Intramolecular rearrangements

It has been shown that S--aminoalkyl derivatives of 2-mercaptopyrimidine and 2-mercapto-4, 6-diaminopyrimidine, containing primary and secondary amino groups, undergo S N rearrangement in neutral or weakly alkaline solution, with the formation of the corresponding aminothiols. In the case of derivatives of 2-mercapto-4, 6-diaminopyrimidine, however, the S N rearrangement proceeds more slowly, apparently as a result of the difficulty of formation of the intermediate cyclic compound.For part II, see [9].     

Polyhedral rearrangements involving five- and six-coordinate carbon

Polyhedral carboranes and metallocarboranes characteristically contain C atoms having coordination numbers of 5 and 6 within the polyhedral surface. Thermal carbon and metal atom migration reactions are observed in these species and we here report results obtained with polyhedra containing 10, 11, 12 and 13 vertices. Thermodynamic activation parameters have been measured for several representative rearrangement reactions and their mechanistic significance is discussed. General rules which appear to govern carbon and metal atom migration reactions are advanced.     

Unexpected nitrosyl-group bending in six-coordinate [M(NO)](6) sigma-bonded aryl(iron) and -(ruthenium) porphyrins.

The six-coordinate nitrosyl sigma-bonded aryl(iron) and -(ruthenium) porphyrin complexes (OEP)Fe(NO)(p-C(6)H(4)F) and (OEP)Ru(NO)(p-C(6)H(4)F) (OEP = octaethylporphyrinato dianion) have been synthesized and characterized. Single-crystal X-ray structure determinations reveal an unprecedented bending and tilting of the MNO group for both [MNO](6) species as well as significant lengthening of trans axial bond distances. In (OEP)Fe(NO)(p-C(6)H(4)F) the Fe-N-O angle is 157.4(2) degrees, the nitrosyl nitrogen atom is tilted off of the normal to the heme plane by 9.2 degrees, Fe-N(NO) = 1.728(2) A, and Fe-C(aryl) = 2.040(3) A. In (OEP)Ru(NO)(p-C(6)H(4)F) the Ru-N-O angle is 154.9(3) degrees, the nitrosyl nitrogen atom is tilted off of the heme normal by 10.8 degrees, Ru-N(NO) = 1.807(3) A, and Ru-C(aryl) = 2.111(3) A. We show that these structural features are intrinsic to the molecules and are imposed by the strongly sigma-donating aryl ligand trans to the nitrosyl. Density functional-based calculations reproduce the structural distortions observed in the parent (OEP)Fe(NO)(p-C(6)H(4)F) and, combined with the results of extended Hückel calculations, show that the observed bending and tilting of the FeNO group indeed represent a low-energy conformation. We have identified specific orbital interactions that favor the unexpected bending and tilting of the FeNO group. The aryl ligand also affects the Fe-NO pi-bonding as measured by infrared and (57)Fe M?ssbauer spectroscopies. The solid-state nitrosyl stretching frequencies for the iron complex (1791 cm(-)(1)) and the ruthenium complex (1773 cm(-)(1)) are significantly reduced compared to their respective [MNO](6) counterparts. The M?ssbauer data for (OEP)Fe(NO)(p-C(6)H(4)F) yield the quadrupole splitting parameter +0.57 mm/s and the isomer shift 0.14 mm/s at 4.2 K. The results of our study show, for the first time, that bent Fe-N-O linkages are possible in formally ferric nitrosyl porphyrins.     

Synthesis and properties of bis(dialkylphosphino)ethane iron dihydrides

Syntheses and properties of the iron bisphosphinoethane complexes FeH₂(PP)₂ and FeHCl(PP)₂[PP?R₂PCH₂CH₂CH₂PR₂, where R?Me (PP?DMPE), Et(PP?DEPE), and n-Pr (PP?DprPE)] are reported. The complexes can be formed by reduction of the corresponding dichlorides FeCl₂(PP)₂ with lithium aluminium hydride in THF solution provided that ethanol or more acidic reagents are not employed during the reaction work-up. The dihydrides are notably basic compounds and can be protonated reversibly by alcohols. The dihydrides exist as equilibrating mixtures of cis and trans isomers in solution. The cis isomers of each of the dihydrides are fluxional on the NMR timescale and NMR studies indicate that the interconversion of cis isomers does not necessarily proceed via the trans isomer.     

