Rearrangements of 4-Quinolylcarbene, 3-Quinolylcarbene, and 2-Quinolylcarbene to 1-Naphthylnitrene and Cyanoindenes by Falling Solid Flash Vacuum Pyrolysis.

The relationship between 4-quinolylcarbene 17, 3-quinolylcarbene 21, 2-quinolylcarbene 25, and 1-naphthylnitrene 35 has been explored experimentally and computationally. The diazomethylquinolines generated from (5-tetrazolyl)quinolines or 1,2,3-triazolo[1,5-a]quinoline by conventional flash vacuum pyrolysis (FVP) were observed by IR spectroscopy. The carbenes were generated by falling solid flash vacuum pyrolysis (FS-FVP). 4-Quinolylcarbene 17 was found to rearrange to 3-quinolylcarbene 21 and then to 2-quinolylcarbene 25, and finally via 1-naphthylnitrene 35 to 1-cyanoindene 36, which then isomerizes to 3- and 2-cyanoindenes 12 and 13. The thermal rearrangement of 2-quinolylcarbene to 1-naphthylnitrene was verified by ESR spectroscopy. The reaction mechanism has been elucidated with the help of calculations of the structures and energies of the quinolylcarbenes and 1-naphthylnitrene and the intervening aza-benzobicyclo[4.1.0]heptatrienes, aza-benzocycloheptatetraenes, and aza-benzocycloheptatrienylidenes and the transition states connecting them at the B3LYP/6-31G* level. The nonobserved 1,2-hydrogen shifts in aza-benzocycloheptatetraenes/aza-benzocycloheptatrienylidenes are found to have very high activation barriers.

Ring Contraction in Arylcarbenes and Arylnitrenes; Rearrangements of 1- and 3-Isoquinolylcarbenes and 2-Naphthylnitrene to Cyanoindenes.

Flash vacuum pyrolysis (FVP) of 1-(5-(13)C-5-tetrazolyl)isoquinoline 18 generates 1-((13)C-diazomethyl)isoquinoline 19 and 1-isoquinolyl-((13)C-carbene) 22, which undergoes carbene-nitrene rearrangement to 2-naphthylnitrene 23. The thermally generated nitrene 23 is observed directly by matrix-isolation ESR spectroscopy, but undergoes ring contraction to a mixture of 3- and 2-cyanoindenes 26 and 27 under the FVP conditions. The (13)C label distribution in the cyanoindenes was determined by (13)C NMR spectroscopy and indicates the occurrence of two parallel paths of ring contraction starting from 1-isoquinolylcarbene; path a via ring expansion to 3-aza-benzo[c]cyclohepta-1,2,4,6-tetraene 32 bifurcating to 2-naphthylnitrene 23 and 2-aza-benzobicyclo[3.2.0]heptatriene 39 (paths a1 and a2); and path b via ring closure of the carbene onto the ring nitrogen, yielding 1-aza-benzo[d]bicyclo[4.1.0]hepta-2,4,6-triene 34 and 3-aza-benzo[d]cyclohepta-2,3,5,7-tetraene 35. Product studies demand that the major path is route a1 via 2-naphthylnitrene 23, which then undergoes direct ring contraction to 1-cyanoindene; but the (13)C label distribution requires that the non-nitrene route b contributes significantly. The two reaction paths are modeled at the B3LYP/6-31G* level. The initially formed carbene 22 is estimated to carry chemical activation of some 40 kcal/mol. This allows both reaction channels to proceed simultaneously under low-pressure FVP conditions. FVP of 3-(5-tetrazolyl)isoquinoline 28 similarly generates 3-diazomethylisoquinoline 29 and 3-isoquinolylcarbene 30, which rearranges to 3- and 2-cyanoindenes 26 and 27.

Triazoloazine-Diazomethylazine Valence Isomerization. [1,2,3]Triazolo[1,5-a]pyridines and 2-Diazomethylpyridines.

