Purinyl N-directed aroylation of 6-arylpurine ribo- and 2'-deoxyribonucleosides, and mechanistic insights.

The purinyl ring contains four embedded nitrogen atoms of varying basicities. Selective utilization of these ring nitrogen atoms can lead to relatively facile remote functionalization, yielding modified purinyl motifs that are otherwise not easily obtained. Herein, we report previously undescribed N-directed aroylation of 6-arylpurine ribo and the more labile 2'-deoxyribonucleosides. Kinetic isotope analysis as well as reaction with a well-defined dimeric, palladated 9-benzyl 6-arylpurine provided evidence for N-directed cyclometallation as a key step, with a plausible rate-limiting C-H bond cleavage. Radical inhibition experiments indicate the likely intermediacy of aroyl radicals. The chemistry surmounts difficulties often posed in the functionalization of polynitrogenated and polyoxygenated nucleosidic structures that possess complex reactivities and a labile glycosidic bond that is more sensitive in the 2'-deoxy substrates.

Covalent and ionic bonding in bi- and tricyclic Group 15 amides: equidistant P-I and As-I bonds and fluxional cations.

A series of trivalent Group 15 bis(tert-butylamido)cyclodiphosph(iii)azane element bi- and tricycles of the formulae {[(tBuNP)2(tBuN)2]ElX}, El = P, As, Sb, Bi, where X = Ph, OPh, OtBu, N3, hexamethyldisilylamide (HMDS), OTf, was synthesized from the corresponding chlorides via salt elimination. The ensuing compounds were studied spectroscopically and X-ray crystallographically with a particular focus on the length of the El-X bond. While the Group 15 element to phenyl and HMDS were of normal lengths and completely covalent, those to azide appeared to be partly ionic. The {[(tBuNP)2(tBuN)2]ElI} showed El-I bonds that were substantially longer than the typical element iodide bonds, suggesting a very high degree of polarity and bordering on ionic bonding. Finally, the triflate [(tBuNP)2(tBuN)2]P] + [SO3CF3]- proved to be an ion pair in the solid state. The antimony analog, however, showed a long covalent Sb-O bond in the solid state, although it appears to dissociate into ions in solution. The phosphonium triflate salt is fluxional and exhibits a previously unseen highly symmetrical structure in solution. The bonding trends from completely covalent to completely ionic are discussed in terms steric restrictions and the delocalization of charge in either the cation or the anion.

The Disappearing Director: The Case of Directed N-Arylation via a Removable Hydroxyl Group.

A facile and broadly applicable method for the regiospecific N-arylation of benzotriazoles is reported. Copper-mediated reactions of diverse 1-hydroxy-1H-benzotriazoles with aryl boronic acids lead to 1-aryl-1H-benzotriazole 3-oxides. A N1-OH → N3 prototropy in the 1-hydroxy-1H-benzotriazoles is plausibly the underlying basis, where the tautomer is captured by the boronic acid, leading to C-N (not C-O) bond formation. Because the N-O bond in amine N-oxides and 1-hydroxy-1H-benzotriazoles can be easily reduced by diboron reagents such as (pinB)2 and B2(OH)4, exposure of the 1-aryl-1H-benzotriazole 3-oxides to B2(OH)4 then leads to facile reduction of the N-O bond resulting in diverse, regiospecifically-arylated benzotriazoles. Thus, the N-hydroxyl group in 1-hydroxy-1H-benzotriazoles acts as a disposable arylation director.

The chameleonic reactivity of dilithio bis(alkylamido)cyclodiphosph(iii)azanes with chlorophosphines.

To synthesize a bis(diphosphinylamine) ligand for use in late-metal catalysis dilithio bis(tert-butylamido)cyclodiphosph(iii)azane was treated with two equivalents of (Ph)2PCl. The chlorodiphenylphosphine attacked the dilithium salt kinetically at phosphorus, followed by a rearrangement to an unsymmetrical P, N-di-substituted product. Solid-state structures of both products were determined, and the activation energy for the rearrangement was measured. To gain further insight into the location of chlorophosphine attack (N versus P) on lithium diamides, dilithio bis(cyclohexylamido)cyclodiphosph(iii)azane or dilithio bis(tert-butylamido)cyclodisilazane was treated with two equivalents of PCl3. In each case only the symmetrically N,N'-substituted bis(PCl2) product was obtained and characterized by X-ray analysis. The structures of these latter compounds are unusual, because in both cases a bicyclic compound with a bis(amido) chelated, central P-Cl moiety was expected based on precedent. The two proximal and juxtaposed PCl2 groups may provide a starting point for potentially novel reactivity studies.

