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Heike Heckroth

Publications and source records attributed to Heike Heckroth.

4 recordsLinked to original sources
PubMed2006

Alpha-carbonyl substituent effect on the lifetimes of triplet 1,4-biradicals from Norrish-Type-II reactions.

Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from alpha-heteroatom-substituted beta-branched butyrophenones 1-6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2-4 (R(alpha)=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 micros). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 micros. Photolyses of the alpha-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid C-O bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3 LYP/6-31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing alpha-oxy-substituted over alpha-amino-substituted radicals.

Journal Article↗
PubMed2003

Photofragmentation of C,N-protected alpha-amino acids: comparing tert-leucine with sulfur-containing amino acids methionine and cysteine.

The photochemical fingerprint for the N-acetyl methyl ester of the aliphatic amino acid tert-leucine 1 was investigated. This reaction path was compared with the electron transfer active amino acids methionine (N-acetyl methyl ester derivative 2a as well as the methyl ester derivative 2b) and the cysteine derivatives 3a and 3b (penicillamine derivative). Photofragmenation of the ester group dominated the photolysis of 1, whereas loss of methylmercaptane was observed for 2a and 3a. Vinylglycine derivatives 11 and 14 were formed to a minor extent. The gem-dimethylated compound 3b gave a solvent-characteristic product pattern with photoelimination products 18 and 19 in acetonitrile and loss of methylmercaptane to give 20 in methanol.

Acetonitriles↗
PubMed2002

Stereoselective synthesis of 2-aminocyclobutanols via photocyclization of alpha-amido alkylaryl ketones: mechanistic implications for the Norrish/Yang reaction.

A series of chiral N-acylated alpha-amino p-methylbutyrophenone derivatives 1a-1h was synthesized from alpha-amino acids via a three-step procedure. These substrates were photolyzed in benzene and gave Norrish II and Norrish I cleavage products as well as the N-acylated 2-aminocyclobutanols that derive from gamma-hydrogen abstraction and 1,4-triplet biradical combination (Yang cyclization). The products were formed with characteristic Yang/cleavage ratios. The quantum yields for the photodecomposition of the N-acetyl-protected substrates 1b,e,f were moderate (12-26%); the diastereoselectivities of the cyclobutanol formation were remarkably high for all substrates. High diastereospecificity was observed for the isoleucine derivatives (2S,3S)-1g and allo-(2S,3R)-1g; the Yang reaction dominated the photochemistry of allo-1g, whereas 1ggave preferentially Norrish II cleavage. The role of hydrogen bonding as one of the stereodirecting effects was demonstrated by comparison of the cyclization efficiency of the valine derivative 1e with 1h,i,j. Also, aromatic beta-keto esters gave the Yang cyclization products in low yields. The diastereoselectivity of the cyclobutanol formation was rationalized by a three-step mechanism where every step is connected with one distinct stereochemical induction mechanism: (a) diastereoselective hydrogen abstraction, (b) conformational equilibration of the 1,4-tetramethylene biradicals (as calculated by semiempiric methods) controlled by hydrogen bonding, and (c) diastereoselective biradical combination (versus cleavage) influenced by spin-orbit coupling controlled intersystem crossing geometries.

Journal Article↗