5940
Chemical and Infrared Spectral Details of Reactions Involving Stereospecific Incorporation of Oxygen- 18 into Substituted Manganese and Rhenium Carbonyl Derivatives via Exchange Reactions with H 2 1 8 0 Donald J. Darensbourg* and Joseph A. Froelich Contributionfrom the Department of Chemistry, Tulane University, New Orleans, Louisiana 70118. Received March 3, 1977
Abstract: The reactions of substituted group 7 B metal carbonyl cationic derivatives, [M(CO)5L]+ (M = Mn, Re and L = phosphine, C5H5N, CH3CN) and [ Mn(C0)4(diphos)]+, with labeled water are described. The carbonyl ligands in these derivatives are found to undergo facile oxygen-exchange reactions with H 2 l 8 0 , presumably through hydroxycarbonyl intermediates, to afford CI8O enriched species. In all cases investigated, the carbonyl sites with the higher CO stretching force constant (implying more positive character at carbon as well as a more stable LUMO) were found to be more susceptible to oxygen exchange, thus leading to stereospecifically labeled species. In addition, the reactivity of the substrate toward oxygen exchange with water was found to decrease in the order [M(C0)6]+ > [M(CO)sL]+ >> [M(CO)4(L-L)]+. The preparation of cis[Mn(C0)4(I3CO)L]+ (L = PMe2Ph, PPh3, and AsPh3) species is reported along with oxygen-I8 exchange reactions of these derivatives to yield mixed labeled (13C1sO)metal carbonyls. The incorporation of oxygen-I 8 into metal carbonyl cations was found to be greatly accelerated by the addition of small quantities of triethylamine.
Although [Re(Co),]+ is perfectly stable in water, Muetterties has observed that this species exchanges oxygen with oxygen atoms in water.' We have previously reported on the stereospecific incorporation of oxygen- 18 into manganese and rhenium pentacarbonyl derivatives via exchange reactions with H2I80, where the equatorial C O positions were enriched at a faster rate than the axial C O position.2 These processes were proposed to proceed through an intermediate resulting from hydroxide attack at a carbonyl carbon site, which is in chemical equilibrium with the starting material and where proton exchange readily occurs between the two oxygen atoms (eq 1). M(C0)sL'
+ H2I80
-
(M(C0)4(L)C(=O)180HJ M(C0)4(CI80)L+ (1) ( M = Mn, Re and L = phosphine, pyridine, CH3CN) -+
I n the manganese hexacarbonyl cationic derivative a secondary process occurs as well as the oxygen exchange reaction which involves @-hydrogentransfer in the [Mn(C0)5COOH] intermediate with concomitant formation of HMn(C0)S and C02.3Similarly, the reaction of Fe(C0)5 with -OH to afford Fe(C0)dH- and C 0 2 has been proposed to proceed through the intermediate [ Fe(C0)4COOH]-.4 Hydroxycarbonyl intermediates have also been implicated in the reaction of [PtCl(CO)(PEt,)2]+ and water to yield the Pt(I1) hydride species and C 0 2 5and in the catalytic reduction of nitric oxide by carbon monoxide using [RhC12(CO)2]-.6 Hui and Shaw' have also proposed the intermediacy of a hydroxycarbonyl species, [ M(CO)sCOOH]-, in substitution reactions of the group 68 hexacarbonyls with phosphines in the presence of aqueous sodium hydroxide. Recently, we have postulated an analogous sulfhydrocarbonyl intermediate in the reaction of [M(C0)5L]+ species (M = Mn, Re) with NaSH to afford H Mn(C0)dL derivatives and C O S 8 On the other hand a stable Ir(lI1) hydroxycarbonyl species, IrCl2(COOH)(CO)(PMe2Ph)2,has been prepared from the reaction of the dicarbonyl cation with water.9 Indeed this reaction was found to be readily reversible in that the dicarbonyl cation was re-formed upon reaction of the hydroxycarbonyl derivative with dry HCI. I n addition to our interest in the preparation of stereospecifically labeled metal carbonyl derivatives with either I3CO or CI8O for both photochemical and thermal mechanistic studies,'O-'swe have been for a long period interested in unJournal of the American Chemical Society
derstanding the electronic factors responsible for controlling the specific site of nucleophilic attack in metal carbonyl derivatives.16-18This report represents a detailed extension of our earlier communication,2 as well as further studies defining the mechanism of these metal-bound carbon monoxide oxygen exchange reactions with water.
