Allylic platinum

JCD549, 1988JCD427, 19950M2538> are intermediate between those observed for /3-allyl complexes (ca. 4000 Hz for unsubstituted allyl platinum complexes, 3500—4000 Hz for 2-alkoxyallyl platinum complexes) and platinacyclobutane complexes (<2000 Hz) <1993OM3019>. These data clearly support a contribution from the 3-allyl-like coordination mode in metallacyclobutanone complexes. 

New reports continue to appear, although many pertinent issues remain. Platinacyclobutane complexes 157 have been isolated from the alkylation of cationic 3-allyl platinum complexes with ketene silyl acetals and silyl enol ethers... 

Bis(pinacolato)diborane(4) selectively adds to terminal aikenes and cyclic aikenes having internal strain to provide bis(boryl)alkanes in 76-86% yields 85-89 in the presence of a catalytic amount of Pt(dba)2 at 50 °C67 (Scheme 16). It is interesting to mention that Pt(dba)2 directed 1,2-addition to certain conjugated dienes, whereas 1,4-addition through a 7i-allyl-platinum(II) intermediate is an energetically more favorable process. The 1,4-addition to penta-1,3-diene at 80 °C with Pt(PPh3)4 gives 90, but the same reaction with Pt(dba)2 selectively produced the 1,2-addition product 91 at room temperature (Scheme 16). 

Bis-pi-allyl Pd and Pt complexes have been found to catalyze the addition of allyl tributyltin to aldehydes [26]. These catalysts are formed in situ from Pd- and PtCl2-phosphine complexes and the allylstannanes (Fig. 7, step 1). The allylation step is depicted as a metallocene reaction of the aldehyde and an his-allylmetal complex (Fig. 7, step 3). The catalyst is regenerated by attack of the allylic stannane on the alcoholate-palladium complex formed in step 3. Representative additions of allyl and methallyl tributyltin to aldehydes with the pi-allyl platinum catalyst are summarized in Table 14. 

In contrast with palladium(II), platinum(II) shows little tendency to form TT-allylic complexes. Only one tt-allylic platinum(II) complex, tr-allyl-7T -cyclopentadienylplatinum(II) has been prepared so far (4), We have now shown that several acyclic and cyclic 1,3-dienes react with sodium chloro-platinite in alcohols to give products of composition [PtCl (diene)l these are very insoluble and do not appear to involve tt -allylic bonding to the platinum i.e, they are true olefin complexes. 

Bis(acac-/i-allyl-platinum) (95) contains bridging allyl groups, each of which forms a a-bond to one metal atom and a w-bond to the other. Each platinum atom is bonded to two oxygen atoms, a carbon atom, and an... 

T. Suzuki and H. Fujimoto, Inorg. Chem., 1999, 38, 370-382. Mechanisms of the Nucleophilic Substimtion of the AUyl Carbons of ( 7r-Allyl)platinum and (rr-Allylfpal-ladium Complexes. 

Allyl Complexes. Allyl complexes of thorium have been known since the 1960s and are usually stabilized by cyclopentadienyl ligands. AEyl complexes can be accessed via the interaction of a thorium haUde and an aHyl grignard. This synthetic method was utilized to obtain a rare example of a naked aHyl complex, Th(Tj -C2H )4 [144564-74-9] which decomposes at 0°C. This complex, when supported on dehydroxylated y-alumina, is an outstanding heterogeneous catalyst for arene hydrogenation and rivals the most active platinum metal catalysts in activity (17,18). 

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

When used at room temperature in the presence of an active platinum catalyst in an inert solvent, e.g., acetone or ethyl acetate, oxygen will oxidize nonhindered, saturated hydroxyl groups and exposed allylic alcohols. This reagent has found extensive use in sugar chemistry and is particularly suited for the selective oxidation of either 3a- or 3j -alcohols of steroids. Other hydroxyl groups on the steroid skeleton are much less sensitive to oxidation. As a result, this reaction has been used extensively in research on polyhydroxy cardiac-active principles, e.g., the cardenolides and bufadienolides, where the 3-hydroxyl group is easily oxidized without extensive oxidation or dehydration of other hydroxyl groups. The ordinarily difficult selective oxidation of the... 

