Reversible 1,1‐Hydroboration: Boryl Insertion into a CN Bond and Competitive Elimination of HBR2 or RH

Boranes with the general formula of HBR₂ have been found to undergo a facile 1,1‐hydroboration reaction with pyrido[1,2‐a]isoindole ( A ), resulting in insertion of a BR₂ unit into a C? N bond and the formation of a variety of BN heterocycles. Investigation on the thermal reactivity of the BN heterocycles revealed that these molecules have two distinct and competitive thermal elimination pathways: HBR₂ elimination (or retro‐hydroboration) versus R? H elimination, depending on the R group on the B atom and the chelate backbone. Mechanistic aspects of these highly unusual reactions have been established from both experimental and computational evidence. Adduct formation between HBR₂ and A was found to be the key intermediate in 1,1‐hydroboration of A .     

Single Oxygen‐Atom Insertion into PB Bonds: On‐ and Off‐Metal Transformation of a Borylphosphine into a Borylphosphinite

An oxygen atom is selectively inserted into the P?B bond of a borylphosphine ( L₁ ) by reaction with Me₃NO to afford the corresponding borylphosphinite ( L₂ ). This transformation can also be effected when L₁ is coordinated to rhodium. The ν(CO) values for trans‐[RhCl(CO)(L)₂] reveal very different electronic properties for coordinated L₁ and L₂ which translate into the strikingly different performances of the complexes [RhCl(L)(cod)] (L= L₁ or L₂ , cod=1,5‐cyclooctadiene) in hydrosilylation and hydroboration catalysis.     

Direct Trifluoromethylthiolation and Perfluoroalkylthiolation of C(sp2)H Bonds with CF3SO2Na and RfSO2Na

A new method for CF₃SO₂Na‐based direct trifluoromethylthiolation of C(sp²)? H bonds has been developed. CF₃SSCF₃ is generated in situ from cheap and easy‐to‐handle CF₃SO₂Na, and in the presence of CuCl can be used for electrophilic trifluoromethylthiolation of indoles, pyrroles, and enamines. The method has been extended to perfluoroalkylthiolation reactions using R_fSO₂Na.     

The Cross‐Dehydrogenative Coupling of CH Bonds: A Versatile Strategy for CC Bond Formations

Over the last decade, substantial research has led to the introduction of an impressive number of efficient procedures which allow the selective construction of C? C bonds by directly connecting two different C? H bonds under oxidative conditions. Common to these methodologies is the generation of the reactive intermediates in situ by activation of both C? H bonds. This strategy was introduced by the group of Li as cross‐dehydrogenative coupling (CDC) and discloses waste‐minimized synthetic alternatives to classic coupling procedures which rely on the use of prefunctionalized starting materials. This Review highlights the recent progress in the field of cross‐dehydrogenative C? C formations and provides a comprehensive overview on existing procedures and employed methodologies.     

Gold‐Catalyzed C(sp3)H/C(sp)H Coupling/Cyclization/Oxidative Alkynylation Sequence: A Powerful Strategy for the Synthesis of 3‐Alkynyl Polysubstituted Furans

In sharp contrast to the gold‐catalyzed reactions of alkynes/allenes with nucleophiles, gold‐catalyzed oxidative cross‐couplings and especially C? H/C? H cross‐coupling have been under represented. By taking advantage of the unique redox property and carbophilic π acidity of gold, this work realizes the first gold‐catalyzed direct C(sp³)? H alkynylation of 1,3‐dicarbonyl compounds with terminal alkynes under mild reaction conditions, with subsequent cyclization and in situ oxidative alkynylation. A variety of terminal alkynes including aryl, heteroaryl, alkenyl, alkynyl, alkyl, and cyclopropyl alkynes all successfully participate in the domino reaction. The protocol offers a simple and region‐defined approach to 3‐alkynyl polysubstituted furans.     

BH,CH,and BC Bond Activation: The Role of Two Adjacent Agostic Interactions

Tuning the nature of the linker in a L～BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ‐C? H and ε‐B_sp2? H, agostic interactions. Such a unique coordination mode stabilizes a 14‐electron “RuH₂P₂” fragment through connected σ‐bonds of different polarity, and affords selective B? H, C? H, and B? C bond activation as illustrated by reactivity studies with H₂ and boranes.     

Direct Trifluoromethylthiolation of Alcohols under Mild Reaction Conditions: Conversion of ROH into RSCF3

A direct process for the trifluoromethylthiolation of allylic and benzylic alcohols under mild conditions has been developed. A wide range of free alcohols underwent nucleophilic substitution in the presence of stable CuSCF₃ and BF₃ ? Et₂O to give the corresponding products in good to excellent yields.     

From Anilines to Isatins: Oxidative Palladium‐Catalyzed Double Carbonylation of CH Bonds

A novel palladium‐catalyzed C? H double carbonylation introduces two adjacent carbonyl groups for the synthesis of isatins from readily available anilines. The reaction proceeds under atmospheric pressure of CO with high regioselectivity and without any additives. Density functional theory investigations indicate that the palladium‐catalyzed double carbonylation catalytic cycle is plausible.     

Iridium‐Catalyzed Synthesis of Diaryl Ethers by Means of Chemoselective CF Bond Activation and the Formation of BF Bonds

Transition‐metal‐catalyzed C? F activation, in comparison with C? H activation, is more difficult to achieve and therefore less fully understood, mainly because carbon–fluorine bonds are the strongest known single bonds to carbon and have been very difficult to cleave. Transition‐metal complexes are often more effective at cleaving stronger bonds, such as C(sp²)? X versus C(sp³)? X. Here, the iridium‐catalyzed C? F activation of fluorarenes was achieved through the use of bis(pinacolato)diboron with the formation of the B? F bond and self‐coupling. This strategy provides a convenient method with which to convert fluoride aromatic compounds into symmetrical diaryl ether compounds. Moreover, the chemoselective products of the C? F bond cleavage were obtained at high yields with the C? Br and C? Cl bonds remaining.     

