Monophosphine complex

The search for even more active and recyclable ruthenium-based metathesis catalysts has recently led to the development of phosphine-free complexes by combining the concept of ligation with N-heterocyclic carbenes and benzyli-denes bearing a coordinating isopropoxy ligand. The latter was exemplified for Hoveyda s monophosphine complex 13 in Scheme 5 [12]. Pioneering studies in this field have been conducted by the groups of Hoveyda [49a] and Blechert [49b], who described the phosphine-free precatalyst 71a. Compound 71a is prepared either from 56d [49a] or from 13 [49b], as illustrated in Scheme 16. 

A first evaluation of complex 71a by Blechert et al. revealed that its catalytic activity differs significantly from that of the monophosphine complex 56d [49b]. In particular, 71a appears to have a much stronger tendency to promote cross metathesis rather than RCM. Follow-up studies by the same group demonstrate that 71a allows the cross metathesis of electron-deficient alkenes with excellent yields and chemoselectivities [50]. For instance, alkene 72 undergoes selective cross metathesis with 3,3,3-trifluoropropene to give 73 in excellent yield and selectivity. Precatalyst 56d, under identical conditions, furnishes a mixture of 73 and the homodimer of 72 (Scheme 17) [50a]. While 56d was found to be active in the cross metathesis involving acrylates, it failed with acrylonitrile [51]. With 71a, this problem can be overcome, as illustrated for the conversion of 72—>74 (Scheme 17) [50b]. 

Phosphine sulfides or selenides are well-known ligands in gold(I) chemistry. With monophosphine complexes of the type [AuX(SPR3)] (X = C1, Br, CN PR3 = PPh3, PCy3, PPh2Py,... 

Generally, monophosphine complexes can be generated by decomposition of suitable precursors, among which the most notable are palladacycles (Section 9.6.3.4.7). A spectacular example makes use of spontaneous disproportionation of a dimeric complex of Pd1 with very bulky ligands to give one of the most reactive catalytic systems known so far, which catalyzes the fast crosscoupling of arylboronic acids with aryl chlorides and hindered aryl bromides at room temperature (Equation (28)) 389... 

Two-coordinate Au(I) monophosphine complexes are usually not active in vivo because they are too reactive, e.g., toward plasma components such as albumin, and do not reach cancer cells. In contrast, diphosphine complexes such as 36, 37, and 38 are active in vivo. The... 

A monophosphine complex is formed when 3 is mixed with three equivalents of a zinc(ii)salphen complex and half an equivalent of Rh(acac)(CO)2 (acac = acetyl acetonate), whereas the assembly based on template 4 and the zinc(n)salphen complexes forms a bis-phosphine rhodium species. In the latter case, the bisphosphine rhodium complex is completely encapsulated by six salphen building blocks. This difference in mono- versus diphosphine ligation to the Rh -center and, to a lesser extent, the difference in electronic features (and thus donating properties of the phosphine) between template ligands 3 and 4, can be used to induce a different catalytic behavior. 

Palladium-monophosphine complexes catalyse trans-selective arylative, alkenyla-tive, and alkylative cyclization reactions of alkynals [e.g. (68)] and alkynones with organoboronic reagents. These reactions afford six-membered allylic alcohols (69) (g) and/or their five-membered counterparts (70), whose ratios are dramatically affected... 

A quantitative study of the dissociation of dinitrogen from the complexes [M(N2)2(dppe)2] in THF shows that it occurs too slowly to be the initiating step (16) and it must be the protonation of one of the dinitrogen ligands that triggers the sequence of reactions leading to ammonia in the monophosphine complexes. 

Both nickel and palladium monophosphine complexes catalyze the dimerization of methyl acrylate the tail-to-tail dimer is a nylon-6,6 precursor. The cycle involves... 

A report of the X-ray crystallographic studies of enan-tiopure ( -allyl)Fe(CO)2(NO) complex (170) has appeared, though httle detail was provided. The same report described the CD spectrum of this complex in more detail the negative band at ca. 350 mn and the positive band at ca. 450 nm can be used to assign the configuration of the complex. Diastere-omeric complexes exhibit the opposite Cotton effect. The crystal structures of corresponding monophosphine complexes (145) have been determined. ft is possible to consider these complexes as either trigonal bipyramidal (bidentate allyl) or tetrahedral see Tetrahedral) (monodentate allyl), with the central carbon of the allyl closer to the iron atom (2.084 A) than the terminal carbon atoms (2.117 and 2.142 A). These complexes are chiral at the iron atom, and it has proved possible to separate the diastereomeric complexes formed by enantiomerically pure aminophosphines. 

The monophosphine complexes Os3(/u-H)2(CO)9L react with diazomethane to give N2 and a wide range of homologous alkenes. The rates of diazomethane decomposition correlate well with the electronic nature of L rather than its size, with less donating ligands showing higher rates. 

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... 

A similar example is seen in the [Pd2(dba)3]-catalyzed hydroboration of 2-methyl-l-buten-3-ynes [274]. While PPhj and PPh2(CgF5) favor the 1,4-addition product allenylborane 100 all diphosphines yield the 1,2-addition product ( )-dienylborane 102 exclusively (Table 1-13). This remarkable difference in selectivity is explained based on an 1,3-enyne monophosphine complex 103 and an alkynyl diphosphine complex 104 as intermediates. Dppf exhibits the best product yield among the phosphines tested. Similar observation was noted in the asymmetric hydroboration (Scheme 1-44) [275]. The action of catecholborane on 1-phenyl-1,3-butadiene also proceeds regioselectively to give, after oxidation, anti-l-phenyl-l,3-butanediol... 

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