Olefins reaction with carbenes

The fact that Schrock s proposed metallocyclobutanes decomposed to propylene derivatives rather than cyclopropanes was fortunate in that further information resulted regarding the stereochemistry of the olefin reaction with the carbene carbon, as now the /3-carbon from the metal-locycle precursor retained its identity. The reaction course was consistent with nucleophilic attack of the carbene carbon on the complexed olefin, despite potential steric hindrance from the bulky carbene. Decomposition via pathways f-h in Eq. (26) was clearly confirmed in studies utilizing deuterated olefins (67). 

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... 

Heteroatom Wittig chemistry also includes reactions of N-sulfonyl imines. It was demostrated that these compounds underwent olefination reactions with nonstabilized phosphonium ylides under mild conditions to afford an array of both Z- and E-isomers of 1,2-disubstituted alkenes, allylic alcohols, and allylic amines.Additionally, studies of the reactions of 5-bromo-4,6-dimethyl-2-thioxo-l,2-dihydropyridine-3-carboni-trile and thiazolidinone with phosphorus ylides have proved the formation of new phosphonium ylides. Annulations via P-ylides are a common occurrence in the literature. For example, on photochemical irradiation, phosphonium-iodonium ylides were shown to undergo 1,3-dipolar cycloaddition reactions with triple bonds, via a carbene intermediate, to yield furans. " Even more common are the reactions of Morita-Baylis-Hillman (MBH) acetates and carbonates. Zhou et al. demostrated that these substrates were able to generate very reactive 1,3-dipoles in the presence of tertiary phosphines the dipoles then underwent cycloaddition reactions to yield annulation products (Scheme 16). ... 

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. 

Tnmethyl(trifluororaethyl)tin can also be prepared via in situ formation and capture of tnfluorometbide by trimethyltin chlonde [13, 14] (equation 9) This tin analogue has been used as a precursor for difluorocarbene either by thermal decomposition or by reaction with sodium iodide m 1,2-dimethoxyethane This carbene generation procedure has been used to study difluorocarbene selectivity with steroidal olefins [75] (equation 10). 

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. 

The Diels-Alder reaction of activated olefins is considered as one of the most useful and predictable reactions in organic synthesis. The electron-acceptor character of the pentacarbonylmetal fragment makes a,/J-unsaturated carbene complexes ideal substrates for the [4S+2C] cycloaddition reaction with dienes. 

The reaction of methyl acrylate and acrylonitrile with pentacarbonyl[(iV,iV -di-methylamino)methylene] chromium generates trisubstituted cyclopentanes through a formal [2S+2S+1C] cycloaddition reaction, where two molecules of the olefin and one molecule of the carbene complex have been incorporated into the structure of the cyclopentane [17b] (Scheme 73). The mechanism of this reaction implies a double insertion of two molecules of the olefin into the carbene complex followed by a reductive elimination. 

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... 

EPR studies of diphenylmethylene and a number of other arylmethylenes have indicated that these carbenes have triplet ground states.<30) Photolysis of diphenyldiazomethane in olefin matrices results in the formation of triplet diphenylmethylene, which undergoes primarily abstraction reactions with the olefins. Cyclopropanes are produced as minor products. 

Electron-rich olefins such as 36 have been used by Lappert in the synthesis of a great number of mono-, bis-, tris-, and tetrakiscarbene complexes from various transition metal species (62). Ru, Os, and Ir carbene complexes have been prepared from reactions with these olefins, e.g.,... 

The Lewis acid-Lewis base interaction outlined in Scheme 43 also explains the formation of alkylrhodium complexes 414 from iodorhodium(III) meso-tetraphenyl-porphyrin 409 and various diazo compounds (Scheme 42)398), It seems reasonable to assume that intermediates 418 or 419 (corresponding to 415 and 417 in Scheme 43) are trapped by an added nucleophile in the reaction with ethyl diazoacetate, and that similar intermediates, by proton loss, give rise to vinylrhodium complexes from ethyl 2-diazopropionate or dimethyl diazosuccinate. As the rhodium porphyrin 409 is also an efficient catalyst for cyclopropanation of olefins with ethyl diazoacetate 87,1°°), stj bene formation from aryl diazomethanes 358 and carbene insertion into aliphatic C—H bonds 287, intermediates 418 or 419 are likely to be part of the mechanistic scheme of these reactions, too. 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

Thus, reactions affording either cyclopropanes or propylenes would most likely represent forms of termination of metathesis activity. As a corollary, any catalytic conversion of cyclopropanes to metathesis olefins via Eq. (26) would seem to require decomposition of the metal-carbene species in order to regenerate a naked metal species (M ) capable of further reactions with cyclopropanes. Of course, bimolecular carbene decomposition to yield an olefin as in Eq. (11) (e.g., ethylene from 2M=CH,) is one accepted process which could account for regeneration of M ... 

