Phosphine intermediates

In Figure 1, the observed reactions are indicated in the context of a proposed catalytic scheme for terminal hydroformy-lation. The reaction pathway involving the alkyl phosphine intermediate (VIII) is the most likely. Overall, the results of the NMR studies provide consistent explanations for the process parameters of selective hydroformylation, particularly of the low pressure continuous product flashoff process (.5,15). it was shown that, in contrast to prior indications (3), the tris-phosphine complex (I) is a remarkably stable and favored species in the presence of excess phosphine and H2. This complex (I) is postulated to have a key role in the reversible generation and... 

It is obvious that such equilibria would exist for all the other catalytic intermediates. The result of all this is coupled catalytic cycles and many simultaneous catalytic reactions. This is shown schematically in Fig. 5.5. The complicated rate expressions of hydroformylation reactions are due to the occurrence of many reactions at the same time. As indicated in Fig. 5.5, selectivity towards anti-Markovnikov product increases with more phosphinated intermediates, whereas more carbonylation shifts the selectivity towards Mar-kovnikov product. This is to be expected in view of the fact that a sterically crowded environment around the metal center favors anti-Markovnikov addition (see Section 5.2.2). 

From the point of view of generality, it is desirable if a phosphine intermediate with more than one easily interchanged group could be synthesized asymmetrically. This has been realized to some extent with the discovery that PI1PCI2 or PhP(NEt2>2 reacts with cheap and readily available ephedrine to form a... 

Whereas tetrahedral chelate complexes Pd(L—L)2 do not react with Phi, 1 1 mixtures of L—L with Pd(dba)2 give (L—L)Pd(dba) and its dissociation product, 2-coordinate (L—L)Pd. Both 3- and 2-coordinate species oxidatively add iodoben-zene, though the latter species are more reactive.57 By contrast, the isolable 14-electron complex Pd[P(o-tol)3]2 reacts with p-Bu QHiBr via a dissociative pathway that produces a mono-phosphine intermediate ... 

One major advantage offered by the dppf ligand in Rh-catalyzed olefin hydroformylation is exemplified in its higher linear aldehyde selectivity when present in a dppf Rh ratio of 1.5 or higher [37,242]. This result leads to the proposed key intermediate of a Rh dimer with both chelating and bridging phosphine in the catalytic cycle. It also confirms the significance of the tris (phosphine) moieties at the point when the aldehyde selectivity is determined, i.e., the step in which the hydride is inserted into the M-olefin bond. This involvement of a dinuclear or tris (phosphine) intermediate appears to differ from the intermediate RhH(CO)(PR 3)z (olefin) (which is converted into the square planar Rh(R)(CO)(PR 3)2 by hydride insertion) commonly accepted for hydroformylation catalyzed by monophosphine complexes. P NMR studies also established the existence of the equilibrium in which the disphosphine can be... 

For the reactions with tertiary phosphines, intermediates of the form [HPt(/r-H)( -dppm)2PtH(PR3)]+ have been detected, and equilibrium constants for their formation at low temperature have been determined (110). Exchange of PR3 takes place between the two platinum centers. The elimination of H2 has been shown to be intramolecular, and the rate-determining step is the elimination itself. Rearrangement to a second intermediate without a bridging hydride is believed to occur prior to reductive elimination, but it is not possible to determine with certainty whether the elimination occurs at one platinum center or by means of a concerted dinuclear process. Mechanisms involving each type of elimination process have been proposed (Scheme 8) (110). 

Dingwall, J.G., Ehrenfreund, J., and Hall, R.G., Diethoxymethylphosphonites and phosphinates. Intermediates for the synthesis of a,P- and y-aminoalkylphosphonous acids. Tetrahedron, 45, 3787, 1989. Mohrle, H., and Vetter, W., Participation of phosphonate neighbour groups with dehydrogenations of amines, Z. Naturforsch., 43b, 1662, 1988. 

Solvolysis of the a-haloalkylphosphinic esters (97) and (98) with methoxide ion in methanol leads to the rearranged phosphonate esters (99) and (100). The formation of these products can be readily accommodated in terms of a cyclic phosphinate intermediate. 

Denney and his co-workers (91) showed that reduction of trans- -decalyl h droperoxide by triphenylphosphine proceeds with retention of configuration and that reductions of this type carried out in ethyl alcohol-H20 gave 0 -free phosphine oxide and alcohol. Provided that similar mechanisms operate for both phosphites and phosphines, intermediates such as 18 are unlikely, since valency expansion should lead to inversion if SN2 or to racemization if SNl and should also be expected to furnish more than one alcohol. Furthermore, if 18 is formed, then it must decompose to products faster than the hydroxide ion equilibrates with 0 . Denney and co-workers (91) hold that the most reasonable mechanism involves attack by phosphite on the hydroxyl oxygen to give intermediate 19, which yields products by a simple proton transfer, or a simultaneous proton transfer via the transition state 20. 

The formation of mixed anhydrides 2 is critical for overall yield of the conversion of H-phosphonate monoesters 1 into H-phosphonothioate 4. The transformation of an H-phos-phonate into an H-phosphonothioate function in the ribo series proved to be a stereoselective process. In contradistinction to (his transformation, synthesis of the ribonucleoside 3 -H-phosphonothioate via phosphinate intermediates [202] hardly showed any stereoselectivity. 

Toyobo s HEIM II (former GH) is apparently based on the following chemistry via a phosphinate intermediate dihydrooxaphosphaphenanthrene oxide (DOPO) [63562-31-2] produced from o-phenylphenol and phosphorus trichloride (135) ... 

Phosphorus-Containing Printed Wiring Boards. This same phosphinate intermediate, dihydrooxaphosphaphenanthrene oxide (DOPO), is also the subject of substantial efforts in the Far East and Europe to react it into epoxy printed wiring boards and encapsulated electronic circuit elements as a... 

A catalytic Aza-Wittig reaction was developed for the synthesis of 4-3//-quinazolinones, involving formation of the corresponding imino-phosphine intermediate in the first step (Scheme 63). ... 

A plausible mechanism is the activation of the hydroxyl group via a cationic phosphine intermediate, followed by a halide nucleophilic substitution (Scheme 42.11). 

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