Catalysis platinum

A further improvement in platinum catalysis is claimed from use of tin(Il) haUde and phosphine ligands which are rigid bidentates, eg, l,2-bis(diphenylphosphinomethyl)cyclobutane (27). High rates for a product containing 99% linear aldehyde have been obtained. However, a pressure of 10 MPa (1450 psi) H2 CO is requited. 

Thus, all five triple bonds in decamethyl[5]pericycline 3 can be completely hydrogenated to yield the corresponding saturated cyclic hydrocarbon 78 under platinum catalysis (Pt02, Adam s catalyst), albeit with some difficulty (Fig. 6). 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

Although Echavarren et al. reported similar conversions by platinum catalysis, lower selectivity was observed [165-167]. Computational studies then suggested the initial formation of a cyclopropyl carbenoid intermediate 213, as shown in Scheme 8.28. 

Previously reported platinum catalysts contain monodentate phosphines or bidentate phosphines and weakly coordinating anions (we have argued on the basis of P spectroscopy characteristics that the hitherto popular SnCl j anion used in platinum catalysis should also be regarded as a weakly coordinating anion indeed trifiate complexes of diphosphines also give active catalysts). 

In addition to the synthetic route shown in Scheme 32, Armstrong approached the synthesis from the other direction, by converting the initial resin-bound iodide to a supported pinacolatoboronate 30 under platinum catalysis (Scheme 33). This was then coupled in the usual way with a solution-phase aryl iodide in high yield, and with much more satisfactory results than were obtained with the vinylboronates 29. This chemistry was later shown to be useful in solution for one-pot biaryl synthesis by genera-... 

O. Deutschmann, L.I. Maier, U. Riedel, A.H. Stroemman, and RW. Dibble. Hydrogen Assisted Catalytic Combustion of Methane on Platinum. Catalysis Today, 59 141-150,2000. 

A further improvement in platinum catalysis is claimed from use of tin(TI) halide and phosphine ligands which are rigid bidentates, e.g.. 1,2-bis(cliphenylphosphinomethyl)cyclobutane. 

Rearrangements of 1,3-diynes (43) to dienynes (45a) and (45b) have been carried out catalytically a metal-carbene intermediate (44) is likely to be involved.41 Interestingly, gold catalysis provides mainly (45a) whereas platinum catalysis under a CO atmosphere yields predominantly (45b). 

While considerable rate data for sintering of supported nickel and platinum catalysis are available from previous studies, their interpretation, comparison, and correlation is complicated by experimental limitations which include (1) lack of systematic or statistical experimentation, and (2) nonstandard and difficult-to-compare measurements of dispersion, crystallite diameter and crystallite size distribution. 

The hydrosilation of 1-alkynes can give either Z-or E-isomers (see (E) (Z) Isomers) of the linear vinylsilane or the Markownikow product (equation 5). Reaction of 1-hexyne with trichlorosilane under platinum catalysis conditions gives a mixture of 1- and 2-silyl-l-hexene (78 22). From the E-stereochemistry of the 1-silyl isomer, a yyn-addition mechanism can be assumed. 

Both disilazanes A and B, which are readily prepared and isolated, may be cocondensed with other polymeric polysilazanes. By cocondensation, the controlled introduction of either (or both) vinyl and silyl hydride groups can be accomplished readily. In turn, these groups provide a convenient cross-linking mechanism for these polymers via platinum catalysis or thermal inducement of hydrosilation. This method eliminates the need for cross-linking via hydrolysis or oxidation, which introduces undesirable oxygen bonds. Other advantages will be treated in a later section. 

Platinum catalysis, 489 490 (see also Meta I-catalyzed reactions)... 

Similar findings have been reported by Ishikawa and coworkers. Using the disilacyclobutene (100) as the key reactant, they have found that disilacyclopentane and -ene products, e.g., (102)-(107), can be obtained using non-enolizable ketones <930M87>, alkenes, dienes <930M4987, 950mii4>, or alkynes <95JOM(499)35> with nickel or platinum catalysis—presumably via metal insertion intermediates such as (101) ( heme 12). 

Platinum catalysis of hydroformyiation has seen remarkable progress in the last twenty years. The metal has extremely low or nonexistent activity. A phosphine complex, [Ph3P)3PtH] PF6, is a low activity catalyst at 150°C and 10 MPa of H2/CO. A phosphine platinum(II) iodide, (Ph3P)2M2. is somewhat better, giving 89% conversion of olefin to aldehydes at 180°C and 25 MPa pressure in 1 hr in dimethylformamide solvent. 

Coumarins can be obtained directly, in a one-pot procedure, from phenols and a propiolate, using palladium or platinum catalysis. ... 

Heyns, K., and Paulsen, H., Selective Catalytic Oxidation of Carbohydrates, Employing Platinum Catalysis, 17, 169-221... 

Organic iodides with 6-hydrogen atoms can be carbonylated under platinum catalysis (Scheme 60). The enhanced stability of platinum-hydrogen bonds with respect to palladium-hydrogen bonds... 

Hydrosilylation of allylic amines proceeds under platinum catalysis with the opposite regioselectivity to their oxygen analogs [133], The intermediate l-aza-2-silacyclobu-tanes resulting from a 4-ejco-trig addition are thermally stable and may be purified by distillation. Alternatively, direct oxidation using the Tamao conditions provides the corresponding 1,2-amino alcohols with excellent syn selectivity (Scheme 10-111). 

---