Dienes coordination

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

By far the greatest number of diene-metal complexes are of the type (diene)Fe(CO)3. All of these complexes exhibit the s-cis rf -diene coordination mode. 

The Cr(CO)6 and Cr(CO)3(arene) catalysts hydrogenate conjugated dienes by 1,4-addition of hydrogen. The diene coordinates in a cisoid configuration (equation 7)23. This... 

Stereoregular Polymerization of Dienes. Coordination polymerization exhibiting high degree of chemo- and stereoselectivity is the method of choice to synthe-size c -1,4-polybutadiene the commercially most important product (see... 

The origin of the stereoregulation in conjugated diene coordination polymerisation was recognised by Porri et al. [7,41] who explained the different modes of formation of the isotactic and the syndiotactic polymers in the presence of various transition metal-based catalysts. 

This type of -coordination is often encountered for complexes of naphthalene An + 2= IOtc), where some aromaticity An + 2 = 6tc) is retained in the second ring of the fused arene. The drive towards buta-1,3-diene coordination may be sufficient to favour rearrangement of non-conju-gated dienes, or the ring opening of cyclopropylalkenes (Figures 6.2, 6.6,... 

During diene coordination to a metal atom with a Cl ligand, the n allylic attachment is transformed into a a bond between C and the transition metal, , whereas with the latter catalyst it is transformed into a a bond between 3C and ... 

Dienes coordinated to iron tricarbonyl do not undergo Diels-Alder reactions and a number of interesting annulation reactions of groups attached to iron tricarbonyl diene complexes have been developed. Cyclohexadiene iron tricarbonyl complexes having a pendant alkene undergo annulation reactions to form spirocychc compounds (Scheme 160). Related reactions of allylic thioesters afford spirocychc thialac-tones. 

Conjugated 1,3-dienes can coordinate to a single metal atom or may bridge two metals of the cluster. Most commonly the diene coordinates to a single metal atom in the cis conformation, as in (29). The diene is found in the s-trans conformation bridging two metals in Os3(CO)io(l,3-butadiene) (30). 

Attempts have been made to copolymerize conjugated dienes with olefins but there are no data on polymerization rates or reactivity ratios. They are ill suited for copolymerization in that polymerization rates are markedly reduced by the presence of the conjugated diene and the copolymers are heterogeneous in composition and may be crosslinked. The reasons for this behaviour have not been established but a possible explanation is that the conjugated diene coordinates preferentially with the catalyst and so excludes the olefin, but has a slow insertion rate compared with the more reactive olefin. 

The success of this triple catalytic system relies on a highly selective kinetic control. From a thermodynamic point of view, there are 10 possible redox reactions that could occur in this system. However, the energy barrier for six of these [O2 + diene, Oj + Pd(0), etc.] are too high and only the kinetically favored redox reactions shown in Scheme 8-12 occur. A likely explanation for this kinetic control is that the barrier is significantly lowered by coordination. Thus, diene coordinates to Pd(II), Pd(0) coordinates to BQ, HQ coordinates to and O2 coordinates to ML" . 

The preparalion of cyclic or acyclic (diene)RuCp X complexes 125 (Cp = C5H5 or CsMes) via direct complexation has been reported (Scheme 26) . This may be accomplished using either (l,5-cod)RuCpX (126) or the tetrameric species [Cp Ru(//3 — X)]4 (127). Complexes 125 exhibit the s-c zj -diene coordination mode as evidenced by X-ray diffraction analysis. The ligands are oriented such that the open end of the diene is eclipsed with the Ru—X bond. Treatment of complexes 125 with AgOSO2CF3 or AgO2CCF3 effects replacement of the halide ligand X with either a triflate or trifluoroacetate ligand. Oxidation of diene complexes 125 (R = Me), which lack alkyl substituents... 

Mono-diene complexes of zirconocene and hafnocene have been prepared by two methods [129-131 ], viz the photochemical reaction of diphenylzirconocene in the presence of diene and the reaction of metallocene dichlorides with diene magnesium adduct. The structures and reactivity of s-cis-dicm complexes indicate that the metal-lacyclopentene (B) is the preferred canonical form. Complexes of the type Cp2Zr(j-/ra/25-1,3-diene), have been prepared they were the first examples of this mode of coordination (C). Insertion of unsaturated compounds into a diene coordinated to zirconocene results in regioselective C—C bond formation [132-136]. 

Conjugated dienes coordinated to palladium(Il) undergo nucleophilic addition of oxygen and nitrogen nucleophiles from the opposite side of the palladium center to give Ti-allylpalladium complexes. Subsequent reaction of the 7C-allyl complexes with another molecule of nucleophile results in the 1.4-addition of two kinds of nucleophiles to dienes. The direction of the second nucleophilic attack is dependent on the nature of the nucleophiles as well as the reaction conditions. Representative examples... 