Experimental Section Material and Equipment. Mnz(CO)lo, Re2(CO)lo, bis( I ,2-diphenylphosphino)ethane, triphenylarsine, and dimethylphenylphosphine were purchased from Strem Chemical Co. Triphenylphosphine was the generous gift of M and T Chemical Co. Nitrosylhexafluorophosphate (NOPF6) was obtained from Aldrich, whereas NH4PF6 was purchased from Ozark-Mahoning Co. Carbon monoxide, 90% cnriched in ' T O , was acquired from Prochem. H2180 (96.5% by weight oxygen-18) and D2I80 (99.0% by weight oxygen-18) were obtained from Monsanto Research Corp. (Mound Laboratory) and Norsk Hydro (Norway), respectively. Reagent grade acetonitrile (Matheson Coleman and Bell) was purified by refluxing over sodium hydride or calcium sulfate and distilled prior to use. Melting points were taken in open capillary tubes on a ThomasHoover apparatus and are uncorrected. Elemental analyses on the coniplexes were performed by Galbraith Laboratories, Inc., Knoxville, Tenn. Preparations. All operations on complexes in solution were carried out under an atmosphere of nitrogen. Manganese pentacarbonyl bromide and Mn(C0)4( L)Br derivatives were prepared according to the procedures in the 1iterat~re.I~ The M(IZCO)sL+ (M = Mn, Re) complexes were synthesized by methods we have previously described.?" Mn(CO),(diphos)Br was prepared from Mn(C0)sBr and bis( I ,2-diphenylphosphino)ethane in T H F under a C O atmosphere as described by Angelici and Brink.21The material was purified by recrystallization from CHzCl2/hexane. [ Mn(CO).~(diphos)j[PFb]. This complex was prepared by employing ; I procedure analogous to that described by Anglin and Graham22for the synthesis of the rhenium derivative. Mn(CO)3(diphos)Br (0.76 g, I .2 mmol) dissolved in 50 mL of benzene under a nitrogen atmosphere underwent a color change from yellow to red upon the addition of 2 g of AIC13. The solution was then pressurized with C O (-2-3 atm) and heated to 60 OC for 1 h. Upon cooling to room temperature, the solvent was removed in vacuo. Addition of a methanolic solution of N H J P F ~(10.0 mmol in IO mL) to the residue dissolved in 20 mL of methanol resulted in immediate precipitation of white crystals. These were collected by filtration and washed with methanol and hexane, mp >232 OC. Anal. Calcd for C30H2404P3F6Mn: C, 50.72; H. 3.41. Found: C, 50.72; H, 3.40.
/ 99:18 / August 31. 1977
594 1 [ C ~ S - M D ( C ~ ) ~ ( ~ ~ C OThese ) L I Pcomplexes, F~~ where L = MezPhP, PhjP, Ph3As, were prepared in low yields (10-15%) from thecorresponding cis-Mn(C0)4(L)Br species and I3CO (90% enrichment) in the presence of AIC13 as described above for the preparation of the [ Mn(CO)r(diphos)] [PF,] derivative. The stereochemical site of I3CO addition was determined by infrared spectroscopy in the u(C0) region. There was no infrared spectral evidence for rearrangement of the stereospecifically incorporated I3CO in these derivatives in solution at ordinary temperatures. IR spectra (u(C0) in acetonitrile): [MezPhPMn(CO)s]+ 2135.6 (m), 2078.2 (w, sh), 2062 (sh), 2050.3 (s), 2018.9 (s) cm-l; [Ph3PMn(C0)5]+ 2134.9 (m), 2080 (w, sh), 2065 (sh), 2052.1 (s), 2021.0 (s) cm-l; [Ph3AsMn(CO)s]+ 2135.1 (m), 2080 (w, sh), 2064 (sh), 2053.9 (s), 2021.8 (s) cm-l. Reactions of [Mn(CO)4(diphos)][PF6] with H2I80. [Mn(CO)l(diphos)][PF6](O.I 14 g, 0.160 mmol) was dissolved in 4.0 mL of dry acetonitrile under nitrogen with stirring and 0.15 mL (8.3 mmol) of 95% H2I80 was added using a microsyringe. The reaction was monitored by periodically withdrawing samples and observing the u(C0) infrared spectra over an 80-day period. The complex was found to be perfectly stable in acetonitrile under an inert atmosphere for at least 3 months. A simiiar reaction was carried out where [Mn(CO)4(diphos)][PF,] (0.0526 g, 0.074 mmol) was dissolved in 2.0 mL of dry acetonitrile ABS. with 0.05 m L (2.5 mmol) of 95% H2I80 with the addition of 5.0 pL of triethylamine (0.036 mmol). The rate of oxygen-18 incorporation was markedly enhanced in the presence of triethylamine