Alkenyl carbenes reactions

Alkylideneaminocarbene complexes 76, which are aza analogs of alkenyl-carbene complexes, upon reaction with alkynes primarily give formal [3+2] cycloadducts analogous to the 1-aminocarbene complexes (Scheme 16) [74,75]. Aumann et al. proposed that this should be considered as a formal 1,3-dipo-... 

Amino(aryl)carbene complexes prefer cyclopentannulation over benzannulation. Amino(alkenyl)carbene complexes may react in a benzannulation reaction. 

Wulff et al. examined the necessary reaction conditions for a,fi-unsaturated aminocarbene complexes to react in a benzannulation reaction [23]. The reaction of dimethylamino(alkenyl)carbene complexes 18 with terminal alkynes in non-coordinating and non-polar solvents afforded phenol products in acceptable yields (Scheme 12). 

The observation by Fischer et al.18 that the 4,1-addition of dimethylamine to compound la is thermodynamically controlled at 20°C, whereas 2,1-addition/elimination is kinetically controlled at -115°C, turned out to be limited to few cases.20 It has been shown9a 9b 42 112 113 that for most cases, three competing reaction paths must be considered (i) 2,1-addition/elimina-tion with formation of (l-amino)alkynylcarbene complexes (= 2-amino-l-metalla-l-en-3-ynes) 98 (ii) 4,1-addition to give [(2-amino)alkenyl]carbene complexes (= 4-amino-l-metalla-l,3-butadienes) 96 and (iii) 4,1-addition/ elimination to (3-amino)allenylidene complexes (= 4-amino-l-metalla-1,2,3-butatrienes) 99 (Scheme 33, M = Cr, W). The product ratio 96 98 99 depends on the bulk of substituents R and R1, as well as on the reaction conditions. Addition of lithium amides instead of amines leads to predominant formation of allenylidene complexes 99.112 Furthermore, compounds 99 also can be generated by elimination of ethanol from complexes 96 with BF3 or AlEt3114 and A1C13,113 respectively. 

Addition of alkynes to [2-(amino)alkenyl]carbene complexes ( )-96a does not afford cyclohexadienones (as might have been anticipated in case of a DOtz-type reaction),3 but gives cyclopentadienes 101 instead (Scheme 35) 3r,9a,9h jt js imp0rtant to note that the assembly of the cyclopentadiene... 

The cycloaddition and annelation reactions can be conducted concurrently in a one-pot operation to provide a regioselective synthesis of complex bicyclic systems. This method is an attractive alternative to preparation of a,p-alkenyl carbene complexes from cyclo-hexenyllithiums. 

The reactions of alkenyl carbene complexes with isonitriles can produce a variety of products whose distribution is sensitive to the nature of the isonitrile. Reacting 3 equiv. of aryl isonitrile with the alkenyl complex (383) produces the dihydro- y-carbolinone (385). 74 The ketenimine in (384) is liberated with a second equivalent of the isonitrile, and the the third equivalent undergoes a formal [4 -i- 2] cycloaddition with the free ketenimine. With sterically encumbered isonitriles, the ketenimine is more rapidly liberated from the metal by a second isonitrile and 2 equiv. are required to drive the reaction to completion. Free ketenimines that are generated in this manner can be trapped via their [4 + 2] cycloadditions with maleic anhydride. 7 Primary and secondary alkyl isonitriles will react with alkenyl carbene complexes to give cycloadducts that can be converted to pyrroles or products that contain five-member ring carbo-cycles. 7 ... 

Alkenylcyclopropane-l -carbonitriles were obtained in moderate to good yields, in some cases even with good diastereoselectivity, by the photolysis of 3/f-pyrazole-5-carbonitriles in unsaturated solvents (Table 4). On UV irradiation, the pyrazoles isomerize to jS,> -unsaturated a-diazocarbonitriles, which cannot be isolated but decompose rapidly to l-alkenyl(cyano)car-benes, which then either cyclize to cyclopropenecarbonitriles 1 or react with the alkene substrate to give 1 -alkenylcyclopropane-1 -carbonitriles 2. As a rule, the cyclopropanation of electron-rich alkenes, dienes, thiophene, and furan dominates the intramolecular carbene reaction. A generalized procedure can be found in Houben-Weyl, Vol. 19b, p 1209. 

Method A and C have been used for intramolecular cyclopropanation reactions. Thus, 2-silabicyclo[3.1.0]hexanes 5b,c and 2-silabicyclo[4.1.0]heptane 5d have been prepared in moderate yields by treatment of alkenyl(chloromethyl)dimethylsilanes 4b-d with sodium. In the case of 4d, C-H insertion of the intermediate carbene competes with cyclopropanation, and for 4e, it becomes the sole carbene reaction. The corresponding allylsilane 4a does not undergo intramolecular cyclopropanation, and in the case of vinylsilane 6, intramolecular cyclopropanation is probably involved in the formation of silylcyclopropane 7. 

