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18.6 Examples of Transition-Metal-Catalyzed Reactions 845 18_BRCLoudon_pgs4-3.qxd 11/26/08 9:09 AM Page 845 18.6 EXAMPLES OF TRANSITION-METAL-CATALYZED REACTIONS 845 PROBLEMS 18.14 A student has written the following ligand substitution reaction, claiming that it changes the oxidation state of the metal by one unit. What is wrong with this reasoning? Cl_ Pd(PPh3)4 ClPd(PPh3)3 PPh3 + LT + 3 18.15 The Wilkinson catalyst chlorotris(triphenylphosphine)rhodium(I), ClRh(PPh3)3, brings about the catalytic hydrogenation of an alkene in homogeneous solution: R R ClRh(PPh3)3 $CHA C) 2 RCH2CH2R (18.40) + HH) $ (a) Using the following mechanistic steps as your guide, draw structures of the transition- metal complexes involved in each step. Give the electron count and the metal oxidation state at each step. 1. oxidative addition of H2 to the catalyst 2. ligand substitution of one PPh3 by the alkene 3. 1,2-insertion of the alkene into a Rh H bond and readdition of the previously ex- pelled PPh3 ligand L 4. reductive elimination of the alkane product to regenerate the catalyst (b) According to the known stereochemistry of the 1,2-ligand insertion and reductive elim- ination steps, what would be the stereochemistry of the product if D2 were substituted for H2 in the reaction? 18.6 EXAMPLES OF TRANSITION-METAL-CATALYZED REACTIONS A. The Heck Reaction In the Heck reaction, an alkene is coupled to an aryl bromide or aryl iodide under the influence of a Pd(0) catalyst. H3C $ Pd P L Lc 34 CH3 (catalyst) CH3 (CH3CH2)3N % H2C A CH2 % HBr (18.41) CH C' N (solvent) i ++3 i neutralized by Br 18 h, 125 °C CH A CH % % 2 the (CH CH ) N (86% yield) 3 2 3 (The aryl substituents of the phosphine ligands used in the catalyst in this case are o-tolyl (that is, o-methylphenyl) groups rather than phenyl groups, but phenyl groups are also sometimes used.) The reaction is named for Richard F. Heck (b. 1931), who discovered the reaction in the early 1970s while a professor of chemistry at the University of Delaware. (A Japanese chemist, T. Mizoroki, simultaneously discovered the reaction, but it is generally known as the Heck reaction.) The Heck reaction has proven to be one of the most useful processes for form- ing carbon–carbon bonds to aromatic rings and even, occasionally, to vinylic groups. The mechanism of the Heck reaction is outlined in the following equations. You should identify the process or processes involved in each step (L tri-o-tolylphosphine ligands; the steps in Eq. 18.42b are numbered for reference). = 18_BRCLoudon_pgs4-3.qxd 11/26/08 9:09 AM Page 846 846 CHAPTER 18 • THE CHEMISTRY OF ARYL HALIDES, VINYLIC HALIDES, AND PHENOLS. TRANSITION-METAL CATALYSIS The actual catalytically active species is believed to be PdL2, which is formed by two lig- and dissociations: L L L L L L L L L L Pd L L Pd L L Pd L (18.42a) L L L L + L + The PdL2 thus generated enters into the catalytic cycle. CH2 L L Ar (2) Ar L L L L L (1) H2C CH2 H2C (3) L Pd Ar Br L Pd L Pd L + L L L Br L Br L + CH2 ArCHCH2 L H H L L L L L (4) CH (5) L L L Pd Pd L Pd L "H Ar (6) L Br L Br L Br ArCH CH2 + L H L L L L (7) L Pd L L Pd HBr (18.42b) +reacts with L Br L (CH3CH2)3N 3 PROBLEM 18.16 Characterize each step of the mechanism in Eq. 18.42b in terms of the fundamental processes discussed in the previous section. Give the electron count and the oxidation state of the metal in each complex. Another example of the Heck reaction illustrates two important aspects of the reaction. I Pd(OAc)2 catalyst (CH3CH2)3N M HI (18.43) dimethylformamide + 15 h, 100 °C + reacts with (CH3CH2)3N cyclohexene iodobenzene (2-cyclohexenyl)benzene (excess) (72% yield) First, the catalyst is not Pd(0), but rather a Pd(II) species. (Pd(OAc)2 is used because it is a con- venient and easily handled Pd derivative.) In some cases (typically with iodobenzenes as the aryl halides), the reaction can be run with Pd(II), but it is believed that the Pd(II) is reduced to Pd(0), perhaps by a few molecules of alkene that are converted into vinylic acetates; Pd(0) is the actual catalyst. Addition of an oxidizable ligand such as PPh3 can also serve to reduce the Pd(II). Be- cause a very small amount of Pd is used, the by-products of these reactions are also formed in very small amounts. Second, the alkene double bond in the product is not at the site of coupling, but rather one carbon removed. What has happened here? 18_BRCLoudon_pgs4-3.qxd 11/26/08 9:09 AM Page 847 18.6 EXAMPLES OF