Abstract
Riley oxidation of advanced heterocyclic intermediates (dihydrodipyrrins and tetrahydrodipyrrins) is pivotal in routes to synthetic hydroporphyrins including chlorins, bacteriochlorins, and model (bacterio)chlorophylls. Such macrocycles find wide use in studies ranging from energy sciences to photomedicine. The key transformation (–CH3 → –CHO) is often inefficient, however, thereby crimping the synthesis of hydroporphyrins. The first part of the review summarizes 12 representative conditions for Riley oxidation across diverse (non-hydrodipyrrin) substrates. An interlude summarizes the proposed mechanisms and provides context concerning the nature of various selenium species other than SeO2. The second part of the review comprehensively reports the conditions and results upon Riley oxidation of 45 1-methyltetrahydrodipyrrins and 1-methyldihydrodipyrrins. A comparison of the results provides insights into the tolerable structural features for Riley oxidation of hydrodipyrrins. In general, Riley oxidation of dihydrodipyrrins has a broad scope toward substituents, but proceeds in only modest yield. Too few tetrahydrodipyrrins have been examined to draw conclusions concerning scope. New reaction conditions or approaches will be required to achieve high yields for this critical transformation in the synthesis of hydroporphyrins.
Highlights
The oxidation of active methylene groups with selenium dioxide was reported in the open literature in 1932 [1]
A comparison of the results provides insights into the tolerable structural features for Riley oxidation of hydrodipyrrins
Riley oxidation of dihydrodipyrrins has a broad scope toward substituents, but proceeds in only modest yield
Summary
The oxidation of active methylene groups with selenium dioxide was reported in the open literature in 1932 [1]. While studies of the eponymous SeO2 -mediated oxidation [1,2,3,4,5,6,7,8] and companion chemistry occupied Harry Lister Riley (1899–1986) [9] for a relatively short period, the 1932 publication [1] had perhaps the deepest impact among his more than four decades of publications. Our focus here is to comprehensively review the use of Riley oxidation in a key transformation leading to synthetic analogues of the native photosynthetic pigments. The scope of this targeted review covers the totality of all work on this topic since inception, which was in 2001
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