Showing papers in "Journal of the American Chemical Society in 1968"

Trimethylene and the addition of methylene to ethylene

Abstract: Extended Hiickel calculations on a distorted cyclopropane indicate the presence of a singlet trimethylene intermediate with a CCC angle of 125”, trigonal terminal methylene groups coplanar with the carbon skeleton. This molecule has a high barrier to internal rotation and a low barrier to conrotatory reclosure to cyclopropane. The first excited configuration of trimethylene and cyclopropane is a floppy molecule with no rotational barriers. The electronic structure of trimethylene is unusual with a symmetric x-type level above an antisymmetric combination. Similar level orderings, implying conrotatory closing and concerted 1,2 addition, are found in other “1,3 dipoles.” The potential surface for the addition of methylene to ethylene is explored in detail. The most symmetrical approach is symmetry forbidden, and the reaction path is unsymmetrical. It begins as a x approach and terminates as U. Because of the electronic structure of trimethylene it is possible for this unsymmetrical approach to be stereospecific. The specificity of singlet and triplet methylene additions is attributed not to the difference in spin, but to the difference in the spatial part of the wave function. The ring-opened form of cyclopropanone has an electronic structure different from that of trimethylene and other 1,3 dipoles, It is consistent with the valence-bond formulation of an oxy anion of allyl cation. A consequence of this electronic structure is a disrotatory closure back to the cyclopropanone and propensity to concerted 1,4 addition. The extended Hiickel calculations make cyclopropanone and allene oxide unstable with respect to oxyallyl (the ring-opened form). In fact they give no stability for cyclopropanone with respect to conversion to oxyallyl. A x-electron SCF-CI calculation has the ground state of oxyallyl, a triplet, with a singlet only 0.1 eV above. n this paper two aspects of cyclopropane chemistry I are discussed: the question of the existence and electronic structure of a trimethylene intermediate CH2CH2CH2, and the detailed transition-state geometry and specificities observed in the addition of methylenes to ethylenes. An important and connected problem, the cis-trans thermal isomerization of substituted cyclopropanes and the competing rearrangement to propylenes, has not yet been considered in detail.

Synthesis and properties of pentaamminepyridineruthenium(II) and related pentaammineruthenium complexes of aromatic nitrogen heterocycles

Abstract: Synthesis and Properties of Pentaamminepyridinenthenium ( 11) and Related Pentaammineruthenium Complexes of Aromatic Nitrogen Heterocycles P. Ford,l* De F. P. Rudd,lb R. Gaunder, and H. Taube Contribution from the Department of Chemistry, Stanford University, Stanford, California 94305. Received September 11, 1967 Abstract: The syntheses of pentaammineruthenium(I1) and pentaammineruthenium(II1) complexes of pyridine and analogous aromatic nitrogen heterocycles are reported here. The pentaammineruthenium(I1) complex of each of these heterocyclic ligands has an intense absorption band in the visible region which, on the basis of evidence presented below, is assigned to a metal-to-ligand charge-transfer transition. The pentaamminepyrazineruthenium- (11) complex ion can be protonated in a reversible fashion, presumably at the noncoordinated nitrogen of the 1,4- diazine. The pK, of this complex demonstrates that the dipositive ion is approximately two orders of magnitude more basic than the free ligand. This result and related data are interpreted in terms of ground-state ir back-bonding from the ruthenium(I1) to the aromatic ligand. W e undertook the present work in the hope that com- parison of the reactivity of various ruthenium- (111) complexes, which have low-spin d5 electronic con- figurations, to analogous cobalt(II1) complexes (low- spin d6) would improve our understanding of the role of electronic configuration in the mechanism of reductions of these complexes. Pentaammineruthenium(II1) com- plexes of nitrogen heterocycles such as pyridine deriva- tives are of special interest because it is reported that reductants such as chromium(I1) can attack at ligand sites remote from the metal ion center in certain Co(II1) analogs. Remote attack may require transfer of the reducing electron through the conjugated n orbitals of the aro- matic ligand. It has been suggested that the ruthenium- (111) complexes having a n-symmetry (t2,) acceptor orbital might demonstrate dramatically different re- activity characteristics in reductions by remote attack from Co(II1) analogs having u-symmetry (or e,) acceptor orbitals. In the course of investigating these problems, we have synthesized a number of pentaammineru- thenium(I1) and pentaammineruthenium(II1) complexes of aromatic nitrogen heterocycles. These have been found to have interesting physical and spectral proper- ties; their description and interpretation form the sub- ject of this paper. Results Spectral Characteristics. The spectrum of penta- amminepyridineruthenium(I1) perchlorate in aqueous solution is shown in Figure 1. Characteristic of the spectrum of this complex and of the pentaammine- ruthenium(I1) complexes of related aromatic nitrogen heterocycles is an intense absorption band in the visible range having an extinction coefficient approximately lo4 M-’cm-’. Comparison ofthe spectrum in the visible range for pentaammineruthenium(I1) perchlorate in aqueous solution to the spectra of pentaamminepyridine- ruthenium(II1) perchlorate (Figure 1) and of hexaam- mineruthenium(I1) perchlorate (Table I) in aqueous (1) (a) NSF Postdoctoral Research Associate, Stanford University; (b) NSF Science Faculty Fellow and Visiting Scholar at Stanford University. (2) F. R. Nordmeyer and H. Taube, J . Am. Chem. Soc., 88, 4745, solutions demonstrates that among these the band occur- ring at 407 mp is unique to the Ru(I1)-pyridine species. Somewhat analogous absorptions have been observed in the spectra of other pyridine complexes with d6 metal ions. For example, the spectra of pyridine complexes of chloroiridium(II1) show intense visible range absorp- tions which are suggested by Jorgensen3 to arise from metal-to-ligand charge transfers. Table I. Spectra of Pentaammineruthenium Complexes Complex Lax, m r (log emax) (NH3)eRu2+ a ( N H ~ ) ~ R u ~ + ~ 320 (2.00) ( N H ~ ) ~ R U ( ~ Y ) 407 (NH~)~Ru(PY)~+ Pyridineb Pyridine (H+) T. Meyer, Ph.D. Dissertation, Stanford University, 1966, p 30. band of pyridine spectrum: H. H. Jaff6 and M. Orchin, “Theory and Application of Ultraviolet Spectroscopy,” John Wiley and Sons, Inc., New York, N. Y., 1962, p 363. “0,O) The only other high-intensity absorption apparent in the electronic spectrum of the pentaamminepyridine- ruthenium(I1) complex occurs in the ultraviolet region of the spectrum at 244 mp. The position of this band and its intensity, E 4.6 X lo3 M-’ cm-I, are similar to those for free pyridine and for protonated pyridine (Table I) implying that this transition is essentially localized in the aromatic ligand and is presumably analogous to the T absorption seen in the free ligand. Similar n* bands occur in the spectrum of pentaamminepyridine- ruthenium(II1); on the other hand, transitions in this region for hexaammineruthenium(I1) and the hexa- ammineruthenium(II1) are of much lower intensity (Table I). The absorption bands for pyridine, when it is free, when it is protonated, and when it is bound to ruthen- ium(III), are close in energy while the analogous band for the pyridine bound to ruthenium(I1) is shifted to higher energy. (3) C. K. Jprgensen, Acta Chem. Scand., 11, 151 (1957). Ford, Rudd, Gaunder, Taube 1 Pentaamminepyridineruthenium(II)

