Organometallics 1986,5, 391-393
-
391
"RCH~OCU" Cu(0) + RCHzOH + RCH(0) (1) Three pieces of evidence strongly implicate a P-elimination in the decomposition of t r a n ~ - R 0 I r ( C O ) ( P P h ~ ) ~ , The alkoxides were prepared by reactions similar to R = Me, n-Pr, or i-Pr. (1)The decomposition only occurs those previously reported.15J6 The spectroscopic paramfor alkoxides which contain @-hydrogens.(2) The decometers are consistent with the formulation trans-ROIrposition produces an iridium hydride, HIr(C0)(PPh,),, the (CO)(PPh3)2.29 Decomposition reactions of the alkoxides expected complex from a &hydrogen elimination. (3) For are carried out as follows: a solution of 0.10 g of the iridium the n-propoxy and isopropoxy complexes the expected alkoxide complex and 0.08 g of PPh3 (-2-3 equiv) in 20 organic products (propanal and acetone, respectively) are mL of cyclohexane are placed in a pressure tube fitted with observed in high yield. The alcohol product that is also a Teflon stopcock and allowed to stir at 70 "C for several observed may result from a binuclear elimination between hours. To detect the organic products by gas chromathe alkoxide complex and the formed hydride. tography (Varian 2440 FID, 1 2 f t Carbowax, 60 "C), the reaction is carried out in 10 mL of HPLC grade toluene Acknowledgment. We acknowledge the donors of the (Aldrich) which was dried over CaHz and distilled directly Petroleum Research Fund, administered by the American onto the alkoxy complex. Chemical Society, and the Alfred P. Sloan Foundation for The alkoxides which contain P-hydrogens trans-ROIrpartial support of this research. A loan of IrC13.XH20was (CO)(PPh,), (R = Me, i-Pr, and n-Pr) decompose at generously provided by Johnson Matthey Corp. moderate rates at 70 "C; in the presence of PPh, the irdium product is HIr(CO)(PPh,), identified by comparison to independently prepared samples.,O Similar reactions are not observed for R = H, Ph, or t-Bu. For trans-iAn Unusual Solvent Effect In the Reaction of PrOIr(CO)(PPh,), the decompositionleads to acetone and Ethylmagneslum Bromide wlth Triethylborane In Ethyl 2-propanol (eq 2). The acetone and 2-propanol were Ether or Tetrahydrofuran. A Simple Direct Route to tr~ns-i-PrOIr(CO)(PPh~)~ + PPh3 Tetraorganylborate Complexes HIr(CO)(PPh,), + CH3C(0)CH3+ CH3CHOHCH3 (2) formed in comparable amounts (total yield of the two is Herbert C. Brown' and Uday S. Racherla 90% as an average of three decompositions). Similar deRichard 6. Wetherill Laboratory, Purdue University composition of tr~ns-n-PrOIr(CO)(PPh,)~ leads to propWest Lafayette, Indiana 47907 anal (92%). We have been unable to detect formaldehyde Received September IO, 1985 during decompositionof trans-MeOIr(C0)(PPh3)2.31The observation of ketone and alcohol for R = i-Pr indicates Summary: The reaction of organic halides, magnesium that decomposition of these alkoxides may be similar to turnings, and boron trifluoride etherate yields triorganylcopper(1) alkoxides where both products were also obboranes quantitatively in anhydrous ethyl ether (modified served.' As suggested previously the alcohol may arise from fission of the metal-alkoxide bond with hydrogen organometallic method) but leads to quantitative