2-Diazomethylpyridines 1D and 6D, the valence isomers of [1,2,3]triazolo[1,5-a]pyridines 1T and 6T, have been observed directly at ∼2080 cm(-1) by a combination of mild flash vacuum pyrolysis (FVP) at 200-600 °C with low temperature IR spectroscopy. Calculations confirm a ca. 17 kcal/mol barrier for the formation of 2-diazomethylpyridine 1D from [1,2,3]triazolo[1,5-a]pyridine 1T, the diazo compound lying ca. 5 kcal/mol above the triazole. In the higher temperature range (400-600 °C) 2-diazomethylpyridine 1D eliminates N2 with formation of 2-pyridylcarbene 2 and rearrangement to 1-cyanocyclopentadiene 4. 2-Diazomethylpyridine 1D undergoes 1,3-dipolar cycloaddition with tetracyanoethylene (TCNE) at 20-90 °C to yield 3-(2-pyridyl)cyclopropanetetracarbonitrile 11 and 3-(tricyanovinyl)-[1,2,3]triazolo[1,5-a]pyridine 13T via unobserved pyrazolines 10 and 12. FVP of triazole 13T affords an IR absorption at 2080 cm(-1) ascribed to the corresponding diazo compound 13D.

Intramolecular interactions in aminoacyl cyclic-3',5'-nucleotides.

Polymerization of amino-acid acyl cyclic-3',5'-nucleotides is postulated to be the origin of RNA and associated protein in prebiotic molecular evolution. The enthalpy change in the intramolecular interaction between the nucleotide base and the amino-acid side chain determines the stability of the particular complex, resulting in a preferred association (or coding) of a base for a particular amino acid. The compounds studied were glycine acyl cyclic-3',5'-guanylate where the strong hydrogen bond between protonated glycine and guanine N7 gives an enthalpy change of -0.05 h. Similarly, hydrogen bonds in l-lysine acyl cyclic-3',5'-adenylate give an enthalpy change of -0.06 h. Hydrophobic interactions in l-phenylalanine acyl cyclic-3',5'-uridylate give an enthalpy change of -0.02 h and the corresponding value for l-proline acyl cyclic-3',5'-cytidylate is -0.01 h. These interactions were expected to be modified as the genetic code became a duplet and finally a triplet code. The interactions have been shown to be feasible from the overall enthalpy changes in the ZKE approximation at the MP2/6-31G* level.

A plausible prebiotic synthesis of pyridoxal phosphate: vitamin B6 - a computational study.

A set of chemical reactions is proposed to account for the formation of pyridoxal phosphate, Vitamin B6, from a primeval atmosphere composed of cyanoacetylene, diacetylene, carbon monoxide, hydrogen, water, and a phosphoric acid. The reactions have been shown to be feasible from the overall enthalpy changes in the ZKE approximation at the HF and MP2/6-31G* level.

An ab initio computational study of thiamin synthesis from gaseous reactants of the interstellar medium.

A set of chemical reactions is proposed to account for the formation of thiamin derivatives from gaseous reactants that have been identified in the interstellar medium, and may have been relevant to a prebiotic atmosphere. The gaseous mixture consisted of methanimine, acetonitrile, cyanoacetylene, ammonia, acetylene, allylene, hydrogen sulfide, thioformaldehyde, and hydrogen in the presence of water. Most of the reactions appear to be exothermic. The reactions have been shown to be feasible from the overall enthalpy changes in the ZKE approximation at the HF and MP2/6-31G(*) level.

Possible origin for porphin derivatives in prebiotic chemistry--a computational study.

A set of chemical reactions is postulated to account for the formation of the macrocyclic porphin structure, basic to the pyrrole derivatives chlorophyll, protoporphyrin, heme and bilirubin, important in photosynthesis, respiration and digestion. A set of equations is given for the prebiotic synthesis of porphin derivatives from the simple molecules; cyanoacetylene, diacetylene, carbon monoxide and ammonia that have been detected in space. A number of isomers of hydrogenated porphin arise which may lose hydrogen to give ultimately porphin and its dehydrogenated derivative. The reactions, while not unique, provide a pathway which has been shown to be feasible from the overall enthalpy changes in the ZKE approximation at the HF and MP2/6-31G* level.