Generating Stereodiversity: Diastereoselective Fluorination and Highly Diastereoselective Epimerization of α-Amino Acid Building Blocks.

Diastereoselective fluorination of N-Boc ( R)- and ( S)-2,2-dimethyl-4-((arylsulfonyl)methyl)oxazolidines and a previously unknown diastereoselective epimerization at the fluorine-bearing carbon atom α to the sulfone was realized. Diastereoselectivities of both reactions were excellent for benzothiazolyl sulfones, allowing access to two enantiomerically pure diastereomers from one chiral precursor. To demonstrate synthetic utility, the benzothiazolyl sulfones were converted to diastereomerically pure ( S, S)- and ( R, S)-benzyl sulfones via sulfinate salts and to amino acids. To understand the diastereoselectivities, DFT analysis was performed.

Reactions of Germylenes and Stannylenes with Halo(hydrocarbyl)- and Chloro(amino)phosphines: Oxidative Addition versus Ligand Transfer.

Oxidative addition (OA) is an important elementary step in chemistry, but it has been studied mainly in the context of transition-metal-catalyzed reactions and mainly with carbon-X substrates (X = halogen, H). Reports of main-group metal compounds undergoing OA are rare by comparison, and those involving phosphorus-halogen substrates are rarer still. Acyclic and cyclic diazagermylenes and -stannylenes react with chloro(hydrocarbyl)phosphines with the intermediacy of oxidative addition products. Stannylenes react faster than germylenes, and these reactions are first-order in both reactants and slowed by steric bulk. Kinetic data and the structures of intermediates and products had suggested an adduct/insertion mechanism for these reactions. To gain further insight into these transformations, the work presented herein was extended to chloro(hydrocarbyl)phosphines with varying organic substituents. These studies confirmed prior conclusions concerning the rate-diminishing effect of steric bulk, and the rate dependence on leaving groups also seems to suggest adduct/insertion or SN2 mechanisms. Importantly, these new data now also point to associative decomposition pathways. In the course of the investigation, it was discovered that aliphatic chloro(amino)phosphines react differently with the carbene analogues, giving oxidative addition products for germylenes but metathesis reactions for stannylenes.

Ruthenium-Catalyzed C-H Bond Activation Approach to Azolyl Aminals and Hemiaminal Ethers, Mechanistic Evaluations, and Isomer Interconversion.

C(sp3)-N bond-forming reactions between benzotriazole and 5,6-dimethylbenzotriazole with N-methylpyrrolidinone, tetrahydrofuran, tetrahydropyran, diethyl ether, 1,4-dioxane, and isochroman have been conducted using RuCl3•3H2O/t-BuOOH in 1,2-dichloroethane. In all cases, N1 and N2 alkylation products were obtained, and these are readily separated by chromatography. One of these products, 1-(isochroman-1-yl)-5,6-dimethyl-1H-benzotriazole, was examined by X-ray crystallography. It is the first such compound to be analyzed by this method, and notably, the benzotriazolyl moiety is quasi-axially disposed, consistent with the anomeric effect. This has plausible consequences, not observed previously. In contrast to other hemiaminal ether-forming reactions, which proceed via radicals, this Ru-catalyzed process is not suppressed in the presence of a radical inhibitor. Therefore, an oxoruthenium-species-mediated rapid formation of an oxocarbenium intermediate is believed to occur. In the radical-trapping experiment, previously unknown products containing both the benzotriazole and the TEMPO unit have been identified. In these products, it is likely that the benzotriazole is introduced via a Ru-catalyzed C-N bond formation, whereas C-O bond-formation with TEMPO occurs via a radical reaction. We show that reactions of THF with TEMPO are influenced by ambient light. A competitive reaction of THF and THF-d8 with benzotriazole indicated that C-H bond cleavage occurs ca. 5 times faster than C-D cleavage. This is comparable to other metal-mediated radical reactions of THF, but lower than that observed for a reaction catalyzed by n-Bu4N+I-. Detailed mechanistic experiments and comparisons are described. The catalytic system was also evaluated for reactions of benzimidazole, imidazole, 1,2,4-triazole, and 1,2,3-triazole with THF, and successful reactions were achieved in each case. In the course of our studies, we discovered an unexpected but significant isomerization of some of the benzotriazolyl hemiaminal ethers. This is plausibly attributable to the pseudoaxial orientation of the heterocycle in the products and the stability of oxocarbenium ions, both of which can contribute to C-N bond cleavage and reformation. Predominantly, the N2-isomers rearrange to the N1-isomers even upon storage at low temperature! This previously unknown phenomenon has also been studied and described.

