Organometallics 1989,8, 2821-2831
2821
Alkyne/Alkene Coupling Reactions Leading to 1,3-Diene and 2,4-Pentadienyl Complexes of Rhenium(I I I ) . Activation of Dichloro(g5-pentamethylcyclopentadienyl) (g2-alkyne)rhenium( I I I ) by Bransted Acid Catalysis Wolfgang A. Herrmann, * Roland A. Fischer, and Eberhardt Herdtweck Anorganisch-chemisches Institut der Technischen Universitat Mijnchen, Lichtenbergstrasse 4, 0-8046 Garching, Germany Received March 17, 1989
Alkynelalkene coupling reactions leading to 1,3-diene-and 2,4-pentadienyl-typecomplexes 9d-1 and loa-d, respectively, of rhenium(III), arise from activation of dichloro(q5-pentamethylcyclopentadienyl)(q2-alkyne)rhenium(III) (2a-d) by catalytic amounts of Brernsted acids. These latter alkyne complexes of rhenium exhibit not the slightest reactivity toward alkenes if acids are absent. Isotopic labeling and trapping experiments suggest that initial HC1 elimination from the starting alkyne complexes 2a-d is promoted by trace amounts of strong Brernsted acid. This mechanistic key step is also supported by the observed product distribution that depends on the nature of the alkyne and alkene substituents as well as on the geometry around the metal center. In addition, derivativesof 2a-d without chloride ligands (e.g., the glycolato complex 5) do not allow alkynelalkene coupling reactions. The new 1,3-diene and 2,4-pentadienyl complexes of trivalent rhenium were characterizedby their NMR and mass spectra as well as by two X-ray structure analyses. (q5-C5Me5)ReCl2(q4-(E~)-3,4-dimethylhexa-2,4-diene) (Sg, Me = CHJ crystallizes in the monoclinic space group E 1 / c , with a = 859.4 (1)pm, b = 2745.4 (4) pm, c = 855.5 (1) pm, /3 = 117.73 (l)’,and 2 = 4. Pertinent bond distances are Re-C(2) = 222.0 (3), Re-C(3) = 236.4 (3), C(2)-C(3) = 143.8 (4), and C(3)-C(4) = 139.6 (4) pm. The dihedral angle between the C(2)/Re/C(5) and C(1-.8)-planes is 95.8 (1)’. These data are in agreement with the description of this compound as a a4-s-cis-dienecomplex. (~5-C5Me5)ReCl(~5-(E,E)-3,4-diphenylhexa-2,4-dien-6-yl) (rod)crystallizesalso in the monoclinic space group P2,lc, with a = 1605.2 (2) pm, b = 1022.7 (3) pm c = 1567.6 (1)pm, /3 = 113.35 (l)’, and 2 = 4. Pertinent bond distances are Re-C(l) = 242.1 (6), Re-C(2--6) = 218.3 (3), C(l)-C(2) = 153.1 (4), C(2)-C(3) = 143.6 (4), C(3)-C(4) = 149.0 (3), C(4)-C(5) = 141.9 (4), and C(5)-C(6) = 144.5 (3) pm. Due to this specific ligand, the 2,4-pentadienyl skeleton defined by atoms C(2-6) is not coplanar, thus contrasting the common 2,4-pentadienyl ligands typical of the so-called “open ferrocenes”.
Introduction
Scheme I
Starting from the organometallic oxide (q5-C5Me5)Re0 (1); we have recently opened up synthetic routes for Reni alkyne complexes of type (q5-C5Me5)ReX2(q2-R1C=CR2) (2a-f; X = C1, Br, I; R’, R2 = alkyl, aryl; numbering scheme see Table I).2 We became interested in a comparison of catalytic reactions, has found its way into the textbook their chemical and structural properties with those of reknowledge of modern organometallic ~ h e m i s t r y . ~Since lated alkyne complexes of the “early” and “late” transition the new alkyne complexes 2a-f can be considered as simple metals. Representing a typical reaction of transition-metal model systems of CC coupling processes a t rhenium cenalkyne complexes, the coupling of the coordinated alkyne ters, we have subjected them to an investigation of their with other unsaturated molecules such as alkenes, alkynes, reactivity. nitriles, carbon monoxide, carbonyl compounds, or hetUp to the present, stoichiometric and catalytic alkyne/ erocumulenes (e.g., carbon dioxide, isocyanates), even in alkene coupling has mainly been used in preparative organic c h e m i ~ t r y . ~Most of the known metal-centered coupling reactions of these classes of compounds lead preferentially to metallacyclo-2-pentenes.3 There are only (1)(a) Herrmann, W. A. J. Organomet. Chem. 1986, 300, 111. (b) few reports on subsequent rearrangements to 1,3-diene Herrmann, W. A.; Okuda, J. J.Mol. Catal. 1987,41,109. (c) Herrmann, W. A,; Herdtweck, E.; Floel, M.; Kulpe, J.; Kiisthardt, U.;Okuda, J. ligands via 1,3-hydrogen shift proce~ses.~ A formal rep-
Polyhedron 1987,6,1165. (d) Herrmann, W. A. Comments Inorg. Chem. 1988,7,73. (e) Herrmann, W. A. Angew. Chem. 1988,100,1269Angew. Chem., Int. Ed. Engl. 1988,27, 1297. (2) (a) Herrmann, W. A,; Fischer, R. A.; Herdtweck, E. J.Organomet. Chem. 1987,329, C1. (b) Herrmann, W. A.; Fischer, R. A.; Herdtweck, E. Angew. Chem. 1987,99,1283; Angew. Chem., Int. Ed. Engl. 1987,26, 1284. (c) Herrmann, W. A.; Fischer, R. A.; Felixberger, J. K.; Paciello, R. A.; Herdtweck, E. Z . Naturforsch. 1988,B43,1391. (d) Herrmann, W. A.; Fischer, R. A.; Herdtweck, E. Angew. Chem. 1988,100,1566; Angew. Chem. Int. Ed. Engl. 1988,27, 1563. (e) Herrmann, W. A.; Fischer, R. A,; Amslinger, W.; Herdtweck, E. J. Organomet. Chem. 1989,362,333.
