Cyclic pd n Systems

The replacement of a p orbital of a cyclic n system by one dn orbital may result in two extreme directions of the d orbital, either radial or tangential orientation. 

A radially directed d orbital shows the same symmetry behaviour as the replaced p orbital 33. Therefore the properties of this cyclic (pd)n system are basically the same as in a jr system containing only p orbitals, except for the 

If the orbital is directed tangentially, all MO s of the it system contain an additional node which is caused by the symmetry behaviour of the tangential d orbital. But, as this additional node is not located at a fixed position, one obtains a delocalized (pd)n system showing the same properties as a Mobius it system 34 which may be obtained by gradually twisting all p orbitals of a tt system through a small angle in the same direction 1161. 

The HMO-orbital energies for idealized Mobius-type (pd)n systems (9) may be given in closed form 1,6 (5). 

But at a d-orbital center both types of (pd)n-bonding are possible, generally with different probability because of differences in (pd)n overlap, which depends on the geometry of the cycle. If both radial and tangential d orbitals may be equally involved in tt bonds, the situation is best decribed by an orthogonal linear combination of both d orbitals (6) 

---

The participation of inner d orbitals in bonding may be observed with transition metal atoms of the third or higher rows of the periodic table. If these transition metal atoms or ions are coordinated by heteroatoms of ligands which are themselves connected by conjugated multiple bonds, one obtains chelate rings of different ring sizes which may form cyclic (pd)n systems. Some examples of such compounds with different coordinating heteroatoms are collected in Fig. 2. 

Fig. 9. Specific HMO 7r-bond energies E /N for idealized types of cyclic (pd)n systems in dependence on the ring size N for optimum 7t-electron occupation number...

The results of the HMO 7r-electron considerations for idealized cyclic (pd)n systems may be summarized as follows ... 

In most of the cyclic (pd) systems which may be formed by use of either outer or inner d orbitals, cyclic delocalization through the d-orbital center is possible and leads to an increase in stability, but in no case will the (pd)n system alone lead to an extraordinary ground-state stabilization which can be classified as aromatic. 

Intramolecular arylation of G-H bonds gives cyclic aromatic compounds. In this intramolecular arylation, the carbon-palladium cr-bond is first formed by the oxidative addition of Pd(0) species and then the resulting electrophilic Pd(n) species undergoes the intramolecular G-H bond activation leading to the formation of the palladacycle, which finally affords the cyclic aromatic compounds via reductive elimination.87 For example, the fluoroanthene derivative is formed by the palladium-catalyzed reaction of the binaphthyl triflate, as shown in Scheme 8.88 This type of intramolecular arylation is applied to the construction of five- and six-membered carbocyclic and heterocyclic systems.89 89 89 ... 

Fig. 2. Examples of transition metal chelates with unsaturated-ligand n systems which may use inner d orbitals for cyclic (pd) bonding...

The assumption of an angle of 90° between the a bonds at the metal makes descriptions 44 and 45 equivalent. Model 44 has to be preferred if one constructs the 7T MO s from individual AO s. The (pd)n-overlap integral is not reduced and it leads to the prediction of no cyclic conjugation across the metal. The resulting six-membered linear polyene-type ir system is occupied by eight 7r electrons, i.e. two electrons enter antibonding MO s. 

The 6/Pd-system is the most efficient catalyst for the coupling of cyclic amines and aryl triflates [42a]. In the case of electron-rich or electronically neutral aryl triflates, the reaction can be performed at room temperature with NaOt-Bu as base, Eq. (60). In reactions of electron-deficient aryl triflates, use of K3PO4 as base and running the reaction at 80 °C results in clean C-N bond formation. 

More recently, however, results with monooelfins indicate that trans addition can occur to both Pd(II) and Pt(II) n complexes. The first demonstration was the trans addition of amines to Pt(II) complexes 215), and more recently the trans attack of acetate on cyclohexene has been demonstrated (Section III, A, 2, a). However, cis attack can also occur such as the addition of phenylpalladium to cyclohexene (Section III, A, 4) or addition of PdCOOR to cyclic olefins (Section III, A, 3). Also, in the exchange studies (Section III, B, 1) the stereochemistry indicates that some nucleophiles can attack cis or trans. Thus chloride ion containing acetic acid can attack the Pd(II) olefin n complex from either inside or outside the coordination sphere. What are the factors involved The most important appears to be the ability of the nucleophile to coordinate to Pd(II). Thus, phenyl is covalently bonded to Pd(II) and is therefore always in the coordination sphere. Chloride is both inside the coordination sphere as well as outside the coordination sphere and can thus attack both cis and trans. Acetate is not complexed to Pd(II) in chloride-containing media, and thus can only attack trans. On the other hand, in chloride-free acetic acid, acetate is both inside and outside the coordination. Stereochemical results indicate that in this system acetate can attack in both a cis and trans fashion. 

The Buchwald series of bulky phosphines containing the tunable bi-aryl moiety have also been shown to allow for the efficient animation of aryl chlorides as well.57 In particular, Pd/L3 catalyzed the reaction between secondary cyclic amines and most aryl chlorides at 80 °C, Secondary acyclic amines and anilines tend to couple best with neutral to electron rich aryl chlorides. Primary aliphatic amines are quite limited to either the amination of o-tolyl chlorides, or more activated primary amines such as benzyl amine. A better ligand for this transformation was reported to be L4 as the amination could be conducted at room temperature in many cases.58 Base sensitive R1 required the use of K3PO4 rather than NaOf-Bu. Most amines work well with this catalyst system, except for unactivated primary aliphatic amines (e.g., n-hexyl amine). Allyl amine, hydrazines, imines, benzyl amine, and (EtO)2CH2NH2 all couple well with aryl chlorides under the... 

---