Photoinduced radical tandem annulation of 1,7-diynes: an approach for divergent assembly of functionalized quinolin-2(1H)-ones

The first photocatalytic trichloromethyl radical-triggered annulative reactions of amide-linked 1,7-diynes with polyhalomethanes were established for the flexible assembly of functionalized quinolin-2(1H)-ones with generally acceptable yields. With the installation of the aryl group (R1) into the alkynyl moiety, C-center radical-initiated Kharasch-type addition/nucleophilic substitution/elimination cascade to produce quinolin-2(1H)-ones-incorporating gem-dihaloalkene, whereas three examples of polyhalogenated quinolin-2(1H)-ones were afforded when amide-linked 1,7-diynes bearing two terminal alkyne units were subjected to BrCX3 by exploiting dry acetonitrile as a solvent.


Introduction
Aza-heterocyclic compounds are found in a wide variety of natural drugs and biologically active molecules, many of which are pharmacologically important (Pozharskii et al., 1997;Wen et al., 2022;Zhao et al., 2023;Liu et al., 2023a;Liu et al., 2023b).Among these, quinolin-2(1H)-one and its analogs are an important class of nitrogen-containing heterocycle scaffolds and are widely encountered in a myriad of pharmaceutical molecules and synthetic compounds (Sliskovic et al., 1991;Suzuki et al., 2001;Bach et al., 2002;Kuethe et al., 2005) which display versatile biological and pharmacological activities (McQuaid et al., 1992;Michael, 1995;Peifer et al., 2008), such as P2X7 receptor antagonist, rebamipide, and MAP kinase inhibitor (Figure 1) (Maignan et al., 2016;Tan et al., 2016;Miliutina et al., 2017;Wu et al., 2020).Various synthetic strategies have been achieved to construct the skeleton of such heterocycles, organic synthesis to materials science (Rogawski, 2006;Meanwell, 2011) and can act as interesting synthetic intermediates in various chemical transformations for producing other useful molecules (Leriche et al., 2003;Okutami and Mori, 2009).Traditional approaches for the preparation of gem-dihaloalkenes include Wittig-type reactions, Julia-Kocienski reaction (Zhao et al., 2010;Chelucci, 2012;Zheng et al., 2013;Gao et al., 2015), and carbene insertion (Zeng et al., 2021) (Scheme 1A).With two geminal halogen atoms linked by an alkenyl carbon, these compounds exhibit higher reactivity for the oxidative addition of transition metal complexes than the corresponding monohaloolefins (London et al., 2014;Tian et al., 2016;Daniel et al., 2019), and the halogen atoms can be replaced by nucleophilic reagents through the additional elimination pathway (Yokota et al., 2007;Ichikawa et al., 2008).Despite significant progress in this field, the development of a new strategy for synthesizing a variety of valuable gem-dihaloalkenes remains a pressing need.To the best of our knowledge, the design and assembly of products incorporating a gem-dihaloalkene moiety and a quinolin-2(1H)-one skeleton using diynes as starting materials have not yet been reported.

Results and discussion
Initially, N-benzyl-N-(2-ethynylphenyl)-3-phenylpropiolamide 1a and CBrCl 3 2a were selected as representative substrates under the irradiation of 30 W blue LEDs to identify the reaction conditions (Table 1).With eosin Y or Mes-Acr + ClO 4 − as photocatalysts, the reaction in the presence of K 2 CO 3 in acetonitrile at room temperature did not detect the desired product 3a (entries 1-2).
Having establishing the optimal reaction conditions, we then evaluated the substrate scope and generality of an array of amidelinked 1,7-diynes for this photocatalytic radical tandem annulation toward synthesizing quinolin-2(1H)-ones bearing gem-dihaloalkenes; the results are summarized in Scheme 2. First, CBrCl 3 (2a) reacted with 1,7-diynes 1 to investigate the influence of different the electronic properties and positions of substituents in the arylalkynyl units (R 1 ), and all of them conveniently participated in the current cascade cyclization with acceptable yields.Both electron-donating (such as methyl 1b, methoxy 1c, and tert-butyl 1d) and electron-withdrawing (fluoro 1e) groups located at the para-or meta-position of the arylalkynyl moiety all performed well in this transformation, affording the corresponding gem-dichloroalkenes 3b-3e in 49%-59% yields.However, the obvious impact on steric hindrance and electronic effect was demonstrated because arylalkynyl with orthosubstituted or strong electron-withdrawing groups were suppressed during the reaction process, delivering almost no desired product.Subsequently, 1,7-diynes with different benzyl groups of nitrogen atoms could perform smoothly under standard conditions.The benzyl group bearing a functional group, including ether (o-methoxy 1f), alkyl (p-methyl 1i), and halogen (m-fluoro 1g, m-chloro 1h, p-fluoro 1j, p-chloro 1k, and p-bromo 1l), proved to be good candidates for the reaction, enabling their addition-cyclization to render the desired products 3f-3l with yields ranging from 48% to 66%.Subsequently, we chose methyl (1m and 1n) as the representative functional group to introduce the C4 or C5 position of the internal arene ring of 1,7diynes to investigate its synthesis efficiency.The corresponding products 3m-3n were isolated in 41% and 46% yields, respectively.Furthermore, for the replacement of the benzyl group with a methyl group on the nitrogen atoms, amide-tethered 1,7-diynes 1o was a good reaction analog, giving the product 3o with a yield of 59%.Similarly, the substrate scope of this method was further assessed by taking advantage of CBr 4 as the gem-dibromination reagent for assembling gem-dibromovinyl-incorporating quinolin-2(1H)-ones.Frontiers in Chemistry frontiersin.org We found that 1,7-diynes 1 with varied substitution patterns could effectively take part in the current system, furnishing corresponding products 3p-3s in 48%-65% yields.Unfortunately, N-unprotected amide-linked 1,7-diyne 1p and ester-linked 1,7-diyne 1q did not yield desired products.In addition, the preformed substrate 1r with two internal alkyne moieties was an unreactive reactant under standard conditions, and 1,7-diyne 1r was recovered, showing that terminal alkynes on starting material play an important role in this transformation.
To further expand the range of substrates for this transformation, amide-linked 1,7-diynes with two terminal alkynyl moieties 1s were subjected to the reaction of CBrCl 3 under the above optimal conditions, but the reaction was completely suppressed.Surprisingly, the reaction can proceed smoothly in the presence of dry acetonitrile, and the unprecedented polyhalogenated quinolin-2(1H)-ones 4a was obtained in 54% yield via 1,5-(SN″)-substitution (Scheme 3A).Furthermore, a moderate chemical yield was observed for the 1,7-diynes with a methyl group located at the 5-position of the internal arene ring 1t for the assembly of the polyhalogenated

