Palladium-Catalyzed Migratory Insertion of Isocyanides for Synthesis

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Palladium-Catalyzed Migratory Insertion of Isocyanides for Synthesis of C-Phosphonoketenimines Qiang Yang, Chong Li, Ming-Xing Cheng, and Shang-Dong Yang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01253 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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Palladium-Catalyzed Migratory Insertion of Isocyanides for Synthesis of C-Phosphonoketenimines Qiang Yang,† Chong Li,† Ming-Xing Cheng,† Shang-Dong Yang*†‡ †State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. China. ‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China. ABSTRACT: An efficient method for the synthesis of C-phosphonoketenimines through palladium-catalyzed migratory insertion of isocyanides has been developed for the first time. This procedure tolerates wide functional-groups and has a good atom economy. Further transformations of the products which are useful building blocks for the β-aminophosphonates, βaminovinylphosphonates and C-phosphorylated tetrazoles indicate potential synthetic utility. KEYWORDS: C-phosphonoketenimines, palladium, isocyanides, phosphonylated, synthetic method

Ketenimines are an important class of compounds as useful synthetic intermediates or synthons in chemistry. Since Staudinger reported the first ketenmines in 1920,1 much of the knowledge of these analogous compounds has been flourished within the last hundred years.2 Due to their unique properties related to the presence of an ethylene-like C-C double bond and an azomethine-like C-N double bond, ketenimines have opened a wide range of available reaction paths. It has been proved that ketenimines can undergo many useful reactions such as nucleophilic additions, radical additions, biradical cyclizations, electrocyclizations, sigmatropic rearrangements and cycloaddition reactions.2c-d,3 On the other hand, isocyanides4 which may function as a nucleophile, an electrophile and a radical are ideal reagents for the synthesis of ketenimines.2d,5 Organophosphorus compounds,6 such as phosphonates and phosphine oxines, are useful in medicinal chemistry,7 agricultural chemistry,8 materials,9 biochemistry10 and organic synthesis.11 Considering these diverse applications, development of new and efficient methods to synthesis of organophosphorus compounds has became an intensive topic of organic synthetic chemistry.12 A ketenimine bearing a phosphoryl group as a multifunctional substituent has been reported years ago.13 Compared with partial ketenimines which are difficult to isolate just as intermediates in reactions,2g,14 phosphonoketenimines are more stable and easier to control. Notably introduction of a phosphoryl group bonded to the ketenimine would be expected to improve not only the reactivity of the ketenimine but also its value as a synthetic reagent.13a However, such reports are particularly rare, especially for the transition metal catalyst reports are none. In 1979, Ohshiro and Motoyoshiya reported the first synthesis of Cphosphonoketenimines (Scheme 1, path a).13a Then an alternative preparation of C-phosphonoketenimines had been report-

ed by Kolodyazhnyi and Yakovlev in 1980 (Scheme 1, path b).13c Among these approaches suffer from limitations such as complicated procedures and narrow substrate scope. Considering the importance of ketenimines and continuing our research interest in synthesis of new-style organophosphorus compounds,15 for the first time, we report an efficient method to synthesize stable ketenimines beaing a phosphoryl group through palladium-catalyzed migratory insertion of isocyanides with α-halophosphonates and α-halophosphine oxides. Scheme 1. Methods Phosphonoketenimines

for

the

Synthesis

of

C-

The investigation was started from tert-butyl isocyanide (1a) with diethyl (bromo(phenyl)methyl)phosphonate (2aa) in the presence of 10 mol % Pd(OAc)2 and two equivalents of K2CO3 in 1,4-dioxane under argon atmosphere at 85 ºC (see the Supporting Information, Table S1, entry 1). To our delight, diethyl (2-(tert-butylimino)-1-phenylvinyl)phosphonate (4a) was isolated in 23% yield. Encouraged by this result, we further screened solvents, catalysts and bases and used different ratio of 1a and 2aa under an argon atmosphere extensively.

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Unfortunately, we had done our best to increase the yield of 4a but Table 1. Optimization of Reaction Conditionsa

a

Reaction conditions: 1a (0.2 mmol), 2ab (0.3 mmol), [Pd], base (1.4 equiv), solvent (2mL), Temperature, Ar, 30h. bIsolated yields based on 1a. c1 equivalent H2O was added. dUnder air atmosphere. en.r. = no reaction.

just got in 55% under the reaction conditions with [Pd(C3H5)Cl]2 (5 mol %), Cs2CO3 (1.4 equiv) and 1a/2aa (1.0/1.5) in 1,2-dichloroethane under Ar atmosphere at 85 ºC (see the Supporting Information, Table S1, for details). While we have done this reaction, we detected that some debrominated by-products of diethyl benzylphosphonate has also been produced simultaneously in the reaction system. This discovery prompted us to consider that the carbon bromine bond of 2aa is too active to decompose. In light of this

