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When a dimethyl group was added to C exo, the reaction no longer proceeds this way, and was able to polymerize lactide, δ-valerolactone, and ω-pentadecalactone. While this broadened the scope and speed of the polymerization, the reaction was difficult to control due to the formation of an enolate intermediate.
IMes is a free NHC that can be used as a ligand. Other NHCs have been isolated as the free ligands. [6] Aside from IMes, another important NHC ligand is IPr, which features diisopropylphenyl groups in place of the mesityl groups. [1] [7] NHCs with saturated backbones include SIMes and SIPr.
Ever since the first crystalline carbene structure was isolated by Arduengo ins 1990, tuning different properties of NHCs has been a popular area of study in main group chemistry. [4] The first NHC boryl anion was synthesized by Segawa in 2006. [ 1 ]
When the alkyl group bridges two main group elements, the bonding is called three-center two-electron bonds. This pattern is seen for dimethyl beryllium and trimethylaluminium. In the case of methyl lithium, the methyl group can be shared (bonded to) three Li centers. These bonding aspects influence the structures: Trimethylaluminium, dimethyl ...
This process can be used in conjunction with the in situ generation of free carbenes. Pd-NHC complexes can also be synthesized through transmetalation with silver-NHC complexes. The transmetallated NHCs can either be isolated for subsequent reaction with palladium in a two-step method, or generated in the presence of palladium in a one-pot ...
Copper-free variations to the Sonogashira reaction have been developed to avoid the formation of the homocoupling products. [ 19 ] [ 25 ] There are other cases when the use of copper should be avoided, such as coupling reactions involving substrates which potential copper ligands, for instance free-base porphyrins .
This effect can be used to stabilize highly reactive main group and transition metal compounds. [6] [22] Because excessive steric hindrance can be an issue for some reactivity, NHCs and CAACs bearing substituents with multiple spatial conformations (e.g. cyclohexyl) offer "flexible steric bulk" for catalysis.
A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. [1] Carbene complexes have been synthesized from most transition metals and f-block metals, [2] using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. [1]