C–N and C–P bond formation via cross dehydrative coupling reaction: an efficient synthesis of novel 3,4-dihydroquinazolines

Abstract

A new methodology has been developed to construct C–N and C–P bonds through direct coupling of C–OH and N–H/P–H bonds via dehydrative cross coupling reaction. Furthermore, this protocol offers an efficient and straightforward way to access a biologically important nitrogen-heterocycle, namely, 3,4-dihydroquinazoline utilizing either an inexpensive iron catalyst or under metal-free conditions.

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3,4-Dihydroquinazolines are an important class of nitrogen heterocycles and exist as a core structure in natural alkaloids.¹ These chromophores also display different type of biological activities such as trypanothione reductase (TryR) inhibitor,² BACE-1 inhibitor,³ T-type calcium channel blocking agent⁴ and especially as anti-cancer agents.⁵ Due to broad application possibilities, considerable efforts have been made towards the synthesis of these scaffolds.⁶ However, the problems associated with these methods are as follows: multi-step synthetic procedures, use of toxic, harmful and unstable starting materials, such as carbodiimide, isocyanate, azide, and isocyanate derivatives, which heavily impede further biological applications. Therefore, the development of operationally simple, straight forward and eco-friendly protocols using readily available, inexpensive and innocuous precursors to access these scaffolds are highly desirable.

Organic compounds containing carbon–nitrogen (C–N) and carbon–phosphorus (C–P) bonds have shown broad practical applications in medicinal chemistry,^2–5,7 material science⁸ and particularly as chiral ligands in asymmetric organic synthesis.⁹ Although numerous methods have been developed for the construction of these bonds over several years,¹⁰ the discovery of new protocols using a direct cross coupling strategy involving readily available substrates with C–H, N–H and P–H bonds are highly welcome.

In recent years, construction of carbon–carbon and carbon–heteroatom bonds through the cross dehydrative coupling approach has emerged as a fascinating and powerful tool in modern organic synthesis.¹¹ This strategy employs abundantly available and relatively inoffensive alcoholic substrates as coupling partners and avoids the use of undesirable pre-formed functionalities such as organic halides/pseudo organic halides and organometallic reagents. Other interesting feature of this reaction is the release of water (H₂O) as the only by-product, which makes this strategy more environmentally benign and atom economical. Over the past few years, number of methods involving transition metal, non-transition metal and metal-free approaches have been disclosed to couple relatively activated alcoholic pro-electrophiles bearing a sp³ C–OH bond (benzylic, propargyl and allylic alcohols) with a variety of pro-nucleophiles (Nu-H) such as alkynes,¹² alkenes,¹³ phenol,¹⁴ N,N-dimethyl aniline,¹⁵ indoles,¹⁶ active methylene compounds,¹⁷ ketones,^17a β-keto phosphonates,¹⁸ aldehydes,¹⁹ arenes and heteroarenes (Scheme 1, I).²⁰ In addition, the method is also explored to couple sp² C–OH bond of tautomerizable nitrogen heterocycles with different carbon based pro-nucleophiles (Scheme 1, II).^11a,21 However, to the best of our knowledge this strategy has not been investigated for the carbon–hetero atom bond formation.

Scheme 1 Direct coupling of C–OH with C–H/N–H/P–H bonds to generate C–C, C–N and C–P bonds via dehydrative cross coupling reaction.

In continuation of our recent efforts towards the synthesis of biologically relevant nitrogen based heterocycles through cross coupling reactions,²² we herein wish to report, for the first time, the construction of C–N and C–P bonds via a dehydrative cross coupling approach and thereby accessing structurally diverse C4-amidyl and C4-phosphonated-3,4-dihydroquinoline derivatives involving either an inexpensive iron catalyst or a metal free system (Scheme 1, III).

In our previous reports,^22c,d we have shown the formation of an α-amino hydroxide (3a) and peroxide (3aa) intermediates and its subsequent coupling with various carbon-based pro-nucleophiles (Nu-H) to generate new C–C bonds without the aid of a metal-catalyst to afford C4-substituted-3,4-dihydroquinazolines (Scheme 2). In continuation of our work, we further investigated the reaction of these intermediates 3a and 3aa with benzamide (4a) to form new C–N bond, which leads to introduction of nitrogen functionality in the C4-position of the dihydroquinazolines using 10 mol% of a copper catalyst in 2 mL DCE at 75 °C. To our delight, the desired coupled product was obtained in 20% isolated yield (Scheme 2). Interestingly, a drastic improvement in yield (85%) was observed when the reaction was performed with pure α-amino hydroxide (aminol, 3a) intermediate (ESI† for further details). Nevertheless, we further decided to optimize the reaction to achieve an optimal procedure (Table 1).