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initial finding, we switched 2aa to diethyl (chlro(phenyl)methyl)phosphonate (2ab) which had lower reaction activity. To our surprise, we were pleasant to get the yield of 4a in 84% (Table 1, entry 1) under existing conditions. Following other solvents such as toluene, 1,4-dioxane and CH3CN were also screened, and DCE was found to be the best choice (Table 1, entries 2-4). A variety of Pd were also tested, and results showed that Pd(C3H5)Cl2 was still the most suitable catalyst (Table 1, entries 5-7). Though Pd(dba)2 could also give the product 4a in 82%, it was extremely sensitive to the air. Changing the base to K3PO4, K2CO3 or KOAc did not have a marked effect on the reaction outcome (Table 1, entries 8-10). Reducing the loading of [Pd(C3H5)Cl]2 decreased efficiency of reaction (Table 1, entries 11-12). Decreasing the reaction temperature was also unfavorable (Table 1, entries 13). Next, we tested the influence of H2O and O2. It is noteworthy that H2O had little effect on the reaction (Table 1, entry 14).14k,16 However, when the reaction was conducted in air, we could hardly detect the product (Table 1, entry 15). What’s more, palladium and base were essential for the transformation indicated by the control experiments (Table 1, entries 16-17). Having the optimized reaction conditions in hand, the scope of various isocyanides was tested. As shown in Scheme 2, alkyl isocyanides such as tert-butyl- (1a), cyclohexyl- (1b), 1,1,3,3-tetramethylbutyl- (1c) and 1-adamantyl-isocyanide (1d) led to the desired products in moderate to good yields (55%−84%). Electron-poor aromatic isocyanide (1e) also smoothly afforded the phosphonoketenimine. Unfortunately, benzyl isocyanide (1f) could not react in our reaction under the standard conditions. Scheme 3. Synthesis of C-Phosphonoketenimines from tertButyl Isocyanide 1a and α-Halobenzylphosphonates and αHalophosphine oxides a,b

Scheme 2. Investigation of Various Isocyanides and αChlorobenzylphosphonate 2aba,b

a

Reaction conditions: 1 (0.2 mmol), 2ab (0.3 mmol), [Pd(C3H5)Cl]2 (5 mol %), Cs2CO3 (1.4 equiv), 1,2-dichloroethane (2 mL), 85 ºC, Ar, 30h. bIsolated yields of 4 based on 1.

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a

a

Reaction conditions: 1a (0.2 mmol), 2 (0.3 mmol), [Pd(C3H5)Cl]2 (5 mol %), Cs2CO3 (1.4 equiv), 1,2-dichloroethane (2 mL), 85 ºC, Ar, 30h. bIsolated yields of 4 based on 1a.

Next, we subsequently investigated a series of αchlorophosphonates and α-chlorophosphine oxides (Scheme 3). To our delight, the procedure tolerated a variety of phosphonates such as dimethyl-, diisopropyl-, dibutyl- and dibenzyl phosphonates, which all converted into the desired products (4g-4j) in good yields. αChlorobenzyldiphenylphosphine oxide and αchlorobenzyldinaphthylphosphine oxide also afforded excellent yields of the products (4k, 4l). Other phosphonates, such as diisopropyl phenylphosphinate (2ob) and chiral (1S, 2S, 5R)-(–)-menthoxylphenylphosphinate (2pb), were also compatible with the reaction and obtained the desired products (4o, 4p) in good yields. Additionally, the reaction could also be carried out with α-bromophosphonates and α-bromophosphine oxides leading the yields up to 85 % (4k). It is worth noting that α-phosphoryl benzyl chloride got much higher yields of the products than α-phosphoryl benzyl bromide except dibenzyl (halo(phenyl)methyl)phosphonate (2j). To further explore the substrate scope, we tested the scope with regard to substrates on the aromatic ring (Scheme 4). Substrates at the ortho-, meta-, and para position with a range of electron-rich and electron-poor functional groups were tolerated and afforded the desired products (5a-5m) in moderate

Reaction conditions: 1a (0.2 mmol), 3a-3p (0.3 mmol) or 3q (0.15 mmol) , [Pd(C3H5)Cl]2 (5 mol %), Cs2CO3 (1.4 equiv), 1,2dichloroethane (2 mL), 85 ºC, Ar, 30h. bIsolated yields of 5 based on 1a.