A long sought goal in mechanistic polymer chemistry has been the determination of those factors which lead to cis, trans or vinyl structures in diene polymers. Various proposals have been made jnd are summarized in the comprehensive revie edited by Saltman. The simplest proposal, advanced by Cossee and Arlman, assigns the dominant role to the nature of the diene coordination. In this mechanism bidentate coordination, e.g. of necessity involving the cisoid conformation of the diene, would lead to cis polymer. 

Reaction schemes for hydrogenation of dienes using dihydride catalysts usually follow the basic hydride or unsaturated pathways of Scheme 1 species 2 will be, however, a hydrido cr-alkenyl or a 7r-allyl intermediate [see equation (f) below]. Transfer of the second hydrogen yields a monoene product if diene coordination is preferred over monoene coordination, selective reduction of diene to monoene can occur. Some Rh (and Ir ) dihydride catalysts that effect such catalysis are listed in Table 2, although systems based on the Cr and Fe carbonyl have been studied most (see also 14.3.3.5) . 

The chemistry of dienes coordinated to the cationic CpMo(CO)j fragment has been exploited many times for complex molecule synthesis. Originally, Faller showed that the cationic molybdenum complex in Equation 11.44 undergoes nucleophilic attack by hydride, deuteride, methyl lithium, and enamines to produce the Ti -allyl complex. As expected, attack of the nucleophiles occurs at a terminal position and exclusively from the face opposite the metal. Trityl cation abstracts a hydride from this allyl product from the face opposite the metal to regenerate a diene complex. Pearson has used this sequence of nucleophilic attack and hydride abstraction to synthesize substituted cyclohexenes with control of stereochemistry as shown in Scheme 11.6. ... 

As discussed in Chapter 3, olefins and dienes bind to electron-poor metal centers by a flow of electrons from the olefin iT-system to the metal and from the metal to the olefin t -system. Thus, olefins bound to electron-rich and strongly backbonding metal centers react with protons and electrophiles directly at the metal-carbon bond. However, olefins and dienes coordinated to electron-poor metal centers are less reactive toward electrophiles than those bound to electron-rich metal centers or even free olefins and dienes. However, electron-poor olefin and diene complexes do imdergo reactions with electrophiles at the coordinated ligand by an indirect pathway. This indirect pathway occurs by insertion of the olefin or diene into the bond formed by attack of the electrophile at the metal. 

Bimetallic Complexes A novel type of bridging buta-1,3-diene coordination in the dimeric species [Cp2 H-(l,2-ti-2-Zr) 4-Ti-l-Al)-C4H6)(p-Cl)AlQ2] was rcpcHted272. 

Conjugated dienes coordinated to a transition metal can readily be transformed into a rr-allylmetal complex by functionalization at the 4-position.f This makes dienes useful substrates for catalytic transformations since the 7r-allyl complex formed can undergo further reaction. t A number of Pd-catalyzed reactions of conjugated dienes are known that proceed via rr-allylpalladium intermediates and lead to useful 1,4- or 1,2-funtional-ization of the diene. There are two types of reactions of this kind (i) Pd(0)-catalyzed reactions that involve initial oxidative addition and (ii) Pd(II)-catalyzed reactions that involve electrophilic activation of the diene by the metal followed by nucleophilic attack. This section deals with C— N and C—O bond formation via these two types of reactions. This topic has previously been reviewed in connection with Pd-catalyzed additions to conjugated dienes. [5] [n] -jijg present review will focus mainly on the work published since 1997 but will also briefly discuss previous work. 

Nucleophilic attack on dienes coordinated to Pd(ll) is facile [Eq. (26)] (Takahashi and Tsuji, 1968). 

It is possible that the acylation of dienes coordinated to tricarbonyliron might be improved as a synthetic reaction by carrying out the reaction in the presence of carbon monoxide. This would facilitate formation of a more stable intermediate XXIII. Treatment with base, followed by removal of the tricarbonyliron group, should provide a ready synthesis of 2,4-dienones [Eq. (62)]. However, acylations of cyclohexadieneiron derivatives have so far not given favorable results. 

In 1973, the homo- and copolymerization of it-(2,A-hexa- eay acrylate)tri-carbonyliron with styrene, methyl acrylate, acrylonitrile, and vinyl acetate was reported by Pittman and coworkers. It was reported that these polymers (47) could be protonated to produce the Jt-allyliron derivatives. Nakamura and coworkers have also reported the synthesis and elecirochemical properties of polymers containing dienes coordinated to iron tricarbonyl moieties in the polymer sidechains. ... 

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