scholarly journals Reactions of Titanocene- and Zirconocene-Bis(trimethylsilyl)acetylene Complexes with Selected Heterocyclic and Aromatic NH and OH Acid Compounds

2007 ◽  
Vol 72 (4) ◽  
pp. 475-491 ◽  
Author(s):  
Perdita Arndt ◽  
Vladimir V. Burlakov ◽  
Ulrike Jäger-Fiedler ◽  
Marcus Klahn ◽  
Anke Spannenberg ◽  
...  
Keyword(s):  
Bond Activation ◽  
Reaction Pathway ◽  
Reaction Products ◽  
X Ray ◽  
X Ray Crystallography

The titanocene complexes Cp'2Ti(η2-Me3SiC2SiMe3) (Cp' = Cp (1), Cp* (2)) react with pyrrole under the formation of the titanium(III) mono-N-pyrrolides Cp'2Ti(NC4H4) (Cp' = Cp (6), Cp* (7)); whereas the corresponding zirconocene system Cp2Zr(η2-Me3SiC2SiMe3)(thf) (3) forms in a different reaction pathway first the Cp2Zr(NC4H4)[C(SiMe3)=CH(SiMe3)] (8) and then the zirconium(IV) bis-N-pyrrolide Cp2Zr(NC4H4)2 (11). With Cp*2Zr(η2-Me3SiC2SiMe3) (4) and pyrrole, the zirconium(IV) mono-N-pyrrolide with an agostic alkenyl group Cp*2Zr(NC4H4)[C(SiMe3)=CH(SiMe3)] (9) was obtained. In the reaction of the ethylenebistetrahydroindenyl (ebthi) complex rac-(ebthi)Zr(η2-Me3SiC2SiMe3) (5) with 2,3,5,6-tetrafluoroaniline under N-H bond activation, a complex with an agostic alkenyl group rac-(ebthi)Zr(NH-C6HF4)[C(SiMe3)=CH(SiMe3)] (10) was formed. Compound 10 reacts with additional 2,3,5,6-tetrafluoroaniline to give the bisanilide rac-(ebthi)Zr(NH-C6HF4)2 (12) which was obtained directly from 5 with two equivalents of 2,3,5,6-tetrafluoroaniline. In reactions of complex 5 with unsubstituted aniline to rac-(ebthi)Zr(NH-C6H5)2 (13) and with pentafluorophenol to bisphenolate rac-(ebthi)Zr(O-C6F5)2 (14), no intermediates could be isolated. The new reaction products 6, 9, 10, 12, 13 and 14 were investigated by X-ray crystallography.

scholarly journals Multi Component Reactions under Increased Pressure: On the Mechanism of Formation of Pyridazino[5,4,3-de][1,6]naphthyridine Derivatives from Reaction of Malononitrile, Aldehyde and 2-Oxoglyoxalarylhydrazones in Q-Tubes

2017 ◽  
Author(s):  
Majdah A. AL-Johani ◽  
Khadijah M. Al-Zaydi ◽  
Sameera M. Mousally ◽  
Norah F. Alqahtani ◽  
Mohamed H. Elnagdi
Keyword(s):  
Crystal Structure ◽  
Reaction Pathway ◽  
Aromatic Aldehydes ◽  
Reaction Products ◽  
Mechanism Of Formation ◽  
X Ray ◽  
Alternate Pathway ◽  
Considerable Research ◽  
Increased Pressure ◽  
Chemical Evidence

The considerable biological and medicinal activities of pyridazines has stimulated considerable research on efficient syntheses of these derivatives. In the last decade, microwave irradiation has generally been used for the energy source. As demonstrated in recent studies, pressure reactor “Q-tubes” may be used to accelerate several of these reactions in a more optimal and safer manner (compared to microwaves). In these studies there has been postulated a pathway for the formation of pyridazino[5,4,3-de][1,6]naphthyridine derivatives . In this paper we consider this pathway, and an alternate pathway, for several reactions. Contrary to the suggestion in these studies the pathway in which initial dimerization of malononitrile was postulated could be excluded based on chemical evidence. The reactions performed were the reaction of arylhydrazonals 1a,b with benzylidinemalononitrile which afforded in Q-tube the 3-acyl-4-aryl-1-phenyl-6-amino-1,4-dihydropyridazines, and the reaction of arylhydrazonals 1a,b, malononitrile 9 and aromatic aldehydes 10a-g in Q-tubes which afforded the tricyclic systems 12a-n whose structure could be established by X-ray crystal structure determination. In conclusion, we have added to the work of the recent studies by excluding a reaction pathway for one of their reaction products.