with a concomitant production of metal-hydride complex being observed. Reactions of [M(Co)sL][PF6] with H2'*0. [M(C0)5L] [PF,] and M complexes ( M = Mn; L = MeZPhP, Ph3P, CH3CN, C ~ H S N = Re; L = Ph3P, MezPhP, CH3CN) were reacted with H2I80 in much the same manner as that described above for the [Mn(C0)4(diphos)] [PFb] derivative. In a typical experiment, [Me*PhPMn(CO)s] [PF,] (0.040 g, 0.084 mmol) was dissolved in 1 .O mL of acetonitrile containing 0. I O cm3 of H2180 (5.0 mmol) with stirring. The enrichment was monitored by IR spectroscopy in the v(C0) region. Similar experiments were carried out with the stereospecifically "CO labeled species, e.g., C ~ ~ - [ M ~ ( C O ) ~ ( ~ ~ [PF,]. CO)PM~ZP~] A heterogeneous reaction was carried out between [Mn(CO)5PPh3][PF6] and H2I80 by adding 5.0 mL of dry hexane to a degassed sample of the pentacarbonyl complex (0.122 g, 0.212 mmol) I I I I I I followed by the addition of 0.05 mL (2.5 mmol) of 95% H2l8OO. The 2160 2120 slow production of oxygen- I8 enriched cis-HMn(CO)4PPh, species Figure 1. High-frequency region of the u(C0)spectra for the incorporation was followed by withdrawing samples of the solution phase with a ofoxygen-18 into [MezPhPMn(CO)s]+.The numbers correspond to the hypodermic syringe at various time intervals and obtaining their inisotopic species listed in Table I. frared spectra in the u(C0) region. Infrared Measurements and Vibrational Analysis. The infrared spectra were recorded on a Perkin-Elmer 521 spectrophotometer was possible to assign all the observed bands to the various equipped with a linear absorbance potentiometer. The spectra were oxygen- 18 enriched species by noting the rates of appearance calibrated against a water vapor spectrum below 2000 cm-' and and decay of bands concomitantly aided by calculations inagainst a CO spectrum above 2000 cm-l. Matched sodium chloride volving a restricted C O force field. Table I lists the results for cells were used in the measurements. the seven observed isotopic species of [Me*PhPMn(CO)s]+ Initial CO stretching force constant calculations on the [Menriched with C'*O. (CO)5L]+ ( M = Mn and Re, L = Lewis base) species were performed Although there are calculated u(C0) band positions in using the Cotton-Kraihanzel approach23employing the CIhO fremixed axial-equatorial CI8O substituted species which are quency data. On the other hand, initial C O stretching force constant quite similar to those of equatorially only substituted species, calculations on [ Mn(C0)4(diphos)]+ were carried out using the ClhO frequency data and a modified Cotton-Kraihanzel procedure refined the high-frequency band at 2094.1 cm-' assigned to the all by Jernigan, Brown, and Dobson24and employing a program develCI8O molecule, 7, does not begin to grow in until the preoped in our l a b o r a t o r i e ~The . ~ ~ trial force constants were refined using dominant species in solution is the equatorially substituted the C i 8 0frequency data and an iterative computer programZhthat tetra-CI80 derivative, 6. That all the observed bands are coradjusts a set of force constants common to a group of isotopically rectly assigned to oxygen-18 enriched carbonyl stretching visubstituted molecules to give simultaneously a least-squares fit bebrations in the parent derivative is revealed experimentally by tween the observed and calculated frequencies for all the molecules. carrying out the reverse reaction on the highly enriched species The trial force constants were generally refined to reproduce the obwith H2I60, the [Me2PhPMn(Ci60)5]+derivative being afserved u ( C 0 ) vibrations within an average of
2070.5
2002.9
1963.7
1967.9
1994.4
1974.8
1953.3
1977.9
1967.9
1953.3
- L l
2044s
UObserved frequencies are listed in parentheses directly below the calculated values. Italicized frequencies were employed as input data in the force field fit (k,(eq) = 16.55,k,(ax) = 16.90,k,(ax-eq) = 0.25,,k,'(eq-eq) = 0.37,,kt(ax-ax) = 0.48,).bTwo grossly overlapped bands centered -2007 c ~ n - ~ .
