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Sep 26, 2014 - Depalladation of Neutral Monoalkyne- and Dialkyne-Inserted. Palladacycles and Alkyne Insertion/Depalladation Reactions of. Cationic ...
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Depalladation of Neutral Monoalkyne- and Dialkyne-Inserted Palladacycles and Alkyne Insertion/Depalladation Reactions of Cationic Palladacycles Derived from N,N′,N″‑Triarylguanidines as Facile Routes for Guanidine-Containing Heterocycles/Carbocycles: Synthetic, Structural, and Mechanistic Aspects Priya Saxena,† Natesan Thirupathi,*,† and Munirathinam Nethaji‡ †

Department of Chemistry, University of Delhi, Delhi 110 007, India Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India

‡

S Supporting Information *

ABSTRACT: Depalladation of the monoalkyne-inserted cyclopalladated guanidines [κ2(C,N)Pd(2,6-Me2C5H3N)Br] (I and II) in PhCl under reflux conditions and that of the dialkyne-inserted cyclopalladated guanidine [κ2(C,N):η2(CC)PdBr] (III) in pyridine under reflux conditions afforded a guanidine-containing indole (1), imidazoindole (2), and benzazepine (3) in 80%, 67%, and 76%, yields, respectively. trans-[L2PdBr2] species (L = 2,6Me2C5H3N, C5H5N) were also isolated in the aforementioned reactions in 35%, 42%, and 40% yields. Further, the reaction of the cyclopalladated guanidine [κ2(C,N)Pd(μ-Br)]2 (IV) with AgBF4 in a CH2Cl2/MeCN mixture afforded the cationic pincer type cyclopalladated guanidine [κ3(C,N,O)Pd(MeCN)][BF4] (4) in 85% yield and this palladacycle upon crystallization in MeCN and the reaction of [κ2(C,N)Pd(μ-Br)]2 (V) with AgBF4 in a CH2Cl2/MeCN mixture afforded the cationic palladacycles [{κ2(C,N)Pd(MeCN)2][BF4] (5 and 6) in 89% and 91% yields, respectively. The separate reactions of 4 with 2 equiv of methyl phenylpropiolate (MPP) or diphenylacetylene (DPA) and the reaction of 5 with 2 equiv of MPP in PhCl at 110 °C afforded the guanidine-containing quinazolinium tetrafluoroborate 7 in 25−32% yields. The reaction of 6 with 2 equiv of DPA under otherwise identical conditions afforded the unsymmetrically substituted guanidinium tetrafluoroborate 8, containing a highly substituted naphthalene unit, in 82% yield. Compounds 1−8 were characterized by analytical and spectroscopic techniques, and all compounds except 4 were characterized by single-crystal X-ray diffraction. The molecular structures of 2 and 3 are novel, as the framework in the former arises due to the formation of two C−N bonds upon depalladation while the butadienyl unit in the latter revealed cis,cis stereochemistry, a feature unprecedented in alkyne insertion chemistry. Plausible pathways for the formation of heterocycles/carbocycles are proposed. The influence of substituents on the aryl rings of the cyclopalladated guanidine moiety and those on alkynes upon the nature of the products is addressed. Heterocycles 1 and 7 revealed the presence of two rotamers in about a 1.00:0.43 ratio in CDCl3 and in about a 1.00:0.14 ratio in CD3OD, respectively, as detected by 1H NMR spectroscopy while in CD3CN and DMSO-d6 (1) and CD3CN and CDCl3 (7), these heterocycles revealed the presence of a single rotamer. These spectral features are attributed to the restricted C−N single-bond rotation of the CN3 unit of the guanidine moiety, which possibly arises from steric constraint due to the formation of a N−H···Cl hydrogen bond with CDCl3 (1) and N−H···O and O−D···O hydrogen bonds with CD3OD (7).

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substituents.6−9 The ring size of the “[κ2(C,N)Pd]” unit and the steric/electronic properties of the C,N-chelate in this unit and steric and electronic properties of the alkynes regulate the mode and degree of insertion and course of depalladation and thus the frameworks of the resulting organic products. A variety of alkyne-inserted [C,N] palladacycles that contain amino, imino, pyridyl, and azo functionalities have been subjected to the insertion−depalladation sequence, and from these reactions, numerous hetero- and carbocycles have been

INTRODUCTION 1,1- and 1,2-insertion reactions of [C,N] palladacycles with small molecules such as CO, isocyanide, allenes, alkenes, alkynes, and arynes are some of the most important methodologies in organometallic/organic chemistry for the synthesis of a variety of hetero- and carbocycles through a stoichiometric insertion and depalladation sequence.1−5 The hetero- and carbocycles prepared through a small molecule insertion−depalladation sequence are difficult to prepare through other routes. The alkyne insertion of [C,N] palladacycles is the most widely studied insertion reaction, as alkynes can act as electrophiles or nucleophiles depending upon the © XXXX American Chemical Society

Received: August 18, 2014

A

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isolated.10−14 The palladium-catalyzed pathway involving alkyne and amine, imine, or amide partners that leads to the formation of hetero- and carbocycles is also a rapidly growing field.15−18 The structural and mechanistic information obtained from the stoichiometric alkyne insertion−depalladation sequence is suggested to pave the way to a better understanding of the mechanism of formation of hetero- and carbocycles in palladium-catalyzed reactions.16f Depalladation of monoalkyne- and dialkyne-inserted [C,N] palladacycles has been reported to form hetero- and carbocycles more frequently than that of trialkyne-inserted [C,N] palladacycles.6−9 Palladium-mediated synthesis of heterocycles and carbocycles was achieved by two different routes. In the first route, monoalkyne- and dialkyne-inserted palladacycles were depalladated at relatively high temperature, resulting in the formation of hetero- and carbocycles through reductive elimination of palladium accompanied by C−N bond formation involving the coordinated nitrogen and carbon atoms. In the second relatively milder route, alkyne insertion was performed on more reactive precursors, namely [κ2(C,N)PdL2]+[WCA]− and [κ2(C,N)Pd(μ-I)]2 (C,N = cyclopalladated amine, imine, pyridine; L = Lewis base; WCA = weakly coordinating anion) to afford alkyne-inserted palladacycles in situ, which subsequently undergo depalladation to afford hetero- and carbocycles as outlined in the first route.6−9 Recently, we studied alkyne insertion reactions of cyclopalladated guanidines of the types [κ2(C,N)Pd(μ-Br)]2 and [κ2(C,N)Pd(Lewis base)Br] to afford a variety of alkyneinserted palladacycles with unanticipated frameworks, mainly due to ring contraction cum amine−imine tautomerization of alkyne-inserted palladacyclic intermediates in conjunction with the nature of the alkyne, coordinated anion, and other reaction conditions.19,20 We have chosen the monoalkyne- and dialkyneinserted cyclopalladated guanidines I−III for depalladation reactions following the first route, as such an attempt would afford guanidine-containing heterocycles/carbocycles (see Chart 1).20 We have also used the cyclopalladated guanidines

cyclic chemistry.24,25 Line drawings of a few medicinally important guanidine-containing heterocycles are shown in Chart 2.24a,b Herein, we report the synthesis and characterChart 2

ization of three new cationic cyclopalladated guanidines, four new heterocycles, and one carbocycle. Line drawings of the new compounds 1−8 are shown in Chart 3. The determination of the molecular structures of 1−3, 5−7, and 8·1.5H2O by singlecrystal X-ray diffraction in conjunction with the knowledge gained from alkyne insertion chemistry from the literature allowed us to propose plausible mechanisms of the formation of guanidine-containing heterocycles and the carbocycle.