Mapping the conformational itinerary of β-glycosidases by X-ray crystallography

2003 ◽  
Vol 31 (3) ◽  
pp. 523-527 ◽  
Author(s):  
G.J. Davies ◽  
V.M.-A. Ducros ◽  
A. Varrot ◽  
D.L. Zechel
Keyword(s):  
Transition State ◽  
Reaction Pathway ◽  
Tight Binding ◽  
Three Dimensional ◽  
Intermediate Structure ◽  
Active Centre ◽  
X Ray ◽  
Solution Structures ◽  
X Ray Crystallography ◽  
Covalent Intermediate

The conformational agenda harnessed by different glycosidases along the reaction pathway has been mapped by X-ray crystallography. The transition state(s) formed during the enzymic hydrolysis of glycosides features strong oxocarbenium-ion-like character involving delocalization across the C-1–O-5 bond. This demands planarity of C-5, O-5, C-1 and C-2 at or near the transition state. It is widely, but incorrectly, assumed that the transition state must be 4H3 (half-chair). The transition-state geometry is equally well supported, for pyranosides, by both the 4H3 and 3H4 half-chair and 2,5B and B2,5 boat conformations. A number of retaining β-glycosidases acting on gluco-configured substrates have been trapped in Michaelis and covalent intermediate complexes in 1S3 (skew-boat) and 4C1 (chair) conformations, respectively, pointing to a 4H3-conformed transition state. Such a 4H3 conformation is consistent with the tight binding of 4E- (envelope) and 4H3-conformed transition-state mimics to these enzymes and with the solution structures of compounds bearing an sp2 hybridized anomeric centre. Recent work reveals a 1S5 Michaelis complex for β-mannanases which, together with the 0S2 covalent intermediate, strongly implicates a B2,5 transition state for β-mannanases, again consistent with the solution structures of manno-configured compounds bearing an sp2 anomeric centre. Other enzymes may use different strategies. Xylanases in family GH-11 reveal a covalent intermediate structure in a 2,5B conformation which would also suggest a similarly shaped transition state, while 2S0-conformed substrate mimics spanning the active centre of inverting cellulases from family GH-6 may also be indicative of a 2,5B transition-state conformation. Work in other laboratories on both retaining and inverting α-mannosidases also suggests non-4H3 transition states for these medically important enzymes. Three-dimensional structures of enzyme complexes should now be able to drive the design of transition-state mimics that are specific for given enzymes, as opposed to being generic or merely fortuitous.

Novel Intramolecular CAryl–S Bond Activation by an Electron Rich, Ring-Expanded-NHC-Rh centre: A Combined Experimental and DFT Study

2011 ◽  
Vol 64 (8) ◽  
pp. 1141 ◽  
Author(s):  
Abeer Binobaid ◽  
Kingsley J. Cavell ◽  
Mikhail S. Nechaev ◽  
Benson M. Kariuki
Keyword(s):  
Bond Activation ◽  
Reaction Pathway ◽  
Heterocyclic Carbenes ◽  
Dft Study ◽  
Dft Studies ◽  
X Ray ◽  
Carbene Complex ◽  
Donor Capacity ◽  
Insight Into ◽  
Sulfur Bond

The reaction of (o-MeSPh)-N-functionalized tetrahydropyrimidinium salts with KN(SiMe3)2 and [Rh(COD)Cl]2 in THF leads to the formation of a novel dimeric RhIII bis-carbene complex. The reaction involves the unexpected cleavage/oxidative addition of the aryl-sulfur bond to give dimeric metallated RhIII with bridging MeS< moieties. This unusual reaction is probably a consequence of the sterically imposing structure and strong donor capacity of ring-expanded N-heterocyclic carbenes (RE-NHCs). An X-ray structure of the [(Ph,DIPP-NHC)Rh(Cl)(SMe)2] product complex has been obtained, and DFT studies were undertaken to gain an insight into the reaction pathway.

Combining X-ray crystallography and single-crystal spectroscopy to probe enzyme mechanisms

2009 ◽  
Vol 37 (2) ◽  
pp. 378-381 ◽  
Author(s):  
Arwen R. Pearson ◽  
Robin L. Owen
Keyword(s):  
Data Collection ◽  
Single Crystal ◽  
Diffraction Data ◽  
Reaction Pathway ◽  
X Ray ◽  
X Ray Crystallography ◽  
Enzyme Systems ◽  
Catalytic Intermediates ◽  
Trapping Methods ◽  
Induced Changes

The combination of X-ray crystallography and rapid cryo-trapping methods has enabled the visualization of catalytic intermediates in a variety of enzyme systems. However, the resolution of the X-ray experiment is not always sufficient to precisely place the structure on the reaction pathway. In addition, many trapped intermediates are X-ray-sensitive and can decay during diffraction data collection, resulting in a final structure that may not be representative of the initial trapped species. Complementary methods, such as single-crystal spectroscopy, provide a means to precisely identify the cryo-trapped species as well as detect any X-ray-induced changes during diffraction data collection.

