Olefins coupling

The selective intramolecular arene C-H/olefin coupling reaction (Murai reaction) represents one of the most important discoveries in catalytic C-H functionalization chemistry (Equation (94)). 

The relatively low mass balances of the reactions with vinylsilane terminating groups raised questions about the overall utility of vinylsilane groups with the anodic olefin-coupling reaction. Were the low mass balances due to an... 

TABLE 6 Product Distribution Upon Photolysis of at-Alkyl Dibenzylketones in Zeolites Yields of Olefin, Coupling Product (AB) and Rearrangement Product 7 (see Scheme l)a b... 

As discussed in connection with olefin-coupling reactions and shown in Fig. 4, the coupling of vinyl Grignard reagents is stereospecific and dependent upon the transition metal catalyst used (32, 33). The dimerization of ethylene, shown in Fig. 6, was also shown to produce primarily the terminal olefin 1-butene (35). The size of the metal has also been shown to influence the course of the catalyzed oligomerization reactions of butadiene. When bis-(ir-allyl) metal complexes are used as... 

Intramolecular anodic olefin coupling reactions involving allyl- (equation 18) and vinyl-silanes (equation 19) can lead to good yields of quaternary carbons with control of the relative stereochemistry19,20. This is the first example of an electrochemical reaction that makes use of a temporary silicon tether. 

In 1986, Belotti, Pete and Portella reported that intramolecular ketone/olefin coupling could be achieved via photoinduced electron transfer (irradiation of an aliphatic ketone in HMPA)78. Several examples of this chemistry are highlighted below (Scheme 26). Intramolecular additions to C=C and allenes were also reported with yields in the range 70-80% however, additions to C=N were unsuccessful. 

A number of intramolecular ketyl anion/olefin coupling reactions promoted by Sml2 have been reported since 1985. In general, Sml2 reactions give extremely high yields and exhibit high diasteroselectivity. 

Several examples of intramolecular ketyl anion/unactivated olefin coupling reactions were reported by the Molander group, one of which is illustrated in equation 2884. An interesting facet of this reaction is that it is possible to further react the cyclized samarium(III) intermediate (47) with a variety of electrophiles, thereby enhancing the... 

Analogous intramolecular chelation-controlled ketone/olefin couplings with Sml2, in which Sm+3 was complexed in a cyclic manner to the ketyl anion and a /1-carbonyl of an ester or amide functionalilty, were reported as early as 1987 (Scheme 31)86. The cyclized samarium intermediate 49 could be further reacted with added electrophiles such... 

For additional examples of ketone/olefin coupling reactions promoted by Sml2, see ... 

Scheme 1. The proposed reaction pathway of the ruthenium-catalyzed C-H/olefin coupling by means of the chelation-assistance.

This C-H/olefin coupling can be extended to coupling with acetylenes [6], The reaction of aromatic ketones with internal acetylenes gives the ortho alkenylated product in high yield (Scheme 2), but reaction with terminal acetylenes does not afford the coupling product. With terminal acetylenes, dimerization of acetylenes occurs as a predominant reaction. 

Scheme 3. Proposed stepwise pathway for the ruthenium-catalyzed C—H/olefin coupling.

In (C5Me5)Rh(C2H3SiMe3)2-catalyzed C-H/olefin coupling the effect of the coordination of the ketone carbonyl is different from that in the ruthenium-catalyzed reaction [10], In the rhodium-catalyzed reaction all C-H bonds on the aromatic ring are cleaved by the rhodium complex without coordination of the ketone carbonyl. Thus, C-H bond cleavage and addition of Rh-H to olefins proceed without coordination of the ketone carbonyl. After addition of the Rh-H species to the olefin, a coordinatively unsaturated Rh(aryl) (alkyl) species should be formed. Coordination of the ketone carbonyl group to the vacant site on the rhodium atom leads... 

Although there are several examples of transition metal-catalyzed addition of C-H bonds to acetylenes, there is neither experimental evidence nor theoretical consideration in respect of a reaction mechanism for C-H/acetylene coupling. This coupling reaction is believed to proceed through a pathway similar to that proposed for C-H/olefin coupling. 