to excellent yields. Generally, the substrates with electronwithdrawing groups, such as fluor- (3c, 3k), trifluoromethyl(3f) and nitro- (3g) gave higher yields of the desired products than that of electron-donating groups (3a-b, 3h, 3j). Although chloro- or bromo substrates on aromatic ring (3d-e, 3i, 3m) can undergo oxidative addition onto Pd(0), they were tolerated and had little effect on the yields of products under our conditions. In addition, pyridine, thiophene and furan substrates (3n-3p) were also compatible with the reaction. In particular, compound 3q with two groups of α-chlorobenzylphosphonates could also run smoothly to afford the product (5q). Importantly, the reactions can also be extended to synthesize other ketenimines (scheme 5). Such as ethyl 2-chloro-2phenylacetate (6), α-benzoyl benzyl chloride (7) and (chloromethylene)dibenzene (8) afforded ketenimines (9-11) in moderate to good yields. Scheme 5. Synthesis of other keteniminesa

Scheme 4. Synthesis of C-Phosphonoketenimines from tertButyl Isocyanide 1a and Diethyl (chloro(aryl)methyl) phosphonate a,b a

Reaction conditions: 1a (0.2 mmol), 6-8 (0.3 mmol), [Pd(C3H5)Cl]2 (5 mol %), Cs2CO3 (1.4 equiv), 1,2-dichloroethane (2 mL), 85 ºC, Ar, 30h. bIsolated yields of 9-11 based on 1a.

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Since phosphonoketenimines are a significant class of reactive species and useful synthetic intermediates,5a we illustrated some synthetic utilities with our reaction product (Scheme 6). As a transformation, β-aminophosphonates which have interesting and diverse biological and biochemical properties17 could be easily obtained by hydrolysis of phosphonoketenimines (eq 1).11b Besides the obvious hydrolysis, phosphonoketenimines could also be used for nucleophilic addition reaction (eq 2).13b,13d In addition, more useful heterocycle formations could be reached using phosphonoketenimines as annulation reagents. For instance, diethyl (2-(tert-butylimino)1-phenylvinyl)-phosphonate (4a) reacted with trimethylsilyl azide in tert-butanol afforded C-phosphonylated tetrazole in excellent yield (eq 3). 5a

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which might be intermediate D tautomeric produced intermediate E. The above results evidence that this transformation possibly involving the intermediate C, D, and E. O Ph

P(OEt)2 Pd(tBuNC)2 [C-X]

Figure 1. Analytical Data of ESI-MS Scheme 8. Proposed Mechanism

Scheme 6. Transformations of C-Phosphonoketenimines

In order to explore the possible mechanism, several reactions were performed as shown in scheme 7 (see the Supporting Information, Scheme S1, for details). Benzyl, as we all know, is easy not only to occur oxidative addition by the transition metal, but also to generate benzyl radical. Thus we carried out our reactions in a series of radical-trapping experiments. Such as (1-cyclopropylvinyl)benzene (eq.1) and diethyl 2,2-diallylmalonate (eq. 2) had little effect on the yield of the 4a. These evidences indicate that the radical pathway isn’t involved in the reactions. Scheme 7. Investigation of the Reaction Mechanism

To further understand the catalytic cycle of the palladiumcatalyzed for synthesis of C-phosphonoketenimines, the model reaction was monitored by mass spectrometry experiment (Figure 1). First, the ESI/MS showed a set of signal peaks at m/z 499.1413, which corresponds to [C−X]+. The signal might mean that Pd(0)(tBuNC)2 could react with 2 by oxidative addition to form intermediate C. Another set of signal peaks at m/z 582.2149 might be the intermediate D, which resulted from the migratory insertion of 1 with intermediate C. However, we found a very weak signal peaks at m/z 416.0665,

On the basis of these results and previous reports,18 a proposed reaction mechanism was shown in Scheme 8. First, palladium(II) and isocyanide afforded palladium(0)-isocyanide intermediate B in situ. Second, intermediate B reacted with 2 to give a α-phosphonylated palladium-benzyl intermediate C by oxidative addition. Subsequently, intermediate C was smoothly converted to α-phosphonylated benzylimidoyl intermediate D by 1,1-insertion of isocyanide 1. Finally, the desired product 4 was obtained by β-hydride elimination of intermediate D and given palladium(0)-isocyanide B to recycle into the reaction. Meanwhile, intermediate D could also proceed by leaving ligands to form intermediate E, followed by β-hydride elimination to give the desired product 4 and recycle into the reaction. In summary, an effective synthesis of Cphosphonoketenimines via palladium-catalyzed migratory insertion of isocyanides has been developed for the first time. The reactions are operationally simple, tolerate wide functional groups and have a good atom economy. This research opens a new door to synthesize new ketenimines. Further transformations of the C-phosphonoketenimines also indicate potential synthetic utility.

ASSOCIATED CONTENT Supporting Information. Detailed experimental procedures, 13 spectra data for all compounds, copies of 1H, C, 31P and 19F NMR spectra (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author *[email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We are grateful for the NSFC (Nos. 21272100 and 21472076) financial support.

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