Influence of Carbon Group Substituents on Bond Shift and Electrochemical Reduction of Cyclooctatetraene

Abstract

The free energies of activation for bond shift in the carbon group substituted cyclooctatetraenes (COT−M(CH3)3) in THF-d 8 at 298 K have been determined to be 16.4, 16.2, 16.2, and 18.1 kcal/mol for M = Si, Ge, Sn, and C, respectively, and 15.6 kcal/mol for CH3−COT. These data permit an interpretation of the previously reported opposite orders for the ease of the first and second electrochemical reductions in the Si, Ge, and Sn compounds. It is postulated that the order of the first reduction potential is controlled by a decrease in overlap between the substituent and the ring π orbitals in the order Si > Ge > Sn, whereas the second reduction potential is controlled by the energy gap between the symmetric π HOMO of the COT radical anion and an interacting substituent σ* orbital of π symmetry (εσ * π − επ), which increases in the order Sn < Ge < Si. HF/3-21G molecular orbital calculations indicate that the high barrier for t-Bu−COT primarily reflects steric effects in the transition state.

Preparation and Properties of Centrally Bridgehead‐Substituted Hexacyclo[4.4.0.02,1.03,5.04,8.07,9]decanes (“Diademanes”) and Related (CH)10Hydrocarbons

“…[4,15]). The starting materials, trimethylsilyl-(4 b) [16] and (acetoxymethyl)cyclooctatetraene (4 c) were prepared from the known bromocyclooctatetraene [17] and cyclooctatetraenylmethanol, [18] respectively, by established procedures (see Experimental Section). Prolonged heating of solutions of 4 b or 4 c with maleic anhydride in benzene afforded the cycloadducts 5 b and 5 c in 75 and 46 % yields, respectively (Scheme 2), which were transformed into the corresponding basketane derivatives 6 b and 6 c by acetone-sensitized photocyclization.…”

“…Organic extracts were dried over MgSO 4 . Trimethylsilylcyclooctatetraene [16] (4 b): n-Butyllithium (101 mmol, 44 mL of a 2.3 m solution in hexane) was added dropwise at À 78 8C, under an atmosphere of argon, to a stirred solution of bromocyclooctatetraene [17] (18.3 g, 100 mmol) in anhydrous diethyl ether (100 mL). After additional stirring at À 55 to À 60 8C for 2 h, the reaction mixture was cooled to À 78 8C, treated dropwise with a solution of Me 3 SiCl (10.8 g, 100 mmol) in Et 2 O (50 mL), and stirred for an additional 2 h at 0 8C.…”

Transannular Reactions of Two Parallel 1,3‐Butadiynes: Syntheses, Structures, and Reactions of 1Azacyclotetradeca‐3,5,10,12‐tetrayne Derivatives

“…In the latter case we expect for the 13 C NMR spectrum a chemical shift in the order of δ =148 ppm for the central carbons and δ =100 ppm for the peripheral carbons 20. For the cyclooctatetraene nuclei in 25 and 25′ we expect δ values between 120 and 150 ppm 21. The values observed for our product fit somewhat better to 25 and 25′ .…”

Transannular Reactions of Two Non‐Parallel 1,3‐Butadiyne Units − Syntheses, Structures and Protonation Reactions of 1‐Isopropyl‐1‐azacyclopentadeca‐3,5,11,13‐tetrayne and 1‐Isopropyl‐1‐azacyclohexadeca‐3,5,12,14‐tetrayne

“…[20] For the cyclooctatetraene nuclei in 25 and 25' we expect d values between 120 and 150 ppm. [21] The values observed for our product fit somewhat better to 25 and 25'.…”