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RESULTS AND DISCUSSION Syntheses. During the low-yield (23%) synthesis of I from the insertion reaction of the 2,6-lutidine-coordinated monomeric analogue of V with MPP in CH2Cl2 under ambient conditions, we observed the formation of the adduct trans[(2,6-Me2C5H3N)2PdBr2] in 36% yield.20 We speculated that the low yield of I and the formation of the adduct are due to spontaneous depalladation of I, and this information provided a clue that palladacycle I could serve as a suitable precursor for the heterocycle 1. Hence, palladacycle I was subjected to depalladation in PhCl under reflux conditions for 10 h to afford the indole 1 in 80% yield along with the formation of trans[(2,6-Me2C5H3N)2PdBr2] in 35% yield. Richeson and coworkers synthesized a guanidine-containing indole from the guanylation reaction of indole with N,N′-diisopropylcarbodiimide in the presence of catalytic amounts of LiN(SiMe3)2/ TMEDA.26 Palladacycle II was also subjected to depalladation in PhCl under reflux conditions for 4.5 h, and the resulting mixture was subjected to column chromatography to afford the imidazoindole 2 in 67% yield along with the formation of trans-[(2,6Me2C5H3N)2PdBr2] in 42% yield and N,N′-bis(o-anisyl)urea27 (β form) in 16% yield. The formation of an imidazoindole derivative such as 2 through depalladation of a monoalkyneinserted [C,N] palladacycle is unprecedented in the literature. The diinserted palladacycle III was depalladated in pyridine under reflux conditions for 5 h, and the resulting reaction mixture upon workup afforded the benzazepine derivative 3 in 76% yield along with the formation of trans-[(C5H5N)2PdBr2] in 40% yield. Upon perusal of the synthetic routes reported for benzazepine derivatives,16g,28 we conclude that the synthetic route reported herein for 3 is an elegant, reliable route for guanidine-containing benzazepine. The successful isolation of heterocycles 1−3 following the first route prompted us to explore the second route in order to understand whether such a strategy would afford hetero- and

Chart 1

IV and V as starting materials for more reactive substrates of the type [κ2(C,N)PdL2]+[WCA]−, which would enable us to isolate guanidine-containing hetero- and carbocycles through the second route, as outlined above (see Chart 1).21,22 Nitrogen-containing heterocycles are medicinally important compounds,23 and more importantly, guanidine-containing heterocycles are considered as privileged scaffolds in heteroB

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Chart 3

carbocycles with distinct frameworks. Hence, palladacycle IV was treated with AgBF4 in a CH2Cl2/MeCN mixture to afford a solid, which upon crystallization in a CH2Cl2/n-hexane mixture afforded the cationic pincer type palladacycle 4 in 85% yield. Palladacycle 4 upon crystallization in MeCN afforded 5 in 89% yield. Alternatively, the reaction of IV with AgBF4 in a CH2Cl2/ MeCN mixture and subsequent crystallization of the resulting product from the same solvent combination afforded 5 in 90% yield. Analogously, the reaction of V with AgBF4 in a CH2Cl2/ MeCN mixture readily afforded 6 in 91% yield. The separate reactions of 4 with 2 equiv of MPP and DPA in PhCl under reflux conditions for 5 h and the reaction of 5 with 2 equiv of MPP under the aforementioned conditions followed by workup of the reaction mixture afforded the quinazolinium derivative 7 in 25−32% yields. The reaction of 6 with 2 equiv of DPA in PhCl under reflux conditions for 3 h afforded the annulated guanidinium salt 8 in 82% yield. Molecular Structures. The molecular structures of 1−3, 5−7, 8·1.5H2O, and N,N′-bis(o-anisyl)urea27a were determined by single crystal X-ray diffraction and are shown in Figures 1−6 and Figure S1 in the Supporting Information. There are two molecules in an asymmetric unit of 1, 2, and 5 but the structural features are discussed only for one molecule. The molecular structure of 1 consists of an indole ring wherein the nitrogen atom of the ring is simultaneously a part of the guanidine unit. Both the amino nitrogen atoms and the central carbon atom of the guanidine unit are planar. In 1, the C1−N1 distance 1.441(3) Å is longer than but the C1−N3 distance 1.370(3) Å is comparable with the C−N single-bond distances of the CN3 unit of N,N′,N′′-tri(2-tolyl)guanidine, LH22‑tolyl (1.377(4) and 1.380(4) Å29), and the C1−N2 distance 1.263(3) Å in the former is shorter than that reported for the latter (1.282(4) Å). The lone pair on N1 is involved in π delocalization of the indole ring to attain the aromatic character, and hence the lone pair is not available for n−π conjugation with the CN π* orbital of the guanidine unit, which explains the trend in bond distances observed for the CN3 core. The molecular structure of 2 consists of a dihydroimidazo[1,5-a]indole framework wherein both the amino nitrogen atoms of the guanidine unit are part of the dihydroimidazole ring while the imino nitrogen atom is not. Interestingly, the carbon atom C23 of the alkenyl unit of the central ring bears one methyl ester substituent, whereas the second ester group

Figure 1. ORTEP representation of 1 at the 50% probability level. Hydrogen atoms, except that on the amino nitrogen atom, are removed for clarity. Selected bond distances (Å) and angles (deg): C(1)−N(1) = 1.441(3), C(1)−N(2) = 1.263(3), C(1)−N(3) = 1.370(3); N(1)−C(1)−N(2) = 125.8(2), N(2)−C(1)−N(3) = 124.1(2), N(1)−C(1)−N(3) = 110.2(2).

which was present in II has lost the OMe unit in the course of the formation of the dihydroimidazole ring in 2. In 2, both the C1−N1 distance 1.414(4) Å and the C1−N3 distance 1.417(4) Å are longer than the C−N single-bond distances of the CN3 unit of N,N′,N′′-tris(2-anisyl)guanidine, LH22‑anisyl (1.372(4) and 1.384(4) Å29), and the C1−N2 distance 1.249(4) Å in the former is shorter than that reported for the latter (1.272(4) Å). The lone pair on N3 is involved in π-electron delocalization of the indole ring, and that on N1 is involved in an interaction with the CO π* orbital and π* orbitals of the o-anisyl ring to which it is attached. Hence, n−π conjugation involving the lone pair of the amino nitrogen atoms with the CN π* orbital of the guanidine unit is minimal. The molecular structure of 3 consists of a benzo[b]azepine core wherein one of the amino nitrogen atoms of the guanidine C

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the aromatic system in 1 and 2 but they are not in 3. The seven-membered heterocyclic ring in 3 revealed a pseudo-boat conformation with C16, C17, C33, and C36 forming the basal plane (mean deviation 0.0377 Å), while C23, C26, and N1 lie at 0.656(3), 0.728(3), and 0.663(2) Å, respectively, out of the basal plane (see Figure 7). The angle between the mean plane formed by the basal atoms of the boat and the CN3 unit of the guanidine moiety is 70.42(8)°. To the best of our knowledge, 3 represents the first crystallographically characterized heterocycle that possesses a butadienyl unit with cis,cis stereochemistry obtained through the depalladation route (see later). Diinserted [C,N] palladacycles upon depalladation or the reaction of cyclopalldated [κ2(C,N)PdL2]+[WCA]− with 2 equiv of alkynes are, on the basis of the literature, anticipated to afford (i) heterocycles wherein the butadienyl unit possesses cis,cis stereochemistry, (ii) heterocycles with a cyclobutene unit11a,12a (see structures A and B in Chart 4), and (iii) heterocycles with unanticipated frameworks.10g,11g The heterocycles A and B are considered as valence isomers; the thermal conversion of the latter to the former28a,30 is known in the literature, and thermal conversion of the former to the latter has been proposed in the literature.12a The formation of a heterocycle such as 3 which possesses the framework A in the depalladation reaction of diinserted [C,N] palladacycles has never been reported in the literature, although Pfeffer and coworkers erroneously claimed the formation of azacyclooctatriene from depalladation of a diinserted [C,N] palladacycle;12a later this compound turned out to be a benzofulvene derivative, as has been proven by Heck and co-workers through a structural determination.10e Thus, 3 represents the first heterocycle formed through a dialkyne insertion−depalladation sequence leading to C−N bond formation with the [C,N] chelate unit remaining intact during the depalladation reaction. In 2013, the molecular structures of two benzazepines obtained through entirely different routes were reported.16g Further, to the best of our knowledge, there exists no report of structural characterization of indole, imidazoindole, and benzo[b]azepine derivatives containing a guanidine unit. The noncovalent interactions present in the crystal lattices of 1−3 are shown in Figures S2−S6 in the Supporting Information. The palladium atom in 5 and 6 is surrounded by the imino nitrogen atom, the aryl carbon atom, and the nitrogen atoms of two MeCN groups and thus revealed a distorted-square-planar geometry (mean deviations 0.0284 Å (5) and 0.0220 Å (6)). As anticipated, the Pd1−N5 distance 2.136(6) Å in 5 and the corresponding distance in 6 (2.140(4) Å) are longer than the Pd1−N4 distances in 5 (2.063(8) Å) and 6 (2.006(3) Å), owing to greater trans influence of the aryl carbon. The degree of n−π conjugation involving the lone pair of the amino nitrogen atoms with the CN π* orbital of the guanidine unit can be estimated from the values of ΔCN and ΔCN′, which have been defined in our previous publication.21 The values of ΔCN and ΔCN′ in 5 (ΔCN′ = 0.158(11) Å; ΔCN = −0.029(12) Å) suggest a greater extent of n−π conjugation involving the lone pair of the endocyclic amino nitrogen atom with the CN π* orbital of the guanidine unit than that involves the lone pair of the exocyclic amino nitrogen atom. In contrast, the values of ΔCN and ΔCN in 6 (ΔCN′ = 0.054(8) Å; ΔCN = 0.054(7) Å) are comparable with each other. It should be noted that two crystallographically distinct molecules in 5 are linked to each other via a pair of N−H···π interactions and, to improve this interaction, the exocyclic N(H)Ar unit could have twisted (see Figure S7 in the Supporting Information). This could result in a

Figure 2. ORTEP representation of 2 at the 50% probability level. Hydrogen atoms are removed for clarity. Selected bond distances (Å) and angles (deg): C(1)−N(1) = 1.414(4), C(1)−N(2) = 1.249(4), C(1)−N(3) = 1.417(4); N(1)−C(1)−N(2) = 131.8(3), N(2)− C(1)−N(3) = 125.0(3), N(1)−C(1)−N(3) = 103.2(3).