Heavier carbene analogues and their derivatives as κ-El, κ2-El,N, and κ2-N,N' ligands: different reactivity patterns for acyclic and cyclic diamidogermylenes and -stannylenes.

Because of the increasing importance of N-heterocyclic carbenes in organometallic chemistry we investigated the ligand properties of structurally-related acyclic and cyclic heavier carbene analogues with transition metal chlorides. Acyclic {(Me(3)Si)(2)N}(2)El, El = Ge and Sn, react with CuCl with transfer of one (Me(3)Si)(2)N ligand to yield the known copper tetramer {(Me(3)Si)(2)NCu}(4). The cyclic Me(2)Si(µ-N(t)Bu)(2)Ge, by contrast, binds copper through germanium only, furnishing a tetranuclear ladder structure with both terminal and bridging germylenes. The tin homologue, however, inserts into the CuCl bond, and the ensuing {Me(2)Si(µ-N(t)Bu)(2)SnCl}(-) ions then coordinate one copper ion via their tin atoms while sandwiching the remaining three copper ions in an unprecedented κ(2)-N,N' fashion. Chemically-harder Cr(II)--created in a redox reaction of Me(2)Si(µ-N(t)Bu)(2)Sn with CrCl(3)(THF)(3)--is not coordinated by tin, but chelated by both nitrogen atoms of one {Me(2)Si(µ-N(t)Bu)(2)SnCl}(-) ion and more weakly through the tin-bound chloride.

Syntheses and structures of cationic and neutral, homo- and heteroleptic tert-butoxides of the group 4 metals.

The utility of tri-tert-butoxystannate as a chelating tridentate ligand for group 4 metals was investigated. The highly Lewis acidic metals degraded the stannate ion in a series of tert-butoxide abstraction steps to produce a variety of group 4 tert-butoxides. A total of 1 equiv of NaSn(O(t)Bu)(3) reacted with cis-MCl(4)(THF)(2) [M = Zr (1), Hf (2)] in THF solutions to furnish the salts fac-{[M(O(t)Bu)(3)(THF)(3)](SnCl(3))}, which are separated ion pairs featuring weakly coordinating trichlorostannate ions. Neutral complexes, namely, [M(O(t)Bu)(2)Cl(2)(THF)(2)] [M = Zr (3), Hf (4)], were isolated when 2/3 equiv of sodium stannate was used in these reactions. Titanium tetrachloride formed analogues neither of 1 and 2 nor of 3 and 4, but Ti(O(t)Bu)(3)Cl reacted with silver triflate to give [Ti(O(t)Bu)(2)(OTf)(2)(THF)(2)] (5). Anion exchange of triflate for trichlorostannate transformed 1 to the contact ion pair fac-[Zr(O(t)Bu)(3)OTf(THF)(2)] (6). A total of 2 equiv of NaSn(O(t)Bu)(3) reacted with cis-MCl(4)(THF)(2) to give the complexes fac-[Sn(mu-O(t)Bu)(3)M(O(t)Bu)(3)] [M = Zr (7), Hf (8)]. Tri-tert-butoxystannate may be used as a selective alkoxylating agent for group 4 metals, and it can be transferred to these metals intact if their Lewis acidity is appropriately attenuated as in fac-{[M(O(t)Bu)(3)(THF)(3)](SnCl(3))}. Single-crystal X-ray studies revealed distorted octahedral coordination geometries for all compounds (1-8), with 1, 2, 7, and 8 being crystallographically C(3) symmetric.