(0 However, one requirement must be fulfilled; the intermediates along
the initial carbon-carbon bond formation reactions must provide the possibility of 1,3-H-migration to achieve a rearrangement of the ligand with respect to r-conjugation. In fact, facile intramolecular coupling of alkyne and allyl, simultaneously bonded to a ($-C5Me5)ReC1fragment, does occur.
0276-7333/89/2308-2821$01.50/0
(3) Review: Otauka, s.;Nakamura, A. Adu. Organomet. Chem. 1976, 14,246. Textbooks: (a) Winter, M. J. In Chemistry of the Metal-Carbon Bond; Hartley, F. D., Hrsgb.; 1985; Vol. 3, p 259. (b) Yamamoto,A. Organotransition Metal Chemistry-Fundamental Concepts and Applications; Wiley: New York, 1986. (c) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry, 2nd ed.; University Science Books: Mill
Valley, CA, 1987. (4) Review: Vollhardt, K. P. C. Angew. Chem. 1984,96,525; Angew. Chem., Int. Ed. Engl. 1984,23,523. Examples: (a) Negishi, E.; Holmes, S. J.; Tour, J. M.; Miller, J. A. J. Am. Chem. SOC.1986, 107, 2568. (b) Negishi, E.; Cerderbaum, F. E.; Takahashi, T. Tetrahedron Lett. 1986, 27,2829. (c) Nugent, W. A.; Calabrese,J. C. J.Am. Chem. Soc. 1984,106, 6422. (d) Tamao, K.; Kobayashi, K.; Ito, Y. Ibid. 1988,110, 1286.
0 1989 American Chemical Society
2822 Organometallics, Vol. 8, No. 12, 1989
Herrmann et al. Scheme I1
\
R r
k.
CI1
2e,2t (X=Br I )
I
I
CI
Figure 1. Molecular structure (ORTEP drawing) of the 3-hexyne (2b-Et) complex of formula (~5-C,Me5)ReC12(~2-C2H5C=CC2H5) in the crystal. Hydrogen atoms are omitted for better clarity. Shown are the 50% thermal ellipsoids. Selected bond distances (pm) and angles (deg): Re-Cl(1) = 244.2 (I),Re-Cl(2) = 236.6 (l),Re-C(3) = 196.1 (3),Re-C(4) 196.9 (3),C(3)-C(4) = 132.6 (4); CI(l),Re,CI(2) = 83.60 (3).2a
Table I. Numbering Scheme of the Rel*'-Alkyne Complexes of Type (ss-CsMes)ReXa(rla-RIC~Rz) (2a-f)' compd no.
X
2a
c1 c1 c1 c1
2b 2c 2d 2e 2f
Br I
R' Me
Et
Me
Me
Me Me
Ph Me Me
"Abbreviations: Me = CH,; Et = C2H5; Ph
Scheme 111
R2
Et Ph
Ph
?a
Ea
= C,HS.
resentation of this latter type of reaction-insertion of alkynes into olefinic CH bonds-is shown in Scheme I. In this paper we describe CC coupling reactions of nonactivated alkenes with alkynes, occurring at Re"' centers yielding to coordinated l,&diene derivatives as shown in Scheme I.
Results and Discussion A. Activation of Re"' Alkyne Complexes by Bransted Acid Catalysis. The rhenium complexes (v5-C5Me5)ReC12(v2-R1C=tR2) (2a-d) are remarkably inert for addition reactions of unsaturated functionalized molecules to form metallocyclic products. This observation is all the more surprising since this type of reaction is typical of both "early" and "late" transition-metal alkyne c~mplexes.~ Rhenium thus seems to be an exception here, which in fact could arise from the rather strong bonding of the alkyne to the d4 Ren1center. According to an X-ray structure analysis of the 3-hexyne derivative 2b-Et (Et = C2H5), the average Re-C(3)/C(4) distance of 196.5 (4) pm is very short indeed.2a It is interesting to note that the analogous tantalum(II1) compounds (q5-C5Me5)TaCl2(v2-RC=CR)exhibit comparably short metal-to-carbon distances as well (206.7 (6)/207.5 (6) pm, for R = C,H59. Likewise, reactivity toward either ethylene or an excess of the free alkyne was not observed. Only the Ta"' complex of the parent acetylene (v5-C5Me5)TaC12(q2-HC=CH) takes up ethylene in toluene solution at 80 OC (!) to form the metallacyclo( 5 ) (a) Wakatauki, Y.; Aoki, K.; Jamazaki, H. J. Am. Chem. SOC. 1979, 101,1123. (b) Allen, S.R.; Green, M.; Moran, G.; Orpen, A. G.; Taylor, G. E. J. Chem. SOC., Dalton. Trans. 1984,441. (c) Green, M.; Nagele, K. R.; Woolhouse, C. M.; Williams, D. J. J. Chem. SOC.,Chem. Commun. 1987,1793.(c) Trost, B. M.; Lautens, M. J.Am. Chem. SOC.1985,107, 1781. (d) Trost, B.M.; Chen, S.-F. Ibid. 1986,108,6053. (e) Trost, B. M.; Tour, J. M. Ibid. 1987,109, 5268. (0Trost, B. M.; Lautens, M. Tetrahedron Lett. 1985,26, 4887. (g) Klang, J. A,; Collum, D. B. Organometallics 1988,7,1532. (6)Smith, G.; Schrock, R. R.; Churchill, M. R.; Youngs, W. J. Inorg. Chem. 1981,20,387.
2a,2c-d
9d,9k-I
loa-d
Table 11. Numbering Scheme of the 1,3-DieneComplexes of Type (rls-C~Mer)ReXI(r14-C(R1)RL--C(Ra)C(R4)=C(R6)H) (9a-n)" compd no.
9a 9b 9c 9d 9e 9f 9g 9h 9i 9k 91 9m 9n
X C C1 C1 C1 C1 C1 C1 C1
C1 C1 C1 Br Br
R'
R2
R3 l H H H H H Me H Me H H Me Me H Me Me H Me Me H Me Me Me H Me Me Me Me H Me Ph H Ph Ph H Me Me H Ph Ph
R' H Me H H Me H Me Me Me H H H H
R5 H H Me H H Me Me Me Me H H H H
aAbbreviations see Table I.
