Diastereoselectivity in the rhodium-catalyzed hydroformylation of (+)(R)-1-phenylethyl vinyl ether

The degree of diastereoselectivity in the rhodium-catalyzed hydroformylation of (+)(R)-1-phenylethyl vinyl ether is much higher than that in the hydroformylation of the related olefin, 4-phenyl-1-pentene.     

Domino reaction sequences in the rhodium-catalyzed hydroformylation of 3-acetyl-1-allylpyrrole: a short route to 5,6,7,8-tetrahydroindolizines

When 3-acetyl-1-allylpyrrole (1) was subjected under hydroformylation conditions, with Rh₄(CO)₁₂ as catalyst precursor, to 30 atm CO/H₂ (1:1) total pressure and 140 °C, an equimolar mixture of the isomeric 5,6,7,8-tetrahydroindolizines 4′ and 5′ was obtained as the almost exclusive product. In both cases a domino hydroformylation/cyclization on the α pyrrole positions by the aldehyde 3 carbonyl group occurs which involves different intermediates: while 4′ is generated via the dihydroindolizine 4, 5′ forms via direct reduction of 8-hydroxytetrahydroindolizine 5, a structure that has never been observed before from 1-allylpyrroles under oxo conditions.     

A combined experimental and computational investigation of the microscopic external heavy atom effect in van der Waals clusters

We present a combined experimental and computational study of the external heavy atom effect in van der Waals clusters of para-difluorobenzene (pDFB) with rare-gas atoms. Experimentally, clustering with rare-gas atoms is observed to shorten significantly the S1 fluorescence lifetime compared with that of the pDFB monomer, an effect we interpret in terms of an enhancement of the S1-T1 intersystem crossing rate. In order to test the validity of this widely held assumption, we have calculated the S1-T1 spin-orbit coupling matrix elements in the X-pDFB complexes (X=Ne, Ar, Kr) using a multiconfigurational linear response approach.     

A Dissociative Route to C?H Bond Activation: Ligand Cyclometalation in cis‐Dimethyl[(sulfinyl‐κS)bis[methane]][tris(2‐methylphenyl)phosphine]platinum(2+)

The dialkyl compound cis‐dimethyl[(sulfinyl‐κS)bis[methane]][tris(2‐methylphenyl)phosphine]platinum(2+) (cis‐[Pt(Me)₂(dmso)(P(o‐tol)₃]; 1 ) has been isolated from the reaction of cis‐dimethylbis[(sulfinyl‐κS)bis[methane]]platinum(2+) (cis‐[Pt(Me)₂(dmso)₂]) with tris(2‐methylphenyl)phosphane (P(o‐tol)₃). Restricted rotation around the P? C_ipso bonds of the phosphane ligand generates two different conformers, 1a and 1b , in rapid exchange in non‐polar solvents at low temperature. Strong through‐space contacts between the ortho‐Me substituent groups on the ligand and the cis‐Me groups in the coordination plane were determined, which proved useful for identifying the atropisomers formed. At room temperature, ¹H‐NMR spectra of 1 maintain a ‘static’ pattern upon onset of easy and rapid ortho‐platination, leading to [[2‐[bis(2‐methylphenyl)phosphino‐κP]phenyl]methyl‐κC]methyl[(sulfinyl‐κS)bis[methane]]platinum(2+) ( 2 ), a new C,P‐cyclometalated compound of platinum(II), with liberation of methane. The process has been studied by ¹H‐ and ³¹P{¹H}‐NMR in CDCl₃, and kinetics experiments were performed by conventional spectrophotometric techniques. The first‐order rate constants k_c decrease with the addition of dimethyl sulfoxide until the process is blocked by the presence of a sufficient excess of sulfoxide. This behavior reveals a mechanism initiated by ligand dissociation and formation of a three‐coordinate species. The value of the rate constant for dimethyl sulfoxide dissociation k₁ has been measured independently over a wide temperature range by both ¹H‐NMR ligand exchange (isotopic labeling experiments) and ligand substitution (stopped‐flow pyridine for dimethyl sulfoxide substitution). The rates of the two processes are in reasonable agreement at the same temperature, and a single Eyring plot can be constructed with the two sets of kinetics data. However, the value of the derived dissociation constant at 308 K (k₁=6.5±0.3 s^?1) is at least two orders of magnitude higher than that of cyclometalation (k_c=0.0098±0.0009 s^?1 at 308 K). Clearly, the dissociation step is not rate‐determining for cyclometalation. A multistep mechanism consistent with mass‐law retardation is derived, which involves a pre‐equilibrium that controls the concentration of an unsaturated three‐coordinate, 14‐electron T‐shaped cis‐[PtMe₂{P(o‐tol)₃}] intermediate. Cyclometalation is initiated in this latter by an agostic interaction with the σ(C? H) orbital of a methyl group. Oxidative addition of the C? H bond follows, yielding a cyclometalated‐hydrido 16‐electron Pt(IV) five‐coordinate intermediate. Finally, reductive elimination and re‐entry of dimethyl sulfoxide with liberation of methane should yield the cyclometalated species 2 .     