Figure 3. ORTEP representation of 3 at the 50% probability level. Hydrogen atoms, except that on the amino nitrogen atom, are removed for clarity. Selected bond distances (Å) and angles (deg): C(1)−N(1) = 1.387(2), C(1)−N(2) = 1.281(2), C(1)−N(3) = 1.375(2); N(1)−C(1)−N(2) = 117.6(1), N(2)−C(1)−N(3) = 131.1(2), N(1)−C(1)−N(3) = 111.3(1).

moiety is part of the seven-membered heterocyclic ring. The locations of the methyl ester and phenyl substituents on the butadienyl part of the seven-membered heterocyclic ring in 3 are identical with those found in the butadienyl part of its precursor III. Interestingly, the butadienyl unit in 3 revealed cis,cis stereochemistry, while that in III revealed cis,trans stereochemistry. The C−N distances of the CN3 unit in 3 are comparable with those reported for LH22‑anisyl.29 These features as opposed to those discussed for 1 and 2, are attributed to the fact that amino nitrogen atom(s) are part of D

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Figure 4. ORTEP representations of the cations of 5 (a) and 6 (b) at the 50% probability level. Hydrogen atoms, except those on the amino nitrogen atoms and the anion, are removed for clarity. Selected bond distances (Å) and angles (deg) for 5/6: Pd(1)−N(1) = 1.981(6)/2.014(3), Pd(1)−N(4) = 2.063(8)/2.006(3), Pd(1)−N(5) = 2.136(6)/2.140(4), Pd(1)−C(17) = 1.968(9)/1.982(4), C(1)−N(1) = 1.296(9)/1.297(5), C(1)−N(2) = 1.454(7)/1.351(6), C(1)−N(3) = 1.267(8)/1.351(5); C(17)−Pd(1)−N(4) = 95.3(3)/90.6(2), N(4)−Pd(1)−N(5) = 88.2(3)/ 86.8(1), N(1)−Pd(1)−N(5) = 91.9(2)/92.9(1), C(17)−Pd(1)−N(1) = 84.6(3)/89.8(2), C(17)−Pd(1)−N(5) = 174.9(3)/177.2(2), N(1)− Pd(1)−N(4) = 179.2(4)/178.2(2).

Figure 5. ORTEP representation of the cation of 7 at the 50% probability level. Hydrogen atoms, except those on the amino nitrogen atom and the anion, are removed for clarity. Selected bond distances (Å) and angles (deg): C(1)−N(1) = 1.407(4), C(1)−N(2) = 1.341(4), C(1)−N(3) = 1.302(4); N(1)−C(1)−N(2) = 115.5(3), N(1)−C(1)−N(3) = 122.2(3), N(2)−C(1)−N(3) = 122.3(3). Figure 6. ORTEP representation of the cation of 8·1.5H2O at the 50% probability level. Hydrogen atoms, except those on the amino nitrogen atoms, water molecule, and the anion, are removed for clarity. Selected bond distances (Å) and angles (deg): C(1)−N(1) = 1.334(3), C(1)− N(2) = 1.337(3), C(1)−N(3) = 1.336(3); N(1)−C(1)−N(2) = 122.2(2), N(2)−C(1)−N(3) = 119.8(2), N(1)−C(1)−N(3) = 117.9(2).

poorer n−π conjugation involving the exocyclic amino nitrogen atom with the CN π* orbital of the guanidine unit. The molecular structure of 7 consists of a quinazolinium cation and a BF4− anion. Interestingly, the exocyclic N(H)Ar unit is coplanar with the quinazolinium ring (mean deviation 0.1034 Å), while the phenyl ring attached to C23 makes an angle of 80.66(9)° and the o-anisyl ring attached to N1 makes an angle of 78.00(9)°. The C1−N1 distance 1.407(4) Å is longer than the C1−N2 distance 1.341(4) Å, due to n−π conjugation only from the exocyclic amino nitrogen atom with the CN π* orbital involving the C1−N3 bond, as there is no lone pair on N1. The salt 8·1.5H2O is nothing but an unsymmetrically substituted guanidinium cation with a BF4− anion. Two of

the nitrogen atoms of the guanidinium cation are linked to the o-tolyl substituent, while the third nitrogen atom is linked to a highly substituted naphthalene ring. The CN3 unit is planar, as anticipated for a charge-delocalized guanidinium cation with comparable C−N distances. The formation of highly substituted naphthalene derivatives through dialkyne insertion E

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also been proposed to explain the presence of two isomers for a related 2-substituted amide-based dihydroindole derivative.31 Surprisingly, the 1H NMR spectra of 1 in CD3CN and DMSO-d6 revealed the presence of only one species (see the Experimental Section). The NH proton of the guanidine moiety in 1 can form a hydrogen bond with all three of the aforementioned deuterated solvents. However, the C−N singlebond rotation is restricted only in CDCl3, possibly due to its greater steric bulk. Factors such as Lewis basicity, dielectric constant, and steric bulk of the solvent have been shown to affect the relative population of rotamers.32,33 Thus, two rotamers are observed in CDCl3, while only one is observed in CD3CN and DMSO-d6. The 1H NMR spectrum of 4 revealed one singlet at δ 2.55 assignable to CH3 protons of MeCN and three downfield shifted singlets at δ 3.78, 3.87, and 4.24 assignable to OCH3 protons of the guanidine unit. The estimated ratio of OCH3:CH3 protons of 3:1 clearly indicated a pincer type framework for the cation of 4. Moreover, the 13C{1H} NMR spectrum of 4 revealed one singlet at δ 3.3 assignable to the CH3 carbon of MeCN and three singlets at δ 56.0, 56.2, and 60.1 assignable to OCH3 carbons of the guanidine unit, which further supported the pincer type framework for the cation of 4. Previously, we identified the neutral pincer type palladacycle [Pd{κ3 (C,N,O)-C 6 H 3 (OMe)-3(NHC(NHAr)(NAr))-2}(OAc)] as one of the species formed through the neighboring group participation of the OMe group in the NAr unit of the acetate analogue of IV through variable-temperature 1H NMR measurements.21 The 1H NMR spectrum of 5 revealed two singlets at δ 2.00 and 2.51 assignable to CH3 protons of two distinct MeCN groups and three downfield-shifted singlets at δ 3.77, 3.87, and 4.18 assignable to OCH3 protons of three distinct anisyl substituents of the guanidine moiety. The estimated ratio of OCH3/CH3 protons of 3/2 clearly indicated a chelate type framework for the cation of 5. Moreover, the 13C{1H} NMR spectrum of 5 revealed two singlets at δ 1.70 and 3.17 assignable to the CH3 carbon of two distinct MeCN groups and three singlets at δ 55.87, 55.92, and 57.81 assignable to OCH3 carbons of the guanidine unit, and this spectral feature further supported a chelate type framework for the cation of 5. The 1H and 13C{1H} NMR spectra of 6 indicated a chelate type framework such as that present in 5. 1 H NMR spectra of 7 in CDCl3 and CD3CN revealed the presence of one species, while that in CD3OD revealed the presence of two isomers in about a 1.00:0.14 ratio. Further, the presence of two isomers of 7 in CD3OD was supported by 13 C{1H} NMR spectroscopy. The observed spectral behavior of 7 is possibly ascribed to the ability of CD3OD to form intermolecular N−H···O and O−D···O hydrogen bonds with the proton of the N(H)Ar moiety and the oxygen atom of the o-OMe group of the anisyl ring of the same moiety. As a result, the C−N(H)Ar single bond of the CN3 unit restrictively rotates on the NMR time scale, resulting in the formation of two rotamers, namely E and F, as shown in Scheme 2. Mechanistic Aspects of Depalladation. The formation of 1 involves C−N bond formation through nucleophilic attack of the imino nitrogen atom on the palladated carbon followed by reductive elimination of palladium in the form of Pd0L accompanied by the loss of HBr. Oxidative addition of HBr to Pd0L species could form “PdHBr(L)” species, and this hydrido species subsequently recombines with itself to form trans[L2PdBr2] (L = 2,6-Me2C5H3N), Pd(0), and H2, as discussed

Figure 7. Selected portion of the molecular structure of 3 illustrating the pseudo-boat conformation of the seven-membered ring.

