Bithienylsilanes: unexpected structure and reactivity

5-(2,2′-Bithienyl)hydrosilanes were prepared by reaction of bithienyl lithium with chlorodimethylsilane, dichloromethylsilane and trichlorosilane. It was shown that 5-(2,2′-bithienyl)dimethylsilane possesses higher reactivity in the hydrosilylation reaction of monosubstituted acetylene derivatives compared with (2-thienyl)dimethylsilane. The elongation of the π-conjugated chain leads to increasing selectivity of the hydrosilylation reaction. An unusual structure for bis[5-(2,2′-bithienyl)]methylsilane has been established by X-ray analysis.     

Low-Temperature Photochemistry of Previtamin D: A Hula-Twist Isomerization of a Triene

What is the standard course of the cis – trans isomerization of nonpolar polyenes? Many results support a pathway in which only a central CH group rotates out of the plane, while the remaining parts of the molecule reorient within the plane (“hula-twist” mechanism). The intermediate structure through which the molecule passes corresponds to the geometry predicted for the conical intersection of the S₁ and S₀ potential energy surfaces (see picture).     

Biomimetically inspired total synthesis and structure activity relationships of 1-O-methyllateriflorone. 6 pi electrocyclizations in organic synthesis

The total synthesis of 1-O-methyllateriflorone (2) is described. The construction of the cage-like domain of the molecule involved a biomimetic Claisen/Diels-Alder cascade, whereas the novel spiroxalactone framework was generated by an intramolecular Michael reaction within precursor 16a involving the carboxylate residue as the nucleophile. This finding might bear on the biosynthetic pathway by which nature forms lateriflorone. Described herein is also an interesting cascade sequence involving facile 6 pi electrocyclizations which leads to complex benzopyran systems. The biological evaluation of a small library of lateriflorone analogues and related systems establishing the first SAR within this class of compounds is also included. Among the most active compounds against tumor cells are 2, 16b, 56, 58, and 59.     

Some fundamental questions in chemical reactivity

This paper provides an overview of recent progress on some fundamental questions in chemical reactivity. The following issues are emphasized: (1) the choice of electronic structure level for dynamics study; (2) Diels-Alder reactions; (3) combined molecular orbital/molecular mechanics methods; (4) solvent effects; (5) S_N2 reactions; (6) dynamics simulations; (7) proton-transfer reactions; and (8) femtochemistry. Received: 25 November 1997 / Accepted: 23 December 1997     

Some probability relations in multicomponent copolymers

It was previously shown that for a stationary random copolymer of A, B, and C, we have in general p(AB) + p(AC) = p(BA) + p(CA), etc., in place of p(AB) = p(BA) which is valid for a stationary binary copolymer. Here, p(AB) for example, is the probability that a randomly picked pair of consecutive comonomers in the polymer consists of an A followed by a B. For a stationary ternary copolymer produced by a first-order Markovian addition mechanism, we show that P_ABP_BCP_CA/P_ACP_CBP_BA = k, where k is a constant characteristic of a particular set of three monomers but independent of its composition. Here, P_AB is the conditional probability of finding a monomer of B given that its immediate predecessor is an A. We further show that if the individual rate constants of the monomer additions involved take a special form such as used in the Alfrey-Price Q–e scheme, then we have k = 1 irrespective of the kinds of monomers, and in addition we have p(AB) = p(BA), p(AC) = p(CA), etc. Thus, although these latter results were previously proposed by Ham as an alternative basis to supplant the Q–e scheme, they may rather be regarded as mathematical consequences of special assumptions adopted for the form of the individual rate constants. For a stationary random copolymer of four components A, B, C, and D, we have p(AB) + p(AC) + p(AD) = p(BA) + p(CA) + p(DA), etc., in general. For a first-order Markovian four-component copolymer, we show that there are seven different combinations of the conditional probabilities that are constants (k1, k2,…, k1) independent of the monomer composition. Again, if we assume the same special form for the rate constants involved, we find that all the seven constants k1, k2, …, k7 reduce to unity and p(XY) = p(YX) for X,Y, = A, B, C, D.     

