Carbenes halocarbenes

More useful for synthetic purposes, however, is the combination of the zinc-copper couple with methylene iodide to generate carbene-zinc iodide complex, which undergoes addition to double bonds exclusively to form cyclopropanes (7). The base-catalyzed generation of halocarbenes from haloforms (2) also provides a general route to 1,1-dihalocyclopropanes via carbene addition, as does the nonbasic generation of dihalocarbenes from phenyl(trihalomethyl)mercury compounds. Details of these reactions are given below. 

Carbene itself is extremely reactive and gives many side reactions, especially insertion reactions (12-19), which greatly reduce yields. When it is desired to add CH2 for preparative purposes, free carbene is not used, but the Simmons-Smith procedure (p. 1088) or some other method that does not involve free carbenes is employed instead. Halocarbenes are less active than carbenes, and this reaction proceeds quite well, since insertion reactions do not interfere.The absolute rate constant for addition of selected alkoxychlorocarbene to butenes has been measured to range from 330 to 1 x 10 A few of the many ways in... 

Which isomer is predominantly formed depends on R, R, and on the method by which the carbene or carbenoid is generated. Most studies have been carried out on monosubstituted species (R = H), and in these studies it is found that aryl groups generally prefer the more substituted side (syn addition) while carbethoxy groups usually show anti stereoselectivity. When R = halogen, free halocarbenes show little or no stereochemical preference, while halocarbenoids exhibit a preference for syn addition. Beyond this, it is difficult to make simple generalizations. 

Decomposition of diazo compounds by iron porphyrins is a convenient method for the synthesis of non-heteroatom carbene-iron porphyrins [22]. Reaction of [Fe(F2o-TPP)] [F20-TPP = meso-tetrakis(pentafluorophenyl)porphyrinato dianion] with diazo compounds N2C(Ph)R (R = Ph, C02Et, C02CH2CH=CH2) under an inert atmosphere afforded complexes [Fe(F2o-TPP)C(Ph)R] in 65-70% yields (Scheme 2). Like the halocarbene complex [Fe(TPP)(CCl2)], [Fe(F2o-TPP)CPh2] reacted with Melm to afford six-coordinate species [(MeIm)Fe(F2o-TPP)CPh2] in 65% isolated yield. 

Further studies were carried out with halocarbene amides 34 and 357 Although again no direct spectroscopic signatures for specifically solvated carbenes were found, compelling evidence for such solvation was obtained with a combination of laser flash photolysis (LFP) with UV-VIS detection via pyridine ylides, TRIR spectroscopy, density functional theory (DFT) calculations, and kinetic simulations. Carbenes 34 and 35 were generated by photolysis of indan-based precursors (Scheme 4.7) and were directly observed by TRIR spectroscopy in Freon-113 at 1635 and 1650 cm , respectively. The addition of small amounts of dioxane or THF significantly retarded the rate of biomolecular reaction with both pyridine and TME in Freon-113. Also, the addition of dioxane increased the observed lifetime of carbene 34 in Freon-113. These are both unprecedented observations. 

The methods for generating these types of ions are the same as those described above for diradicals and carbenes. Eor example, the reaction of dichloromethane with O leads to the formation of CCl2. Thus, Jesinger and Squires have used CID of halocarbene anions to determine the thermochemical properties of carbynes (Eq. 5.16). ... 

Halide displacement from the carbene ligands of Ru, Os, and Ir halocarbene complexes by N-, O-, and S-based nucleophiles frequently leads to the formation of new heteroatom-substituted carbene complexes. This important class of reactivity will be discussed in more detail in Section V,D, but it is appropriate here to illustrate the scope of this method with several examples ... 

Although we are not specifically concerned here with kpp and the kinedcs of carbene-pyridine ylide formation, we note that the magnitude of is directly related to the structure and reactivity of the carbene. fcpyr ranges from 105 M s-1 for ambiphilic alkoxycarbenes to 109-10I° M-1 s 1 for electrophilic halocarbenes or alkylcarbenes. Very nucleophilic carbenes (MeOCOMe) do not react with pyridine.13... 

