RESEARCH
Pi-Allyl Route Leads to Rearrangements Mechanism explains products formed by reactions of allyl alcohols catalyzed by noble metal salts The rearrangement of allyl alcohols by soluble noble metal catalysts can be explained by a pi-allyl mechanism. Experimental results obtained from studies by Dr. R. E. Rinehart and Dr. R. W. Fuest of United States Rubber Co.'s (Wayne, N.J.) research center support this mechanism. The rearrangement is catalyzed by salts such as rhodium chloride and ruthenium chloride. The reaction can be interpreted by a mechanism which involves competitive loss of hydride or hydroxide to the metal atom to form pi-allyl complex intermediates, Dr. Rinehart told the Fourth Annual Metropolitan Regional Meeting of the ACS, held at Stevens Institute (Hoboken, N.J.). Dr. Rinehart and Dr. Fuest illustrate the usefulness of the pi-allyl concept with data they have obtained from the isomerization of trans-butene-2 by alcoholic rhodium nitrate. All three normal butenes are isomerized by alcoholic rhodium solutions to an equi-
librium mixture of £rans-butene-2, cisbutene-2, and butene-1. Since no skeletal rearrangement occurs in the reaction, no isobutylene is produced. After one hour's reaction time, gas chromatographic analysis shows that a small amount (0.4%) of butene-1 and no detectable cis-butene-2 form. Even after a little more than two hours, the concentration of butene-1 (the isomer least favored at equilibrium) is twice the concentration of ds-butene-2. After several hours' reaction time, the concentration of the ds-butene-2 exceeds the concentration of butene-1. The pi-allyl mechanism offers a reasonable explanation for the experimental results, Dr. Rinehart and Dr. Fuest believe. In the pi-allyl mechanism, the original pi complex consists of olefin bonded to a coordinated metal species, which is not a hydride. Transfer of a hydride from the allylic position of the pi complex leads to a C 3 structure coordinated to the metal-pially 1 complex.
Pi-Allyl Complex Mechanism Explains Rearrangement of Allyl Alcohol
H H
Hydride Transfer ^
Allyl Alcohol
Propionaldehyde, Acrolein
H
Hydroxide Transfer
Propane, Propylene
Pi Complex Carbonyl Transfer Ethylene, Ethane
Rearrangement of allyl alcohol catalyzed by soluble noble metal catalysts gives a variety of products. In the mechanism proposed by Dr. R. E. Rinehart and Dr. R. W. Fuest, the metal first forms a pi complex with the alcohol. Next, three different pi-allyl complexes can form. Hydride can migrate to the metal to produce a hydroxy pi-allyl complex which leads to aldehyde products. Hydroxide may be lost to the metal to form a pi-allyl complex which produces propane and propylene. Loss of carbonyl to the metal can lead to formation of ethylene and ethane 40
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The organic portion of a pi-allyl complex has a structure that preserves the original olefin's steric features. Thus, the pi-allyl complex from transbutene-2 can give a pi complex only of tfran.s-butene-2 or butene-1, but not of ds-butene-2. It's possible to prepare cis-butene-2 from trans-butene-2, but only by going through the butene-1 pi complex, Dr. Rinehart explains. The pi-allyl mechanism proposes that a hydride migrates from olefin to metal. However, species other than hydride of comparable ligand strength (such as hydroxide) might, if present, migrate to the metal in competition with hydride migration. Allyl Alcohol. Like butene, allyl alcohol can lose hydride to the metal to form a hydroxy pi-allyl complex. This leads to aldehyde products, chiefly propionaldehyde. Some acrolein also forms. Instead of a hydride transfer, hydroxide may be lost to the metal. Thus allyl alcohol's rearrangement forms some propane and propylene, Dr. Rinehart and Dr. Fuest find. The U.S. Rubber pair find an interesting side reaction during the rearrangement of allyl alcohol. When an alcohol reacts with a noble metal compound or complex, there's always the possibility of a loss of carbonyl to the metal giving a metal carbonyl complex. Some of this decarbonylation occurs with allyl alcohol in the presence of rhodium chloride, since Dr. Rinehart and Dr. Fuest have found ethylene and ethane as products. Methallyl alcohol (methyl group substituted for hydrogen at allyl alcohol's C-2) reacts in the presence of rhodium chloride in a way similar to that of allyl alcohol, they find. In their gas chromatographic analyses of the volatile gases produced in the reaction, Dr. Rinehart and Dr. Fuest have found isobutylene (formed by hydroxide removal). Smaller amounts of propylene and propane (formed from loss of carbon monoxide) were also found. The volatile liquids formed in the methallyl alcohol reactions all have the isobutyl carbon structure. The principal product was isobutyraldehyde. Methacrolein in smaller quantities was also detected. Cyclopropyl Carbinol. The U.S. Rubber chemists have also studied the more complicated rearrangement of cyclopropyl carbinol with noble metal salts. Some of the reactions found for allyl or methallyl alcohol should also occur with the cyclic compound, they reasoned. For example, with cyclo-
propyl carbinol, there might also be a hydride transfer competing with hydroxide transfer. In the rhodium chloride-catalyzed rearrangement of cyclopropyl carbinol, some of the products have an isobutyl or isobutylene structure. Isobutylene, propane, and propylene form. Other gaseous products are n-butenes and 1,3-butadiene. Rearrangement products found in the volatile liquid fraction from the cyclopropyl carbinol reactions include isobutyraldehyde and methacrolein. Also, the straight-chain C 4 aldehydes, butyraldehyde and crotonaldehyde, have been found. In explaining the formation of some of these products, Dr. Rinehart and Dr. Fuest say that rupture of the cyclopropyl bond can occur at the allylic position accompanied by a hydride shift. This produces a pi-allyl complex from which straight-chain C 4 aldehydes and hydrocarbons form. Rupture at the homoallylic position, they add, proceeds by a homo pi-allyl complex. This would produce branched-chain products. For example, isobutylene forms by homoallylic bond rupture with a hydroxide transfer, they explain. The products with branched-chain structures are in marked contrast to the rearrangement products of cyclopropylcarbinyl cation. Under carbonium ion conditions, cyclobutyl derivatives, but no isobutyl derivatives, usually form. To get more insight on the pi-allyl and homo pi-allyl mechanisms, Dr. Rinehart and Dr. Fuest are currently studying the reactions of 5-hydroxynorbornene-2 with rhodium chloride.
