Carbocyclic carbene

Cp or Cp with elimination of N2 forming the corresponding carbenes Mn(CO)2(CR RML. The carbene precursor C H [C(N2)Ph]2 under similar conditions provides a bis-carbene complex. Reaction of the 7-membered carbocyclic carbene 69 with CO results in the n -ketene derivative 70. ... 

The simplest stable singlet carbene derived from a cyclic precursor is cyclo-propylidene. Like other carbocyclic carbenes " it does not contain a... 

The simplest stable singlet carbene derived from a cyclic precursor is cyclo-propylidene. Like other carbocyclic carbenes, it does not contain a heteroatom within the ring and is therefore not further discussed here. The first cyclic carbene based on a four-membered heterocycle was described in 2004. Deprotonation of iminium salt 87a with KHMDS gave carbene dimer 88a=88a while the same reaction with 87b, bearing even bulkier N,N -substituents, led to the isolation of 88b (Scheme 1.11). Attempts to deprotonate 87a with KOt-Bu resulted in the opening of an endocyclic P-N bond, demonstrating the electrophilic character of the phosphorus atom in this compound. 

Keywords Fischer carbene complexes Cycloaddition reactions Carbocycles Heterocycles... 

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

Catalytic reactions of disubstituted styrenyl substrates diminish oligomeric product formation because of the presence of ethylene. That is, if the initial transformation of the Ru-carbene occurs with the undesired regiochemis-try (e.g., 49->52 in contrast to 49->51, Scheme 13), dimerization and oligomerization may predominate, particularly in situations where reclosure of the carbocyclic ring is relatively slow (e.g., cycloheptenyl substrates). In contrast, as illustrated in Scheme 14, in the presence of ethylene atmosphere, the unwanted metal-carbene isomer 52 may rapidly be converted to triene 53. The resulting triene might then react with LnRu=CH2 to afford metal-carbene 51 and eventually chromene 41. 

Reactions of Titanium Carbene Complexes with Carbon—Carbon Double Bonds Table 14.1. Preparation of carbocyclic compounds from alkenyl thioacetals. 

Diazo compounds have been extensively used in the preparation of three-membered carbocycles either as carbene sources or as precursors for 1-pyrazolines or 3//-pyrazoles. Nitrogen extrusion from pyrazolines is particularly valuable for the synthesis of alkylcy-clopropanes, since the direct carbene route is impractical, as a matter of fact, owing to rapid intramolecular processes in alkylcarbenes. The cycloaddition of diazo compounds to unsaturated bonds to give 1-pyrazolines and 3/f-pyrazoles usually proceed in a concerted manner, and hence is stereospecific. In the subsequent nitrogen extrusion from the adducts,... 

These reactions are driven by a combination of factors, including a loss of ring strain or the release of a volatile olefin such as ethylene. They can also be kinetically controlled by the formation of a less reactive carbene complex. An important feature of RRM is the catalytic transfer of stereocentres from the corresponding substituted carbocycles, i.e. the chirality embedded in the carbocyclic starting material is completely transferred to the product side chain. This allows chirality to be introduced by means of side chains at the carbocycle. Synthetically, this... 

A common procedure used to forge carbocyclic systems is represented by the ring-closing metathesis reactions exploiting various Grubbs-type carbene catalysts. Based on this cyclization technology, Skaanderup and Madsen [71], on the way to polyhydroxy-nortropane calystegines, reported the asymmetric synthesis of aminated 6a-carbahexoseptanose derivatives 311. 

The benzannulation reaction tolerates phenyl carbene complexes bearing both electron-donating [35a] and -withdrawing [35b] substituents. Additionally, carbene complexes containing condensed systems such as naphthalene and other carbocyclic hydrocarbons as well as heterocydes have also been successfully submitted to the reaction (see Section 8.5). 

Bi- and tricyclic carbocyclic ring systems can be easily obtained from (1-alkynyl)carbene complexes in two or more reaction steps without isolation of intermediate products. For example, the bicyclic 1,3-dienone 44 has been generated from compound le in a two-step cycloaddition/annulation procedure involving [4+2] cycloaddition of a diene to the C=C bond of a (l-alkynyl)carbene complex and a subsequent Dotz reaction.68 Furthermore, a 1,4-dioxy dihydronaphthalene 45 was obtained from the silyl derivative lg in a similar reaction sequence. In contrast to the methyl derivative 44, the corresponding silyl derivative generated from lg undergoes facile... 

RCM has attracted much attention and has seen a tremendous increase in synthetic applications over the last decade <2000CR2963, 2006JOM(691)5129>. In this reaction, two C-C multiple bonds, such as double and double, or double and triple in the same molecule, are converted to unsaturated carbocycles or heterocycles in the presence of a metal carbene complex. The versatility of Schrock s molybdenum catalyst and Grubbs ruthenium complexes 68 and 69 (Scheme 10) in carbo- and heterocyclizations, respectively, of very different ring sizes were demonstrated <2000CR2963, 2006JOM(691)5129>. 

Examples of the vinylcyclopropene photorearrangement are known where the chromophore is external to the three-membered ring. The results suggest that the carbene mechanism operates and that it is not contingent upon the chromophore being a part of the carbocycle. For example, direct photolysis of 245d leads to three photoproducts (equation 79) and the presence of an allene requires a [1,2]H shift in the carbene intermediate. 

Mo2(C5Hs)2(CO)/ or [Mo( -MeC=CMe)(PhS(0)SPh)( /-C5H5)] produce complexes derived from fission of the cyclopropene C(l)-C(3) bond. However, treatment of the last of these carbocycles with the binuclear rhodium complex [Rh2(ju-CO)2( /-C5Me5)2] results in the cleavage of the carbon-carbon double bond and formation of the bridged carbene complex 313. ... 

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