Iron topochemistry and surface reactivity of amphibole asbestos: relations with in vitro toxicity

Chemical reactivity of asbestos tremolite from Italy and USA localities and Union Internationale Contre le Cancer (UICC) crocidolite was studied in relation to Fe content, oxidation state, and structural coordination. Direct correlation between amount of Fe²⁺ at the exposed M(1) and M(2) sites of the amphibole structure and fiber chemical reactivity was established. The in vitro toxicity of the same samples was investigated on human alveolar A549 cell line. Relationship between crystal-chemical features and cell toxicity is not straightforward. UICC crocidolite has Fe content and chemical reactivity largely higher than that of tremolite samples, but all show comparable in vitro toxic potential. Results obtained evidenced that Fe topochemistry is not a primary factor for induced cell toxicity, though it accounts for asbestos chemical reactivity (and possibly genotoxicity).     

NaHg: A variant of the cesiumchloride structure structural relations and electronic structure

The crystal structure of the gold-bronce coloured, metallic NaHg, which was already determined earlier, is characterized by nearly square planar Hg₄ clusters condensed to zig-zag chains via common edges. We discuss the hitherto unknown relation of the orthorhombic NaHg structure to the CsCl type (B2). and present calculations of the electronic structures for the hypothetical B2 structure as well as for the real orthorhombic structure by means of the relativistic LMTO method. A clear tendency is observed towards a population of the mercury 6 p-states, which are empty in the free atom in its ground state. This population is the most important contribution to the bonding between the mercury atoms in the cluster chains. Thus the simple chemical picture, whereafter a partial electron transfer should take place from the electropositive sodium to mercury, is in accordance with the calculation results.     

A bipyridyl thorium metallocene: synthesis, structure and reactivity

The synthesis, structure and reactivity of a new bipy thorium metallocene have been studied. The reduction of the thorium chloride metallocene [η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)ThCl(2) (1) with potassium graphite in the presence of 2,2'-bipyridine gives the purple bipy metallocene [η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)Th(bipy) (2) in good yield. Complex 2 has been fully characterized by various spectroscopic techniques, elemental analysis and X-ray diffraction analysis. Complex 2 reacts cleanly with trityl chloride, silver halides and diphenyl diselenide, leading to the halide metallocenes [η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)ThX(2) (X = Cl (1), Br (3), I (4)) and [η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)Th(F)(μ-F)(3)Th[η(5)-1,3-(Me(3)C)(2)C(5)H(3)](F)(bipy) (5), and selenido metallocene [η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)Th(SePh)(2) (6), in good conversions. In addition, 2 cleaves the C[double bond, length as m-dash]S bond of CS(2) to give the sulfido complex, [η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)ThS (7), which further undergoes an irreversible dimerization or nucleophilic addition with CS(2), leading to the dimeric sulfido complex {[η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)Th}(μ-S)(2) (8) and dimeric trithiocarbonate complex {[η(5)-1,3-(Me(3)C)(2)C(5)H(3)](2)Th}(μ-CS(3))(2) (10) in good yields, respectively.     

Quadricyclanes. Part II: Electronic structure and chemical reactivity

The electronic structure of quadricyclane and 3-methylidenequadricyclane obtained by photoelectron spectroscopy, is used as a basis for the discussion of cycloadditions to these systems. The electronic structure of 3-heteroquadricyclanes, arrived at by theoretical calculations, agrees well with that expected from the above measured systems. A surprising outcome is that the orbital most responsible for the observed 2,4-cycloadditions to these heterosystems in not the HOMO but the third highest orbital which lies well below the former. This strongly suggests that these 2,4-cycloadditions proceed not in a concerted fashion but presumably involve as rate-determining step the formation of a resonance-stabilized zwitterionic intermediate. The nature of this intermeiate is discussed and the feasability of its formation investigated on the basis of thermochemical considerations.     