-
2-pentene of
c -
the formula
~
(v5-C5Me5)Ta(CH=
CHCH2CH2)C12.6 By way of contrast, the chloride ligands of 2a-d undergo facile substitution reactions as summarized in Scheme 11. Thus alkyl (3a,b),oxo (4), alkoxy (51, bromo (2e),and iodo derivatives (2f) are obtained in rather good yields.2 Unlike the aforementioned tantalum complex, rapid substitution of the alkyne ligand of 2d takes place when carbon monoxide is present (1 atm, room temperature); the trans isomer of ( V ~ - C ~ M ~ ~ ) R ~(6)C results ~ ~ ( Cfrom O ) ~this displacement reaction in 90% isolated yield. If the substitution reactions listed in Scheme I1 follow an associative mechanism, it is obvious that the a-donor vs *-acceptor properties of the entering ligand are of crucial importance. A cyclization reaction between the coordinated alkyne and the free alkene, for example, requires precoordination of the alkene molecule. The absence of such a vacant site to coordinate the alkene to the compounds (v5-C5Me5)ReCl2(v2-R1C=CR2)(2a-d) would explain why the coupling reactions do not occur. Generation of a vacant site would possibly entail subsequent CC
Rhenium 1,3-Diene and 2,4-Pentadienyl Complexes
Organometallics, Vol. 8, NO. 12, 1989 2823
Table IV. Isolated Y.ields of ComDounds 9a-n and 10a-e Table 111. Numbering Scheme of the 2,4-Pentadienyl Complexes of Type (#-CsMes)ReX(qs-CH8CHCR1CR%HCH2) 2,4-penta(10a-e). 1,3-diene dienyl complex complex compd no. X R' R2 alkyne complex no. no. 1Oa c1 Me M alkene (% yield) (% yield) (no.) 10b c1 Me Ph ethylene 10a (10) 9d (85) 1oc c1 Ph Me 2a 9e (30) b propene 10d c1 Ph Ph 2a 9f (25) 1Oe Br Me Me (2)-2-butene 2a 9g (88) Abbreviations see Table I. (E)-a-butene 2a 9h (64) 2a 3-methylbutadiene(l,3) 9i (35) 3-methylpentadiene(2,4) 2a no reaction coupling processes at the metal, regardless of the bonding (E,E)-hexadiene(2,4) 2a 9c (83) properties of the alkyne ligands of the Re"' complexes ethylene 2c 9k (35) 10b (18) 2a-f. 1Oc (23) When a bright yellow solution of 2a in toluene is first 2d ethylene 91 (27) 10d (70) 2d butadiene (1,3) saturated with ethylene and then treated at room tem9a (95) 2d 2,3-dimethylbutadiene( 1,3) perature with a catalytic amount of tetrafluoroboric acid, 1Oe (5) 9b (67) 2e ethylene 9m (82) the color of the reaction mixture changes within a few 2e (n-2-butene 9n (88) minutes to wine-red. Both the red l,&diene complex (E)-2-butene 2e no reaction (t15-C5Me,)ReC1z[~4-CH(CHJ=C(CH3)CH=CHz] (9d) and 2f ethylene no reaction the yellow 2,4-pentadienyl complex (tlS-C5Me5)ReCl[s53a ethylene 9d (36) 10a (31) ethylene 5 (CH,)CHC(CH,)C(CH,)CHCH,I (loa) are obtained upon no reaction
chromatographic workup, with the product yields amounting to 85% and lo%, respectively (Scheme 111). 9d and 10a result from CC coupling processes of the coordinated alkyne (compound 2a) and ethylene. A series of 1,3-diene-complexes(numbering scheme see Table 11) and 2,4-pentadienyl-complexes (numbering scheme see Table 111) could be synthesized analogously. I t is to be noted that no such reactions take place in the absence of strong acids. (Weaker acids, e.g. CF,COOH, do not work in this sense.) 2a was isolated unchanged after treatment with 2,3-dimethylbutene(2) and a trace amount of HBF4, while 3-methylbutene(2) gave the expected diene complex 9i. From these results it becomes obvious that the alkene has to have at least one hydrogen substituent at the double bond; otherwise the coupling reaction to yield the l,&diene complexes is not feasible. The observed alkyne/alkene coupling to yield coordinated l,&dienes refers directly to the overall reaction of Scheme I. Insertion of an alkene into one of the Re-C(alkyne) bonds of 2a followed by a second insertion of alkene into the other Re-C(a1kyne) bond and with subsequent elimination of HC1 leads, after isomerization, to complex loa, which exhibits a substituted 2,4-pentadienyl ligand. The relative amounts of 1,3-diene and 2,4-pentadienyl complexes formed in this reaction depend on the nature of the alkyne complex and the free alkene. Table IV summarizes the yields that were obtained after chromatographic workup of the crude products. The spectroscopic and analytical data of 9a-n and loa-e are available as supplementary material to this paper. A detailed discussion of the X-ray structures of 9g and 10d is given below. B. Possible Reaction Mechanisms. The role of the small amount of acid, necessary to induce CC coupling reactions like that shown in Scheme 111, is of primary interest. A series of experiments have been performed in order to gain some insight into the reaction mechanism(& First, protonation of the Lewis basic rhenium center, followed by a hydride migration to the alkyne ligand to yield a a-vinyl intermediate b (Scheme IV), is an obvious pathway worthy of consideration. Recent examples of protonation of metal-attached alkynes to form (reactive) vinyl species were given by Werner et al.' and Green et al.* (7) Wolf, J.; Werner, H. Organometallics 1987,6,1164and references cited therein.
Yields were determined by weight determination of the isolated products as they were obtained from chromatographic workup of the crude reaction mixtures (see Experimental Section). Only traces of an isomeric mixture. Scheme IV
d
& :' I
I
I
I 1. Labeling Experiments. However, when a solution of 2a in toluene-d8 was treated with deuterated acids (e.g., excess DzS04)in the presence of ethylene, no deuterium (8) (a) Green, MTReeverC.; Woolhouse, C. XIIIth International Conference on Organometallic Chemistry, Torino, Sept 1988,Abstract No. 311. (b) Conole, G. C.; Green, M.; McPartlin, M.; Reeve, C.; Woolhouse, C. J. Chem. SOC.,Chem. Commun. 1988,1310.
