Organometallics 1983,2, 638-649
638
Synthesis and Reactivity of Cyclohexenylmanganese Tricarbonyl, a Complex Containing a Two-Electron, Three- Cent er Mn***H***CI nt eract ion M. Brookhart,' W. Lamanna, and Allan R. Pinhas Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 275 14 Received October 4, 1982
The cation (ben~ene)Mn(CO)~+ undergoes stepwise, vicinal addition of 2 equiv of hydride in an exo fashion to yield a unique transition-metal anion, ( l,3-cyclohexadiene)Mn(C0)3-. The diene anion is highly reactive. Exposure to oxygen results in oxidation of the metal and liberation of free 1,3-cyclohexadiene. Protonation yields an unusual, bridged (cyclohexenyl)Mn(CO)3species possessing an aliphatic, endo C-H bond that is activated via coordination to manganese. The three-center Mn.-H-C interaction in this complex renders the bridging hydrogen acidic and permits facile removal by base to regenerate the diene anion. Alkylation with Me1 or MeOS02CF3resulb in methyl addition to the endo side of the ring and coordination of a second endo C-H bond. A second deprotonation/alkylation sequence can be achieved to give ring-dialkylated cyclohexenyl products. The coordinated C-H bond of the bridged cyclohexenylspecies is replaced by external ligands L (L = CO, P(OMe)3)to give (cyclohexenyl)Mn(C0)3Ladducts. Hydride addition to the r-allyl unit of the tetracarbonyl species results in reduction of the polyolefin to cyclohexene. Thermolysis of the phosphite adduct causes loss of CO and formation of the bridged complex (cyclohexenyl)Mn(CO)2P(OMe),. The parent complex C,HsMn(CO)3reacts with diazomethane in an unexpected fashion providing an alternate and complimentary method of ring methylation. Reactions with activated olefins appear to proceed via a free radical mechanism resulting in transfer of H2 across the double bond.
Introduction Transition-metal-assisted reactions of arenes with nucleophiles have received particular attention as a unique and potentially useful method of arene reduction and/or functionalization. Nucleophilic addition to transitionmetal arene complexes typically yields the corresponding exo-substituted cyclohexadienyl complexes:
I
M
I
M
The arene complexes (C6H6)Fe(C5H5)+,'C6H6Mn(C0)3+,1 and C6H6Cr(C0)32and their ring-substituted derivatives are among the most thoroughly studied in this regard. In each case, the exo-substituted cyclohexadienyl complexes can be converted back to the corresponding substituted arenes in either complexed or uncomplexed form. In the iron system, oxidation with N-bromosuccinimide results in endo hydrogen abstraction and formation of the substituted arene ~ o m p l e xhowever, ;~ competing abstraction of the added exo Nu is often a p r ~ b l e m . A ~ similar conversion is possible in the manganese systems via thermal isomerization of the exo-substituted cyclohexadienyl complex followed by abstraction of the resulting 6-ex0 hydrogen using the trityl ~ a t i o n .Alternatively, ~ direct oxidation of the 6-exo-substituted complex with cerium(1) Green, M. L. H.; Davies, S.G.; Mingos, D. M. P. Tetrahedron 1978, 34, 3047 and references therein. (2) (a) Semmelhack, M. F.; Hall, H. T.; Farina, R.; Yoshifuji, M.; Clark, 1979,101,3535. G.; Bargar, T.; Hirotau, K.; Clardy, J. J.Am. Cheq. SOC. (b) Semmelhack, M. F.; Hall, H. T.; Yoshituji, M.; Clark, G. Zbid. 1975, 97,1247. (c) A substituted cyclohexadiene can be obtained by the single reduction/protonation sequence on an &ne chromium tricarbonyl complex: Semmelhack, M. F.; Harrison, J. J.; Thebtaranoth, Y. J. Org. Chem. 1979,44, 3275. (3) Khand, I. V.; Pauson, P. L.; Watts, W. E. J. Chem. SOC. C 1969, 2024. (4) Nesmeyanov, A. N.; Vol'Kenau, N. A.; Shilovtaeva, L. S.; Petrakova, V. A. J. Organomet. Chem. 1975, 85, 365. (5) Pauson, P. L.; Munro, G. A. M. 2. Anorg. Allg. Chem. 1979, 458, 211.
