596
Organometallics 1986, 5 , 596-598
+
+
Table I. Electrochemical Values for Bis(hexamethy1benzene)chromium 1 /O and 1 /2+ Room-Temperature Redox Processes" peak position V peak height WA ox. step 1+/0 2+/ 1+
E , cathodic
E , anodic
i, cathodic
i, anodic
-1.661 -0.195
-1.600 -0.124
0.40 0.16
0.35 0.36
scan rate, m V / s 50 200
Up, mV 61 71
ipcli,,
1.03 0.44
El/%v -1.630 -0.160
"Concentration = 7.97 mM. 0.OM)v
-0.875
I -
-1.7x) 1+/0
r
-2.625
-3.m
I n
/
I I
,' 2+/1+
/'
t
I
\
I+/O
'\
,/4
+OICXJV -0038 -0175
-0312
-3450V
c--e--t---t--r -145OV-1550
i8y)
-1750 - 1 8 5 0 V
Figure 2. A typical current-voltage curve of bis(hexamethy1benzene)chromiumrecorded at (a) 50 mV/s over the full range exhibiting both 1+/0 and 2+/1+ redox couples, (b) 50 mV/s over the range exhibiting the 1+/0 redox couple, and (c) 200 mV/s over the range exhibiting the 2+/1+ redox couple.
Model RE 0074 X-4 Recorder. After the sample (0.03 g, 7.97 mM) was introduced into the electrochemical cell a cyclic voltammogram, CV, containing both redox potentials was recorded. Each redox couple was then individually investigated. Both the 1+/0 and 2+/1+ redox couples were recorded separately a t 50, 100, 200, 500, and 1,000 mV/s to test for peak position/scan rate dependency. The anodic peak position for each individual CV was essentially unaffected by and independent of the scan rate. Peak currents were dependent on the scan rate. Scan rates of each individual CV used in the determination of electrochemical parameters were chosen to maximize the peak currents and cathodic and anodic peak height parity. Figure 2 represents (a) a cyclic voltammogram (CV) recorded at a scan rate of. 50 mV/s and exhibiting both redox processes, (b) a detailed CV of the 1+/0 redox couple recorded at 50 mV/s, and (c) a detailed CV of the 2+/1+ redox couple recorded at 200 mV/s. Table I summarizes the electrochemical parameters obtained. Electron Spin Resonance. A saturated solution of bis(hexamethy1benzene)chromium in methanol was recorded on a Varian E-9 EPR spectrometer with a Varian E-101 Microwave bridge operated a t room temperature. The ESR spectrum of bis(hexamethy1benzene)chromium(1+)was consistent with that reported by B r ~ b a k e r .The ~ g value was 1.9893 (lit.9 g = 1.9867). The observation confirms that our bis(hexamethy1benzene)chromium sample forms the expected stable 1+ cation in solution. The 1+/0 and 2+/1+ redox potentials have peak separations, AE, of 61 and 71 mV, respectively. These values are well within those expected for a one-electron Nernstian process at room temperature. For example, Saji3 and Treichel" report the 1+/0 redox couple for bis(benz(9) Brubaker, C.; Li, T.; Kung, W.; Ward, D.; McCulloch, B. Organometallics 1982, 1, 1229. (10) Treichel, P.; Essenmacher, G.; Efner, H.; Klabunde, K. Inorg. Chim. Acta 1981, 48, 41.
ene)chromium in acetonitrile at 58 and 114 mV, respectively. Peak separation, the effect of the scan rate, and the ratio of cathodic peak current, ipc, to anodic peak current, ipa, form the basis of the argument for the reversibility for the 1+/0 couple of bis(hexamethy1benzene)chromium and the quasi-reversibility of the corresponding 2+/ 1+ couple. The peak current ratio for the 2+/1+ redox couple is less than unity. It is not unexpected that at moderate scan rates of 200 mV/s at room temperature that some of the 2+ cation might disproportionate before the return scan occurs because this chromium species is highly unstable under normal conditions. We can speculate that the presence of the 12 electronreleasing methyl groups on the two complexed rings provides sufficient electron density to the chromium cation to permit the formal, further oxidation of the 1+ species in a potential range that can be attained in this solvent system. Unpublished work from these laboratories on the electrochemistry of the monocations and magnetic properties of progressively methylated derivatives of bis(benzene)chromium(O) seems to support this general conclusion. Work in the extension of the range of oxidation states for bis(arene)chromium complexes continues.
