Insertion of ethyl diazoacetate into the platinum–carbon bond of Pt(diphosphine)(halide)(aryl) complexes. X-ray structure of the Pt{(2<Emphasis Type="BoldItalic">S</Emphasis>,4<Emphasis Type="BoldItalic">S</Emphasis>)-bdpp)}I(Ph) complex

Pt(diphosphine)X(aryl) complexes [diphosphine = 1,3-bis(diphenylphosphino)propane (dppp), 2,4-bis(diphenylphos phino)pentane (bdpp); aryl = phenyl, 2-thiophenyl; X = Cl, I] have been reacted with ethyl diazoacetate in chloroform. It has been revealed by in␣situ n.m.r. studies that the starting compounds insert the carbene, formed from ethyl diazoacetate, into the Pt–aryl group resulting in Pt(diphosphine)X{CH(aryl)COOC₂H₅}. Depending on the reaction conditions (reaction time, ratio of the reactants) and the ligands various side-reactions have been observed: (i) the formation of Pt(diphosphine)X₂ in chloroform, (ii) the insertion of the :CHCOOC₂H₅ fragment into the Pt–halide bond of the dihalogeno complexes Pt(diphosphine)X₂ resulting in the exclusive formation of Pt(diphosphine)X(CHXCOOC₂H₅). Diastereoselective insertion reactions have been observed in the presence of (S,S)-bdpp as diphosphine. The Pt{(S,S)-bdpp)}I(Ph) complex has been characterized by X-ray crystallography.     

Cobalt(II) and nickel(II) chloride and bromide complexes of 2-(2′-methyl-8′-quinolyl)benzoxazole and 2-(2′ or 4′-methyl 8′-quinolyl)benzimidazole

Summary Cobalt(II) and nickel(II) halide complexes of the ligands 2-(2-methyl-8-quinolyl)benzoxazole (mqbo), 2-(2-methyl8quinolyl)benzimidazole (mqbi) and 2-(4-methyl-8-quinolyl)benzimidazole (mqbi) were synthesized and characterized by analytical, thermogravimetric, conductivity and magnetic data, and i.r. and electronic spectra.The ligands are bidentate N-donors yielding complexes where the coordination geometry depends on the metal ion and steric hindrance. All the cobalt complexes have formula [CoL₂X₂] and distorted tetrahedral geometry. Different types of nickel compounds were obtained: i) complexes of formula NiLX₂ · n H₂O (or EtOH) (L = mqbo or mqbi; n=0–1.5) which arepseudo-tetrahedral or five-coordinate and ii) complexes NiL₂X₂ · n H₂O (L = mqbi, n=3 or 4) where the metal is bound to four nitrogen atoms and the overall coordination geometry is tetragonal. The structural changes occurring after removal of water or alcohol from the complexes are also reported.     

Säurekatalysierte Umlagerung von 4-Allyl-cyclohex-2-en-1-olen; Beispiele für ladungskontrollierte [3s, 4s]-Umlagerungen von cyclischen Allylkationen