Reversal of polarization in amidophosphines: neutral- and anionic-kappaP coordination vs. anionic-kappaP,N coordination and the formation of nickelaazaphosphiranes.

Nickel(ii) chloride reacts with the bis(tert-butylamino)diazadiphosphetidine {Bu(t)(H)NP(micro-NBu(t))(2)PN(H)Bu(t)} to form trans-[{Bu(t)(H)NP(micro-NBu(t))(2)PN(H)Bu(t)}(2)NiCl(2)]. In solution and the solid-state each heterocyclic ligand coordinates nickel through one phosphorus atom only. For comparison the solid-state structure of the known trans-[NiCl(2)(PEt(3))(2)] was also determined and it was found that the two complexes have almost identical bond parameters about nickel. The nickel-amidophosphine complexes [{Bu(t)OP(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))], [(PBu(n)(3))ClNi{Bu(t)NP(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))], and [{Me(2)Si(micro-NBu(t))(2)PNBu(t)}NiCl(PBu(n)(3))] were synthesized and X-ray structurally characterized. In these mono- and di-nuclear nickel complexes the nickel ions are coordinated in pseudo square-planar fashions, by one trialkylphosphine ligand, one chloride ligand and one kappaP,N-coordinated amidophosphine moiety from tert-butylamido-substituted heterocycles. Attempts to create nickel complexes chelated in a kappa(2)P fashion by the o-phenylenediamine-tethered mono- and di-anionic 1-{Me(2)Si(micro-NBu(t))(2)PN} 2-{Me(2)Si(micro-NBu(t))(2)PNH}C(6)H(4) and 1,2-{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4), respectively, afforded instead [1,2-{Me(2)Si(micro-NBu(t))(2)PN}{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4)NiCl] and [1,2-{Me(2)Si(micro-NBu(t))(2)PN}{Me(2)Si(micro-NBu(t))(2)PN}C(6)H(4)Ni{PEt(3)}], each complex having kappaP,N and kappaP coordinated amidophosphine ligands.

Inorganic heterocyclic bis(phosphines): syntheses and structures of a 1,2-bis(diazasilaphosphetidino)ethane and its nickel, molybdenum, and rhodium complexes.

Synthesis and characterization of a new, highly electron-rich, chelating bis(phosphine), based on the ethanediyl-linked inorganic heterocycle [Me(2)Si(mu-N(t)Bu)(2)P], are reported. Treatment of nickel chloride with this bis(phosphine) afforded square-planar cis-[[Me(2)Si(mu-N(t)Bu)(2)PCH(2)](2)NiCl(2)], which features isometric nickel-chloride (2.2220(8) A) and nickel-phosphorus (2.1572(8) A) bonds. The ligand reacted with cis-[(piperidine)(2)Mo(CO)(4)] to form colorless cis-[[Me(2)Si(mu-N(t)Bu)(2)PCH(2)](2)Mo(CO)(4)], which has distorted octahedral geometry and long Mo-P bonds (2.5461(18) A). Because of its potential applications in hydrogenation catalysis cis-[[Me(2)Si(mu-N(t)()Bu)(2)PCH(2)](2)Rh(COD)]BF(4) was synthesized. This square-planar, cationic rhodium(I) complex, having symmetrical Rh-P (2.250(2) A) and Rh-C (2.305(6) A) bonds, is structurally related to bis(phospholano)- and bis(phosphetano)rhodium species.

Syntheses and structures of P-anilino-P-chalcogeno- and P-anilino-P-iminodiazasilaphosphetidines and their group 12 and 13 metal compounds.

