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Download Article (PDF) Carbon-Carbon Bond Formation by Reductive Coupling with Titanium(II) Chloride Bis(tetrahydrofuran)* John J. Eisch**, Xian Shi, Jacek Lasota Department of Chemistry, The State University of New York at Binghamton, Binghamton, New York 13902-6000, U.S.A. Dedicated to Professor Dr. Dr. h. c. mult. Günther Wilke on the occasion of his 70th birthday Z. Naturforsch. 50b, 342-350 (1995), received September 20, 1994 Carbon-Carbon Bond Formation, Reductive Coupling, Titanium(II) Chloride, Oxidative Addition, Carbonyl and Benzylic Halide Substrates Titanium(II) bis(tetrahydrofuran) 1, generated by the treatment of TiCl4 in THF with two equivalents of n-butyllithium at -78 °C, has been found to form carbon-carbon bonds with a variety of organic substrates by reductive coupling. Diphenylacetylene is dimerized to ex­ clusively (E,E)-1,2,3,4-tetraphenyl-l,3-butadiene; benzyl bromide and 9-bromofluorene give their coupled products, bibenzyl and 9,9'-bifluorenyl, as do benzal chloride and benzotrichlo- ride yield the l,2-dichloro-l,2-diphenylethanes and l,l,2,2-tetrachloro-l,2-diphenylethane, respectively. Styrene oxide and and ris-stilbene oxide undergo deoxygenation to styrene and fra/«-stilbene, while benzyl alcohol and benzopinacol are coupled to bibenzyl and to a mix­ ture of tetraphenylethylene and 1,1,2,2-tetraphenylethane. Both aliphatic and aromatic ke­ tones are smoothly reductively coupled to a mixture of pinacols and/or olefins in varying proportions. By a choice of experimental conditions either the pinacol or the olefin could be made the predominant product in certain cases. The reaction has been carried out with heptanal, cyclohexanone, benzonitrile, benzaldehyde, furfural, acetophenone, benzophenone and 9-fluorenone. In a remarkable, multiple reductive coupling, benzoyl chloride is converted into 2,3,4,5-tetraphenylfuran in almost 50% yield. The stereochemical course of two such couplings, that of diphenylacetylene to yield exclusively (E,E)-1,2,3,4-tetraphenyl-l,3-buta­ diene and that of acetophenone to produce only racem/c-2,3-diphenyl-2,3-butanediol, is inter­ preted to conclude that the couplings proceed via two electron transfer pathways (TET) involving titanium(IV) cyclic intermediates of the titanirene and the oxatitanacyclopropane type, respectively. The monomolecular hydrodeoxygenation or bi- termining the reducing action of the resulting re­ molecular reductive coupling of a wide gamut of agent is uncertain. The ill-defined nature of such organic substrates has been found to occur by the reductants is readily evident from the numerous action of various reactive metals, metal hydrides titanium-based reagents reported to be formed or subvalent metal complexes [1,2]. Such reducing when TiCl4, TiCl3 or CpTiCl2 is treated with, agents often are employed in heterogeneous reac­ among others, RLi, RMgX, R3AI, LiAlH4, Li, K, tion media either as highly dispersed metal par­ Mg or Zn [2], Outstanding among these reducing ticles or as metals adsorbed on solid supports such combinations for its versatility in organic synthesis as graphite. In many other cases, the reducing is the McMurry Reagent, a black suspension of agent is generated, in situ, by treating a transition some form of titanium(O) generated when a 4:1 metal salt with a main group metal, metal hydride mixture of LiAlH4 and TiCl4 is added to THF [2], or metal alkyl. Although it is certain that the tran­ With this backdrop and in connection with our sition metal center is thereby reduced, the exact investigation of new routes to transition metal oxidation state formed is often uncertain and the borides [3], we recently found that titanium(II) role of the main group metal reductant in de­ chloride could be readily synthesized from ti- tanium(IV) chloride by simply adding two equiv­ alents of a metal alkyl to TiCl4 in toluene or tetra- * XIII Communication of the series, “Organic Chemis­ hydrofuran (eqs 1-3): try of Subvalent Transition Metal Complexes”; XII The titanium(II) chloride bis(tetrahydrofuran) 1 Communication: J. Am. Chem. Soc. 108, 7763 (1986). formed in eq. 1 could be obtained free of LiCl ** Reprint requests to Prof. J. J. Eisch. and analytically pure by evaporating the THF and 0932-0776/95/0300-0342 $06.00 © 1995 Verlag der Zeitschrift für Naturforschung. All rights reserved. J. J. Eisch et al. ■ Carbon-Carbon Bound Formation by Reductive Coupling with TiCl2-2THF 343 TiC14 + 2 BunLi ----- — ------► TiCI2-2THF + 2 BunH (D - 2 LiCl 1 TiCl4 + 2 H2C=C H C H ,M gC l — _ u ■» TiCl2*2THF*2MgCl2*4rHF (2) * * L3H6 2 TiCL + 2 Me-iAl ------------------- ► TiCl2*Me2AlCl + 2 CH4 (3) - Me2AlCl extracting 1 into toluene. The titanium(II) chloride This observation is consistent with a 2:1 stoichi­ 2 formed in eq. 2 was weakly complexed with the ometry of reaction and the formation of tetrachlo- magnesium chloride by-product and