Carbene reaction with olefins

The preparation of cyclopropane derivatives has been greatly facilitated by the development of carbene-type intermediates (see Chapter 13) and their ready reaction with olefins. The preparation of phenylcyclopropane from styrene and the methylene iodide-zinc reagent proceeds in only modest yield, however, and the classical preparation of cyclopropane derivatives by the decomposition of pyrazolines (first employed by Buchner in 1890) is therefore presented in the procedure as a convenient alternative. 

In contrast to alkoxycarbene complexes, most aminocarbene complexes appear too electron-rich to undergo photodriven reaction with olefins. By replacing aliphatic amino groups with the substantially less basic aryl amino groups, modest yields of cyclobutanones were achieved (Table 10) [63], (Table 11) [64]. Both reacted with dihydropyran to give modest yields of cyclobutanone. Thio-carbene complexes appeared to enjoy reactivity similar to that of alkoxycar-benes (Eq. 15) [59]. 

One of the earliest reported thermal reactions of Fischer carbene complexes was the reaction with olefins to give cyclopropanes [127]. More recently it has been shown that photolysis accelerates inter molecular cydopropanation of electron-poor alkenes [128]. Photolysis of Group 6 imine carbenes with alkenes... 

In marked contrast to the results of Gassman and Schrock, major differences were noted by Casey and co-workers in a series of studies utilizing phenylcarbene-substituted W(0) complexes in reactions with olefins. The H NMR spectra of new phenylcarbene tungsten and iron (69) complexes indicate a substantial positive charge residing on the carbene carbon, and as expected, these complexes readily form ylides on reaction with phosphines ... 

Among typical carbon-carbon bond (C-C) formation reactions with carbenes, the cyclopropanation reaction with olefins has been well studied including its application to industrial processes. The second typical reaction of carbenes is the insertion reaction into the carbon-hydrogen bond (C-H) which seems to be a direct and efficient C-C bond forming reaction. However, its use for synthetic purpose has often been limited due to low selectivity of the reactions.3... 

Substituted cyclopropylidenes have been shown to participate in both inter-and intramolecular addition reactions with olefins. The resulting products are spiropentane derivatives as well as carbene dimers which are formed as side-products [99, 100]. In the absence of olefinic reaction products the latter may even become the main products [99 b],... 

Important evidence in support of the chain-reaction theory was the synthesis of carbene complexes, such as diphenylcarbenepentacarbonyl-tungsten(O) (4), and their reaction with olefins. The results in Eq. (12.16) demonstrate69 the scission of the carbon-carbon double bond of the alkene and the combination of the ethyl-idene fragment with the diphenylcarbene group of 4 ... 

Other Carbene or Carbenoid Reactions with Olefins... 

The potential of laser flash photolysis in the study of carbene reactions with heteroatoms has come to be recognized in recent years. A number of kinetic studies using this technique have been carried out with carbene precursors in nitrile solvents.122-127 An absorption band at 470 nm was observed in the laser flash photolysis of diazofluorene (246) in inert solvents. This band was assigned to triplet fluorenylidene (247). In acetonitrile, however, a second band was also detected at 400 nm and whose buildup is concurrent with the decay at 470 nm.122 Laser flash experiments in other nitrile solvents (i.e., benzonitrile and pivalonitrile) also produced a transient absorption band which is very similar to that observed in acetonitrile. The band at 400 nm was assigned to an intermediate nitrile ylide (248). This absorption could be quenched on addition of an electron-deficient olefin providing good support for its... 

The mechanism indicates that intermediate formation of a carbene, but tests to confirm the presence of the carbene by possible addition reactions with olefins were negative. The formation of both cyclohexene and cyclohexene are depicted as having the carbene as a common precursor. The experimental data do not, however, rule out the formation of these molecules from diffei ent precursors. For example, cyclohexend could be formed directly from the excited diazirine molecule. Again, further consideration of this point will be deferred until later. 

Phenylthiocarbene, generated by phase-transfer catalysis from (phenylthio)chloromethane adds stereospecifically to ( )-1,2-diphenylethene to provide, 2-trans-2,3-cis-, 2-diphenyl-3-(phenylthio)cyclopropane1. The simple diastereoselectivity of reactions with olefins such as cyclohexene and phenylethene is moderate to good, favoring the endo- or cis-products. For further examples of sulfur-substituted carbenes, see Houben-Weyl Vol. E 19b, pp 1682-1745. 

Turning to carbene related reactive species, alkylidene carbenoids like 26 (X = halogen, OR, NR2) are particularly valuable for preparative purposes since they can undergo cycloaddition reactions with olefins (to methylenecyclopropanes), isomerizations (to alkynes by the so-called Fritsch-Buttenberg-Wiechell rearrangement), and dimerization (to [3]cumulenes). Although carbenoids have been studied extensively by NMR spectroscopy [23], the first X-ray structural analysis of a stable carbenoid, 27, as a TMEDA 2THF complex has been reported only recently [24]. 

The reaction is of great interest because the strongest bond in alkene, the C=C bond, is broken during the reaction. The widely accepted so-called Chauvin mechanism [42] suggests that transition-metal carbene complex acts as catalyst by undergoing a [2 -I- 2J cycloaddition reaction with olefin, via a metallacyclobutane intermediate ... 

