystal and molecular structure of the dinitrato(trimesitylarsine)mercury(II) dimer are reported. The compound [HgAs(mesityl)3(NO3)2]2 belongs to the monoclinic space group P21/c[Formula: see text] with a = 20.001(2), b = 17.591(5), c = 16.553(2), β = 99.991(6), and Z = 4; the asymmetric unit contains two independent half dimers. The structure was refined to a final R of 0.0458 for 2631 observed reflections measured by diffractometer with 2 < θ < 20°. The crystal structure consists of discrete centrosymmetric dimeric molecules of [HgAs(mesityl)3(NO3)2]2 separated by normal van der Waals distances. The mercury coordination in both dimers I and II is characterized by three strong, nearly coplanar bonds (in I Hg—As 2.476(3), Hg—O 2.41(2) and 2.20(2), and in II Hg—As 2.482(3), Hg—O 2.28(2) and 2.39(2), and three weaker Hg—O bonds in the range 2.55–2.90 Å. Both independent trimesitylarsine ligands have the same regular propeller conformation (Hg—As—C—C torsion angles of 45 to 50°) and the largest C—As—C angles (mean value 112.5(8)°) yet determined. The As(mesityl)3 cone angle is 202°. The two independent dimers differ primarily in the orientation of the non-bridging nitrato groups. Nitrato vibrational and arsine 1H nmr data are also presented for [HgAs(mesityl)3(NO3)2]2, which becomes monomeric in dichloroethane solution.

Conformational behaviour of medium-sized rings. Part 9. Disalicylides and trisalicylides

The temperature dependences of the 1H n.m.r. spectra of di-o-thymotide (13) and di-o-carvacrotide (14) have been interpreted in terms of ring inversion (12a)⇌(12b) between enantiomeric boat conformations. Comparison of the ΔG‡ values (17.7 and 18.4 kcal mol–1, respectively) for this conformational change suggests that it takes place by a pseudorotational process involving folded boat conformations (17) in which the substituents (methyl and isopropyl, respectively) on C-3 enter into 1,5-interactions with the carbonyl oxygen atoms in the eight-membered rings at the rate-determining transition states. The temperature dependences of the 1H n.m.r. spectra of tri-o-cresotide (6) and tri-m-cresotide (7) provide further evidence that a mechanism involving pedalling of trans-ester linkages through the mean plane of the twelve-membered ring accounts for the conformational changes between enantiomeric propeller (4) and helical (5) conformations. The ΔG‡ values for interconversion and inversion processes of eight- and twelve-membered ring compounds show an informative dependence upon the steric demands of the ortho-substituents on the aromatic rings.

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.

X-ray structure determinations of the solid state conformations of tri- [formula omitted]-carvacrotide and of tri-3- and tri-6-methylthiosalicylides

In the solid state, tri-6-methylthiosalicylide ( 5 ) adopts only one helical conformation, tri-3-methylthiosalicylide ( 4 ) exists as both enantiomeric helical conformations, and tri- o -carvacrotide ( 2 ) provides the first example of the unique situation where both enantiomeric propeller conformations and both enantiomeric helical conformations coexist.

The crystal and molecular structure of triphenylsilythiirane

Triphenylsilylthiirane, crystallized from anhydrous pentane, has the cell dimensions: a 16.452(6), b 11.440(2), c 19.192(7) Å; β 105.39(3)°; V 3483(2) (Å 3; Z  8; space group C2/ c. The structure was solved by direct methods and refined anisotropically down to R  0.094. The triphenylsilyl group has a propeller conformation. The thiirane ring shows a significant thermal motion.

Structural studies of steric effects in phosphine complexes. Part 6. The synthesis, characterization and molecular structure of the dinitro(trimesitylphosphine)mercury(II)dimer

The synthesis and crystal and molecular structure of the dinitro(trimesitylphosphine)mercury(II) dimer is reported. The compound [HgP(mesityl) 3(NO 3) 2] 2 belongs to the monoclinic space group P2 1/ c (C 5 2h, No. 14) with a = 19.940(3), b = 17.588(2), c = 16.403(2) Å, β = 100.36(1)° and Z = 4; the asymmetric unit contains two independent half dimers. The structure was refined to a final R of 0.056 for 4220 observed reflections measured by diffractometer. The mercury coordination in both dimers I and II is characterized by three strong nearly coplanar bonds (in I HgP 2.395(3), HgO 2.180(12) and 2.257(8), and 2.342(9)) and three weaker HgO bonds in the range 2.58–2.90 Å. Both independent trimesityl phosphine ligands have the same regular propeller conformation (HgPCC torison angles 42 to 49°) and the largest CPC angles (mean value 113.2(5)°) and cone angle (θ = 208°) yet determined. The main difference between the two independent dimers is the orientation of the non-bridging nitrato groups. Nitrato vibrational and phosphine 1H and 31P n.m.r. data are also presented for [HgP(mesityl) 3(NO 3) 2] 2.

Conformation of cis-stilbene and some derivatives studied by nuclear magnetic relaxation measurements

The spin–lattice relaxation times and nuclear Overhauser enhancements (NOE) have been measured for the olefinic protons of cis-stilbene and a few derivatives in dilute solution in CDCl3. The results confirm the propeller conformation suggested previously on the basis of the electronic spectrum. The angle of twist is in the range 20–40°.

A theoretical study of internal rotation in fragments of aromatic polyimide chains, containing ether and sulphide groups

A theoretical examination has been carried out of internal rotation in fragments of polyimide chains containing phenylene-oxide-phenylene and phenylene-sulphide-phenylene groupings. Diphenyl ether (DPE) and diphenyl sulphide (DPS) were taken as models. For analysis of internal rotation the classical conformational method at different values of the torsional potential U OC about the OC bonds was used, as well as the EHT, MINDO/2, CNDO/2 and PCILO quantum chemical methods. The results are presented in the form of potential maps describing the dependence of the energy of DPE on the angles of internal rotation around the OC bonds. The potential maps take different forms depending on the value of U OC or the quantum chemical method used, at some values of U OC the classical maps being similar to the quantum chemical maps. Comparison of the potential maps with one another and with experimental measurements showed that the most realistic results are given by the CNDO/2 and EHT methods and the corresponding classical barriers. According to these maps the “propeller” equilibrium conformation is characteristic of DPE, in which internal rotation is fairly free. Potential maps were calculated for DPS by the EHT method and by the classical method for various values of U SC. The calculations showed that DPS also has the equilibrium propeller conformation, but hindrance to free rotation is greater in DPS than in DPE.