Alkene-Based Radical Cascades for Oxidant-Free Aromatization
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Solution Overview
Problem
Current methods for synthesizing polyaromatic compounds face challenges in controlling reactivity and achieving efficient aromatization, particularly with alkenes, which require additional oxidizing steps and lack effective control over fragmentation processes.
Innovation Solution
The method involves chemoselective interaction of aromatic enynes with radicals derived from stannanes, utilizing 2c,3e Through-Bond interactions for β-C—C bond scission to facilitate aromatization without external oxidants, allowing alkenes to serve as synthetic equivalents of alkynes, enabling the synthesis of Sn-functionalized naphthalene derivatives and extended polyaromatics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If alkenes are used as precursors for conjugated systems, then the synthesis route is simpler and avoids external oxidants, but alkenes cyclize to products that require an extra oxidizing step for aromatization
Solution Approach 1:
The radical intermediate generated during enyne cyclization performs β-C—C bond scission on itself to achieve aromatization, eliminating the need for external oxidizing agents. The substrate's own radical center drives the fragmentation and aromatization process in a self-service manner.
Solution Approach 2:
The radical intermediate formed during cyclization acts as an intermediary species that enables both the cyclization and subsequent aromatization steps. This radical intermediary facilitates the transformation from non-aromatic to aromatic product without requiring external reagents.
2Adaptability or versatility
If traditional radical transformations are used with alkenes, then standard reagents can be applied, but control over fragmentation processes is lacking
Solution Approach 1:
The substrate is designed with specific structural features at the β-position (weak C—C bonds, radical-stabilizing groups) that create local reactivity differences. This local quality enables selective β-C—C bond scission while maintaining compatibility with standard radical reagents like Bu3SnH.
Solution Approach 2:
The reaction conditions and substrate structure are optimized to control the timing and selectivity of fragmentation. By adjusting parameters such as radical stabilizing groups and bond strength at specific positions, precise control over when and where fragmentation occurs is achieved while maintaining versatility with standard reagents.
3Object-affected harmful factors
If alkenes are used instead of alkynes, then reduced chemical functionality is achieved, but alkenes cannot serve as direct precursors for conjugated systems
Solution Approach 1:
Instead of using alkenes as reduced precursors that require oxidation to achieve aromaticity, the invention inverts the approach by using radical fragmentation to directly generate aromatic systems from alkene-based enynes. This transforms the traditional oxidation pathway into a fragmentation-driven aromatization process.
Solution Approach 2:
The enyne substrate structure serves multiple functions: it provides both the alkene and alkyne components needed for cyclization, and the alkene portion simultaneously serves as the precursor for the radical intermediate that drives aromatization. This multi-functionality allows alkenes to act as direct precursors for conjugated aromatic systems.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables efficient synthesis of Sn-functionalized naphthalene derivatives and polyaromatics by selectively promoting fragmentation and aromatization, overcoming previous limitations in using alkenes as precursors for conjugated systems.
Implementation Method 1
chemoselective interaction of aromatic enynes with radicals derived from stannanes
Implementation Method 2
the pool of four equilibrating isomeric radical intermediates is selectively depleted through the matched 5-exo trig cyclization
Implementation Method 3
utilizing 2c,3e Through-Bond interactions for β-C—C bond scission to facilitate aromatization
Implementation Method 4
Aromatization of the latter occurs via β-C—C bond scission which is facilitated by 2c,3e-Through-Bond (TB) interactions
Data Source
AI summary
Disclosed are methods for rerouting radical cascade cyclizations by using alkenes as alkyne equivalents. The reaction sequence is initiated by a novel 1,2 stannyl shift which achieves chemo- and regioselectivity in the process. The radical “hopping” leads to the formation of the radical center necessary for the sequence of selective cyclizations and fragmentations to follow. In the last step of the cascade, the elimination of a rationally designed radical leaving group via β-C—C bond scission aromatizes the product without the need for external oxidant. The Bu3Sn moiety, which is installed during the reaction sequence, allows further functionalization of the product via facile reactions with electrophiles as well as Stille and Suzuki cross-coupling reactions. This selective radical transformation opens a new approach for the controlled transformation of enynes into extended polycyclic structures of tunable dimensions.