Valence Isomerization of a 1,3-Diphosphacyclobutane-2,4-diyl: Photochemical Ring Closure to 2,4-diphosphabicyclo[1.1.0]butane and Its Thermal Ring Opening to gauche-1,4-Diphosphabutadiene

The bond stretch isomer 1,3-diphosphacyclobutane-2,4-diyl 1 was transformed photochemically to give the previously unknown 2,4-diphosphabicyclo[1.1.0]butane 2 , which itself can be converted thermally into gauche-1,4-diphosphabutadiene 3 . The crystal structures of these three energy-rich valence isomers of 1,2-diphosphete have been determined. R=SiMe₃; Mes*=2,4,6-tBu₃C₆H₂.     

Synthesis and properties of air-stable 1,3-diphosphacyclobutane-2,4-diyls and the related compounds

A kinetically stabilized phosphaalkyne bearing a bulky Mes^∗ (2,4,6-tri-t-butylphenyl) group is useful compound to prepare an enormous number of highly stable 1,3-diphosphacyclobutane-2,4-diyls through reactions with a lithium reagent and an electrophile. By utilizing this synthetic protocol, we prepared several non-symmetrical 1,3-diphosphacyclobutane-2,4-diyls in which the substituents on the phosphorus are different. Furthermore, we succeeded in preparation and characterization of novel air-tolerant symmetrical 2,4-bis(2,4,6-tri-t-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyls bearing the identical alkyl substituents on the phosphorus atoms. Structures and properties of the 1,3-diphosphacyclobutane-2,4-diyls indicate characters as singlet ground-state carbon centered biradicals. In addition to those biradicals, we succeeded in preparation and isolation of a novel P-heterocyclic air-stable neutral radical as well as a P-heterocyclic cation radical.     

Synthesis of phosphorus ylides bearing a P-H bond from a kinetically stabilized 1,3,6-triphosphafulvene

In mixing 2,4,6-tris(2,4,6-tri-tert-butylphenyl)-1,3,6-triphosphafulvene with alkyllithium compounds and acetic acid, both of nucleophilic alkylation and electrophilic protonation occurred at the exo sp2-phosphorus atoms to afford [2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclopentadienylidene](alkyl)(2,4,6-tri-tert-butylphenyl)phosphoranes which are phosphorus ylides that bear a P-H bond. A phosphorus ylide bearing both P-H and P-F bonds was obtained by reaction of 2,4,6-tris(2,4,6-tri-tert-butylphenyl)-1,3,6-triphosphafulvene with hydrogen tetrafluoroborate, and the structure was determined by X-ray crystallography. Both P=C double bond and P(+)-C(-) zwitterionic character was indicated by the metric parameters. The isolated phosphorus ylide bearing a P-H bond, [2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclopentadienylidene](2,4,6-tri-tert-butylphenyl)phosphorane, showed no isomerization by H-migration to the corresponding phosphinodiphospholes, probably due to the pi-accepting ability of the unsaturated PC bonds and aromaticity of the C3P2 ring. The ylide structure and aromaticity of 2,4-diphosphacyclopenta-2,4-dienylidenephosphorane was characterized by theoretical calculations. In addition, the regioselective protonation of the lithiated phosphinodiphospholes generated from the 1,3,6-triphosphafulvene is discussed.     

Dissociation of 2,4-bis(2,4,6-tri-tert-butylphenyl)-cyclo-1,3-dipnicta-2,4-diazanes (pnict = P, As, Sb) imposed by substituent steric strain: a cyclobutane/olefin analogy

Three 2,4-bis(Mes)-cyclo-1,3-dipnicta-2,4-diazanes, 2PnX, have been synthesized as 1,3-dichloro- (Pn = As; X = Cl) or 1,3-bis(triflato)- (Pn = P, Sb; X = OTf) derivatives (Mes = 2,4,6-tri-tert-butylphenyl; OTf = triflato = trifluoromethanesulfonoxy). The compounds have been structurally characterized as cyclodimers, but spectroscopic characterization of melts and solutions indicates facile dissociation of the diphosphadiazane 2POTf derivative to give the corresponding iminophosphine 1POTf and of the diarsadiazane 2AsCl derivative to give the corresponding iminoarsine 1AsCl. 1,3-Bis(triflato)-2,4-bis(Mes)-cyclo-1,3-distiba-2,4-diazane (2SbOTf) does not dissociate in the melt or in solution. The presence of the sterically bulky Mes substituent does not preclude association of N-Pn olefin analogues to give single bonded cyclobutane analogues. The facile dissociation of 2POTf and isolation of 1POTf implicates a relatively high degree of substituent steric strain in the dimer. In comparison, dissociation of 2AsCl is only apparent in the melt and in solution, likely the result of the lower substituent strain in the larger N(2)As(2) framework. The largest N(2)Pn(2) framework in 2SbOTf provides sufficient space for the Mes substituents, and the monomer is not observed under the conditions examined.     

