Ring inversion in benzopentathiepin

DFT calculations for non-benzo pentathiepin: Greer, A. J. Am. Chem. Soc. 2001, 123, 10379

Geometry optimization by DFT/PBE/Λ1 (PRIRODA program [Chem. Phys. Lett. 1997, 281, 151; Russ. Chem. Bull. 2005, 54, 820])
with thermochemical ΔG (298.15 K) and Grimme's D3(BJ) dispersion corrections [J.Chem. Phys. 2010, 132, 154104; J. Comp. Chem. 2011, 32, 1456] for the energy of optimized geometry

Basis Λ1 [Chem. Phys. Lett. 2005, 416, 116; Theor Chem Acc (2019) 138: 40]: H (6s,2p)→[2s,1p]; C,N,O (10s,7p,3d)→[3s,2p,1d]

On figure(s) below click on the level title to download xyz file	==>
Click on energy level to view 3D structure in browser (run JSmol)	==>

Animation of A-B-C-A1 process

IRC in concatenated xyz format: A-B-C-A1, A-B-B1-A1, A-C-B-B1-C1-A1, B-B1, B-C-B2

Unimolecular paths for ring inversion in Greer's pentathiepin

E.M. Brzostowska, M. Paulynice, R. Bentley, A. Gree, Chem. Res. Toxicol. 2007, 20, 1046-1052

On figure(s) below click on the level title to download xyz file	==>
Click on energy level to view 3D structure in browser (run JSmol)	==>

Animation of Greer's process (3a-3b-3c IRC)

Animation of low-energy process (3a-3bi-3b-3c IRC)

IRC in concatenated xyz format: A-B-C-A1, B-C-B2

Ring inversion in pentathiepan

On figure(s) below click on the level title to download xyz file	==>
Click on energy level to view 3D structure in browser (run JSmol)	==>

Animation of low-energy process (a-b-x-x1-b1-a1 IRC)

