Tethered bis-

Scheme 4 Intramolecular Zn-MesSiCl-promoted reductive coupling of 0,0 -tethered bis(V-arylidene valinates)...

The [4+ 4]-cycloaddition reaction of tethered bis-dienes has been used by Wender and co-workers in total synthesis as exemplified in syntheses of ( )-salsolene oxide and (-l-)-asteriscanolide (Scheme 28). In the synthesis of ( )-salsolene oxide, a nickel(0)-catalyst cleanly effects the cycloaddition of the two conjugated dienes in compound 93 to afford the bicyclo[5.3.1]undecadiene in a good yield and with moderate selectivity.99 The first synthesis of (-l-)-asteriscanolide was accomplished in only 13 steps. The key [4+ 4]-cycloaddition reaction efficiently set the requisite eight-membered ring of (-l-)-asteriscanolide in good yield and with excellent diastereoselectivity.100 The diastereoselective [4 + 4]-cycloaddition has also been applied to the synthesis of the core ring system found in several sesterterpenes such as the ophiobolins (Scheme 28).101... 

Scheme 1.3. General synthetic scheme for the regio- and stereoselective macrocyclization of Cgo by Bingel addition of tethered bis-malonates.

Scheme 1.24. Regioselective addition to Cgo of a 2,3-O-isopropylidene-L-threitol-tethered bis-azide under formation of diaza-bishomo-[60]fullerene 252, and regioselective interconversion between the azahomo- (252) and the epiminofullerene (254) isomers.

The addition of an excess of gaseous H2S to a toluene solution of (CsMe4SiMe2NMe2)2TiCl2 affords the sulfido-tethered bis-Cp compound (GsMe4SiMe2S)2Ti (Scheme 616) by cleavage of both Si-N and Ti-Gl bonds. 

The five-coordinate zirconocene dichloride 1178 contains a tethered bis(Cp)-phosphine ligand it was prepared by salt metathesis of ZrCl4(THF)2 with Li2[PhP(CFI2CF[2C5F[4)2].911 Coordination of the phosphorus atom to the metal center was confirmed by its X-ray structure. Dissolution of 1178 in wet MeOH followed by evaporation of the solvent yielded the cationic chloro-aqua complex 1179 shown in Scheme 277 as the chloride salt. Treatment of an aqueous solution of 1178 with excess potassium thiocyanate gives the bis(isothiocyanato) complex 1180. 

An early example of amidate ligands being utilized with early transition metals was reported in 2001 by Giesbrecht et al. [10a] A tethered bis(amidate) ligand was used to prepare a dimeric species (Figure 14), which was tested for ethylene polymerization. In the presence of 500 equiv. of MAO at 1 atm ethylene gas only modest catalytic activity was observed. [10a]... 

Figure 14 Tethered bis(amidate) titanium complex for preliminary studies as initiators for ethylene polymerization. Ar=4-tert-butylphenyl, X=NMe or Cl [10a]...

Scheme 7-10 Extensions of the Bingel macrocyclization preparation of a higher adduct of Cgy [25] and use of tethered bis(jHceto esters) instead of tethered bismalonates [74]. THP=3,4,5, tetrahydro-2//-pyran-2-yl.

Formation of Bis-adducts by Double Diels-Alder Addition of Tethered Bis(buta-l,3-diene)s to C o... 

Nishimura and co-workers used bis(o-quinodimethanes) connected by a.n+dioxa-methylene tethers for the regioselective bis-functionalization by double Diels-Alder addition (Scheme 7-14) [93]. The tethered bis(o-quinodimethanes) were intermediately formed by heating the bis(bromomethyl) derivatives 72a-d together with C, in toluene in the presence of KI and [18]crown-6 [90]. With an 0-(CH2)2-0 tether (72a), the cis-2 and cw-3 bis-adducts 73a and ( )-74a were isolated in 10% and 8% yield, respec-... 

Scheme 7-14 Regioselective bis-functionalizations by double Diels-Alder addition of tethered bis(o-quinodimethanes) to Cgo [93, 94].

Nakamura and co-workers synthesized the oligomethylene-tethered bis(cyclopropenone acetals) 88a-c to generate by thermolysis minute amounts of nucleophilic vinylcarbenes which subsequently underwent two-fold [3+2] cycloadditions to 6-6 bonds of Cgo (Scheme 7-16) [98,99]. Thus, reagent 88a, with a -(CH2)3- tether, afforded the Q-sym-... 

Tethered bis(8-quinolinolato)chromium(lll/ll) complexes as asymmetric catalytic redox systems 07CL1082. 

Figure 15. Intramolecu lar photocycloaddition of a three-atom tethered bis-2-pyridone and the thermal/photochemical transformation of cis [4+4] product 42 into trans product 41...

In a related work, Fu and eo-workers realized the AFC alkylation of indoles with p,y-unsaturated a-keto esters by using heteroarylidene-tethered bis(ox-azoline) 71a and Cu(OTf)2 in DCE at -78 Note that pyrrole was applied successfully in this catalytie system to afford the corresponding produets 73 in high yields with excellent enantioselectivity (Scheme 6.26). 

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