Preparation of 2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutenes from 2-(2,4,6-tri-tert-butylphenyl)-1-phosphaethyne and alkyllithiums

2-(2,4,6-Tri-tert-butylphenyl)-1-phosphaethyne was allowed to react with 0.5 eq. of alkyllithium to afford 2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutenes through an intramolecular cyclisation of a 1,3-diphosphabuta-1,3-diene intermediate.     

Synthesis, structure, and reactivity of novel 3,4-diphosphacyclobutenes bearing silyl groups

[reaction: see text] Copper-mediated homocoupling of sterically hindered 2-(2,4,6-tri-tert-butylphenyl)-1-trialkylsilyl-2-phosphaethenyllithiums afforded 1,2-bis(trialkylsilyl)-3,4-diphosphacyclobutenes (1,2-dihydrodiphosphetenes) through a formal electrocyclic [2+2] cyclization in the P=C-C=P skeleton as well as 2-trimethylsilyl-1,4-diphosphabuta-1,3-diene. Reduction of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes followed by quenching with electrophiles afforded ring-opened products, (E)-1,2-bis(phosphino)-1,2-bis(trimethylsilyl)ethene and (Z)-2,3-bis(trimethylsilyl)-1,4-diphosphabut-1-ene. The structures of the ring-opened products indicated E/Z isomerization around the C=C bond after P-P bond cleavage of 5, and the isomerization of the P-C=C skeleton. Ring opening of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes affording (E,E)- and (Z,Z)-1,4-diphosphabuta-1,3-dienes was observed upon desilylation.     

Syntheses and reactivity of novel unsaturated cyclic compounds containing phosphorus atoms

Kinetic stabilization of compounds containing heavy main-group elements through the use of bulky substituents is of current interest. The widely used 2,4,6-tri-t-butylphenyl group (Mes*) is recognized as a powerful bulky protecting group and has enabled us to successfully prepare various types of phosphorus compounds with unusual structures. When a phosphaalkyne carrying Mes* was treated with tBuLi and then quenched with MeOH, a 1,3-diphosphacyclobutene was obtained, whereas, when MeI was used as a quencher, a 1,3-diphosphacyclobutane-2,4-diyl was formed almost quantitatively as a stable biradical compound. The reaction mechanism for the formation of both compounds can be explained by a phosphide intermediate, which can be formed via dimerization of the phosphaalkyne promoted by tBuLi. Some other diphosphacyclobutane-2,4-diyls with various substituents were prepared in a similar fashion and showed interesting reactivities including ring expansion, oxidation, isomerization and so on.     

SYNTHESIS AND CHARACTERIZATION OF KINETICALLY STABILIZED 1,3-DIPHOSPHACYCLOBUTENE DERIVATIVES

1-(2,4,6-Tri-tert-butylphenyl)-2-phosphaethyne (1) was allowed to react with 0.5 equiv of an alkyllithium and subsequently with an alcohol to afford a bulky 1,3-diphosphacyclobutene, and its structure and coordination properties on transition metals were investigated. On the other hand, 1 was allowed to react with an alkyllithium and iodomethane to form a stable biradical, 1,3-diphosphacyclobutane-2,4-diyl.     

Mannich Reaction as a Convenient Route to New Macrocyclic Compounds Containing an Uracil Fragment

Mannich reaction of 1,3-bis[4-(2-mercaptomethylphenoxy)butyl]-6-methyl-1,2,3,4-tetrahydropyrimidine-2,4-dione, formaldehyde, and N-benzylamine afforded a 24-membered macrocyclic compound containing an uracil fragment, 1,3-(11-benzyl-6,7:15,16-dibenzo-5,17-dioxa-9,13-dithia-11-aza-6,15-heneicosadiene-1,21-diyl)-6-methyl-1,2,3,4-tetrahydropyrimidine-2,4-dione.     

