Key step rearranges the relatively rigid bicyclo [4.3.1.] skeleton to the more flexible hydroazulene system |
Hydroazulene
Bicyclo[4.3.1]keto ester derivative
Y Subsequent , modification ! at C-7
R=CH z AcOCH z -
r^^
X=GH3S03-
/ Bulnesol
methylene as the protected side-chain precursor in the cyclohexanone start ing material was governed by the necessity that it survive a variety of vigorous reagents—strong base, con centrated sulfuric acid, and lithium aluminum hydroxide. Initially the Northwestern chemists tried just a phenoxymethylene group at C-4 of the starting compound. But they found that ring sulfonation oc curred during their attempts to cyclize the related diketo ester (in concen trated sulfonic acid). Substitution of chlorine on the aromatic ring of the phenoxymethylene side chain retarded the sulfonation reaction, so they settled on the p-chlorophenoxymethylene group in the starting compound. In one of their key steps—reduction of the bicyclic keto ester to the cor responding diol with lithium aluminum hydride—the Northwestern chemists found, to their gratification, that es sentially one isomer was formed. The relative stereochemistry of the product was confirmed by conversion to a delta-lactone. This product could be formed from only one of the four theo retically possible isomers of the diol, Dr. Marshall asserts. A second key step—skeletal rear rangement—turns out to be surpris ingly facile and remarkably specific, Dr. Marshall says. The rearrange ment of the diol to the hydroazulene product went smoothly via treatment of the mesylate derivative of the diol monoacetate in a solution of refluxing acetic acid-sodium acetate. 52 C&EN APRIL 8, 1968
I
\ £ Ho- \
1
I
\
H
*'CH3 3
C H 3
The rearrangement may proceed in two stages. This possibility rests on the formation of an isolable tertiary acetate product when rearrangement is conducted at low temperature. This is not a necessary mechanism, Dr. Marshall points out. At higher temperature an incipient cationic in termediate could lead directly to the observed olefin. In either event, Dr. Marshall adds, rearrangement pro ceeds by migration of that carboncarbon bond which is trans to the departing mesylate group. After rearrangement to the hydro azulene, there was still one fine point to overcome to have a product iden tical to natural bulnesol. This was isomerization of C-7, and was ac complished with relative ease, via basic equilibration of the C-7 carboxylic ester.
research laboratory, Midland, Mich. Pentacyanocobaltate (II) is a known homogeneous catalyst and will reduce activated carbon-carbon double bonds. The reaction of hydrogen with this catalyst to form hydridopentacyanocobaltate(III) under average experi mental conditions, Dr. Wymore notes, results in more than 90% of the cobalt being present as the hydride at equilibrium. Dr. Wymore's work, drawing upon earlier research efforts of Dr. Otto Piringer of Romania and others, was aimed at heightening catalytic action by replacing the cyanide in Co(CN) 5 _ 3 with groups which are weaker pi acids or acceptors, thereby destabilizing the cobalt hydrogen bond. He found that it was impossible to remove all of the cyanide ion; at least two or three cyanides appear to be required before the cobalt-hydrogen bond will form; without the inter mediate hydride there is no catalytic reaction. The overall reaction rate depends upon both the concentration and reactivity of the hydride, so if its concentration is too low the system won't have a fast rate even if it con tains a very reactive cobalt hydride. The general formula for this group of catalysts is one part cobalt to one to two parts nitrogen chelator to one to four parts cyanide. The nitrogen chelator, Dr. Wymore notes, can be almost any ligand containing two nitrogen donors, such as glyoximes, but is generally an amine. The total number of groups added is usually five to six relative to the cobalt. Ethylenediamine, triethylenetetramine, and 2,2 / -dipyridyl would make good catalysts though base must be added to the EDA system to pre vent hydrolysis of the cyanide ion.
Ligand substitution speeds catalysis 1 EC 1 ΟΟΤΗ
ACS N A T , 0 N A L
MEETING Inorganic Chemistry
Improved catalytic action, resulting in faster rates of reduction of carboncarbon double bonds, can be achieved by replacing part of the cyanide in pentacyanocobaltate with ligands such as 2,2'-dipyridyl, ethylenediamine, or o-phenanthroline, says Dr. C. Elmer Wymore of Dow Chemical's physical
REVEALS RATIO. Dr. Wymore's work shows cyanide-ligand ratio is critical
Absorption spectra correlate with catalyst activity of various ligand-substituted pentacyanocobaltates
II
^ ^ ^ ^
Co(dipyMCN)2
6 0 0 Μμ
0.05M Co/2dipy/XCN
8 0 0 MM
1 0 0 0 M/*
1200 Mi»
Wave length
Some materials which Dr. Wymore has reduced in his work are crotonaldehyde, acrylates, styrene, and buta diene. The kinetic results for the reduc tion of styrene using a catalyst solu tion prepared with the ratio C o / 2 d i p y / 2CN (plus base), in a 50-50 (by vol ume) ethanol-water medium at 25° C. with 1 atm. hydrogen, can be repre sented by:
CO 2Co + H 2 : ^ 2CoH
K«.