Chart 4

of [C,N] palladacycles followed by depalladation of the resulting product has been reported earlier,10a−c,e−h,11f and two such carbocycles have been structurally characterized.10a,11f The noncovalent interactions present in the crystal lattices of 6, 7 and 8·1.5H2O are shown in Figures S8−S10 in the Supporting Information. NMR Studies. The 1H NMR spectrum of 1 in CDCl3 revealed the presence of two isomers in about a 1.00:0.43 ratio, and this ratio was found to be independent of the concentration. Further, the presence of two isomers of 1 in CDCl3 was supported by 13C{1H} NMR spectroscopy. The two solution species are ascribed to the presence of two rotamers, C and D, which interconvert via C−Nindole single-bond rotation of the CN3 unit, as this bond is longer than the remaining two C− N bonds (see above and also Scheme 1). In an ideal Scheme 1

representation, in the major rotamer indicated as C, the C NAr unit and the phenyl group on the pyrrole ring are located on the same side, while in the minor isomer indicated as D, the aforementioned units are located on the opposite side. In solution, the C−N bond rotation may be restricted due to steric constraints. Such a C−N single-bond rotation in solution has F

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Scheme 2

Scheme 4

by Vicente and co-workers34 and subsequently by us.20 The formation of 2 from II is believed to occur in two steps, as shown in Scheme 3. The first step involves the formation of the Scheme 3

the formation of a diradical intermediate in the formation of an unusual dimer of the type [μ2-κ1:κ1(C,N)Pd(PPh3)Br]2 from the bridge-splitting reaction of an eight-membered monoalkyne-inserted cyclopalladated amine of the type [κ2(C,N)Pd(μ-Br)]2 with PPh3.36 The aforementioned literature precedence and cis,trans stereochemistry of the butadienyl unit in III versus cis,cis stereochemistry of the same unit in 3 indirectly support the decoordination and recoordination pathway proposed in Scheme 4. Indirect evidence for the formation of the intermediate K stems from the fact that Heck and co-workers also suggested a cis,cis-diene-containing [C,N] palladacycle as the intermediate in the formation of a tetrasubstituted naphthalene derivative from depalladation of a diinserted [C,N] palladacycle that contains the diene in cis,trans stereochemistry.10e Moreover, the molecular structure of one diinserted palladacycle wherein the butadienyl unit revealed cis,cis stereochemistry is known which further supports the proposed structure for the intermediate K.37 A plausible mechanism of formation of 7 is illustrated in Scheme 5. This mechanism is somewhat analogous to that invoked by Pfeffer and co-workers to explain the formation of a benzo[b]quinolizinium salt from the insertion reaction of cationic cyclopalladated 2-benzylpyridine with ethyl 3-phenylpropiolate followed by depalladation.11e Insertion of MPP or DPA into the Pd−C bond of 4 or 5 can give rise to the eightmembered palladacyclic intermediate L, which can subsequently undergo Pd−N bond cleavage followed by C−N bond formation involving the β carbon atom of the olefinic unit and subsequent migration of π electrons from the olefinic unit to the Pd−C single bond to afford the six-membered palladacarbene intermediate M. The intermediate M upon losing the proton, Pd(0), and other byproducts can give rise to the heterocycle 7. Pfeffer and co-workers explained the

intermediate G, the structural analogue of 1, and this intermediate subsequently loses MeOH intramolecularly to afford 2. N,N′-Bis(o-anisyl)urea is believed to form through partial hydrolysis of the guanidine moiety present in II. It should also be noted that depalladation of cyclopalladated amine,10g,11a,12a imine,10a,b,m,13 pyridyl,11b,j and azo14 compounds in the presence of alkyne afforded six- or seven- rather than five-membered nitrogen-containing heterocycles or their salts. Only Pfeffer and co-workers reported the formation of 2,3-disubstituted indoles from the alkyne insertion−depalladation sequence involving cyclopalladated N-phenyl-2-pyridylamine of the types [{κ2(C,N)Pd(MeCN)2]+[BF4]− and [{κ2(C,N)Pd(μ-I)]2 in the presence of alkynes.11i However, neither the molecular structures nor the probable mechanism of formation of the 2,3-disubstituted indoles was reported. The gross resemblance of our work to that reported by Pfeffer and co-workers is ascribed to the presence of an endocyclic amino (NH) functionality in the six-membered “[κ2(C,N)]Pd” ring of the precursors from which indoles have been obtained. A plausible pathway for the formation of 3 is outlined in Scheme 4. In the presence of an excess of boiling pyridine, palladacycle III dechelates through Pd−N bond cleavage to afford the acyclic intermediate H. This intermediate upon homolytic cleavage of one of the π bonds of the butadienyl unit can give rise to the diradical intermediate I. This intermediate upon C−C bond rotation can give rise to the more ordered diradical intermediate J. Thereafter, re-formation of the double bond and recoordination of the imino nitrogen atom to the palladium can give rise to the intermediate K, which upon reductive elimination, as occurred in the formation of 1 and 2, can give rise to 3. Palladium is recovered as the previously known adduct trans-[(C5H5N)2PdBr2]35 during the formation of 3. It should be noted that Vicente and co-workers invoked G

dx.doi.org/10.1021/om500837v | Organometallics XXXX, XXX, XXX−XXX

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Scheme 5. Plausible Mechanism of Formation of 7a

a

which are neutral analogues of O, from the monoalkyne insertion reactions of six-membered cyclopalladated guanidines, cis-/trans-[(C,N)PdBrL].20 The intermediate O upon insertion with a second molecule of DPA can give rise to the intermediate P. The intermediate P transforms to 8 via the spirocyclic intermediate Q and the intermediate R, as previously suggested in the formation of annulated products from dialkyne-inserted [C,N] palladacycles.10a−c,e,f The insertion reactions of 4 or 5 with DPA afforded 7, albeit in low yield, while the insertion reaction of 6 with DPA afforded 8 in good yield. The difference in the nature of the depalladated products can be explained by invoking greater steric pressure in the intermediate N than in the intermediate L due to the presence of the sterically more hindered o-tolyl substituent of the guanidine unit in the former. Thus, steric factor appears to facilitate ring contraction cum amine−imine tautomerism to afford the guanidinium salt 8 (see Schemes 5 and 6). These examples illustrate the influence of aryl substituents of the guanidine moieties in 4/5 and 6 upon the nature of depalladated products.

Only the cation is shown for clarity.

formation of the benzo[b]quinolizinium salt in low yield due to its formation from the minor regioisomer of the alkyne-inserted palladacycle.11e An analogous explanation can be invoked for the low yield of 7 from the insertion reaction of 4 or 5 with MPP. However, the low yield of 7 from the reaction of 4 or 5 with DPA can be attributed to our inability to isolate the crystallized product from the mother liquor after the isolation of the first crop of crystals due to the gradual transformation of the mother liquor to a sticky form. A plausible mechanism for the formation of 8 is illustrated in Scheme 6. Palladacycle 6 undergoes an insertion reaction with

■

CONCLUSION Mono- and diinserted palladacycles I−III upon depalladation followed by C−N bond formation resulted in the formation of heterocycles 1−3 in high yield. The cationic pincer type palladacycle 4 and chelate type palladacycles 5 and 6 were isolated in high yield, and these palladacycles upon an alkyne insertion−depalladation sequence afforded the heterocycle 7 in low yield and the carbocycle 8 in high yield. Synthetic routes reported herein are believed to enrich the chemistry of nitrogen-containing heterocycles in general and guanidinecontaining heterocycles in particular. The new compounds have been characterized by analytical and spectroscopic techniques. Molecular structures of seven new compounds have been determined by single-crystal X-ray diffraction. The molecular structures of 2 and 3 are novel, as the framework in the former arises due to the formation of two C−N bonds upon depalladation, while the butadienyl unit in the latter revealed cis,cis stereochemistry, a feature unprecedented in alkyne insertion chemistry. Heterocycles 1 and 7 revealed the presence of two rotamers in solution due to the restricted C−N singlebond rotation of the CN3 unit, depending on the steric bulk and hydrogen bond forming capability of the solvents. Plausible mechanisms for the formation of heterocycles 1−3 and 7 and the carbocycle 8 have been outlined. The nature of substituents in the alkynes as well as those on the nitrogen atoms of the guanidine moiety in 4/5 and 6 influence the course of the insertion/depalladation process and hence nature of the depalladated products (compare 2 versus 3 and 7 versus 8).

