Insertion process, carbene-like

Surprisingly, the indolizidine aziridine 39 was synthesized in 88% yield, but was found to be an uncatalyzed reaction, providing the product upon diazo-transfer at 0°C to oxime 37. Two plausible mechanisms are invoked for this carbene reaction sequence (1) a carbenoid insertion process or (2) collapse of the azomethine ylide formed upon addition of the diazoamide. The uncatalyzed process is likely due to a dipolar cycloaddition of the diazo and pendent oxime. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

Cycloadditions to the cycloproparenyl n framework with dienes and carbenes that likely proceed via norcaradiene intermediates have been discussed (Section V.A.3). Presented here are those processes that involve insertion into the strained three-membered ring a bond. 

The FAB MS spectra of the alkaline buffer photolysis products indicated that innermolecular reactions took place between the carbene 64 guest and the CyD hosts (Fig. 13). Such supramolecular processes might entail carbene 64 insertions into the C-H bonds of the CyDs, but more likely they involve insertions into the hosts O-H bonds (Scheme 16). However, no 64- CyD primary insertion products, e.g., 79, were... 

Indenes, like cyclobutenones and furans, are common side-products in the reaction of chromium arylalkoxycarbene complexes with alkynes, especially internal alkynes [9]. The in-dene structure comes about by a process that is very similar to naphthol formation annula-tion to the aryl ring still occurs, but without carbon monoxide insertion, and, instead, bond formation takes place directly between an alkyne carbon and the aryl carbon ortho to the metal carbene substituent [Eq. (18)] [4]. Scheme 5-1 shows two pathways that have been suggested for this transformation beginning from the vinylcarbene intermediate 3, naphthol formation can be diverted to intermediate 8, either by direct cyclization (3 -+ 8) or through the chromacyclohexadiene (3->6- 8). Aromatization and decomplexation yield the indene [7 b, d, 43], More detailed mechanistic analyses consider the roles of the stereochemistry of 3, as an ( )- or (Z)-vinylcarbene, as well as the coordination of external ligands, in the production of indenes, naphthols, furans, cyclobutenones, and other common side-products [8 a, 9, 13, 44],... 

As a final entry in this section, we felt it appropriate to turn to an example that perhaps reflects a likely focus of metathesis explorations in the future. While all of our preceding examples have illustrated metathesis reactions exclusively of olefinic substrates, other unsaturated precursors can engage in productive metathesis events when suitably activated by appropriate metal-carbene complexes. One of these processes, the enyne metathesis reaction, is shown in Scheme 17. After insertion of the mthenium alkylidene... 

Sol 3. (d) Diazo compounds liberate the free carbenes under photochemical conditions that are in singlet states. This process can be directed toward the formation of a triplet carbene using a triplet sensitizer like benzophenone. For triplet carbenes, the aUcene insertion reaction is nonstereospecific. Therefore, the reaction of Z-but-2-ene with triplet carbene gives a mixture of cisHrans-1,2-dimethylcyclopropane. 

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