Transition metal-catalyzed direct functionalization of C–H bonds is one of the key emerging strategies that is currently attracting tremendous attention with the aim to provide alternative environmentally friendly and efficient ways for the construction of C–C and C–hetero bonds. In particular, the strategy involving regioselective C–H activation assisted by various functional groups shows high potential, and significant achievements have been made in both the development of novel reactions and the mechanistic study. In this review, we attempt to give an overview of the development of utilizing the functionalities as directing groups. The discussion is directed towards the use of different functional groups as directing groups for the construction of C–C and C–hetero bonds via C–H activation using various transition metal catalysts. The synthetic applications and mechanistic features of these transformations will be discussed, and the review is organized on the basis of the type of directing groups and the type of bond being formed or the catalyst.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

Ruthenium-Catalyzed Reactions for Organic Synthesis.

Alkylation reactions represent an important organic transformation to form C–C bonds. In addition to conventional approaches with alkyl halides or sulfonates as alkylating agents, the use of unactivated olefins for alkylations has become attractive from both cost and sustainability viewpoints. This Review summarizes transition-metal-catalyzed alkylations of various carbon–hydrogen bonds (addition of C–H bonds across olefins) using regular olefins or 1,3-dienes up to May 2016. According to the mode of activation, the Review is divided into two sections: alkylation via C–H activation and alkylation via olefin activation.

Transition-Metal-Catalyzed C–H Alkylation Using Alkenes

Transition metal-catalyzed C-H functionalization has evolved into a prominent and indispensable tool in organic synthesis. While nitrogen, phosphorus and sulfur-based functional groups (FGs) are widely employed as effective directing groups (DGs) to control the site-selectivity of C-H activation, the use of common FGs (e.g. ketone, alcohol and amine) as DGs has been continuously pursued. Ketones are an especially attractive choice of DGs and substrates due to their prevalence in various molecules and versatile reactivity as synthetic intermediates. Over the last two decades, transition metal-catalyzed C-H functionalization that is directed or mediated by ketones has experienced vigorous growth. This review summarizes these advancements into three major categories: use of ketone carbonyls as DGs, direct β-functionalization, and α-alkylation/alkenylation with unactivated olefins and alkynes. Each of these subsections is discussed from the perspective of strategic design and reaction discovery.

Transition metal-catalyzed ketone-directed or mediated C–H functionalization

The direct functionalization of C–H bonds by formation of a C–C bond is an interesting and important alternative to coupling reactions involving halide derivatives. These reactions are complex but during the past few years a number of new systems have been developed which display promising scope. This microreview will briefly mention the different mechanisms leading to C–H activation and describe the new systems leading to the catalytic formation of carbon–carbon bonds.

Catalytic Formation of Carbon–Carbon Bonds by Activation of Carbon–Hydrogen Bonds

Polymers [Ph(COMe)CH 2 CH 2 Si(Me) 2 -R-Si(Me) 2 CH 2 CH 2 ] n (R= O, CH 2 CH 2 ) have been obtained by ruthenium-catalyzed polyaddition of acetophenone with the corresponding divinyl compounds

Synthesis of High Molecular Weight Copolymers by Ruthenium-Catalyzed Step-Growth Copolymerization of Acetophenone with .alpha.,.omega.-Dienes

This paper reports a novel ruthenium catalyzed regioselective copolymerization reaction between acetophenone and α,ω-dienes such as divinyltetramethyldisiloxane and divinyldimethylsilane which leads respectively to copoly(3,3,5,5-tetramethyl-4-oxa-3,5-disila-1,7-heptanylene/2-acetyl-1,3-phenylene) and copoly(3,3-dimethyl-3-sila-1,5-pentanylene/2-acetyl-1,3-phenylene). This reaction involves the ruthenium catalyzed insertion of the carbon-carbon double bonds of α,ω-dienes into the aromatic C−H bondsortho to the acetyl group of acetophenone. Similar ruthenium catalyzed reactions between acetophenone and alkenes to yield monomericortho alkyl substituted acetophenones have been recently reported.1

Ruthenium catalyzed regioselective copolymerization of acetophenone and α,ω-dienes

This paper reports a novel ruthenium catalyzed regioselective copolymerization reaction between anthrone, fluorenone or xanthone and α,ω-dienes such as 1,3-divinyltetramethyldisiloxane and 3,3,6,6-tetramethyl-3,6-disila-1,7-octadiene. This reaction involves the ruthenium catalyzed regioselective Anti-Markovnikov insertion of the carbon-carbon double bonds of the α,ω-dienes into the aromatic carbon-hydrogen bonds which are ortho to the carbonyl group of anthrone, fluorenone or xanthone. Similar ruthenium catalyzed copolymerization reactions between acetophenone and α,ω-dienes [1] have been recently reported as have reactions between acetophenone and alkenes to yield monomeric ortho-alkyl substituted acetophenones [2].

Ruthenium catalyzed regioselective copolymerization of anthrone, fluorenone, or xanthone with α,ω-dienes

Ruthenium-catalyzed step-growth copolymerization ofp-(dialkylamino)acetophenones and α,ω-dienes such as 1,3-divinyltetramethyldisiloxane gives copolymers which have respectable molecular weights. The synthesis and characterization of these copolymers is reported.

Ruthenium-Catalyzed Regioselective Step-Growth Copolymerization of p-(Dialkylamino)acetophenones and .alpha.,.omega.-Dienes

Chemical modification of unsaturated polysiloxanes with pendant or terminal vinyl groups and polycarbosilanes with pendant Si-vinyl groups has been achieved by a novel ruthenium catalyzed reaction. Dihydridocarbonyltris(triphenylphosphine)-ruthenium catalyzes the addition of the ortho C−H bond of 2′-methylacetophenone across the C−C double bond of pendant or terminal vinyl groups of polymers such as copoly(dimethylsiloxane/vinylmethylsiloxane), polydimethylsiloxane which is terminated with vinyldimethylsiloxy groups and copoly(vinylmethylsilylene/1,4-phenylene).

Hongjie Guo

Papers

Synthesis of High Molecular Weight Copolymers by Ruthenium-Catalyzed Step-Growth Copolymerization of Acetophenone with .alpha.,.omega.-Dienes

Ruthenium catalyzed regioselective copolymerization of acetophenone and α,ω-dienes

Ruthenium catalyzed regioselective copolymerization of anthrone, fluorenone, or xanthone with α,ω-dienes

Ruthenium-Catalyzed Regioselective Step-Growth Copolymerization of p-(Dialkylamino)acetophenones and .alpha.,.omega.-Dienes

Ruthenium catalyzed chemical modification of unsaturated polymers