TRANSITION-METAL-CATALYZED REACTIONS 847 This sort of product, which occurs commonly with cyclic alkenes in the Heck reaction, is a direct consequence of the stereochemistry of certain steps in the mechanism. The insertion step (step 3 in Eq. 18.42b) must occur in a syn manner because the reaction is intramolecular. Hence, in the initially formed insertion complex, the Pd and the phenyl group become cis sub- stituents on a cyclohexane ring. the only b-hydride available for syn-elimination I H L Pd" L H H L Pd (18.44) L H H "I H Pd and phenyl are cis The subsequent b-elimination is also a syn process. Hence, only a hydride cis to the Pd is “el- igible” for elimination. When a noncyclic alkene is used in the Heck reaction, internal rotation is possible so that the hydride on the carbon at which insertion occurs can be eliminated. Pd" Ph Pd" Ph L L syn internal H H insertion L rotation CCA H CC) ) H HH HH Pd" H Pd" H syn L b-elimination L L H CC) ) Ph H Ph (18.45) CCA HH HH When the starting material is a cyclic alkene, as in Eq. 18.43, an analogous internal rotation is prevented by the ring. The only cis b-hydride available for elimination is the one (shown in red in Eq. 18.44) on the other b-carbon. Elimination of this hydride yields an alkene in which the carbon at the insertion point—the one attached to the phenyl—is not part of the double bond, but is one carbon removed. We can summarize this in the following way, with the inser- tion point marked with an asterisk (*): H H H HH* * syn syn Ph insertion H Ph b-elimination L Pd Ph L Pd L Pd H (18.46) L L L L L "I "I "I When the Heck reaction is applied to unsymmetrically substituted alkenes, such as an alkene of the form R CHACH2, two products are in principle possible, because insertion L 18_BRCLoudon_pgs4-3.qxd 11/26/08 9:09 AM Page 848 848 CHAPTER 18 • THE CHEMISTRY OF ARYL HALIDES, VINYLIC HALIDES, AND PHENOLS. TRANSITION-METAL CATALYSIS might occur at either of the alkene carbons. It is found that when the R group is phenyl, CO2R (ester), CN, or another relatively electronegative group, the aryl halide tends to react at the un- substituted carbon; that is, the product is R CHACH Ar, usually the E (or trans) stereoisomer. When R alkyl, mixtures of productsL are oftenL observed (Problem 18.17). = PROBLEMS 18.17 When iodobenzene and propene are subjected to the conditions of the Heck reaction, two constitutionally isomeric products are formed. What are they? Why are two products formed? 18.18 What two sets of aryl bromide and alkene starting materials would give the following com- pound as the product of a Heck reaction? H % "C OCH3 C % ( % "H 18.19 The product of a Heck reaction is, like the starting material, an alkene. Why doesn’t a Heck reaction of the product compete with the reaction of the starting alkene? 18.20 What product is expected when cyclopentene reacts with iodobenzene in the presence of tri- ethylamine and a Pd(0) catalyst? B. The Suzuki Coupling The Suzuki–Miayura coupling reaction (usually referred to as the Suzuki reaction or the Suzuki coupling) is a Pd(0)-catalyzed process in which an aryl or vinylic boronic acid (a compound of the form RB(OH)2, where R an aryl or vinylic group) is coupled to an aryl or vinylic iodide or bromide in the presence of= a base, which is in many cases aqueous sodium hydroxide or sodium carbonate. The reaction can be used to prepare three types of com- pounds: biaryls—compounds in which two aryl rings are connected by a s bond; aryl-substi- tuted alkenes; and conjugated alkenes. Eq. 18.47 illustrates the preparation of a biphenyl. Pd(OAc)2 (0.3 mole %) PPh3 Na2CO3 NaOH + B(OH)2 + Br CH O propanol–water phenylboronic acid p-bromobenzaldehyde CH O ++Na Br B(OH)3 (18.47) boric acid 4-phenylbenzaldehyde (a biphenyl; 86% yield) The Pd(0) catalyst can be Pd(PPh3)4, the same catalyst used in the Heck reaction, or the Pd(0) can be formed in the reaction flask from Pd(OAc)2, a strategy that is also used in the Heck re- action, as in the preceding example. Eq. 18.48 shows the preparation of an aryl-substituted alkene. 18_BRCLoudon_pgs4-3.qxd 11/26/08 9:09 AM Page 849 18.6 EXAMPLES OF TRANSITION-METAL-CATALYZED REACTIONS 849 HH H H Pd(PPh3)4 B(OH)2 (4.5 mole %) CC KOH + CC (18.48) H2O/THF CH3 CH3O Br CH3 4-methoxybenzene- (Z)-1-bromo-1-propene boronic acid CH3O (Z)-1-methoxy-1-propenylbenzene (cis-anethole; 62% yield) As this example illustrates, the coupling occurs with retention of alkene stereochemistry.
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