formation abstraction or from a binuclear reaction between the of tetraorganylborate complexes in tetrahydrofuran. A formed hydride and remaining alkoxide. detailed study of the reaction of ethylmagnesium bromide To check the latter possibility, we have examined the with triethylborane revealed that essentially no reaction reaction between HIr(CO)(PPh3), and trans-i-PrOIroccurs in ethyl ether, EtMgBr BEt,, but complete com(co)(PPh3)2. bination occurs in tetrahydrofuran, Et,BMgBr. This detrans-i-PrOIr(CO)(PPhJ2 HIr(C0)(PPh3)3 velopment provides a simple, convenient route for the i-PrOH ? (3) synthesis of tetraorganylborate complexes. Reaction 3 proceeds readily at room temperature to give Recently we described a general quantitative synthesis a good yield of 2-propanoland an unidentified air-sensitive of triorganylboranes via a modified organometallic method' iridium compound. No trace of acetone was observed in (eq 1). this binuclear elimination. This binuclear elimination of alcohol from a metal hydride and a metal alkoxy could 3RX + 3Mg BF3.OEt2 R3B 3MgFX (1) arise from a hydrogen bridging (as a hydride) between the 90-99 % two iridium atoms or a hydrogen bond interaction (as a During this study, we discovered an unusual solvent proton) between the HIr and the oxygen of the alkoxy. effect. llB NMR examination of the reaction mixture Further experiments are in progress to delineate the revealed that the reaction of ethyl bromide, magnesium mechanism of the binuclear elimination. Use of transturnings, and boron trifluoride etherate (taken in 3.5:3.5:1 CD301r(CO)(PPh3)2 or ~~u~~-(CD,),CDOI~(CO)(PP~~)~ in molar ratio) in ethyl ether forms triethylborane (loo%), C6H12 leads to D I I - ( C O ) ( P P ~identified ~)~ by comparison but in tetrahydrofuran, the reaction leads to the exclusive to an independently prepared sample and previous reformation of the bromomagnesium tetraethylborate-THF port.33,34 complex2 (eq 2).
-
+
+
-+
-
+
(29) The carbonyl stretching frequency (CCH12)and NMR spectra (in benzene-&-each spectra has a multiplet at -7 ppm): trans-CH,OIr(CO)(PPh,), 1951 cm-', 3.4 ( 8 ) ppm; trans-n-PrOIr(CO)(PPh,)z, 1951 1951 cm-', 3.65 (t), 1.16 (m), 0.60 (t) ppm; trans-t-BuOIr(CO)(PPh3)2, cm-l, 0.89 (e) ppm; trans-PhOIr(CO)(PPh3)2,1958 cm-'; trans-i-PrOIr(CO)(PPh&, 1945 cm-', 0.72 (d), 4.0 (septet) ppm; trans-HOIr(C0)(PPh3)2,1923 cm-'; HIr(CO)(PPh3)3,2070 (IR-H), 1933 cm-' (C-0). (30) Wilkinson, G.Znorg. Synth. 1972,13, 127. (31) Formaldehyde undergoes reactions with many of the iridium complexes through an apparent initial oxidative addition.32 (32) Bernard, K. A.; Atwood, J. D., manuscript in preparation. (33) Vaska, L. J. Am. Chem. SOC.1966,88, 4100. (34) Based on the absence of the Ir-H stretch at 2070 cm-', we estimate the amount of HIr(CO)(PPh,), at less than 10%.
0276-7333/86/2305-0391$01.50/0
35EtBr
+ 3 5Mg +
+
Et@ t 3MgFBr 100%
BF3.0Etz 2h
1 t 0 5 EtMgBr
EtqBMgBrsnTHF t 87 5% l/sBF3 'THF t '1/BMgFBr 12 5%
(2)
(1) Brown, H.C.; Racherla, U. S. J. Org. Chem., in press. (2) Both reactions were performed at 0.25 M concentration. By llB NMR, all of the species formed,namely, triethylborane (6 86.6), BF,.OEt, (6 O), and the ate complex (6 -16.6), could be readily distinguished.
