Carbenes four-membered ring carbene

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

The generally accepted mechanism is a chain mechanism, involving the intervention of a metal-carbene complex (126) and a four-membered ring containing... 

In most transition metal-catalyzed reactions, one of the carbene substituents is a carbonyl group, which further enhances the electrophilicity of the intermediate. There are two general mechanisms that can be considered for cyclopropane formation. One involves formation of a four-membered ring intermediate that incorporates the metal. The alternative represents an electrophilic attack giving a polar species that undergoes 1,3-bond formation. 

The (phosphino)(silyl)carbene 2a has been shown to exhibit this diagnostic reactivity. Under rather drastic conditions (300°C, 10 2 mm Hg), 2a has been converted into the azaphospholidine 37, in high yield, as a mixture of four diastereomers, which result from the presence of three asymmetric centers.4 The high regioselectivity of the carbene insertion (no formation of a four-membered ring was detected) is intriguing, especially since the same regioselectivity is observed upon thermolysis of bis(diisopropyl-amino)phosphinodiazomethane li.4 Yet, in contrast, four-membered heterocycles were obtained with other phosphinocarbenes (Sections V,C,1 and 3). 

Examples of the formation of four-membered rings by intramolecular C-H insertion of electrophilic carbene complexes are listed in Table 4.4. 

Nitropropane, 200 Simmons-Smith reagent, 275 Four-membered rings Chromium carbene complexes, 82 Menthol, 172... 

On the other hand, TV-tetraalkyl derivatives151 152 are quite well known (equation 2) but only in one case have ligands of this type been reported to bind in the bidentate chelating mode to a central atom153 to form a four-membered ring. The more normal use of such compounds, as ligands, is in carbene formation154 via metal-central carbon atom bond. 

The generation of four-membered ring systems can be accomplished by a cycloaddition process under photochemical conditions or with special substrates under thermal conditions. Iron-vinylidene complexes belong to such a class of special substrates where a thermal [2 + 2]-cycloaddition is possible. If imines are used, a hetero-[2 + 2]-cycloaddition with an iron-vinylidene complex leads to an iron-carbene complex attached to an azetidine ring system, as reported by Barrett and coworkers (Scheme 9.20) [46, 47]. The oxidation of these iron-carbene complexes leads to [3-lactams 27. Interestingly, the application of 2-thiazolines generates penam... 

We have expanded our collection of stereoselective reactions even more in the making of alkenes by the Wittig reaction (chapter 15), from acetylenes (chapter 16), by thermodynamic control in enone synthesis (chapters 18 and 19) and in sigmatropic rearrangements (chapter 35). We have seen that such E- or Z-alkenes can be transformed into three-dimensional stereochemistry by the Diels-Alder reaction (chapter 17), by electrophilic addition (chapters 23 and 30), by carbene insertion (chapter 30) and by cycloadditions to make four-membered rings (chapters 32 and 33). 

Any reaction that makes new bonds so efficiently and with so little reagent and so little waste is obviously very important. The yield is also rather good What happens is a metathesis—an exchange of groups between the two arms of the molecule. First, the carbene complex adds to one of the alkenes in what can be drawn as a [2 + 2] cycloaddition (Chapter 35) to give a four-membered ring with the metal atom in the ring. 

The authors proposed the following mechanism to explain the unusual organic transformations. The /z-peroxo-/i-oxo complex, 15, reacts in a biradical fashion with the protons on a methylenic carbon to generate a carbene and two hydroxy radicals that recombine to give a gem-diol. This diol will in turn collapse to the ketone product. The diketonization of acetylenes was explained in a similar fashion with the formation of a four-member ring intermediate (Scheme 2) (36, 37). 

Complexes of type (90) represent a new class of zirconium hydrocarbyl derivatives in which a strong metal carbene interaction in a pincer hgand structure is formed. The crystal structure of such complexes shows a short metal carbon link common to two fused four-membered rings which are almost coplanar. 

In penicillins of the type (370) the position a to the carbonyl in the four-membered ring bears a hydrogen and a nitrogen substituent. This requires that unsubstituted amino carbene complexes of the type (369) be readily available and was the driving force for the development of the procedure for the prep-... 

The crystal structure of 2-lithiated l-methyl-l,3-benzazaphosphole 35 showed it to have a dimeric structure with two THF molecules associated with each lithium. The two lithium atoms and the two C-2 atoms form a four-membered ring <20020M912>. In contrast, the N-lithiated derivative of 2,5-dimethyl-l//-l,3-benzazaphosphole exists as a monomeric species 36 in the solid state <2002JOM(646)113>. The carbene-type complex 37 derived from the lithiated species 35 on reaction with tungsten hexacarbonyl was also studied by X-ray crystallography. 

So why does silicon do it and carbon not The answer is not found in the energetic differences between the two four-membered rings and their corresponding carbenes since ah initio calculations revealed that on this basis the isomerization of 8 to carbene 9 would be favoured by more than 10 kcal/mol (Eqs. 11 and 12). 

N — H insertion generally proceeds unidirectionally, C—H insertion leading to a four-membered ring is not infrequently accompanied by C—H insertion, yielding a five-membered ring, and also by aromatic cycloaddition, ylide formation, etc. Few other carbene reactions are employed in the synthesis of azetidines, and their mechanisms are not always clear. 

Photolytic and catalytic decomposition of a-diazo esters produces )8-lactones, which are formed via intramolecular C—H insertion of a carbene or carbenoids. Tertiary alkyl esters of diazomalonic acid are decomposed by rhodium acetate with exclusive formation of the four-membered ring 211. This suggests a smooth insertion into the C—H bond activated by the adjacent oxygen atom (90TL1023). jS-Lactone 212 was obtained by photolysis of diazo malonic ester 213 (71CC577). 

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