Cyclic Alkyl(aryl)boranes for 1,1-Carboboration of Monoalkynyltin Compounds

2013 ◽  
Vol 68 (5-6) ◽  
pp. 493-502 ◽  
Author(s):  
Bernd Wrackmeyer ◽  
Peter Thoma ◽  
Wolfgang Milius
Keyword(s):  
Solid State ◽  
High Selectivity ◽  
Ring Expansion ◽  
Membered Ring ◽  
Reaction Products ◽  
State Structure ◽  
Alkyl Aryl ◽  
X Ray ◽  
X Ray Crystallography ◽  
Nmr Techniques

Two cyclic alkyl(aryl)boranes, a 1-bora-indane derivative 1, and a tricyclic derivative 4, containing the boron atom in a six-membered ring, were structurally characterized by NMR techniques in solution. The solid-state structure of the 1-bora-indane 1 was determined by X-ray crystallography. The reactivity of these cyclic alkyl(aryl)boranes towards monoalkynyltin compounds, Me3Sn-C≡C-Me and Me3Sn-C C-Fc (Fc=ferrocenyl), was studied using multinuclear magnetic resonance methods (1H, 11B, 13C, 119Sn NMR). Novel alkenylboranes were formed by 1,1-carboboration reactions. This process involves an expansion of both five- and six-membered rings. Insertion into the respective B-C(aryl) bond was preferred with high selectivity. In the case of the six-membered ring in 4, the ring expansion to seven-membered rings proved to be readily reversible, and the thermodynamically stable reaction products were formed by ring contraction and concomitant transfer of the exocyclic B-nPr group

Structural verification of a tetrahydrotetrazole compound

2019 ◽  
Vol 75 (9) ◽  
pp. 1208-1212
Author(s):  
Gary W. Breton ◽  
Lauren A. Hahn ◽  
Kenneth L. Martin
Keyword(s):  
Half Life ◽  
Reaction Pathway ◽  
Membered Ring ◽  
Crystal Phase ◽  
Previous Attempt ◽  
X Ray ◽  
X Ray Crystallography ◽  
H Nmr ◽  
Spectral Evidence

Tetrahydrotetrazoles are five-membered-ring heterocycles containing four contiguous saturated nitrogen atoms. Very few examples of such compounds have been reported in the literature. Our previous attempt at the synthesis of a member of this class of compound suggested that the N—N bonds may be more labile than expected. This finding raised the question as to whether the structures of any of the previously reported tetrahydrotetrazoles had been properly assigned. We have reproduced the synthesis of a reported tetrahydrotetrazole, namely 1,2-di-tert-butyl 3-phenyl-1H,2H,3H,10bH-[1,2,3,4]tetrazolo[5,1-a]isoquinoline-1,2-dicarboxylate, C25H30N4O4, and have now confidently confirmed its structure via X-ray crystallography. However, while sufficiently stable in the crystal phase, we discovered that it remains very labile in solution (having a half-life of only 15 min at 20 °C in CDCl3). A tentative reaction pathway for its dissociation based on 1H NMR spectral evidence is provided.

Unusual Weiss–Cook condensation of dimethyl 2,3-dioxobutanedioate and dimethyl 3-oxoglutarate

1994 ◽  
Vol 72 (4) ◽  
pp. 1162-1164 ◽  
Author(s):  
Ghislain Deslongchamps ◽  
Daniel Mink ◽  
Paul D. Boyle ◽  
Nina Singh
Keyword(s):  
Sodium Chloride ◽  
Dimethyl Sulfoxide ◽  
Reaction Pathway ◽  
Quantitative Yield ◽  
X Ray ◽  
X Ray Crystallography ◽  
Kinetically Controlled

