Heavier alkaline earth catalysts for the intermolecular hydroamination of vinylarenes, dienes, and alkynes

The heavier group 2 complexes [M{N(SiMe(3))(2)}(2)](2)(1, M = Ca; 2, M = Sr) and [M{CH(SiMe(3))(2)}(2)(THF)(2)] (3, M = Ca; 4, M = Sr) are shown to be effective precatalysts for the intermolecular hydroamination of vinyl arenes and dienes under mild conditions. Initial studies revealed that the amide precatalysts, 1 and 2, while compromised in terms of absolute activity by a tendency toward transaminative behavior, offer greater stability toward polymerization/oligomerization side reactions. In every case the strontium species, 2 and 4, were found to outperform their calcium congeners. Reactions of piperidine with para-substituted styrenes are indicative of rate-determining alkene insertion in the catalytic cycle while the ease of addition of secondary cyclic amines was found to be dependent on ring size and reasoned to be a consequence of varying amine nucleophilicity. Hydroamination of conjugated dienes yielded isomeric products via η(3)-allyl intermediates and their relative distributions were explained through stereoelectronic considerations. The ability to carry out the hydroamination of internal alkynes was found to be dramatically dependent upon the identity of the alkyne substituents while reactions employing terminal alkynes resulted in the precipitation of insoluble and unreactive group 2 acetylides. The rate law for styrene hydroamination with piperidine catalyzed by [Sr{N(SiMe(3))(2)}(2)](2) was deduced to be first order in [amine] and [alkene] and second order in [catalyst], while large kinetic isotope effects and group 2 element-dependent ΔS(++) values implicated the formation of an amine-assisted rate-determining alkene insertion transition state in which there is a considerable entropic advantage associated with use of the larger strontium center.     

Reactions of 2-hydroxybenzonitrile with isocyanates

Triethylamine catalyzes the reaction of 2-hydroxybenzonitrile ( 1 ) with aryl isocyanates to form the corresponding carbamates 2a-c , as well as the cyclization of the latter compounds to either 4[N-(N-arylcarbamoyl)-imino]-3-aryl-2H-1,3-benzoxazin-2-ones 4a-c , or 4-arylamino-2H-1,3-benzoxazin-2-ones 7a-c , depending on the reaction temperature. Under analogous conditions, the carbamates obtained from 1 and 2-chloroethyl isocyanate, 3-chloropropyl isocyanate and ethyl isocyanatoacetate undergo a double cyclization yielding imidazo- and pyrimido[1,2-c|1,3]benzoxazinones 13a,b,17 . Upon heating in phenyl ether, compounds 7a-c , rearrange to 2-(2-hydroxyphenyl)-4(3H)-quinazolinones 10a-c .     

Microwave-initiated hydroamination of 2-ethoxypropenal with secondary amines

Reactions of 2-ethoxyprop-2-enal with cycloaliphatic secondary amines (morpholine, piperidine, pyrrolidine) follow 1,4- or 1,2-addition pattern with subsequent condensation of the adduct with the initial amine to produce isomeric 2-ethoxyprop-2-ene-1,1-diamines and 2-ethoxyprop-1-ene-1,3-diamines. These reactions are accelerated by a factor of 15–30 under microwave irradiation and in the presence of water. The regioselectivity of primary nucleophilic attack varies over a wide range, depending on the amine basicity and reactant ratio. The reaction of 2-ethoxyprop-2-enal with pyrrolidine and water at a ratio of 1: 10: 10 was characterized by increased regioselectivity of 1,4-addition (up to 75%). Unlike cycloaliphatic amines, 2-ethoxyprop-2-enal reacted with N-methylaniline under microwave irradiation at a lower rate to give 2-ethoxy-N ¹,N ¹,N ³-trimethyl-N ¹,N ¹,N ³-triphenylpropane-1,1,3-triamine.     

Cp2TiCl2-Catalyzed reaction of grignard reagents with isocyanates,formation of reduction-coupling product of isocyanates

Phenylisocyanate and phenylthioisocyanate react with Grignard reagents in the presence of a small amount of Cp₂TiCl₂ at room temperature to afford reduction-coupling products, N-methyl-N, N′-diphenylurea and N-methyl-N, N′-diphenylthiourea.     

Asymmetric hydroamination/cyclization catalyzed by group 4 metal complexes with chiral biaryl-based ligands

Group 4 metal complexes based on chiral biaryl ligands are readily prepared by a convenient amine elimination procedure, and they are efficient catalysts for the asymmetric hydroamination/cyclization of aminoalkenes. The biaryl-based ligands are highly modular enabling facile tuning of the catalyst reactivity and selectivity. The corresponding heterocyclic products have been obtained in excellent enantiomeric excesses (up to 93%) using sterically hindered C₂-symmetric titanium and zirconium mesitoylamidate complexes such as [1,1′-(C₁₀H₁₀)₂-2,2′-{NCO(2,4,6-Me₃C₆H₂)}₂]M(NMe₂)₂ (M = Ti, Zr). These results are presented in this short review.     

