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Transition Metal Catalyzed Alkylation at Sp3 , Sp2 , and Sp Carbons

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Transition Metal Catalyzed Alkylation at Sp3 , Sp2 , and Sp Carbons Transition Metal Catalyzed Alkylation at sp 3─ , sp 2─ , and sp ─ Carbons Nobuaki Kambe, 1 Jun Terao, 2 and Takanori Iwasaki 1 1 Department of Applied Chemistry, Faculty of Engineering, Osaka University Suita, Osaka 565 0871, Japan 2 ─ Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University Nishikyo ─ ku, Kyoto 615 ─ 8510, Japan (Received August 19, 2011; E ─ mail: [email protected]) Abstract: Transition metal catalyzed alkylation reactions resulting in cross ─ coupling and multicomponent ─ coupling are described. These reactions proceed efciently under mild conditions using a combination of tran- sition metal catalysts and alkyl (pseudo)halides in the presence of Grignard reagents and represent the practi- 3 2 cal routes to the alkylation of sp ─ , sp ─ , and sp ─ carbons. Anionic transition metal complexes play important roles as active catalytic species for S N2 and electron transfer processes in reactions with alkyl halides. The car- bometalation of C ─ C unsaturated bonds with alkylmetal species is also a promising tool for the introduction of alkyl groups. alkyl halides containing a β ─ hydrogen resulted in homocou- 1． Introduction pling and/or disproportionation with the formation of only 4 Transition metal catalyzed cross ─ coupling and multicom- small amounts of cross ─ coupling products (Scheme 1). ponent ─ coupling reactions have evolved as powerful tools for carbon ─ carbon bond formation, and have routinely been Scheme 1. Metal ─ catalyzed reaction of ArMgX with organohalides. applied to the synthesis of functional materials and bioactive compounds as well as various synthetic intermadiates. 1 In these synthetic manipulations of organic compounds pro- moted by transition metals, bond disconnection and reconnec- tion usually takes place on unsaturated carbons such as those Even after the breakthrough development of novel cata- found on aryl, alkenyl, and alkynyl substrates. Alkyl halides lytic systems for the cross ─ coupling of organo(pseudo)halides 5 and pseudo halides (Alkyl ─ X) are readily available and versa- with organometallic reagents in the 1970s, alkyl halides were tile alkylating reagents but have not often been used in transi- not yet recognized as suitable reagents for cross ─ coupling, due tion metal catalyzed reactions, for the following major reasons. to the reasons mentioned above, except for the case of Cu. The oxidative addition of alkyl halides to transition metals is During the past decade, however, remarkable progresses have 2 slower and less efcient than the cases of aryl and vinyl halides been achieved in the eld of cross ─ coupling using alkyl halides and, probably more importantly, β ─ hydrogen (or a hetero- by developing efcient catalytic systems that employ new atom) elimination from the alkylmetal intermediates that are ligands. 6 formed in catalytic cycles readily takes place to give olens. 2.1 Ni and Pd Catalyzed Cross ─ coupling Reactions Electron transfer from a metal to alkyl halides and homolytic About a decade ago, we found that Ni catalyzes the cross ─ M ─ C bond cleavage of alkylmetal intermediates lead to dis- coupling of Grignrad reagents with alkyl halides in the pres- proportionation and homocoupling. Furthermore, the reduc- ence of 1,3 ─ butadiene as an additive without the need for tive elimination of alkyl groups on metals is slower than that phosphine ligands. For example, the reaction of n ─ decyl bro- 2 3 for (sp )C units. mide with n ─ butyl Grignard reagent proceeded efciently in The present article summarizes our investigations into the presence of isoprene and a catalytic amount of NiCl 2 in 7 transition metal catalyzed carbon ─ carbon bond forming reac- THF at 25 ℃ to give tetradecane in high yield (eq. 1). In the tions involving an alkyl group(s) as the reacting partner(s), absence of isoprene, reduction and elimination predominated which include not only cross ─ coupling but also the alkylation and signicant amounts of decane and decenes were produced at unsaturated carbons of alkenes, dienes, allenes, alkynes etc. in the reactions. Unsubstituted 1,3 ─ butadiene showed a higher 3 activity for this cross ─ coupling reaction. 