The P-anilino-P-chalcogeno(imino)diazasilaphosphetidines [Me(2)Si(mu-N(t)Bu)(2)P=E(NHPh)] (E = O (3), S (4), Se (5), N-p-tolyl (6)) were synthesized by oxidizing the P-anilinodiazasilaphosphetidine [Me(2)Si(N(t)Bu)(2)P(NHPh)] (2) with cumene hydroperoxide, sulfur, selenium, and p-tolyl azide, respectively. The lithium salt of 4 reacted with thallium monochloride to produce ([Me(2)Si(mu-N(t)Bu)(2)P=S(NPh)-kappaN-kappaS]Tl)(7), which features a two-coordinate thallium atom. Treatment of 4-6 with AlMe(3) gave the monoligand dimethylaluminum complexes ([Me(2)Si(mu-N(t)Bu)(2)P=E(NPh)-kappaN-kappaE]AlMe(2)) (E = S (8), Se (9), N-p-tolyl (10)), respectively. In these complexes the aluminum atom is tetrahedrally coordinated by one chelating ligand and two methyl groups, as a single-crystal X-ray analysis of 8 showed. A 2 equiv amount of 4-6 reacted with diethylzinc to produce the homoleptic diligand complexes ([Me(2)Si(mu-N(t)Bu)(2)P=E(NPh)-kappaN-kappaE](2)Zn)(E = S (11), Se (12), N-p-tolyl (13)). A crystal-structure analysis of 11 revealed a linear tetraspirocycle with a tetrahedrally coordinated, central zinc atom.

Monomeric, Four-Coordinate Group 4 Metal Complexes with Chelating Bis(tert-butylamido)cyclodisilazane Ligands: Syntheses and Molecular Structures of {(MeSiN(t)Bu)(2)(N(t)Bu)(2)}MCl(2) and {(MeSiN(t)Bu)(2)(N(t)Bu)(2)}MMe(2), M = Zr, Hf.

The interaction of (MeSiN(t)Bu)(2)(N(t)BuLi)(2) with MCl(4) in hot toluene produces {(MeSiN(t)Bu)(2)(N(t)Bu)(2)}MCl(2,) M = Zr (1), Hf (2), in good yields. The light-yellow and colorless solids are isostructural and crystallize in the monoclinic crystal system, space group P2(1)/n, with four molecules in the unit cell. The unit cell dimensions for 1 (293 K) are a = 9.174(6) Å, b = 18.027(13) Å, c = 16.515 Å, beta = 98.81(6) degrees, and those for 2 (213 K) are a = 9.1871(1) Å, b = 17.8553(2) Å, c = 16.4770(3) Å, beta = 99.339(1) degrees. The metal atoms are pseudotetrahedrally coordinated by cyclodisilazane and chloride ligands but have one additional weak bonding interaction with a cyclodisilazane ring-nitrogen atom. Treatment of 1 or 2 with 2 equiv of MeMgCl in ether affords the corresponding dimethyl species {(MeSiN(t)Bu)(2)(N(t)Bu)(2)}MMe(2), M = Zr (3), Hf (4). These isostructural, colorless organometallic compounds are remarkably stable to atmospheric oxygen and moisture. They crystallize in the monoclinic space group C2/c, Z = 4. The cell parameters for 3 (298 K) are a = 18.867(2) Å, b = 9.361(1) Å, c = 18.059(3) Å, beta = 119.49(1) degrees, and those for 4 (212 K) are a = 18.8066(4) Å, b = 9.3208(2) Å, c = 17.8725(3) Å, beta = 119.508(1) degrees. The dimethyl species are structurally very similar to the dichloro complexes 1 and 2, with the notable exception that the metal centers in these organometallic compounds are truly four-coordinate.

Formation of Metal Clusters or Nitrogen-Bridged Adducts by Reaction of a Bis(amino)stannylene with Halides of Two-Valent Transition Metals.