Acknowledgment. We thank Mr. Ted Pettijohn for his assistance with the ESR experiments. The Robert A. Welch Foundation and the National Science Foundation provided generous support. Registry No. Bis(hexamethy1benzene)chromium (0), 1215666-0; bis(hexamethy1benzene)chromium (l+), 12243-39-9.
Influence of Dienes on the Cobalt Carbonyl Catalyzed Reactlon of Mercaptans with Carbon Monoxide Shlomo Antebi and Howard Atper"' Ottawa-Carleton Chemistry Institute Department of Chemistry, University of Ottawa Ottawa, Ontario, Canada K I N 984 Received October 29, 1985
Summary: Thio esters are obtained in good to excellent yields by the cobalt carbonyl catalyzed carbonylation of mercaptans in the presence of 2,3dimethyl- 1,&butadiene or 2,3-dimethoxy-1,&butadiene. Diene-cobalt carbonyl complexes [i.e., (diene-Co(CO),),] are probably the key catalytic species in these reactions.
Many investigations have been carried out on metalcatalyzed reactions of substrates bearing nitrogen and oxygen atoms. Examples include the cobalt carbonyl catalyzed reductive carbonylation of Schiff bases with organoboranes,l the conversion of amines to formamides John Simon Guggenheim Fellow, 1985-1986. H.; Amaratunga, S. J . Org. Chem. 1982, 47, 3593.
( 1 ) Alper,
0276-7333/86/ 2305-0596$01.50/0 0 1986 American Chemical Society
Organometallics, Vol. 5, No. 3, 1986 597
Communications
Table I. P r o d u c t s Obtained from the Reaction of M e r c a p t a n s w i t h Diene, CO, a n d CO*(CO)~ products, % yieldb RSH, R =
Ph p-CH3C6H4
dienen
thio ester
disulfide
CHD DME CHD CHD(N2)' CHD(NJd In
20 87 22 8
10 3 16
37 13 61 82 24 58 14 55 24 84 24 63 85 13 59 5
15
6 35 87 20
5
3
I
p-BrC,& p-FC6H4
2-C10H7 P-CH~OC~H~CH~ p-CH,CBH,CH2
DM DME CHD DM CHD DM CHD DME CHD DM DME CHD DME CHD
addition product
sulfide
12
4 27
14 18
4
12
5 5 27 14
2 5 12 18
55 3 15
CHD = 1,3-cyclohexadiene; I = isoprene; In = indene; DM = 2,3-dimethyl-1,3-butadiene; DME = 2,3-dimethoxy-1,3-butadiene. bProducts were identified by comparison of boiling points and spectral data (IR, NMR ['H, '%I, MS) with literature values. Yields are of pure materials. Nz atmosphere. - d Without C O ~ ( C O ) ~
or ureas induced by various metal complexes,2 and the ruthenium complex catalyzed carbonylation of methyl ether to methyl a ~ e t a t e . ~Few examples are known of catalytic processes in which one of the reactants contains sulfur as the heteroatom, since the presence of such a heteroatom frequently poisons the catalyst. Recently, we found that cobalt carbonyl is a fine catalyst for the desulfurization and carbonylation of benzylic mercaptans and thiophenols to carboxylic ester^.^ COZ(CO)~, HzO
RSH co R'oH 58-61 atm, 190 " C WRCOOR' + H2S Hydrocarbons were formed when the reaction was effected in benzene instead of aqueous alcoh01.~ The byproduct is carbonyl sulfide, while hydrogen sulfide was the gaseous product of the reaction in aqueous alcohol. +
RSH
+
+ CO
CodC0)8/H,O C6H6,185-190" C , 61 atm
RH
+ COS
It seemed of value to attempt the mercaptan reaction with carbon monoxide in benzene, in the presence of conjugated dienes. Conceptually, the diene can complex to the cobalt catalyst to generate a new perhaps more active catalytic species, intercept an organocobalt intermediate, or experience addition of the mercaptan affording an unsaturated sulfide.6 We now wish to report that the presence of a diene causes a dramatic change in reaction course and that the product yields are sensitive to the nature of substituent groups on the diene. When p-toluenethiol (1, R = p-CH3C6H4)was carbonylated in the presence of 1,3-cyclohexadiene and cobalt carbonyl, at 58 atm and 185-190 "C, the thio ester 2, R = p-CH3C6H4,was formed in 22% yield together with diene, 185-190"C RSH + CO Co,(CO),, 55-61 atm RCOSR + COS 2 1 (2) Sheldon, R. A. "Chemicals from Synthesis Gas"; D. Reidel Publishing Co.; Dordrecht, Holland, 1983;pp 167-184. (3)Braca, G.; Sbrana, G.; Valentini, G.; Andrich, G.; Gregorio, G. J . Am. Chem. Sac. 1978,100, 6238. (4)Shim, S. C.; Antebi, S.; Alper, H. J . Org. Chem. 1985,50, 147. (5)Shim, S.C.; Antebi, S.; Alper, H. Tetrahedron Lett. 1985,26,1935. (6) E.g.: Claisse, J. A.; Davies, D. I. J. Chem. SOC.1965,4894.Saville, B. J. Chem. SOC.1962,5040.
