Boronic aryl ether formation

Elimination to yield alkenes can be induced thermally or by treatment with acids or bases (for one possible mechanism, see Figure 3.39) [138,206]. Less common thermal demetallations include the thermolysis of arylmethyloxy(phenyl)carbene complexes, which can lead to the formation of aryl-substituted acetophenones [276]. Further, (difluoroboroxy)carbene complexes of molybdenum, which can be prepared by treating molybdenum hexacarbonyl with an organolithium compound and then with boron trifluoride etherate at -60 °C, decompose at room temperature to yield acyl radicals [277]. 

Boron trifluoride etherate was used in conjunction with the reducing agent borane to rearrange aromatic O-triisopropylsilyl ketoximes to cyclic and acyclic aniline derivatives. The steric hindrance of the substituents on the silicon atom, the size of the aliphatic ring and the presence of alkoxy substituents on the aryl group played important roles in the aniline formation. 

Cyclohexanone (202) was converted to compound (203) whose transformation to cyclohexanone (204) was accomplished in three steps. It underwent cyclialkylation with boron trifluoride etherate affording the cyclized product (205) (R=R,=OMe) in 64% yield along with naphthalene (206) (R=Ri= H,H). Compound (205) on heating under reflux with DDQ in benzene produced ketone (207) whose tosylhydrazone on treatment with sodium cyanoborohydride afforded reduced product (208). Deprotection of the aryl methyl ethers and oxidation with ceric ammonium nitrate led to the formation of miltirone (197). 

The photochemistry of aluminum aryls bears certain features in common with boron aryls the low reactivity of the monomeric triaryl-metallic, the formation of biaryls in tetracoordinate systems having proximate aryl groups and the formation of subvalent metallic products (22-24). Thus, when irradiated at 254 nm in ethyl ether solution, triphenylaluminum ethyl etherate underwent no formation of biphenyl or aluminum metal. On the other hand, the irradiation of triphenylaluminum (99) in benzene or toluene solution, in which the aluminum aryl is largely dimeric, led to the production of biphenyl (45%) and aluminum metal. When such a reaction mixture was filtered to remove the aluminum and the filtrate treated with D2O, hydrogen gas was evolved that consisted of >98% H—D. The biphenyl now isolated was 20% undeuterated and 80% monodeuterated further spectral comparisons showed that the deuteron was at the 2-position. These findings indicated the formation of an aluminum-hydride and an o-biphenylyl aluminum bond [Eq. (47)]. The source of the hydrogen in H—D was not the... 

There are two reports of the application of this chemistry to the preparation of symmetrical biaryl ethers. One approach by Prakash and co-workers involves an in situ oxidation of the boronic acid with H2O2 (0.25 equiv.), followed by addition of 4A molecular sieves, Cu(OAc)2 (0.5 equiv.) and Et3N (0.5 equiv.) [17]. The protocol developed by Sagar et al. entails simply stirring the boronic acid (1 equiv.), Cu(OAc)2 (1 equiv.), EtjN (5 equiv.), and water (10 equiv.) in methylene chloride and acetonitrile [18] (Equation 3). However, notably, biaryl ether formation is a typical side reaction in O-and N-arylations. 

The interaction of 2-alkyl-A -thiazolines (196) and aryl isocyanates in the presence of catalytic amounts of boron trifluoride etherate, or methyl isocyanate, produces cyclic 2 2 adducts (197), e.g. 6, 8a -dimethyl-3-methylcarbamoyl-5 -oxospiro(thiazolidine-2,7 -perhydrothiazolo[3,2-c]pyri-midine). This contrasts with the formation of the simple 2 1 adducts (198) in the absence of the catalyst (see these Reports, Vol. 2, p. 627). The 2 2 adducts may have one of several possible structures their formulation as (197) is based on their spectral properties and the results of their... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

Introduction and stereochemical control syn,anti and E,Z Relationship between enolate geometry and aldol stereochemistry The Zimmerman-Traxler transition state Anti-selective aldols of lithium enolates of hindered aryl esters Syn-selective aldols of boron enolates of PhS-esters Stereochemistry of aldols from enols and enolates of ketones Silyl enol ethers and the open transition state Syn selective aldols with zirconium enolates The synthesis of enones E,Z selectivity in enone formation from aldols Recent developments in stereoselective aldol reactions Stereoselectivity outside the Aldol Relationship A Synthesis ofJuvabione A Note on Stereochemical Nomenclature... 

In 2000, Guy reported the stoichiometric coupling of alkane thiols and arylboronic acids, which was initially thought to be mediated by Cu(ll) [71]. Liebeskind proposed that the reaction was more likely catalyzed by Cu(l), generated by oxidation of the alkane thiols into dialkyl disulfides. Based on this hypothesis, Liebeskind predicted that disulfides and disulfide equivalents should be effective reagents for thioether formation [34]. This process would constitute a modification of the Chan-Evans-Lam, which involves the coupling of arylboronic acids and amines or alcohols in the presence of tertiary amine bases, generating aryl amines and ethers, respectively. Indeed, the coupling of diphenyl disulfide with phenyl boronic acid would yield diphenyl sulfide. 

A simple modification of the reaction sequence shown above has allowed for the synthesis of e f-narciclasine (cnt-188). Thus, as shown in Scheme 20, Suzuki-Miyaura cross-coupling of the previously prepared 2-bromocyclohex-2-enamine 209 with the readily synthesised aryl boronic acid 212 afforded the expected lactam 213 (63%). Once again, treatment of the last compound with TMS-Br resulted in exhaustive cleavage of the MOM ether residues and, this time, the formation of the target compound cnt-188 (48%). 

Cross-coupling reactions leading to the formation of C-X (X = heteroatom) bonds catalyzed by Pd(dba)2 have been reported. Aniline derivatives have been prepared via reaction of amine nucleophiles with aryl halides in the presence of Pd(dba)2 and phosphines, especially P( Bu)3. Likewise, diaryl and aryl alkyl ethers are produced from aryl halides (Cl, Br, I) and sodium aryloxides and alkoxides under similar conditions. Conditions effective for the coupling of aryl chlorides with amines, boronic acids, and ketone enolates using an easily prepared phosphine chloride as a ligand have recently been uncovered (eq 22). The preparation of aryl siloxanes and allyl boronates via Pd(dba)2-catalyzed C-Si and C-B coupling have been reported as well. 

An efficient method for intramolecular direct arylation was employed on a doubly functionalized caUx[4]arene fixed in the cone conformation (Scheme 3.25). Dihydroxycalixarene 113 was obtained from dibromide 112 in 84% yield using lithiation, calixboronate formation and the final oxidative carbon-boron bond cleavage. The hydroxy groups of 113 were reacted with benzyl bromide (Williamson ether synthesis) and CS2CO3 in acetone to produce corresponding... 

The reaction of fluorinated nitro(orthonitrophenyl)imidazole with aryl boronic acids under Suzuki-Miyaura conditions is unusual in that it leads to displacement of fluoride and formation of O-arylated products, such as (106). It is possible that the pathway involves initial reaction of substrate with base to give a nitrophenyl derivative, which reacts by an Ar mechanism with more substrate to give a diphenyl ether before... 

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