Bridging bis

Finally, the 1,3-dione systems prepared by Cram and Alberts deserve special note . These compounds, referred to as hexahosts are similar to the polymer-bound material illustrated as Compound 29 in Chap. 6. The synthesis is based on a methylene-bridged bis-dithiane unit. One of these may be cyclized with a polyethylene glycol, or more than one unit may be incorporated to give multiple 1,3-dione binding sites in the macrocycle. The former case is illustrated in Eq. (3.46). 

If a bridged bis-crown is used instead of diaza-18-crown-6, the cryptand contains two macrorings facing each other (see Table 8.5). Note also that the 2,2 -binaphthyl unit has been used extensively by Cram and his coworkers to provide chirality to mono-cyclic systems as well (see Sect. 3.13). 

Other interesting synthetic applications of the ketone-derived enamine alkylation are found in the monomethylation of steroid enamines (249), extension of the benzylation reaction (250) to a ferrocene derivative (251), the use of a-bromoesters (252) and ketones (252) or their vinylogues (25J), in the syntheses of alantolactone (254-256), isoalantolactone (257), and with a bridged bis-enamine (258). The use of bifunctional alkylating agents is also seen in the introduction of an acetylenic substituent in the synthesis of the characteristic fragrant constituent of jasmine (259), the synthesis of macrocyclic ketolactones (260), the use of butyrolactone (261), and the intermolecular or intramolecular double alkylations of enamines with dihalides (262). 

Dilution of a solution of diazonium salts in sulfuric acid with water affords triazene derivatives (95IZV1315,95RCB1269). Treatment of a bridged bis(amino) furazans 200 with nitrous acid in acetic or hydrochloric acid gives macrocyclic triazene 201 (Scheme 132) (95MI4). 

Ethylene-bridged bis-indenyl zirconocene dichloride-methylalu-moxane system. 

On the way to cydo-Cig, l,6-bis(triisopropylsilyl)-l,3,5-hexatriyne (2) [50] was subjected to reaction with [Co2(CO)8], followed by ligand exchange with the bridging bis(diphenyphosphino)methane (dppm) ligand, whereupon smooth deprotection yielded the stable dark-red dicobalt complex 3 (Scheme 1). Oxida-... 

Among these in situ protocols are those using ionic liquids as the solvent, or as both the solvent and the ligand. It was shown that the use of PdCOAc) in imidazolium-based ionic liquids forms in situ NHC-Pd(II) species [42], The use of methylene-bridged bis-imidazolium salt ionic liquids to form chelated complexes has also been reported [43], although better results have been obtained when Bu NBr is used as the solvent [44] and imidazolium salts were added together with PdCl in catalytic amounts [45]. Other related catalytic species such as bis-NHC complexes of silica-hybrid materials have been tested as recyclable catalysts [46,47]. 

Pyrazolato-bridged bis(tacn) ligands and their dinickel complexes are mentioned in Sections 6.3.4.12.6 and 6.3.4.12.7. 

A dithiolate ligand forms a structurally characterized dizinc bisligand complex with both of the ligands bridging, bis //-[(dimercaptomethylene)propanedinitrilato-1S, S ,] tetrakis(4-methylpyridine) dizinc.578... 

As shown in the two examples described here, formation of the benzene nucleus by trimerization of alkynes is usually catalyzed by a Co-complex. However, Und-heim and coworkers [276] have recently shown that a Ru "-complex can also be used. Reaction of the triyne 6/4-9, which was prepared from SchollkopPs bislactim ether 6/4-8 [277] with Grubbs I catalyst 6/3-13, led to 6/4-10 in an excellent yield of 90%. Hydrolysis of 6/4-10 gave the desired as-indacene-bridged bis(a-amino acid) derivative 6/4-11 (Scheme 6/4.3). 

A double set of signals in the 13C NMR spectra of THF and pyridine complexes of bridged bis-l-boraadamantanes initially prepared via an allylboron-acetylene condensation-hydroboration sequence shows that these compounds consist of a mixture of racemic and meso-imms <1998IZV728, 2000IZV501, B-2003MI94>. THF complexes show nB NMR shifts at about 12 ppm, whereas the pyridine analogues resonate at about —3 ppm (Table 4). 

Table 4 Boron and carbon chemical shifts in bridged bis-1 -boraadamantane complexes...

Scheme 61).22 149 150 152 229 230 The methylene-bridged bis(oxazoline)s (70) can afford either neutral copper complexes of the semicorrin type (72) or cationic copper complexes (73). Cyclopro-panation with copper complexes of type (70) shows similar stereochemistry to that with the corresponding copper semicorrin complexes.229-231 Alternatively, bis(oxazoline) ligand (71), bearing... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

In view of the versatility of A-heterocyclic carbenes as ligands and their structural diversity in silver(i) coordination chemistry, an extension of the work to ligands with two or more carbene moieties was reported. A dinuclear silver(i) complex 52 (Figure 21) with an o-phenylenedimethylene-bridged bis(carbene) ligand has been synthesized in 66% yield from silver(i) oxide and the bis(imidazolium) salt.88 The reaction to synthesize 52 has to be carried out in... 

A homoleptic methylene-bridged bis(carbene) dinuclear silver(i) complex, 55, was isolated in 81% yield from a two-step reaction, as shown in Scheme 13.100 The structure of 55 reveals a boat conformation with the two methylene groups protruding away from the Ag2 core in one direction. The Ag-C distances of 2.081(2)-2.095(2) A were in the normal range for silver(i) TV-heterocyclic carbene complexes, while an uncommonly short intramolecular Ag-Ag contact of 3.2039(3) A was observed in the homoleptic silver(i) carbene complexes. 

An acid-catalyzed double-Michael addition of water to the bridged bis-dioxine moiety in a larger macrocyclic framework has been described by the Kollenz group (Scheme 6.134) [269]. While conventional reaction conditions failed to provide any of the desired functionalized 2,4,6,8-tetraoxaadamantane product, microwave heating of the hydrophobic macrocyclic bisdioxine in a 1 1 mixture of 1,2-dichloroethane and acetic acid containing excess concentrated hydrochloric acid at 170 °C for 40 min provided a 35% isolated yield of the desired oxaadamantane compound. 

The stereoselectivity established for the trident series, was tested using the C-bridged bis-aziridine 58 which produced the ONCNO-[5]polynorbomane 115 by reaction with the O-bridged dipolarophile 36 (two equivalents) in 70% yield (Scheme 19). However, as noted above, replacing the central C-bridge with the A-Z bridge caused loss in stereoselectivity. Accordingly the type B approach was preferred. 

X-ray studies confirm Bi2(ju,-OR)2 centrosymmetric dimers 4 in the solid state (24). Terminal alkoxides are generally bound more tightly than bridging units [terminal Bi-0 2.064(7) A bridging Bi-0 2.188(7) and 2.688(7) A], The bismuth oxide alkoxide cluster compounds [Bi6(p,3-07)(p,3-OC6F5) Bi(OC6F6)4 3(sol)2] [sol]2 (sol = thf or toluene) were also isolated as minor products (25). Both structures contain the unusual octahedral I Bi, (/x3-07 X/x3-OCeF.-) I" cores 5, which... 

---