Studies of inter- and intramolecular interaction in mononucleotides by proton magnetic resonance.

Abstract: The interaction and conformation of 5’, 3’-, and 2’-nucleoside monophosphates in aqueous solutions have been studied mainly by pmr at varying concentrations and pH. The chemical shifts of the base protons and H-1 ’ protons of the adenine nucleotides were found to shift upfield at increasing concentration in a manner analogous to adenosine. This result indicates that the AMP molecules also associate to form vertical stacks with similar geometry in solution, a conclusion also supported by the vapor pressure osmometry data. The phosphoryl substitution at the 5 ’ or 3‘ position reduces the tendency of association to about 30-40z as compared to the corresponding adenine nucleosides. Substitution of the phosphoryl group at the 2‘ position has a greater influence in reducing the association, especially in the dianionic form. The 5 ‘-phosphoryl group (not the 3‘or 2’-phosphate) was found to have a specific deshielding effect on the H-8 proton (and not H-2) of the 5’-purine nucleotides and on the H-6 proton (and not H-5) of the 5’-pyrimidine nucleotides. This deshielding effect is largest (-0.2 ppm) when the phosphate is in the dianion form (above pD 7.4), less when in the monoanion form (-0.12 ppm, below p D 5.9), and least as the monomethyl ester (-0.07 ppm). These data indicate that the 5’-nucleotides must be in the “anti” conformation. The mechanism of this phosphate deshielding effect is discussed based on the observation that this effect appears to be dependent on the “acidity” of the sensitive protons. Comparison of the pmr data on the ribose and deoxyribose 5’-nucleotides suggests the presence of intramolecular hydrogen bonding between the 2‘-OH group and the N-3 of the purine or the 2-keto of the pyrimidine. or the past few years the properties of various purine F bases a n d purine and pyrimidine nucleosides in aqueous solutions have been studied extensively by vapor pressure osmometry a n d by proton magnetic resonance. *-’ The results conclusively show that these compounds, especially the purines and the purine nucleosides, associate extensively in water primarily by way of vertical stacking of base rings. In the studies of cooperative binding of adenosine to polyuridylic acid,* the stacking energy has been shown to be the major factor contributing to the conformational stability of nucleic acid. In the continuation of our recent research on the purine a n d pyrimidine nucleosides by pmr,607 we report here the investigation on the solution properties of purine and pyrimidine mononucleotides. We have studied the influence of the charged phosphate group upon the stacking interactions of these mononucleotides. I n addition, we have found a specific intramolecular deshielding of the H-6 proton of pyrimidine nucleotides a n d the H-8 proton of purine nucleotides by the 5’-phospha te group. This finding indicates t ha t in aqueous solution the 5 ’-nucleotides are in “anti” conformation. Also, comparison of the da ta on the deoxyribonucleotides a n d the ribonucleotides suggests the possibility of intramolecular hydrogen bonding of the 2’-hydroxyl g roup to the bases in these compounds. (1) (a) Supported in part by a program project grant, National Institutes of Health (GM 10802-04), and by grants from the National Science Foundation (GB-5483 and GB-767). (b) Resented in part at the Ninth Annual Biophysics Meeting, San Francisco, Calif., 1965. (2) P. 0. P. Ts’o, I. S. Melvin, and A. C. Olson, J . Amer. Chem. SOC., 85. 1289 (1 963). (3) P. 0. P. Ts’o and S. I. Chan, ibid., 86, 4176 (1964). (4) S. I. Chan, M. P. Schweizer, P. 0. P. Ts’o, and G. K. Helmkamp, (5) M. P. Schweizer, S. I. Chan, and P. 0. P. Ts’o, ibid., 87, 5241 (6) 0. Jardetzky, Biopolym. Symp., 1, 501 (1964). (7) A. D. Broom, M. P. Schweizer, and P. 0. P. Ts’o, J . Arner. Chem. (8) W. M. Huang and P. 0. P. Ts’o, J . Mol. Biol., 16, 523 (1966). ibid., 86, 4182 (1964).