Carbenes alkene synthesis

Acyloxy)alkenyl carbene complexes, synthesis, 223-224 Alkenes, bishydroxylation cycloreversion of rhenium diolates, 148-156... 

This section only covers [2+ l]-cycloaddition reactions performed with carbenes. The synthesis of trifiuoromcthyl-subsliluted three-membered-ring compounds has been achieved by reaction of carbciies with trifiuoronicthyl-subslituled unsaturated systems and by reaction of trifluoro-melhyl-substituled carbenes with alkenes. 

Considerably better yields are obtained when the alkene is significantly strained, as in the synthesis of [3]peristylane (5). Note in this case that flash-vacuum pyrolysis is the method of choice for the final step. The intramolecular addition step in the synthesis of ( —)-cyclo-copacamphene (8) and in a very similar synthesis of (+ )-cyclosativene presumably also gain assistance from ring strain. It should be noted that in these last two syntheses, the diazoalkane-alkene intramolecular cycloaddition succeeded where attempted carbene-alkene cycloadditions had failed. 

The most common application of carbenes in synthesis is in the formation of three-membered rings by addition to multiple bonds. This is a typical reaction of all carbenes that do not undergo intramolecular insertion. Generation of the carbene in the presence of an alkene gives a cyclopropane product. Addition of halocarbenes to alkenes is a stereospecific cis reaction, but this is not necessarily the case with all carbenes. Hence Z-2-butene 109 gives the cyclopropane 110, in which the two methyl groups remain cis to one another (4.86). The stereospecificity... 

Recent efforts have achieved Z-selective ruthenium carbene catalysts, providing a catalyst solution to a general problem in stereoselective alkene synthesis Diastereoselectivity in Olefin Metathesis Development of Z-Selective Ru Catalysts Vol 1, Chapter 3 Grubbs, Handbook of Metathesis, 2nd Edition, Volume 2, Chapter 7. As this is a relatively new field, mechanistic studies are... 

The McMurry alkene synthesis has been reviewed, considering the mechanistic alternatives of carbene or pinacolate intermediates. ... 

Che C-M, Huang J-S (2002) Ruthenium and osmium porphyrin carbene complexes synthesis, structure, ad connection to the metal-mediated cyclopropanation of alkenes. Coord Chem Rev 231 151-164... 

Taige MA, Ahrens S, Strassner T. Platinum(ll)-bis-(N-heterocyclic carbene) complexes synthesis, structure and catalytic activity in the hydrosilylation of alkenes. J Organomet Chem. 2011 696 2918-2927. 

Abstract In this chapter, alkene synthesis based on the reactiOTi of //-tosylhydrazones is described. The reactivity of tosylhydrazones is determined by either the acidity of a-proton and hydrazcMie proton or the electropositivity of the carbon of C=N bond. This leads to diverse reactivities and a series of N-tosylhydrazone-based olefination methodologies. Both non-catalytic and transition metal-catalyzed olefinations from Al-tosylhydrazones are introduced in this chapter. Most of the transition metal-catalyzed reactions proceed via metal carbene transformations. The synthesis of alkenes through Pd-catalyzed cross-coupling reactions of Af-tosylhydrazones is particularly attractive and will be discussed in detail. 

Keywords Alkene synthesis Cross-coupling Metal carbene /V-Tosylhydrazones... 

The synthesis of aziridines through reactions between nitrenes or nitrenoids and alkenes involves the simultaneous (though often asynchronous vide supra) formation of two new C-N bonds. The most obvious other alternative synthetic analysis would be simultaneous formation of one C-N bond and one C-C bond (Scheme 4.26). Thus, reactions between carbenes or carbene equivalents and imines comprise an increasingly useful method for aziridination. In addition to carbenes and carbenoids, ylides have also been used to effect aziridinations of imines in all classes of this reaction type the mechanism frequently involves a stepwise, addition-elimination process, rather than a synchronous bond-forming event. 

Platinum (II) carbenes should be mentioned as <7-bonded organometallics. An important general synthesis by cleavage of an electron-rich alkene affords a pair of isomers, the trans-form isomerizing to the thermodynamically more stable cw-form on heating (Figure 3.61). 

Reactions involving free carbenes are very exothermic since two new theoretical treatment of the addition of singlet methylene to ethylene suggests that there is no activation barrier.168 The addition of carbenes to alkenes is an important method for synthesis of many types of cyclopropanes and several of the methods for carbene generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9, at the end of this section, gives a number of specific examples. 

The addition of dichlorocarbene, generated from chloroform, to alkenes gives dichlorocyclopropanes. The procedures based on lithiated halogen compounds have been less generally used in synthesis. Section D of Scheme 10.9 gives a few examples of addition reactions of carbenes generated by a-elimination. 

Under special conditions carbenes have been observed to dimerize to alkenes as illustrated in the synthesis of tetramesitylethylene (2.19)234). 

A rhodium-catalyzed intramolecular C-H functionalization has been employed for the synthesis of bicyclic imidazoles. The alkene acts as an anchor to the metal, directing the C-H functionalization process, which involves the formation of an Rh(l) carbene intermediate (Equation (118)).107... 

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. 

Initial reports of cross-metathesis reactions using well-defined catalysts were limited to simple isolated examples the metathesis of ethyl or methyl oleate with dec-5-ene catalysed by tungsten alkylidenes [13,14] and the cross-metathesis of unsaturated ethers catalysed by a chromium carbene complex [15]. With the discovery of the well-defined molybdenum and ruthenium alkylidene catalysts 3 and 4,by Schrock [16] and Grubbs [17],respectively, the development of alkene metathesis as a tool for organic synthesis began in earnest. 

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