Allyl complexes structure

Allylic acetoxy groups can be substituted by amines in the presence of Pd(0) catalysts. At substituted cyclohexene derivatives the diastereoselectivity depends largely on the structure of the palladium catalyst. Polymer-bound palladium often leads to amination at the same face as the aoetoxy leaving group with regioselective attack at the sterically less hindered site of the intermediate ri -allyl complex (B.M. Trost, 1978). 

Figure 25 Molecular structure of bis(allyl) complex [Bu C(NPr )2]V(>/ -allyl)2. ° ...

In 1989 we reported on the synthesis and structure of the first l,3-diphospha-2-sila-allylic anion 3a [4], mentioning its value as a precursor for phosphino-silaphosphenes. In analogy to 3a we obtained the anions 3b-f [5] by treatment of 4 equivalents of the lithium phosphide 1 with the adequately substituted RSiC, of which 3b and 3c were investigated by X-ray analyses. The very short P-Si bond lengths (2.11-2.13 A) of 3a-c and the almost planar arrangement of Pl-Sil-P2-Lil indicate the cr-character of the Lithium P-Si-P allyl complex. 

Based on the discussed acylpalladium 7i-allylic complex (Scheme 5.22) and the reported X-ray structure of the (R)-MOP—Pd 7i-allylic complex [31], the acylpalladium (R)-MOP Ti-allylic complex C (Scheme 5.24) is proposed for the formation of the (R)-product. Complex D, which would give the (S)-product, suffers from steric compression between the MeO-naphthyl ring and the acyl group, while there is no such steric interaction in complex C. Thus, reductive elimination of Pd(0) from C would preferentially yield the... 

Figure 4. Structural data obtained for the cationic r 3-allyl complexes 26 (R = Me or CH2Ph), illustrating the differing Pd-C bond lengths which result from unfavorable steric interactions.

Other conjugated diene systems readily react with either H4Ru4-(CO)i2 or Ru3(CO)i2, to yield 7r-allyl complexes. Thus, butadiene yields the crotyl derivative HRu3(CO)9C4H5. The X-ray structure of the related adduct formed from cis-trans- or trans-trans-2,4-diene is shown in Fig. 12. The hydride is considered to bond to the Ru(l)-Ru(2) edge, as this is the longest metal-metal bond in the structure and in the... 

Figure 16. Optimized structures of the endo (lOa-anf and exo (IQb-anti) r -silyll allyl complexes with 4-(dimethylamino)-styrene as a substrate. Highlighted in (a) is the steric contact between the hydrogen of the benzylic carbon of the substrate and a hydrogen of the mesityl substituent 12 63 A) for the endo-anti intermediate, lOa-on/i. Highlighted in (b) is the steric contact between on hydrogen of the methyl group of the substrate and a hydrogen of the mesityl suhstituent 12 47 A) for the exo-anti silyl-allyl intermediate, Wb-anti. Hydrogen atoms are not shown except those whose interactions are highlighted.

Some of the evidence for such structures comes from the change in product distribution of the butenes as a function of cyanide concentration when butadiene is hydrogenated with pentaeyanocobaltate(II) catalyst or when the a butenyl complex is reduced with the hydride complex [HCo(CN)5] . Thus 1-butene is the major product in the presence of excess CN, and major product in the absence of excess cyanide. The 1-butene presumably arises from the cleavage of a tr complex, and the 2-butene via an intermediate w-allyl complex. The Tr-allyl complexes of cobalt tricarbonyl are well-characterized and can be prepared either from butadiene and HCo(CO)4 or from methallyl halide and NaCo(CO)4 [49). 

Additional mechanistic insights were gained when Hartwig and coworkers isolated and characterized the first 7t-allyl complexes that are chemically and kinetically competent to be intermediates in iridium-catalyzed allylic substitution [46]. These complexes were prepared independently from allylic electrophiles that are more reactive than allylic carbonates. The isolation and structural characterization of these species provided a detailed view into the origins of enantioselectivity. 

The cationic polymerization of 2-vinyltetrahydrofuran 1 is marred by side reactions due to the allylic structure of the substituent and the participation of the ring (22). The oligomers obtained have therefore a complex structure. 

Addition reactions of allylic boron compounds have proven to be quite general and useful. Several methods for synthesis of allylic boranes and boronate esters have been developed.36 37 The reaction has found some application in the stereoselective synthesis of complex structures. 

This reaction has been extensively studied " and I discuss it here merely to point out certain pitfalls. The conversion of a Ni(0) bis (butadiene) cranplex XIX to a Ni(II) bis- r-allylic complex XX (the structures are drawn withmit stereochemical implication) by reductive coupling appears at first sight to be an allowed process each ligand contributes four electrons and the metal contributes two, making ten in all. Thus Eq. (1) is satisfied, provided the metal itself introduces no discontinuity ... 

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

The nuclear (proton) magnetic resonance spectra of allylic complexes strongly favor a structure in which all three carbon atoms of the allylic radical are symmetrically bonded to the transition metal M as shown in (XLII). The value of nmr spectroscopy in confirming the presence of a... 

Pd(H) complexes with strongly electron-withdrawing ligands can insert into the allylic C—H bond (path c) to form directly the Jt-allyl complex via oxidative addi-tion.502,694,697 Pd(OOCCF3)2 in acetic acid, for example, ensures high yields of allylic acetoxylated products.698 The delicate balance between allylic and vinylic acetoxylation was observed to depend on substrate structure, too. For simple terminal alkenes the latter process seems to be the predominant pathway.571... 

The formation of metallacyclobutane through a ir-allyl complex without involvement of carbene species [Eq. (12.19)] was also suggested as the initiation step for systems where carbene formation from the alkene is difficult to occur for structural reasons 72-74... 

Stereoselectivity, or the formation of a cis-1,4 or tram-1,4 unit, is connected to the structure of the chain end 7t-allyl complex (Scheme 13.8). The syn-allyl (52) and the anri-allyl (53) form, which are in equilibrium,383 give rise to the formation of the trans-1,4 or cis-1,4 groups, respectively. ... 

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