82215-69-8Relevant academic research and scientific papers
Intramolecular O-H...O hydrogen-bond-mediated reversal in the partitioning of conformationally restricted triplet 1,4-biradicals and amplification of diastereodifferentiation in their lifetimes
Moorthy, Jarugu Narasimha,Samanta, Subhas,Koner, Apurba L.,Saha, Satyajit,Nau, Werner M.
supporting information; experimental part, p. 13608 - 13617 (2009/02/06)
The photoreactivity and nanosecond transient phenomena have been investigated for a rationally designed set of ketones 4-9 in order to gain comprehensive insights concerning the influence of intramolecular hydrogen bonding on (i) the lifetimes of triplet 1,4-biradicals and (ii) the partitioning of the latter between cyclization and elimination. Comparisons of the photochemical results and lifetime data for the biradicals of ketones 6 versus 8 and 7 versus 9 revealed a remarkable influence of hydrogen bonding when superimposed upon steric factors: while 6 and 7 yielded cyclobutanols in poor yields, cyclization was found to be overwhelmingly predominant for 8-anti and moderately so for 9-anti, with a high stereoselectivity in the formation of cyclobutanols (>95% for 8-anti). The diastereochemistry in the case of 8 permitted the occurrence of fragmentation or cyclization almost exclusively (>90% cyclization for 8-anti and >75% elimination for 8-syn). Significantly, the intramolecular hydrogen bonding in the biradicals of 8 and 9 was found to reverse their partitioning between cyclization and elimination compared with the behavior of the biradicals of ketones 3; the ketones 8-anti and 9-anti underwent cyclization in benzene, predominantly leading to cyclobutanols with syn stereochemistry between the C2 and C3 substituents. In accordance with photoproduct profiles, an unprecedented ～2-fold difference in the lifetimes of the intermediate diastereomeric triplet biradicals of ketones 8 in nonpolar solvents (e.g., τsyn = 123 ns and τanti = 235 ns in cyclohexane) was observed via nanosecond laser flash photolysis, while no such difference in lifetimes was found for the triplet biradicals of acetoxy ketones 9. The intriguing diastereodifferentiation in the lifetimes of the diastereomeric triplet 1,4-biradicals of 8 and the product profiles of ketones 6, 7, and 9 are best reconciled via a unified mechanistic picture in which superposition of steric factors over varying magnitudes of O-H...O hydrogen bonding selectively facilitates a particular pathway. In particular, the diastereodifferentiation in the photochemical outcomes for the diastereomers of ketone 8 and in the lifetimes of their triplet biradicals can be understood on the basis of rapid deactivation of the 8-syn triplet biradical via fragmentation and slow cyclization of the 8-anti triplet biradical from chair- and twist-boat-like hydrogen-bonded conformations, respectively. The photolysis in polar aprotic solvents such as DMSO and pyridine was found to reverse the chemoselectivity, yielding reactivity paralleling that of ketones 3, for which the steric factors between the C2 and C3 substituents control the photochemical outcome.
STEREOSELECTIVE ALDOL CONDENSATIONS OF ORGANOTIN REAGENTS WITH ALDEHYDES
Labadie, Sharada S.,Stille, J. K.
, p. 2329 - 2336 (2007/10/02)
The reaction of the enolstannanes of cyclohexanone or propiophenone with various aldehydes under kinetic control (-78 deg C) gave predominately the threo aldols, diastereoselectivity as high as 95:5 being achived.At higher temperatures (+45 deg C) predominate erythro selectivity was observed.The enolstannane of propiophenone exists as an equilibrium mixture of O-Sn (probably the E-isomer) and C-Sn derivatives.Reaction at -78 deg C takes place rapidly with the O-Sn enolate, further reaction requiring isomerization of the C-Sn to the O-Sn enolate.The Pd catalyzed condensation of cyanomethyltributyltin with reactive aldehydes, such as nitrobenzaldehydes, took place at ambient temperatures in polar solvents to give high yields of condensation products.No reaction occured with aldehydes such as benzaldehyde.Only low stereoselectivity (10-34percent ee) was observed when (-)DIOP or (-)BPPM were utilized as chiral phosphine ligands.
DIASTEREOSELECTIVE ALDOL CONDENSATIONS OF TIN ENOLATES WITH ALDEHYDES
Shenvi, Sharada,Stille, J. K.
, p. 627 - 630 (2007/10/02)
The reaction of tin enolates of cyclohexanone or propiophenone with benzaldehyde at -78 deg C gives predominately the threo aldol diastereomer.
