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Skeletal ring enlargement is gaining renewed interest in synthetic chemistry and has recently focused on insertion of one or two atoms. Strategies for heterocyclic expansion through small-ring insertion remain elusive, although they would lead to the efficient formation of bicyclic products. Here, we report a photoinduced dearomative ring enlargement of thiophenes by insertion of bicyclo[1.1.0]butanes to produce eight-membered bicyclic rings under mild conditions. The synthetic value, broad functional-group compatibility, and excellent chemo- and regioselectivity were demonstrated by scope evaluation and product derivatization. Experimental and computational studies point toward a photoredox-induced radical pathway.
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Dimeric boroles: effective sources of monomeric boroles for heterocycle synthesis
Monomeric boroles have been gaining attention as reagents for the synthesis of heterocycles due to their ability to insert atoms into the BC 4 ring in a single step. Although unique boron frameworks can be accessed via this methodology, the products feature aryl substitution on the carbon centers as steric bulk is required to preclude borole dimerization. This work demonstrates that insertion chemistry is possible with Diels–Alder dimeric boroles and that such reactivity is not exclusive to monomeric boroles with bulky groups. With 1-phenyl-2,3,4,5-tetramethylborole dimer, the formal 1,1-insertion of a nitrene and sulfur generate the six-membered aromatic 1,2-azaborine and 1,2-thiaborine, respectively. The isolation of the 1,2-thiaborine enabled the synthesis of an η 6 -chromium complex. Benzophenone and diphenylketene readily insert a CO unit to generate BOC 5 seven-membered rings confirming dimeric boroles can serve as monomeric synthons in 1,2-insertion reactions. An epoxide did not furnish the anticipated eight-membered BOC 6 ring, instead provided a bicyclic system with a BOC 3 ring. The insertion chemistry was demonstrated with two other borole dimers featuring different substitution with diphenylketene as a substrate. This work elevates borole insertion chemistry to a new level to access products that do not require bulky substitution.
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- Award ID(s):
- 1753025
- PAR ID:
- 10147245
- Date Published:
- Journal Name:
- Chemical Science
- Volume:
- 11
- Issue:
- 1
- ISSN:
- 2041-6520
- Page Range / eLocation ID:
- 126 to 131
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9sc04053f