2． Alkylation at sp Carbons by Cross coupling Reactions Using Alkyl Hali─des ─ Alkyl halides and Grignard reagents have been widely employed in organic reactions as carbon electrophiles and nucleophiles, respectively. The chemical behaviors of transition metals in reactions between these two reagents have been extensively studied in the 1940s as the pioneering work by Kharasch and co ─ workers. They revealed that ArMgBr reagents could be cross ─ coupled with vinyl halides in the pres- ence of Co, Cr, or Cu salts as the catalysts, however the use of Vol.69 No.11 2011 （ 77 ） 1271 有機合成化学69-11＿10論文_Kambe.indd 77 2011/10/20 16:40:48 This reaction exhibited an interesting chemoselectivity in an allyl moiety. Thus, this catalytic cycle proceeds via Ni(II) ─ 2 16 which the (sp )C ─ Br bond survived intact in the present system Ni(IV) complexes having 16 electrons. (eq. 2). Alkyl chlorides and tosylates also underwent this The cross ─ coupling of Grignard reagents with alkyl bro- 3 cross ─ coupling reaction, giving rise to the desired products in mides and tosylates was examined using various η ─ allylnickel 8 3 good yields (eq. 3). Even alkyl uorides could be cross ─ cou- and η ─ allylpalladium complexes as catalysts and the yields pled with alkyl Grignard reagents in the presence of bisdienes were shown in eq. 6. 17 When nickel and palladium complexes 9,10 1 which functioned more efciently than butadiene (eq. 4). containing one allyl ligand, such as (C 3H 5NiCl) 2 (6b) and Interestingly, the relative rates of cross ─ coupling decreased in (C 3H 5PdCl) 2 (6e), were used, MeMgBr was coupled with decyl the order R ─ Br＞R ─ F＞R ─ Cl, demonstrating the potent syn- bromide to give the expected cross ─ coupling products in good thetic utility of alkyl uorides as alkylating reagents in transi- yields, whereas the use of n BuMgCl resulted in the reduction tion metal chemistry. The strong interaction between the leav- and elimination of decyl bromides, probably due to the β ─ ing F anion and the Mg cation at the transition state in elimination of the allyl(butyl)metal intermediates. Nickel and 3 comparison with heavier halides provides a reasonable expla- palladium complexes containing no η ─ allyl ligand (6c and 6f) 3 nation for the high reactivity of alkyl uorides (vide infra). were not effective. Bis(η ─ allyl)nickel and ─ palladium com- Palladium also catalyzes the cross ─ coupling reaction of alkyl plexes (6a and 6d) exhibited excellent catalytic activities for tosylates and bromides with Grignard reagents in the presence these alkyl ─ alkyl coupling reactions. These results support the 11 of 1,3 ─ butadiene. catalytic pathway depicted in Scheme 2 and indicate that both Bisdiene 2 was also effective for the Ni ─ catalyzed cross ─ allyl groups on the metal play important roles but the ethylene coupling reaction of organozinc reagents with alkyl halides tether (CH 2CH 2) connecting two allyl ligands of 3 in Scheme 2 12 (eq. 5). This catalytic system tolerates a wide variety of func- is not essential. Ni complexes were found to catalyze cross ─ tional groups, including nitriles, ketones, amides, and esters. 13 coupling at lower temperatures and showed a higher perfor- mance than the corresponding Pd complexes. Theoretical calculations were performed based on the pathway shown in Scheme 2 using bis(allyl)Ni complexes (3, 6a, 6g), a Me anion instead of Grignard reagents (RMgX), and MeCl. 18 From the calculated free energy changes for each process along the reaction pathway it was proposed that the oxidative addition step (i.e., 4→5 in Scheme 2) was likely to be the rate determining step. It was suggested that allyl ligands inuence the reaction rates of this process. Actually the free energy changes for the oxidative addition of MeCl to nickelate complexes 4a, 7a, 7g leading to the corresponding (allyl) 2NiMe 2 decrease in the order 4a＞7a＞7g, indicating that a strained Scheme 2. Catalytic cycle for Ni ─ catalyzed cross ─ coupling reactions using alkyl halides via an η 1,η 3─ octadienediylnickel com- plex. A plausible reaction pathway for this Ni catalyzed cross ─ coupling is depicted in Scheme 2. Ni(0), formed by the reduc- tion of NiCl 2 with Grignard reagents, reacts with 2 moles of 14 1,3 ─ butadiene to afford the bis(π ─ allyl)nickel complex 3, 1 3 which reacts with Grignard reagents to form the anionic η ,η ─ octadienediylnickel complex 4. 15 This complexation might enhance the nucleophilicity of Ni toward alkyl halides. Cou- pling products are formed by the nucleophilic substitution of alkyl halides on the nickel of 4 yielding the dialkylnickel com- plex 5, followed by reductive elimination. 1,3 ─ Butadienes play an important role in the conversion of Ni(0) to Ni(II), which is inert toward oxidative addition with organic halides but reacts readily with R ─ MgX to form anionic complex 4, and sup- presses the β ─ hydrogen elimination process by occupying the 1 3 coordinating site on the nickel via the dynamic η ─ η shift of Figure 1. Theoretical calculations of bis(allyl)Ni intermadiates. 1 272 （ 78 ） J. Synth. Org. Chem., Jpn. 有機合成化学69-11＿10論文_Kambe.indd 78 2011/10/20 16:40:56 form of 7g is the most reactive for this step. Interestingly, 4a LFNO showed a slightly higher reactivity than LFPO. The 1 3 3 3 and 7g possess η ─ ,η ─ coordination of allyl ligands but η ─ ,η ─ present coupling reaction was found to be catalyzed by trace coordination seems possible for 7a probably due to its high amounts of Ni or Pd that had been leached from perovskites exibility. The reductive elimination of ethane from with a very high TON (ca. 10 7). (allyl) 2NiMe 2 is exergonic and should be a rapid process.
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