Scheme 2 Synthesis of 3,4-dihydroquinazoline derivatives via the cross coupling reaction.

Table 1 Optimization of reaction conditions^a

Entry	Catalyst	Conditions	Time (h)	Yield^b (%)
a Reaction conditions: (i) 2,3-diphenyl-3,4-dihydroquinazolin-4-ol 3a (0.25 mmol), catalyst (10 mol%), benzamide 4a (0.5 mmol), solvent (2 mL), 75 °C.b Yields based on NMR analysis of the reaction mixture.c Isolated yield.
1	Cu(OTf)2	DCE	12	>95
2	Cu(OAC)2	DCE	12	>95
3	CuBr	DCE	12	>95
4	FeCl3	DCE	12	>95
5	Fe(OAC)2	DCE	12	>95
6	FeCl2	DCE	12	>95
7	—	DCE	12	>95
8	—	DCE	2	<40
9	FeCl2	DCE	2	93^c
10	FeCl2	CH3CN	2	N.D
11	FeCl2	EtOH	2	N.D
12	FeCl2	Toulene	2	81^c

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An improvement in yield was observed when the reaction was performed for 12 hours (Table 1, entry 1). We noticed a similar catalytic activity with other copper salts (Table 1, entries 2 and 3). As the iron salts are cheap and less-toxic, further reactions were examined with iron salts. It was found that iron catalysts were also effective for this transformation (Table 1, entries 4–6). Surprisingly, the reaction proceeded even in the absence of the iron catalyst (Table 1, entry 7). To find out the role of the catalyst, two parallel reactions were performed at the shorter interval of time (2 h) (Table 1, entries 8 and 9). The reactions that was carried out with the catalyst provided higher conversion (93%, Table 1, entry 9) than the one without catalyst (<40%, Table 1, entry 8). This clearly shows the influence of the metal catalyst in enhancing the rate of the reaction. Regarding the role of solvents, no desired product was observed in CH₃CN and EtOH (Table 1, entries 10 and 11), whereas good conversion was observed when toluene was used as a solvent (Table 1, entry 12). Finally, we chose to perform the reactions at the optimized conditions of 10 mol% of FeCl₂, 2 equivalent of benzamide (4a), and 1 equivalent of aminol (3a) in 2 mL of DCE at 75 °C for 2 hours (Table 1, entry 9).

Under the optimized reaction conditions, we investigated the scope of the reaction employing various 2,3-diaryl-3,4-dihydroquinazolin-4-ols (3) and amides, and the results are summarized in Table 2. The aminol (3a) reacted smoothly with benzamide containing electron releasing and withdrawing substituents and gave the corresponding coupled products in good to excellent isolated yields (Table 2, 5a–g). In the case of hetero aromatic amide, such as nicotinamide, it coupled successfully with 3a and resulted in the corresponding product 5h in excellent yield (Table 2, 5h). In addition, the reaction was investigated to couple 2,3-aryl-3,4-dihydroquinazolin-4-ol derivatives (3) bearing electron donating and withdrawing group on the aryl ring, such as methyl, methoxy, tertiary-butyl, fluro and trifluoromethyl with benzamide, and corresponding products were obtained in good to excellent yields (Table 2, 5i–n). Interestingly, aliphatic amides namely, acetamide and propionamide were also ideal coupling partners for this new methodology and reactions with 3 provided the coupled products in the range of 68–81% isolated yields (Table 2, 5o–u). Further, the single crystal X-ray analysis of compounds 5i and 5p clearly reveals the incorporation of benzamide and acetamide groups into the quinazoline moiety (Table 2).

Table 2 Dehydrative cross coupling reaction between 2,3-diaryl-3,4-dihydroquinazolin-4-ols and amides^a,b

a Reaction conditions: (i) 2,3-diaryl-3,4-dihydroquinazolin-4-ol 3 (0.25 mmol), amide 4 (0.5 mmol, 2 equiv.), FeCl2 (10 mol%), 1,2-dichloroethane (2 mL), 75 °C, 2 h.b Isolated yields.

