Envelope conformers

Envelope (Section 3 6) One of the two most stable conforma tions of cyclopentane Four of the carbons in the envelope conformation are coplanar the fifth carbon lies above or be low this plane... 

Saturated five-membered heterocyclic compounds are non-planar, existing in half-chair or envelope conformations. The far-IR spectra of THE and 1,3-dioxolane (127) show both to have barriers of ca. 0.42 kJ moP ... 

Whereas the aromatic systems are planar, fully reduced five-membered rings have non-planar envelope conformations, as is discussed in Section 4.01.4.3. 

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

Reaction of [Rh(/z-Cl)(CO)2]2 with sodium pyrazolate leads to 206 (85CJC699). The Rh2N2Cl ring has the envelope conformation. The rhodium atom has distorted square-planar coordination. The molecules in the crystalline lattice form onedimensional stacking units with alternating rhodium atoms in the binuclear units, intermolecularly interacting in a zigzag chain. 

Ho HT, Fan L, Nowicka-Sans B, McAuhffe B, li CB, Yamanaka G, Zhou N, Fang H, Dicker 1, Dalterio R, Gong YF, Wang T, Yin Z, Ueda Y, MatiskeUa J, Kadow J, Qapham P, Robinson J, Colonno R, Lin PF (2006) Envelope conformational changes induced by human immunodeficiency virus type 1 attachment inhibitors prevent CD4 binding and downstream entry events. 

The ring templates can be further used to construct larger, multicyclic systems as illustrated in Fig. 7.8. For norbornane, two fitting conformahons of cyclopentane in the envelope conformation can be joined in order to construct the complete 3D structure of norbornane. In this case, this is the only low-energy conformahon known for norbornane due to its rigidity. 

The preferred TS is a chair with the enolate oriented syn to the bulky pyrrolidine substituent. It was suggested that the syn acylation occurs through an envelope conformation of the pyrrolidine ring with the nitrogen electron pair oriented axially. 

P2j Z = 2 Dx = 1.33 R = 0.09 for 792 intensities. The molecule is tricyclic, with a five-membered and a seven-membered ring fused to the ribofuranose ring. Both five-membered rings have an envelope conformation. The seven-membered ring has the same conformation as is found in 2,5-arabinosylcytidine monophosphate.79 ... 

The lowest energy calculated S5+ structure has Cs symmetry43 and is best described as a five-membered ring in the envelope conformation. 

The only known complex with a (RSb)5 ligand is cyclo-[ i-(Me3SiCH2Sb)5-Sb1,Sb3- W(CO)5 2].125 The molecular structure is depicted in Fig. 25. The antimony ring adopts a slightly distorted envelope conformation with a Sb4/Sb3 dihedral angle of 129.1°. The metalcarbonyl units are in 1-3 positions trans to each other. 

Three new spirans, including 131 and 132, incorporating a 1,5-benzodithiepin system have been synthesised from methyl benzynes. The structure of 131 was confirmed by X-ray crystallography, and the seven-membered ring was shown to have a chair conformation and the five-membered ring an envelope conformation . 

Stereodynamics of Cyclic Nitronates From general considerations and according to X-ray diffraction data (263a), molecules of hve-membered cyclic nitronates should adopt an envelope conformation with the C-5 atom deviating from the plane. This atom fluctuates together with its substituents (R4 and R5) (Scheme 3.83, process a). 

P212121 Z = 4 Dx = 1.21 R = 0.05 for 1,605 intensities. The carbon chain has a sickle conformation, with C-1-C-2-C-3-C-4, C-2-C-3-C-4-C-5 torsion angles of —60°, -50°, respectively. The dioxolanes have a twist and an envelope conformation, and are related by an approximate, two-fold axis along C-3-C-4. The double-bond stereochemistry is Z. 

P2t Z = 4 Dx = 1.32 R = 0.059 for 1,985 intensities. The configuration of C-2 is (S). The two symmetry-independent molecules in the crystal structure have similar, but not identical, conformations. The gulonolactone and isopropylidene rings have envelope conformations. There appears to be a significant difference (of 5 pm) in two of the isopropylidene C-O bond-lengths in the two molecules. 

The structures of the salts (131)168 and (132),159 a chelated ylide,160 and the insecticide coroxon161 have been determined. The structures of the cis- and trans-isomers of the cyclic phosphonamide (133) differed in that the cw-isomer has a half-chair conformation whereas the trans-isomer has an envelope conformation. Both compounds had hydrogen bonds of approximately 269 pm.162 The phosphadiazane (134)... 

The PE spectra of four 2-substituted 1,3-dimethylimidazolidines (58) have been recorded and analysed using AMI and PM3 quantum chemical calculations106. A single broad band is found for the two hn ionizations and the energies of the two mn orbitals are split by only 0.0-0.3 eV indicating little n/n interaction. This is consistent with envelope conformations of the five-membered ring of 58 and an axial-equatorial orientation of the two N-methyl groups. 

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