Propargyl complexes

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

Central carbon addition to 3-allenyl/propargyl complexes 594... 

Central carbon addition to 73-allenyl/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... 

Virtually all of the transition metal-propargyl complexes investigated react readily with either neat SO2 or SO2 in organic solvents according to Eq. (24), where M = CpFe(CO)2 (111, 124), CpMo(CO)3 (111, 124),... 

CH20S(0)CH=CMn(C0)6 124). The parent propargyl complex was isolated in each case. 

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... 

Electron-rich aromatic componnds are readily alkylated at room temperature with propargyl complexes to form, after demetallation, 2-propyne-snbstituted aromatic compounds. Depending on the snbstitution pattern of the alkyne complex and the aromatic substrate, alkylation occurs at times with very high regioselectivity. This type of annulation was used in a synthesis of psendoipterosin... 

The method is limited because of the requirement for stoichiometric reagents, but impressive applications in the preparation of complex molecules have appeared. For example, a model system for ene-diyne antibiotics has been prepared by an intramolecular route (Scheme 19). Added versatility arises from the electophilic coupling of vinyl-propargyl complexes, such as in Friedel-Crafts acylation conditions. 

The most direct and successful synthesis of metal propargyl complexes involves nucleophilic attack of a metal carbonylate anion at a propargyl halide (9a-c,IO) [Eqs. (1) and (2)]. The propargylic products isolated... 

Only recently have early transition metal propargylic complexes been recognized. The lanthanide alkyls [LnCH(SiMe4)2(i7 -C5Me5)2j (Ln = La. Ce) react with the 2-alkynes MeC=CR (R = Me, Et, "Pr) to afford 1,2-disubstituted 3-alkylidenecyclobutenes. The first step in this catalytic cy-... 

Osella and co-workers have demonstrated the effective dehydroxylat-ing ability of FefCO) . After refluxing [Co2(CO)6(/i-i7-,i7--EtC=C-CH(0H)CH3)] with Fe(CO)s in acetone, high yields of the heterometallic dimer [CoFe(CO)5(/i-i7, i7 -MeHCC=CEt)] (42) were isolated 27a) [Eq. (15)]. These investigators further showed that heterobimetallic propargyl complexes of the type [FeCo(CO)6(/i-i7%V R2CC=CR)] (42) and [FeCo-... 

Metal Fragment Condensation Reactions with Propargylic Complexes... 

Both terminal allenyl and propargyl complexes are well known. However, structural data for both types of complexes remain scarce (Table 11). 

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