Palladium complexes, cyclohexyl

This result has been explained72 390 by the particular instability of the alkene complex resulting from exocyclic addition (14) relative to endocyclic addition (15) in the cyclohexyl system. This rationale is supported by the results obtained by the use of the considerably more sterically bulky tri-o-tolylphos-phine instead of triphenylphosphine. A 13 83 exocyclic endocyclic ratio is obtained with the bulky phosphine. This result is nicely explained by the ability of the bulky phosphine to magnify the instability of the endocyclic alkene-palladium complex (14) relative to the exocyclic alkene complex (15) and favor endocyclic addition. 

The first multiple insertion of isocyanide into the Pd-C bond of organopal-ladium complexes was reported by Yamamoto in 1970 (Scheme 21) [30]. The reaction of one, two, and three equivalents of cyclohexyl isocyanide with the trans-iodobis(phosphine)methylpalladium(II) complexes 17 yielded the monoimino (18), bisimino (19), and trisimino (20) palladium complexes, re-... 

Multiple, successive insertion of isonitriles into the palladium-carbon o--bond has also been found with organopalladium(II)bis(phosphine) complexes. However, unlike the Ni- catalyzed polymerization, only single, double, and triple insertion reactions of isonitriles have been reported for Pd-mediated reactions. For instance, reaction of methylpalladium(II) complex 2 with cyclohexyl isonitrile in ratios of 1 1, 1 2, and 1 3 afforded selectively single (3), double (4), and triple (5) insertion products, respectively (Scheme 3). However, no further insertion of cyclohexyl isonitrile to 5 took place. No reactivity of the triple insertion complex 5 was explained by the intramolecular coordination of the nitrogen atom of the third imino group, which formed a stable five-mem-bered chelating palladium complex. 

The addition of cyclohexyl isocyanide to several palladium(II) methyl complexes has proven particularly interesting 169, 170). From... 

The thermally stable nickel and palladium hydride complexes, trans-[MHX(PR3)2], where M = Ni or Pd, R = cyclohexyl or isopropyl, and X = halogen, have been prepared by various methods.1-5 Hydrido[tetra-hydroborato( 1 - )] complexes can be prepared from them by metathet-ical reactions.3 The hydrido[tetrahydroborato( 1 — )]bis(tricyclohexylphos-... 

Carbon-transition metal bonds can also be formed by cocondensing transition-metal atoms with isonitriles. Thus, using Ni and Fe with t-butyl isocyanide, methyl isocyanide, cyclohexyl isocyanide and vinyl isocyanide the NiL4 and FeL, complexes are formed. Palladium cocondensed with isonitriles yields PdLj polymeric structures with terminal and bridged isonitrile ligands ... 

Hydrides. The hydrides have the trans-structure, and the cis-species appear not to exist.28 Most of the studies have been on platinum compounds since the comparable palladium (and also nickel) hydrido complexes are usually less stable thermally some compounds such as ra w-PdClH(PR3)2 with R = cyclohexyl or Ph have been made.29... 

Treatment of the diamide 77 with dibutyltin dichloride affords the 2,2-bis(2-[4(/ ),5(5)-diphenyl-1,3-oxazolinyl])propane 78, while successive reaction of 77 with mesyl chloride and aqueous ethanolic sodium hydroxide yields the diastereomer 79 <96TI3649>. The (+)- and (-)-forms of the chiral oxazoline 80 were used as ligands for palladium catalysed allylic amination reactions thus the acetate 81 and benzylamine gave the optically active amine 82 in excellent enantiomeric excess <97JOC55Q8>. The enantioselective catalytic alkylation of aldehydes RCHO (R = n-heptyl, Ph, cyclohexyl or PhCH=CH) with allyluibutyltin in the presence of chiral bis(oxazolinyl)zinc complexes, c.g., 83, leads to alcohols 84 in 40 6% enantiomeric excess <97TL145>. 

In the other study [41], Pd-bis(azido) compounds [Pd(dppn)(N3)2], [Pd(dppf)(N3)2], and [Pt(l-dpn)(SMe2)(N3)2] [dppn, l,8-bis(diphenylphosphino)naphthalene dppf, l,10-bis(diphenylphosphino)ferrocene 1-dpn 1-diphenyl-phosphinonaphthalene] underwent [2 -p 3] cycloaddition with isocyanides R NC (R = cyclohexyl, rBu, 2,6-dimethylphenyl) to convert azido ligands to five-membered, C-coordinated tetrazolate rings. In a related study [42], alkynyl palladium(II)-azido species of the type [Pd(N3)(C=CR)PMe3] reacted with rBuNC to give corresponding complexes with C-bound tetrazolates. 

Schemes 58-62. A new non-rigid phosphine ligand was synthesized and reacted with Fe(CO)5 to form the mononuclear iron complex (Equation (81)). Phosphino-oxazoline ligands were used as assembling ligands for hetero-metallic complexes, where the phosphorus atom binds to iron and the nitrogen atoms act as donor atoms to copper, cobalt, or palladium (Scheme 58). The copper complex catalyzes cyclopropanation and Diels-Alder reactions. When 2-(A -diphenylphosphinomethyl-A -cyclohexyl)aminopyridine (NNP) reacts with Fe(CO)5 in ethanol, /ra .r-(OC)3Fe(NNP)2 is formed (Scheme 59). This monometallic complex can then be reacted with a copper salt in CH2GI2 to form a complex having an Fe-Cu dative bond. The complex was demonstrated to be an efficient catalyst for the cyclopropanation of styrene by ethyl diazoacetate and for the Diels-Alder reaction of cyclopentadiene and methacrolein. No other heterometallic complexes have been shown to have such reactivity. Previously known...

Other metals can catalyze Heck-type reactions, although none thus far match the versatility of palladium. Copper salts have been shown to mediate the arylation of olefins, however this reaction most probably differs from the Heck mechanistically. Likewise, complexes of platinum(II), cobalt(I), rhodium(I) and iridium(I) have all been employed in analogous arylation chemistry, although often with disappointing results. Perhaps the most useful alternative is the application of nickel catalysis. Unfortunately, due to the persistence of the nickel(II) hydride complex in the catalytic cycle, the employment of a stoichiometric reductant, such as zinc dust is necessary, however the nickel-catalyzed Heck reaction does offer one distinct advantage. Unlike its palladium counterpart, it is possible to use aliphatic halides. For example, cyclohexyl bromide (108) was coupled to styrene to yield product 110. 

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