Cycloadditions carbene synthesis

Keywords Fischer carbenes Template synthesis Cocyclization Cycloaddition Cyclopentadienes Cyclopentenones Domino reactions... 

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. 

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

Other examples of [2C+2S+1C0] cycloaddition reactions have been described by Herndon et al. by the use of chromium cyclopropyl(methoxy)carbenes. These complexes react with alkynes releasing ethene and forming cyclopenta-dienone derivatives, which evolve to cyclopentenone derivatives in the presence of chromium(O) and water [122] (Scheme 76). This reaction has been extended to intramolecular processes and also to the synthesis of some natural products [123]. These authors have also described another process involving a formal [2C+2S+1C0] cycloaddition reaction. Thus, the reaction of methyl and cyclo-propylcarbene complexes with phenylacetylene derivatives does not afford the expected benzannulated products, and several regioisomers of cyclopentenone derivatives are the only products isolated [124] (Scheme 76). 

Another example of a [2s+2sh-1c+1co] cycloaddition reaction was observed by Barluenga et al. in the sequential coupling reaction of a Fischer carbene complex, a ketone enolate and allylmagnesium bromide [120]. This reaction produces cyclopentanol derivatives in a [2S+2SH-1C] cycloaddition process when -substituted lithium enolates are used (see Sect. 3.1). However, the analogous reaction with /J-unsubstituted lithium enolates leads to the diastereoselective synthesis of 1,3,3,5-tetrasubstituted cyclohexane- 1,4-diols. The ring skeleton of these compounds combines the carbene ligand, the enolate framework, two carbons of the allyl unit and a carbonyl ligand. Overall, the process can be considered as a for-... 

Besides the formation of carbenes from diazo compounds and the hydroformyla-tion, rhodium (as described previously for palladium) has also been used as catalyst in domino processes involving cycloadditions. Thus, Evans and coworkers developed a new Rh(I)-catalyzed [4+2+2] cycloaddition for the synthesis of eight-membered rings as 6/2-105 using a lithium salt of N-tosylpropargylamines as 6/2-104, allyl carbonates and 1,3-butadiene (Scheme 6/2.22) [221]. The first step is an al-... 

The earlier examples of [2 + 1] cycloaddition of a carbene (or carbenoid) on the double bond of alkylidenecyelopropanes to yield spiropentane derivatives were observed as undesired side reactions in the synthesis of alkylidenecyelopropanes through the addition of a carbene to a substituted allene [161]. In some cases the spiropentane derivative was obtained as the major product [161a, c] especially when a large excess of the carbene reagent was used. For example, when methyl 3,4-pentadienoate (610) was treated with a ten-fold excess of methylene iodide and zinc-copper couple the two products 611 and 612 were isolated in 1 4.5 ratio (Scheme 86) [161a]. 

The presence of a cyclopropyl moiety in the carbene complexes makes them useful for synthesis. The cyclopropylcarbene complexes 95 undergo a cycloaddition reaction with alkynes to give the cyclopentenones 96 [51]. The reaction course is explained as being metallacyclopentene fragmentation. (Scheme 34)... 

Cycloaddition of the carbene chromium complexes 97 with CO incorporation provides a versatile method for naphthol synthesis, in which the metallacy-clic intermediates 99 are involved [47]. An alternative entry to 101 is achieved by metal carbonyl-catalyzed rearrangement of the cyclopropenes 98 via the same metalla-cyclobutenes 99 and vinylketene complexes 100 [52], Mo(CO)6 shows a higher activity than Cr(CO)6 and W(CO)6. The vinylketene complex 103 is formed by the regioselective ring cleavage of 1,3,3-trimethylcyelopropene 102 with an excess of Fe2(CO)9 [53]. (Scheme 35 and 36)... 

Independently Volpin17 synthesized diphenyl cyclopropenone from diphenyl-acetylene and dibromo carbene (CHBr3/K-tert.-butoxide). This reaction principle of (2 + 1) cycloaddition of dihalocarbenes or appropriate carbene sources ( caibenoids ) to acetylenic triple bonds followed by hydrolysis was developed to a general synthesis... 

In addition to conventional generation of carbenes from nitrodiazo compounds (22) (79), target intermediates C can be prepared by oxidation of functionalized AN CH2XNO2 with phenyliodonium diacetate. The reactions of Rhodium intermediates with certain olefins afford the corresponding cyclopropanes (23). The cycloaddition reaction was performed in the presence of a catalyst. (The successful synthesis of nitrocyclopropanes from trinitromethane derivatives and nitroacetic ester was also documented (81)). 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

The use of group 6 heteroatom stabilized carbene complexes (Fischer carbene complexes) in organic synthesis is relatively recent and, in spite of that, it has already produced impressive synthetic results [1]. These versatile organometallic reagents have an extensive chemistry, and they are probably one of the few systems that undergo cycloadditions of almost any kind. For instance, [1+2], [2+2], [3+2], [3+3], [4+1], [4+2], [4+3], [6+3]... 

Keywords Absolute configuration, Amines, Amino acids, Carbenes, Cascade reactions, 2-chloro-2-cyclopropylideneacetates. Combinatorial libraries. Cycloadditions, Cyclobutenes, Cyclopropanes, Diels-Alder reactions. Heterocycles, Michael additions. Nitrones, Nucleophilic substitutions, Peptidomimetics, Palladium catalysis. Polycycles, Solid phase synthesis, Spiro compounds. Thiols... 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

The carbonyl ylide generated from metal carbene can also add to C=0 or C=N bonds. The [2 + 3]-cycloaddition of carbonyl ylide with G=0 bond has been used by Hodgson and co-workers in their study toward the synthesis of zaragozic acid as shown in Scheme n 27a,27d Recently, a three-component reaction approach to syn-a-hydroxy-f3-amino ester based on the trapping of the carbonyl ylide by imine has been reported.The reaction of carbonyl ylide with aldehyde or ketone generally gives l,3-dioxolanes. Hu and co-workers have reported a remarkable chemoselective Rh2(OAc)4-catalyzed reaction of phenyl diazoacetate with a mixture of electron-rich and electron-deficient aryl aldehydes. The Rh(ii) carbene intermediate reacts selectively with electron-rich aldehyde 95 to give a carbonyl ylide, which was chemospecifically trapped by the electron-deficient aldehyde 96 to afford 1,3-dioxolane in a one-pot reaction (Equation (12)). 

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