The acid-catalysed rearrangement of the cyclohex-2-en-1-ols 15 , d₃- 15 , 16 , 17 and 19 , the cyclohexa-2,5-dien-1-ols 20 and 21 , and also the allyl alcohols 22 and 23 (Scheme 3), using 98-percent sulfuric acid/acetic anhydride 1:99 at room temperature, was investigated. From the rearrangement of 4-allyl-4-phenyl-cyclohex-2-en-1-ol ( 15 ), with reaction times greater than 2 hours a single product is obtained, 4-allyl-biphenyl ( 50 ) in 33% yield (Scheme 9). With reaction times below 2 hours the acetate 53 from 15 was isolated, and this could be converted into 50 . The reaction of 2′,3′,3′-d₃-15 in Ac₂O/H₂SO₄ lead to 1′,1′,2′-d₃-50 (Scheme 11). The rearrangement of 4-allyl-4-methyl-cyclohex-2-en-1-ol (16) (Scheme 14) yielded 39% of the corresponding acetate 60 and 30% of 4-allyl-toluene ( 6 ), which also resulted by a rearrangement of 60 under the reaction conditions. These rearrangements are all [3s,4s]-sigmatropic reactions, which proceed via the cyclohexenyl cation a (Scheme 12, R = C₆H₅, CH₃). In Ac₂O/H₂SO₄ the allyl-cyclohexadienes primarely formed subsequently undergo dehydrogenation to yield the benzene derivatives 6 , 50 and d₃- 50 . From the rearrangement of 4,4-diphenyl-cyclohex-2-en-1-ol ( 19 ) at 0° a reaction mixture is obtained which consists of the acetate 55 , 2,3-diphenyl-cyclohexa-1,4-diene ( 57 ) and o-terphenyl ( 56 ) (Scheme 10). Both 55 and 57 are converted under the reaction conditions to o-terphenyl ( 56 ). No 4-(1′-methylallyl)-biphenyl is obtained from the rearrangement of 4-crotyl-4-phenyl-cyclohex-2-en-1-ol ( 17 ). In this case, apart from the corresponding acetate 64 , a single product 5-(1′-acetoxyethyl)-1-phenyl-bicyclo[2.2.2]oct-2-ene ( 65 ) (Scheme 16) was obtained; under the reaction conditions the acetate 64 rearranges to 65 . The rearrangement of 4-allyl-4-phenyl-cyclohexa-2,5-dien-1-ol ( 20 ) gives, as expected, not only 4-allyl-biphenyl ( 50 ) but also 2- and 3-allyl-biphenyl ( 51 and 52 ) and biphenyl (Scheme 13). 4-Benzyl-4-methyl-cyclohexa-2,5-dien-1-ol (syn- and anti- 21 ) gave in Ac₂O/H₂SO₄ at 10° as rearrangement products 93% of 2-benzyltoluene ( 97 ) and 7% of 4-benzyl-toluene ( 98 ) (Scheme 21). Hence [1,4]-rearrangements in cyclohexadienyl cations, seems to occur only to a limited extent. The alicyclic alcohols 22 and 23 (Scheme 18) gave, in Ac₂O/H₂SO₄, as main product the corresponding acetates 73 and 75 , as well as small amounts of olefins 74 and 76 formed by dehydration i.e. [3,4]-rearrangements occur in these systems. Also no [3,4]-rearrangements were observed in solvents reactions of either 4,4-dimethyl-hepta-1, 6-dien-3-yl tosulate (79; see Scheme 19) or its corresponding alcohol 24.     

Synthesis, electrochemistry, and crystal and molecular structures of some molybdenum(0) arene derivatives with fluorinated and phenyl-substituted arene ligands

Compounds of general formula Mo(η⁶-arene)(CO)₃, arene=diphenyl, 1; 1,3,5-triphenylbenzene, 2; C₆H₅F, 3; C₆H₅CF₃, 4, have been prepared in good yields by reacting fac-Mo(CO)₃(DMF)₃, DMF=N,N-dimethylformamide, with BF₃ · OEt₂ and the appropriate arene. The crystal and molecular structures of 1, 3, and 4, are reported. The dinuclear derivative Mo₂(η⁶:η⁶-C₆H₅-C₆H₅)(CO)₆, 5, was obtained by thermal reaction of Mo(η⁶-toluene)(CO)₃ with Mo(η⁶-diphenyl)(CO)₃. An electrochemical study has been performed on the new complexes, showing that the dimolybdenum complex undergoes a single two-electron reduction at about the same potential as the corresponding dichromium complex, the molybdenum dianion being less stable than the chromium analogue.     

A novel general route for the synthesis of C-glycosyl tyrosine analogues

A general method for the synthesis of C-glycosyl amino acids is described here. The stereoisomerically pure tyrosine analogues alpha-L-4 and beta-L-6 are prepared in reasonable overall yields from allyl derivatives 10 and 11. The key step is a benzannulation procedure which is employed in the creation of the aromatic ring that bears the amino acid function.     