H2I80 in 2.0 mL of CH$N), the rate of oxygen-18 incorporation being markedly enhanced in the presence of triethylamine. For example, after 3 h of reaction time oxygen exchange has proceeded to approximately the same extent as that which took 78 days under similar conditions in the absence of triethylamine. This acceleration in rate can be attributed to an increase in the concentration of the reactive hydroxide species with added amine base. At the same time, however, these conditions increase the relative rate of P-hydrogen transfer in the hydroxycarbonyl intermediate with formation of C 0 2 and metal hydride as compared to oxygen exchange (eq 3).49 That is, although the reaction path leading to metal [ M n ( C O ) , ( d i p h o s ) ] ++ H,"0
-
? }
{(diphos)Mn(CO)3C"OH
2100
Figure 5. u(C0) spectra as a function of time for the incorporation of oxygen-18 into [Mn(CO)d(diphos)]+in acetonitrile: ( A ) 9 days, (B)39 days, (C)49 days, and (D)78 days (overlay - - -,after 3 h with added EtjN).
(hydroxide ion) assistance in the p-elimination process. In other words, in the presence of relatively large concentrations of hydroxide ion, the hydroxycarbonyl intermediate may be attacked by -OH, presumably at the carboxylic carbon atom^,^*,^^ leading to the formation of metal hydride and carbon dioxide.54It should be pointed out here parenthetically that reactions of metal carbonyls with potassium hydroxide have been known for some time to afford metal carbonyl hydride derivatives on a preparative scale, e.g., Fe(C0)s and Cr(C0)6 yield [HFe(C0)4)]- and [Cr2H(CO)lo]-, respecti~ely.~,~~ Alternatively, the hydroxide ion (or Et,N itself) may serve to deprotonate the hydrocarbonyl intermediate, followed by release of C 0 2 with hydrolysis of the reduced metal species, i.e.56.57 0
I1
MnCOH + -OH (diphos)Mn(CO),H + CO,
2000 CM-l dCC0
0
II
MnCO- + H,O
-
Mn- + co, Hz\
[( d i p h o ~ ) M n ( C O ) , ( C ' ~ O ) ] +
MnH + -OH
hydride formation is not noted for the reaction of [Mn(C0)4(diphos)]+ with water even over reaction periods as long as 78 days (where extensive oxygen- 18 exchange has occurred), in the exchange reaction carried out in the presence of triethylamine substantial metal-hydride formation was obser~ed.~~,~~ This observation that the hydroxycarbonyl intermediate is relatively more reactive toward &hydrogen transfer vs. oxygen exchange in the presence of added base as compared to reactions carried out in the absence of base may be due to base
In conclusion, there is an obvious similarity between this process (metal hydride formation and COz elimination) and the metal carbonyl cluster catalyzed energy-important water gas shift reaction (CO H20 F! C 0 2 H2).S8We have, therefore, initiated a study of oxygen-exchange reactions between metal carbonyl clusters and labeled water.
+
+
Acknowledgment. The financial support of the National Science Foundation through Grant C H E 76-04494 is greatly appreciated.
Darensbourg, Froelich
/ Exchange Reactions with H2' 8O
5946
References and Notes E. L. Muetterties. Inorg. Chem., 4, 1841 (1965). D. J. Darensbourgand D. Drew, J. Am. Chem. SOC.,98, 275 (1976). D. J. Darensbourg and J. A. Froeiich, J. Am. Chem. Soc., 99, 4726
(1977). See review by F. Calderazzo on metal carbonyls in I. Wender and P. Pino, "Metal Carbonyls in Organic Synthesis", Interscience, New York, N.Y.,
1968.