As shown in Figure 9.2(a), an X substituent, which has a p orbital, or other suitable doubly occupied orbital that will interact with the n bond, raises the 2p orbital of the carbene, thereby increasing the separation of the 2p and ip" (a) orbitals. The ground state of an X -substituted carbene becomes a singlet, and many carbenes in this class are known. The most familiar of these are the halocarbenes. 

K. N. Houk, N. G. Rondan, and J. Mareda, Theoretical Studies of Halocarbene Cycloaddition Selectivities. A New Interpretation of Negative Activation Energies and Entropy Control of Selectivities, Tetrahedron 1985, 41, 1555. Calculations on carbene addition reactions led to a general explanation of why it is possible for very exothermic, bimolecular reactions to have negative activation enthalpies. 

CHi generated by photolysis of CH2N2 in the liquid phase is indiscriminate—totally non-selective—in its reactivity (p. 199). CH, generated in other ways and other carbenes are less reactive and insert in the order tertiary > secondary > primary.232 Halocarbenes insert much less readily, though a number of instances have been reported.233 Nevertheless, even for less reactive carbenes, the insertion reaction has seldom been used for synthetic purposes.234 The carbenes can be generated in any of the ways mentioned in Chapter 5 (p. 198). For the similar insertion of nitrenes, see 2-12. 

Negative activation energies for cycloaddition reactions of some carbenes are reported60 and they confirm the presence of a pre-association equilibrium on the reaction pathway. In addition, the entropy control of the cycloaddition of halocarbenes to the C=C double bond was extensively reported61 and explained by the presence of a weakly bound intermediate complex (25), which is reversibly formed and is probably a CT complex62. Its presence is also supported by direct kinetic data59,63. 

Concurrent cyclopropanation by carbenes and carbanions has been investigated.163 It has been demonstrated that the deliberate addition of halide ions afforded concurrent cyclopropanation of electron-poor alkenes by an equilibrating mixture of phenyl-halocarbenes and phenyldihalomethide carbanions, permitting smooth modulation of selectivity between electron-rich and electron-poor alkenes. 

New carbene complexes may arise from the modification of a preformed carbene ligand. Examples have already been met in the case of halocarbenes (Figure 5.10) however, further examples will be discussed under the topic of carbene reactivity. 

IS the most popular, one-step method for m ag fluorinated cyclopropanes and cyclopropenes a-Fluorocarbenes are particularly well behaved, because they all have singlet ground states [/, 2] and therefore usually add stereospecifically to alkenes and do not insert into C-H bonds competitively with addition Moreover, quantitative competition studies of carbene additions to alkenes near room temperature show that a-fluorocarbenes are more selective than other a-halocarbenes, with difluorocarbene being the most selective electrophihc carbene known [3, 4] The relative selectivities, however, can be quite temperature dependent [5, d] The numerous preparations and cycloaddmons of fluorocarbenes have been reviewed thoroughly [7, 8 9,10 ... 

Cyclopropanation of carbon-carbon double bonds with methylene can be achieved in several ways. Aside from the carbene or carbenoid additions, activated multiple bonds may be cyclopropanated by reaction with ylides or by a synthetic route via 1-pyrazolines (see Sections III. B and III. D in this chapter, and also Chapter 9). Addition of halocarbenes followed by hydrogenolysis provides a further route for the cyclopropanes (Section V.A). 

Heterosubstituted carbenes as C(l) unit In 1963 Ohno reported the cycloaddition of dichlorocarbene onto enamines (128). Subsequently, the formation of aminocyclopro-panes (129) from 128 and halocarbenes was investigated by several groups ° (equation 32). CHHala and t-BuOK. 22s, 230,233,235.242 NaOCMe2Et2 BuLi or... 

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