Structure, spectroscopy, and spectral tuning of the gas-phase retinal chromophore: the beta-ionone "handle" and alkyl group effect

The low-lying singlet states (i.e. S0, S1, and S2) of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct have been studied in isolated conditions by using ab initio multiconfigurational second-order perturbation theory. The computed spectroscopic features include the vertical excitation, the band origin, and the fluorescence maximum of both isomers. On the basis of the S0-->S1 vertical excitation, the gas-phase absorption maximum of PSB11 is predicted to be 545 nm (2.28 eV). Thus, the predicted absorption maximum appears to be closer to that of the rhodopsin pigment (2.48 eV) and considerably red-shifted with respect to that measured in solution (2.82 eV in methanol). In addition, the absorption maxima associated with the blue, green, and red cone visual pigments are tentatively rationalized in terms of the spectral changes computed for PSB11 structures featuring differently twisted beta-ionone rings. More specifically, a blue-shifted absorption maximum is explained in terms of a large twisting of the beta-ionone ring (with respect to the main conjugated chain) in the visual S-cone (blue) pigment chromophore. In contrast, the chromophore of the visual L-cone (red) pigment is expected to have a nearly coplanar beta-ionone ring yielding a six double bond fully conjugated framework. Finally, the M-cone (green) chromophore is expected to feature a twisting angle between 10 and 60 degrees. The spectroscopic effects of the alkyl substituents on the PSB11 spectroscopic properties have also been investigated. It is found that they have a not negligible stabilizing effect on the S1-S0 energy gap (and, thus, cause a red shift of the absorption maximum) only when the double bond of the beta-ionone ring conjugates significantly with the rest of the conjugated chain.     

Synthesis,chemical, and electrochemical characterization of the [Ag13{μ3-Fe(CO)4}] n− (n = 3, 4, 5) cluster anions: X-Ray structural determination of [N(PPh3)2]3 [Ag12(μ12-Ag){μ3Fe(CO)4}8]1

The oxidation of the [Fe(CO)₄]^2– dianion with Ag⁺ salts occurs through a particularinner-sphere mechanism, which involves an intermediate cascade of silver clusters stabilized by Fe(CO)₄ ligands. The last detectable Ag-Fe cluster of the sequence is the [Ag₁₃{-Fe(CO)₄}₈]^3– trianion, which has been selectively obtained by using ca. 1.7 equivalents of Ag⁺ per mole of [Fe(CO)₄]^2–. The [Ag₁₃{-Fe(CO)₄}₈]^3–- trianion has been isolated in a crystalline state with several quaternary cations, and has been characterized by X-ray diffraction studies of its bis(triphenylphosphine)iminium salt. [N(PPh₃)₂]₃ [Ag₁₃{ ₃-Fe(CO)₄}₈]·2(CH₃)₂CO, monoclinic, space group P2₁ (No.4),a = 16.284(2) Å,b =18.767(5) Å,c = 25.905(4) Å, = 90.46(1)°,V = 7916(3) ų,Z = 2,R = 0.0324. The molecular structure of the anion consists of a centered cuboctahedron of silver atoms with the triangular faces capped by Fe(CO)₄ units. Chemical reduction of ( Ag₁₃{ ₃-Fe(CO)₄}₈]^3– affords the corresponding [Ag₁₃{ ₃-Fe(CO)₄)₈]^4–, which in turn gives [Ag₁₃{ ₃-Fe(CO)₄)₈]^5– and [Ag₆{ ₃-Fe(CO)₄}₄]^– upon further reduction. Electrochemical investigations confirm the reversibility of the [Ag₁₃{ ₃-Fe(CO)₄}₈]^3–/4– redox change. Furthermore, in spite of some electrode poisoning effects, evidence of the existence of the [Ag₁₃{ ₃-Fe(CO)₄}₈]^5– pentaanion was obtained. The yet structurally uncharacterized [Ag₆{ ₃-Fe(CO)₄)₄]^2– dianion is quantitatively obtained by reaction of [Fe(CO)₄]^2– with ca. 1.5 equivalents of Ag⁺ or by addition of one equivalent of Ag⁺ to solutions of the [Ag₅{Fe(CO)₄}₄]^3– trianion. All attempts to isolate its quaternary salts as crystalline materials failed owing to formation of amorphous insoluble precipitates. The above series of ₃-Fe(CO)₄ octa-capped cuboctahedral Ag₁₃ clusters can be envisioned as the Ag⁺ . Ag and Ag^– cryptates of the [Ag₁₂{}₃-Fe(CO)₄}₈]^4– cryptand. respectively.Dedicated to Prof L. F. Dahl on his 65th birthday.     