attack at metal centers4"
H. C. Clark and W. J. Jacobs, Inorg. Chem., 9, 1229 (1970). D. E. Hendriksen and R . Eisenberg, J. Am. Chem. SOC., 98, 4662
(1976). K-Y. Hui and B. L. Shaw, J. Organomet. Chem., 124,262 (1977). J. A. Froelich and D. J. Darensbourg, Inorg. Chem.. 16,960 (1977). A. J. Deeming and B. L. Shaw, J. Chem. SOC.A, 443 (1969). D. J. Darensbourg, M. Y. Darensbowg, and R. J. Dennenberg, J. Am. Chem. Soc., 93,2807 (1971). D. J. Darensbourg and H. H. Nelson, IIi, J. Am. Chem. SOC., 98, 6511
(1974). W. J. Knebel, R. J. Angelici, 0. A. Gansow. and D. J. Darensbourg, J. Organomet. Chem., 66, C11 (1974). D. J. Darensbourg, G. R. Dobson, and A. Moradi-Araghi, J. Organomet. Chem., 116,C17-(1976). D. J. Darensbourg and A. Saker, J. Organomet. Chem., 117, C90 119761. , D. J. Darensbourg, H.H. Nelson, Ill,and M. A. Murphy, J. Am. Chem. SOC., 99, 896 (1977). D. J. Darensbourg and M. Y. Darensbourg, Inorg. Chem., 9, 1691 - I
(1970). M. Y. Darensbourg. H. L. Conder. D. J. Darensbourg, and C. Hasday, J. Am. Chem. SOC.,95, 5919 (1973). D. Drew, M. Y. Darensbourg, and D. J. Darensbourg, J. Organomet. Chem.,
_..
(34)W. B. Perry, T. F. Schaaf. W. L. Jolly, L. J. Todd, and D. L. Cronin, Inorg. Chem., 13,2038 (1974). (35) D. L. Lichtenberger and R. F. Fenske, Inorg. Chem., 15, 2015 (1976). (36)K. Fukui, T. Yonezawa, C. Nagata, and H. Singu, J. Chem. Phys., 22, 1433 (1954). (37)G. Klopman and R. F. Hudson, Theor. Chim. Acta, 8, 165 (1967). (38)L. Salem. Chem. Brit., 449 (1969). (39)This consideration excludes reactions proposed to involve nucleophilic
85. 73 . - I19751 I . -
- I
(a) E. W. Abel and G. Wilkinson, J. Chem. SO~.,1501 (1959):(b)R. J. Angelici and F. Basoio, J. Am. Chem. SOC.,84,2495 (1962). (20)D. Drew, D. J. Darensbourg, and M. Y. Darensbourg, Inwg. Chem., 14,1579
11975).
k. J. Angeiici and R. W. Brink, Inorg. Chem., 12, 1067 (1973). J. R . Anglin and W. A. G. Graham, J. Am. Chem. SOC., 98,4678(1976). F. A. Cotton and C. S. Kraihanzel, J. Am. Chem. SOC.,84,4432(1964). R. T. Jernigan, R. A. Brown, and G. R. Dobson, J. Coord. Chem., 2, 47 (1972). C. L. Hyde and D. J. Darensbourg. Inorg. Chem.. 12, 1075 (1973). J. H. Schachtschneider and R. G. Snyder, Spectrochim. Acta, l9,85.117
(1963). More detailed C"0 enriched spectra for [Me2PhPMn(CO)s] [PFs] in the entire v(C0) region may be found in ref 2. In addition, we have observed that this reaction proceeds -4.5 times slower in D2180as compared with H2% The details of this isotope effect on the oxygen-exchange process wili be the subject of a later publication. R. J. Angelici and L. J. Blacik, Inorg. Chem., 11, 1754 (1972). G. R. Dobson and J. R. Paxson, J. Am. Chem. SOC.,95,5925(1973). R. J. Angelici, Acc. Chem. Res., 5, 335 (1972). T. F. Block, R. F. Fenske. and C. P. Casey, J. Am. Chem. Soc., 98, 441
(1976).
