Adamantyl complexes

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of stmcture. The most useful compounds do not necessarily have the simplest stmctures, but are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hy-drogenolysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and ally], readily cleaved by Pd-catalyzed isomerisation. 

Diethylzinc l,3-bis(l-adamantyl)imidazol-2-ylidene adduct is known (93JOM(462)13). Bis(l,3-diphenylimidazol-2-ylidene) mercury(II) carbene complexes follow from the corresponding imidazolium perchlorate and mer-cury(II) chloride (68AGE682). 

The reactions of the isopropyl-substituted 3-dimethylaminopropenyli-denechromium complex 109 with terminal alkynes 90 bearing a bulky substituent (e.g., R=ter -butyl, mesityl, adamantyl etc.), in the presence of moist pyridine, yield 2-(acylmethylene)pyrrolidines 110 (Scheme 23) [84]. The dihydroazepinetricarbonylchromium complexes 111 were found to be the key... 

For a chiral molybdenum-based catalyst available in situ from commercial components, see (a) Aeilts SL, Cefalo DR, Bonitatebus PJ, Houser JH, Hoveyda AH, Schrock RR (2001) Angew Chem Int Ed 40 1452 (b) For the first enantiomerically pure solid-sup-ported Mo catalyst, see Hultzsch KC, Jernelius JA, Hoveyda AH, Schrock RR (2002) Angew Chem Int Ed 41 589 (c) For a chiral Mo catalyst, allowing RCM to small- and medium-ring cyclic amines, see Dolman SJ, Sattely ES, Hoveyda AH, Schrock RR (2002) J Am Chem Soc 124 6991 (d) For a novel adamantyl imido-molybdenum complex with advanced selectivity profiles, see Tsang WCP, Jernelius JA, Cortez GA, Weatherhead GS, Schrock RR, Hoveyda AH (2003) J Am Chem Soc 125 2591... 

This metathetical route was recently extended to the synthesis of amidinato complexes of Al, Ga, and In containing very bulky amidinate ligands with 2,6-Pr CgHs, adamantyl, m-terphenyl or 9-tiiptycenyl substituents at the central carbon atom. ... 

We further synthesized unsymmetrical MiniPHOS derivatives 13b (Scheme 13) [30]. Thus, enantioselective deprotonation of l-adamantyl(dimethyl)phos-phine-borane (74, R = 1 -Ad), followed by treatment with ferf-butyldichlorophos-phine or 1-adamantyldichlorophosphine, methylmagnesium bromide and bo-rane-THF complex afforded the optically active diphosphine-boranes 82b as a mixture with the corresponding raeso-diastereomer. Enantiomerically pure unsymmetrical MiniPHOS-boranes 82b were obtained by column chromatography on silica gel or separation by recycling preparative HPLC. 

Ru—C(carbene) bond distances are shorter than Ru—P bond lengths, but this can simply be explained by the difference in covalent radii between P and The variation of Ru—C(carbene) bond distances among ruthenium carbene complexes illustrates that nucleophilic carbene ligands are better donors when alkyl, instead of aryl, groups are present, with the exception of 6. This anomaly can be explained on the basis of large steric demands of the adamantyl groups on the imidazole framework which hinder the carbene lone pair overlap with metal orbitals. Comparison of the Ru—C(carbene) bond distances among the aryl-substituted carbenes show... 

With the same tactic, a meta-stable 3-coordinate iron(III) imido complex was reported by Holland and coworkers [40]. Using the diketiminate-supported dinitrogen iron(I) complex [L T eNNFeU ] as a source of iron(I) precursor [L = 2,4-bis (2,6-diisopropylphenylimino)pent-3-yl], the addition of adamantyl azide (AdN3) in... 

Fast, selective conversion of internal alkenes to linear esters is catalyzed by Pd11 complexes of chelating bis(phospha-adamantyl)diphosphines, e.g., the mesa (168) and racemic (169) isomers, and the catalysis is acutely sensitive to the ligand backbone and even to the diastereomer used the results are compared with those for the P Bu2 analogous.686... 

In contrast to the Simmons-Smith reagent and similar carbenoids, which are reactive and therefore difficult to characterize, adducts of the fV-heterocyclic l,3-diorganylimidazol-2-ylidenes are remarkably stable. The first iV-heterocyclic carbene complex of zinc, namely the l,3-di(l-adamantyl)imidazol-2-ylidene diethylzinc complex 48 (Figure 22), was reported by Arduengo et al. in 1993.98 Because of the general utility of these iV-heterocyclic... 

A non-covalently functionalized dendrimer was also applied in a continuous allylic amination reaction.[33] PPI dendrimers functionalized with urea adamantyl groups can act as host molecules for phosphorus ligands equipped with acetyl urea groups (Figure 4.18). The so formed supramolecular complex was reacted with a palladium precursor... 

Plenio et al. tested an adamantyl phosphine ligand bound to soluble polystyrene (Figure 4.43) in various palladium-catalyzed C-C coupling reactions.[62] The retention of metal complexes of the polymer-bound phosphine ligand were determined to be higher than 99.95%. 

Monomeric carbene complexes with 1 1 stoichiometry have now been isolated from the reaction of 4 (R = Bu, adamantyl or 2,4,6-trimethylphenyl R = H) with lithium l,2,4-tris(trimethylsilyl)cyclo-pentadienide (72). The crystal structure of one such complex (R = Bu) revealed that there is a single cr-interaction between the lithium and the carbene center (Li-C(carbene) 1.90 A) with the cyclopentadienyl ring coordinated in an if-fashion to the lithium center. A novel hyper-valent antimonide complex has also been reported (73). Thus, the nucleophilic addition of 4 (R = Mes R = Cl) to Sb(CF3)3 resulted in the isolation of the 1 1 complex with a pseudo-trigonal bipyramidal geometry at the antimony center. 

In 1993 Burk, Brown, and coworkers confirmed that DuPHOS complexes exhibit the same anti-lock-and-key mechanistic motif as seen for aryl phosphine ligated catalysts [41], In 1998 by Burk and coworkers reported an unexpected and interesting result [34], With substrates having R<, = aryl, selectivity of 99% e.e. for the S product resulted from (S,S)-Me-DuPHOS-Rh hydrogenations, but the R product was obtained with similarly high enantioselectivity when Ra = t-Bu or adamantyl. In other words, the simple change of an aryl substituent to a bulky alkyl completely reverses the sense of enantioselection. 

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