Subject carbenes

Carbene chemistry in general has been the subject of various reviews. ... 

Vinyl cyclopropanes tethered to an aUcyne chain 127 were also subjected to the cycloisomerisation reaction in presence of the NHC-Ni catalyst system (Scheme 5.34) [39], The product formation depends on the substrate used and the NHC hgand. When SIPr carbene is used, three different products were obtained depending on the size of the R group attached to the alkyne moiety. If R is small (like a methyl) product 128 is obtained exclusively. If R is Et or Pr a mixture of 128 and 129 is obtained in 3 2 to 1 2 ratio, respectively. However, when R is large groups such as Bu or TMS only product 130 is obtained. When IfBu carbene 131 is used as the ligand, cycloisomerisation of 127 afforded product 128 exclusively, regardless of substituent size (Scheme 5.34) [39]. 

This article is an attempt at evaluating new important features of tin(II) chemistry the central point is the interrelationship between molecular structure and reactivity of molecular tin(II) compounds. To define these compounds more closely, only those are discussed which are stable, monomeric in solvents and which may be classified as carbene analogs21. Thus, not a complete survey of tin(II) chemistry is given but stress is laid on the structures and reactions of selected compounds. A general introduction to the subject precedes the main chapters. For comparison, also solid-state tin(II) chemistry is included to demonstrate the great resemblance with molecular tin(II) chemistry. Tin(II) compounds, which are either generated as intermediates or only under definite conditions such as temperature or pressure, are not described in detail. 

Only limited number of neutral monocyclic 11,3,2 diazaphosphole representatives have been reported, which have mostly been prepared by [4+1] cyclocondensation of diaminomaleodinitrile (DAMN) with P(III) reagent and the alkylation of the initially formed 1,3,2-diazaphospholide [2, 4, 7], During recent times, 67t-aromatic [l,3,2]diazaphospholium ions of type 46 [45], more often represented as cyclic phosphenium cation 47 [46,47], have attracted more attention due to their isoelec-tronic nature with Arduengo carbenes . Nature of bonding and aromaticity of these cations have been the subject of several experimental and theoretical studies (Structure 2) [48-52],... 

The preparation of [5]rotane 81 (pentaspiro[2.0,2.0.2.0.2.0.2.0]pentadeeane), again includes one step of carbene addition, by the Simmons-Smith reaction, to 13-methyle-ne-tetraspiro[2.0.2.0,2.0.2.1]tridecane 8026). A full paper on the subject appeared later 27 >. 

The Ru porphyrin complex (8) has also been used as a catalyst for the cyclization of allylic diazoacetates,258 albeit with limited success only the cyclization of F-cinnamyl diazoacetate shows high enantioselectivity (Scheme 81). It is noteworthy that a carbenoid species prepared from allyl o-phenyl-o-diazoacetate and complex (8) has been isolated and subjected to X-ray diffraction analysis, though it does not undergo the desired cyclization. In the structure, the carbene plane lies almost halfway between the two adjacent Ru—N bonds. 

Although not the central subject of this review, several thorium dihaptocarbamoyl complexes ( 4) Th(n2-CONR2)/ were also examined with respect to thorium hydride-catalyzed reduction. Under 0.75 atm H2 and over the course of several days at temperatures as high as 100°C, no hydrogenation was observed. These results are in accord with other spectral, structural, and chemical data (14) indicating the importance of carbamoyl resonance hybrids O and P, and that the carbene-like reactivity is significantly reduced in comparison to the acyls (14) ... 

Equilibrium studies. Acidities of thiazolium cations, such as 213, cannot be measured in basic aqueous solution because the hydroxyl adduct ( pseudobase ) 216 of the thiazolium cation is formed rapidly and is subject to base-catalyzed ring opening (Scheme 36).151 In DMSO, formation of the carbene dimer 217 from 213 and 214 is a complicating factor.8 If indicator anion (In") was added to a solution of 213, a very rapid drop in absorbance was followed by a somewhat... 

It is obvious, that matrix isolation spectroscopy is the method of choice for the study of carbenes. The large number of carbenes identified by matrix isolation illustrates the strength of this analytical tool, as demonstrated by an excellent review on this topic by Sander et al. in 199314. In this comprehensive article all the work carried out until that date has been summarized. The goal of our review is to report on our own contributions to this field and to complete Sander s overview — taking over his classification of individual carbenes — by results achieved by us in this subject matter during the most recent epoch. 

Vinylidenecarbene or allenylidene3 (R)2C=C=C has a lance-shaped, unsubstituted and sp-hybridized carbene center and, therefore, will not be easily subject to steric hindrance in its insertion reactions. On this assumption, (2-methyljpropenylidenecarbene or its carbenoid was chosen as a prototype of typical vinylidenecarbenes and its insertion reaction with several different types of alkoxides was investigated by employing two methods (A and B, Scheme 10) for carbene generation.20 The insertion products 20 were obtained almost exclusively except lithium allyloxide (Table 4, entry 10).21 By-products such as propargyl ether and allenyl ether were not formed at all. To be noted here, in... 

