Cycloaddition propargyl complexes

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

Another type of SO2 reaction with organometallics is cycloaddition to 2-alkynyl (propargyl) complexes of Fe, Mn, Mo and An example is the reaction of... 

Electrophilic attack also occurs at the -Y-position of propargyl ligands, and this reaction converts propargyl complexes to allene complexes. An example of this protonation process is shown in Equation 12.61. This reaction has been used to initiate cycloaddition reactions. An example of this type of cycloaddition is the [3-t2] reaction of p-toluenesulfonyl isocyanate with the propargyl complex in Equation 12.62. Propargyl and Ti -allyl ligands are about equally reactive toward such cycloadditions. 

In another conceptually novel [5 + 2]-process, Tanino and co-workers synthesized cycloheptene derivatives by stereoselective [5 + 2]-cycloadditions involving hexacarbonyldicobalt-acetylene complexes as the five-carbon component and enol ethers as the two-carbon component (Schemes 22 and 23).60 61 The role of the dicobalthexacarbonyl complex is to facilitate formation and reaction of the propargyl cation putatively involved as an intermediate in this reaction. The dicobalthexacarbonyl moiety can be removed using various conditions (Scheme 24) to provide alkane 60, alkene 62, and anhydride 63. 

Allenyl cations. In the presence of a zinc chloride-ether complex allenyl cations (a) can be generated from a propargyl halide such as (1). The cation can undergo either [3 + 2]- or [2 + 2]cycloaddition with an alkene. When the R group is aryl, [2 + 2]cycloaddition is the major pathway (2) when R is CHj or C,H3, [3 + 2]-cycloaddition becomes the major reaction (3) (equation 1).1... 

Although the [2+2] cycloaddition continues to dominate the methodology for the synthesis of boracyclobutene and metallacyclobutene complexes, conceptually new and potentially general alternatives have recently been introduced. In particular, the central carbon alkylation of electrophilic propargyl and allenyl complexes has significantly enriched the palette of available metallacyclobutenes, raising considerable promise for the development of new reactions of relevance to organic synthesis. 

The thiolate-bridged diruthenium complex 101 can promote a cycloaddition reaction between propargylic alcohols and 1,3-dicarbonyl compounds to provide 3-acyM//-pyrans in excellent yield (Scheme 33). The reaction proceeds via formation and alkylation of the allenylidene complex 102 to form the vinylidene intermediate 103, which upon cyclization furnishes 4//-pyrans (Scheme 33) <2004JOC3408>. 

The acid catalyzed cycloaddition of propargylic alcohols with phenols gives a mixture of 2H- and 477-chromenes <2001SC439>. The thiolate-bridged diruthenium complex 101 can effectively mediate the cycloaddition of aryl propargylic alcohols with phenols or naphthols to afford 4-aryl-4//-chromenes (Equations 57 and 58) <2002JA7900>. 

Recently, cyclopropane derivatives were produced by a ruthenium-catalyzed cyclopropanation of alkenes using propargylic carboxylates as precursors of vinylcarbenoids [51] (Eq. 38). The key intermediate of this reaction is a vinylcarbene complex generated by nucleophilic attack of the carboxylate to an internal carbon of alkyne activated by the ruthenium complex. Then, a [2+1] cycloaddition between alkenes and carbenoid species affords vinylcyclo-propanes. 

Cycloaddition reactions of a,/3-unsaturated chromium and tungsten complexes have been studied to a great extent and have been reviewed.3 -6 Our report on cycloaddition of (l-alkynyl)carbene complexes is restricted to a short abstract and an update including more recent results. A most remarkable feature of [4+2] cycloadditions of 1,3 dienes to C=C bonds of (l-alkynyl)carbene complexes, e.g., li, is that such reactions proceed under very mild conditions, compared to those for reactions of propargylic esters, e.g., 41. Thus, formation of a Diels-Alder adduct, e.g., a norbornadiene derivative 42, can be achieved at 25°C via carbene complexes instead of at 190°C via the direct route (Scheme 15).68 Ligand disengagement from compound 40 can be achieved in various ways, e.g., by formation of an ester 43 through oxidation of the Cr=C bond, or by formation of an allyl silane 4369 or a stannane.70 71... 

Substituted phthalides can be prepared in a [2+2+2] cycloaddition of two molecules of methyl propiolate with propargyl alcohol. This cyclotrimerization, which is catalyzed by a cobalt(ll)-DPPE complex in the presence of catalytic amounts of zinc, affords the reaction products in acceptable to good yields (Equation 150) <2005CC4955>. 

Metal-allyl complexes of formulas CpFe(CO)aCHaC(R) = CR R" and CpMo(CO)3CHaC(R) = CR R", as well as analogous propargyl and cyclopropylmethyl complexes, react with (CN)aC=C(CN)a to give cycloaddition products 21a, 56, 120, 121) of the type discussed in Section V. 

Alkynols complexed to cobalt can be oxidized to alkynals without decomplexation. Propargyl aldehydes are protected from polymerization upon complexation with Co2(CO)6. These aldehydes smoothly undergo Wittig-type reactions. Carbonyl-ene reactions have been demonstrated (Scheme 194). Complexation to cobalt protected the enyne in complex (132) from Michael-type reactions (Scheme 195). Alkenyl-substituted complexes undergo [3 + 2]cycloadditions with nitrile A-oxides (Scheme 196). 

The reactions with tertiary amines or phosphines that have no active hydrogen atoms result in platinacyclobutene cations, a rare species for late transition metal (Scheme 39). Substituted carbanions are added to the jj -aUenyl/propargyl platinum complex to yield the neutral substituted- ) -TMM derivatives that undergo huther [3 + 2] cycloaddition with good tt-acids as TCNE or maleic anhydride to produce highly substituted cyclopentanoids (Schemes 40, 41). 

An interesting transformation involving the indole nucleus was found from propargylic carboxylates to give tetracychc compounds with Au(I) (equation 85). This reaction proceeds by an allene-gold complex in equilibrium with the aUcenyl-gold species, which reacts intramolecularly with the indole to form the product. When the reaction of these substrates is performed with dichloro(pyridine-2-carboxylato)gold(III) or Pt(II) as catalysts, products in equation (86) are obtained instead. This new reactivity can be explained by a formal [3 + 2] cycloaddition of 1,3-dipole... 

An additional advantage of the intramolecular protocol stems from the opportunity to prepare easily the required polyfunctional precursors via cobalt carbonyl stabilized propargyl cations. The approach based on the tandem utilization of Co-mediated alkylation and Pauson-Khand annulation was developed in Schreiber s studies to elaborate short pathways for the synthesis of polycyclic compounds. An example of the efficiency of this protocol is the two-step transformation of the acyclic precursor 409 into the tricyclic derivative 410. The cobalt-complexed acetal 409 was first transformed into the cyclooctyne derivative 411 via intramolecular reaction of the in situ generated propargyl cation 409a with the allylsilane moiety. Cyclooctyne 411 underwent smooth cycloaddition in the presence of carbon monoxide to give the target compound 410 with excellent stereoselectivity. 

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