When the cyclic bis(amino)stannylene Me(2)Si(NtBu)(2)Sn is allowed to react with metal halides MX(2) (M = Cr, Fe, Co, Zn; X = Cl, Br [Zn]) adducts of the general formula [Me(2)Si(NtBu)(2)Sn.MX(2)](n) are obtained. The compounds are generally dimeric (n = 2) except the ZnBr(2) adduct, which is monomeric in benzene. The crystal structures of [Me(2)Si(NtBu)(2)Sn.CoCl(2)](2) (triclinic, space group &Pmacr;1; a = 8.620(9) Å, b = 9.160(9) Å, c = 12.280(9) Å, alpha = 101.2(1) degrees, beta = 97.6(1) degrees, gamma = 105.9(1) degrees, Z = 1) and of [Me(2)Si(NtBu)(2)Sn.ZnCl(2)](2) (monoclinic, space group P2(1)/c; a = 8.156(9) Å, b = 16.835(12) Å, c = 13.206(9) Å, beta = 94.27(6) degrees, Z = 2) were determined by X-ray diffraction techniques. The two compounds form similar polycyclic, centrosymmetrical assemblies of metal atoms bridged by chlorine or nitrogen atoms. While in the case of the cobalt compound Co is pentacoordinated by three chlorine and two nitrogen atoms, in the zinc derivative Zn is almost tetrahedrally coordinated by three chlorine atoms and one nitrogen atom. The iron derivative [Me(2)Si(NtBu)(2)Sn.FeCl(2)](2) seems to be isostructural with the cobalt compound as can be deduced from the crystal data (triclinic, a = 8.622(7) Å, b = 9.158(8) Å, c = 12.353(8) Å, alpha = 101.8(1) degrees, beta = 96.9(1) degrees, gamma = 105.9(1) degrees, Z = 1). If NiBr(2), PdCl(2), or PtCl(2) is combined with the stannylene, the reaction product is totally different: 4 equiv of the stannylene are coordinating per metal halide, forming the molecular compound [Me(2)Si(NtBu)(2)Sn](4)MX(2), which crystallizes with half a mole of benzene per molecular formula. The crystal structures of [Me(2)Si(NtBu)(2)Sn](4).NiBr(2).(1)/(2)C(6)H(6) (tetragonal, space group I4(1)/a, a = b = 43.86(4) Å, c = 14.32(2) Å, Z = 16) and [Me(2)Si(NtBu)(2)Sn](4).PdCl(2).(1)/(2)C(6)H(6) (tetragonal, space group I4(1)/a, a = b = 43.99(4) Å, c = 14.318(14) Å, Z = 16) reveal the two compounds to be isostructural. The molecules have an inner Sn(4)M pentametallic core (mean distances: Sn-Ni 2.463 Å, Sn-Pd 2.544 Å) with the transition metal in the center of a slightly distorted square formed by the four tin atoms, the distortion from planarity resulting in a weak paramagnetism of 0.2 &mgr;(B) for the nickel compound. The halogen atoms form bridges between two of the tin atoms and have no bonding interaction with the transition metal. The nickel compound has also been prepared by direct interaction of Br(2) or NR(4)Br(3) with [Me(2)Si(NtBu)(2)Sn](4)Ni as a minor product, the main products being Me(2)Si(NtBu)(2)Sn(NtBu)(2)SiMe(2,) Me(2)Si(NtBu)(2)SnBr(2), NiBr(2) and SnBr(2). Other metal clusters have been obtained by the reaction of Me(2)Si(NtBu)(2)Sn with tetrakis(triphenyphosphine)palladium or by the reaction of Me(2)Si(NtBu)(2)Ge with RhCl(PPh(3))(3). In the first case Ph(3)PPd[Sn(NtBu)(2)SiMe(2)](3)PdPPh(3) (rhombohedral, space group R3c, a = b = 21.397(12) Å, c = 57.01(5) Å, alpha = beta = 90 degrees, gamma = 120 degrees, Z = 12) is formed and is characterized by X-ray techniques to be composed of a central PdSn(3)Pd trigonal bipyramid with the tin atoms occupying the equatorial positions (Pd-Sn = 2.702(5) Å). In the second reaction all the triphenylphosphine ligands are replaced from rhodium and Rh[Ge(NtBu)(2)SiMe(2)](4)Cl is formed (monoclinic, space group P2(1)/n, a = 12.164(2) Å, b = 23.625(5) Å, c = 24.128(5) Å, beta = 102.74(3) degrees, Z = 4). The central core of this molecule is made up of a rhodium atom which is almost square planarly coordinated by the germanium atoms, two of which are bridged by chlorine (mean Ge-Rh = 2.355 Å).