Carbon-hydrogen bond functionalization is a variant of reaction in which Carbon–hydrogen bond is displaced with a carbon–X bond (X being Carbon, Oxygen, or Nitrogen). This term commonly indicates that a transition metal is instrumental in the cleavage process. When this occurs, it typically implies that the hydrocarbon was the first to react primarily to a metal catalyst to produce an organometallic complex in which the hydrocarbon is directed to the innermost part of the metal, either via an intervening “alkane or Arene complex” or as a gradation state that leads to an “M−C” intermediate. This is the preliminary stage usually called the C-H activation and sometimes addressed as C-H functionalization, which can then feature in the resulting reactions to create a functionalized product. The most vital of this description is the prerequisite that, as the C-H cleavage occurs, the hydrocarbyl species continues to be merged with the inner-sphere.
A duplicate description, as scholars who have utilized the term named it, means any organic transformation whereby the end result in the subsequent change of a relatively stationary C–H bond into a functional group, notwithstanding the nature of the system, or with the singular mind towards it. Particularly, this description does not entail the dispensation of a transition metal to the hydrocarbon in the reaction simulation.
In fact, this enlarged definition contains a much-subsumed one in its background. Yet, it would comprise an iron-catalyzed alkane functionalization reaction that moves across the oxygen rebound mechanism such as cytochrome P450 enzymes and their synthetic analogs), where a metal and carbon combination is not portrayed as being included.
Besides, the ligand-based involvement of many metal carbene species alongside hydrocarbons would be included with this classification even though some are mechanically vague. A sizeable sum of the divide has made C-H activation to mean the hydrocarbon cleaving step of any bond that becomes the functionalization of a hydrocarbon group (or any observable matter of C–H bond cleavage, like H/D exchange). Still, others retain the original stiff description of the term C–H activation, while using C–H functionalization in its encompassing definition.
History and Scholarly Perspectives
The pioneering idea of the C–H activation reaction is often linked to a German chemist, Otto Dimroth (28 March 1872 – 16 May 1940). In 1902, he documented that Benzene reacted with mercury(II) acetate and that a lot of electrophilic metal centers observe this Friedel-Crafts-like reaction.
Joseph Chatt, (6 November 1914 – 19 May 1994), a well-known British researcher in the area of inorganic and organometallic chemistry, observed the addition of C-H bonds of naphthalene by Ru(0) complexes, which indicates that Chelation-assisted C-H activations are prevalent.
In 1969, it was introduced into the chemistry sphere that potassium tetrachloroplatinate induced isotope scrambling between methane and heavy water. This was the work of Alexander Shilov (January 1, 1930 – June 6, 2014) a Russian chemist who further proposed a pathway that was to consist of the binding of methane to Pt(II). Subsequently, in 1972, what was later known as the Shilov group was able to procreate methanol and methyl chloride in like reactions comprising of a stoichiometric quantity of potassium tetrachloroplatinate, catalytic potassium hexachloroplatinate, methane, and water.
Yet, as a result of the fact that the work of Shilov was publicized in the Soviet Union during the Cold War era, his theory was generally disregarded by Western scientists. This system is one of the few grass-root catalytic systems for alkane functionalization.
Besides, as it has been proven by history that some revelations in C-H activation were being made in partnership with those of cross-coupling. Yuzo Fujiwara reported, in 1969, that the synthesis of (E)-1,2-diphenylethene from benzene and styrene with Pd(OAc) 2 and Cu(OAc) 2, is a procedure very similar to that of cross-coupling.
On the category of oxidative addition, Malcolm Leslie Hodder Green (FRS FRSC), Emeritus Professor of inorganic chemistry at the University of Oxford, in 1970 reported on the Photochemical insertion of tungsten (as a Cp2WH 2 complex) in a benzene C–H bond and George M. Whitesides in 1979 was the pioneer of the intramolecular aliphatic C–H activation
Moreover, Robert George Bergman also documented the first transition metal-mediated intermolecular C–H activation of unactivated and generally saturated hydrocarbons by oxidative addition. Using a Photochemical approach, photolysis of Cp*Ir(PMe 3)H 2, where Cp* is a Pentamethylcyclopentadienyl ligand, capitalizing to the coordinately unsaturated species Cp*Ir(PMe 3) which reacted via oxidative addition with Cyclohexane and Neopentane to form the corresponding hydridoalkyl complexes, Cp*Ir(PMe 3)HR, where R = cyclohexyl and neopentyl, respectively.
It was discovered that the reaction of the same hydrocarbon with Cp*Ir(CO) 2 upon irradiation to afford the related alkylhydrido complexes Cp*Ir(CO)HR, where R = cyclohexyl and neopentyl, respectively. Following the latter reaction, it is presumed to begin via the oxidative summation of alkane to 16-electron iridium(I) intermediate, Cp*Ir(CO), established by irradiation of Cp*Ir(CO) 2.
Mechanisms for Carbon-hydrogen bond functionalization can be grouped into three conclusive segments:
(i) Oxidative addition, whereby a low-valent metal center inserts into a C-H bond, which splits open the bond and dissolves the metal. Here is the symbol of its reaction L n M + RH → L n MR(H)
(ii) Electrophilic activation in which an electrophilic metal confronts the hydrocarbon, relocate a proton. It is represented like this: L n M+ + RH → L n MR + H+. The underlying substance undertakes an S Ear-type mechanism.
(iii) Sigma-bond metathesis, which advances through a “four-centered” transition state in which bonds dissolve and manifest in a single step. It is represented like this: L n MR + R’H → L n MR’ + RH
Carbon-hydrogen bond functionalization is a segment of successive development that will keep pushing the limits of chemical reactions to the extent that C–H bonds may soon be pointed out as omnipresent functional groups. Such an ability to upturn a species of C–H bonds, brings about new worldviews complex organic synthesis.
The direct plans for arranging molecules are readily shown from a geometrical viewpoint and should direct the viral simplification of synthetic sequences. Unhinging the reaction of omnipresent bonds will also affect the pathway of molecular design in diverse characters of research because pioneering synthetic processes could affect how we make the materials we might need.
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