2824 Organometallics, Vol. 8, No. 12, 1989
Herrmann et al.
incorporation was observed in the final products 9d and loa. Neither 9d nor loa did undergo acid-catalyzed H/D exchange in deuterated solvents. It can therefore be taken for granted that loss of deuterium during the workup procedures does not occur. Beyond that, temperaturedependent NMR experiments do not convey any indication for the appearance of vinyl species. Compound 2a and 2d were treated with acids (CF3COOH, HBF4.- O(C2H5)2, CF3S03H)in various solvents (CD2CI2,C6D5CD3,CdD80). Thus one has to assume at least that the equilibrium between the hypothetical intermediates a and b (Scheme IV) lies almost quantitatively on the side of intermediate a, even at low temperatures. When 2a, dissolved in toluene, was treated with excess tetrafluoroboric acid in the presence of perdeuterated ethylene, all four deuterium atoms showed up in the product 9d-d,. Seuen out of eight deuterium atoms of the two C2D4 molecules, that react with 2a (Scheme 111), occurred in 10a-d7, while one deuterium atom is obviously lost as DCI. The results of the labeling experiments were obtained by 'H NMR, 13CNMR, and EI-MS techniques after separation and purification of the reaction products. These experiments furthermore show that the rearrangement and elimination processes-succeeding the initial coupling reactions-proceed intramolecularly. Also, they do not rely on the presence of acid; otherwise, H/D exchange should occur which evidently is not the case. Vinyl intermediates such as b are 16-electron species, thus providing a vacant coordination site for the entering ethylene molecule. However, it is more likely that protonation of 4e ligands leads to v2-vinyl species, thus avoiding the unsaturated 16e count of the +vinyl structurea8 Even if b would be present only at low concentrations, insertion of the ethylene molecule into the rhenium-vinyl bond would yield some alkyl intermediate h (Scheme IV). Activation of the vinylic hydrogen by the rhenium would then yield intermediate i, which in turn could undergo deprotonation to form the neutral intermediate 1. Intramolecular rearrangement of 1 would finally give the isolable product 9d without H/D exchange. Up to this stage, this mechanism is in agreement with the results of the labeling experiments. However, /3-hydrogen activation is known to be quite favorable at rhenium c e n t e r ~ . ~ JIntermediate ~ h exhibits hydrogen or deuterium atoms, respectively, in the ,&position. For this reason formation of product 9d could also traverse intermediate k. This latter mechanism would give rise to H/D exchange at the carbon atom C(1). One must at least expect a competition between cyclometalation (h i) and S-hydrogen activation (h k). But if this is the case, one should observe H/D exchange at the carbon atom C(l), due to the contribution of path h k. 2. Intermolecular Competition Experiments and Trapping Reactions. A competition experiment was performed in which an excess of both the free alkyne, e.g. butyne(2), and ethylene was present in addition to 2a. When the toluene solution was treated with stoichiometric amounts of HBF,, a yellow oil separated immediately. It was identified as the bis(r-alkyne) complex [ (v5-C5Me5)ReCl(butyne(2)12] [BF,] (8b).2"When the free alkyne was replaced by an excess of acetonitrile, however, the related complex [(~5-C5Me5)ReCl(butyne(2))(CH,CN)] [BF,] (7b) was obtained in almost quantitative yield. In neither case were CC coupling products such as rhenacyclopentadienes
-
-
-
(9) Yang, G. K.; Bergman, R. G. Organometallics 1986, 4 , 129. (IO) Herrmann, W. A.; Felixberger, J. K.; Herdtweck, E.; SchBfer, A.; Okuda, J. Angew. Chem. 1987, 99,466; Angew. Chem., Int. Ed. Engl.
1987, 26, 466.
7c
Figure 2. Scheme V
Scheme VI I
cat. HBF,
I
l+
H C,H, Toluene
5-Ht
no reaction
detected. It is to be noted that all these substitution reactions do not take place in the absence of acid; for example, ionic 7b is not obtainable when 2a is treated at room temperature with acetonitrile in the presence of hexafluorophosphate (from [N(n-C4H9)4]+[PF6]-) as the counterion. Acid-catalyzed reaction of complex 2d with conjugated dienes such as 1,3-butadiene or 2,3-dimethyI-l,&butadiene entails rapid and complete substitution of the alkyne ligand while cocyclization reactions no longer come to the fore (Scheme V). These experiments as well as the aforementioned NMR results, the isotopic labeling, and the acid-promoted substitution reactions strongly contradict the occurrence of vinyl intermediates b and alkyl intermediates h as discussed above (cf. Scheme IV). There remains a second pathway starting from key intermediate a. Supposing that species a undergoes HCl elimination (Scheme IV, first vertical row), the observed substitution products would result from trapping reactions of the presumably highly reactive, cationic intermediate c of formula [ (v5-C5Me5)ReC1(v2-RC=CR)]+. The occurrence of the ionic compounds 7b and 8b may thus be explained. (Note that intermediates of type c, trapped with appropriate ligands, would give products of type y, according to Scheme IV.) These latter compounds can also be made via reaction of 2a with appropriate silver salts in the presence of the respective ligand.2b In the absence of coordinating ligands, however, attempted abstraction of one chloride group from 2a by means of silver reagents (e.g., AgESbF,]) did not prove possible. 3. Intramolecular Competition Experiments. When the 0,O-glycolate complex 5 was used instead of the dichloro analogue 2a, no reactivity toward ethylene was observed (Scheme VI). 5 was instead isolated unchanged. However, protonation of the chelating glycolate ligand at one oxygen atom could be confirmed by 'H NMR spectroscopy. At very low temperatures (-95 "C, toluene-d8), 5 and its 0-protonated form 5-[H+] can be discriminated within the scope of the NMR time scale. A fast equilibrium between 5 and 5-[H+] is established (Scheme VI) at higher temperatures around ambient. The chelating ligand