Synthese und Reaktionen 8-gliedriger Heterocyclen aus 3-Dimethylamino-2,2-dimethyl-2H-azirin und Saccharin bzw. Phthalimid

Synthesis and Reactions of 8-membered Heterocycles from 3-Dimethylamino-2,2-dimethyl-2H-azirine and Saccharin or Phthalimide 3-Dimethylamino-2,2-dimethyl-2H-azirine ( 1 ) reacts at 0-20° with the NH-acidic compounds saccharin ( 2 ) and phthalimide ( 8 ) to give the 8-membered heterocycles 3-dimethylamino-4,4-dimethyl-5,6-dihydro-4 H-1,2,5-benzothiadiazocin-6-one-1,1-dioxide ( 3a ) and 4-dimethylamino-3,3-dimethyl-1,2,3,6-tetrahydro-2,5-benzodiazocin-1,6-dione ( 9 ), respectively. The structure of 3a has been established by X-ray (chap. 2). A possible mechanism for the formation of 3a and 9 is given in Schemes 1 and 4. Reduction of 3a with sodium borohydride yields the 2-sulfamoylbenzamide derivative 4 (Scheme 2); in methanolic solution 3a undergoes a rearrangement to give the methyl 2-sulfamoyl-benzoate 5 . The mechanism for this reaction as suggested in Scheme 2 involves a ring contraction/ring opening sequence. Again a ring contraction is postulated to explain the formation of the 4H-imidazole derivative 7 during thermolysis of 3a at 180° (Scheme 3). The 2,5-benzodiazocine derivative 9 rearranges in alcoholic solvents to 2-(5′-dimethylamino-4′,4′-dimethyl-4′H-imidazol-2′-yl) benzoates ( 10 , 11 ), in water to the corresponding benzoic acid 12 , and in alcoholic solutions containing dimethylamine or pyrrolidine to the benzamides 13 and 14 , respectively (Scheme 5). The reaction with amines takes place only in very polar solvents like alcohols or formamide, but not in acetonitrile. Possible mechanisms of these rearrangements are given in Scheme 5. Sodium borohydride reduction of 9 in 2-propanol yields 2-(5′-dimethylamino-4′,4′-dimethyl-4′H-imidazol-2′-yl)benzyl alcohol ( 15 , Scheme 6) which is easily converted to the O-acetate 16 . Hydrolysis of 15 with 3N HCl at 50° leads to an imidazolinone derivative 17a or 17b , whereas hydrolysis with 1N NaOH yields a mixture of phthalide ( 18 ) and 2-hydroxymethyl-benzoic acid ( 19 , Scheme 6). The zwitterionic compound 20 (Scheme 7) results from the hydrolysis of the phthalimide-adduct 9 or the esters 11 and 12 . Interestingly, compound 9 is thermally converted to the amide 13 and N-(1′-carbamoyl-1′-methylethyl)phthalimide ( 21 , Scheme 7) whose structure has been established by an independent synthesis starting with phthalic anhydride and 2-amino-isobutyric acid. However, the reaction mechanism is not clear at this stage.     

Cascade radical reactions via alpha-(arylsulfanyl)imidoyl radicals: competitive [4 + 2] and [4 + 1] radical annulations of alkynyl isothiocyanates with aryl radicals

Aryl radicals react with 2-(2-phenylethynyl)phenyl isothiocyanate through a novel radical cascade reaction entailing formation of alpha-(arylsulfanyl)imidoyl radicals and affording a new class of compounds, i.e. thiochromeno[2,3-b]indoles. These derivatives are formed as mixtures of substituted analogues arising from competitive [4 + 2] and [4 + 1] radical annulations. The isomer ratio is strongly dependent on the aryl substituent and is correlated to its capability to delocalize spin density. The presence of a methylsulfanyl group in the ortho-position of the initial aryl radical results in complete regioselectivity and better yields, as the consequence of both strong spin-delocalization effect, which promotes exclusive [4 + 1] annulation, and good radical leaving-group ability, which facilitates aromatization of the final cyclohexadienyl radical. Theoretical calculations support the hypothesis of competitive, independent [4 + 2] and [4 + 1] annulation pathways. They also suggest that rearrangement onto the sulfur atom of the [4 + 1] intermediate does not occur via a sulfuranyl radical but rather through either a transition state or a sulfur-centered (thioamidyl) radical; the latter is possibly the preferred route in the presence of an o-methylsulfanyl moiety that can act as a leaving group in the final ipso-cyclization process.