(IV)/sulfuric acid results in conversion all the way to the free substituted arene.6 Mild oxidation of the anionic cyclohexadienylchromium complexes with iodine also gives the free substituted arene via endo hydrogen removal and cleavage from the metal.2b This latter reaction has been used extensively by Semmelhack to perform net nucleophilic aromatic substitutions in the chromium system. In a few rare cases, activation of an arene ligand toward addition of 2 equiv of nucleophile and formation of a diene complex has been demonstrated. Wilkinson7has shown that hydride reduction of the dication (C6H6)2R~2+ leads to mixtures of (benzene)(l,3-cyclohexadiene)ruthenium and dicyclohexadienylruthenium. It is not clear, in this case, whether the diene complex arises from initial addition of 2 equiv of hydride to a single ring or thermal isomerization of the dicyclohexadienyl complex. Vollhardt8 has reported double nucleophilic addition to the (benzene)cyclopentadienylcobalt(2+) dication upon treatment with methoxide or cyclopentadienide. Nucleophilic addition occurs vicinally and stereospecificallyexo on the arene ring. Liberation of the diene by oxidative demetalation could not be achieved. Most recently Maitlis has observed double hydride addition to the benzene rings in the complexes (C5(CH3),)MC6H6+(M = Ir, Rh) and (C6(CH3)& RuC6H:+ using NaAlH2(0CH,CH20CH3)2.9 The direct conversions of arenes to 1,3-cyclohexadienes (substituted or unsubstituted) via transition-metal-mediated double nucleophilic additions are particularly interesting from a synthetic standpoint in that they represent a possibly versatile complement to the Birch reduction. Although the current reports clearly indicate that such additions are possible, examples are limited and the chemistry of the resulting diene adducts has been virtually unexplored. (6) (a) Walker, P. J. C.; Mawby, R. J. J. Chem. Soc., Chem. Commun. 1972,330. (b) Mawby, A.; Walker, P. J. C.; Mawby, R. J. J. Organomet. Chem. 1973,55, C39. (7) Jones, D.; Pratt, L.; Wilkinson, G. J. Chem. Soc. 1962, 4458. (8) Lai, Y.-H.; Tam. W.; Vollhardt, K. P. C. J . Organomet. Chem. 1981, 216, 97. (9) Grundy, S. L.; Maitlis, P. M. J. Chem. SOC.,Chem. Commun. 1982, 379.
0276-7333/83/2302-0638$01.50/00 1983 American Chemical Society
Synthesis of Cyclohexenylmanganese Tricarbonyl The results described in this manuscript demonstrate that 2 equiv of hydride can be added to the cationic arene complex (benzene)Mn(C0)3+to effect reduction to the (1,3-~yclohexadiene)manganese tricarbonyl anion in high yields. The reduction proceeds in a stepwise fashion through a neutral cyclohexadienylmanganese tricarbonyl intermediate and represents the first example of double nucleophilic addition to a monocationic arene complex. The (1,3-~yclohexadiene)manganese tricarbonyl anion is the first anionic diene complex to be characterized. Although thermally quite stable, this species is highly reactive and undergoes a series of unique and interesting conversions. Exposure to oxygen results in oxidation of the metal and liberation of free 1,3-cyclohexadiene. Protonation yields an unusual bridged cyclohexenyl species possessing an aliphatic endo C-H bond that is activated via coordination to manganese. The three-center, two-electron MnH.-C interaction in this complex renders the bridging hydrogen acidic and permits facile removal by base to regenerate the diene anion. Alkylation of the diene anion results in electrophilic addition to the endo side of the ring and coordination (and activation) of a second endo C-H bond to give monoalkylated cyclohexenyl derivatives. A second deprotonation/alkylation sequence can also be achieved to give ring-dialkylated cyclohexenyl products. In addition, the coordinated C-H bond of the bridged cyclohexenyl species is easily displaced by external ligands L to give (~yclohexenyl)Mn(CO)~L species. Hydride addition to the a-allyl unit of this species results in further reduction of the polyolefin to cyclohexene. Overall, the reactions reported herein enable a series of potentially useful, manganese-mediated transformations for the selective reduction of benzene to 1,3-cyclohexadiene,cyclohexene, and stereoselectively functionalized derivatives. The net electrophilic substitution reactions of the coordinated C-H bond of the bridged cyclohexenyl species are particularly interesting in that they provide a unique and selective method for the functionalization of aliphatic carbon centers and the formation of carbon-carbon bonds. The chemistry provides valuable insight into the nature of the Mn.-H-.C interaction in these complexes and suggests in a general way how this new mode of C-H bond activation may be of value in transition-metal-mediated organic syntheses. A preliminary account of part of this work has appearedlo as well as a detailed spectroscopic and X-ray structural analysis" of the (6-endo-methylcyclohexeny1)manganese tricarbonyl complex containing a three-center, two-electron Mn.-H-C bond. Pauson has recently reported similar reductions of (arene)manganesetricarbonyl cations using lithium aluminum hydride.12
Results and Discussion Hydride Reduction of (Benzene)manganese Tricarbonyl Hexafluorophosphate. Generation of (Cyc1ohexadiene)manganeseTricarbonyl Anion and Cyclohexenylmanganese Tricarbonyl. Excess lithium triethylborohydride or potassium triisopropoxyborohydride react in a stepwise fashion with (benzene)manganese tricarbonyl hexafluorophosphate, 1, in tetrahydrofuran (THF) to transfer 2 equiv of hydride. The product (1,3cyc1ohexadiene)manganese tricarbonyl anion, 2, resulting from double vicinal addition of hydride to the benzene (10) Lamanna, W.; Brookhart, M. J. Am. Chem. SOC.1981,103,989. (11) Brookhart, M.; Lamanna, W.; Humphrey, M. B. J. Am. Chem. SOC.1982,104, 2117. (12) Bladon, P.;Munro, G. A. M.; Pauson, P. L.; Mahaffy, C. A. L. J. Organomet. Chem. 1981,221, 79.
