Alkenes carbene insertion

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Photoelimination of nitrogen from diazirines, for example, proceeds via carbene intermediates. 3-tert-Butyldiazirine (395) is converted into the cyclopropane 396 and the alkene 397 with the formation of the carbene insertion product being favored from the singlet state.328 3-Cyclopropyl-3-chlorodiazirine (398) has similarly been converted to the carbene 399 which undergoes both rearrangement to l-chlorocyclobutene(400)329 and addition... 

To date, the most frequently used ligand for combinatorial approaches to catalyst development have been imine-type ligands. From a synthetic point of view this is logical, since imines are readily accessible from the reaction of aldehydes with primary or secondary amines. Since there are large numbers of aldehydes and amines that are commercially available the synthesis of a variety of imine ligands with different electronic and steric properties is easily achieved. Additionally, catalysts based on imine ligands are useful in a number of different catalytic processes. Libraries of imine ligands have been used in catalysts of the Strecker reaction, the aza-Diels-Alder reaction, diethylzinc addition, epoxidation, carbene insertions, and alkene polymerizations. 

Dehydrogenative Cj cyclization (25, 26). Its probable pathway is an alkene-alkyl insertion (5). A carbene-alkyl insertion mechanism may eventually also be possible. 

Carbenes are only rarely detected, let alone isolated and characterized. Kinetically favorable exothermic reactions (among them cycloaddition with alkenes and insertion into CH bonds) generally preclude this. l,3-Diadamantylimidazol-2-ylidene, 8Ad, is an exception, in that it forms a stable solid, the crystal structure of which has been determined. ... 

Many of the limitations of C—C bond formation by C —H insertion outlined for intermolecular reactions (Section 1.2.1.) can be overcome by making the reaction intramolecular. Thus, hydrogen atom abstraction followed by intramolecular radical-radical coupling or radical addition to an alkene are increasingly popular processes. Two-electron carbene insertions, either thermal or transition metal catalyzed, have also been used extensively. In either case, ring construction involves net C—C bond formation at a previously unactivated C-H site. 

An interesting Fe-catalyzed SN2 -like carbene insertion reaction using diazo compounds and allyl sulfides (the Doyle-Kirmse reaction) was reported by Carter and Van Vranken in 2000 [20], Various allyl thioethers were reacted with TMS-diazomethane in the presence of catalytic amounts of Fe(dppe)Cl2 to furnish the desired insertion products with moderate levels of stereocontrol [Equation (7.6), Scheme 7.14]. The products obtained serve as versatile synthons in organic chemistry, e.g. reductive desulfurization furnishes lithiated compounds that can be used in Peterson-type oleftnations to yield alkenes [Equation (7.7), Scheme 7.14] [21]. 

These carbene (or alkylidene) complexes are used as either stoichiometric reagents or catalysts for various transformations which are different from those of free carbenes. Reactions involving the carbene complexes of W, Mo, Cr, Re, Ru, Rh, Pd, Ti and Zr are known. Carbene complexes undergo the following transformations (i) alkene metathesis (ii) alkene cyclopropanation (iii) carbonyl alkenation (iv) insertion to C—H, N—H and O—H bonds (v) ylide formation and (vi) dimerization. Their chemoselectivity depends mainly on the metal species and ligands, as discussed in the following sections. 

Some metals form stable complexes of carbenes, a notable example being rhodium. Treatment of the diazocompound with catalytic rhodium acetate gives the cyclopropane 52 via the carbene complex 53. Notice that the carbene inserts in only the less hindered of the two alkenes.12 This style of chemistry is treated in more detail in Strategy and Control. 

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). 

In principle, triplet carbene insertions should follow a two-step radical pathway analogous to their insertion into alkenes. However, very few triplet carbene insertions into C-H bonds have been observed, and the stereochemical consequence of the two-step mechanism (which should result in mixtures of stereoisomers on insertion into a C-H bond at a stereogenic centre) has never been verified. 

Explain why the carbene 4.123, anomalously for an X-substituted carbene, inserts into an alkene ... 

The first phosphonia-alkene (155) has been prepared as shown an X-ray crystal structure reveals a twist of 60° around the double bond The diphosphino-diazomethane (156) upon heating in benzene gave (157) by carbene insertion into one of the isopropyl groups. The first diphosphirene (158) has been prepared and... 

A study in chemoselectivity during carbene insertion into alkenes. 

The word carbene derives from the name given to free, disubstituted carbon compounds with general structure 3 (X and Y same as in l).2 Because the central carbon atom does not possess an octet of electrons, free carbenes are electron deficient and extremely reactive. They are so reactive that some carbenes insert themselves into normally inert alkane C-H bonds (equation 10.1) or react with alkenes to form cyclopropanes (equation 10.2), a synthetically useful transformation. 

Alkenes with a fairly unreactive double bond additionally form the products of carbene insertion into the C-H bond (see Houben-Weyl, Vol. E19b, pp 1494 and 1495), e.g. formation... 

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