Chromene-Annulated Bacteriochlorins

Syntheses and optical
properties of mono- and bis-chromene-annulated
bacteriochlorins are described. Known monochromene-annulated meso-(pentafluorophenyl)­chlorin is susceptible to a regioselective
OsO4-mediated dihydroxylation, generating two monochromene-annulated
trihydroxybacteriochlorin stereoisomers: either the newly introduced vic-cis-diol functionality is on the same side as the vic-cis-diol moiety the chromene-annulation was based on
or on the opposite side. Treatment of the two isomers with heat or
base generates different sets of bis-chromene-annulated bacteriochlorin
stereo- and regioisomers. Detailed 1D and 2D 1H and 19F NMR spectroscopic investigations allowed the characterization
of the isomers that formed. The regioselectivity of the second annulation
reaction was rationalized computationally on steric grounds. The bacteriochlorin-type
optical spectra of the mono- and bis-chromene-annulated bacteriochlorins
are modulated as a result of the annulation, with each isomer possessing
a unique spectrum, attributed to the effects the regiochemically distinct
annulations have on the conformation of the chromophore. The formation
of a bis-chromene-annulated chlorin from the bacteriochlorins is also
described, including its X-ray crystal structure, revealing some details
of the metrics of the chromene-annulated moiety. The vic-diol functionality of monochromene-annulated trihydroxybacteriochlorins
is also susceptible to oxidation and ring-expansion reactions, generating
chromene-annulated pyrrole-modified chlorins incorporating oxazolone
and morpholine moieties. The work expands the body of work on the
synthesis and optical fine-tuning of meso-aryl-substituted
bacteriochlorins.