The Weiss–Cook condensation of dimethyl 2,3-dioxobutanedioate with two equivalents of dimethyl 3-oxoglutarate in aqueous bicarbonate produces an "abnormal" product, pentamethyl cis-3-(carbomethoxymethyl)-3,7-dihydroxy-2-oxabicyclo-[3.3.0]oct-7-ene-1,4,5,6,8-pentacarboxylate 7, whose structure has been determined by X-ray crystallography. Rrapcho decarbomethoxylation (sodium chloride, aqueous dimethyl sulfoxide, 140 °C) of this compound produces dimethyl cis-3,7-dioxobi-cyclo[3.3.0]octane-1,5-dicarboxylate 4 in quantitative yield. These results suggest that compound 7 may be the product of a kinetically controlled reaction pathway involving 4-hydroxypent-2-en-1-one intermediate 5. Heating of 7 in dimethyl sulfoxide may have promoted thermodynamic equilibration to a Weiss–Cook product, which then decarbomethoxylated to produce 4.

scholarly journals A synthetic, catalytic and theoretical investigation of an unsymmetrical SCN pincer palladacycle

2016 ◽  
Vol 3 (4) ◽  
pp. 150656 ◽  
Author(s):  
Gavin W. Roffe ◽  
Sarote Boonseng ◽  
Christine B. Baltus ◽  
Simon J. Coles ◽  
Iain J. Day ◽  
...  
Keyword(s):  
Density Functional ◽  
Bond Activation ◽  
Reaction Pathway ◽  
Moderate Activity ◽  
Aim Analysis ◽  
X Ray Crystallography ◽  
Aryl Bromides ◽  
Activity Density ◽  
The Stability

The SCN ligand 2-{3-[(methylsulfanyl)methyl]phenyl}pyridine, 1, has been synthesized starting from an initial Suzuki–Miyaura (SM) coupling between 3-((hydroxymethyl)phenyl)boronic acid and 2-bromopyridine. The C–H activation of 1 with in situ formed Pd(MeCN) 4 (BF 4 ) 2 has been studied and leads to a mixture of palladacycles, which were characterized by X-ray crystallography. The monomeric palladacycle LPdCl 6, where L-H = 1, has been synthesized, and tested in SM couplings of aryl bromides, where it showed moderate activity. Density functional theory and the atoms in molecules (AIM) method have been used to investigate the formation and bonding of 6, revealing a difference in the nature of the Pd–S and Pd–N bonds. It was found that S-coordination to the metal in the rate determining C–H bond activation step leads to better stabilization of the Pd(II) centre (by 13–28 kJ mol −1 ) than with N-coordination. This is attributed to the electron donating ability of the donor atoms determined by Bader charges. The AIM analysis also revealed that the Pd–N bonds are stronger than the Pd–S bonds influencing the stability of key intermediates in the palladacycle formation reaction pathway.

Reaction pathway of ADP-ribose pyrophosphatase, revealed by time-resolved X-ray crystallography

2008 ◽  
Vol 64 (a1) ◽  
pp. C271-C272
Author(s):  
N. Kamiya ◽  
K. Kai ◽  
N. Nakagawa ◽  
S. Kuramitsu ◽  
I. Miyahara
Keyword(s):  
Reaction Pathway ◽  
Time Resolved ◽  
X Ray ◽  
X Ray Crystallography

Anta and its Oxidation Products

1995 ◽  
Vol 418 ◽  
Author(s):  
Kien-Yin Lee ◽  
Richard Gilardi ◽  
Michael A. Hiskey ◽  
James R. Stine
Keyword(s):  
High Explosive ◽  
Ammonium Persulfate ◽  
Nitrogen Molecule ◽  
Heat Of Formation ◽  
Oxidation Products ◽  
Reaction Products ◽  
High Nitrogen ◽  
X Ray ◽  
X Ray Crystallography ◽  
Crystal Density

Abstract5-Amino-3-nitro-lH-1,2,4-triazole (ANTA) is a molecule with high stability. Aside from being an insensitive high explosive (IHE), it is also used as a synthon for other potential new IHEs. The crystal structure of ANTA was resolved by X-ray crystallography. However, when ANTA was recrystallized from 2-butanone, crystals with molecular packing characterized by extended planar sheets were obtained (ß-ANTA). The crystal density of ß-ANTA is 1.73 g/cm3, which is less dense than α-ANTA (p = 1.82g/cm3).The high-nitrogen molecule, 5,5'-dinitro-3,3'-azo-1,2,4-triazole (DNAT) was calculated to have a high density and a positive heat of formation (AHf). In an attempt to prepare DNAT, we have studied the oxidation of ANTA with different oxidizers. It was found that DNAT is the reaction product when the potassium salt of ANTA was oxidized with potassium permanganate. However, when ANTA was oxidized with ammonium persulfate in aqueous medium, one of the reaction products obtained was the azoxy moleclue of DNAT (DNAzT). We are unable to determine the crystal densities of either DNAT and DNAzT because the crystals obtained were solvated with the crystallization solvents.

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