Diazocarbene

To date, bis(diazomethyl)silanes represent the only precursors to silylene-dicarbenes. It should be noted, however, that the extrusion of N2 from the two diazo functions can occur more or less simultaneously or successively. In the latter case, a diazocarbene is formed in the first place, which is supposed to undergo a fast carbene reaction before the generation of a carbene from the second diazo function occurs. This should be kept in mind for all transformations which are explained mechanistically with the participation of silylene-dicarbenes. There are even examples where only one of the two diazo groups enters carbene chemistry at all (see equations 35 and 36 below). 

Bis[diazo(trimethylsilyl)methyl] phosphines 239 loose only one equivalent of dinitrogen on warming and are transformed into l,2,4(k3)-diazaphospholes 240139 (equation 81). It is obvious to explain this cyclization by intramolecular ketazine formation of a diazocarbene intermediate, but in light of the preceding discussion, speculations about the bond state of this phosphinocarbene (phosphavinyl ylide or phosphaacetylene) are allowed. 

Fundamental Aspects of the Metal-Catalyzed C-H Bond Functionalization by Diazocarbenes Guiding Principles for Design of Catalyst with Non-redox-Active Metal (Such as Ca) and Non-Innocent Ligand... 

I 2 Fundamental Aspects of the Metal-Catalyzed C-H Bond Functionalization by Diazocarbenes... 

Scheme 2.3 Schematic presentation of the transition-metaRatalyzed C-H bond alkylation by diazocarbene.

Bearing in mind that the Fe-center does not change its oxidation state during the course of the reaction, we tested the combination of Ca " and the same non-innocent PDI ligand and found it to be an efficient catalyst for benzylic C-H functionalization by diazocarbenes. In the other words, we wanted to prove that the entire C-H bond alkylation by diazocarbenes can exclusively rely on the redox power of the PDI ligand, without involvement of redox properties of the metal. A word of caution this work should not be seen as an attempt to propose a practical Ca catalyst for the C-H alkylation because of the existence of many competing reactions thaat are not subject of this chapter and were not studied. These efforts should be seen as a computational test of an extreme approach to C - H alkylation. 

In this chapter, we have used computational tools to study the (PDI)Ca-catalyzed benzylic C-H bond functionalization by diazocarbenes. Here, we use two different diazo compounds, unsubstituted N2CH2 and donor-donor N2CPh2, with dichloromethane (CH2CI2) as the solvent. 

Potential Energy Surface of the [(PDI)Ca(THF)3] Catalyzed C-H Bond Alkylation of MeCHjPh by Unsubstituted NjCHj Diazocarbene... 

We start our discussion by exploring mechanisms and controlling factors of the [(PDI)Ca(THF)3]-catalyzed C-H bond alkylation of MeCHjPh by unsubstituted N2CH2 diazocarbene. First, we discuss the diazocarbene decomposition by [(PDIlCalTHFlg], 4. The calculated energetics, spin population, and geometric... 

Figure 2.3 Potential energy surface of the reaction, as well as schematic presentation of [(PDIjCalTHFjj] catalyzed donor-donor (D/D)- the reactants, intermediates, transition states, substituted NjCPhj diazocarbene decom- and product of this reaction. Energies are position and metallocarbene formation presented as A/fgjjCAGgjjIfAGjoi]...

Thus, consistent with the aforementioned analyses, the substitution of H by Ph in carbene carbon makes diazocarbene decomposition thermodynamically more feasible and irreversible. 

In this chpter, we analyzed electronic factors affecting the mechanism of transition-metal-catalyzed C-H bond alkylation by diazocarbene precursors. We demonstrated that efficient transition-metal-catalyzed alkylation of C-H, C=C, and C=C bonds by diazocarbenes requires a fine balance ... 

Thus, the relatively weak Schrock-type oilandrelatively strong Fischer-Tropsch type of metallocarbenes are more attractive for the C-H, C=C, and C=C bond alkylation by diazocarbenes. The aforementioned delicate balance can be achieved by utilizing combination of... 

On the basis of these analyses, we hypothesized the use of an earth-abundant transition-metal complex containing a non-redox-active metal center and redox-active (non-innocent) ligand as a catalyst. We demonstrated our prediction on [(PDl)Ca(THF)3], where PDI is a non-innocent pyridine-2,6-diimine ligand, and the catalyzed benzyhc (of the MeCH2Ph substrate) C-H bond alkylation by unsubstituted and diphenyl (termed the iio or-(io or)-substituted diazocarbene precursors, N2CH2 and N2CPh2. 

---