Stable cyclic carbenes and related species beyond diaminocarbenes

Abstract

The success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of various systems. Spectacular results in this area have been achieved using cyclic diaminocarbenes (NHCs) as a result of their strong σ-donor properties. Although it is possible to cursorily tune the structure of NHCs, any diversity is still far from matching their phosphorus-based counterparts, which is one of the great strengths of the latter. A variety of stable acyclic carbenes are known, but they are either reluctant to bind metals or they give rise to fragile metal complexes. During the last five years, new types of stable cyclic carbenes, as well as related carbon-based ligands (which are not NHCs), and which feature even stronger σ-donor properties have been developed. Their synthesis and characterization as well as the stability, electronic properties, coordination behavior, and catalytic activity of the ensuing complexes are discussed, and comparisons with their NHC cousins are made.

Figure 1

Solid-state structure of PHC³ showing the very weak pyramidalization of the phosphorus centers.

Figure 2

Calculated data for the parent saturated NHC and CAAC. Schematic representations of phosphines, NHCs, and CAACs, showing their very different steric environments.

Figure 3

Solid-state structure of enantiomerically pure CAAC².

Figure 4

Molecular structure of [RhCl(CO)(CAAC²)] showing the metal–hydrogen interactions.

Figure 5

X-ray crystal structure of: left: [Pd(allyl)(CAAC²)]BF₄ (anion omitted for clarity), and right: complex 18 characterized only by ¹H NMR spectroscopy (right).

Figure 6

X-ray crystal structure of [Au(CAAC³)(η²-toluene)]B(C₆F₅)₄ (CAAC³_Au+)

Figure 7

Solid-state structures of palladium complexes showing the different steric environments provided by small CAAC¹ (left), bulky rigid CAAC² (middle), and flexible CAAC⁵ (right).

Figure 8

Solid-state structures of CP²-LiBF₄ polymer 33 (left) and tertiary complex 34 with the BF₄ anion omitted for clarity (right).

Figure 9

Solid-state structure of CP².

Figure 10

Molecular structure of CBA³ in the solid state.

Figure 11

Molecular structures of CBA³(H⁺) (left; the Dipp groups on the oxygen atoms have been removed for clarity), and CBA⁴(H⁺) (right) in the solid state.

Figure 12

Solid-state structure of CCDP⁴.

Figure 13

Molecular structure of CVP² in the solid state.

Figure 14

X-ray crystal structures of [RhCl(CO)₂(CBA³)] and [RhCl-(cod)(CBA⁵)].

Figure 15

Molecular structure of aNHC² in the solid state.

Scheme 1

Crystallographically characterized carbenes known before 2000 (A–I), and discovered between 2000 and 2004 (J–M), with the carbene bond angle given in parentheses. Dipp =2,6-iPr₂C₆H₃, Dtbp =2,6-tBu₂C₆H₃.

Scheme 2

Contraction of the carbene bond angle of K upon complexation. nbd = norbornadiene.

Scheme 3

Cyclic carbenes and related species discussed in this Review.

Scheme 4

Computational data for PHC^1–3 and triazolin-5-ylidene D. S =singlet, T =triplet.

Scheme 5

Spontaneous ring closure of phosphorus analogues of amidinium salts, which can be prevented by the ring structure of PHC(H⁺)s.

Scheme 6

Synthetic routes to PHC^3–5(H⁺). Mes* =2,4,6-tBu₃C₆H₂, Tf = trifluoromethanesulfonyl, DBU =1,8-diazabicyclo[5.4.0]undec-7-ene.

Scheme 7

Influence of the counteranion/base combination on the deprotonation of PHCs; preparation of PHC³ and PHC⁴. Ar =2,4,6-tBu₃C₆H₂, HMDS = 1,1,1,3,3,3-hexamethyldisilazane.

Scheme 8

Decomposition of PHC³ in solution, and resonance form PHC⁴.

Scheme 9

Synthesis of a Zr-PHC complex by reduction of a thioacetal. Cp =cyclopentadienyl.

Scheme 10

Acyclic (amino)(phosphino)carbenes such as J feature an active lone pair of electrons on the P center. cod =1,5-cyclooctadiene.

Scheme 11

Synthesis of phosphaformamidinates 8 and N-PHC^1-3(H⁺). Mes =2,4,6-Me₃C₆H₂, Mes* = 2,4,6-tBu₃C₆H₂

Scheme 12

Deprotonation of N-PHC^1,2(H⁺).

Scheme 13

Deprotonation of N-PHC³(H⁺), spectroscopic characterization, and rearrangement of N-PHC³. TMP =tetramethylpiperidide, LDA =lithium diisopropylamide.

Scheme 14

Evolution of persistent (alkyl)(phosphino)- and (alkyl)-(amino)carbenes in solution.

Scheme 15

First synthetic route for the preparation of the small CAAC¹.

Scheme 16

Hydro-amination (left) and hydro-iminiumation (right).

Scheme 17

Synthesis of enantiomerically pure CAAC², which illustrates the hydro-iminiumation route.

Scheme 18

Rigid CAAC^2–4, as well as NHC B² and CAAC⁵ with flexible steric bulk.

Scheme 19

Synthesis of 14-electron [RhCl(CO)(CAAC²)].

Scheme 20

Metal-catalyzed coupling of enamines and terminal alkynes.

Scheme 21

[Au(CAAC³)(NH₃)]B(C₆F₅)₄-catalyzed NH₃ hydroamination of alkynes and allenes.

Scheme 22

Hydroamination of alkynes with diethylamine by using 5 mol% CAAC³_Au+.

Scheme 23

Hydroamination of allenes with diethylamine by using 5 mol% CAAC³_Au+.

