Bacteriochlorin syntheses - Status, problems, and exploration

Abstract

Bacteriochlorins – Nature’s near-infrared (NIR) chromophores – are distinguished by an intense ([Formula: see text] [Formula: see text]105 M[Formula: see text]cm[Formula: see text] long-wavelength absorption band in the [Formula: see text]700–1000 nm. The development of routes to prepare synthetic, tailorable bacteriochlorins holds promise for multiple disciplines where NIR-light-promoted photoactivity is of interest. A de novo route to bacteriochlorins equipped with a stabilizing gem-dimethyl group in each pyrroline ring was discovered in 2003. Continued development in this arena over 20 years has led to additional routes as well as methods to install substituents at selected positions about the perimeter of the macrocycle. The present paper reports studies that highlight substantial limitations of existing synthetic routes, including stymied access to multi-bacteriochlorin arrays and the inability to install (in a rational way) distinct groups at opposite sides of the macrocycle. The origins of the limitations are traced to particular stages of the chemistry ranging from derivatizing pyrroles, creating pyrrolines, constructing and elaborating dihydrodipyrrins, coupling dihydrodipyrrins, and forming macrocycles. Through exploration of a dozen aspects of bacteriochlorin syntheses, 60 new compounds (and nine known compounds via improved syntheses) have been prepared and characterized; the data include 20 single-crystal X-ray diffraction analyses. The research taken together points to areas of focus to fulfill the promise of this fascinating class of compounds.

Extension of nature's NIR-I chromophore into the NIR-II region

“…1(C)). Self-condensation 29 of 7 was carried out with catalysis by BF 3 ·OEt 2 at room temperature, which gave (as expected) 22 bacteriochlorin BrNp-BC as a single band (for evidence of atropisomers, see the Experimental section) in 12% yield.…”

“…The prior synthesis of annulated bacteriochlorins Phen-BC and Benz-BC relied on the approach shown in Scheme 1. 21 A bromo-substituted dihydrodipyrrin ( I ) 21 was converted via a Northern–Southern route 22 to the corresponding 2,12-dibromobacteriochlorin ( II ). For annulation, the latter was treated with the arene, which contained one bromine atom and one boronic acid moiety that together define the spacing of adjacent annulation sites.…”

“…A general challenge was to obtain sufficient 2,12-dibromobacteriochlorin ( II ) for expansive studies of reaction conditions that might ameliorate the coupling with V . An alternative route to 2,12-dibromobacteriochlorin ( II ) was explored beginning with pyrrole VI and synthon VII to obtain a bromo-substituted dihydrodipyrrin ( VIII ) for use via an Eastern–Western route, 22 but this approach was not fruitful. The synthetic approaches via II to obtain annulated bacteriochlorins were discontinued (see ESI,† Section S1).…”