Liquid 0F2-Silica Mixture Explodes Spontaneous explosion can occur in a liquid OF 2 -silica gel mixture under certain conditions, according to Dr. Florence I. Metz and her co-workers at Midwest Research Institute (Kansas City, Mo.). During the course of electron paramagnetic resonance studies of liquid OF 2 , the MRI scientists found that a mixture of 6 0 / 8 0 mesh silica gel and liquid O F 2 at 254 torr in a 3- or 4-mm. (inside diameter) tube exploded when the temperature exceeded 77° K. To date, no explosion has occurred in 5-mm. tubes under identical conditions. Other tube diameters have not been tried.
Tetranitrogen Tetrasulfide Contains Some Bonding Between Sulfur Atoms Calculations indicate that the molecule has appreciable pi-electron derealization The tetranitrogen tetrasulfide (N 4 S 4 ) molecule is an eight-membered ring of alternating sulfur and nitrogen atoms with some bonding between sulfur atoms at opposite apexes (C&EN, Feb. 8, page 3 7 ) . In addition, the molecule contains considerable pi-electron derealizations—some valence electrons are not localized on any particular atom but move through the whole molecule. Dr. Almon G. Turner of Polytechnic Institute of Brooklyn has reached these conclusions from self-consistent field (SCF) molecular orbital calculations. His calculations were made excluding electron repulsions. Inorganic chemists have been interested in the geometrical and electronic structure of tetranitrogen tetrasulfide for some time, Dr. Turner told the Fourth Annual Metropolitan Regional Meeting of the ACS, held at Stevens Institute (Hoboken, N.J.). In the past, there has been some controversy over whether the four nitrogen atoms are coplanar or the four sulfur atoms are in one plane. Chemists now generally accept the geometrical structure arrived at by Dr. B. D. Sharma and Dr. Jerry Donohue at the University of Southern California, Los Angeles. From the results of a three-dimensional x-ray study, the USC pair concluded that the molecule's four nitrogen atoms lie in a plane, while the sulfur atoms lie above and below the plane. Dr. Turner's molecular orbital calculations also favor the coplanar nitrogen structure over the coplanar sulfur. Using the extended Huckel theory developed by Dr. Roald Hoffman and Dr. W. N. Lipscomb of Harvard University and the SCF method, Dr. Turner has carried out molecular orbital calculations on N 4 S 4 using several different sets of parameters and the geometrical structure of Dr. Sharma and Dr. Donohue. From his calculations, Dr. Turner views the N 4 S 4 molecule as having considerable pi-electron derealization. This arises from the overlap of 3p and 3d orbitals of the sulfur atoms with s
STRUCTURE. The geometrical structure of the tetranitrogen tetrasulfide molecule contains four coplanar nitrogen atoms, according to a three-dimensional x-ray study by Dr. B. D. Sharma and Dr. Jerry Donohue at the University of Southern California, Los Angeles. Molecular orbital calculations by Dr. Almon G. Turner of Polytechnic Institute of Brooklyn support the coplanar nitrogen structure. His calculations also indicate that there is some bonding between the sulfur atoms located on the same side of the nitrogen atom plane (shown by broken lines)
and p orbitals of the nitrogen atoms, he explains. His calculations also indicate that the nitrogen atoms are the electron-rich centers in the system and the sulfur atoms are electron deficient. His calculations show that there is some bonding between the sulfur atoms located on the same side of the plane defined by the nitrogen atoms. The bonding arises mostly by overlap of p orbitals on each of the sulfur atoms. However, about one third of the bonding comes from the participation of the d xy orbital on each sulfur. The molecule's S—S bond is about 15% as strong as its N—S bond. Thus it's considerably weaker than an S—S single bond, Dr. Turner says. Alkaline hydrolysis of tetranitrogen tetrasulfide gives dithionite, S 2 0 4 - 2 , which contains a sulfur-to-sulfur single bond, he notes. Dr. Turner's calculations give no evidence for the presence of chemical bonds between nitrogen atoms in tetranitrogen tetrasulfide. This includes nitrogen atoms located either diagonally or those bonded to the same sulfur atom. FEB.
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