Cyclometalation, Pd-complexes

Murahashi, S.-I. Tamba, Y. Yamamura, M. Yoshimura, N. Reactions of cyclometalated Pd complexes with organolithium compounds or Grignard reagents. Selective ortho alkylation and arylation of benzaldehydes, azobenzenes, and tertiary benzylic amines. J. Org. Chem. 1978, 43, 4099-4106. 

Scheme 2.62 Activation of DMPP towards Diels-Alder reactions by cyclometallated Pd complexes.

Cyclometallation and RE of the cyclopropane should give PtHClL2 the phosphine must cyclometallate in the -CH2Nb case, which would release CHsNb and leave a cyclometalated Pd complex. 

Many of the reactions outlined in Section 4 for Pd-C bonds occur in cyclometalated complexes. However, the existence of the Pd-C bond in a chelate ring imposes some kinetic stability on the bond. In general, mild acids and bases and oxygen are unreactive with cyclometalated palladium complexes. More vigorous reagents will lead to useful reactions, however. 

The Murai reaction (Scheme 4), the replacement of an ortho-CH on an aromatic ketone by an alkyl group derived from a substrate olefin, is catalyzed by a variety of Ru complexes. This C bond formation occurs via chelate directed C-H bond activation (cyclometalation) in the first step, followed by alkene insertion into RuH and reductive elimination of the alkylated ketone. In a recent example of the use of a related cyclometalation in complex organic synthesis, Samos reports catalytic arylation (Suzuki reaction) and alkenylation (Heck reaction) of alkyl segments of a synthetic intermediate mediated by Pd(II). 

Following some earlier observations [18], the chemistry of cyclopalladated complexes has been greatly developed in the last ten years [31]. Recently, systematic investigations on the luminescence of cyclometallated Pd(II) complexes have also appeared. Most of the available data are gathered in Table 2. 

Several Pd(II) cyclometallated azobenzene complexes, having general structures (azb )PdBr(L) and (azb")PdBr(L) (12) where L = 4-stylbazole, have... 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

A series of cyclometalated Pd(ii) complexes which contain both (C N N)-7i-extended and chloride or pentafluorophenylacetylide ligands... 

Figure 2.2 Cyclometallated Pd(II) dimers used to resolve racemic phosphines. Only one enantiomer of the chiral amine and only the cis isomer for each complex are represented. Usually, R = R = Me.

Cyclometalated-NHC complexes of Pd 19 and 20 (Figure 9.4) were tested in Heck coupling of -butyl acrylate with several aryl bromides and chlorides. At... 

It was shown in stoichiometric studies (Scheme 12.21) as well as a catalytic reaction [eqn (12.10)] that NHC-Pd complex 63 was a capable catalyst for the halogenation of arenes. It should be noted that complex 64 could be isolated and characterized, and underwent reductive elimination to form 65 only upon warming in solution. Complex 65 was found to be a slower bro-mination catalyst than [Pd(OAc)2], which had been previously used for the same reaction. This was attributed to slower cyclometalation with a more electron-rich Pd, which is proposed to be the rate-limiting step in this type of reaction. ... 

In transition metal complexes of suitable geometry the metal may undergo intramolecular oxidative insertion into C-H bonds. Intermediates of Pd-catalyzed C-C bond formation can also undergo such cyclometalations to yield palladacycles . This can give rise to unexpected products or, if the palladacycles are too stable, the catalyst will be consumed and no further reaction will occur. At high temperatures reductive elimination from such complexes can occur to yield cyclic products. 

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