Substituted diaryldiazomethanes and diazofluorenes: structure, reactivity and stability

The synthesis of several substituted diaryldiazomethanes and diazofluorenes, and an assessment of their structure, reactivity and stability, is reported.     

Chemistry of boryllithium: synthesis, structure, and reactivity

A series of lithium salts of boryl anion, boryllithiums, were synthesized and characterized by NMR spectroscopy and crystallographic analysis. In addition to the parent boryllithium compound 35a, structural modification of boryllithium, using saturated C-C and benzannulated C=C backbones in the five-membered ring and mesityl groups on the nitrogen atoms, also allowed generation of the corresponding boryllithium. The solid state structures of boryllithium showed that the boron-lithium bond is polarized where the boron atom is anionic in all (35a x DME)(2), 35a x (THF)(2), 35b x (THF)(2), and 35c x (THF)(2) when compared to the structures of hydroborane 38a-c and optimized free boryl anion opt-46a-c. Dissolution of the isolated single crystals of (35a x DME)(2) and 35a x (THF)(2) in THF-d(8) showed that the boron-lithium bond remained in solution and free DME or THF molecules were observed. Temperature-dependent (11)B NMR chemical shift changes of 35a were observed in THF-d(8) or methylcyclohexane-d(14), suggesting a change of chemical shift anisotropy around the boron center. The HOMO of opt-35a x (THF)(2) had a lone pair character on the boron atom, as observed for phenyllithium, whereas the HOMO of hydroborane 38a corresponds to the pi-orbital of the boron-containing five-membered heterocycle. The polarity of the B-Li bond, estimated by AIM analysis, was similar to that of alkyllithium. Boryllithiums 35a and 35b behave as a base or a boron nucleophile in reaction with organic electrophiles via deprotonation, S(N)2-type substitution, halogen-metal exchange or electron-transfer, 1,2-addition to a carbonyl group, and S(N)Ar reaction. In the case of the reaction with CO(2), intramolecular cyclization followed by CO elimination from borylcarboxylate anion and subsequent protonation gave hydroxyboranes 64a and 64b. The characters of the carbonyl groups in the borylcarbonyl compounds 60a, 60b, 61, 62, and 63a, which were obtained from the reaction of boryllithiums 35a and 35b, were investigated by X-ray crystallography, IR, and (13)C NMR spectroscopy to show that the boryl substituent weakened the C=O bond when compared to carbon substituted analogues.     

Benzyl cobaloximes with thiodioximes: Synthesis, structure and reactivity

Five benzyl cobaloximes with different thiodioximes, BnCo[d(SR)gH]₂Py, have been synthesized and four of these complexes have been characterized by X-ray. The reactivity of these complexes towards molecular oxygen has been studied. The puckering of the Co(dioxime)₂ unit, caused by dioxime side chain, the SR group, significantly influences the Co-C bond reactivity. Structural features in one of the oxygen inserted cobaloximes have been studied to confirm if puckering of dioxime is the guiding factor. The reactivity is also affected, to some extent, by the C-H…π interaction between the benzyl and the dioxime moiety.     

Amino substituted bisketenes: generation, structure, and reactivity

Hitherto unknown diamino-substituted bisketenes with both free (14) and tethered (16) amino substituents have been generated by using laser flash photolysis for ring opening of the corresponding cyclobutenediones. The time-resolved kinetics of ring closure of the amino bisketenes back to the cyclobutenediones were measured by IR or UV spectroscopy, and give first-order rate constants which vary by a factor of 7.5x10(4), and the bis(Me2N) bisketene 14 is the most reactive in ring closure that has been reported. Rate constants for ring closure of these and previously observed bisketenes vary by a factor of 10(13). The dialkylamino bisketenes 16 (R=Me, n-Bu) with tethered substituents and restricted geometries are less reactive than the bis(Me2N) bisketene 14 by factors of 1700 and 540, respectively. Computational results obtained with DFT methods suggest angle strain in the tethered cyclobutenediones 15 inhibits facile cyclization of bisketenes 16.     