p-tolyl disulfide (16% yield) and a product (6% yield) resulting from 1,2-addition of the thiol to 1,3-cyclohexadiene. The latter adduct could be obtained in high yield, in the absence of cobalt carbonyl, by using a nitrogen atmosphere! The yield of thio ester in these reactions is dependent on the structure of the diene. While indene and 2-methyl-1,3-butadiene (isoprene) are somewhat less or more effective than l,&cyclohexadiene, 2,3-dimethyl-1,3butadiene affords thio esters in improved yields (55433%). Even better is 2,3-dimethoxy-1,3-butadiene, since thio esters are obtained in excellent yields (82-87%) when the mercaptan carbonylation is run in the presence of this diene. The carbonylation reaction is applicable to a variety of thiophenols and benzylic mercaptans (see Table I for yields of pure products), but alkanethiols are inert under these reaction conditions. Note that hydrocarbons were not isolated in these reactions. Carbonyl sulfide was the gaseous product, confirmed by mass spectrometric analysis of the evolved gases (intense signal a t m l e 60). It is likely that diene-cobalt carbonyl complexes are the true catalysts in the carbonylation reaction. Such complexes are known and are easily prepared by the reaction of the diene with C O ~ ( C O )Complex ~ ~ ~ ~ 3, synthesized
c H3 3
according to Winkhaus and Wilkinson; was employed as the catalyst for the carbonylation of p-methoxybenzenethiol in benzene [in the absence of diene and Co,(CO),]. The thio ester was obtained in 48% yield, which compared (7) Winkhaus, G.; Wilkinson, G. J . Chem. SOC.1961,602. (8)McArdle, P.;Manning, A. R. J. Chem. SOC.A 1970,2123.
598
Organometallics 1986, 5 , 598-601
U
RSCo(C0)4
co
RSCCo(C0)4
I0/
-
RCo(C0)4
2 R /IC C o ( C 0 ) 4
-COS
7
6
5
0
I/ RCCo(CO)4 +
0
I1
RCSR t HCo(CO),
RSH
7 2HCo(CO)s
-
+
Co2(CO)s
H2
quite favorably with the results (58%) of the reaction effected under the usual conditions (Le., mercaptan/ CO/ C O ~ ( C Odiene). )~/ There is a previous description of the conversion of mercaptans to thio esters by cobalt carbonyl. However, very drastic conditions were required (approximately 1000-atm pressure and 275 0C).9 In addition, a recent patentlo claims that thiophenols react with 1,3-butadiene, synthesis gas, and then carbon monoxide, in the presence of cobalt carbonyl and pyridine, a t 900 atm and 135 "C, to give thio esters of pentenoic acid (not formed in the absence of pyridine). The diene, in this case, is incorporated in the product. The formation of thio esters can be explained by the pathway outlined in Scheme I. Reaction of thiol with C O ~ ( C Owould ) ~ give the thioalkylcobalt complex 4 and HCo(CO),. Carbonylation of 4 to 5, followed by expulsion of carbonyl sulfide, would generate the dkylcobalt complex 6. Complex 7 would result from carbonylation of 6, and subsequent reaction of 7 with additional thiol would afford the thio ester. The disulfide may be formed by reaction of 4 with RSH, while sulfide would arise by analogous reaction of 6 with RSH." Why does the presence of the diene have such a significant influence on the facility of the carbonylation reaction (and blocks the desulfurization process)? The diene-complexed analogue of 6 would be 8 (illustrated for 2,3-dimethyl-1,3-butadiene).It is conceivable that one function of the diene ligand is to enhance the ligand migration step (to 9) and thereby promote thio ester production by subsequent reaction of 9 with thiol. The CH3