Although alkylidenecarbenes (R)2C=C and carbenoids 22-24 have an ip-hybridized carbene center similar to that of vinylidenecarbenes, the reactivity will be subject to the steric influence of substituents R3 and R4 because its location is closer to the carbene center than vinylidenecarbenes (Scheme 11). The steric effect was exerted in the reactions of 2-methylpropenylidene 22 generated from 2-methyl-1-chloropropene and butyllithium (BuLi) (Scheme ll).22 23 The results are summarized in Table 5. A more detailed discussion on the stereoselectivity of this reaction will be revisited in Section HI. A. 

This section diverges from the main subject but, being studied together with the insertion of (phenylthio)carbenes with alkoxides, is worth describing here due to its synthetic versatility as C-C bond forming reactions. 

The subject of this chapter is carbenes with aryl substituents (aromatic carbenes). These materials are short-lived reactive intermediates in which the normal tetravalency of carbon is reduced by two. Carbenes have been the object of speculation and investigation for more than 80 years. Nevertheless, there still is considerable uncertainty about their chemical and physical properties. In the last five years the pace of research in carbene chemistry has quickened. This is a consequence of the development of high-speed pulsed lasers that permit, for the first time, direct observation of carbenes under the conditions in which they react. This research has provided new information on the effect of structure on the chemical and physical properties of carbenes. 

The presentation of carbene chemistry in this chapter is by no means complete or exhaustive. The subject is simply too large to be covered in detail within the allotted space. However, there are available comprehensive monographs and other more sharply focused reviews covering features of carbene chemistry neglected here (Closs, 1968 Bethell, 1969 Kirmse, 1971 Moss and Jones, Jr., 1973 Jones, Jr. and Moss, 1975, 1978, 1981, 1985 Abramovitch, 1980 Griller et a ., 1984a Eisenthal et al., 1984 Wentrup, 1984). 

Despite the complications, and with a few reservations (see DPM below), the chemical properties of singlet and triplet carbenes are generally distinct and separable. For the most part, triplet carbenes behave as biradicals, and the singlets as electrophiles. Of course there are exceptions, but this generalization appears to be true for the aromatic carbenes which are the subject of this report. In the remaining parts of this section we discuss some of the particular reactions used to characterize the carbenes listed in Tables 1 and 2. 

Cyclohexane and cyclohexane-d12 have been used as the probe for crossover and, hence, the reactive multiplicity of the subject carbenes. By combining direct and triplet-sensitized generation of the carbene with kinetic analysis from laser spectroscopy and the results of the crossover experiments, a rather complete picture of the reaction of aromatic carbenes with hydrocarbons emerges. 

This review focuses on the cross-metathesis reactions of functionalised alkenes catalysed by well-defined metal carbene complexes. The cross- and self-metath-esis reactions of unfunctionalised alkenes are of limited use to the synthetic organic chemist and therefore outside the scope of this review. Similarly, ill-defined multicomponent catalyst systems, which generally have very limited functional group tolerance, will only be included as a brief introduction to the subject area. 

A relatively unique type of reactive metabolite is carbene, i.e., a divalent carbon, which is a proposed intermediate in the oxidation of methylene dioxy-containing compounds. A methylenedioxy group in aromatic compounds is subject to O-dealkylation, e.g., 3,4-methylenedioxyamphetamine, as shown in Figure 8.20. The process generates formic acid and the catechol metabolite as final products. However, in the course of the reaction, a... 

An alternative mechanistic scenario for the initial steps of this reaction (associative route) was the subject of a study by TDS [27, 39], It considers the possibility that the cycloaddilion with alkynes takes place initially by direct reaction with the coordinatively saturated chromium carbene complex 9. 

Steric effects were also found to be important for determining the reactivity of rhodium complexes containing N-heterocyclic carbene (NF1C) ligands [47] (Scheme 10), which have been the subject of intense in-... 

Acceptor-substituted carbene complexes are electrophilic intermediates which react readily with lone pairs, giving the corresponding ylides. These can be valuable intermediates, capable of undergoing a broad range of synthetically useful transformations. This subject has been treated in several reviews [38,995,1077-1079,1086]. 

The subject of this book has been organized in three main sections preparation and applications of heteroatom-substituted carbene complexes (Fischer-type carbenes), non-heteroatom-substituted carbene complexes, and acceptor-substituted carbene complexes. In each section the different types of reaction have been ordered either according to the mechanism or according to the type of product. In addition to a selection of illustrative examples, several experimental procedures have been included. These were chosen taking into account safety, availability of starting materials, relevance of the products, and general interest. 

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

When 2,2-dichloro-3-phenylpropanal 203 is subjected to standard reaction conditions with chiral triazolium salt 75c, the desired amide is produced in 80% ee and 62% yield Eq. 20. This experiment suggests that the catalyst is involved in an enantioselec-tive protonation event. With this evidence in hand, the proposed mechanism begins with carbene addition to the a-reducible aldehyde followed by formation of activated car-boxylate XLII (Scheme 32). Acyl transfer occurs with HOAt, presumably due to its higher kinetic nucleophilicity under these conditions, thus regenerating the carbene. In turn, intermediate XLin then undergoes nucleophilic attack by the amine and releases the co-catalyst back into the catalytic cycle. 

A difference in reaction efficiency was observed depending on the catalyst used. Imidazolium salt 305 provides the highest yield of desired product. When preformed complex 307 is subjected to the reaction conditions, fran -2-ethylcyclohex-anol is detected by gas chromatography in 76% yield (Eq. 30). Alkylation starting with free carbene 306 results in only 28% yield of desired alkylated epoxide. 

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