Scheme 6. Plausible Mechanism of Formation of 8a

■

a

EXPERIMENTAL SECTION

(E)-Methyl 1-(N,N′-Bis(o-tolyl)carbamimidoyl)-7-methyl-2phenyl-1H-indole-3-carboxylate (1). Palladacycle I (100 mg, 0.128 mmol) was dissolved in chlorobenzene (15 mL) in a 25 mL round-bottom flask to give a clear solution. The round-bottom flask was fitted to a condenser-guard tube setup, and the solution was simultaneously stirred and refluxed for 10 h and then cooled. The blackish yellow solution thus obtained was filtered, and the filtrate was left undisturbed. Yellow crystals of the adduct trans-[(2,6Me2C5H3N)2PdBr2] were obtained after 1 day. The crystals were washed with several aliquots of n-hexane and subsequently recrystallized from CH2Cl2. Yield (adduct·1.5CH2Cl2): 35% (27 mg, 0.044 mmol). The volatiles from the mother liquor were removed under vacuum to afford a light yellow solid, which was crystallized from a

Only the cation is shown for clarity.

DPA to afford the eight-membered palladacyclic intermediate N, which upon ring contraction followed by amine−imine tautomerization can give rise to the intermediate O. Ring contraction and amine−imine tautomerization or only amine− imine tautomerization is frequently invoked to explain the formation of small molecule inserted products in stoichiometric19−22,38 and catalyzed39 reactions mediated by palladium. In the past, we have isolated palladacycles such as I and II, H

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yellow solid was crystallized from a CH2Cl2/toluene mixture under ambient conditions to afford 2 as yellow cuboidal crystals and N,N′bis(o-anisyl)urea as white flaky crystals. The two types of crystals were separated with the aid of a brush. The yellow crystals were washed with a minimum amount of methanol and dried. Yields: 67% (80 mg, 0.164 mmol; 2); 16% (11 mg, 0.040 mmol; N,N′-bis(o-anisyl)urea). Characterization Data for 2. Mp (DSC): 221.23 °C. Anal. Calcd for C27H23N3O6 (mol wt 485.49): C, 66.80; H, 4.77; N, 8.66. Found: C, 66.47; H, 5.04; N, 8.74. IR (KBr, cm−1): ν(CO) 1764 (s), 1718 (vs); ν(CN) 1708 (vs). 1H NMR (CDCl3, 400 MHz): δ 3.60, 3.65, 3.93 (each s, 3 × 3H, OCH3), 3.99 (s, 3 H, C(O)OCH3), 6.30 (d, JHH = 8.1 Hz, 1 H, ArH), 6.45 (d, JHH = 8.0 Hz, 1 H, ArH), 6.61−6.69 (m, 2 H, ArH), 6.72−6.79 (m, 2 H, ArH), 7.01 (d, JHH = 8.0 Hz, 1 H, ArH), 7.05 (apparent t, JHH = 7.3 Hz, 1 H, ArH), 7.11 (d, JHH = 7.3 Hz, 1 H, ArH), 7.36 (t, JHH = 8.1 Hz, 1 H, ArH), 8.03 (d, JHH = 8.0 Hz, 1 H, ArH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 52.00 (C(O)OCH3), 54.78 (OCH3), 55.04 (OCH3), 57.34 (OCH3), 109.22 (C), 109.72 (CH), 110.31 (CH), 110.97 (CH), 116.28 (CH), 119.37 (CH), 119.44 (CH), 120.66 (C), 120.96 (CH), 124.08 (CH), 125.75 (CH), 129.73 (CH), 130.71 (CH), 131.24 (C), 133.03 (C), 133.33 (C), 133.40 (C), 148.14 (C), 148.27 (C), 154.64 (C), 157.54 (CO), 163.43 (CO). MS (HRMS) m/z (relative intensity %) [ion]: found, 486.1646 (100) [M + H]+; calcd, 486.1665. Characterization Data for N,N′-Bis(o-anisyl)urea (β Form).27 Anal. Calcd for C15H16N2O3 (mol wt 272.30): C, 66.16; H, 5.92; N,10.29%. Found: C, 65.89; H, 6.21; N, 9.97. 1H NMR (CDCl3, 400 MHz): δ 3.88 (s, 2 × 3 H, OCH3), 6.89 (dd, JHH = 7.3, 1.5 Hz, 2 × 1 H, ArH), 7.00 (dquint, JHH = 8.1, 1.8 Hz, 2 × 2 H, ArH), 7.11 (br, 2 × 1 H, NH), 8.12 (dd, JHH = 7.7, 1.8 Hz, 2 × 1 H, ArH). (E)-Dimethyl 1-(N,N′-Bis(3-methoxyphenyl)carbamimidoyl)9-methoxy-2,4-diphenyl-1H-benzo[b]azepine-3,5-dicarboxylate (3). Palladacycle III (200 mg, 0.226 mmol) was dissolved in pyridine (15 mL) in a 25 mL round-bottom flask to afford a clear solution. The round-bottom flask was fitted to a condenser-guard tube setup, and the solution was simultaneously stirred and refluxed for 5 h. The red solution thus obtained was filtered, and the filtrate was left undisturbed. The formation of a yellow crystalline solid was observed after 1 day. The solid was separated, washed with several aliquots of nhexane, and subsequently recrystallized from CH2Cl2 to afford the adduct trans-[(C5H5N)2PdBr2]. Yield: 40% (33 mg, 0.078 mmol). The volatiles from the mother liquor were removed under vacuum to afford a reddish yellow solid. The solid was subjected to column chromatography over alumina. A yellow band was eluted with an ethyl acetate/n-hexane (3/97, v/v) mixture, and the eluent was evaporated under vacuum to afford a yellow solid. The solid was crystallized from a CH2Cl2/toluene mixture at ambient temperature over a span of several days to afford 3 as irregular yellow crystals. Yield: 76% (120 mg, 0.172 mmol). Characterization Data for 3. Mp (DSC): 208.39 °C. Anal. Calcd for C42H37N3O7 (mol wt 695.76): C, 72.50; H, 5.36; N, 6.04. Found: C, 72.73/72.50; H, 5.41/5.59; N, 6.00/5.83. IR (KBr, cm−1): ν(NH) 3323 (m); ν(CO) 1734 (s, sh), 1720 (vs); ν(CN) 1627 (vs). 1H NMR (CDCl3, 400 MHz): δ 3.20, 3.36 (each s, 2 × 3 H, C(O)OCH3), 3.66, 3.68, 3.82 (each s, 3 × 3 H, OCH3), 6.37 (t, JHH = 7.3 Hz, 1 H), 6.49 (d, JHH = 7.3 Hz, 2 H), 6.54 (t, JHH = 7.0 Hz, 2 H), 6.60 (d, JHH = 7.3 Hz, 1 H), 6.66 (d, JHH = 7.3 Hz, 1 H), 6.75 (br, 2 H), 6.90 (d, JHH = 7.4 Hz, 1 H), 7.28 (br, 4 H), 7.34 (d, JHH = 7.7 Hz, 1 H), 7.39 (t, JHH = 7.3 Hz, 2 H), 7.56 (d, JHH = 8.0 Hz, 1 H), 7.76−7.79 (m, 2 H), 7.82 (d, JHH = 7.3 Hz, 2 H) (ArH and NH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 51.71 (C(O)OCH3), 52.07 (C(O)OCH3), 55.16 (OCH3), 55.63 (OCH3), 55.93 (OCH3), 109.48 (CH), 110.76 (CH), 111.64 (CH), 119.58 (CH), 120.02 (CH), 120.26 (CH), 120.52 (CH), 120.59 (CH), 121.68 (CH), 122.66 (CH), 127.50 (CH), 127.75 (CH), 128.11 (CH), 128.31 (CH), 128.95 (CH), 129.23 (C), 129.66 (CH), 129.88 (C), 130.03 (CH), 132.20 (C), 133.41 (C), 135.18 (C), 137.08 (C), 139.04 (C), 141.77 (C), 145.29 (C), 147.67 (C), 150.51 (C), 150.76 (C), 150.99 (C), 155.62 (C), 167.62 (CO), 169.22 (CO). MS (ESI+) m/z (relative intensity %) [ion]: 718 (40) [M + Na − H]+, 696 (100) [M]+, 638 (19) [M + H − C(O)OMe]+, 378 (8) [LH32‑tolyl]+.

