Carbene insertions migratory

It is notable that two different types of dienes have been produced depending on the structure of a,p-unsaturated substrates. Similar reaction mechanisms can be proposed oxidative addition-Pd carbene formation-migratory insertion affords intermediate E. Diene A is released with subsequent p-H elimination for the cyclic or linear substrates without hydrogen at the 8-position. Otherwise, complex E prefers to undergo rearrangement to give intermediate G for cyclic... 

Weak base such as cesium carbonate has been utilized in this reaction to generate diazo compounds in situ from tosylhydrazones through the Bamford-Stevens reaction. The reaction is initiated by palladium-promoted decarboxylation of propargylic carbonate to form propargylpalladium complex A, which then tautomerizes to afford allenylpalladium intermediate B. Subsequently, the common carbene formation-migratory insertion-p-hydride elimination occurs to afford various vinylallenes (Fig. 30). 

Besides dissociation of ligands, photoexcitation of transition metal complexes can facilitate (1) - oxidative addition to metal atoms of C-C, C-H, H-H, C-Hal, H-Si, C-0 and C-P moieties (2) - reductive elimination reactions, forming C-C, C-H, H-H, C-Hal, Hal-Hal and H-Hal moieties (3) - various rearrangements of atoms and chemical bonds in the coordination sphere of metal atoms, such as migratory insertion to C=C bonds, carbonyl and carbenes, ot- and P-elimination, a- and P-cleavage of C-C bonds, coupling of various moieties and bonds, isomerizations, etc. (see [11, 12] and refs, therein). 

Different types of migratory insertions are exemplified by reactions 10, 11, and 12 in the Table II. An evaluation scheme is given in Matrix 6. In the present study we consider this reaction according to its general mechanism, shown in Eq. (6), where L is a neutral ligand (olefin, carbon monoxide, carbene, etc.) which becomes X (anionic c-donor ligand). X is also an anionic a-donor ligand. 

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

Danoponlos et al. reported the crystal structure of a palladium pincer carbene complex that is the product of intramolecular 1,2-methyl migration from palladium to the carbene carbon atom, a process also referred to as a migratory insertion of the carbene into the Pd-methyl bond [434] (see Figure 3.150). The importance of this compound stems from the fact that it was the first unambiguous experimental evidence for this process actually to take place after it had been suspected for several years with the suspicion being backed by several theoretical calculations [447-449]. 

The Cp 2" D 2 compounds also display a rich CO migratory insertion see Migratory Insertion) chemistry. The key products in this chemistry are carbene-like jj -acyl complexes (30). 

The migratory aptitude of R in (11) varies widely with its structure (see Section 3.9.2.1), the shift of an alkoxy group being among the slowest. The formation of alkoxyketenes in the photolysis of alkyl diazoacetates is a fairly recent discovery. The major competing reactions of the carbene precursor are insertions into the C—H and O—H bonds of alcohols employed as solvents and ketene traps. The extent of Wolff rearrangement varies with structure ethyl diazoacetate (20-25%), phenyl diazoacetate (45-60%), and A -methyldiazoacetamide (30%). These reactions are of limited synthetic interest at present. 

Alkyne polymerization in organic media has been reviewed [131]. A large variety of catalysts has been reported to polymerize alkynes in organic media. Similar to the polymerization of olefins, early transition metal as well as late transition metal catalysts are effective for this polymerization. Depending on the nature of the metal, two different mechanisms of polymerization have been suggested polymerization via a metal alkyl intermediate, or via a metal carbene (Scheme 7.9). With metal alkyl complexes, polymerization proceeds via migratory insertion of the alkyne into the metal-carbon bond [path (a) in Scheme 7.9] whereas with metal carbenes the mechanism is equivalent to that of metathesis [path (b)]. 

These two observations were explained by the intermediacy of carbene species. Nucleophilic P-addition generates a vinyliodonium ylide - iodoallene intermediate (83), which loses iodobenzene giving the alkylidenecarbene (84). If either the substituent R or the nucleophile Nu has a high migratory aptitude, a 1,2-shift leads to the rearranged alkyne products. However, when the substituent of the alkylidenecarbene has a y-hydrogen and if both the substituent and nucleophile have a poor migratory aptitude, intramolecular 1,5-C-H insertion takes place to form the cyclopentene derivatives. If a viable external trap is present, intermolecular insertion then occurs preferentially, 7,21,22,24... 

Transition-metal carbene complexes undergo insertion reactions with C-H compounds. For example, under the action of PF or HCl, the rhodium cyclopen-tadienyl carbene complex gives the products of an migratory insertion of the CPh unit into an Ar/j -C-Hbond of the cyclopentadienyl ring [64] ... 

Other reports were more fortuitous, as in the unexpected formation of 565 (Scheme 55) upon alkylation of Bp Rh(CO)(py) (566,) with methyl iodide. The mechanism for this was established to be intramolecular (from cross-over experiments with and H-labelled 566), and proposed to proceed by oxidative addition of Mel, with subsequent migratory insertion of CO. The nature of the hydride migration process from boron remains to be determined. On prolonged heating (45 °C), 565 evolves into 567 a rare example of reverse a-hydride migration from metal to carbene. 

Also noteworthy are some alkylidenes that exemplify rare reactivity for metal hydrides. The first is the cyclic carbene complex 565, the formation of which is itself unusual, proceeding as it does from the interaction of Bp Rh(CO)(py) (566) and methyl iodide. This is proposed to involve the oxidative addition of Mel and subsequent migratory insertion of CO, though at what stage the B-H activation occurs remains to be determined. More significant, however, is that on heating to 45 °C, 565 irreversibly evolves into the alkyl complex 567 via a rare reverse a-hydride migration onto the alkylidene carbon (Scheme 55, Section II-D.2). 

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