The ruthenium-catalyzed addition of C-H bonds in aromatic ketones to olefins can be applied to a variety of ketones, for example acetophenones, naphthyl ketones, and heteroaromatic ketones. Representative examples are shown in the Table 1. Terminal olefins such as vinylsilanes, allylsilanes, styrenes, tert-butylethy-lene, and 1-hexene are applicable to this C-H/olefin coupling reaction. Some internal olefins, for example cyclopentene and norbornene are effective in this alkylation. The reaction of 2-acetonaphthone 1 provides the 1-alkylation product 2 selectively. Alkylations of heteroaromatic ketones such as acyl thiophenes 3, acyl furans, and acyl pyrroles proceed with high yields. In the reaction of di- and tri-substitued aromatic ketones such as 4, which have two different ortho positions, C-C bond formation occurs at the less congested ortho position. Interestingly, in the reaction of m-methoxy- and m-fluoroacetophenones C-C bond formation occurs at the congested ortho position (2 -position). 

Several related examples of transition metal-catalyzed addition of C-H bonds in ketones to olefins have been reported (Table 2) [11-14]. The alkylation of diterpenoid 6 with olefins giving 7 proceeds with the aid of Ru(H)2(CO)(PPh3)3 (A) or Ru(CO)2(PPh3)3 (B) as catalyst [11], Ruthenium complex C, Ru(H)2(H2)(CO) (PCy3)2, has catalytic activity in the reaction of benzophenone with ethylene at room temperature [12]. The alkylation of phenyl 3-pyridyl ketone 8 proceeds with A as catalyst [13], Alkylation occurs selectively at the pyridine ring. Application of this C-H/olefin coupling to polymer chemistry using ce,co-dienes such as 1,1,3,3-tetramethyl-l,3-divinyldisiloxane 11 has been reported [14]. 

The additive-modified catalyst mixture developed for intramolecular olefin coupling is sufficiently active to catalyze the analogous intermolecular reaction. A variety of five-membered N-heterocydes can be catalytically alkylated at the C-2 position (Table 2) [8], Additional functional groups on the heterocyde[15] (Table 2, Entry 1) and on the olefin (Table 3) are well tolerated. Products corresponding to linear addition are usually obtained exclusively, even when the olefin is rapidly isomerized under the reaction conditions. 

C-H bonds. In this section, among the transition-metal-catalyzed C-H/olefin coupling, ruthenium-catalyzed reactions will mainly be discussed. 

Chelation-assisted C-H/olefin coupling can be applied to the atroposelective alkylation of biaryl compounds. The reaction of 2-(l-naphthyl)-3-methylpyri-dine with ethylene using [RhCl(coe)2]2 where coe is cyclooctene, and PCy3 results in the formation of an ethylation product in 92% yield (Eq. 21) [20]. In place of the PCy3 ligand, the use of (R) - (1 - [ (S) - 2- diphenylphosphino ] ferro-cenyl)ethyl methyl ether [(R),(S)-PPFOMe] leads to the atropselective alkyla-... 

The C-H/olefin coupling of aryloxazolines proceeds with unusual product selectivity. In this case, alkylation products, i.e., formally dehydrogenation products, are obtained as a major product (Eq. 22) [11]. These types of dehydrogenation compounds are believed to be formed via a carbometalation pathway. The first example of this type of alkenylation of arenes with olefins using palladium(II) complexes via C-H bond cleavage was reported in 1967 [32]. Later, several efforts were made to perform this reaction in a catalytic manner [33]. In 2001, Milstein et al. [34] reported the oxidative alkenylation of arenes with olefins using a Ru/02/C0 catalyst system (Eq. 23). Details of the reaction mechanism have not been elucidated. 

Scheme 1 Possible reaction pathways for the C-H/olefin coupling...

These reactions can be applied to an acyclic system [66]. When reactions of tran5-4,4-dimethyl-l-phenyl-l-peneten-3-one with styrene and tri-ethoxyvinylsilane are conducted using RuH2(CO)(PPh3)3 as the catalyst, the expected olefinic C-H/olefin coupling products are obtained in good yields (Eq. 39) [66]. 

Intramolecular olefinic C-H/olefin coupling with the aid of Ru(CO)3(PPh3)2, which is also effective for the reaction of aromatic ketones with olefins, yields the carbocyclic compounds in excellent yield (Eq. 42) [67]. This type of cy-clization reaction can be extended to an asymmetric version when the [RhCl(coe)2]2/PPFOMe catalyst system is employed [68]. 

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