Nucleophilic attack toward group 4 metal complexes bearing reactive 1-Aza-1,3-butadienyl and imido moieties

Unique (1-aza-2-butenyl)titanium complexes bearing a phosphonium ylide moiety [Ti=NTbt{C(Me)(PR3)CH=C(Me)N(Mes)}Cl] (3-5, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes = 2,4,6-trimethylphenyl) were formed by the nucleophilic attack of PMe3, P(n-Bu)3, and 1,2-bis(dimethylphosphino)ethane (dmpe) toward the corresponding (1-aza-1,3-butadienyl)titanium complex, [Ti=NTbt{C(Me)CHC(Me)N(Mes)}(mu-Cl)2Li(tmeda)] (2a). The reaction of a lithium beta-diketiminate, [Li{N(Tbt)C(Me)CHC(Me)N(Mes)}] (1) with [TiIICl2(dmpe)2] also resulted in the formation of the same complex 5. Density functional theory calculation indicated that the negative charge of the model molecule of 3 was slightly delocalized to the C3N plane. In addition, the calculation of the model molecule of 2a suggested the electrophilicity of 2a at the carbon atom connecting to the titanium atom. Interestingly, the reaction of zirconium and hafnium analogues (2b and 2c) with PMe3 and dmpe did not proceed. In contrast to the cases of phosphine reagents, pyridine which was found to undergo the nucleophilic attack toward the titanium center of 2a gave the pyridine-coordinated titanium-imide [Ti=NTbt{C(Me)CHC(Me)N(Mes)}Cl(py)] (7).     

The first structurally authenticated alkali metal 1,3-diphosphaallyl complex [[Bu(t)C(PMes)2]Li(thf)3] (Mes = 2,4,6-Me3C6H2): an alternative synthetic approach to substituted 1,3-diphosphaallyl complexes

Reaction of the phosphavinyl Grignard reagent [Z-MesP=C(Bu(t))MgBr.OEt2]2 (Mes = 2,4,6-Me3C6H2) with MesPCl2 affords the corresponding 1,3-diphosphapropene compound [Z-MesP=C(Bu(t))P(X)Mes] (X = Cl, Br); subsequent reaction with two equivalents of elemental lithium in thf affords the title compound [[Bu(t)C(PMes)2]Li(thf)3], which contains an asymmetric eta1-1,3-diphosphaallyl ligand.     

Photochemistry of heterocycles: VIII. photocycloaddition of acetylenic compounds to 1,3-dimethyl-6-azathymine

The photocycloadditions of 1,3-dimethyl-6-azathymine to 2-butyne-1,4-diol and 1,4-dimethoxyl-2-butyne have been studied. Three novel compounds, 7,8-bis(hydroxymethyl)-3,5-dioxo-2,4, 6-trimethyl-1,2,4-triazabicyclo [4,2,0]-7-octene (1), 7,8-bismethoxymethyl-3,5-dioxo-2,4,6-trimethyl- 1,2,4-triazabicyclo[4,2,0]-7-octene (2) and 8,9-bismethoxymethyl-4-oxo-1,3,5-trimethyl-7-oxa-2,3,5-triazaspiro[5,3]- 1,8-nonadiene (3) were obtained. The proposed reaction mechanism, which includes excited triplet complexes and biradicals as intermediates, was supported by kinetic and photophysical studies.     

Synthesis of some 1,3,8-trisubstituted pyrimido[4,5-c][2,7]-naphthyridin-6-ones as potential antitumor agents

Condensation of three 2,4-disubstituted 6-aminopyrimidines with methyl 1-benzyl-4-oxo-3-piperidinecarboxylate afforded, in each case, new tricyclic, angular 1,3,8-trisubstituted pyrimido[4,5-c][2,7]naphthyridin-6-ones. 2,4,6-Triaminopyrimidine gave the 7,8,9,10-tetrahydrocyclo condensed product 5 as anticipated. However, the use of 2-amino-4-oxo- or 2,4-dioxo-6-aminopyrimidine afforded the dehydrogenated, 9,10-dihydrotricyclic products 11 and 12 . The growth of leukemia L1210 cells in culture were inhibited 50% by the 1,3-diamino analog 5 at 2 × 10⁻⁶M and by the 1,3-dioxo analog 12 at 10⁻⁵M.     

