Rhodium-catalyzed intermolecular C-H silylation of arenes with high steric regiocontrol

Why You Really Should Consider Using Palladium-Catalyzed Cross-Coupling of Silanols and Silanolates

The transition metal-catalyzed cross-coupling of organometallic nucleophiles derived from tin, boron, and zinc with organic electrophiles enjoys a preeminent status among modern synthetic methods for the formation of carbon-carbon bonds. In recent years, organosilanes have emerged as viable alternatives to the conventional reagents, with the added benefits of low cost, low toxicity and high chemical stability. However, silicon-based cross-coupling reactions often require heating in the presence of a fluoride source, which has significantly hampered their widespread acceptance. To address the "fluoride problem", a new paradigm for palladium-catalyzed, silicon-based cross-coupling reactions has been developed that employs a heretofore underutilized class of silicon reagents, the organosilanols. The use of organosilanols, either in the presence of Brønsted bases or as their silanolate salts, represents an operationally simple and mild alternative to the fluoride-based activation method. Organosilanols are readily available by many well-established methods for introducing carbon-silicon bonds onto alkenes, alkynes, arenes and heteroarenes. Moreover, several different protocols for the generation of alkali metal salts of, vinyl-, alkenyl-, alkynyl-, aryl-, and heteroarylsilanolates have been developed and the advantages of each of these methods have been demonstrated for a number of different coupling classes. This review will describe the development and implementation of cross-coupling reactions of organosilanols and their conjugate bases, silanolates, with a wide variety of substrate classes. In addition, application of these transformations in the total synthesis of complex natural products will also be highlighted. Finally, the unique advantages of organosilicon coupling strategies vis a vis organoboron reagents are discussed.

Pd(ii)-catalyzed remote regiodivergent ortho- and meta-C-H functionalizations of phenylethylamines

Site selectivity control is of vital importance in the direct functionalization of inert C-H bonds. Reported here is a novel example of remote regiodivergent ortho- and meta-C-H bond functionalizations of phenylethylamine derivatives by using a novel 2-cyanobenzoyl group as the original directing functionality, where the regioselectivity was adjusted by a methylation. The potential of the method was exemplified by sequential functionalizations of both ortho- and meta-C-H bonds of a phenylethylamine derivative in a streamlined manner.

Selective Aromatic C-H Hydroxylation Enabled by η6-Coordination to Iridium(III)

We report an aromatic C-H hydroxylation protocol in which the arene is activated through η⁶-coordination to an iridium(III) complex. η⁶-Coordination of the arene increases its electrophilicity and allows for high positional selectivity of hydroxylation at the site of least electron density. Through investigation of intermediate η⁵-cyclohexadienyl adducts and arene exchange reactions, we evaluate incorporation of arene π-activation into a catalytic cycle for C-H functionalization.

Remote para-C-H Functionalization of Arenes by a D-Shaped Biphenyl Template-Based Assembly

Site-selective C-H functionalization has emerged as an efficient tool in simplifying the synthesis of complex molecules. Most often, directing group (DG)-assisted metallacycle formation serves as an efficient strategy to ensure promising regioselectivity. A wide variety of ortho- and meta-C-H functionalizations stand as examples in this regard. Yet despite this significant progress, DG-assisted selective para-C-H functionalization in arenes has remained unexplored, mainly because it involves the formation of a geometrically constrained metallacyclic transition state. Here we report an easily recyclable, novel Si-containing biphenyl-based template that directs efficient functionalization of the distal p-C-H bond of toluene by forming a D-shaped assembly. This DG allows the required flexibility to support the formation of an oversized pre-transition state. By overcoming electronic and steric bias, para-olefination and acetoxylation were successfully performed while undermining o- and m-C-H activation. The applicability of this D-shaped biphenyl template-based strategy is demonstrated by synthesizing various complex molecules.

Ligand-Enabled Meta-C-H Alkylation and Arylation Using a Modified Norbornene

2-Carbomethoxynorbornene is identified as a more effective transient mediator to promote a Pd(II)-catalyzed meta-C(sp(2))-H alkylation of amides with various alkyl iodides as well as arylation with previously incompatible aryl iodides. The use of a tailor-made quinoline ligand is also crucial for this reaction to proceed.

Synthesis and structure of rhodium(i) silyl carbonyl complexes: photochemical C-F and C-H bond activation of fluorinated aromatic compounds

