Solution and X-ray structure of triethylsilylmethyl-substituted tropylium cation

The beta-silyl tropylium cation 9 has been successfully generated in the presence of the noncoordinating counterion tetrakis(pentafluorophenyl)borate in benzene and toluene. The (13)C and (29)Si NMR spectra of 9 demonstrate that the beta-silicon substituent interacts strongly with the aromatic pi-system of the cation by hyperconjugation. The crystal structure of 9 shows structural effects consistent with this interaction. [structure--see text]     

Determining the sigma-donor ability of the cyclopropane C-C bond

The low temperature crystal structures of ester and ether derivatives of varying electron demand, derived from cyclopropylmethanol and dicyclopropylmethanol , have been determined. These structures show a very strong response of the C-OR bond distance to the electron demand of the OR substituent, demonstrating the strong sigma-donor ability of the strained C-C bonds in the cyclopropane ring.     

Manganese(I) poly(mercaptoimidazolyl)borate complexes: spectroscopic and structural characterization of Mn...H-B interactions in solution and in the solid state

The manganese(I) tricarbonyl complexes (Bm(R))Mn(CO)3(R = Me, Bz, But, p-Tol) and (PhBmMe)Mn(CO)3, the first bis(mercaptoimidazolyl)borate derivatives for this metal, have been readily prepared and fully characterized. In particular, the presence of three-center-two-electron Mn...H-B interactions in these species, both in solution and in the solid state, has been investigated using a combination of IR and NMR spectroscopies and, in the case of the methyl-, tert-butyl- and para-tolyl-substituted derivatives, by X-ray crystallography. To complement these synthetic and structural studies, the tris(mercaptoimidazolyl)borate complexes (TmMe)Mn(CO)3(R = Me, Bz, But, p-Tol) and (PhTm(Me))Mn(CO)3, as well as the related pyrazolylbis(mercaptoimidazolyl)borate (pzBmMe)Mn(CO)3, have also been synthesized and characterized by a combination of analytical and spectroscopic techniques.     

Synthesis and structure of novel organocycloborates

A series of alpha,omega-boryl(bromo)alkanes of the general formula R2B-(CH2)n-Br (n = 3, 4, 5, 6) was obtained in high yield following a standard hydroboration protocol. Upon treatment with Mg turnings and formation of the respective Grignard species, all alkanes with n = 4 to 6 underwent an unprecedented boron-centered cyclisation reaction with formation of boratacyclopentanes, -hexanes, and -heptanes, respectively. All new compounds were isolated in high yields as colourless, crystalline materials and characterised in solution by multinuclear NMR spectroscopy. Four representative examples were chosen for X-ray diffraction studies, thus providing the first structurally characterised ring systems of that size at a tetraalkyl borate centre.     

Asymmetric decarboxylative Claisen rearrangement reactions of sulfoximine-substituted allylic tosylacetic esters

Allylic acetate esters containing a variety of N-arylsulfonyl sulfoximines on the acetyl residue have been prepared and submitted to the decarboxylative Claisen rearrangement reaction. Rearranged products were isolated in generally good yields, and diastereoselectivities up to 82:18 have been obtained. The N-(2,4,6-triisopropylphenylsulfonyl)-S-phenyl sulfoximine moiety gave the best selectivity. The stereochemistry of the major isomer was established by X-ray crystallography. A model to explain the stereochemical course of the rearrangement is proposed.     

Application of the Dötz reaction to construction of a major portion of the ansa macrocycle (-)-kendomycin

[Reaction: see text] A D?tz reaction employing a terminal alkyne and a Fischer-type alkenylchromium carbenoid led to a pentasubstituted benzene from which a major portion of the Streptomyces metabolite (-)-kendomycin was synthesized.