Mild and Selective Activation and Substitution of Strong Aliphatic CF Bonds

A procedure for chemoselectively manipulating the strong aliphatic C?F bond with direct transformation into a C?N bond under mild conditions is reported. The activation and subsequent substitution of primary alkyl fluorides is mediated by La[N(SiMe₃)₂]₃, and results in high to excellent yields of tertiary amines. The methodology displays high selectivity towards the C(sp³)?F bond, and a variety of secondary amines are applicable as nucleophiles. Mechanistic investigations reveal a reaction that is first order with respect to [La[N(SiMe₃)₂]₃], [R¹R²NH], and [alkyl fluoride], and a 6‐membered cyclic transition state is proposed. In addition, ¹H NMR spectroscopy shows that La[N(SiMe₃)₂]₃ is the active species involved in the substitution and that protonolysis of the amine, yielding La[NR¹R²]₃, lowers the reactivity.     

Catalytic Methylation of CH Bonds Using CO2 and H2

Formation of C? C bonds from CO₂ is a much sought after reaction in organic synthesis. To date, other than C? H carboxylations using stoichiometric amounts of metals, base, or organometallic reagents, little is known about C? C bond formation. In fact, to the best of our knowledge no catalytic methylation of C? H bonds using CO₂ and H₂ has been reported. Described herein is the combination of CO₂ and H₂ for efficient methylation of carbon nucleophiles such as indoles, pyrroles, and electron‐rich arenes. Comparison experiments which employ paraformaldehyde show similar reactivity for the CO₂/H₂ system.     

Azaphilic versus Carbophilic Coupling at CN Bonds: Key Steps in Titanium‐Assisted Multicomponent Reactions

Consecutive C‐ and N‐arylation of N‐heterocyclic nitriles is mediated by titanium(IV) alkoxides. The carbo‐ and azaphilic arylation step may be separated by choosing the order in which the two equivalents of aryl transfer reagent are added. In the course of this transformation, the ancillary N‐heterocycle acts as both a directing anchor group and electron reservoir. In the selectivity‐determining step, the selectivity is governed by a choice between (direct) C‐ and Ti‐arylation; the latter opens up a reaction pathway that allows further migration to the nitrogen atom. The isolation of metal‐containing aggregates from the reaction mixture and computational studies gave insights into the reaction mechanism. Subsequently, a multicomponent one‐pot protocol was devised to rapidly access complex quaternary carbon centers.     

Heteroatom‐Guided,Palladium‐Catalyzed,Site‐Selective CH Arylation of 4H‐Chromenes: Diastereoselective Assembly of the Core Structure of Myristinin B through Dual CH Functionalization

A highly site‐selective, heteroatom‐guided, palladium‐catalyzed direct arylation of 4H‐chromenes is reported. The C?H functionalization is driven not only by the substituents and structure of the substrate but also by the coupling partner being used. The diastereoselective assembly of the core structure of Myristinin B has been achieved by using a dual C?H functionalization strategy for regioselective direct arylation.     

Coordination and Activation of AlH and GaH Bonds

The modes of interaction of donor‐stabilized Group 13 hydrides (E=Al, Ga) were investigated towards 14‐ and 16‐electron transition‐metal fragments. More electron‐rich N‐heterocyclic carbene‐stabilized alanes/gallanes of the type NHC?EH₃ (E=Al or Ga) exclusively generate κ² complexes of the type [M(CO)₄(κ²‐H₃E?NHC)] with [M(CO)₄(COD)] (M=Cr, Mo), including the first κ² σ‐gallane complexes. β‐Diketiminato (′nacnac′)‐stabilized systems, {HC(MeCNDipp)₂}EH₂, show more diverse reactivity towards Group 6 carbonyl reagents. For {HC(MeCNDipp)₂}AlH₂, both κ¹ and κ² complexes were isolated, while [Cr(CO)₄(κ²‐H₂Ga{(NDippCMe)₂CH})] is the only simple κ² adduct of the nacnac‐stabilized gallane which can be trapped, albeit as a co‐crystallite with the (dehydrogenated) gallylene system [Cr(CO)₅(Ga{(NDippCMe)₂CH})]. Reaction of [Co₂(CO)₈] with {HC(MeCDippN)₂}AlH₂ generates [(OC)₃Co(μ‐H)₂Al{(NdippCme)₂CH}][Co(CO)₄] ( 12 ), which while retaining direct Al?H interactions, features a hitherto unprecedented degree of bond activation in a σ‐alane complex.     

Magnesium Catalysis for the Hydroboration of Carbodiimides

A β‐diketiminato magnesium alkyl complex, [CH{C(Me)NDipp}₂}MgnBu] (Dipp=2,6‐iPr₂C₆H₃), was shown to be an effective pre‐catalyst for the first reported catalytic hydroboration of alkyl‐ and aryl‐substituted carbodiimides with pinacol borane (HBpin). The catalytic reactions proceed under mild conditions to afford the corresponding N‐borylated formamidine compounds in good yields. The reactions were observed to proceed through the intermediacy of magnesium amidinate and formamidinatoborate intermediates and an example of one of these latter species has been structurally characterised by an X‐ray diffraction analysis. Crucially, no formation of the N‐boryl formamidine products was observed in the absence of additional equivalents of the carbodiimide and HBpin substrates. This observation, supported by the evolution of a sigmoidal kinetic profile for the hydroboration of dicyclohexylcarbodiimide, has been rationalised as the consequence of an allosteric effect of the pinacol borane and carbodiimide on the magnesium formamidinatoborate intermediates.