Hari K. Gupta

Do aromatic transition states lower barriers to silatropic shifts? A synthetic, NMR spectroscopic, and computational study

Abstract In a previous calculational study at the semi-empirical unrestricted Hartree-Fock (UHF) level of theory, we predicted that strategically incorporating an aromatic ring onto indenyltrimethylsilane would lower the barrier for [1,5]-silatropic shifts around the five-membered ring through retention of aromatic character in the transition state and in the intermediate iso -indene. We now report the synthesis and dynamic behavior of the tricyclic system angular trimethylsilylbenzindene, and also the tetracycle trimethylsilylcyclopenta[ l ]phenanthrene. Incorporation of one aromatic ring onto the indenyl ligand does lower the ΔG ≠ value for [1,5]-silatropic shifts to 21.9±0.5 kcal mol −1 , compared to 24 kcal mol −1 for the indenyl system. Addition of a second aromatic ring further lowers the barrier to 17.6±0.2 kcal mol −1 , good agreement with calculations. The intermediate iso -indenes for both systems have been trapped as their Diels-Alder adducts with tetracyanoethylene.

The barriers to trimethylsilyl migrations in indenes and benzindenes: Silatropic shifts via aromatic transition states

Abstract The barriers to migration of a trimethylsilyl group around the five-membered ring of indene, and of linear and angular benzindenes, have been calculated at the unrestricted Hartree-Fock (UHF) level of theory by using AMPAC and MOPAC. It is found that the retention of aromatic character in the transition state and in the intermediate isoindene lowers the barrier for the silatropic shifts. A preliminary experimental study on a trimethysilyl derivative of angular benzindene reveals that Me 3 Si migration is indeed a facile process. However, in the corresponding bis-trimethylsilyl systems, rearrangements which require the intermediacy of geminal Me 3 Si groups have rather high barriers.

The Structure of a Seven-Bladed Propeller: C7Ph7+ is Not Planar

The first nondisordered structure of a free tropylium cation has been unequivocally determined by single-crystal X-ray diffraction. The solid-state structure of the heptaphenylcycloheptatrienyl cation, C7Ph7+, in [C7Ph7+][CF3CO2−]⋅2 CF3CO2H reveals a severely crowded system in which the central ring is nonplanar, and the propeller blades (the peripheral phenyl substituents) are arranged in a slightly tilted fashion about the seven-membered ring (right).

Steric control in the formation of Co2(CO)6-alkyne complexes from Group 14 tetraalkynes and their reactions with acid

Abstract A series of tetrakis(trimethylsilylethyne) derivatives of Group 14 metals (2–4) was prepared. Co2(CO)6 complexes 5–10 were synthesised by the reaction of 2–4 with Co2(CO)8. From the silyl and germyl based compounds 2 and 3, either one or two alkynes could be complexed with Co2(CO)6. In contrast, the tin derived compound 4 could accommodate up to four Co2(CO)6 complexes. The longest wavelength UV-Vis absorbances of the silicon and germanium-based complexes were consistent with multiple, non-conjugated Co2(CO)6 chromophores. The tetrakis Co2(CO)6 complex 10, however, absorbs at a much longer wavelength suggesting conjugation of Co2(CO)6 complexes through the tin. The reactivity towards protonolysis of the uncomplexed alkynes 2–4 is a consequence of the hyperconjugative stabilisation of the intermediate β-vinyl cation (the β-effect): Sn(CCSiMe3)3>SnOTf(CCSiMe3)2>SiMe3>Ge(CCSiMe3)3. The reactivity of the Co2(CO)6 complexes, however, was quite different from the reactions of 2–4 and from analogous all-carbon systems. Treatment of 5–10 with strong acid led neither to protiodemetallation of the complexed or non-complexed alkynes but to decomplexation of the cobalt. Similarly, ligand metathesis reactions between 10 and Ph2SiCl2 were not observed. The normal reactivity of silylalkynes towards electrophiles, which was expected to be enhanced by the presence of the cobalt complex, was diminished by the particular steric environment of the molecules under examination (5–10). As a result, the favoured reaction under these conditions was decomplexation of the cobalt.

A synthetic and structural study on metal cluster complexes of allyl—alkynyl—silanes: Does protonation lead to metal-stabilized silyl cations?

Abstract The mono-alkynes Me3SiC≡CSiMe2(CH2CH=CH2) 7 and (CH2=CHCH2)Me2Si-C≡CSiMe2(CH2CH=CH2) 8, and the di-alkyne (CH2=CHCH2)Me2SiC≡C-C≡CSiMe2(CH2CH=CH2) 9, have been prepared and treated with Co2(CO)8 and Mo2(CO)4Cp2, respectively, to give the corresponding dimetalla-alkyne tetrahedral complexes. Two of the clusters have been structurally characterised: [Cp2Mo2(CO)4]Me3SiC≡CSiMe2(CH2CH=CH2), 11, crystallised in the space group C2/c with a = 15.058(3) A, b = 12.474(2) A, c = 29.128(6) A, β = 100.12(3)°, V = 5386(2) A3, Z = 8. [Co2(CO)6]2((CH2=CHCH2)Me2SiC≡C-C≡CSiMe2(CH2CH=CH2)), 13, crystallised in the space group C2/c with a = 12.604(3) A, b = 15.447(3) A, c = 18.382(4) A, β = 107.20(3)°, V = 3418(12) A3, Z = 4. Treatment of each of these clusters with HBF4 yielded the corresponding fluorosilane with concomitant formation of propene. It was shown that for [Co2(CO)6] (CH2=CHCH2)Me2SiC≡C-C≡CSiMe2(CH2CH=CH2) 15, reaction with only one equivalent of HBF4 yields the fluorosilane with loss of propene at the non-complexed alkynyl terminus. The relevance of these results to the possible intermediacy of a metal-stabilized silylium ion is discussed.