CHCl3/heptane mixture to afford 1 as white cuboidal crystals over a span of several days. Yield: 80% (50 mg, 0.102 mmol). The composition of the adduct was confirmed by comparing its 1H NMR data and melting point with those of the authentic sample reported in the literature.20 Characterization Data for 1. Mp (DSC): 201.91 °C. Anal. Calcd for C32H29N3O2 (mol wt 487.59): C, 78.82; H, 5.99; N, 8.62. Found: C, 78.66; H, 6.03; N, 8.49. IR (KBr, cm−1): ν(NH) 3380 (m); ν(C O) 1686 (s); ν(CN) 1663 (vs). 1H NMR spectra of 1 in CDCl3 solution at 20.0 × 10−3 mM and 273 × 10−3 mM concentrations revealed the presence of two isomers, hereafter indicated as isomers 1 and 2 in about a 1.00:0.43 ratio, as estimated from the integrals of alkyl protons. 1H NMR (CDCl3, 400 MHz): δ 1.43 (s, 3 H, CH3, isomer 2), 1.58 (br, 3 H, CH3, isomer 1), 2.19 (s, 3 H, CH3, isomer 2), 2.25 (br, 3 H, CH3, isomer 1), 2.69 (s, 3 H, CH3, isomer 1), 2.85 (s, 3 H, CH3, isomer 2), 3.66 (s, 3 H, OCH3, isomer 1), 3.68 (s, 3 H, OCH3, isomer 2), 6.01 (br, 1 H, isomer 2), 6.04 (d, JHH = 8.0 Hz, 1 H, isomer 1), 6.35 (s, 1 H, isomer 2), 6.45 (s, 1 H, isomer 1), 6.67 (br, 1 H, isomer 1), 6.74 (t, JHH = 7.7 Hz, 1 H, isomer 2), 6.86 (t, JHH = 7.0 Hz, 2 H, isomer 1), 6.91 (d, JHH = 6.6 Hz, 2 H, isomer 1), 6.96 (d, JHH = 8.0 Hz, 2 × 1 H, isomers 1 and 2), 7.02 (t, JHH = 7.0 Hz, 2 × 1 H, isomers 1 and 2), 7.13 (t, JHH = 7.3 Hz, 2 × 1 H, isomer 2), 7.22 (q, JHH = 7.6 Hz, 3 H (isomer 1), 6H (isomer 2)), 7.31 (t, JHH = 7.7 Hz, 2 H, isomer 1), 7.37 (t, JHH = 7.3 Hz, 1 H (isomer 1), 2H (isomer 2)), 8.17 (d, JHH = 8.0 Hz, 2 × 1 H, isomers 1 and 2), 8.42 (br, 2 × 1 H, isomers 1 and 2) (ArH and NH). 1H NMR (CD3CN, 400 MHz): δ 1.46, 2.28, 2.72 (each s, 3 × 3 H, CH3), 3.60 (s, 3 H, OCH3), 5.95 (d, JHH = 7.4 Hz, 1 H, ArH), 6.58 (t, JHH = 7.3 Hz, 1 H, ArH), 6.80 (t, JHH = 7.3 Hz, 1 H, ArH), 6.99−7.05 (m, 4 H, ArH), 7.18 (t, JHH = 7.9 Hz, 1 H, ArH), 7.23 (d, JHH = 7.3 Hz, 2 H, ArH), 7.28 (dt, JHH = 7.8, 2.0 Hz, 3 H, ArH), 7.41 (t, JHH = 7.6 Hz, 1 H, ArH), 7.73 (s, 1 H, NH), 8.08 (d, JHH = 7.9 Hz, 1 H, ArH), 8.18 (d, JHH = 7.9 Hz, 1 H, ArH). 1H NMR (DMSOd6, 400 MHz): δ 1.38, 2.29, 2.74 (each s, 3 × 3 H, CH3), 3.54 (s, 3 H, OCH3), 5.84 (d, JHH = 8.0 Hz, 1 H, ArH), 6.57 (d, JHH = 7.3 Hz, 1 H, ArH), 6.75 (t, JHH = 7.3 Hz, 1 H, ArH), 6.96−7.04 (m, 4 H, ArH), 7.15 (t, JHH = 7.3 Hz, 1 H, ArH), 7.20−7.31 (m, 5 H, ArH), 7.43 (t, JHH = 7.6 Hz, 1 H, ArH), 7.83 (d, JHH = 8.0 Hz, 1 H, ArH), 8.03 (d, JHH = 6.7 Hz, 1 H, ArH), 9.42 (s, 1 H, NH). The 13C{1H} NMR assignments of the signals of isomers 1 and 2 were made wherever possible. 13C{1H} NMR (CDCl3, 100.5 MHz): δ 17.29 (CH3, isomer 2), 18.23 (CH3), 18.27 (CH3), 18.48 (CH3), 20.39 (CH3, isomer 2), 50.91 (OCH3), 50.98 (OCH3), 107.33 (C, isomer 1), 107.93 (C, isomer 2), 118.78 (CH), 119.01 (CH), 119.13 (CH), 120.30 (CH), 120.43 (CH), 121.32 (C), 121.87 (C), 123.04 (CH), 123.19 (CH), 123.87 (CH), 124.24 (CH), 124.53 (CH), 126.27 (CH), 127.00 (CH), 127.20 (CH), 127.35 (CH), 127.46 (CH), 127.60 (CH), 128.16 (C), 129.08 (CH), 129.23 (CH), 129.80 (CH), 130.19 (C), 130.52 (CH), 131.28 (CH), 131.70 (CH), 134.01 (C), 135.44 (C), 135.89 (C), 136.90 (C), 137.08 (C), 142.30 (C), 143.87 (C), 144.51 (C), 144.79 (C), 145.07 (C) (ArC, ArCH, CN, CC), 165.14 (CO, isomer 1), 165.25 (CO, isomer 2). MS (ESI+) m/z (relative intensity %) [ion]: 488 (100) [M]+. (Z)-Methyl 5-Methoxy-2-(2-methoxyphenyl)-3-(2-methoxyphenylimino)-1-oxo-2,3-dihydro-1H-imidazo[1,5-a]indole-9carboxylate (2). Palladacycle II (200 mg, 0.246 mmol) was dissolved in chlorobenzene (15 mL) in a 25 mL round-bottom flask to give a clear yellow solution. The round-bottom flask was fitted to a condenser-guard tube setup, and the solution was refluxed for 4.5 h. The blackish yellow solution thus obtained was filtered, and the filtrate was left undisturbed. The formation of yellow crystals of the adduct trans-[(2,6-Me2C5H3N)2PdBr2] was observed after 1 day. The crystals of the adduct were washed with several aliquots of n-hexane and subsequently recrystallized from CH2Cl2. Yield: 42% (50 mg, 0.104 mmol). The volatiles from the mother liquor were removed under vacuum to afford a reddish yellow solid. The solid was purified by column chromatography over alumina. A pale yellow band was eluted with an ethyl acetate/n-hexane (4/96, v/v) mixture. The volatiles from the eluent were evaporated to dryness to afford a yellow solid. Amount: 96 mg. The 1H NMR spectrum of the solid revealed that it is a mixture of 2 and N,N′-bis(o-anisyl)urea in about a 4:1 ratio. The I