0 1986 American Chemical Society
392 Organometallics, Vol. 5, No. 2, 1986
Communications
Further, the use of 4 equiv each of ethyl bromide and magnesium turnings in the above method results in complete conversion to tetraethylborate complex (eq 3). 4EtBr
+ 4Mg + BF3.0Et2
THF, 25 ' C
EtdBMgBpnTHF
+ 3MgFBpnTHF
(3)
We decided to study the reaction of ethylmagnesium bromide (EtMgBr) with triethylboron (EkB) in ethyl ether (EE) and in tetrahydrofuran (THF). l'B NMR readily distinguishes between Et3B and Et4BMgBr. Both Et3B and Et4BMgBr are stable to water at 25 "C. Consequently, the ethane evolved on addition of water provides a quantitative measure of the free EtMgBr in the presence of Et3B and Et4BMgBr. The reaction of EtMgBr3 (1.0 mL, 3.0 M, 3 mmol) with triethylborane (0.29 g, 3 mmol) in ether (10.6 mL) produced a clear upper layer and a much smaller viscous gray-colored lower layer. There was no observable change over 6 h. I'B NMR analysis of the upper layer showed Et3B (6 86.6) exclusively, with not even a trace of the ate complex, whereas analysis of the small lower layer showed only the Et4MgBr.nOEh complex (6 -16.6). In a duplicate experiment, water (5 mL) was added to the reaction mixture and the resulting ethane evolved determined to be 2.9 mmol., This experiment establishes that Et4BMgBr.nOEh is completely insoluble in ether, and the amount formed is only 0.1 mmol or approximately 4%5 (eq 4). EE, 6 h
Et3B + EtMgBr&Et,BMgBr.nOEt, (4) 96% 96% 4% Under the same conditions, the reaction of EtMgBr3 (1.0 mL, 3.0 M, 3 mmol) with Et3B (0.29 g, 3 mmol) in T H F (10.6 mL) afforded in 6 h a heterogeneous reaction mixture, a clear upper layer and a white crystalline solid. l'B NMR analysis of the upper layer showed exclusively the Et,BMgBr.nTHF complex (6 -16.5). The solid was next isolated and characterized by 'H NMR and microanalysis to be the complex E ~ B M ~ B P ~ T HOnce F . ~ again, in a duplicate experiment, water was added to the reaction mixture. The ethane evolved was 0.1 mmol, corresponding to the formation of 2.9 mmol of Et4BMgBr, a conversion of 96% (eq 5). THF, 6 $ Et3B + EtMgBr , - c Et4BMgBr.4THF (5) 96 % 4% 4 70 At this stage it was clear that the formation of Et4BMgBr from Et3B and EtMgBr is highly unfavorable in ether but highly favorable in THF. It appeared that the better coordinating properties of THF6 might be responsible. To test this conclusion, the following experiments were run. The addition of EtMgBr in ether (1.0 mL, 3.0 M, 3 mmol) to Et3B (0.29 g, 3 mmol) in a mixture of ether (9.6 mL) and THF (0.87 g, 1.0 mL, 12 mmol) a t 25 "C afforded in 6 h 89% of the Et,BMgBr.CTHF complex (eq 6). (3) For the nature of the Grignard reagent in ether solvents, see: (a) Ashby, E. C. Q.Reu., Chem. SOC. 1967,21, 259. (b) Pure A p p l . Chem. 1980, 52, 545. For simplicity, ethylmagnesium bromide is represented throughout this paper as EtMgBr, although this may not always be the reactive species in solution. (4) Brown, H. C.; Kramer, G. W.; Levy, A. B.; Midland, M. M. "Organic Syntheses via Boranes"; Wiley-Interscience: New York, 1975. (5) 'H NMR spectrum was consistent with the structure Et4BMgBr.4THF: S 0.85-1.65 (m,' 20 H), 1.8-2.05 (m, 16 H), 3.75-4.05 (m, 16 H). The results of microanalysis also matched with this structure. Calcd for [Et4BMgBr.4THF]: C, 55.47; H, 10.08; Br, 15.38; Mg, 4.68. Found: C, 54.14; H, 10.23; Br, 15.07; Mg 4.38. (6) Coates, G. E.; Green, M. L. H.; Powell, P.; Wade, K. "Principles of Organometallic Chemistry"; Methuen and Company: London, 1969.
