Insertion bonds

The first step of the reaction is the oxypalladation of the triple bond with PdCl2 as shown by 228 to form the alkenylpalladium species 229, and the Pd is displaced with proton to regenerate Pd(TI) species and the lactone 224. The alkenylpalladium species 229 can be utilized for further reaction. When allyl chloride (230) is added, double bond insertion is followed by elimination of... 

FIGURE 25.13 Double bonds are introduced into the growing fatty acid chain in E. coli by specific dehydrases. Palmitoleoyl-ACP is synthesized by a sequence of reactions involving four rounds of chain elongation, followed by double bond insertion by /3-hydroxydecanoyl thioester dehydrase and three additional elongation steps. Another elongation cycle produces cA-vaccenic acid. 

Previously, trifluorosilyl groups have been bound to phosphorus (40) and silicon via the SiF (g), fluorine-bond insertion-mechanism (41). The new compound HgCSiFs) is readily hydrolyzed, but it can be stored for long periods of time in an inert atmosphere. It is a volatile, white solid that is stable up to at least 80°C. The preparation of bis(trifluoro-silyDmercury, of course, raises the possibility of (a) synthesis of the complete series of trifluorosilyl, "silametallic compounds, as had previously been done for bis(trifluoromethyl)mercury by using conventional syntheses, and (b) transfer reactions similar to those in Section II, as well as (c) further exploration of the metal-vapor approach. The compound Hg(SiF.,)j appears also to be a convenient source of difluoro-silane upon thermal decomposition, analogous to bis(trifluoromethyl)-mercury ... 

Group-IB (Cu, Ag, Au) or -IIB (Zn, Cd, Hg)-Transition-Metal Bonds 8.3.3. Group IIB-Transition- and Inner-Transition-Metal Bonds 8.3.3.4. by Insertion into a Bond Insertion of a Group-IIB Element... 

S.3.3.4. by Insertion Into a Metal-Metal Bond Insertion of a Group-IIB Element into Transition-Metal to Transitlon-Metal Bonds... 

In 1982, Breslow and coworkers reported the first example of iron-catalyzed nitrene C-H bond insertion [29]. They used [Fe(TTP)] as catalyst and PhINTs as nitrene precursor to achieve C-H bond amination of cyclohexane. However, the product yield was low (around 10%). Subsequently, the same authors found that iminoio-dane 7 derived from 2,5-diisopropylbenzenesuIfonamide underwent intramolecular C-H amination efficiently with [Fe(TPP)Cl] as catalyst at room temperature, giving the insertion product in 77% yield (Scheme 29) [85]. 

The Cossee mechanism has been demonstrated by direct observation of organometallic complexes where a C = C bond inserts itself into an M-C bond as shown in Eq. (7)-(9). A labeling experiment on a cationic platinum complex 111 indicated the reversible insertion of the coordinated alkene into the Pt-C bond as shown in Eq. (7) [142]. 

As mentioned above nonconjugated dienes give stable complexes where the two double bonds can form a chelate complex. A common pathway in palladium-catalyzed oxidation of nonconjugated dienes is that, after a first nucleophilic addition to one of the double bonds, the second double bond inserts into the palladium-carbon bond. The new (cr-alkyl)palladium complex produced can then undergo a /(-elimination or an oxidative cleavage reaction (Scheme 2). An early example of this type of reaction, although not catalytic, was reported by Tsuji and Takahashi (equation 2)12. 

Figure 4.63 displays the optimized CH2=IrH2+ product species, and Fig. 4.64 shows the deep attractive well (>50 kcalmol-1) for bond insertion (4.100), which is of about four times the exothermicity of the corresponding 3Ti reaction (Fig. 4.61). Figure 4.64 also includes the leading donor-acceptor interactions of... 

In contrast to the severe difficulty of cracking a sigma bond, insertion of a transition metal into a pi bond can proceed in facile fashion. This can be illustrated by the attack of Ti on the pi bond of ethylene, which leads to metallacycle formation in the reaction... 

The advantages of this method of carbene synthesis are that reaction can be carried out in neutral solution, and that reaction yields are often dramatically improved. Thus, although reactions of dihalocarbenes generally do not give rise to products corresponding to single bond insertion, Seyferth has reported insertion of phenyl (trihalomethyl) mercury-generated carbenes into... 

The above reactions reinforce the diradical mechanism proposed for the BF3 reaction. Hexafluorobenzene and the various fluorinated ethylenes 73,74 however, react quite differently. The products in these reactions formally correspond to C-F bond insertion by an SiF2 monomer. 

The allylic 2-alkynoates 77 are a group of special enynes with an ester linkage between their double bond and triple bond. When halopalladation of the triple bond is followed by C-C double bond insertion and the cleavage of the carbon-palladium bond, a series of y-lactones (78-81) can be obtained. 

Zirconacyclopentadiene shows a different reactivity towards CO as compared with zirco-nacyclopentane and zirconacyclopentene. Zirconacyclopentane and zirconacydopentene readily react with CO at low temperature to give cyclopentanone and cyclopentenone, respectively. The different reactivity of zirconacyclopentadienes can be explained by comparing the reactivity of the Zr—Csp2 bond with that of the Zr—Csp3 bond. Insertion of CO into the Zr—C bond proceeds readily at low temperature and therefore zirconacydopentane and zirconacyclopentene, which contain Zr—Csp3 bonds, react directly with CO as shown in Eq. 2.65 [45], Zirconacyclopentadienes, on the other hand, do not. 

Recently the primary amine 39 was transformed into indolizidine 40 using an ytterbium catalyst. After a regio-specific C=C bond insertion of the internal olefin into the Ln-N bond, a second insertion is accomplished as illustrated in the proposed mechanism <2004JOC1038> (Scheme 15). 

Under drastic thermal conditions, the iminosilane-LiF adduct 82 eliminates LiF and the iminosilane intermediate 83 rearranges intramolecularly by C-H bond insertion affording 2,2,4,4-tetra-/i /Z-butyl-l,3-diaza-2,4-disilabicy-clo[3.3.0]octane 84 in 87% yield (Equation 6) <1996JOM203>. 

Jacobsen, Panek and co-workers (86) investigated the intermolecular Si-H bond insertion of diazoesters. Bis(oxazolines) and diimines were found to be effective in this reaction, with diimine enf-88a providing optimal selectivities. As expected, enantioselectivity is a function of silane structure, with bulkier silanes providing higher selectivities but lower reactivity. Both CuOTf and Cu(OTf)2 catalyze this reaction but the Cu(II) precursors leads to much lower enantioselectivity (44% vs 83% at -40°C). 

Another example, in which the piperidine cycle is generated de novo, exploits a hetero Diels-Alder cycloaddition of 1 -/r-tolylsulfinyl-1,3-penta-diene 91 with benzylnitrosoformate, that generates an oxazine 92 with complete regioselectivity and 7i-facial diastereoselectivity.69 Osmilation of the double bond inserts stereoselectively two hydroxyl groups on the oxazine skeleton, protection and catalytic hydrogenation finally afforded the enantiomerically pure imino sugars 94 (Fig. 38). 

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