A comprehensive treatment of the benzannulation of Fischer carbene complexes with alkynes is not possible in this review, and thus instead the material presented here will hopefully serve to give the reader an overview of its scope and limitations. The first report of this reaction was in 1975 by Dotz in which he describes the formation of the naphthol chromium tricarbonyl complex (236) from the reaction of the phenyl chromium complex (la) with diphenylacetylene. In the intervening years over 100 papers have been published describing various aspects of this reaction.The reaction of the generic cartene complex (233 Scheme 34) with alkynes will serve to focus the organization of the scope and limitations of the benzaimulation reaction. The issues to be considered are (i) the regioselectivity with unsymmetri-cal alkynes (ii) possible mechanisms (iii) applications in natural product syntheses (iv) the effect of substitution on the aryl or alkenyl substituent of the carbene carbon (v) functionality on the alkyne (vi) effects of the solvent and the concentration of the alkyne (vii) tandem applications with other reactions of carbene complexes (viii) reactions where aromatization is blocked (cyclohexadienone annulation) (ix) annulation of aryl versus alkenyl carbene complexes (x) the effect of the ligands L on the metal (xi) the effect of the ancilliary substituent RX and (xii) reactions with —C X functionality. 

Reaction of the alkenyl carbene complex 117, derived from tetra-O-benzyl-aldehydo-D-hbose, with cyclohexene oxide in the presence of [Cp2TiCl]2 gave the oxa-trans-decalin complex 118, together with a smaller proportion of the epimer at the asterisked carbon. This work was extended to the use of sugar epoxide 119 and the carbene complex 120 to give the dioxabicyclo-system 121. ... 

Alkenyl carbene complex was dissolved in benzene at a concentration of 0.002 M. The red solution was deoxygenated by the freeze-pump-thaw method (four cycles) and heated in an oil bath to 100°C. The solution turned from red to yellow during the reaction. After the completion of the reaction, the solvent was removed, and the residue was taken up in CH2Cl2/ether, and stirred in air overnight. The mixture was then loaded on silica gel and chromatographed to give 40% of p-methoxy phenol derivative and 21% of m-methoxy phenol derivative. 

The metal-carbene complexes are nowadays extensively used as reagents in organic synthesis. Fischer carbene complexes have become valuable building blocks in various stoichiometric reactions such as the aldol reaction, benzannu-lation, cycloaddition, cyclopropanation, and Michael reaction. In particular, Fischer alkenyl carbene complexes have been widely used in cycloadditions [2c,2f,5]. On the other hand, a typical reaction of Schrock carbene complexes is olefin metathesis (Figure 5.2) [6]. Schrock s olefin metathesis catalyst is a high reactivity, but air- and moisture-sensitive reagent. 

A diastereoselective [3+2] cycloaddition is achieved in the reaction of (—)-8-phenylmenthyl derivatives of Fischer alkenyl carbene complex 8 with diazomethanes 9 to afford pyrazoline carbene complexes 11 and 12 (Scheme 5.3) [11]. Dihydropyrazole carbenes 10 are initially formed with high diastereoselectivity (>90% de). Then, the dihydropyrazole carbenes 10 tautomerize to the thermodynamically more stable pyrazoline carbene complexes 11. The pyrazoline carbene complexes 11 are obtained only in moderate yields (29-52%) due to the oxidation to the corresponding pyra-zoles by exposure of air. The isolated yield of 12 is improved... 

The one-pot reaction of (—)-8-phenylmenthyl derivatives of Fischer alkenyl carbene complex 17 with nitrilimines 18 also affords pyrazoline esters 19 with high regioselectivity and diastereoselectivity (Scheme 5.5) [12],... 

On the other hand, the reaction of Fischer alkenyl carbene complex 24 with nitrone 21 does not afford the desired [3-1-2] cycloadduct. Instead, the metal-carbene bond of Fischer alkenyl carbene complex is oxidatively... 

Cyclopentanone derivatives 50 and 53 are stereoselec-tively obtained by the reaction of Fischer alkenyl carbene complexes 47 with chiral pyrrolidine enamines 48 (Scheme 5.12) [21]. The R -substituent on the pyrrolidine... 

The reactions of Fischer alkenyl carbene complexes 67 with 2-siloxy-1,3-diene 68 afford vinylcyclopentene... 

The reaction can be interpreted by a sequence of carbene ligand metathesis to generate alkenyl carbene complexes 73, followed by the metalla-Diels-Alder reaction with a second diene 72 and reductive fragmentation. 

The reaction of chromium alkenyl carbene complexes 81 with allenes, which thermally undergo metathesis, is also catalyzed or mediated to afford formal [3+2] cycloadducts by metal exchange (Scheme 5.18) [28]. The treatment of chromium alkenyl carbene complexes 81 with allenes 82 in... 

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