EE-THF
Et3B + EtMgBr2XhEt4BMgBr.4THF (6) 11% 11% 89 % Moreover, when the Et4BMgBr-4THF (1.56 g, 3 mmol), taken into ether (12 mL), was stirred a t 25 "C for 6 h in ether (12 mL), the Et3B measured by NMR and the EtMgBr found by hydrolysis are both 20% (2~5%) (eq 7). EE
E t 4 B M g B r . 4 T H F eEt3B + EtMgBr (7) 80% 0C'6h20% 20% On the basis of these data, it appears that the formation of Et4BMgBr.nOEt2from a mixture of Et3B and EtMgBr in anhydrous ether (0.25 M) at 25 "C represents a highly unfavorable equilibrium. However, this still does not fully explain why absolutely no ate complex formation was indicated' in our original triorganylborane synthesis in ethyl ether1 (0.25 M, eq 2 and 8). A major difference between Et36 t EtqBMgBr .nOEtp 3.5EtBr t 3 5 M g t BF3.0Et2
EE 2 h
0%
99%
t
t
3MgFBrl
(8)
O.5EtMgBr
the two reactions described by eq 4 and 8 is the presence of solid magnesium halide, assumed to be MgFBr, in the latter experiment. It was thought possible that the EtMgBr was extracted by this solid. This possibility was tested by the following experiment. EtMgBr in ethyl ether (3.0 mL, 3.0 M, 9 mmol) was slowly added to BF3.0Et2 (0.43 g, 3 mmol) in ether (7.6 mL) at a rate such that the ether refluxed gently. The reaction mixture was then stirred for 2 h, and an additional quantity of EtMgBr (1.0 mL, 3.0 M, 3 mmol) in ether was added. Stirring was continued for 2 h, and then the reaction mixture (0.25 M) was allowed to settle. The ether phase was separated. The solid was washed thoroughly with ether (12 mL). The combined ether phase and washings and the solid were separately treated with water. There was obtained 2.7 mmol of ethane from the solid and only 0.4 mmol of ethane from the ether solution. Thus, in the above experiment, approximately 90% of the excess EtMgBr (2.7 mmol) was extracted from the ether solution by the solid (presumably MgFBr). This observation accounts for the failure to observe any formation of Et4BMgBr.nOEt2 in the original triorganylborane synthesis' (eq 2 and 8). In our original study of the synthesis of triorganylboranes by the "modified organometallic route",' we observed the same phenomena for 11different organic halides (alkyl, alicyclic, aryl, and allylic). Consequently, this difference between the behavior of ether and THF on the formation of tetraorganylborate complexes appears to be quite general. This should not be taken to mean that the formation of tetraorganylborate complexes cannot ever occur in ether. Indeed, Kondo and MurahashP used such formation of spiro ate complexes to achieve a valuable conversion of organoboranes into the corresponding Grignard reagents (eq 9). n-n t R3B t 2BrMg(CH?),MgBr
3RMgBr t (CH& B
(CH2)sMgBr
4 W
(9)
We earlier attempted to achieve such interconver~ion.~ Unfortunately, much of our work utilized THF solutions and the undetected formation of "ate" complexes com(7) "B NMR analysis of the residual solid showed in tetrahydrofuran no boron species. (8) Kondo, K.; Murahashi, S.-I. Tetrahedron Lett. 1979, 1237. (9) Buhler, J. D. Ph.D. Thesis, Purdue University, 1973.
393
Organometallics 1986,5, 393-394
plicated the interconversion. Now that this phenomenon is understood, it is evident that it provides the basis for an exceptionally simple synthesis of tetraorganylborates, as well as the basis for achieving a very simple conversion of organoboranes into Grignard reagents (eq 10). R3B + 3R'MgX 3RMgX + R'3B (10)
-
Acknowledgment. We wish to thank the National Science Foundation (Grant CHE-8414171) for financial support of this research.
Stablllratlon of a Large Arsenlc-Oxygen Heterocycle vla Metal Coordlnatlon. The Synthesis and X-ray Crystal Structure of [Mo( CO),],[cyclo -(CH,AsO),] Arnold L. Rhelngold' and Anthony4 DlMalo Depafiment of Chemistty, University of Delaware Newark, Delaware 19716 Received November 5, 1985
Summary: I n the presence of molecular oxygen, Mo(C0)6,and cyc/o-(CH,As), form the first example of a metal-coordinated alkylarsaoxane, [Mo(CO),] [cyclo (CH,AsO),] (l), containing a 12-membered alternating As-0 ring coordinated to two Mo(CO), groups. The As-0 ring exists as a flattened, trans bimetal-capped cuboctahedron with two planes of three arsenic atoms positioned for coordination and a central plane of six oxygen atoms. Crystals of 1 are orthorhombic of space group Cmca, with a = 13.424 (2), A, b = 16.909 (3) A, c = 11.818 (2) A, Z = 4, and V = 2682.6 (7) A3.