that in equa­ rodititanoxane(III) 6. tion 3 formed a stable complex with Me2AlCl 3. Both 2 and 3, when admixed with an excess of b) Scope of organic substrates reducible by 1 R„A1C13_„, function as highly active hetero­ (Table I) geneous Ziegler catalysts for the polymerization of ethylene and higher olefins, as has been prelimi­ a) Hydrocarbons: Although titanium(II) chlo­ narily reported elsewhere [4]. ride in the form of complexes 1, 2 and 3 and ad­ With a well-defined, soluble subvalent titanium mixed with a six to eight-fold excess of Me2AlCl complex in hand, we were well-positioned to ex­ is able to catalyze the polymerization of ethylene plore the scope and the mechanism of reduction and other alpha-olefins [4], complex 1 in THF or of organic substrates by titanium(II) chloride bis- unsolvated TiCl2 suspended in toluene [6] caused (tetrahydrofuran) 1. We report here the results of neither reduction nor oligomerization of such ole­ our investigation thus far. fins as styrene and 1,1-diphenylethylene, even after 24 h in refluxing solution. Diphenylacetylene 7, Results however, underwent a slow bimolecular reduction to yield solely (E,E)-1,2,3,4-tetraphenyl-l,3-buta­ a) Reaction conditions and stoichiometry diene 9 (entry 1 in Table I) upon hydrolysis (eq. 5): Reductions with 1 were initially conducted with That the organotitanium precursor to 9 is most the lithium chloride-free reagent in refluxing tolu­ likely l,l-dichloro-2,3,4,5-tetraphenyltitanole 8 is ene or tetrahydrofuran solution. Since the pres­ supported by the photoreaction of ? 73-allyltitano- ence of the LiCl had no marked effect on the re­ cene 10 with 7, whereby titanole 13 is formed in ducing activity of 1 for most substrates, subsequent 60% yield [4]. The reaction mechanism leading to reductions were carried out directly with the THF 13 involves the photolytic loss of the allyl radical solutions of 1 still containing the suspended LiCl from 10 and the generation of titanocene(II) 11. (eq. 1). The ratio of 1 to the organic substrate This undergoes oxidative addition with 7 to pro­ ranged from 2:1 to 4:1. However, with diaryl ke­ duce titanirene 12, which inserts a further unit of tones, such as benzophenone, failure to remove 7 to produce 13 (Scheme 1). the LiCl prior to reduction led to a less active re­ ß) Halides: Aromatic halides and aliphatic ha­ agent [5]. lides, as typified by p-bromoanisole and 1-bromo- The stoichiometry of one reduction employing 3-phenylpropane, underwent no discernible re­ 1, which was free of LiCl, is significant: a 1:1 ratio duction by 1 during 24 h in refluxing THF. On the of 1 and benzophenone 4 gave a 48% yield of other hand, benzylic halides, such as benzyl bro­ tetraphenylethylene 5 (eq. (4)): mide 14 gave exclusively the bimolecular re- 2 Ph2C = 0 + 4 TiC l2 --------------- ► Ph2C=CPh2 + 2Cl2Ti— O— TiCl2 (4) A 4 1 5 6 344 J. J. Eisch et al. • Carbon-Carbon Bound Formation by Reductive Coupling with TiCl2-2THF Table I. Reduction of organic substrates with TiCl2-2THF 1. Entry Substrate3 Products13 Yield' 1 Diphenylacetylene3 (E,E)-1,2,3,4-Tetraphenylbutadienee 14 2 Benzyl bromide Bibenzyl 100 3 9-Bromofluorene Fluorene 18 9,9 '-Bifluorenyl 82 4 Benzal chloride 1,2-Dichloro-l ,2-diphenylethanesf 97 5 Benzotrichloride l,l,2,2-Tetrachloro-l,2-diphenylethane 92 6 Dichlorodiphenylmethane Tetraphenylethylene 96 7 Benzopinacol Tetraphenylethylene 39 Tetraphenylethane 51 Benzophenone 10 8 Styrene oxide Styrene 90 9 ris-Stilbene oxide rra«s-Stilbeneg 98 10 N,N-Diphenylaminomethyl phenyl sulfide Methyldiphenylamine 15 11 Benzyl alcohol Bibenzyl 70 12 Benzonitrile Benzyl phenyl ketone 10 13 Heptanal 7,8-Tetradecanediol 80 14 Cyclohexanone 1,1 '-Dihydroxydicyclohexyl dicyclohexylidene 60 15 Benzaldehyde rran5-Stilbeneh 98 16 Furfural (E)-l,2-Bis(2-furyl)ethane 95 17 Acetophenone (E)-2,3-Diphenyl-2-butene 88 Acetophenone rac-2,3-Diphenyl-2,3-butanediol 83 (E)-2,3-Diphenyl-2-butene 13 18 Benzophenone Tetraphenylethylene 58 19 9-Fluorenone 9,9 '-Bifluorenylidene 44 20 Benzoyl chloride 2,3,4,5-Tetraphenylfuran 47 a Unless otherwise specified, all reaction were conducted by allowing a 4:1 molar ratio of the LiCl-containing TiCl2 and the organic substrate to reflux in THF solution under an argon atmosphere for 24 h. The individual runs employed about 2.5 mmol of the substrate dissolved in 30 ml THF; b the product were isolated from the hydrolyzed reaction mixture by column chromatography and identified by comparing their TLC, GC, m.p. and JH and 13C NMR spectral properties with those of authentic samples;c the yields are those of the isolated components but are not yet optimized; d a 2:1 ratio of the acetylene to 1 were employed in a 60 h reaction; e a 40 h reaction time was employed; f a 2:1 ratio of racemic and meso isomers resulted; 8 the ds-isomer was present in 3% yield; h less than 1 % of the c/s-isomer was found.
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