Allene complexes bonding

Because of the exceptional C-F bond strength, the successful preparation of a-halocyclopropyl c-complexes is realized by substitution of 1-bromo-l-fluoro-trans-2,3-dimethylcyclopropane 179 with Fp [90], Silica gel column chromatography of the thus obtained cr-complex 180 results in ring opening to the alcohol 181 as a single stereoisomer. The allene complex 182 is produced by treatment with BF3OEt2, indicating that 181 is derived from 182 and water. The 7i-allyl complex 183 is formed by photolysis via a disrotatory process. 

Insertion of an allene molecule into the Pd-H bond, with regeneration of the starting allene complex 8 which then re-enters the catalytic cycle. In the absence of allene, extensive Pd black formation was observed. 

The allene complexes 123-125 were also determined to adopt trigonal bipyramidal structures with the allene unit assuming a stereochemically rigid position in the trigonal plane, even at 110 °C. In the case of 1,1 -dimethylallene (124), coordination is exclusively through the CH2=C-double bond,54,55 while for methylallene (125) at least two of the four possible geometric isomers were obtained, though their stereochemistry remains undetermined, due to the complexity of the spectra (Chart 1). 

The behavior of allene (1,2-propadiene) and its derivatives as ligands in transition metal complexes has been investigated for more than 20 years (197,198). Allene may be coordinated by only one C=C double bond to a metal (197,199), or it may link to two metal centers as a 2 2 or 3 1 electron donor. Dinuclear /i-f/2l2-allene complexes are known for Mo (200,201), W (201), Mn (202-203), and Rh (199,204). In [Fe2(CO)7(/ -f/3 1-C3H4)] a bonding mode (205,206). In the course of the reaction, often two or more allene molecules become connected with transition metal complexes. Two allenes linked by C-2 yield tetramethyl-eneethane (207,208), which is stabilized by coordination on Fe (205,208), Ni (270), or Pd (210,211). 

Recent structural and spectroscopic investigations of organometallic complexes bonding two carbons of an allenic ligand to one rhodium 50 72> 87,95) or platinum atom 58,87,98,132) may have some pertinence to possible bridged intermediates proposed for various electrophilic additions to allenes, and the cr-iron-jr-iron complexes derived from allene and diiron... 

Although the reaction conditions of the acylstannylation of 1,3- and 1,2-dienes cannot be applied to the corresponding reaction of alkynylstannanes as they stand, the alkynylstannylation of 1,2-dienes takes place in the presence of a catalytic amount of a nickel(O) complex coordinated by a 1,3-diphosphinopropane ligand (Scheme 5.7.27). " The substituents on the phosphorus atom of the diphosphine ligands markedly affect the selection of allene double bonds a Ni-DPPP catalyst favors the internal double bond of allenes, whereas l,3-bis(dimethylphosphino)propane (DMPP) prefers to react at the terminal double bonds to give (Z)-alkenylstannanes selectively. Various aryl- and silyl-ethynylstannanes add to mono-, di- and non-substimted allenes. 

Allene complexes containing o-bonded allenes as ligands are also known u9>. 

This situation may be explained by stronger interaction between the middle carbon atom and the metal atom. Such interaction is caused by the presence of two mutually perpendicular bonding and antibonding n orbitals. The terminal carbon atoms have only n and tt orbitals. This is equivalent to the interaction of the metal with the uncoordinated double bond of the allene which also leads to the change of distance d between carbon atoms. The smaller distance is a result of a smaller atomic radii of the middle allene carbon atom which forms two double bonds. The distance and angles in the allene-metal complexes are given in Table 6.25. Structures of some allene complexes are depicted in Figure 6.24. 

The proposed mechanism based on product observation is depicted in (Scheme 16). Regioselective insertion of the proximal double bond of allene 31 into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, HlRhJSiRs, affords intermediate D-L Subsequent allqme insertion forming the cyclic vinyl-Rh complex D-II followed by silane promoted reductive elimination provides the product 32 and the active Rh-Si complex. When allenynes that did not have substitutions on the alkyne terminus or on the distal allene double bonds were subjected to the optimized conditions, complex product mixtures were obtained. These... 

In the allene complex (82) only one of the allene C-C bonds is coordinated to the metal. The Pt-C(central) bond is shorter than the Pt-C(terminal) bond. The C-C-C angle in the allene ligand is 142(3)° and the length of the unco-ordinated C-C bond is 1.31(5) A. 

As shown in eq. (15.26), the iron atom of iron carbonyl is able to form the complex with benzene double bonds and allene double bonds, or carbonyl double... 

As the final conclusion, the authors assumed that a mechanism involving nucleophilic addition of an allene double bond to a phosphinegold-complexed phos-phinegold acetylide to be more likely than an oxidative cyclization or simple nucleophilic addition to phosphinegold-complexed substrate. The corresponding catalytic cycle for the more probable pathway is shown in Scheme 4.9. 

The CH3S -h allene reaction was studied directly and found to exhibit complex non-Arrhenius behavior [92]. Based on the observed dual-activation-energy temperature dependence, Balia et al. [92] proposed that the reaction mechanism consists of two reaction pathways, one involving CH3S addition to an allene double bond to form an adduct, and another involving H atom abstraction from allene. 

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