(2.)CoH + H2C=CHC6H5
^
CoCH2CHzC6Hs
K2
( 3 ) CoH + CoCHzCH2C6H5 J à 2Co +CH3CH2C6H5, where Co represents all active cobalt species. Below the solubility limit of styrene there is a concentration range where the reaction is psuedo zero order with respect to styrene. This is borne out by a considerable zero order portion in most individual runs, Dr. Wymore notes. Below a styrene concentration of about 0.03M the reaction is first order in styrene. Other kinetic data show that the reaction is second order in added cobalt and first order in hydrogen in the region in which it is zero order in styrene. In the concentration range in which the reaction is first order in styrene it is about one-half order in hydrogen and first order in cobalt catalyst. At high styrene concentrations the
slow step is the splitting of hydrogen (reaction 1). As the concentration of styrene is lowered the overall rate of the second reaction becomes slower until finally it is the slow step. Tracer studies using deuterium with H 2 0 EtOH and H 2 with D 2 0 - E t O H show that the hydrogen in the reduced substrate comes from the gas phase supporting homolytic splitting. These studies also show an exchange between the gas phase and the reduced styrene which supports beta addition of the cobalt to the styrene as in the second equation. Absorption spectra of 0.05M catalyst solutions containing two moles of dipyridyl per cobalt and varying ratios of cyanide give peaks which can be correlated with catalyst activity. As more cyanide is added the peak at 650 τημ grows and then de creases while the ΙΙΟΟ-πΐμ, peak in creases. Dr. Wymore notes that the peak at 650 τημ is mainly due to the com plex C o ( d i p y ) 2 ( C N ) 2 , but when base is present part of the peak is caused by a species C o n ( d i p y ) (OH) (X), where X equals water, chloride, or hydroxide. The Dow chemist assigns the band at 1100 τημ to a species containing more cyanide and less dipyridyl. De creasing the dipyridyl increases this band. The shift of the 1100-πΐμ, peak to shorter wave lengths with increasing cyanide ion is due to increasing con centrations of C o ( C N ) 5 - 3 , which has a peak at 967 τημ. The rate of reduction of styrene by the various solutions increases as the cyanide ion concentration increases until a ratio of four is reached and
then it decreases. The decrease is caused by the formation of Co(CN) 5 ~ 3 , which is a slower catalyst. Because it was not possible to ob tain pure solutions of the various species and there is no wave length at which only one species absorbs, it was not possible to obtain ex tinction coefficients and exact con centrations of the various components, Dr. Wymore says. The data do show, however, Dr. Wymore adds, that Co( d i p y ) ( C N ) 3 ~ is the most reactive species. Absorption of hydrogen by catalyst solutions occurs to about 10% of the oretical and none can be ascribed to the C o ( d i p y ) 2 ( C N ) 2 compound. Thus, in the series C o ( d i p y ) 9 ( C N ) 2 , C o ( d i p y ) ( C N ) 3 - , and C o ( C N ) 5 - 3 the amount of reaction with hydrogen increases with increasing number of cyanide groups. This is a result of a stronger cobalt-hydrogen bond in HCo(CN)5-3. The reactivity as a catalyst is in the reverse order be cause of a more reactive cobalt-hy drogen bond in HCo(dipy) ( C N ) 3 . It cannot, Dr. Wymore adds, be known with certainty whether Co( d i p y ) 2 ( C N ) 2 is a catalyst.
Curium emission lines reveal energy transfer
TH
ACS NATIONAL MEETING Nuclear Chemistry and Technology
Intramolecular energy transfer has been demonstrated for the first time in actinide ion (A 3 +) β-diketone com plexes by Dr. L. J. Nugent and his coworkers, J. R. Tarrant, J. L. Burnett, R. D. Baybarz, G. K. Werner, and O. L. Keller, Jr., at the Oak Ridge National Laboratory, Transuranium Research Laboratory, Oak Ridge, Tenn. This at least poses the possibility of transuranium ion liquid lasers, Dr. Nugent says. Such lasers would need no external power source as radioac tive decay would sustain the laser reaction. The Oak Ridge effort might well be the first attempt to couple quantum electronics with trans uranium research. Studies up to this point, Dr. Nugent explains, show that efficient intra molecular energy transfer and subse quent narrow line luminescence oc curs with certain radioactive trans uranium ions, such as curium-244 hexafluoroacetyl-acetonate at room temperature when in solution. This phenomenon is demonstrated APRIL 8, 1968 C&EN 53