Organometallics, Vol. 2, No. 5, 1983 639 ligand is generated in good yields (>61%), based on the yield of 4 upon quenching with H 2 0 (see below). In the
3
presence of 2.5 equiv of hydride the LiBEt,H reduction is complete in ca. 30 min, whereas the KB(i-Pr0)3Hreduction requires at least 15 h. The same anion can be produced quantitatively by similar hydride reduction of cyclohexadienylmanganesetricarbonyl, 3, which has been identified by IR (vco (in THF) 2017 (s) and 1929 (s, br) cm-') as the intermediate in the former reactions. The potassium salt of anion 2 exhibits strong IR bands at 1930, 1840, and 1789 cm-' in THF. The IR spectrum of the lithium salt in the metal carbonyl region is more complex, exhibiting bands at 1929 (s), 1896 (s), 1853 (s), 1831 (s), 1811 (s), and 1758 (s) cm-l. This is presumably due to ion-pairing effects with the more highly coordinating lithium cation. The diene anion is stable for days under a nitrogen atmosphere in THF solution; however, exposure to air results in immediate decomposition. No attempt was made to isolate anion 2 in solid form, although NMR studies of the anion have been carried out (see below). Treatment of THF solutions of 2 with water results in rapid protonation and quantitative conversion to the unusual cyclohexenylmanganese tricarbonyl complex 4, possessing an endo C-H bondwoordinated to manganese. The Mn.-H-.C interaction in 4 is best described as a two-electron, three-center bonding arrangement in which the cr electrons of the carbon-hydrogen bond are shared by the nominally 16-electron metal center. If one considers the a-allyl unit as a bidentate ligand, then the structure of 4 is roughly octahedral with the bridging hydrogen occupying the coordination site approximately trans to one carbonyl and cis to the two remaining carbony1s.l' Although structures of this type are uncommon, a small number of transition-metal complexes possessing coordinated C-H bonds have been reported,13 the closest analogues being cationic (?r-ally1)ironspecies (below) generated upon protonation of the neutral (diene)FeL, c~mplexes.'~J~ //
;I I H
L3 ~ e '
Unlike the thermally unstable iron complexes, bridged structure 4 is stable to temperatures greater than 120 "C. Complete details of the spectroscopic characterization, (13) (a) Brookhart, M.; Whitesides, T. H.; Crockett, J. M. Inorg. Chem. 1976, 15, 1550. (b) Brookhart, M.; Harris, D. L. Ibid. 1974, 13, 1540. (14) (a) Brown, R. K.; Williams, J. M.; Schultz, A. J.; Stucky, G. D.; Ittel, S. D.; Harlow, R. L. J. Am.Chem. SOC.1980,102,981. (b)Williams, J. M.; Brown, R. K.; Schultz, A. J.; Stucky, G. D.; Ittel, S.D. Zbid. 1978, 100, 7407. (c) Ittel, S. D.; Van-Carledge, F. A,; Tolman, C. A.; Jesson, J. P. Ibid. 1978,100,1317. (d) Ittal, S. D.; Van-Catledge, F. A.; Jesson, J. P. Ibid. 1979, 101,6905.
640 Organometallics, Vol. 2, No. 5, 1983
Brookhart, Lamanna, and Pinhas
structure and dynamic behavior of 4 have been previously reported." To aid in understanding the chemistry presented later, it should be noted here that complex 4 exhibits two fluxional isomerization processes detectable by 'H NMR spectroscopy: (1)a low-energy process (AG*= 8.3 kcal/mol) proceeding through the 16-electron *-allyl species 5 and (2) a higher energy process (AG* = 15.4 kcal/mol) proceeding through the diene hydride species 6. The low-energy isomerization is fast on an NMR time
/I
I/ H (COI3Mn,