Phosphorus isocyanates

Compounds containing the PNCO grouping can be obtained e.g. from phosphorus halides and silver cyanate or esters of N-chloroiminocarbonic acid and by thermal decomposition of alkoxycarbonylimino-phosphoranes. The phosphorus (V) isocyanates resemble the “carbon” isocyanates in their chemical properties, whereas the phosphorus (III) isocyanates resemble the phosphorus (III) chlorides. Some of the compounds discussed have insecticidal or cytostatic actions.     

Photo-decarbonylation of β-styryl isocyanates

β-Styryl isocyanate (1, R ? H) and its β-methyl- (1, R ? CH₃) and β-phenyl- (1, R ? C₆H₅) derivatives underwent both extensive polymerization and the loss of the elements of carbon monoxide upon irradiation at 254 nm in cyclohexane. The formation of 2,5-diphenylpyrazine ( 3 ) and indole 4 , (R ? H) from 1 , (R - H) and 2,3-dimethyl-5,6-diphenylpyrazine ( 6 ) and 2-methylindole ( 4 , R ? CH₃) from 1 , (R ? CH₃) provided diagnostic evidence for styryl nitrene ( 2a ) intermediates. The formation of both phenylacetonitrile ( 5 , R ? H) and α-phenylpropio-nitrile ( 5 , R ? CH₃) was assigned to an initial rearrangement of the residue, C₈H₆(R)N?: ( 2 ), into a ketenimine concerted with the elimination of carbon monoxide from 1. Isomerization then produced a nitrile. β3-(β-phenyl)styryl isocyanate ( 1 , R ? C₆H₅) gave no product requiring the intermediacy of a nitrene and/or an azirine. The formation of 2,3,4,5-tetraphenylpyrrole ( 8 ) was assigned to a dimerization of the isocyanate concerted with or following the elimination of the elements of carbon monoxide and isocyanic acid, and the formation of 3-phenylisocarbo-styril ( 9 ) was assigned to a ring-closure of the isocyanate in an excited triplet state. Each isocyanate gave stilbene and trace amounts of oxidative fragmentation into benzaldehyde and benzonitrile. Solvent participation produced benzylcyclohexane and bicyclohexyl. Two unidentified solids, C₁₇H₁₄N₂O and C₁₂H₁₄N₂O, were obtained from 1 , (R ? CH₃).     

Liquid-phase hydroamination of cyclohexanone

Activity and selectivity of supported Ni, Pt, and Pd catalysts were studied in the liquidphase reductive amination of cyclohexanone at temperatures ranging from 100 to 150 °C. The catalyst 20% Ni/SiO₂ is most active and selective providing a maximum yield of cyclohexylamine. The influence of the reaction conditions on the parameters of the catalytic process was studied. A detailed analysis of the reaction products was carried out using ¹³C NMR spectroscopy and gas chromatography coupled with mass spectrometry (GC-MS). This made it possible to refine the reaction mechanism and to identify a new by-product earlier unknown in the literature.     

Nitropyridyl isocyanates

We hereby report the first preparation of 3‐nitro‐4‐pyridyl isocyanate 9 and 5‐nitro‐2‐pyridyl isocyanate 18 . They were formed by Curtius rearrangement of the corresponding acyl azides 8 and 17 , prepared from methyl 3‐nitro‐4‐pyridinecarboxylate 6 via the hydrazide 7 and 5‐nitro‐picolinic acid 16 , respectively. The substrates 6 and 16 were generated by nitration of methyl 4‐pyridinecarboxylate 5 and nitration and oxidation of 2‐picoline 14 . 3‐Nitro‐4‐pyridyl isocyanate 9 can be stored in dry solution and is stable at room temperature for several weeks while 5‐nitro‐2‐pyridyl isocyanate 18 was less stable and should be used for synthetic purposes immediately.     

Substitution of 3H-furan-2-ones in amination and hydroamination reactions

The amination and hydroatnination of 5-alkyl-3H-furan-2-ones and their 3-benzylidene- and 3 formyl-derivatives were studied. The characteristics of the reactions, the requirements for the nature of the nucleophilic reagents, and the reaction conditions leading to the destruction of the furan ring and recyclization to pyrrolidinone and pyrrolinone structures are indicated.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 349–352, March, 1994.     

