Platinum complexes, stereoselection

The stereoselective catalyzed addition of water or methanol to dimethyl acetylenedicarboxylate (DMAD) was reported to yield oxalacetic acid dimethylester or dimethyl methoxyfumarate. The catalyst precursor cis-[Pd(PMe2Ph)2(solvent)2] [BFJj was prepared from ds-[PdCl2(PMe2Ph)2] and AgBp4 (Eq. 6.54). The analogous platinum complex was not effective, however [99]. 

Reaction of the platinum complex with a molar excess of racemic acrylamide 7 leads to four distinct AB quartets in the 31P-NMR spectrum (with associated platinum satellites) due to the four possible diastereomeric species. These are formed by nonselective binding of the Re-and 57-face of both enantiomers. Such behaviour may be expected when the stereogenic center under examination is remote from the metal center. After reaction of the ethene complex with carvone 8 the metal is bound with complete stereoselectivity to the less hindered Si- 5/-face of the endocyclic, electron-poor double bond. Spectra of optically pure and racemic carvone complexes are shown in Figure 14. 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

Direct metallacyclobutene formation by reaction of free tetrafluorocyclopropene and trigonal platinum complexes or tetragonal iridium complexes (equation 230) is also reported in the literature. These reactions may proceed via a labile / -cyclopropene complex intermediate. However, the stereoselectivity observed in the product complexes of... 

Cobalt, rhodium and platinum complexes modified with numerous chiral phosphanes have been used in asymmetric hydroformylation of styrene. The results are compiled in Table 4. Iso-product selectivities of >95% and stereoselectivities of >90% ce are reported, in many cases, however, only with low conversion rates and yields. Early results based on optical rotation measurements had to be reevaluated due to wrongly adopted rotation values for hydra tropaldehyde4-. ... 

Platinum complexes show none of the catalytic activity found in palladium and nickel complexes. This chemical inertness makes platinum a useful model for the more active catalysts. There has been a suggestion that platinum complexes of chiral a-diimines might lead to stereoselective olefin polymerization. Chiral camphor-based ligands have been employed in palladium complexes for ethylene polymerization, but there was no mention of stereoselectivity in hexene polymerizations. 

Entries 1-22 list exo phosphines 200 whereas entries 23-30 are endo phosphines 203 (Scheme 2.62). The number of endo phosphines prepared to date is more limited, probably due to the low stereoselectivity in their formation. Entries 1-10 include exo monophosphines with additional oxygen (entries 1-6), sulfur (entries 3, 5-8) and nitrogen (entries 9 and 10) donor atoms. Entries 11-22 contain a second phosphorus or an arsenic atom, which in some cases is also stereogenic (entries 14-16 and 19 21). The compound in entry 19 is the dimerisation of DMPP, which required the use of a platinum complex. ... 

These facts may well imply that the stereoselectivity for addition of a hydrosilane to the enantiotopic faces of a ketone is different from that of an olefin which is undoubtedly tr-coordinated to the chiral catalyst the difference is again explained in terms of the intermediacy of an a-siloxyalkyl-platinum complex similar to the a-siloxyalkyl-rhodium complex described in the hydrosilylation of terpene ketones (see Scheme 4). 

Han, L.-B., Choi, N., and Tanaka, M., Oxidaative addition of HP(0)Ph2 to platinum(O) and palladium(O) complexes and palladium-catalyzed regio- and stereoselective hydrophosphinylation of alkynes, Organometallics, 15, 3259,... 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

Diboration of a,/ -unsaturated ketones is promoted by platinum(O) complexes. Reaction of 4-phenyl-3-buten-2-one with bis(pinacolato)diboron in the presence of a platinum catalyst affords a boryl-substituted (Z)-boron enolate, that is, a 1,4-diboration product, in high yield with high stereoselectivity (Scheme 8). The isolated boron enolate is easily hydrolyzed by exposure to water, giving / -boryl ketones in high yields.66 Similar diboration of a,/ -unsaturated ketones has also been achieved with Pt(bian)(dmfu) (bian = bis(phenylimino)acenaphthene, dmfu = dimethyl fumarate).67 Although the... 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

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. 

In contrast, cyclizations of /V-alkoxy derivatives with iodine are much less selective (Table 30, entry 3), and similar results are observed with systems containing internal double bonds of either ( )- or (Z)-configuration.238 The stereoselectivity of the aminomercuration reaction of /V-methyl derivatives has been reported to be controlled by the choice of mercury(II) salt and solvent (entries 6 and 7).244 Cyclization with platinum salts showed little selectivity (entry 8).239 An iodocyclization of a complex N-alkyl system has been used in a recent synthesis of (+)-croomine.238... 

Although Pt-alkene complexes are normally not as reactive as the corresponding Pd complexes toward nucleophilic addition, platinum can serve usefully as a catalyst in these reactions.59 The following cyclization was mediated by a Pt-PPP-pincer complex 22, demonstrating Markovnikov addition at each step of the cyclization (equation 8.42).60 The reaction is stereoselective and mimics the cyclization of squalene epoxide to lanosterol, a key step in the biosynthesis of cholesterol. 

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