ABSTRACT
New neutral bis(alkyl) Sc and Y complexes [N,Npy,N-]Ln(CH2SiMe3)2(THF)n [n = 0, Ln = Sc (1Sc), Y (1Y); n = 1, Ln = Y (1YTHF)] stabilized by a tridentate monoanionic amidopyridinate ligand were straightforwardly prepared by alkane elimination, upon mixing ligand [N,Npy,N-]H and metal precursor Ln(CH2SiMe3)3(THF)2 in toluene at 0 °C. Depending on the work-up conditions, yttrium bis(alkyl)s were isolated as either a pentacoordinate Lewis base free complex [N,Npy,N-]Y(CH2SiMe3)2 (1Y) or as a hexacoordinate THF adduct [N,Npy,N-]Y(CH2SiMe3)2THF (1YTHF). For the smaller Sc ion the only solvent-free complex [N,Npy,N-]Y(CH2SiMe3)2 (1Sc) was isolated as a pentacoordinate species irrespective of the reaction/work-up/crystallization conditions applied. Complexes 1Ln (Ln = Y, Sc) and 1YTHF were scrutinized as pre-catalysts in ternary catalytic systems Ln/borate/AliBu3 (borate = [HNMe2Ph][B(C6F5)4] or [Ph3C][B(C6F5)4]), applied to isoprene (IP) polymerization, providing moderate activity albeit high selectivity with predominant formation of 1,4-cis polyisoprene (up to 99%). The same complexes proved to be effcient catalysts also for the intermolecular hydrolelementation of styrene with various EH sustrates (pyrrolidine, morpholine, Ph2PH, PhPH2, PhSH) affording linear anti-Markovnikov addition products exclusively. After a preliminary activation by B(C6F5)3, selected bis(alkyl) complexes from this series have been finally used as valuable pre-catalysts for the CO2 hydrosylilation to CH4 in the presence of organosilanes as reducing agents (PhMe2SiH, PhSiH3, Et2MeSiH).
ABSTRACT
A unique phenomenon of regioisomerism in coordination chemistry was discovered: the reaction of a sterically hindered o-quinone annelated with a dithiete ring with Pd2dba3 in the presence of 1,2-bis(diphenylphosphino)ethane (dppe) gave a mixture of two regioisomers: catecholate 3Cat and dithiolate 3Dit. Both isomers were isolated in crystalline form and characterized by NMR, IR and X-ray diffractometry studies. DFT calculations reveal that the 3Dit species is more thermodynamically stable than the isomer 3Cat. Isomerization of 3Cat to 3Dit in solution was observed.
ABSTRACT
The antimitotic agent combretastatin A4 (CA-4) has been suggested as an antivascular agent for anticancer therapy relatively recently. To reduce systemic toxicity by means of administration in liposome formulations, in this study new lipophilic prodrugs, oleic derivatives of CA-4 and its 4-arylcoumarin analog (CA4-Ole and ArC-Ole, respectively), have been synthesized: Liposomes of 100 nm mean diameter prepared on the basis of egg phosphatidylcholine and phosphatidylinositol from bakers yeast have been shown to include completely up to 10 mol. % of CA4-Ole, or 7 mol. % of ArC-Ole. Also, prodrug bearing liposomes decorated with tetrasaccharide selectin ligand Sialyl Lewis X (SiaLe(x)) have been constructed to achieve targeting to endothelium under neovascularization. The antitumor activity in vivo was studied in the model of slowly growing mouse breast cancer. Under the used dose (22 mg/kg) as well as the regimen of treatment (four injections, one per a week, starting from the appearance of palpable tumors) cytostatic CA-4 did not reveal any anticancer effect, and oppositely even stimulated tumor growth. Liposome formulations of CA4-Ole did not show such stimulation. However, to achieve pronounced antitumor effect, number of injections of liposomes should be apparently elevated. New antimitotic agent ArC revealed cytotoxic activity of only one tenth value obtained for CA-4 in vitro in the culture of human breast carcinoma cells. Nevertheless, in vivo in the mouse model of breast cancer this compound showed antitumor effect under double CA-4 equivalent dose. The results demonstrate availability of SiaLe(x)-liposomes loaded with ArC-Ole: this preparation began to inhibit tumor growth already after the second injection. It is necessary further to choose doses and regimens of administration both for ArC and liposome formulations bearing ArC-Ole.