The rhodium(i) silyl carbonyl complexes [Rh{Si(OEt)3}(CO)(dippp)] () and [Rh{Si(OEt)3}(CO)(dippe)] () (dippp = 1,3-bis(diisopropylphosphino)propane, dippe = 1,2-bis-(diisopropylphosphino)ethane) were synthesized on treatment of the methyl compounds [Rh(CH3)(CO)(dippp)] () or [Rh(CH3)(CO)(dippe)] () with HSi(OEt)3 at low temperature. The methyl complexes and were prepared starting from the binuclear complexes [{Rh(μ-Cl)(dippp)}2] () and [{Rh(μ-Cl)(dippe)}2] (), respectively. The silyl complexes and as well as the precursors [{Rh(μ-I)(dippp)}2] (), [Rh(X)(CO)(dippp)] (: X = CH3, : X = I) and [Rh(X)(CO)(dippe)] (: X = CH3, : X = Cl) were characterized by NMR and IR spectroscopy and the structures in the solid state were determined by X-ray crystallography. The silyl complex converts into the carbonyl-bridged complex [{Rh(μ-CO)(dippp)}2] () above temperatures of -30 °C by loss of the silyl ligand, whereas is more thermally stable and a reaction to the binuclear complex [{Rh(μ-CO)(dippe)}2] () was observed at 50 °C. The silyl complex reacted under irradiation with hexafluorobenzene and pentafluoropyridine to give the C-F activation products [Rh(C6F5)(CO)(dippe)] () and [Rh(2-C5F4N)(CO)(dippe)] (), respectively. As additional products the silyl dicarbonyl complex [Rh{Si(OEt)3}(CO)2(dippe)] () and the cationic complex [Rh2(μ-H)(μ-CO)2(dippe)2](+)[SiF5](-) () were identified. Compound was synthesized independently by treatment of with gaseous CO. In a similar manner, the dippp analogue [Rh{Si(OEt)3}(CO)2(dippp)] () was also prepared starting from . Photochemical reaction of with pentafluorobenzene and 2,3,5,6-tetrafluoropyridine resulted selectively in C-H bond activation to afford and [Rh(4-C5F4N)(CO)(dippe)] (), respectively.

Silylations of Arenes with Hydrosilanes: From Transition-Metal-Catalyzed C¢X Bond Cleavage to Environmentally Benign Transition-Metal-Free C¢H Bond Activation

The construction of carbon-silicon bonds is highlighted as an exciting achievement in the field of organosilicon chemistry and green chemistry. Recent developments in this area will enable the sustainable chemical conversion of silicon resources into synthetically useful compounds. Especially, the catalytic silylation through C¢H bond activation without directing groups and hydrogen acceptors is one of the most challenging topics in organic chemistry and green chemistry. These remarkable findings on catalytic silylation can pave the way to a more environmentally benign utilization of earth-abundant silicon-based resources in synthetic chemistry.

Rhodium-catalyzed enantioselective silylation of arene C-H bonds: desymmetrization of diarylmethanols

We report a Rh-catalyzed, enantioselective silylation of arene C-H bonds directed by a (hydrido)silyl group. (Hydrido)silyl ethers that are formed in situ by hydrosilylation of benzophenone or its derivatives undergo asymmetric C-H silylation in high yield with excellent enantioselectivity in the presence of [Rh(cod)Cl]2 and a chiral bisphosphine ligand. The stereoselectivity of this process also allows enantioenriched diarylmethanols to react with site selectivity at one aryl group over the other. Enantioenriched benzoxasiloles from the silylation process undergo a range of transformations to form C-C, C-O, C-I, or C-Br bonds.

Acceleration Effects of Phosphine Ligands on the Rhodium-Catalyzed Dehydrogenative Silylation and Germylation of Unactivated C(sp(3))-H Bonds

The current work describes the marked rate of acceleration caused by phosphine ligands on the rhodium-catalyzed dehydrogenative silylation and germylation of unactivated C(sp(3))-H bonds. The reactivity was affected by the steric and electronic nature of the phosphine ligands. The use of the bulky and electron-rich diphosphine ligand (R)-DTBM-SEGPHOS was highly effective to yield the dehydrogenative silylation products selectively in the presence of a hydrogen acceptor. An appropriate choice of C2-symmetric chiral diphosphine ligand enables the asymmetric dehydrogenative silylation via the enantioselective desymmetrization of the C(sp(3))-H bond. The unprecedented catalytic germylation of C(sp(3))-H bonds with dehydrogenation was also examined with the combination of the rhodium complex and a wide bite angle diphosphine ligand to provide the corresponding 2,3-dihydrobenzo[b]germoles in good yield.

Modular Approach to Reductive C(sp2)-H and C(sp3)-H Silylation of Carboxylic Acid Derivatives through Single-Pot, Sequential Transition Metal Catalysis

We report a modular approach to catalytic reductive Csp2-H and Csp3-H silylation of carboxylic acid derivatives encompassing esters, ketones, and aldehydes. Choice of either an Ir(I)/Rh(I) or Rh(I)/Rh(I) sequence leads to either exhaustive reductive ester or reductive ketone/aldehyde silylation, respectively. Notably, a catalyst-controlled direct formation of doubly reduced silyl ethers is presented, specifically via Ir-catalyzed exhaustive hydrosilylation. The resulting silyl ethers undergo Csp2-H and benzylic Csp3-H silylation in a single vessel.

para-C-H Borylation of Benzene Derivatives by a Bulky Iridium Catalyst

A highly para-selective aromatic C-H borylation has been accomplished. By a new iridium catalyst bearing a bulky diphosphine ligand, Xyl-MeO-BIPHEP, the C-H borylation of monosubstituted benzenes can be affected with para-selectivity up to 91%. This catalytic system is quite different from the usual iridium catalysts that cannot distinguish meta- and para-C-H bonds of monosubstituted benzene derivatives, resulting in the preferred formation of meta-products. The para-selectivity increases with increasing bulkiness of the substituent on the arene, indicating that the regioselectivity of the present reaction is primarily controlled by steric repulsion between substrate and catalyst. Caramiphen, an anticholinergic drug used in the treatment of Parkinson's disease, was converted into five derivatives via our para-selective borylation. The present [Ir(cod)OH]2/Xyl-MeO-BIPHEP catalyst represents a unique, sterically controlled, para-selective, aromatic C-H borylation system that should find use in streamlined, predictable chemical synthesis and in the rapid discovery and optimization of pharmaceuticals and materials.