Rhenium 1,3-Diene and 2,4-Pentadienyl Complexes remains coordinated to rhenium when protonated at one oxo site. The entering ethylene molecule does not successfully compete with the protonated chelate ligand for one coordination site a t the rhenium center. This interpretation is consistent with the experimental result that no CC coupling products are detected when tetrahydrofuran is used as solvent. In this case tetrahydrofuran may coordinate to intermediate c to give the ionic species 7c (Figure 2). The electron-deficient Re"' center of intermediate c seems to prefer hard a-donor ligands, such as ethers and nitriles, or r-donors such as alkynes (see above), over olefins. 4. Generation of t h e Reactive Intermediate. A possible way to generate the coordinatively unsaturated species c in toluene solution involves elimination of methane from the monomethyl derivative 3a of Scheme I1 via protonation. Treatment of 3a with stoichiometric amounts of HBF,.O(C2H5), in toluene or dichloromethane solutions in the presence of butyne(2) necessarily yields the bis(r-alkyne) complex 7b. The monomethyl derivative 3a reacts under Brransted acid catalysis with ethylene to the same CC coupling products 9d and 10a that also result from the dichloro precursor compound 2a (see previous discussion). The reactivity of the alkyne complexes 2 decreases dramatically by several orders of magnitude when the chloride ligands are exchanged by their heavier homologues bromide or iodide. While the bromo derivative 2e reacts only slowly with ethylene, the iodo compound 2f shows no reactivity at all, even in the presence of high concentrations of acid (Table I). This result may arise from steric reasons. On the other hand, this behavior can also be explained in terms of HX elimination from intermediates of type a. The bond strength of HX decreases from the lighter to the heavier homologues, which means that elimination of HI would be less favorable in an nonpolar solvent than loss of HC1. On the basis of the discussion of the previous paragraphs, HCl elimination from intermediate a to provide the coordinatively unsaturated cationic 16-electron species c (Scheme IV) seems rather plausible. Let us now coordinate the C2H4 molecule at the vacant coordination site of intermediate c, from which the transient species d would then result. Subsequent cyclization yields the rhenacyclopentene(2) (e). This intermediate may be a key species in the sense that different paths could branch out at this point, finally yielding the 1,3-diene and 2,4-pentadienyl complexes 9d-n and lOa-e, respectively. Mixed alkene/alkyne complexes such as intermediate d are potentially reactive molecules. For example, Green et al. recently reported synthesis and reactivity of the molybdenum compound [ (v5-C5H5)Mo(CO) (v2-H2C= CH2)(v2-MeC=CMe)] [BF,] which is an isoelectronic analogue of the supposed intermediate d. In the molybdenum case isoprene substitutes the ethylene ligand and then couples with the alkyne moiety to yield a 1,3,5-hexatriene complex.5c 5. T h e 1,3-Diene Complexes of Type 9. One can discriminate two different mechanisms to explain the rearrangement e f (Scheme IV). First, a P-hydride migration (C(3) Re) could occur to give a transient species similar to k,which then tautomerizes to f. The 1,3-diene complex 9d results from final chloride addition to f. A related rearrangement of a rhenacyclopentane derivative via P-hydride migration has been reported by Bergman et al.9 Note, however, that cationic intermediates with an hydrogen atom at the rhenium site would undergo fast protonation/deprotonation equilibria! Since we did not
--
Organometallics, Vol. 8, No. 12, 1989 2825 Scheme VI1
R'
R'
2a
C
U
detect any H/D exchange under D+/toluene conditions (see above), this otherwise plausible mechanism must not be considered further. The experimentally observed occurrence of all four deuterium atoms of ethylene-d, in the product complex 9d-d4 could be explained with the assumption of a rhenium-assisted, otherwise forbidden," C(1) (e) suprafacial 1,3-hydride shift process C(3) (Scheme IV). From a formal point of view, this isomerization is accompanied by rising the electron count at the rhenium. The 16-electron rhenacyclopentene(3) (e) (Re") rearranges to the 18-electron q4-1,3-diene complex 9d (Re"'). One mechanistic problem warrants further comment. The product complex 9d has two chloride ligands, just like the starting alkyne complex 2a. If HC1 elimination from intermediate a is proposed, the question arises to reconcile the subsequent cationic monochloro intermediates c-f of Scheme IV with the constitution of the product 9d. HC1 or even chloride present in the reaction medium may add to intermediate f to give 9d. However, dinuclear chloride-bridged structures like u (Scheme VII) would explain the final chlorine transfer to f as well. Although we are not able to rule out one of those possibilities, it remains a fact that the monochloro derivative 3a is transformed into both a mono- and a dichlororhenium complex by acid-catalyzed alkyne/alkene coupling. If one is to postulate initial methane elimination from 3a (confirmed by GC/MS studies), it becomes obvious that some kind of chloro-bridged intermediates giving rise to chlorinetransfer reactions must exist. Otherwise, the occurrence of product 9d is hard to understand. 6. The 2,4-Pentadienyl Complexes of Type 10. Only the parent ethylene, C2H4, couples to rhenium-bound alkynes to give the 2,4-pentadienyl complexes of type 10 in reasonable yields (Table I). With increasing number of substituents at the alkene, the tendency toward double insertion into the rhenium-alkyne bonds decreases rapidly. This might well be due to steric hindrance if substituted alkenes are to coordinate to intermediate c. On the other hand, variation of the substituents at the alkyne ligand does also effect the product distribution. However, with increasing steric demand of the substituents at the alkyne group, the amount of 2,4-pentadienyl species increases from 10% (loa) to 65-70% (loa). The hypothetical intermediate e is formally a 14-electron species and therefore unsaturated. The proposed 1,3hydride shift e f discussed above may compete with at least two other reactions that increase the (formal) electron count at rhenium atom equally well. Addition of HC1 or chloride transfer would give intermediates i or 1, respectively. Rearrangement of 1 by a concerted 1,3-hydrideshift would yield 9d. This is an alternative pathway to 9d starting from intermediate e. Coordination of ethylene to e, however, leads to the 16-electron intermediate m while coordination of ethylene to 1 gives the 18-electron species r. Up to now we have no experimental evidence to discriminate between the two sets of intermediates m-o or i-t. The two different pathways have some features in common. In both cases, insertion of ethylene into the
-
-
(11) Woodward, R. B.; Hoffmann, R. Die Erhaltung der Orbitakymmetrie; Verlag Chemie, Weinheim, Germany, 1970.