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Characterization Data for trans-[(C5H5N)2PdBr2]. Mp (DSC): 250.32 °C. Anal. Calcd for C10H10N2Br2Pd (mol wt 424.43): C, 28.30; H, 2.37; N, 6.60. Found: C, 28.62; H, 2.77; N, 6.61. 1H NMR (CDCl3, 400 MHz): δ 7.33 (t, JHH = 7.0 Hz, 2 H, m-PyH), 7.77 (t, JHH = 7.7 Hz, 1 H, p-PyH), 8.89 (d, JHH = 5.1 Hz, 2 H, o-PyH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 125.2, 138.5, 154.5 (PyCH). Palladacycle 4. To a solution of IV (200 mg, 0.178 mmol) in CH2Cl2 (25 mL) was added an MeCN solution (5 mL) of AgBF4 (70 mg, 0.360 mmol), and the resulting solution was stirred at ambient temperature for 20 min in the absence of light. The formation of a light yellow precipitate was observed in the reaction mixture. The solution was filtered, and the volatiles from the filtrate were removed under vacuum to afford a light yellow solid. The solid was crystallized from a CH2Cl2/n-hexane mixture under ambient conditions over a period of several days to afford 4 as a light green powder. Yield: 85% (185 mg, 0.303 mmol). Characterization Data for 4. Mp (DSC): 176.85 °C dec. ΛM (Ω−1 cm2 mol−1) = 159.3 (10−3 M). Anal. Calcd for C24H25N4O3BF4Pd (mol wt 610.70): C, 47.20; H, 4.13; N, 9.17. Found: C, 47.00/46.91; H, 4.17/4.01; N, 8.78/8.77. IR (KBr, cm−1): ν(NH) 3400 (br, w), 3373 (m); ν(CN) 1616 (vs); ν(BF4)− 1060 (br, vs). 1H NMR (CDCl3, 400 MHz): δ 2.55 (s, 3 H, CH3CN), 3.78, 3.87, 4.24 (each s, 3 × 3 H, OCH3), 6.68 (d, JHH = 8.0 Hz, 1 H), 6.74 (d, JHH = 7.4 Hz, 1 H), 6.83−6.87 (m, 2 H), 6.95−7.00 (m, 1 H), 7.02−7.09 (m, 2 H), 7.14 (d, JHH = 3.6 Hz, 2 H), 7.31−7.36 (m, 2 H), 7.43 (d, JHH = 8.1 Hz, 1 H) (ArH and NH), 8.05 (s, 1H, NH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 3.3 (CH3CN), 56.0 (OCH3), 56.2 (OCH3), 60.1 (OCH3), 109.2, 111.6, 112.0, 112.8, 121.3, 122.4, 122.8, 123.9, 124.2, 124.4, 124.5, 126.5, 127.1, 128.0, 133.2, 145.7, 146.8, 152.3, 152.8 (ArC, ArCH, CN and CH3CN). MS (ESI+) m/z (relative intensity %) [ion]: 523 (100) [M − H]+, 510 (47) [M + H − Me]+, 482 (75) [M − H, MeCN]+, 376 (79) [LH2‑anisyl]+, where M is the cation of 4. Palladacycle 5. Method 1. To a solution of IV (200 mg, 0.178 mmol) in CH2Cl2 (25 mL) was added an MeCN solution (5 mL) of AgBF4 (70 mg, 0.360 mmol), and the resulting solution was stirred at ambient temperature for 20 min in the absence of light. The formation of a light yellow precipitate was observed in the reaction mixture. The solution was filtered, and the filtrate was concentrated under vacuum to about 5 mL to afford 5 as light green crystals over a span of several days. Yield: 90% (210 mg, 0.322 mmol). Method 2. Palladacycle 4 (100 mg, 0.164 mmol) was dissolved in MeCN (10 mL) and stored under ambient conditions to afford 5 as light green crystals over a span of several days. Yield: 89% (95 mg, 0.146 mmol). Characterization Data for 5. Mp (DSC): 160.63 °C dec. ΛM (Ω−1 cm2 mol−1) = 154.6 (10−3 M). Anal. Calcd for C26H28N5O3BF4Pd (mol wt 651.76): C, 47.91; H, 4.33; N, 10.74. Found: C, 47.52; H, 4.73; N, 10.35. IR (KBr, cm−1): ν(NH) 3400 (br, w), 3379 (m); ν(CN) 1612 (s); ν(BF4)− 1060 (br, vs). 1H NMR (CDCl3, 400 MHz): δ 2.00, 2.51 (each s, 2 × 3 H, CH3CN), 3.77, 3.87, 4.18 (each s, 3 × 3 H, OCH3), 6.68 (d, JHH = 8.0 Hz, 1 H), 6.78 (d, JHH = 8.1 Hz, 1 H), 6.82−6.86 (m, 2 H), 6.98−7.06 (m, 3 H), 7.12 (d, JHH = 8.0 Hz, 1 H), 7.20 (t, JHH = 8.1 Hz, 1 H), 7.27−7.32 (m, 2 H), 7.40 (d, JHH = 7.3 Hz, 1 H) (ArH and NH), 8.03 (s, 1 H, NH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 1.70 (CH3CN), 3.17 (CH3CN), 55.87 (OCH3), 55.92 (OCH3), 57.81 (OCH3), 107.68, 111.89, 112.02, 117.68, 118.24, 121.04, 122.17, 122.91, 123.46, 124.07, 124.82, 126.84, 127.46, 128.36, 133.77, 145.50, 146.34, 151.88, 153.06 (ArC, ArCH, CN and CH3CN). MS (ESI+) m/z (relative intensity %) [ion]: 523 (33) [M − MeCN, H]+, 510 (49) [M + H − MeCN, Me]+, 483 (100) [(C,N)Pd]+, 376 (94) [LH2‑anisyl]+, where M is the cation of 5. Palladacycle 6. To a solution of V (200 mg, 0.194 mmol) in CH2Cl2 (25 mL) was added an MeCN solution (5 mL) of AgBF4 (76 mg, 0.390 mmol), and the resulting solution was stirred at ambient temperature for 20 min in the absence of light. The formation of a light yellow precipitate was observed in the reaction mixture. The solution was filtered, and the volatiles from the filtrate were removed under vacuum to afford a light yellow solid. The solid was crystallized from a CH2Cl2/n-hexane mixture under ambient conditions over a

period of several days to afford 6 as light yellow-green cuboidal crystals. Yield: 91% (213 mg, 0.353 mmol). Characterization Data for 6. Mp (DSC): 134.20 °C. ΛM (Ω−1 cm2 mol−1) = 143.1 (10−3 M). Anal. Calcd for C26H28N5BF4Pd (mol wt 603.76): C, 51.72; H, 4.67; N, 11.60. Found: C, 51.40; H, 4.71; N, 11.49. IR (KBr, cm−1): ν(NH) 3413 (m), 3200 (m); ν(CN) 1629 (vs); ν(BF4)− 1060 (br, vs). 1H NMR (CDCl3, 400 MHz): δ 1.71 (s, 3 H, CH3), 1.95 (s, 3 H, CH3CN), 2.16, 2.42 (each s, 2 × 3 H, CH3), 2.43 (s, 3 H, CH3CN), 5.86, 6.55 (each s, 2 × 1 H, NH), 6.72 (t, JHH = 7.7 Hz, 1 H), 6.87 (d, JHH = 7.3 Hz, 1 H), 7.13 (d, JHH = 7.4 Hz, 1 H), 7.19 (d, JHH = 6.6 Hz, 1 H), 7.29 (d, JHH = 7.3 Hz, 2 H), 7.31−7.39 (m, 5 H) (ArH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 1.8 (CH3CN), 3.2 (CH3CN), 16.9 (CH3), 17.7 (CH3), 18.4 (CH3), 117.8 (C), 118.5 (C), 120.6 (C), 122.1 (C), 122.3 (CH), 127.2 (CH), 127.6 (CH), 127.8 (CH), 128.0 (CH), 128.2 (CH), 128.6 (CH), 129.7 (CH), 131.9 (CH), 132.1 (CH), 132.7 (C), 133.3 (C), 134.9 (C), 135.6 (CH), 136.4 (C), 143.2 (C), 146.0 (C) (ArC, ArCH, CN, CH3CN). MS (ESI+) m/z (relative intensity %) [ion]: 475 (94) [M − MeCN, H]+, 460 (35) [M − MeCN, Me, H]+, 434 (96) [M − 2MeCN, H]+, 328 (100) [LH2‑tolyl]+, where M is the cation of 6. 8-Methoxy-3-(2-methoxyphenyl)-2-(2-methoxyphenylamino)-4-phenylquinazolin-3-ium Tetrafluoroborate (7). Method 1. To a suspension of 4 (200 mg, 0.327 mmol) in chlorobenzene (25 mL) was added a chlorobenzene solution (5 mL) of MPP (105 mg, 0.656 mmol) in a 50 mL round-bottom flask. The round-bottom flask was attached to a condenser guard tube setup, and the turbid green suspension was simultaneously stirred and heated at 110 °C for 5 h. During the course of the reaction, the suspension became a clear green solution and gradually transformed into a red solution. The solution was filtered, and the volatiles from the filtrate were removed under vacuum to afford a red solid. The solid was crystallized from a CHCl3/ toluene mixture under ambient conditions over a span of several days to afford 7 as red cuboidal crystals. Yield: 30% (54 mg, 0.098 mmol). No more crystals could be further isolated from the mother liquor, as it turned sticky with time. Methods 2 and 3. The heterocycle 7 was also obtained from the reaction of 4 (200 mg, 0.327 mmol) with DPA (117 mg, 0.655 mmol) in chlorobenzene following the aforementioned procedure in 25% yield (45 mg, 0.082 mmol). The reaction of 5 (200 mg, 0.307 mmol) with MPP (100 mg, 0.624 mmol) in chlorobenzene following the aforementioned procedure also afforeded 7 in 32% yield (55 mg, 0.100 mmol). Characterization Data for 7. Mp (DSC): 288.98 °C. ΛM (Ω−1 cm2 mol−1) = 131.6 (10−3 M). Anal. Calcd for C29H26N3O3BF4 (mol wt 551.34): C, 63.18; H, 4.75; N, 7.62. Found: C, 63.09/63.20; H, 4.48/ 4.58; N, 7.50/7.44. IR (KBr, cm−1): ν(NH) 3366 (m); ν(CN) 1624 (vs); ν(BF4)− 1062 (s), 1017 (s). 1H NMR (CDCl3, 400 MHz): δ 3.62, 3.78, 4.15 (each s, 3 × 3 H, OCH3), 6.82−6.84 (m, 1 H), 7.05 (d, JHH = 8.8 Hz, 2 H), 7.11−7.13 (m, 2 H), 7.16 (s, 1 H), 7.19 (d, JHH = 8.8 Hz, 2 H), 7.30−7.48 (m, 4 H), 7.51−7.55 (m, 1 H), 7.78−7.80 (m, 1 H), 7.88, 7.90 (each s, 2 × 1 H), 8.84−8.87 (m, 1 H) (ArH and NH). 1H NMR (CD3CN, 400 MHz): δ 3.67, 3.79, 4.11 (each s, 3 × 3 H, OCH3), 7.02−7.05 (m, 2 H, ArH), 7.14 (apparent q, JHH = 7.4 Hz, 2 H, ArH), 7.19 (d, JHH = 8.8 Hz, 1 H, ArH), 7.23 (dt, JHH = 8.0, 1.5 Hz, 1 H, ArH), 7.37−7.41 (m, 1 H, ArH), 7.43 (dd, JHH = 4.4, 2.2 Hz, 2 H, ArH), 7.46 (br, 1 H, ArH), 7.48 (s, 1 H, ArH), 7.50−7.55 (m, 2 H, ArH), 7.60 (m, 2 H, ArH), 7.91 (s, 1 H, NH), 8.55 (dd, JHH = 8.1, 1.5 Hz, 1 H, ArH). The 1H NMR spectrum of 7 in CD3OD revealed the presence of two isomers, hereafter indicated as isomers 1 and 2, in about a 1.00:0.14 ratio, as estimated from the integrals of OCH3 protons. 1H NMR (CD3OD, 400 MHz): δ 3.63 (s, 3H, OCH3, isomer 1), 3.90, 3.91 (each s, 2 × 3 H, OCH3, isomer 2), 3.92, 3.94 (each s, 2 × 3 H, OCH3, isomer 1), 3.99 (s, 3 H, OCH3, isomer 2), 6.25 (dd, JHH = 8.0, 1.6 Hz, 1 H, ArH, isomer 2), 6.35−6.40 (m, 1 H, ArH, isomer 1), 6.56 (dq, JHH = 7.8, 1.4 Hz, 2 H, ArH, isomer 2), 6.71 (d, JHH = 7.8 Hz, 1 H, ArH, isomer 1), 6.91 (t, JHH = 7.8 Hz, 1 H, ArH, isomer 1), 7.03−7.12 (m, 2 × 4 H, ArH, isomers 1 and 2), 7.14 (d, JHH = 2.3 Hz, 2 × 1 H, ArH, isomers 1 and 2), 7.15 (s, 1 H, NH, isomer 1), 7.16 (s, 1 H, NH, isomer 2), 7.19−7.32 (m, 2 × 5 H, ArH, isomers 1 and 2), 7.34−7.36 (m, 2 × 1 H, ArH, isomers 1 and 2), 7.39−7.46 (m, 2 × 2 J