Monomeric lithium triazapentadienyl complexes

Treatment of 1,3,5-triazapentadienes [N{(C3F7)C(Mes)N}2]H and [N{(C3F7)C(Dipp)N}2]H (where Mes = 2,4,6-Me3C6H2; Dipp = 2,6-Pr(i)2C6H3) with n-BuLi in hexane, followed by the crystallization from hexane-THF mixture afforded the corresponding lithium 1,3,5-triazapentadienyl complexes as their THF solvates. X-Ray crystallographic analyses revealed that [N{(C3F7)C(Mes)N}2]Li(THF)2 and [N{(C3F7)C(Dipp)N}2]Li(THF) are monomeric in the solid state. [N{(C3F7)C(Mes)N}2]Li(THF)2 has a four-coordinate lithium center with a distorted tetrahedral geometry, and features a boat-shaped C2N3Li metallacycle. [N{(C3F7)C(Dipp)N}2]Li(THF) has a three-coordinate lithium atom and a planar, U-shaped C2N3 ligand backbone. The synthesis, solid-state structure, and 1H and 19F NMR spectroscopic details of [N{(C3F7)C(Mes)N}2]H are also reported.     

Catalyst free synthesis of fused pyrido[2,3-d]pyrimidines and pyrazolo[3,4-b]pyridines in water

A one-pot,three-component condensation reaction of an aldehyde,benzoyl acetonitrile(3-oxo-3-phenylpropane nitrile) and 6- amino-1,3-dimethylpyrimidine-2,4(1H,3H)-dione or 3-methyl-1-phenyl-1H-pyrazol-5-amine in water to give fused pyrido[2,3- d]pyrimidines and pyrazolo[3,4-b]pyridines in high yields without any catalyst,is described.     

Synthesis of Some 3-Substituted 1,2-Dihydroindoles

The reaction of 4-oxo-2-(2-oxo-1,2-dihydroindol-3-ylidene)hydrazone-1,3-thiazin-6-methyl carboxylate 2 with hydrazine hydrate in methanol gave 4-oxo-2-(2-oxo-1,2-dihydroindol-3-ylidene)hydrazone-1,3-thiazin-6-carbonylhydrazine 3. Furthermore, the reaction of 3 with carbon disulfide and then hydrazine hydrate afforded 3-[6-(4-amino-5-thioxo-4,5-dihydro-1H-1,2,4-triazol-3-yl)-4-oxo-1,3-thiazin-2-yl] hydrazone-1,3-dihydroindol-2-one 5. the latest reacted with DMAD to give {6-hydroxy-3-[4-oxo-2-(2-oxo-1,2-dihydroindol-3-ylidene)hydrazone-1,3-thiazin-6yl]-[1,2,4]triazolo[3,4-b][1,3,4]thiadiazin-7-ylidene}methoxycarbonylmethylene 6.     

Hypervalent, low-coordinate phosphorus(III) centers in complexes of the phosphadiazonium cation with chelate ligands

Trifluoromethylsulfonyloxy-(2,4,6-tri-tert-butylphenylimino)phosphine, Mes*NPOTf (Mes = 2,4,6-tri-tert-butylphenyl, OTf = trifluoromethanesulfonate, triflate) reacts quantitatively with the multifunctional ligands 2,2'-bipyridine (2,2'-BIPY), N,N,N',N'-tetramethylethylenediamine (TMEDA), 1,2-bis(diethylphosphino)ethane (DEPE), 1,2-bis(diphenylphosphino)ethane (DIPHOS), and N,N,N',N' ',N' '-pentamethyldiethylenetriamine (PMDETA) to give the Lewis acid-base complexes [Mes*NP(2,2'-BIPY)][OTf], [Mes*NP(TMEDA)][OTf], [Mes*NP(DIPHOS)][OTf], [Mes*NP(DEPE)][OTf], and [Mes*NP(PMDETA)][OTf], respectively. Single-crystal X-ray diffraction studies indicate that the closest contact of the ligand donor atoms occurs at phosphorus in all cases, affecting significant displacement of the OTf anion. The resulting cations [Mes*NP(L)]+ are best described as complexes of a neutral chelating ligand on a phosphadiazonium Lewis acceptor, and highlight the potential for electron-rich centers to behave as Lewis acids despite the presence of a lone pair of electrons at the acceptor site. More importantly, the new complexes represent rare examples of systems containing hypervalent, low-coordinate phosphorus(III) centers.     

