65927-08-4Relevant academic research and scientific papers
Solution Photochemistry of 4-Alkylidene-3,3,5,5-tetramethyl-Δ1-pyrazolines
Bushby, Richard J.,Jesudason, Malini V.,Pollard, Michael D.,Shuhaibar, Khamis F.
, p. 2647 - 2654 (1982)
Direct photolysis of 4-(isopropylidene)-3,3,5,5-tetramethyl-1-pyrazoline (11) is believed to proceed via a singlet trimethylene-methane (TMM).It gives mainly the 'least motions' product (isopropylidene)-2,2,3,3-tetramethylcyclopropane (12).This implies that the singlet TMM adopts a bis-orthogonal geometry (21) rather than the mono-orthogonal geometry (20) expected.Direct photolysis of 13 different α-methylated 4-alkylidenepyrazolines (5) shows, that in this series, formation of the 'least motions' product (22) is normally preferred.In the initially formed bis-orthogonal biradicals, the ability of C-CMe2 rotation to complete with ring closure seems to be determined by steric factors.Generally, the tendency to form the 'least motions' product (22) is greatest in those cases where the alkylidene group bears two substituents (X,Y H), less where there is one (X = H, Y H), and least of all where there is none at all (X,Y = H).The two main exceptions to this least motions rule are for 4-(2',2',3',3'-tetramethylcyclopropylidene)-3,3,5,5-tetramethyl-1-pyrazoline (27) and for 3,3,5,5-tetramethyl-1-pyrazolin-4-ylidenemalononitrile (32).In both of these cases there may be special factors which destabilise the 'least-motions' product.By way of contrast to the direct photolysis, triplet sensitised photolysis of 4-(isopropylidene-3,3,5,5-tetramethyl-1-pyrazoline (11) gives a statistical mixture of the deuterated methylenecyclopropanes (12) and (13).This is consistent with the expected planar geometry (19 or the kinetic equivalent thereof) of the triplet TMM biradical.Some examples of the photochemical interconversion of methylenecyclopropanes and one example of a photochemical reduction of a pyrazoline are also reported.
Photolysis of Tetraalkyl-1-pyrazoline-4-thiones. - Diastereoselective Formation of (E)/(Z)-Alkylidenethiiranes from cis- and trans-Tetraalkyl-1-pyrazoline-4-thiones
Quast, Helmut,Fuss, Andreas,Jakobi, Harald
, p. 1747 - 1755 (2007/10/02)
The tetraalkyl-1-pyrazolin-4-ones 5 react with hydrazine to afford the hydrazones 6 which are transformed into the thiones 7 in high yields by treatment with disulphur dichloride in the presence of triethylamine.Selective excitation of the azo chromophor of 7a with 350-nm light gives rise to the isopropylidenethiirane 8, besides molecular nitrogen, in a very clean reaction; at almost quantitative conversions, less than 2percent of byproducts are observed, and about 10percent of 8 have isomerized to the pentadienethiol 9.Photolysis of the stereoisomers cis- and trans-7b yields mixtures of the alkylidenethiiranes (E)- and (Z)-10 with (E)/(Z) ratios of 35:65 and 49:51.The results are interpreted in terms of diastereomeric bis-orthogonal (cis- and trans-21) and mono-orthogonal thioxyallyl diradicals (E)- and (Z)-23 which cyclize to furnish (E)- and (Z)-10.There seems to be a qualitative resemblance between the photochemical and thermal stabilities in the series of 4-substituted tetramethyl-1-pyrazolines, viz. 7a oct-2-ene derivatives. Key Words: 4H-Pyrazole-4-thiones, 3,3,5,5-tetraalkyl-3,5-dihydro- / Thiiranes, 3,3-dialkyl-2-alkylidene/ 1,3-Pentadiene-3-thiol, 2,4-dimethyl- / Photolysis/ Extrusion of molecular nitrogen/ (E)/(Z) Stereoselectivity
