Cr-bonding

Cr(02CCH3)2]2,2H20. Red insoluble compound formed from sodium ethanoate and CrC)2 in aqueous solution. The most stable Cr(II) compound contains a Cr —Cr bond, chromium fluorides... 

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

A rational classification of reactions based on mechanistic considerations is essential for the better understanding of such a broad research field as that of the organic chemistry of Pd. Therefore, as was done in my previous book, the organic reactions of Pd are classified into stoichiometric and catalytic reactions. It is essential to form a Pd—C cr-bond for a synthetic reaction. The Pd— C (T-bond is formed in two ways depending on the substrates. ir-Bond formation from "unoxidized forms [1] of alkenes and arenes (simple alkenes and arenes) leads to stoichiometric reactions, and that from oxidized forms of alkenes and arenes (typically halides) leads to catalytic reactions. We first consider how these two reactions differ. 

The Pd—C cr-bond can be prepared from simple, unoxidized alkenes and aromatic compounds by the reaction of Pd(II) compounds. The following are typical examples. The first step of the reaction of a simple alkene with Pd(ll) and a nucleophile X or Y to form 19 is called palladation. Depending on the nucleophile, it is called oxypalladation, aminopalladation, carbopalladation, etc. The subsequent elimination of b-hydrogen produces the nucleophilic substitution product 20. The displacement of Pd with another nucleophile (X) affords the nucleophilic addition product 21 (see Chapter 3, Section 2). As an example, the oxypalladation of 4-pentenol with PdXi to afford furan 22 or 23 is shown. 

Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

Among several propargylic derivatives, the propargylic carbonates 3 were found to be the most reactive and they have been used most extensively because of their high reactivity[2,2a]. The allenylpalladium methoxide 4, formed as an intermediate in catalytic reactions of the methyl propargylic carbonate 3, undergoes two types of transformations. One is substitution of cr-bonded Pd. which proceeds by either insertion or transmetallation. The insertion of an alkene, for example, into the Pd—C cr-bond and elimination of/i-hydrogen affords the allenyl compound 5 (1.2,4-triene). Alkene and CO insertions are typical. The substitution of Pd methoxide with hard carbon nucleophiles or terminal alkynes in the presence of Cul takes place via transmetallation to yield the allenyl compound 6. By these reactions, various allenyl derivatives can be prepared. 

As portrayed m Figure 2 20 the two carbons of acetylene are connected to each other by a 2sp-2sp cr bond and each is attached to a hydrogen substituent by a 2sp-ls CT bond The unhybndized 2p orbitals on one carbon overlap with their counterparts on the other to form two rr bonds The carbon-carbon triple bond m acetylene is viewed as a multiple bond of the ct + rr + rr type... 

The hydrocarbon shown called vmylacetylene is used in the syn neoprene a synthetic rubber Identify the orbital overlaps involved in How many cr bonds are there in vmylacetylene How many... 

FIGURE 2 20 Bonding in acetylene based on sp hybridization of carbon The carbon-carbon triple bond is viewed as consisting of one cr bond and two tt bonds... 

The triple bond of acetylene has a cr bond component and two tt bonds the two TT bonds are shown here and are perpendicular to each other... 

FIGURE 4 9 tert Butyl cation (a) The positively charged carbon is sp hybridized Each methyl group IS attached to the positively charged carbon by a cr bond and these three bonds he in the same plane (b) The sp hybridized car bon has an empty 2p orbital the axis of which is perpen dicular to the plane of the carbon atoms... 

FIGURE 10 11 The HOMO of 1 3 butadiene and the LUMO of ethylene have the proper symmetry to allow cr bond formation to occur at both ends of the diene chain in the same tran sition state... 

FIGURE 113 (a) The framework of bonds shown in the tube model of benzene are cr bonds (b) Each carbon is sp hybridized and has a 2p orbital perpendicular to the cr framework Overlap of the 2p orbitals generates a tt system encompass mg the entire ring (c) Electrostatic potential map of benzene The red area in the center corresponds to the region above and below the plane of the ring where the tt electrons are concentrated... 

Step 2 The oxidation step can be viewed as a p elimination Water acts as a base to remove a proton from carbon while the O—Cr bond breaks... 

Induced dipole/mduced dipole attraction (Section 2 17) Force of attraction resulting from a mutual and complemen tary polanzation of one molecule by another Also referred to as London forces or dispersion forces Inductive effect (Section 1 15) An electronic effect transmit ted by successive polanzation of the cr bonds within a mol ecule or an ion... 

Figure 19 compares experimental data with calculated curves (80). In the random Co—Cr alloy the Cr atoms are not distributed ia the most suitable way for reduciag the Af of the alloy. Therefore the maximum local content of Cr for this distribution is much higher than ia the case where Cr—Cr bonds are not present. The curve for no Cr—Cr bonds present shows that the Af becomes zero at 25 at. % Cr, based on the fact that for bulk material the measured Af for this composition is zero. Consequently 4 Cr nearest neighbor ia an hep lattice makes the final Af zero. 

Does the overlap of two p orbitals in the fashion shown correspond to a cr bond or to a -tr bond Explain. 

The simplest arithmetic approach subtracts the C—C cr bond energy of ethane (368 kj/mol 88 kcal/mol) from the C=C bond energy of ethylene (605 kJ/mol 144.5 kcal/mol). This gives a value of 237 kJ/mol (56.5 kcal/mol) for the tt bond energy. 

Recall from Section 1.15 that effects that are transmitted by the polarization of cr bonds are called inductive effects. 

Each carbon in propane is bonded to four atoms and is -hybridized. The C—C bonds are cr bonds involving overlap of a half-filled sp hybrid orbital of one carbon with a half-filled sp hybrid orbital of the other. The C—H bonds are cr bonds involving overlap of a half-filled sp hybrid oribital of carbon with a half-filled hydrogen orbital. 

The indicated bond is a cr bond. The carbon of the CH3 group is -hybridized. The carbon to which the CH3 group is attached is ip -hybridized. 

The C—C single bond in vinylacetylene is a cr bond generated by overlap of an sp -hybridized orbital on one carbon with an ip-hybridized orbital on the other. Vinylacetylene has three cr bonds and three -tr bonds. 

Inductive effect (Section 1.15) An electronic effect transmitted by successive polarization of the cr bonds within a molecule or an ion. 

Sigmatropic rearrangement (Section 24.13) Migration of a cr bond from one end of a conjugated jt electron system to the other. The Claisen rearrangement is an example. 

Figure 13.28 Planar structure of (a) [PhSb Mn(CO)2(>j -C5H5)2 ], and (b) [PhAs Cr(CO)5 2]. Note the relatively short Sb-Mn and As-Cr bonds.

Compounds containing M-C cr-bonds are frequently unstable and do not give rise to an extensive chemistry (p. 999). Vanadium forms a neutral (paramagnetic) hexacarbonyl which, though not very stable, contrasts with that of titanium in that it can at least be prepared in... 

The stabilities of the [ML2R2] phosphines increase from Ni to Pt and for Ni" they are only isolable when R is an o-substituted aiyl. Those of Pt", on the other hand, are amongst the most stable cr-bonded organo-transition metal compounds while those of Pd" occupy an intermediate position. 

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