C.P. Casey and C. A. Bunnell, J. Am. Chem. SOC.,98, 436 (1976). X-ray photoelectron spectroscopy has been used as weii to assess the relative atomic charges in (C0)5CrC(OCH3)CH3.34
(40)G. R . Dobson, Acc. Chem. Res., 9, 300 (1976). (41)T. Kruck and H. Hofler, Chem. Eer., 98, 3035 (1963). (42)These 4CO) vibrational modes were calculated somewhat low (3-5 cm-'), nevertheless, the approximate CO stretching force field generally provided calculated v(C0) values (for a total of 30 observed vibrations) within f 1.5 cm-'. (43)Present studies include a search for possible metal catalysts for chemically converting (on a small scale) 13C0 into mixed labeled 13C180and 13C170 species via a process involving reversible binding of CO accompanied by a facile oxygen exchange reaction with H20.It is important to note here that this procedure is limitedto metal carbonyl derivatives where elimination of CO2 from the hydroxycarbonyl intermediate with concomitant metal hydride fotmation is slow relative to oxygen exchange3 We believe as well that this procedure might form the basis for peparing mixed labeled organic derivatives. (44)Approximate CO stretching force constants for the reactive CO grou s in the species, [ Mn(CO)#, [Mn(CO)5PMe2Ph]+.and [ Mn(CO).(diphos)f+ and 16.90mdyn/A, respectively. It should as well be noted are 16.16,17.44, that sterically the carbonyl ligands are more hindered as substitution at the metal center by phosphines occws. This has been shown to be an important consideration in reaction of W(C0)sL derivatives with the more bulky reagent C6H5CH2MgC1.17 (45)These cationic species have been shown to react with a variety of primary and secondary amines, via nucleophilic attack at carbonyl carbon, to give carbamoyl compound^.^^^^^-^^ (46)R. J. Angelici and L. J. Blacik, Inorg. Chem., 11, 1754 (1972). (47)R. W. Brink and R. J. Angelici, Inorg. Chem., 12, 1062 (1973). (48)R. J. Angelici and R. W. Brink, Inorg. Chem., 12, 1067 (1973). (49)/3-Hydrogen transfer has been found to be important in the reaction Of [Mn(CO)e]+ with H20, but much less so in reactions at electron-rich metal carbonyl center^.^ (50) This hydride species, which has previously been reported in the iiterat~re,~' exhibited u(C0) bands at 1992 and 1906 cm-' in acetonitrile. Upon extraction of the complex into hexane v(C0) absorptions at 2002, 1931,and 1922 cm-l were observed in addition to weaker bands attributable to the corresponding oxygen-I 8 enriched species. (51) B. L. Booth and R. N. Haszeldine, J. Chem. SOC.A, 157 (1966). (52) For example, acylmanganese pentacarbonylhas been shown to react with the nucleophilic reagent, NaOCH3,to afford CH3C02CH3in near quantitative yield.53 (53) R. W. Johnson and R. G. Pearson, Inorg. Chem., 10,2091 (1971). (54)T. Kruck and M. Noack, Chem. Ber., 97, 1693 (1964). (55) W. F. Edgeil and B. J. Bulkin, J. Am. Chem. Soc., 88, 4839 (1966). (56)This is similar to the reaction of CpW(C0)3C(0)NHCH3and triethylamine reported by Jetz and A n g e i i ~ i . ~We ' thank a reviewer for pointing this out. (57)W. Jetz and R. J. Angelici, J. Am. Chem. Soc., 94,3799 (1972). (58)R. M. Laine, R. G. Rinker, and P. C. Ford, J. Am. Chem. Soc., 99,252(1977), and references therein.
Asymmetric Hydrogenation. Rhodium Chiral Bisphosphine Catalyst B. D. Vineyard, W. S. Knowles,* M. J. Sabacky, G . L. Bachman, and D. J. Weinkauff Contributionfrom Monsanto Company, St. Louis, Missouri 63166. Received March 3, 1977
Abstract: Enantiomeric excesses o f 95-96% are obtained b y the asymmetric hydrogenation o f a-acylaminoacrylic acids. The olefin configuration, whether E or Z , has a profound influence on the rate and stereospecificity. Excellent selectivity (-90% ee) was obtained w i t h an a-enol ester, which is the first outstanding result w i t h a nonamide substrate. A catalyst picture is presented and the stereochemical results are discussed.
The efficiency of asymmetric hydrogenation in soluble systems has greatly improved since the original development in 1 968.'.2 Enantiomeric excesses of 95-96% are now possible with a-acylaminoacrylic acids.3 The optimum selectivity associated with earlier catalysts required rather mild reaction Journal of the American Chemical Society
/
99:18
conditions, i.e., ambient temperature and pressure. Discovery of the catalyst precursor [rhodium( 1,5-~yclooctadiene)L]+ tetrafluoroborate- (6),3L = 1,2-ethanediyIbis[(o-methoxyphenyl)phenylphosphine], 5, opened new possibilities. The bisphosphine catalyst was not as sensitive to reaction variables
/ August 31, 1977