Scheme 24

One-pot synthesis of allenes from two alkynes and a sacrificial amine by using 5 mol% CAAC³_Au+. THQ = 1,2,3,4-tetrahydroisoquinoline.

Scheme 25

Tandem hydroamination-hydroarylation reaction, promoted by CAAC³_Au+.

Scheme 26

Isolation of gold(I)–(η¹-alkene) complex 20, as well as examples of catalytic hydroammoniumation and methylamination reactions to give 21 and 22, respectively.

Scheme 27

Classical ruthenium olefin metathesis catalysts Gr^1,2 and HG^1,2. CAAC^1,5,6 were used for preparing 24a,b, and CAAC^1,5,6_Ru.

Scheme 28

Application of CAAC^1,5,6_Ru in ring-closing metathesis reactions.

Scheme 29

Comparative catalytic activity of various ruthenium catalysts for the ethenolysis of methyl oleate.

Scheme 30

The E₁ value correlates with the nucleophilicity of the carbenes. The cationic part of the ylide has to be exocyclic to prevent ring-opening processes, as observed for other cyclic carbenes.

Scheme 31

Generation, rearrangement, and trapping of N-YHC⁴.

Scheme 32

Deprotonation of N-YHC⁵(H⁺) affords stable lithium adduct 27, which has an N-YHC as part of a bidentate ligand.

Scheme 33

Synthesis, spectroscopic characterization, and trapping of N-YHC⁶.

Scheme 34

Preparation of the transient N-YHC⁷ and persistent N-YHC⁸, showing the importance of the ring skeleton.

Scheme 35

Three-membered N-heterocyclic carbenes would rearrange into cumulenes.

Scheme 36

CP¹ and CP², as well as the potential precursors CP²(H⁺) and CP²(Cl⁺).

Scheme 37

Singlet–triplet energy gap for CP¹ and CP³, as well as the relative energy of CP¹ isomers 31 and 32.

Scheme 38

Only certain combinations of anion and base allow for the preparation of CP².

Scheme 39

Preparation of CP⁵, which is in photochemical equilibrium with 35.

Scheme 40

Preparation of triafulvalenes 38, and the possible reaction intermediates. Trip =2,4,6-iPr₃C₆H₂.

Scheme 41

Complexes prepared from free CP². tmeda = N,N,N′,N′-tetramethylethylenediamine.

Scheme 42

Isomerization of quadricyclane to norbornadiene promoted by [PdCl₂(CP)] dimers.

Scheme 43

Resonance forms of diaminocyclopropenylidenes; 39, the most severely bent acyclic allene known up to 2008; resonance forms of tetrakis(amino)allene 40 and carbodiphosphorane 41; calculated carbodicarbene 42 and the concept of carbon(0); synthesis of acyclic bent-allene 44.

Scheme 44

Synthesis a dinuclear gold complex of a carbon(0) derivative.

Scheme 45

Smallest ring allenes isolated before 2008, with the C-C-C bond angles.

Scheme 46

Synthesis of CBA²-lithium adduct 50, and stable free cyclic bent-allene CBA³. Ar =2,6-Me₂C₆H₃.

Scheme 47

Synthesis of CBA⁵(H⁺) and persistent cyclic allene CBA5.

Scheme 48

Double protonation of CBA⁵.

Scheme 49

Syntheses of cyclic carbodiphosphoranes CCDP^1–4.

Scheme 50

Rearrangement of CCDP⁴; analogy with PHC³.

Scheme 51

Cyclic vinylidenephosphoranes (CVPs), which are hybrid compounds between CBAs and CCDPs; CVP¹, the only derivative reported^[210] before 2008.

Scheme 52

Synthesis of CVP².

Scheme 53

Complexes synthesized from free CCDP⁴ and CVP². all =allyl.

Scheme 54

First catalytic applications of complexes bearing a CCDP as a ligand.

Scheme 55

Synthesis of complexes used for Suzuki–Miyaura coupling reactions. dba =trans,trans-dibenzylideneacetone.

Scheme 56

Different resonance forms of CBA (derived from pyrazolium salts), NHC B, and aNHC. Synthesis of the abnormal NHC complex 57.

Scheme 57

Synthesis of NHC¹, and evidence for the transient formation of aNHC¹ through the formation of aNHC¹_(Rhcod).^[163c]

Scheme 58

Synthesis of free aNHC² and of its lithium adduct aNHC²_(Li+).

Scheme 59

Rearrangement of aNHC² upon heating in solution.

Scheme 60

Palladium complexes for Suzuki–Miyaura and Heck reactions (59 and 60), as well as for copper- and amine-free Sonogashira coupling reactions (61).

Scheme 61

aNHC complexes 62_aNHC and 63_aNHC are active in hydrogenation and hydrogen-transfer catalysis, respectively, while NHC complexes 62_NHC and 63_NHC are not.

Scheme 62

Comparison of iridium complexes bearing NHC (64), aNHC (65), and pyrazol-3-ylidene (66) ligands for alkylation reactions.

Scheme 63

Comparative activity of ruthenium complexes bearing NHC (67), aNHC (68), and pyrazol-3-ylidene (69) ligands for the β-alkylation of secondary alcohols with primary alcohols. M =major product, m =minor product.

Scheme 64

Symmetric, asymmetric, and average CO stretching frequencies for cis-[RhCl(CO)₂(L)] (top) and cis-[IrCl(CO)₂(L)] complexes (bottom).

Scheme 65

Members of the NHC family reaching the donor strength of CBAs and N-YHCs.

Scheme 66

Calculated first and second proton affinity [kcalmol⁻¹].

Scheme 67

Calculated energy of the HOMO and LUMO (top),^[127,258] and of the singlet–triplet energy gap and HOMO (bottom) for selected carbenes.^[259]