IRC in concatenated xyz format: a-b-x-x1-b1-a1, a-a2, a-x2, x-x3

Stationary points on the PES of some pentathiepins

A	A-B	B	B-B	C	C-A	ref
0.00 0.00^a	16.37 17.43^a	1.96 7.04^a	4.37 3.74^a	12.64 12.41^a	27.19 26.96^a	A. Greer, J. Am. Chem. Soc. 2001, 123, 10379-10386 Calculated (B3LYP/6-31G*): 7(A) 0.00, 16(B) 4.86, TS15(A-B) 16.95, 21(C) 12.94, TS20(C-A) 26.58 kcal/mol. The author believes that the chair-chair interconversions of pentathiepin 7(A) passes through TS20(C-A). The lower energy path through TS15(A-B) is not considered. B.L. Chenard, D.A. Dixon, R.L. Harlow, D.C. Roe, T. Fukunaga, J. Org. Chem. 1987, 52, 2411-2420 Calculated (MP2): 6-C(A) 0.00, 6-TB(B-B) 4.1, 6-B(C) 13.1, 6-TS(1)(C-A) 29.2 kcal/mol. Chair-chair interconversion proceeds via TS(1).
0.71 0.00	20.40 19.01	3.39 3.21 3.47	6.17	12.20	25.76	E.M. Brzostowska, M. Paulynice, R. Bentley, A. Greer Planar Chirality due to a Polysulfur Ring in Natural Pentathiepin Cytotoxins. Implications of Planar Chirality for Enantiospecific Biosynthesis and Toxicity Chem. Res. Toxicol. 2007, 20, 1046-1052 Calculated by B3LYP/6-31G(d): 3a(A) 0.00, TS-1(C-A) 24.7, 3b(B) 5.9, TS-2 19.0, 3c(A') -0.3 kcal/mol. Mechanism 3a⇄TS-1⇄3b⇄TS-2⇄3c is proposed with barrier 24.7 kcal/mol.
0.00	19.12	4.16	6.47	10.83	27.54
0.00	19.68	3.72	6.18	11.18	27.35	Ando W., Kumamoto Y., Tokitoh N., Tetrahedron Letters, 28(41), 4833–4836 (1987) 1H NMR 1.26(s,6H), 1.57(s,6H): barrier > 16 kcal/mol
0.00	29.34	6.33	9.68	10.97	23.11	Tokitoh N., Ishizuka H., Ando W., Chem. Lett. 1988, 657-660 1H NMR 1.46(s,6H), 1.74(s,6H): barrier > 16 kcal/mol
0.00 0.00^a	23.58 27.32^a	5.48 9.91^a	7.33 6.96^a	12.55 11.26^a	25.83 26.59^a
0.00	23.76	6.49	7.78	10.98	27.52
0.00	23.72	5.08		9.17	25.39	O. Sato, T. Saito, M. Iwase, A. Sakai, Heterocycles 2016, 93, 714-722 No data about barrier
0.68 0.00	28.38 27.63	7.82 6.75	9.33 9.45	11.18 10.96	23.72 24.74	P.A. Searle, T.F. Molinski, J. Org. Chem. 1994, 59, 6600-6605 Lissoclinotoxin A is chiral: protons of CH2 group are diastereotopic on the NMR time scale.
						B.S. Davidson, P.W. Ford, M. Wablman, "Chirality in unsymmetrically substituted benzopentathiepins: The result of a high barrier to ring inversion", Tetrahedron Lett. 1994, 35 (39), 7185-7188 R = H (varacin): according to MM calculations (CVFF force field) half-chair conformation 1c(C-A) is higher TS of the ring inversion (34 kcal/mol) R = COOCH2CH2Si(CH3)3: diastereotopic ArCH2 signals do not broaden at 100 °C (barrier > 21 kcal/mol) R = (S)-CONHCHMe(1-naphthyl): slow interconversion of diastereomers at room temperature over a matter of days in deuteriochloroform (barrier ~25 kcal/mol)
0.07 0.00	26.06 25.64	5.58 5.70	7.93 7.67	11.90 11.96	25.23 25.70	R. Sato, H. Ohta, T. Yamamoto, Sh. Nakajo, S. Ogawa, A. Alam Synthesis of novel axially chiral cyclic benzopolysulfides Tetrahedron Letters 48 (2007) 4991–4994 Experimental ∆G₂₉₈^≠ is 24.3 kcal/mol.
0.11 0.00	27.57 26.87	6.64 6.17	8.23 9.11	11.34 11.54	26.35 24.51	B.L. Chenard, D.A. Dixon, R.L. Harlow, D.C. Roe, T. Fukunaga, J. Org. Chem. 1987, 52, 2411-2420 Experimental barrier 19.3 kcal/mol (ref. 17, unconfirmed data; additionally, CH2 is singlet, not AB system [J. Am. Chem. Soc. 1985, 107, 3871-3879 ])
0.00 0.22	28.61 29.76	7.44 7.46	9.70 10.82	10.08 10.15	23.74 23.82	T. Kimura, M. Hanzawa, K. Tsujimura, T. Takahashi, Y. Kawai, E. Horn, T. Fujii, S. Ogawa, R. Sato Preparation and Conformational Analysis of 6,10-Diethyl[1,2,3]trithiolo[4,5-h]benzopentathiepin Monoxides: Isolation and Optical Properties of Chiral Benzopentathiepin Derivatives Bull. Chem. Soc. Jpn., 75, 817–824 (2002) Experimental ∆G₂₉₈^≠ is 23.9 kcal/mol.
0.12 0.00	26.87 26.86	7.37 7.39	8.75 8.84	13.80 14.05	21.93 21.87	Y. Sugihara, H. Takeda, J. Nakayama Synthesis of Pentathiepanes and Isolation of the Conformers Based on High Inversion Barrier of the Pentathiepane Ring Eur. J. Org. Chem. 1999, 597-605; Tetrahedron Letters 39 (1998) 2605-2608 Experimental ∆G₂₉₈^≠ is 24.0 kcal/mol. Mechanism A⇄C⇄24⇄C'⇄A' is proposed, A⇄C stage being limited.
0.00	30.46	6.47	8.51	13.50	16.92
0.00	31.81	7.57	10.17	12.91	17.66	M. Anafcheh, F. Ektefa Cyclosulfurization of C60 and C70 fullerenes: a DFT study Struct Chem (2015) 26:1115–1124 Sheer confusion. Calculated barrier 37 kcal/mol, but depicted in fig. 4 TS is B-B. Low energy TS C-A is not mentioned.
A	A-B	B	B-B	C	C-A	ref

^aPBE/L1//DLPNO-CCSD(T)/cc-pVTZ
Dec 10 2019