Herrmann et al.
2826 Organometallics, Vol. 8, No. 12, 1989 rhenium-vinyl a-bond is required to generate the sevenmembered rhenacycles n and s. Quite a similar intermediate was proposed by Wakatsuki et al. for the insertion of activated alkenes into a cobalt-vinyl a-bond of cobaltocyclo-2-pentenesto give a cobaltacycl0-4-heptene.~As in this particular case, a @-hydridetransfer mechanism would operate in Scheme IV to yield the intermediates o and t, respectively. If intermediate t undergoes reductive alkyl elimination, a l,&diene complex would result as final reaction product, with a new ethyl substituent showing up at the diene moiety. (Note that transfer of C1- to o gives t as well.) This reaction sequence, consisting of &hydride transfer and reductive alkyl elimination, has previously been proposed for the rearrangement of cobaltocyclo-4heptenes to 1,3-diene complexes of cobalt.% Our rhenium case is in contrast to this proposal. Loss of a proton from o or reductive elimination of HC1 from t gives intermediate p. Now a rearrangement of the diene ligand of p takes place. 1,2-Hydride shift, presumably via intermediate q, gives the 2,4-pentadienyl complexes 10a-d as the second type of isolable products. 7. Conclusions. While it is clear that the CC coupling reactions discussed in this paper do not allow for a unambiguous answer in terms of mechanismb), the following experimental results are to be noted: (i) Strong Brernsted acids are catalysts for the activation of the rhenium alkyne complexes 2a-e, which compounds are chemically otherwise rather inert. This catalytic activation has two main synthetic consequences: first, either one chloride ligand or the alkyne ligand is substituted by other ligands, very much depending on the nature of the entering ligand; second, CC coupling processes with nonactivated alkenes to yield novel 1,3-diene and 2,4-pentadienyl complexes of rhenium are thus easily achieved.2f (ii) Elimination of HC1 from a transient species, as in a, evoked by the presence of protons, plays a pivotal mechanistic role. This special acid effect seems to be closely related to halide ligands present in these organorhenium complexes. It should therefore prove possible to apply the general findings of this paper to other organometal halides rather than being restricted only to the element rhenium. This acid effect on the reactivity of the rhenium alkyne complexes 2a-e is in sharp contrast to all the other known acid-promoted CC coupling reactions of complexed alkynes. In those latter cases initial protonation of the alkyne ligand entails subsequent reactions rather than protonation of ancillary ligands. C. The Stereochemistry of the Alkyne/Alkene Coupling Leading to 1,3-Dienes. Treatment of 2a with (Z)-2-butene yields the 1,3-diene complex 9g. By way of contrast, the E isomer of 2-butene yields the isomeric product 9h. A mixture of (E)-and (Z)-2-butene gives a mixture of 9g and 9h in a molar ratio of 2:l ('H NMR). Compounds 9g and 9h can be separated from each other by HPLC at silica gel; they do not interconvert under such conditions, with this result suggesting that the stereochemistry of the alkene is likely to be preserved through all steps of the CC coupling process. This must be the reason why two different 1,3-dienes are formed. The coupling reaction is stereoselective even when alkenes are employed that do not exhibit any stereochemical information. Reaction of 2a with ethylene yields (E)-3methyLpentadiene(2,4). Treatment of 2c with ethylene gives only (Z)-3-phenylpentadiene(2,4)(as (v5-C5MedReC12 complexes 9d and 9i, respectively), thus showing the regioselectiuity of the coupling reaction. These selectivities are governed by the considerable bulk of the permethylated a-aromatic ligand C5Me5. A suprafacial 1,3-
I"
H
91 Figure 3. Scheme VIII
*
I
d'
=' c _
Y-Y cn,
sa
cn,
.
I
d'
v
e'
*--$-
-$
H2&$H,
CH, 'CH,
I
13H-shill
13H-Shill
H'
e - "I;/"
CHH' CH,
w C%
CH,
9h
H?&$A,
w, w, W
hydride shift leads from intermediate e' to the transient metallacyclo-3-pentene (v) of Scheme VIII. The methyl substituents of (n-2-butene would be cis to each other in intermediate v. A formal disrotation along the C(l)-C(2) and C(3)-C(4) bond axes of v with respect to low steric hindrance would require all methyl groups cis to each other in the final diene complex 9g. Exactly this configuration is present in 9g (single-crystal X-ray diffraction study). The methyl substituents of (E)-&butenewould be trans to each other in the transient species (w). This stereochemical relationship is preserved throughout the entire rearrangement process as outlined above (Scheme VIII). The final l,&diene complex 9h should therefore exhibit the structure shown in Scheme VIII. The 'H and 13C NMR data of compound 9h are expectedly consistent with four different methyl groups, two different olefinic protons, and eight different carbon atoms for the 1,3-diene ligand. (The NMR spectroscopic data of compound 9h are available in the supplementary material.) By comparison with the data obtained for compounds 9a-g, one would predict a chemical shift around 6 = 1.8-1.9 ppm for the olefinic proton H'. This upfield shift is due to the effective shielding by the rhenium atom.12 However, the two olefinic protons H' and H2present in 9h exhibit chemical shifts of 6 = 3.17 and 3.76 ppm, respectively (400 MHz, CDCl,, 25 "C). This result does not perfectly match the proposed structure of 9h (Scheme VIII). Unfortunately, no crystals suitable for X-ray diffraction studies have been obtained so far in order to convey an unambigious solution of the structure problem. There might be a rearrangement of 9h to some kind of s-trans-diene complex, which is thought to exhibit lower repulsive interactions with the sterically demanding a-ligand C5Me,. If 9h adopts the configuration of a trans diene, the shielding of H' could be lower, thus explaining the observed chemical shift. In this context, it should be mentioned that complex 91 does also exhibit peculiar NMR spectra: the signal of the (12) The Re-H(21) and Re-H(51) distances are calculated in ideal position with an average value of 210 pm. A further refinement of the olefinic hydrogen atoms H(21) and H(51) bonded to C(2) and C(5), respectively, was not possible.
