Transition metal free

Remarkably, one year later Leadbeater described that biaryls can be synthesized via a Suzuki-type coupling under transition-metal free conditions [51, 52]. The reaction conditions were almost identical to those reported for the ligand-free process, with the difference being that a larger amoimt of Na2C03 and arylboronic acid were used. Only one successful example of a heteroaryl haUde substrate is shown namely, the coupling of 2-bromopyridine with phenylboronic acid (Scheme 32). 3-Bromothiophene did not couple under the same reaction conditions. Unfortimately, attempts to use heteroarylboronic acids such as 3-pyridinylboronic acid, 3-thienylboronic acid, and lH-indol-5-ylboronic acid on 4-bromoacetophenone completely failed. 

A potentially interesting development is the microwave-assisted transition-metal-free Sonogashira-type coupling reaction (Eq. 4.10). The reactions were performed in water without the use of copper(I) or a transition metal-phosphine complex. A variety of different aryl and hetero-aryl halides were reactive in water.25a The amount of palladium or copper present in the reaction system was determined to be less than 1 ppm by AAS-MS technique. However, in view of the recent reassessment of a similarly claimed transition-metal-free Suzuki-type coupling reaction, the possibility of a sub-ppm level of palladium contaminants found in commercially available sodium carbonate needs to be ruled out by a more sensitive analytical method.25 ... 

The X-ray structure of lithium l-(dimethylamido)boratabenzene, reported in 1993, provided the first crystallographic characterization of a transition metal-free boratabenzene (Scheme 13).18a The observed bond lengths are consistent with a delocalized anion and with significant B—N double-bond character. In a separate study, the B—N rotational barrier of [C5H5B—NMeBnjLi has been determined to be 10.1 kcal/mol, and it has been shown that TT-complexation to a transition metal can increase this barrier (e.g., 17.5 kcal/mol for (C5H5B-N(i-Pr)2)Mn(CO)3).24... 

Scheme 6.37 (a) Transition metal-free, (b) copper(l)-catalyzed Sonogashira-type couplings. 

Direct high-temperature microwave-assisted aminations of a variety of aryl halides [257] or aryl triflates [258] under transition metal-free conditions have also been reported, probably involving a benzyne mechanism. 

The Fujiwara group reported a transition metal free calcium-mediated transformation of methane into acetic acid, in the presence of S2Og2 (Equation (13)). 

Transition Metal Free Catalytie Aerohie Oxidation of Aleohols Under 119... 

Giernoth, R. and Bankmann, D., Transition-metal free ring deuteration of imidazolium ionic liquid cations. Tetrahedron Lett., 47,4293,2006. 

Yonker, C. R. and Linehan, J. C., A high-pressure NMR investigation of reaction chemistries in a simple salt hydrate,. Supercrit. Fluids, 29, 257 2004. Mehnert, C. R, Supported ionic liquid catalysis, Chem. Eur. ]., 11,50,2005. Giernoth, R. and Bankmann, D., Transition-metal free synthesis of perdeuter-ated imidazolium ionic liquidsby alkylation and H/D exchange, Eur. J. Org. Chem., 2008 (in print). DOT 10.1002/ejoc.200700784. 

Transition-metal-free cross-coupling reactions. 556... 

A water-mediated, transition metal free reaction ... 

Leadbeater, N.E. and Marco, M., Transition-metal-free Suzuki-type coupling reactions, Angew. Chem., Int. Ed. Engl, 2003,115, 1445-1447. 

Leadbeater, N.E., Marco, M. and Tominack, B.J. (2003) First examples of transition-metal free Sonogashira-type couplings. Org. Lett., 5, 3919. 

An efficient, mild, transition metal-free method for the intermolecular C-N addition of amides and S-N addition of sulfinamides to arynes has been reported (Scheme 69).106... 

As mentioned previously, the role of the metal is not specific. Even transition metal free layered double hydroxides are suitable catalysts for the olefin epoxidation with O2 and a sacrificial aldehyde (205). Leaching of the metal from the solid catalyst is a serious problem since organic acids that are potential metal ligands accumulate during the reaction. Leaching occurs, for example, with the polybenzimidazole-supported Ni2+ catalyst (199). 

With transition-metal free cationic phosphenium, an amino substituent is known to stabilize the cation. Theoretical studies46-50 support the existence of conjugation involving the nitrogen lone pairs and the vacant phosphorus p orbital, and estimate that the N-P+-N lone pair conjugation in [P(NH2)2]+ contributes 68.1 kcal/mol.47... 

The results of X-ray structures mentioned above show that the P-N bond lengths are longer than those of transition-metal free phosphenium, such as [P(N Pr2)2][AlCl4] (1.613 A). Therefore, Ji-donation as depicted in 59 and 60 may not be dominant, in other words, Ji-back donation from a transition metal (57) may be the dominant contribution. 

Chevion. M. A site-specific mechanism for free radical induced biological damage the essential role of redox-active transition metals. Free Radic. Biol Med 5 2" -37 1988. 

The transition metal free zeolites (HMOR, HZSM5, NaY) did not show any C-S hydrogenolysis activity for thiophene, but exhibited remarkable coking, and, the acidic zeolites, additionally some cracking activity. 

Arvela RK, Leadbeater NE, Sangi MS, Williams VA, Granados P, Singer RD (2005) A reassessment of the transition-metal free Suzuki-type coupling methodology. J Org Chem 70 161... 

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