Figure 1. Thermal ellipsoid diagram for ((CH,AsO),[Mo(CO),],I (1)and atom labeling scheme with hydrogen atoms deleted. Bond
distances (A): Mo-As(l),2.556 (1);Mo-As(lb), 2.557 (1);MoAs(2), 2.535 (1);As(1)-0(3),1.793 (2);As(1)-0(4),1.794 (3);As(2)-0(4a), 1.787(3). Bond angles (deg): As(l)-Mo-As(2), 92.8 (0); As(1)Mo-As(lb), 93.7 (0);As(a)-Mo-As(lb), 92.8 (0); Mo-h(1)-0(3), 116.9 (1);Mo-As(l)-0(4), 116.4 (1); Mo-As(2)-0(4a), 118.4 (1); Mo-As(2)-0(4c), 118.4; As(l)-O(B)-As(la), 119.7 (2); As(l)-O(4)-As(2a), 118.1 (2); 0(3)-&(1)-0(4), 100.9 (1);0(4a)-As(2)-0(4c), 101.2 (2).
-
Alkylarsaoxanes, (RAsO),, are thought to exist as cyclotrimers, cyclotetramers, and other more highly associated, interchangeable ring and chain forms,l but none of the proposed structures has been isolated or confirmed ~rystallographically.~f The potential for arsaoxanes to serve as multidentate ligands offers a means for the iso// A lation of discrete species. We now report the synthesis and characterization of a dimolybdenumhexacarbonyl complex of cy~lo-(CH,AsO)~ containing a 12-membered As-0 Figure 2. A projection of the structure of 1 viewed down the heterocycle bridging Mo(CO), groups, ([M~(CO)~]~[cyclo- Mo-Mo' vector. (CH&O)6ll (1). Complex 1 is prepared from toluene solutions of Mo(C1 was separated from the much less soluble 2 in boiling O), and cyclo-(AsCH,), in which various quantities of CH2C12. The product ratio of 1 to 2 varies with the initial dioxygen are dissolved and heated in Carius tubes at 150 oxygen concentration; at the extremes, only 1 is isolated "C for 48 h. Upon slow cooling, pale yellow crystals of l4 in 77% yield (based on (CH3As),) with addition of stoialong with [MO(CO)~]~[~~C~O-(A~CH~)~~] (2)5 are obtained. chiometric quantities of O2 (eq 1)while only 2 is isolated in rigorously degassed systems (eq 2). (1)Durand, M.; Laurent, J.-P. J. Organomet. Chem. 1974,77, 225. Marsmann, H. C.; Van Wazer, J. R. J. Am. Chem. SOC. 1970,92,3969. (2)The literature abounds with names for compounds of formula RAsO: alkylarsaoxane, arsenosoalkane, alkylamine oxide, and alkylarsenious oxide are the most commonly encountered. (3)Arsaoxanes are among the oldest known organometallic compounds. [(CH3),AsOAs(CH3),]: Cadet de Gassincourt, L. C. Mem. Math. Phys. Saoants Etrangers. 1760,3,363.CH,AsO: von Baeyer, A. Justus Liebigs Ann. Chem. 1858,107,279. (4)(1) 'H NMR (CDCl,): 6 1.91;IR YCO 1980 s, 1906 s, 1875 m; decamp. temp, 300 "C. Anal. Calcd C, 14.47;H, 1.81;As, 45.16. Found: C, 14.48;H, 1.88;As, 45.36. (5)(2)'H NMR (benzene-&): 6 1.61 sh, 1.57s; IR vco 1926 8,1868m, 1843 m;decomp temp, 255 OC. Anal. Calcd: C, 15.25;H, 2.38. Found C, 15.27;H, 2.59.
0276-7333/86/2305-0393$01.50/0
{ [MO(CO)~I~(CH~AS)~O) (2)
2
Compound 1 crystallizes as discrete molecules (Figures 1and 2) without significant intermolecular contacts.6 The 0 1986 American Chemical Society
