Ta-alkyne complexes

Nb and Ta alkyne complexes are of interest in part because they have found use in organic synthesis and cyclooligermerization (see Cyclodimerization -tri-merization Reactions) of alkynes. 

Reaction of Ta-alkyne complexes with R R2C=0. The Ta complexes formed from TaCls/Zn with unsymmetrical alkynes react with carbonyl compounds to form two rcgioisomcric allylic alcohols with a ratio depending on the substituents on the alkync (both steric and electronic effects) as well as the size of the substituents in the carbonyl group. The complexes from acetylenic esters react with carbonyl compounds mainly at the position or to the ester group, whereas complexes from acetylenic amides react mainly at the position fi to the amide. 

Reduction of TaCls with Zn in the presence of alkyne RC=CR and solvents (L) is believed to yield Ta chloro alkyne complexes, which react with lithium alkoxides tethered to alkenyl groups to give chemo- and stereoselective addition of terminal alkenyl groups.Alkoxy-directed insertion of C=C bonds into Ta-alkyne complexes plays a critical role through the following mechanism. 

The pursuit of the synthesis of FK506 and rapamycin continues to draw contributions from the carbohydrate area. The C(21)-C(34) fragment 82 of FK506 has been prepared as shown in Scheme 16. Use of a tantalum-alkyne complex to couple aldehyde 80 with alkyne 81 is an especially interesting step. The requisite low valent tantalum species is generated in situ and the Ta-alkyne complex shows reasonable levels of Qam selectivity towards aldehydes. Alkyne 81 was, in turn, prepared from D-quinic acid. 

Varghese V, Saha M, Nicholas KM (1988) Org Synth 67 141 Nicholas KM (1987) Acc Chem Res 20 207 Caffyn AJM, Nicholas KM (1995) Transition metal alkyne complexes transition metal stabilized propargyl system. In Abel EW, Stone FGA, Wilkinson G (eds) Comprehensive organometallic chemistry II, vol 12. Pergamon Press, Oxford, p 685 Schreiber SL, Klimas MT, Sammakia T (1987) J Am Chem Soc 109 5749 Nakamura T, Matsui T, Ta-nino K, Kuwajima I (1997) J Org Chem 62 3032... 

Hydrogen abstraction from / -alkene ligands is another method to yield alkyne complexes. [N3N]Ta(C2H2) is obtained, for example, from the reaction of [N3N]TaCl2 with 2 equiv of CH2=CHMgBr. ... 

With the exception of(fj -C5Me5)Ta(HC=CH)Cl2 (71), the corresponding alkyne complexes of tantalum do not react with olefins 71 cyclizes with ethylene at 80°C to give the tantalacyclopentene 72 (Scheme 25). ... 

Alkyne complexes [N3N]Ta( 72-HC=CH) and [N3N]Ta( 72-C6H4) were prepared via treatment of (15) with 2 eq H2C=CHMgBr and LiPh, respectively (Scheme 15)., 39 In contrast, under the same conditions with H2C=CHMgBr, [N3N ]TaCl2 afforded an approximately equimolar mixture of [N3N ]Ta(HC=CH) and binuclear [N3N ]Ta=CHCH2CH2CH= Ta[N3N ].40... 

Reaction of Ta-alkyne com from TaCIs/Zn with unsymmctncal regioisomeric allylic alcohols with (both steric and electronic effects 11 group. The complexes from acetyl at the position a to the ester gri u mainly at the position to the an... 

Scheme 2 Catalysed cyclotrimerisation reaction of alkynes using bimetallic Mb (1) or Ta (2) complexes...

Alkynes and CO2 can react stoichiometrically in the presence of Ni(0) complexes stabilized by P- or N-ancUlary ligands (dcpe, dppe, bpy, TMEDA, DBU, etc.) to afford unsaturated pentaatomic oxanickelacycles (Scheme 5.10) [1-10]. Oxametallacyclopentenones have also been obtained by reaction of CO2 with Ni- or Ta-aryne [ 1 ] complexes or by reacting the heterocumulene with alkyne complexes of Ti(II) or Zr(ll) [1, 3, 19, 20]. 

The interaction of alkyne complexes with protic acids has been the most thoroughly studied of electrophilic reactions. When these reactions proceed to organic products, alkenes are formed, often stereospecifically. Schwartz and co-workers have studied the reactions of Cp2MH(alkyne) complexes of the early transition metals, Ta (Labinger et al, 1974) and Nb (Labinger and Schwartz, 1975). These complexes gave cis alkenes exclusively in nearly quantitative yield when treated with strong acids, as exemplified by the formation of cis-2-octene from the tantalum-octyne complex. Since the... 

NbBrs, and NbCls-Pl Sn evidently proceeds via cyclotrimerization of diynes, which most probably involve cyclic carbometallation, details are not very clear.246 2463 Related reactions of Ta and Mo complexes were also investigated in this study. Formation of tantallacyclopropenes by complexation of alkynes with Ta complexes has also been reported247 (Scheme 51). In addition to the Ta-catalyzed polymerization of diynes mentioned above, Ta-catalyzed or -promoted cyclotrimerization reactions of alkynes to produce benzene derivatives, a Ta-promoted ethylene... 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

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