The Stereospecific Tert-Butyllithium Reduction of Triene Sulfoxides: Improved Levels of Deuterium Incorporation

Treatment of triene sulfoxides with t-BuLi and methanol-O-d₁ in D₂O-wet ether leads to trienes with practical levels (96%) of deuterium incorporation. Alternatively, successive treatment of triene sulfoxides dwith CH₃Li and then t-BuLi followed by methanol-O-d₁ quench affords similar levels of label incorporation.     

Coordination Polymers with Bipyridyl Diene and Triene Ligands: Synthesis,Structures and Luminescent Properties

Solvothermal reactions of cadmium salts with bipyridyl diene and triene ligands[1,3-bis((E)-2-(pyridin-4-yl)vinyl)benzene-(1,3-bpeb) and 1,6-bis(4-pyridyl)-1,3,5-hexatriene(bphte)] and one bipyridyl triene ligand[1,6-bis(4-pyridyl)-1,3,5-hexatriene-(bphte)] in the presence of auxiliary carboxylic acids including 2,6-dichlorobenzoic acid(2,6-HDCBA) and 2-naphthal-enecarboxylic acid(2-HNCA) gave rise to four coordination polymers[Cd(bphte)(2,6-DCBA)₂]_n(1), {[Cd(bphte)(2-NCA)₂]·5.8H₂O}_n(2),[Cd₂(1,3-bpeb)₂(2,6-DCBA)₄]_n(3) and {[Cd₄(1,3-bpeb)₄(2-NCA)₇(HCOO)]·     

Electronic structure and reactivity of guanylthiourea: A quantum chemical study

Electronic structure analysis of guanylthiourea (GTU) and its isomers has been carried out using quantum chemical methods. Two major tautomeric classes (thione and thiol) have been identified on the potential energy (PE) surface. In both the cases conjugation of pi‐electrons and intramolecular H‐bonds have been found to play a stabilizing role. Various isomers of GTU on its PE surface have been analyzed in two different groups (thione and thiol). The interconversion from the most stable thione conformer ( GTU‐1 ) to the most stable thiol conformer ( GTU‐t1 ) was found to take place via bimolecular process which involves protonation at sulfur atom of GTU‐1 followed by subsequent C? N bond rotation and deprotonation. The detailed analysis of the protonation has been carried out in gas phase and aqueous phase (using CPMC model). Sulfur atom (S1) was found to be the preferred protonation site (over N4) in GTU‐1 in gas phase whereas N4 was found to be the preferred site of protonation in aqueous medium. The mechanism of S‐alkylation reaction in GTU has also been studied. The formation of alkylated analogs of thiol isomers (alkylated guanylthiourea) is believed to take place via bimolecular process which involves alkyl cation attack at S atom followed by C? N bond rotation and deprotonation. The reactive intermediate RS(NH₂)C? N? C(NH₂)₂⁺ belongs to the newly identified ^⊕N(←L)₂ class of species and provides the necessary dynamism for easy conversion of thione to thiol. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

Phosphido pincer complexes of platinum: synthesis, structure and reactivity

A series of platinum(II) complexes supported by the tridentate bis(phosphine)phosphido ligand bis(2-diisopropylphosphinophenyl)phosphide) [(i)Pr-PPP] have been synthesized and characterized (1-4). X-Ray structural studies of [(i)Pr-PPP]PtCl (1) and [(i)Pr-PPP]PtCH(3) (3) complexes show meridional [(i)Pr-PPP] ligands around approximately square-planar platinum centers. Structural data and NMR analysis highlight a strong trans influence for the phosphido phosphorous donor, comparable to that of the anionic aryl carbon of the classic PCP pincer complexes. A series of thermally stable [PPP]Pt(IV) compounds, including [PPP]Pt(CH(3))(2)X [X = I (5) and SbF(6) (6)], were also synthesized. The study of the binding affinity of SO(2) and NO to complex 1 has also been addressed.