Electronic structure of six-coordinate iron(III) monoazaporphyrins

The electronic structures of six-coordinate iron(III) octaethylmonoazaporphyrins, [Fe(MAzP)L 2] (+/-) ( 1), have been examined by means of (1)H NMR and EPR spectroscopy to reveal the effect of meso-nitrogen in the porphyrin ring. The complexes carrying axial ligands with strong field strengths such as 1-MeIm, DMAP, CN (-), and (t)BuNC adopt the low-spin state with the (d xy ) (2)(d xz , d yz ) (3) ground state in a wide temperature range where the (1)H NMR and EPR spectra are taken. In contrast, the complexes with much weaker axial ligands, such as 4-CNPy and 3,5-Cl 2Py, exhibit the spin transition from the mainly S = 3/2 at 298 K to the S = 1/2 with the (d xy ) (2)(d xz , d yz ) (3) ground state at 4 K. Only the THF complex has maintained the S = 3/2 throughout the temperature range examined. Thus, the electronic structures of 1 resemble those of the corresponding iron(III) octaethylporphyrins, [Fe(OEP)L 2] (+/-) ( 2). A couple of differences have been observed, however, in the electronic structures of 1 and 2. One of the differences is the electronic ground state in low-spin bis( (t)BuNC) complexes. While [Fe(OEP)( (t)BuNC) 2] (+) adopts the (d xz , d yz ) (4)(d xy ) (1) ground state, like most of the bis( (t)BuNC) complexes reported previously, [Fe(MAzP)( (t)BuNC) 2] (+) has shown the (d xy ) (2)(d xz , d yz ) (3) ground state. Another difference is the spin state of the bis(3,5-Cl 2Py) complexes. While [Fe(OEP)(3,5-Cl 2Py) 2] (+) has maintained the mixed S = 3/2 and 5/2 spin state from 298 to 4 K, [Fe(MAzP)(3,5-Cl 2Py) 2] (+) has shown the spin transition mentioned above. These differences have been ascribed to the narrower N4 cavity and the presence of lower-lying pi* orbital in MAzP as compared with OEP.     

Intramolecular cyclization and subsequent rearrangements of alkyne-tethered N-heterocyclic carbenes

Alkyne-tethered imidazole and 1,2,4-triazole-based N-heterocyclic carbene precursors have been prepared and studies of the intramolecular reactions of carbenes are performed. Products consistent with intramolecular cyclizations and subsequent rearrangements were observed. Mechanistic studies using crossover experiments showed that the products did arise from intramolecular carbene additions. The reactions are proposed to go through vinylogous diaminocarbene intermediates similar to vinylogous dialkoxycarbenes formed during Boger cycloaddition reactions. Imidazole substituted dienes were observed to be the major products of tandem cyclization and elimination reactions that were observed for imidazole-based N-heterocyclic carbenes.     

An anionic six-coordinate high-spin iron(III) porphyrin

Isolation and characterisation of the tetrabutylammonium salt of difluoro iron(III) tetraphenylporphyrin are described.     

Single- and double-linkage isomerism in a six-coordinate iron porphyrin containing nitrosyl and nitro ligands

Density Functional theoretical calculations confirm the experimental observation that the low-temperature photolysis of (TPP)Fe(NO)(NO2) (as a KBr pellet) results in the generation of linkage isomers involving the axial NO and NO2 groups and suggest the possible formation of the double linkage isomer (TPP)Fe(ON)(ONO). The energy difference between the ground state (porphine)Fe(NO)(NO2) and the double-linkage isomer (porphine)Fe(ON)(ONO) is 1.57 eV, which is comparable to the 1.59 eV calculated previously for the nitrosyl-to-isonitrosyl linkage isomerism in the five-coordinate (porphine)Fe(NO) analogue.     

Intramolecular 2h group-transfer in heterocyclic bridged-ring systems (dyotropic rearrangements)

Diarylnitrilimines, derived by thermolysis of appropriate diaryltetrazoles, form(4+2)π cyclo-adducts with dipolarophiles of the isodrin type, e.g. $\text{(1)}$; the resulting 2-pyrazolines thermally rearrange into pyrazoles by hydrogen group transfer. Kinetic data for examples of this and related reactions are reported.     

Intramolecular thermal transformations of N-phthalimidoaziridines: 1,3-dipolar cyclo-addition and rearrangements*

The intramolecular thermal cycloaddition of N-phthalimidoaziridines at multiple bonds of substituents with the intermediate formation of azomethine ylides leads to condensed pyrrole derivatives, in which the five-membered ring is adjacent to a five-, six-, or seven-membered ring. Rearrangements, which sometimes become the predominant reactions, compete with cycloaddition. Thus, aziridines with aryl substituents readily isomerize to give imines with a 1,2-shift of the phthalimide group to one of the carbon atoms. Aziridines with one electron-withdrawing substituent probably do not open to give 1,3-dipoles but rather undergo a Cope-type rearrangement involving the three-membered ring and C = O bond of the second substituent. Even in intramolecular reactions, very low activity is found for the cyano group triple bond and aromatic ring bonds as dipolarophiles.     

