Platinum complexes alkene

Figure 6-13. Synergic back-bonding in a platinum alkene complex. In (a), the interaction of a (filled) platinum 5d orbital with the tf molecular orbital of the alkene is shown, whilst in (b), the interaction of a dsp hybrid orbital with the n molecular orbital of the alkene is shown. Note that the two interactions result in electron density moving in opposite directions.

It has also been proposed (96) that multiple exchange occurs via formation of platinum-alkene complexes from the alkyl intermediate (XXXI) ... 

Even multiple exchange in methane could be explained this way. For methane, a carbene complex would be postulated, and such complexes are now known (87). One objection to the involvement of platinum-alkene complexes is that they are very stable and once formed might not be expected to react further, as was found when exchange in chloropro-pane was attempted (Section III,C) (55). Also the compound formed is a platinum(O) complex. A mathematical analysis of multiple exchange similar to that developed by Anderson and Kemball (/) has been carried out (82). 

Platinum alkene complexes have been known since 1830 when Zeise s salt was discovered. Alkene complexes of platinum(II) are kinetically more stable than their palladium counterparts, but this feature makes them less attractive for applications in homogeneous catalysis. A number of reviews have been written on alkene complexes.620-623... 

Under certain conditions, platinum alkene complexes can be converted to jr-allyl complexes by hydrogen loss (equation 281).860 Allyl alcohol insertion into a platinum(II) carbonyl bond gives an allyloxycarbonylplatinum(II) intermediate which can be decarboxylated to form the jj3-allyl complex (equation 282).861,863... 

The phosphine-based platinum(O) catalysts do not catalyze the diboration of alkenes because of the high coordination ability of phosphine over the alkene double bond, but platinum(O) complexes without a phosphine ligand such as Pt(dba)2 [128] and Pt(cod)2 [129] are an excellent catalyst allowing the alkene insertion into the B-Pt bond under mild conditions (Scheme 1-30). The diboration of aliphatic and aromatic terminal alkenes takes place smoothly at 50°C or even at room temperature. The reaction is significantly slow for disubstituted alkenes and cyclic alkenes, but cyclic alkenes having an internal strain afford ds-diboration products in high... 

Phosphite complexes of platinum(0) have received substantially less attention than have phosphine complexes.44 [Pt P(OC6H4OMe-2)3 3] can be prepared by reduction of the [PtCl2 P-(OC6H4OMe-2)3 2] complex in the presence of the phosphite or by the reaction of the phosphite with Lris(//2-norbornene)platinum(II) 44 Alkene complexes of bis(phosphite)platinum(II) can be prepared in a similar manner to the analogous phosphine complexes. 

The platinum(0) complex [Pt(PhNO)(PPh3)2] reacts with C02 to afford the metallacyclic nitroso species [Pt 0N(Ph)C(0)0 (PPh3)2] (60), the first example of insertion of C02 into a Pt—N bond.186 Other unsaturated carbon compounds such as CS2 and electrophilic alkenes and alkynes react similarly. The diradical peril uoro-/V,/V -dimethylethane-l,2-bis(amino-oxyl) reacts readily by oxidative addition to the platinum(0) precursor Pt(PPh3)4 to afford the corresponding platinum(lI)-nitroso complex containing a seven-membered chelate ring (61). The resulting complex is stable in air for several days at room temperature.187... 

Platinum(II) complexes with diphosphines based on DIOP (85),315-321 CHIRAPHOS (86),316,320 and bdpp (87)322-325 backbones have been prepared to be used, in the presence of SnCl2, as catalyst precursors in asymmetric hydroformylation of styrene and other alkenes. 

In this case, the bond between the carbon atoms is about the same as it is for a C-C single bond. Moreover, unlike the anion of Zeise s salt, the carbon atoms are in the plane formed by platinum and the other ligands. Clearly, this represents a significant difference from the usual alkene complexes. In essence, a three-membered C-P-C ring is formed. It appears in this case that the ability of the... 

The rearrangement of platinacyclobutanes to alkene complexes or ylide complexes is shown to involve an initial 1,3-hydride shift (a-elimina-tion), which may be preceded by skeletal isomerization. This isomerization can be used as a model for the bond shift mechanism of isomerization of alkanes by platinum metal, while the a-elimination also suggests a possible new mechanism for alkene polymerisation. New platinacyclobutanes with -CH2 0SC>2Me substituents undergo solvolysis with ring expansion to platinacyclopentane derivatives, the first examples of metallacyclobutane to metallacyclopentane ring expansion. The mechanism, which may also involve preliminary skeletal isomerization, has been elucidated by use of isotopic labelling and kinetic studies. 

Ishiyama, T., Yamamoto, M., and Miyaura, N., Diboration of alkenes with bis(pinacolato)diboron catalyzed by a platinum(O) complex, Chem. Commun., 689, 1997. 

Metal chemical shifts have not found extensive use in relation to structural problems in catalysis. This is partially due to the relatively poor sensitivity of many (but not all) spin 1=1/2 metals. The most interesting exception concerns Pt, which is 33.7% abundant and possesses a relatively large magnetic moment. Platinum chemistry often serves as a model for the catalytically more useful palladium. Additionally, Pt NMR, has been used in connection with the hydrosilyla-tion and hydroformylation reactions. In the former area, Roy and Taylor [82] have prepared the catalysts Pt(SiCl2Me)2(l,5-COD) and [Pt()i-Cl)(SiCl2Me)(q -l,5-COD)]2 and used Pt methods (plus Si and NMR) to characterize these and related compounds. These represent the first stable alkene platinum silyl complexes and their reactions are thought to support the often-cited Chalk-Harrod hydrosilylation mechanism. 

The platinum(O) complexes catalyze the addition of 1 to unsaturated hydrocarbons (Scheme 1). The addition to alkynes,7 alkenes,8 1,3-dienes,9 or allenes10 stereoselectively provides cis-addition products. 

The color of the (N,N-chelate)(olefin)platinum(0) complexes described in this section varies from light to dark yellow. These complexes are air stable and are fairly soluble in chloroform or methylene chloride. The stability in solution increases as the electron-withdrawing properties of the alkene increase. For example, the fumarate or maleate derivatives decompose very... 

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