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 ... 

Fragmentation of 29 or 29 accounts for r-butylethene formation during the photolysis of 29. In thermolytic experiments, however, the yield of this olefin decreases as TME is added and carbene 20 is trapped. The inference is that fragmentation during thermolysis at 100°C is, at least partly, a reaction of carbene 20,46 with continued contribution from the fragmentation of 29. ... 

In contrast, 1,2-H shift to olefin 106 is the dominant reaction of carbene 104, and this process is slow enough to be measured by LFP r = 300 ns in cyclohexane and 560 ns in pentane at 25°C.117 There is a polar solvent effect the lifetime decreases to 52 ns in acetonitrile. However, at least in the case of cyclohexane, the lifetime is solvent limited, with a KIE of 1.5 on the lifetime in cyclohexane- (460 ns). Carbene 104 is much longer-lived than dimethylcarbene (r 21 ns in pentane) or methylcarbene (<1 ns).22,89... 

Only little information is available on carbene formation by rearrangement of bridgehead olefins, generated in thermal reactions. A prominent example is the rearrangement of bridgehead olefin 2, obtained as short-lived intermediate from bromosilane 1 by reaction with potassium fluoride in DMSO at 110°C. Carbene... 

The interpretation of the reaction cascade starting with carbenoids of type 73 and subsequently proceeding with carbenes 52 and bridgehead olefins 53 and ending with carbenes 54 would gain substantial support, if we could generate the carbene of type 52 by a different method and then show that similar products were obtained as via the oiganometallic route. 

In general, the reaction of carbanions with carbenes takes place smoothly to form primary product carbanions. However, they react further with electrophiles and this secondary reaction is difficult to control.36 To overcome this disadvantage, carbanions bearing an appropriate leaving group were designed and utilized for olefin synthesis (Scheme 23). 

The properties of carbenes are also expected to depend very greatly on the electronic characteristics of substituents bound to the divalent carbon. For example, many carbenes with heteroatomic elements attached directly to the central carbon are calculated to have single ground states (Mueller et al., 1981). The early, pioneering work on the stereochemistry of the reaction of carbenes with olefins was done with dibromocarbene (Skell and Garner,... 

Two of the most characteristic reactions of carbene complexes are olefin metathesis and olefin cyclopropanation. Olefin metathesis is a reaction in which the C-C double bond of an alkene is cleaved, and one of the resulting alkylidene fragments combines with the metal-bound carbene to form a new alkene. The second alkylidene fragment forms a new carbene eomplex with the metal. Olefin cyclopropanation is a reaction in which a a bond is formed between the metal-bound alkylidene and each of the two carbon atoms of the alkene, to yield a cyclopropane. 

Isonitrile complexes, having a similar electronic structure to carbonyl complexes, can also react with nucleophiles. Amino-substituted carbene complexes can be prepared in this way (Figure 2.6) [109-112]. Complexes of acceptor-substituted isonitriles can undergo 1,3-dipolar cycloaddition reactions with aldehydes, electron-poor olefins [113], isocyanates [114,115], carbon disulfide [115], etc., to yield heterocycloalkylidene complexes (Figure 2.6). 

When thiocarbonyl derivatives are treated with an excess of electrophilic carbene complex, alkenes are usually obtained [1333-1336], The reaction is believed to proceed by the mechanism sketched in Figure 4.18, closely related to the thiocarbonyl olefination reaction developed by Eschenmoser [1337], Few examples have been reported in which stable thiiranes could be isolated [1338], The intermediate thiocarbonyl ylides can also undergo reactions similar to those of carhonyl ylides, e.g. 1,3-dipolar cycloadditions or 1,3-oxathiole formation [1338], Illustrative examples of these reactions are given in Table 4.22. 

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