Alkenyl carbenes

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-... 

J-Oxygen-functionalised sp3 organolithium compounds react with alkenyl-carbene complexes to generate the corresponding cyclic carbene complexes in a formal [3+3] process (see Sect. 2.8.1). In those cases where the organolithium derivative contains a double bond in an appropriate position, tricyclic ether derivatives are the only products isolated. These compounds derive from an intramolecular cyclopropanation of the corresponding cyclic carbene complex intermediate [89] (Scheme 83). 

A greatly enhanced chemoselective formation of phenol is observed for alkoxy(alkenyl)carbene complexes compared to alkoxy(aryl)carbene complexes. This behaviour reflects the ease of formation of the rf-vinylketene complex intermediate E starting from alkenylcarbene complexes for aryl complexes this transformation would require dearomatisation. 

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). 

Scheme 13 Benzannulation of pyrrolidino(alkenyl)carbene complexes...

THE COMPLEX OsCI2(in2-H2)(CO)(P Pr3)2 AS PRECURSOR OF CARBENE, ALKENYL-CARBENE, AND (jl-BIS-CARBENE DERIVATIVES... 

The chemistry of (l-alkynyl)carbene iron complexes is different from that of chromium and tungsten compounds. [4+2] cycloaddition of cyclopentadiene to a (l-alkynyl)carbene iron complex 11,m (R = SiMe3, r-Bu, c-QHn, n-Pr, Ph) affords (l-alkenyl)carbene Fe(CO)4 complexes 55, but these are readily isomerized at 50°C to Fe(CO)2 complexes 57 by insertion of carbon monoxide into the Fe = C bond of an intermediate Fe(CO)3... 

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... 

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

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 ... 

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. 

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