Atropisomerization, C-H activation, and dissociative substitution at some biphenyl platinum(II) complexes

The reaction of 2,2'-dilithiumbiphenyl with cis-[PtCl(2)(SEt(2))(2)] at -10 degrees C in diethyl ether not only leads to the main product [Pt(2)(micro-SEt(2))(2)(bph)(2)], containing the planar 2,2'-biphenyl dianion (bph(2)(-)), but also forms a new dinuclear platinum(II) compound of formula [Pt(2)(micro-SEt(2))(2)(Hbph)(4)], 1a (Hbph(-) = eta(1)-biphenyl monoanion), in which each metal is in a square-planar environment. NMR spectroscopy and molecular mechanics (MMFF) calculations were used to characterize 1a. The results suggest that the favored conformation for the four Hbph biphenyl groups is alphabetabetaalpha. In chloroform solution, 1a undergoes atropisomerization to 1b (alphabetaalphabeta) (k(is) = 1.03 x 10(-)(4) s(-)(1), at 298 K) that subsequently cyclometalates (k(obs) = 4.48 x 10(-)(6) s(-)(1), at 298 K) to yield [Pt(2)(micro-SEt(2))(2)(bph)(2)] and biphenyl. Both processes, atropisomerization and C-H activation, presumably involve preliminary thioether bridge splitting. The dinuclear complex 1a has been shown to be a versatile and useful precursor to a variety of mononuclear eta(1)-biphenyl platinum(II) complexes. By reaction with diethyl sulfide, dimethyl sulfoxide, or with rigid dinitrogen containing ligands, such as 2,2'-bipyridine or 1,10-phenanthroline, complexes cis-[Pt(Hbph)(2)(dmso)(2)] 3, cis-[Pt(Hbph)(2)(SEt(2))(2)] 4, [Pt(Hbph)(2)(bpy)] 5, and [Pt(Hbph)(2)(phen)] 6 were obtained, respectively. The crystal structures of compounds 5 and 6 were determined. Only the head-to-tail isomer of these compounds was recognized in the solid state and in solution, where restricted rotation around the Pt-C bond prevents interconversion to the head-to-head form. A detailed kinetic study of ligand (dmso) exchange and substitution (by 2,2'-bipyridine and 1,10-phenanthroline) has been performed on complex 3 in CDCl(3) and toluene-d(8) by (1)H NMR magnetization transfer experiments, and in toluene by UV/vis spectroscopy, respectively. The rates of both processes show no dependence on ligand concentration, the rate of ligand substitution being in reasonable agreement with that of ligand exchange at the same temperature. The kinetics are characterized by largely positive entropies of activation. The results are consistent with a dissociative mode of activation analogous to the pattern already found for compounds with a similar [Pt(C,C)(S,S)] set of coordinating ligands. The role of ML(3) d(8) T-shaped 14-electron species, as elusive reaction intermediates or structurally characterized compounds, is discussed.     

Non-conventional syntheses of heterocyclic compounds: a new synthesis of 1,3,4-benzotriazepine derivatives

4,5-Dihydro-1,3,4-benzotriazepines were obtained in good yields by intramolecular thermal cyclization of arylazoanils with methyl or ethyl groups in the ortho positions of the aromatic ring on the imine nitrogen.     

Evidence for formation and different evolution of tertiary rhodium alkyl intermediates under rhodium-catalyzed deuterio-(hydro)formylation of 1-(n-pyridyl)-1-phenylethenes

Deuterioformylation of the vinylidenic substrates 1-(n-pyridyl)-1-phenylethenes, in the presence of phosphine modified Rh₄(CO)₁₂ as catalyst precursor, was carried out at 100 atm of CO and D₂ (1:1), 80 °C and at partial and complete substrate conversion.The direct ²H NMR analysis of the crude reaction mixture led to the conclusion that, under these conditions, the branched alkyl rhodium intermediate is formed almost exclusively. It can: (i) β-eliminate, (ii) undergo migratory insertion on CO or (iii) oxidative addition of deuterium to various extents depending on the position of the nitrogen atom with respect to the olefinic double bond, thus accounting for the observed different chemo- and regioselectivities.     

A symmetry problem in the calculus of variations

Let be a ball in ^N, centered at zero, and letu be a minimizer of the nonconvex functional over one of the classesC _M := {w W _loc ^1, () 0 w(x) M in,w concave} orE _M := {w W _loc ^1,2 () 0 w(x) M in,w 0 inL()}of admissible functions. Thenu is not radial and not unique. Therefore one can further reduce the resistance of Newton's rotational body of minimal resistance through symmetry breaking.