Stable Carbenes Didier Bourissou, † Olivier Guerret, ‡ Franc ¸ ois P. Gabbaı ¨, § and Guy Bertrand* ,† Laboratoire d’He ´te ´rochimie Fondamentale et Applique ´e (UPRES-A CNRS 5069), 118 route de Narbonne, Universite ´ Paul Sabatier (Ba ˆt 2R1), F-31062 Toulouse Cedex 04, France, Centre de Recherche Rho ˆne-Alpes, Elf-Atochem, BP 63, F-69493 Pierre Be ´nite, France, and Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255 Received March 2, 1999 Contents I. Introduction 39 II. Influence of Substituents on the Electronic Structure and Stability of Carbenes 39 II.1. Ground-State Carbene Spin Multiplicity 39 II.1.1. Electronic Effects 41 II.1.2. Steric Effects 43 II.2. Concluding Remarks 43 III. Persistent Triplet Carbenes 43 III.1. Synthesis and Stability 43 III.2. Reactivity 44 III.3. Concluding Remarks 46 IV. Stable Singlet Carbenes: Synthesis and Structural Data 46 IV.1. (X,X)-Carbenes: Diaminocarbenes and Other Aminocarbenes 46 IV.2. (Z,Z)-Carbenes: Diborylcarbenes 49 IV.3. (X,Z)-Carbenes 50 IV.3.1. Phosphinosilyl- and Phosphinophosphoniocarbenes 50 IV.3.2. Sulfinylcarbenes 54 V. Reactivity of Stable Singlet Carbenes 55 V.1. 1,2-Migration Reactions 55 V.2. Carbene Dimerization and Related Reactions 57 V.2.1. Carbene Dimerization 57 V.2.2. Carbene-Carbenoid Coupling Reactions 58 V.3. Addition to Multiple Bonds 61 V.3.1. Addition to Carbon-Carbon Double Bonds 61 V.3.2 Addition to Carbonyl Derivatives 62 V.3.3. Addition to Carbon-Heteroatom Triple Bonds 62 V.3.4. Addition to Cumulenes 64 V.3.5. Addition to 1,3-Dipoles 64 V.4. Insertion Reactions 64 V.4.1. Insertion into Unpolarized C-H Bonds 64 V.4.2. Insertion into Polarized X-H Bonds 65 V.4.3. Insertion into Other Bonds 66 V.5. Carbene-Lewis Acid Adducts 66 V.5.1. Protonation of Carbenes 66 V.5.2. Carbene-Group 13 Element Adducts 66 V.5.3. Carbene-Group 14 Element Adducts 68 V.5.4. Carbene-Group 15 Element Adducts 70 V.5.5. Carbene-Group 16 Element Adducts 70 V.5.6. Carbene-Group 17 Element Adducts 70 V.6. Carbene-Lewis Base Adducts 71 V.7. Carbene-Metal Adducts 72 V.7.1. Carbene-Alkali Metal Adducts 72 V.7.2. Carbene-Alkaline Earth Metal Adducts 73 V.7.3. Carbene-Transition Metal Adducts 73 V.7.4. Carbene-Group 11 Metal Adducts 82 V.7.5. Carbene-Group 12 Metal Adducts 83 V.7.6. Carbene-Rare Earth Metal Adducts 84 VI. Conclusion 85 VII. References 86 I. Introduction Beginning as chemical curiosities with the pioneer- ing work of Curtius, 1a Staudinger, 1b and even earlier efforts, 1c carbenes have played an important role as transient intermediates over the last five decades. 2 Introduced by Doering into organic chemistry in the 1950s 3 and by Fischer into organometallic chemistry in 1964, 4 these fascinating species are involved in many reactions of high synthetic interest. In the last 10 years, 5 our understanding of carbene chemistry has advanced dramatically with the prepa- ration of persistent triplet diarylcarbenes I 6 and the isolation of heteroatom-substituted singlet carbenes (II-XIV) 7 (Figure 1). This review summarizes this fruitful part of carbene chemistry, which only started in the 1980s. 8 The influence of the substituents on the ground- state spin multiplicity and stability of carbenes will be discussed first. Subsequently, the results concern- ing persistent triplet carbenes I will be presented. The next chapter will be devoted to the synthesis, structural, and spectroscopic features of singlet car- benes II-XIV. Last, their reactivity will be presented and compared with that observed for transient car- benes. II. Influence of Substituents on the Electronic Structure and Stability of Carbenes II.1. Ground-State Carbene Spin Multiplicity Carbenes are neutral compounds featuring a di- valent carbon atom with only six electrons in its valence shell. Considering a prototype carbene -C ¨-, the carbon atom can be either linear or bent, each geometry describable by a certain degree of hybrid- ization. The linear geometry implies an sp-hybridized 39 Chem. Rev. 2000, 100, 39-91 10.1021/cr940472u CCC: $35.00 © 2000 American Chemical Society Published on Web 12/22/1999