Organometallics, Vol. 8, No. 12, 1989 2827
Rhenium 1,3-Diene a n d 2,4-Pentadienyl Complexes 21
21
Figure 4. Molecular structure (ORTEP drawing; stereoscopic view) of the 1,3-diene complex of formula (q6-C5Me6)ReCl2(q4-CH(Me)=C(Me)C(Me)=CH(Me)) (9g) in the crystal. The hydrogen atoms are omitted for better clarity. Shown are the 50% thermal ellipsoids. Table V. Crystallographic Information about Compounds 9g and 10d 9P 10d formula CBHaZC1Re C18H&lzRe fw,amu 590.2 502.5 cryst color reddish platelets orange needles cryst dimens, mm 0.42 X 0.05 X 0.42 0.45 X 0.25 X 1.10 24 1 T,"C 23 f 1 P 2 1 / ~(NO.14) P 2 1 / ~(NO.14) space group 859.4 (1) 1605.2 (2) Q, pm 2745.4 (1) 1022.7 (3) b, pm 855.5 (1) 1567.6 (1) c, um 117.73 (1) 113.35 (1) 1787 X lo6 2363 X lo6 4 4 D(calcd), g . ~ m - ~ 1.868 1.659 984 1168 F(000),e p(Mo Ka),cm-' 71.89 53.4 scan type w-scan w-scan scan speed, s max 60 max 60 scan width 0.85 + 0.35 tan 8 1.25 + 0.35 tan 8 28 limits, deg 50 60 no. of data collected 14 123; h(0/-22), 6368; h(-10/10), k(-14/14), k(0/32), 1(-22/22) l(-lO/lO) no. of unique data 3112 6419 no. of reflctns used for 2903, I > 0.5u(I) 5963, I > 0.5u(I) refinement no. of refined hydrogen none 7 atoms no. of parameter refined 190 293 R" 0.023 0.035 0.023 0.023 RWb goodness of fit' 2.639 1.864
*
methyl substituents of the r-aromatic ring ligand is shifted from the typical 6 value of ca. 1.75 ppm upfield to 6 = 1.58 ppm. This small shift of A6 = 0.17 ppm might be effected b y the ring current of the phenyl substituent at C(1) (Figure 3). D. Crystal and Molecular Structure of the Diene Complex (9g). Compound 9g crystallizes from dichloromethaneln-hexane mixtures at -35 "C as red, trigonal plates in the monoclinic space group P2,ln. 9g m a y serve as a representative example for the structures of compounds 9a-g and 9k-n. ( C o m p o u n d s 9h and 9i m a y be exceptions; see preceeding paragraph C.) The solid-state molecular structure is shown in Figure 4. Relevant X-ray data are summarized in Table V, while the fractional atomic coordinates are listed in Table VI. Selected interatomic distances and angles are compiled in Table VII. The coordination geometry of the diene ligand is of particular interest. Complex 9g exhibits the 1,3-diene
Table VI. Selected Interatomic Distances and Angles of Compound 9@ Bond Distances (pm) 153.4 (4) 222.0 (3) C(4)-C(8) 236.4 (3) Re-Cl( 1) 244.1 (1) 235.5 (3) Re-Cl(2) 243.8 (1) 222.1 (3) Re-Cp 193.3 143.8 (4) C(11)-C(12) 139.9 (4) 139.6 (4) C(12)-C (13) 144.3 (4) 144.0 (4) C(13)-C(14) 143.4 (4) 149.5 (4) C(14)4(15) 144.6 (4) 152.2 (4) C(15)-C(ll) 142.1 (4) 151.2 (4) C(l)-C(2)-C(3) C(2)-C(3)-C(4) C(2)-C(3)-C(7) C(3)-C(4)-C(5) C(4)-C(5)-C(6) C(5)-C(4)-C(8) Cl(l)-Re-C1(2)
Bond Angles (deg) 120.0 (3) Cl(l)-Re-Cp 117.5 (3) Cl(2bRe-C~ 121.8 (3) Cl(l)-Re-C(2) 118.3 (3) C1(2)-Re-C(5) 119.4 (3) Cp-Re-Cbut 121.8 (3) C(2)-Re-C(5) 79.83 (3) C(3)-Re-C(4)
113.38 112.99 87.04 (8) 86.74 (8) 133.88 76.3 (1) 34.4 (1)
"Cp denotes the center of the r-bonded ligand C6Me6;Cbut denotes the center of the 1,3-diene ligand. Table VII. Fractional Atomic Coordinates and Equivalent Isotropic Temperature Factors for Non-Hydrogen Atoms in 9g with Estimated Standard Deviations in Parentheses' atom X Y z B,, AZ Re 0.44841 (2) 0.13017 (1) 0.54971 (2) 1.795 (3) 3.43 (3) Cl(1) 0.2076 (1) 0.14730 (5) 0.6175 (1) 0.15981 (5) 0.8519 (1) 3.48 (3) Cl(2) 0.6135 (2) 0.2967 (6) 4.4 (1) 0.1083 (7) 0.0605 (2) C(1) 0.0634 (2) 0.4136 (5) 2.8 (1) C(2) 0.3023 (6) 0.0504 (2) 0.5970 (5) 2.8 (1) C(3) 0.3750 (6) 0.0563 (2) 0.7020 (5) 2.7 (1) C(4) 0.5558 (6) 0.0744 (2) 0.6204 (5) 2.8 (1) C(5) 0.6578 (6) 0.8526 (7) 0.0849 (2) 0.7344 (7) 4.3 (1) C(6) 0.0327 (2) 0.6765 (6) 4.2 (1) C(7) 0.2607 (7) 0.0445 (2) 0.8999 (6) 4.0 (1) C(8) 0.6423 (7) 0.1527 (2) 0.3640 (5) 2.5 (1) C(l1) 0.5722 (5) 0.1455 (2) 0.2569 (5) 2.4 (1) C(12) 0.3920 (6) 0.1778 (2) 0.3150 (5) 2.6 (1) C(13) 0.2966 (5) 0.2054 (2) 0.4599 (5) 2.8 (1) C(14) 0.4251 (6) 0.1894 (2) 0.4907 (5) 2.6 (1) C(15) 0.5973 (5) 0.1308 (2) 0.3241 (6) 4.1 (1) C(21) 0.7105 (6) 0.0906 (5) 4.2 (1) 0.1150 (2) C(22) 0.3157 (7) 0.1884 (2) 0.2162 (7) 4.5 (1) C(23) 0.1033 (7) 0.2496 (2) 0.5410 (6) 4.7 (1) C(24) 0.3896 (7) 0.2134 (2) 0.6105 (6) 4.6 (1) C(25) 0.7673 (7)
" Anisotropically refined thermal parameters are defined by their equivalent form: B, = 4/3[a2B(l,l)+ b28(2,2) + ab(cos 7)@(1,2)+ ac (cos p)B(1,3) + bc(cos a)@(2,3)]. ligand in the so-called supin (exo) conformation (I). (The important different structure types I-IV for 1,3-diene coordination geometries are shown in Figure 5.13 The
2828 Organometallics, Vol. 8, No. 12, 1989
Herrmann et al. Table VIII. Selected Interatomic Distances and Angles of Compound 10da
supin-cis
pron-cis
trans
I
II
111
IV
@ = cI,,t..Re 4 CI
Figure 5. Important coordination geometries of 1,3-diene ligands.