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H, ArH, isomers 1 and 2). 13C{1H} NMR (CD3OD, 100.5 MHz): δ 55.84 (OCH3), 55.87 (OCH3), 56.54 (OCH3), 56.58 (OCH3), 57.05 (OCH3), 57.09 (OCH3), 95.70 (C, isomer 1), 96.37 (C, isomer 2), 112.21 (CH, isomer 2), 112.39 (CH, isomer 1), 112.91 (CH), 113.76 (CH), 120.81 (CH), 121.42 (CH), 121.57 (CH), 121.79 (CH), 122.58 (CH), 122.71 (C), 123.14 (C), 123.62 (C), 126.65 (CH), 127.78 (CH), 128.25 (CH), 129.26 (CH), 129.47 (CH), 129.60 (CH), 130.12 (CH), 131.63 (CH), 131.78 (CH), 132.83 (CH), 141.08 (C, isomer 1), 144.65 (C, isomer 2), 147.25 (C, isomer 1), 147.70 (C, isomer 2), 150.84 (C, isomer 1), 150.98 (C, isomer 2), 155.73 (C, isomer 1), 155.82 (C, isomer 1), 157.83 (C, isomer 2). MS (ESI+) m/z (relative intensity %) [ion]: 465 (100) [M]+, where M is the cation of 7. 2-(2-Methyl-5,6,7,8-tetraphenylnaphthalen-1-yl)-1,3-bis(otolyl)guanidinium Tetrafluoroborate (8). To a suspension of 6 (200 mg, 0.331 mmol) in chlorobenzene (25 mL) in a 50 mL roundbottom flask was added DPA (125 mg, 0.701 mmol). The roundbottom flask was fitted to a condenser-guard tube setup, and the suspension was simultaneously stirred and refluxed for 3 h. During the course of the reaction, the suspension turned into a red solution accompanied by the formation of palladium black. The reaction mixture was filtered, and the volatiles from the filtrate were removed under vacuum to afford an orange-yellow solid. The solid was crystallized from a CH 2Cl2/n-hexane mixture under ambient conditions over a span of several days to afford 8·CH2Cl2 as light yellow irregular crystals. Yield: 82% (233 mg, 0.272 mmol). Characterization Data for 8. Mp (DSC): 214.66 °C. ΛM (Ω−1 cm2 mol−1) = 141.9 (10−3 M). Anal. Calcd for C50H42N3BF4·CH2Cl2 (mol wt 771.69 + 84.93): C, 71.51; H, 5.18; N, 4.91. Found: C, 71.72/ 71.55; H, 4.83/5.00; N, 4.91/4.89. IR (KBr, cm−1): ν(NH) 3324 (m, br); ν(CN) 1628 (vs); ν(BF4)− 1084 (s, br). 1H NMR (CDCl3, 400 MHz): δ 2.04, 2.25, 2.59 (each s, 3 × 3 H, CH3), 6.46 (br, 1 H, NH), 6.59 (d, JHH = 6.6 Hz, 1 H, ArH), 6.67 (d, JHH = 7.4 Hz, 1 H, ArH), 6.73 (s, 2 H, ArH), 6.78 (br, 2 H, ArH), 6.81 (apparent t, JHH = 1.5 Hz, 1 H, ArH), 6.86 (apparent t, JHH = 9.2 Hz, 5 H, ArH), 6.93 (br, 4 H, ArH), 7.00−7.03 (m, 2 H, ArH), 7.10−7.23 (m, 11 H, ArH), 7.31 (t, JHH = 7.0 Hz, 1 H, ArH), 7.38, 8.52 (each br, 2 × 1 H, NH). 13C{1H} NMR (CDCl3, 100.5 MHz): δ 17.58 (CH3), 18.00 (CH3), 19.52 (CH3), 124.82, 125.72, 125.77, 126.73, 126.82, 126.87, 126.93, 127.11, 127.24, 127.46, 128.04, 128.28, 128.68, 128.88, 129.10, 129.79, 130.56, 130.76, 130.92, 131.06, 131.34, 132.06, 132.58, 132.80, 134.05, 134.20, 138.90, 139.26, 139.57, 139.90, 141.93, 142.17, 153.21 (ArC, ArCH, CN). MS (ESI+) m/z (relative intensity %) [ion]: 686 (100) [M + H]+, 670 (24) [M − Me]+, 330 (23) [LH32‑tolyl]+, where M is the cation of 8. Crystals suitable for X-ray diffraction were grown from a CHCl3/petroleum ether mixture at 10 °C over a span of several days to afford 8·1.5H2O as light yellow irregular crystals.

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ACKNOWLEDGMENTS N.T. acknowledges the Department of Science and Technology, Delhi, India, for a research grant (grant no. SR/S1/IC-04/ 2010), and P.S. acknowledges the Council of Scientific and Industrial Research for a fellowship. We also acknowledge the University Science Instrumentation Center, University of Delhi, for infrastructure facilities.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Text giving full experimental and crystallographic details pertinent to general considerations, crystallographic parameters of structurally characterized palladacycles (Tables S1 and S2), Figure S1 for molecular structure of N,N′-bis(o-anisyl)urea, Figures S2−S10 for hydrogen bond interactions, Figures S11− S32 for 1H−13C HETCOR and 13C DEPT 90 NMR spectra of 1−3 and 7 and 13C DEPT 90 NMR spectrum of 6, and Figure S33 for the HRMS spectrum of 2, and CIF files giving crystallographic data for 1−3, 5−7, 8·1.5H2O, and N,N′-bis(oanisyl)urea. This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*E-mail for N.T.: [email protected], thirupathi_n@ yahoo.com. Notes

The authors declare no competing financial interest. K

dx.doi.org/10.1021/om500837v | Organometallics XXXX, XXX, XXX−XXX

Organometallics

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dx.doi.org/10.1021/om500837v | Organometallics XXXX, XXX, XXX−XXX