Use of group 4 bis(sulfonamido) complexes in the intramolecular hydroamination of alkynes and allenes

Titanium tetrakis(amido) complexes catalyze the intramolecular hydroamination of alkynes and allenes more efficiently than Cp-based species. We report here that electron-withdrawing and sterically demanding bis(sulfonamido) ligands lead to enhanced catalytic activity. Zirconium analogues have also been prepared, and the tosyl-substituted complex 20 has been structurally characterized. As in the titanium series, bis(sulfonamido) zirconium catalysts are more efficient in the intramolecular hydroamination of allenes than bis(cyclopentadienyl) complex Cp(2)ZrMe(2) (23). Furthermore, these compounds transform 1,3-disubstituted aminoallenes with high stereoselectivity to the Z-allylamines and allow the hydroamination of a trisubstituted allene. Titanium bis(sulfonamido) imido complex 27 was synthesized. It converts aminoallene 10 to cylic imine 11 with a rate comparable to that of tetrakis(amide) 15, supporting the hypothesis of a catalytically active titanium imido intermediate.     

Phosphanides of the Heavier Alkali Metals

A variety of structures are shown in the solid state by the phosphanides MPHR of alkali metals. Whereas oligomeric and polymeric ladder or helical structures based on M–P bonds predominate for compounds of the lighter homologues (M=Li, Na, K), the 2,6-dimesitylphenylphosphanides of Rb and Cs are present as a Rb₄P₄ cubane and a Cs⁺{Cs₂(PHR)₃}⁻ contact ion pair, respectively (the partial structures are shown above). These unexpected new structures show interactions between the metal center and π electrons of the ligand.     

Copper-catalyzed intermolecular hydroamination of alkenes

[reaction: see text] Regioselective additions of arylsulfonamides to vinylarenes, norbornene, and cyclohexadiene were achieved using a copper-diphosphine catayst under mild reaction conditions. These processes appear to be ligand-accelerated.     

Intramolecular hydroamination of 2-(2-phenylethynyl)aniline catalyzed by gold nanoparticles

The possibility of using heterogeneous catalysts with a low content of gold in the intramolecular hydroamination of 2-(2-phenylethynyl)aniline was shown. The catalysts with size of gold particles lower than 3 nm exhibit catalytic activity. The study of efficiency of the heterogeneous Au-containing catalysts on different supports (MCM-41, γ-Al₂O₃(F), NH₄ ⁺-Beta, Diasorbamine-60, APTES-MCM-41, and SH-PMO) revealed that the maximum yields of 2-phenylindole were achieved with gold supported on mesoporous silicate materials. A high degree of dispersion of gold in these catalysts is explained by the presence of amino or thiol anchor groups in the support composition. It was found by diffuse reflectance infrared Fourier transform spectroscopy and XPS and XRD methods that gold in these catalysts exists on the support surfaces as small metal particles and due to their size dimensions they show a decreased electron density, i.e., they are electron-deficient Au^δ+ nanoparticles.     

Intramolecular hydroamination of conjugated enynes

An intramolecular hydroamination of conjugated enyne was developed using commercially available n-BuLi as a precatalyst. This hydroamination reaction led to products with allene and pyrrolidine functional groups. One of the enyne hydroamination products was successfully converted to natural products irniine and irnidine in three steps. The scope and mechanism of the enyne hydroamination are also discussed.     

Rhodium-catalyzed anti-Markovnikov hydroamination of vinylarenes

The transition metal-catalyzed anti-Markovnikov hydroamination of unactivated vinylarenes with a rhodium complex of DPEphos is reported. The reaction of electron-neutral or electron-rich vinylarenes with a variety of secondary amines in the presence of catalyst forms the products from anti-Markovnikov hydroamination in high yields. Reactions of morpholine, N-phenylpiperazine, N-Boc-piperazine, piperidine, 2,5-dimethylmorpholine, and perhydroisoquinoline reacted with styrene to form the amine product in 51-71% yield. Reactions of a variety of vinylarenes with morpholine generated amine as the major product. Reactions of morpholine with electron-poor vinylarenes gave lower amine:enamine ratios than reactions of electron-rich vinylarenes at the same concentration of vinylarene, but conditions were developed with lower concentrations of electron-poor vinylarene to maintain formation of the amine as the major product. Reactions of dimethylamine with vinylarenes were fast and formed amine as the major product. Mechanistic studies on the hydroamination process showed that the amine:enamine ratio was lower for reactions conducted with higher concentrations of vinylarene and that one vinylarene influences the selectivity for reaction of another. A mechanism proceeding through a metallacyclic intermediate that opens in the presence of a second vinylarene accounts for these and other mechanistic observations.     

Copper-catalyzed intramolecular hydroamination of allenylamines to 3-pyrrolines or 2-alkenylpyrrolidines

Copper salts, such as CuCl, CuI, CuCl₂ and Cu(OTf)₂, were used to catalyze the intramolecular hydroamination of allenylamines to provide the corresponding 3-pyrrolines or 2-alkenylpyrrolidines.