carbon atoms C(2-5) of the diene skeleton including their methyl substituents C(1) and C(6-8) are literally coplanar. The Re-C(2-5) distances vary from 222.0 (3) to 236.4 (3) pm; they are still within the range of distances between the rhenium and the carbon atoms C(l1-15) of the aaromatic ring ligand. The CC distances of the 1,3-diene skeleton are elongated to an average value of 142.5 pm. One can define three critical values that characterize the coordination of the 1,3-diene to the (q5-C5Me5)ReC12 fragment:13 (i) Ad = [d(Re-C2) + d(Re-C5)]/2 [d(Re-C3) + d(Re-C4)]/2 = -13.9 pm (ii) A1 = [l(C2-C3)
+ l(C4-C5)]/2
-
l(C3-C4) = +4.3 pm
(iii) 0 = 95.8(1) (dihedral angle between the planes defined by Re/C(2)/C(5) and C(l-8])
According to these data, the small but nevertheless significant distortion from an ideal symmetrical coordination of the 1,3-diene (Ad = A1 = 0) suggests that 9g has a structure intermediate between a q4-s-cis-dienecomplex (I) and a metallacyclopentene(3) (IV) (structure types I and IV are shown in Figure 5).13 Note that the structure of 9g is quite similar to the related analogues (q5-C5R5)MX2(q4-l,3-diene)of identical composition (M = Nb, Ta; X = C1; R = H, CH3).I4 The tantalum compound (q5C5H5)TaC12(~4-1.3-butadiene), for example, shows values of -15.9 pm for Ad, +8.1 pm for Al, and 94.9O for A preference of 15.7 kcal/mol for the supin isomer was established for this complex by virtue of EHMO calculat i o n ~ . 'It~ is ~ presumably the same electronic reason why the 1,3-diene ligand of compound 9g prefers the supin (I) over the pron (11)orientation (Figure 5). The energy gap between "supin" and "pron" may even be larger in our case particularly when steric interactions with the substituents a t the 1,3-diene and the ring ligand are ~0nsidered.l~ E. Crystal and Molecular Structure of the 2,4Pentadienyl Complex (loa). Compound 10d crystallizes from n-hexane/dichloromethane mixtures at -35 "C as orange plates in the monoclinic space group P2,/c. The solid-state molecular structure is shown in Figures 6 and 7. Relevant X-ray data are summarized in Table V while the fractional atomic coordinates are listed in Table VIII. Table IX presents some selected interatomic distances and angles. (13) Review: (a) Yasuda, H.; Nakamura, A. Angew. Chem. 1987,99, 745; Angew. Chem., Znt. Ed. Engl. 1987,26,723. (b) Erker, G.; Kruger, C.; Muller, G. Adu. Orgunomet. Chem. 1985,24, 1. (14) (a) Kai, Y.; Kaehisa, N.; Miki, K.; Kasai, N.; Akita, M.; Yasuda, H.; Nakamura, A. Bull. Chem. SOC.Jpn. 1983,56,3735. (b) Yasuda, H.; Tatsumi, K.; Okamoto, T.; Mashima, K.; Lee, K.; Nakamura, A.; Kai, Y.; Kanehisa, N.; Kasai, N. J.Am. Chem. SOC. 1985,107,2410. (c) Okamoto, T.; Yasuda, H.; Nakamura, A.; Kai, Y.; Kanehisa, N.; Kasai, N. J. Am. Chem. SOC.1988, 110, 5009. (d) Bunker, M. J.; Green, M. L. H. J. Orgunomet. Chem. 1980,192, C6. (15) The dibromo compound 9n is isostructural to 9g. Herrmann, W. A.; Fischer, R. A.; Anwander,R.; Herdtweck, E., unpublished results 1988.
Re-C(l) Re-C(2) Re-C(3) Re-C(4) Re-C(5) Re-C(6) C(l)-C(2) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6) C(3)-C(31) C(4)-C(41) C(l)-C(2)-C(3) C(2)-C(3)-C(31) C(2)-C(3)-C(4) C(3)-C(4)-C(41) C(3)-C(4)-C(5) C(4)-C(5)-C(6) C(5)-C(4)-C(41) C(2)-Re-C(3) C(3)-Re-C(4)
Bond Distances (pm) 328.9 (3)* Re-Cl 219.5 (3) Re-Cp 215.2 (3) Re-C(l1) 219.7 (3) Re-C(12) 219.5 (3) Re-C(13) 218.8 (3) Re-C(14) 153.1 (3) Re-C(15) 143.6 (3) C(ll)-C(12) 148.9 (3) C(12)-C(13) 141.9 (3) C(13)-C(14) 114.5 (3) C(14)-C(15) 149.2 (3) C(15)-C(ll) 149.7 (3)
242.1 (