The reactivity of phosphagermaallene Tip(t-Bu)Ge=C=PMes* with doubly and triply bonded nitrogen compounds

Phosphagermaallene Tip(t-Bu)Ge=C=PMes* (1; Mes* = 2,4,6-tri-tert-butylphenyl, Tip = 2,4,6-triisopropylphenyl) gives, with N-benzylidenemethylamine and pivalonitrile, [2+2] cycloadditions between the Ge=C double bond and the C=N and C≡N unsaturations, leading to the formation of the corresponding four-membered heterocycles 2 and 9. With N-tert-butyl-α-phenylnitrone and benzonitrile oxide, [2+3] cycloadditions occur to form the five-membered ring derivatives 6 and 7. By treatment of 1 with derivatives which possess weak acidic hydrogens in α of the C=N or C≡N multiple bond, two types of reactions were observed: an ene reaction with methyl(benzylideneamino)acetate and a 1,2 addition with acetonitrile to afford azadienyl(germyl)ether (4) and 3-germa-1-phosphapropene (8), respectively. In the case of benzonitrile, phosphagermaallene 1 behaves as a 1,3-dipole, to give, via a cyclic phosphagermacarbene intermediate, the tricyclic derivative 10.     

Synthesis and properties of the first stable silylene-isocyanide complexes

The first stable silylene-isocyanide complexes, [Tbt(Mes)SiCNAr] (5 c: Ar=Tip, 5 d: Ar=Tbt, 5 e: Ar=Mes*; Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes=mesityl, Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) were successfully synthesized by the reaction of a kinetically stabilized disilene, [Tbt(Mes)Si=Si(Mes)Tbt] (1), with bulky isocyanides, ArNC (3c-e). The spectroscopic data of 5 c-e and theoretical calculations for a model molecule indicated that 5 c-e are not classical cumulative compounds but the first stable silylene-Lewis base complexes. The reactions of 5 c-e with triethylsilane and 1,3-dienes gave the corresponding silylene adducts, and they underwent isocyanide-exchange reactions in the presence of another isocyanide at room temperature. These results indicate dissociation of complexes 5 c-e to the corresponding silylene 2 and isocyanides 3 c-e under very mild conditions. The reaction of 5 c with methanol gave the MeOH adduct 16, [Tbt(Mes)SiHC(OMe)NTip], which has a hydrogen atom on the silicon atom. This regioselectivity can be explained in terms of the contribution of zwitterionic resonance structures D and E, which have an anion on the silicon atom. This result indicates that 5 c is not a classical cumulene having Si=C double bonds that should react with methanol to give adducts bearing a methoxyl group on the silicon atom. Although the reactions of 5 c-e with electrophilic reagents such as methanol, hydrogen chloride, and methyl iodide gave the formal silylene adducts, the studies on the reaction mechanism by trapping experiments and the observation of the intermediate suggested that the reaction mainly or partially proceeds by initial nucleophilic attack of the silicon atom, as is the case in the formation of 16 in the reaction of 5 c with methanol. It was revealed that 5 c-e show the nucleophilicity of the silicon atom, most likely resulting from the contribution of the zwitterionic resonance structures D and E.     

Alkylation of Pyrimidine Derivatives with Chloroacetic Acid Esters

Alkylation of 6-methyluracil, 5-hydroxy-6-methyluracil, and 6-methyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-ylammonium sulfate with isopropyl and ethyl chloroacetates afforded previously unknown alkyl 2-(6-methyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-1-yl)acetates, alkyl 2-(1-alkoxycarbonylmethyl-6-methyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-3-yl)acetates, 1,3-bis(alkoxycarbonylmethyl)-6-methyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-ylammonium sulfates, and alkyl 2-[1,3-bis(alkoxycarbonylmethyl)-6-methyl-2,4-dioxo-1,2,3,4-tetrahydropyrimidin-5-yloxy]acetates.