Dithiophosphinates of trivalent iron,ruthenium and osmium

Dithiophosphinates of Fe(III), Ru(III) and Os(III) have been prepared and their ESR studies indicate distortion from octahedral geometry. The tetragonal and rhombic splittings and the orbital reduction factors were calculated from the experimental g values for low-spin Ru(III) and Os(III) complexes. The metal-ligand covalency increases in the order Ru < Os.     

Ligand orientation control in low-spin six-coordinate (porphinato)iron(II) species

The synthesis of a low-spin six-coordinate iron(II) porphyrinate in which the two axial ligands are forced to have a relative perpendicular orientation has been successfully accomplished for the first time. The reaction of four-coordinate (tetramesitylporphinato)iron(II) with 2-methylimidazole leads to the preparation of [Fe(TMP)(2-MeHIm)(2)] which cocrystallizes with five-coordinate [Fe(TMP)(2-MeHIm)]. The six-coordinate complex accommodates the sterically crowded pair of imidazoles with a strongly ruffled core and relative perpendicular orientation. This leads to shortened equatorial bonds of 1.963(6) A and slightly elongated axial Fe-N bond lengths of 2.034(9) A that are about 0.04 A shorter and 0.03 A longer, respectively, in comparison to those of the bis-imidazole-ligated iron(II) species with parallel oriented axial ligands. The Mossbauer spectrum shows a pair of quadrupole doublets that can be assigned to the components of the cocrystallized crystalline solid. High-spin five-coordinate [Fe(TMP)(2-MeHIm)] has DeltaE(Q) = 2.25 mm/s and delta = 0.90 mm/s at 15 K. The quadrupole splitting, DeltaE(Q), for [Fe(TMP)(2-MeHIm)(2)] is 1.71 mm/s, and the isomer shift is 0.43 mm/s at 15 K. The quadrupole splitting value is significantly larger than that found for low-spin iron(II) derivatives with relative parallel orientations for the two axial ligands. Mossbauer spectra thus provide a probe for ligand orientation when structural data are otherwise not available.     

Intramolecular electrophilic rearrangements in saturated acyclic systems. Reactions of bis-carbanions with isocyanates

The reactions of mono- and bis-isocyanates with bis-carbanions whose COOAlk groups at the anionic centers do not contain acidic hydrogen atoms proceed unusually to form bis-carbamates (including oligomeric carbamates) due to migration of the alkoxycarbonyl groups from the carbon to the nitrogen atom.     

Neutral and dianionic iron and ruthenium 1,4-diphosphabutadiene complexes

Reaction of a tetraphenyl-1,2-diphosphinyl dianion with [FeCl2(THF)1.5] or [Ru(COD)(acac)2] yields dianionic complexes which further react with Ph3SnCl to afford the corresponding bis(1,4-diphosphabutadiene) complexes.     

Synthesis and dehydrocoupling reactivity of iron and ruthenium phosphine-borane complexes

The Fe and Ru phosphine-borane complexes CpM(CO)2PPh2 x BH3 (1, M = Fe, 4, M = Ru) were synthesized utilizing the reaction of the phosphine-borane anion Li[PPh2 x BH3] with the iodo complexes CpM(CO)2I. The Fe complex 1 reacted with PMe3 to yield CpFe(CO)(PMe3)(PPh2 x BH3) (2) and CpFe(PMe3)2(PPh2 x BH3) (3) whereas the Ru species 4 gave only CpRu(CO)(PMe3)(PPh2 x BH3) (5). The complexes 1-5 were characterized by 1H, 11B, 13C and 31P NMR spectroscopy, MS, IR and X-ray crystallography for 1 to 4, and EA for 1, 2 and 4. The reactivity of 1 and 4 towards PPh2H x BH3 was explored. Although no stoichiometric reactions were detected under mild conditions, both 1 and 4 were found to function as dehydrocoupling catalysts to afford Ph2PH x BH2 x PPh2 x BH3 in the melt at elevated temperature (120 degrees C). The carbonyl Fe2(CO)9 also functioned as a dehydrocoupling catalyst under similar conditions. Complex 1 and Fe2(CO)9 represent the first reported active Fe complexes for the catalytic dehydrocoupling of phosphine-borane adducts.     

Intramolecular Diels-Alder reactions using chiral ruthenium Lewis acids and application in the total synthesis of ent-ledol

One-point binding chiral ruthenium Lewis acids incorporating the C(2)-symmetric electron-poor bidentate phosphinite ligand BIPHOP-F and a Cp or an indenyl 'roof' can efficiently catalyze asymmetric intramolecular Diels-Alder reactions of trienes to form bicyclic adducts with good to excellent asymmetric induction. This reaction forms the key step in a total synthesis of ent-ledol in 96% ee. The synthesis also helps to clarify the stereochemical assignment of ledol and inconsistencies in the measured optical rotation.