CH3
RCo(C0h 8
CH3
C H3
R-C-Co(C0)2
I/
0
9
presence of electron-donating substituents on the diene promotes the carbonylation of thiols. However, caution should be exercised in attributing the reactivities of different dienes to inductive effects alone. It may simply be (9) Holmquist, H. E.; Carnahan, J. E. J . Org. Chem. 1960, 25, 2240. (10) Kadelka, J.; Schwarz, H. H. Ger. Offen. DE 3 246 149,1984, Chem. Abstr. 1985, 102, 5914k. (11)Another mechanism which should be considered is reaction of the in situ generated HCo(CO), with the diene to form a (u-a1lyl)cobalt complex.12 This interception of HCO(CO)~ would prevent its reaction with RCo(CO), to form alkane. It should be noted, however, that the carbonylation of mercaptans to thio esters can be catalyzed by 3 in the absence of added diene. (12) Ungvary, F.; Mark6, L. Organometallics 1984, 3, 1466.
0276-7333/86/2305-0598$01.50/0
a consequence of the ease of in situ generation of dienecobalt complexes such as 3 (while 2,3-dimethyl-1,3-butadiene forms 3 in good yield with cobalt carbonyl, the complex formed from 1,3-~yclohexadieneis obtained in low yield).: A t very high pressures (1000 atm), the carbonylation of RCo(CO), to RCOCo(CO), is probably fast, accounting for subsequent thio ester formation on reaction with RSH. However, at low pressures, the reaction of RCo(CO), with HCo(CO), to give RH and regenerate CO,(CO)~ is apparently more rapid than the carbonylation of RCO(CO)~ to RCOCo(CO),. Finally, it is noteworthy that thiophenol has been reported to react with CO,(CO)~,under nitrogen, to give benzaldehyde in 36% yield and benzene in 1.6% yield.13 Aldehydes were not detected in any of our reactions. The following general procedure was used: a mixture of the mercaptan (10 mmol), diene (10-12 mmol), cobalt carbonyl (0.5 mmol), water (2 mL), and benzene (30 mL) was heated overnight at 185-190 "C and 55-61 atm. After cooling to room temperature, the mixture was analyzed by gas chromatography and then worked up by distillation or by column chromatography. In conclusion, this study has demonstrated that dienes are influential in the cobalt carbonyl catalyzed reaction of mercaptans and that thio esters can be formed by this catalytic process which is simple in both execution and workup. Acknowledgment. We are indebted to British Petroleum and to the Natural Sciences and Engineering Research Council, for support of this research. (13)Klumpp, E.; Bor, G.; MarkB, L. Chem. Ber. 1967, 100, 1451.
Organolanthanide and Organoactinide Oxidative Additions Exhibiting Enhanced Reactivity. 3. Products and Stoichiometries for the Addition of Alkyl and Aryl Halides to (C,Me,),Yb"*OEt, and Evidence for Facile, Inner-Sphere (C,Me,),YbI1'R and (C,Me,),YbI1'X RX"Yb"' Grignard" Reactions
+
R. G. Finke,' S. R. Keenan, and D. A. Schiraldit Department of Chemistry, University of Oregon Eugene, Oregon 97403
P. L. Watson*§ Central Research and Development Department Experimental Station E. I. du Pont de Nemours and Company Wilmington, Delaware 19898 Received June 11, 1985
Summary: (C,Me,),Yb".OEt, undergoes atom abstraction, oxidative-additionreactions with alkyl and aryl halides at rates 103-106 faster than typical d-block transition metals according to the generalized stoichiometry 1.O(C,Me,),Yb".OEt, (1.0 a)RX (1.0 -
+
+
-
'Current address: Celanese Fiber Operations-269D, P.O. Box 32414, Charlette, NC 28232. 5 Contribution $3057.
0 1986 American Chemical Society