Deprotonative C-H silylation of functionalized arenes and heteroarenes using trifluoromethyltrialkylsilane with fluoride

A highly selective C-H silylation reaction of functionalized arenes and heteroarenes was developed using Ruppert-Prakash reagent (TMSCF3) activated by alkali metal fluoride. TMSCF3 is considered to play dual roles as a precursor of a mild base and also as a silicon electrophile. The silylation is compatible with sensitive functional groups such as halogen and nitro groups.

Computational organic chemistry: bridging theory and experiment in establishing the mechanisms of chemical reactions

Understanding the mechanisms of chemical reactions, especially catalysis, has been an important and active area of computational organic chemistry, and close collaborations between experimentalists and theorists represent a growing trend. This Perspective provides examples of such productive collaborations. The understanding of various reaction mechanisms and the insight gained from these studies are emphasized. The applications of various experimental techniques in elucidation of reaction details as well as the development of various computational techniques to meet the demand of emerging synthetic methods, e.g., C-H activation, organocatalysis, and single electron transfer, are presented along with some conventional developments of mechanistic aspects. Examples of applications are selected to demonstrate the advantages and limitations of these techniques. Some challenges in the mechanistic studies and predictions of reactions are also analyzed.

Synthesis and characterization of hexaarylbenzenes with five or six different substituents enabled by programmed synthesis

Since its discovery in 1825, benzene has served as one of the most used and indispensable building blocks of chemical compounds, ranging from pharmaceuticals and agrochemicals to plastics and those used in organic electronic devices. Benzene has six hydrogen atoms that can each be replaced by different substituents, which means that the structural diversity of benzene derivatives is intrinsically extraordinary. The number of possible substituted benzenes from n different substituents is (2n + 2n(2) + 4n(3) + 3n(4) + n(6))/12. However, owing to a lack of general synthetic methods for making multisubstituted benzenes, this potentially huge structural diversity has not been fully exploited. Here, we describe a programmed synthesis of hexaarylbenzenes using C-H activation, cross-coupling and [4+2] cycloaddition reactions. The present method allows for the isolation and structure-property characterization of hexaarylbenzenes with distinctive aryl substituents at all positions for the first time. Moreover, the established protocol can be applied to the synthesis of tetraarylnaphthalenes and pentaarylpyridines.

Iridium-catalyzed silylation of aryl C-H bonds

A method for the iridium-catalyzed silylation of aryl C-H bonds is described. The reaction of HSiMe(OSiMe3)2 with arenes and heteroarenes catalyzed by the combination of [Ir(cod)(OMe)]2 and 2,4,7-trimethylphenanthroline occurs with the aromatic compound as the limiting reagent and with high levels of sterically derived regioselectivity. This new catalytic system occurs with a much higher tolerance for functional groups than the previously reported rhodium-catalyzed silylation of aryl C-H bonds and occurs with a wide range of heteroarenes. The silylarene products are suitable for further transformations, such as oxidation, halogenation, and cross-coupling. Late-stage functionalization of complex pharmaceutical compounds was demonstrated.

Transition metal catalyzed meta-C–H functionalization of aromatic compounds

This review summarizes synthetic methods for directing group guided, transition metal catalyzed meta-C–H functionalization of aromatic compounds.

Direct amidation of the phenylalanine moiety in short peptides via Pd-catalyzed C–H activation/C–N formation

The direct amidation of dipeptides through direct aryl C–H transformation of the phenylalanine residue via Pd catalysis.

Reductive arene ortho-silanolization of aromatic esters with hydridosilyl acetals

The design and application of a single-pot, reductive arene C–H bond silanolization of esters for synthesis of ortho-formyl arylsilanols.

Enantioselective synthesis of planar-chiral benzosiloloferrocenes by Rh-catalyzed intramolecular C–H silylation

The first synthesis of planar-chiral benzosiloloferrocenes was achieved by the intramolecular reaction of 2-(dimethylhydrosilyl)arylferrocenes.

Efficient Rh-catalyzed C–H borylation of arene derivatives under photochemical conditions

A new catalyst for efficient C–H borylation reactions of (hetero)arenes in the presence of light is described. Various borylated arenes and heteroarenes are obtained in good yield using trans-[Rh(PMe₃)₂(CO)Cl] as an active photocatalyst and HBPin as an economic boron source under mild conditions.