Mechanistic experiments (A-C).
Frontiers in Chemistry frontiersin.orgproducts 4b-4c (Scheme 3B).The structures of densely functionalized quinolin-2(1H)-ones 3 and 4 were fully characterized by their NMR spectroscopy and HRMS date, and two cases of 3a and 4a were confirmed by X-ray diffraction analysis (see Supplementary Material).The gram-scale experiments for the preparation of 3a on a 4.0 mmol scale were conducted under optimal conditions, and the product was delivered with a comparable yield (59%, Scheme 4A).The practicality of this methodology was further studied through the synthetic application of products.For example, the double nucleophilic vinylic substitution reaction 3a and p-toluenethiol proceeded smoothly by means of sodium hydride as base, which led to the product 5 in 81% yield (Scheme 4B) (Jiang et al., 2017).
Several control experiments were performed to gain insights into the reaction pathway mechanism.First, the use of a radical inhibitor TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) successfully suppressed the reaction process, and the result confirmed that a trichloromethyl radical may be involved in these transformations (Scheme 5A).Next, the reaction occurred in the presence of H 2 O 18 , and the product containing O 18 was isolated in 54% yield and identified by HR-MS (Scheme 5B).In addition, when dry CH 3 CN was employed as a solvent under standard conditions, the reaction progress was completely inhibited (Scheme 5C).These two survey results showed that the oxygen source of the carbonyl group in target products comes from water.Finally, several fluorescence quenching experiments indicated that CBrCl 3 2a was a more efficient quencher of the excited state of fac-Ir(ppy) 3 * than 1,7-diyne 1a (Figure 2).
In light of these findings and previous related works (Wang et al., 2021;Wu et al., 2021;Zheng et al., 2021), we propose a plausible mechanism for this photo-catalyzed annulation of 1,7diynes, as shown in Scheme 6.The photocatalytic cycle was initiated by the activation of Ir(III) with blue light irradiation to form the excited state Ir(III)* species, which reacts with BrCCl 3 to yield trichloromethyl radical A and a bromine anion, together with Ir(IV) complex via a single electron transfer (SET).Next, the Stern-Volmer analysis for fac-Ir(ppy) 3 with 1a and BrCCl 3 2a.

SCHEME 6
Plausible reaction pathway for forming 3 and 4.
radical A can be trapped by the terminal carbon-carbon triple bond of 1,7-diyne 1 to provide the alkenyl radical B, which undergoes 6exo-dig cyclization to give intermediate C. The resulting bromine anion was oxidized by Ir(IV) complex to produce Br radical (Bacauanu et al., 2018;Wang et al., 2019), followed by radical cross coupling with C to obtain intermediate D and regenerate Ir(III) species.Subsequently, the intermediate D reacts with OH − from H 2 O to afford the intermediate E through 1,5-(S N ″)substitution, which eliminates one molecule of HBr to assemble the desired product 3 (path i).The latter process, different from the above, undergoes 1,5-(S N ″)-nucleophilic substitution with excess Br − in the photocatalytic system to give polyhalogenated products 4 (path ii).
SCHEME 2Substrate scope for synthesizing product 3 SCHEME 4Scaled-up preparation (A) and product transformation (B).

TABLE 1
Optimization conditions for forming 3a a .