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After being successful in the synthesis of structurally diverse C4-amidyl-dihydroisoquinazoline derivatives through this strategy, we planned to couple 3 with other pro-nucleophile, such as dialkyl phosphite, to generate C4-phosphonated-3,4-dihydroquinoline derivatives via C–P bond formation. Under the standard reaction conditions, 3a with dimethylphosphite, resulted in the couple product 7a in high yield (>90%). However, during further investigations it was observed that this particular cross coupling works very well even in the absence of a catalyst. For example, performing the reaction at 75 °C for 2 hours in the absence of a catalyst provided almost a quantitative yield of the desired product (Table 3, 7a). This catalyst free protocol to generate C4-phosphonated-dihydroquinazoline derivatives was extended to couple 2,3-aryl-3,4-dihydroquinazolin-4-ol (3) and dialkyl/diaryl phosphate derivatives and the results are summarized in Table 3.

Table 3 Dehydrative cross coupling reaction between 2,3-diaryl-3,4-dihydroquinazolin-4-ols and phosphites^a,b

a Reaction conditions: (i) 2,3-diaryl-3,4-dihydroquinazolin-4-ol 3 (0.25 mmol), phosphite 6 (0.5 mmol, 2 equiv.), 1,2-dichloroethane (2 mL), 75 °C, 2 h.b Isolated yield.

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Dialkyl phosphites (6), such as diethyl, dibutyl and diisopropyl phosphite, were coupled with 2,3-aryl-3,4-dihydroquinazolin-4-ol derivatives (3) and the corresponding products were obtained in good to excellent isolated yields (Table 3, 7a–j). The substituent on the aryl ring of 3, did not have considerable influence over the yields of the phosphite coupled products. When diphenylphosphite was used as a phosphite variant, the yield of the corresponding product was lowered; this could be due to the steric hindrance of the phenyl group of diphenylphosphite (Table 3, 7k). Finally, the formation of the C–P bond was confirmed by single crystal X-ray analysis of compound 7e (Table 3).

Based on our^22c,d and others²³ experimental results we wish to propose a plausible mechanism for the formation of products 5 and 7 (Scheme 3). Aminols are known to exist in the thermal equilibrium with the iminium ion even in the absence of a metal catalyst and the formed iminium ion can be irreversibly trapped with pro-nucleophiles to give the amine adduct.²³ Similarly, under metal/catalyst free conditions the hydroxyl group of 3a is substituted by pro-nucleophiles, (Nu-H) such as amide (4) and phosphite (6), to afford the final products 5 and 7 through the stabilized iminium ion (In1). GC-MS analysis of the blank reaction without a nucleophile in the DCE solvent also shows the formation of iminium ion species, supporting the present mechanism.

Scheme 3 Plausible mechanism for the formation of C4-substituted-3,4-dihydroquinazolines via cross dehydrative coupling reaction.

The final products (dihydroquinazolines) are able to show stereo isomers at the C4-position, and the diastereomeric products were formed in 1 : 1 ratio in all cases, which clearly reveal that the formation of planar iminium/carbocation species (In1/In2) during the course of the reaction. In the presence of an iron catalyst [Fe], the conversion of hydroxy intermediate (3a) into the iminium ion (In2) is accelerated through heterolytic cleavage of the OH group, which is subsequently trapped with pro-nucleophiles (Nu-H).^22d,23

In conclusion, we have demonstrated an efficient and direct method for the synthesis of structurally diverse C4-amidyl and C4-phosphonyl-3,4-dihydroquinazoline derivatives via dehydrative cross coupling reaction, utilizing either an inexpensive iron catalyst or under metal-free conditions. All the products were isolated in good to excellent yields and some of these new structures were confirmed by single crystal X-ray analysis (Tables 2 and 3). Further studies are ongoing to find out the exact mechanism for the products formation, as well as to expand the scope of this method with other pro-nucleophiles.

Acknowledgements

G. S. and T.A. thank University Grants Commission (UGC) and R.A.K and A.S. thank Council of Scientific and Industrial Research (CSIR) for the award of Research Fellowships. K.R.R and S.T.L. thank GITA/CII (India) and NSC (Taiwan) for support under the India-Taiwan S&T cooperation programme.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures, ¹H, ¹³C and ³¹P NMR copy of all new compounds and crystallographic details (CCDC and CIF) of compounds 5i, 5p and 7e. CCDC 996992, 996993 and 996994. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra10203g

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