Dan and Henbesi (ii) demonslraied ihai ihe amount of salts remaining in platinum oxide catalysts had an important bearing on the hydrogenation-hydrogenolysis ratio of allylic functions. Hydrogenolysis is inhibited by salts remaining from the catalyst preparation or by salts such as sodium nitrite, cyanide, or hydroxide added later. 

The formed methylcyclohexane carbocation eliminates a proton, yielding 3-methylcyclohexene. 3-Methylcyclohexene can either dehydrogenate over the platinum surface or form a new carbocation by losing H over the acid catalyst surface. This step is fast, because an allylic car-bonium ion is formed. Losing a proton on a Lewis base site produces methyl cyclohexadiene. This sequence of carbocation formation, followed by loss of a proton, continues till the final formation of toluene. 

Catalytic hydrogenation with platinum liberates the hydrocarbon from methylcobalamin (57) and from alkyl-Co-DMG complexes (161), but not from pentacyanides with primary alkyl, vinyl, or benzyl ligands, though the cr-allyl complex yields propylene (109). Sodium sand gives mixtures of hydrocarbons with the alkyl-Co-salen complexes (64). Dithioerythritol will liberate methane from a variety of methyl complexes [cobalamin, DMG, DMG-BF2, G, DPG, CHD, salen, and (DO)(DOH)pn] (156), as will 1,4-butanedithiol from the DMG complex (157), and certain unspecified thiols will reduce DMG complexes with substituted alkyl ligands (e.g., C0-CH2COOH ->CH3C00H) (163, 164). Reaction with thiols can also lead to the formation of thioethers (see Section C,3). 

Lines represent [1 1 0] directions of underlying platinum. A unit cell ofthe adsorbate structure is drawn, (b) Diagram ofthe proposed (y/7 x yj7) R19.T model for the Jt-allyl structure in Figure 7.18a. Inset shows bonding structure. 

Fiirstner and coworkers developed a new Pt- and Au-catalyzed cycloisomerization of hydroxylated enynes 6/4-141 to give the bicylo[3.1.0]hexanone skeleton 6/4-143, which is found in a large number of terpenes [317]. It can be assumed that, in the case of the Pt-catalysis, a platinum carbene 6/4-142 is formed, which triggers an irreversible 1,2-hydrogen shift. The complexity of the product/substrate relationship can be increased by using a mixture of an alkynal and an allyl silane in the presence of PtCl2 to give 6/4-143 directly, in 55 % yield (Scheme 6/4.36). 

Similar reactions can be utilized to prepare allyl complexes of platinum and palladium. In this case, the product can exist in two isomers as described earlier. Analogous reactions can be used to prepare the tris allyl complexes of several metals. 

The electrolysis of alkyl halides on platinum cathode and tin anode has been mentioned above. A completely different mechanism is associated with alkylation on tin cathodes. Electroreduction of allyl bromide on tin electrodes yields tetraallylstannane (Ca 90%). This is done in acetonitrile solutions with LiClC>4, Et4NBr or BujNBr as electrolyte and followed by CV with Ag/AgBr reference. Yields decrease to 78% in DMF. The proposed mechanism67 in this case is ... 

Shortly after Trost s works, two investigations demonstrated the high reactivity of platinum halide salts for this type of reactions. Blum reported a PtCU-catalyzed rearrangement of allyl propargyl ethers to 3-oxabicyclo[4.1.0]-heptenes (Scheme 79).294 This series of reactions also represented the premiere entry into the versatile formation of cyclopropyl products based on skeletal rearrangements of enynes.295 This intriguing aspect is discussed further. 

Platinum-catalyzed allylation of aldehydes with allyltin reagents was first reported in 1995.4S7 457b,483 483a Ar0matiC) aliphatic, a,/3-unsaturated aldehydes and even cyclohexanone undergo allylation with allyltributyltins in the presence of PtClgtPP 113)2 >n THF at room temperature or higher temperature (Equations (123) and (124)). Allylplatinum species are considered to be the active intermediates on the basis of related mechanistic studies on palladium catalysis. 

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