Until the last decade, product studies formed the main evidence for carbene formation singlet carbenes formed cyclopropanes from alkenes stereospecifically, while triplet carbenes formed cyclopropanes non-stereospecifically. Formation of a cyclopropane (though not by addition to an alkene) via a carbocation route was demonstrated and, more recently, it has been shown that p values for insertion-addition selectivity and for cyclopropanation stereoselectivity vary as to photochemical or thermal generation of the carbene. The authors of this latter study suggest that a ground state diazo compound could be masquerading as a carbene in its thermal reaction with olefins, possibly by electrocyclic... 

The reactions of metal carbene complexes with olefins can be readily followed by H NMR spectroscopy. An early example was the reaction of W(=C//CMe3)(=0)(Cl)2(PEt3)2/AlCl3 (5 12.03) with but-l-ene to give... 

Nevertheless, the use of relative reactivities to characterize carbenic philicity is restrictive the apparent philicity is related to the alkenes selected for the relative reactivity measurements. What if the set of alkenes were expanded by the addition of an even more electron-deficient alkene Such a test was applied in 1987 [65], using a-chloroacrylonitrile, 26, which is more 7t-electron deficient than acrylonitrile, 27. We found that PhCF or PhCCl added 15 or 13 times, respectively, more rapidly to 26 than to 27. In preferring the more electron-deficient olefin, the carbenes exhibited nucleophilic character. However, because they also behave as electrophiles toward other alkenes (Table 4), they must in reality be ambiphiles. In fact, we now realize that all carbenes have the potential for nucleophilic reactions with olefins the crucial factor is whether the carbene s filled a orbital (HOMO)/alkene vacant Ji orbital (LUMO) interaction is stronger than the carbene s vacant p orbital (LUMO)/aIkene filled k orbital (HOMO) interaction in the transition state of the addition reaction. [63]... 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

The acid-induced reaction of aryldiazomethanes with olefins gives arylcyclo-propanes in addition to olefins and esters. The cyclopropanes are formed stereo-specifically and their yields are largest in reactions with olefins which on cation addition give secondary carbonium ion centres. The use of deuteriated acids leads to partial incorporation of deuterium in the cyclopropane adducts, whereas the use of [a- H]-phenyldiazomethane leads to partial loss of deuterium, suggesting a slow proton transfer from the acid to the diazo-compound a carbenoid rather than a free carbene appears to be involved. 

Complexes of nucleophilic carbenes are expected to react, like ylids, with electrophiles whereas complexes of electrophilic carbenes are expected to react, like carbocations, with nucleophiles and bases. All the complexes of terminal carbenes have in common the reactions with olefins, although their nature also varies. The principles of these reactions are detailed here, and application in catalysis and organic synthesis, are exposed in Parts IV and V respectively. Reactions of metal-carbene complexes leading to metal-carbyne complexes are mentioned in section 2. 

Reactions of carbene complexes with olefins and various modes of evolution of metallocycles... 

The reactions of metal-carbene complexes with olefins are important in catalysis and organic synthesis. The first reaction disclosed by Pettit in 1966 involved transient [FeCp(CO)2(=CH2)] that reacted with styrene to give phenylcyclopropane resulting from methylene transfer from the metallacarbocation to the olefin, a reaction that was shown later to be general with iron-methylene complexes of this series. ... 

More recently, Zibinsky and Fokin obtained synthetically useful Rh (11) carbenes from7V-(l,2,4-triazolyl)-substituted 1,2,3-triazoles and Rh(II) carboxylates [45], The resulting carbenes provided ready asymmetric access to secondary homoaminocy-clopropanes 68 (80-95% ee, dr >20 1) via reactions with olefins and also engaged in efficient transannulation reactions with nitriles (Scheme 5.47). 

Several diazo compounds, carbene precursors, containing iron, silicon, cadmium, zinc, germanium, lead, tin, mercury, silver, or lithium are knovm. Ferrocenylphenylcarbene and ferrocenylmethylcarbene are found to behave similar to diphenylcarbene in their reactions with olefins (Scheme 54). ... 

Carbene Reactions. The best procedure for preparing dihalocarbene adducts of olefins consists in stirring a haloform—methylene chloride solution with an excess of concentrated aqueous caustic soda in the presence of hen 2y1triethy1 amm onium chloride. Even stericahy hindered and electronically deactivated compounds give excellent yields (32). Mixed dihalocarbenes, CXY (X,Y = E, Cl, Br, I), except for CE2, can be prepared. 

Contents Introduction and Principles. - The Reaction of Dichlorocarbene With Olefins. - Reactions of Dichlorocarbene With Non-Olefinic Substrates. -Dibromocarbene and Other Carbenes. - Synthesis of Ethers. - Synthesis of Esters. - Reactions of Cyanide Ion. - Reactions of Superoxide Ions. - Reactions of Other Nucleophiles. - Alkylation Reactions. - Oxidation Reactions. - Reduction Techniques. - Preparation and Reactions of Sulfur Containing Substrates. -Ylids. - Altered Reactivity. - Addendum Recent Developments in Phase Transfer Catalysis. 

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