Metalation directing metalating group

Ziegler-Natta catalyst for polymerization of alkenes. Considerable attention has been directed to double-bonded Fischer carbenes of Cr and W, the Schrock carbenes of Ta and Ti, and cyclic polyene ligands of Fe, Co, Cr, and U. Carbonyls of transition metals from groups 6 to 10 of the periodic table include both the monomeric compounds such as Cr(CO)g, Fe(CO)5, Ni(CO)4 and those with two metal groups such as Mn2(CO)io and Co2(CO)s, which is used industrially for hydroformylation. Although their source has not been identified, it has been shown that volatile compounds from landfills contain carbonyls of Mo and W (Feldmann and Cullen 1997). 

The stereochemistry of the reaction depends on the Lewis acid. Protic acids favor retention of configuration, as does TMSOTf. Most metal halides give mixtures of inversion and retention, but A1(CH3)3 gives dominant inversion.142 Inversion is suggestive of direct carbonyl group participation. 

One possible mechanism is electrophilic attack of the complexed carbene carbon atom at the terminal carbon of the diene. The resulting zwitterionic intermediate can now eliminate the metallic group (CO)5M directly, or, alternatively, the metallic group can migrate to yield a new, more stable zwitterion (stabilization of the allyl cation by the heteroatom X). 

For conversion of functionalized diorganozincs into tertiary amines, aromatic compounds which contain a directed metallation group, such as Af,Af-dialkylbenzamides, methoxymethyl phenyl ether, phenyl oxazolines and phenyl Af,Af-dialkylcarbamates, were ortho-lithiated, transmetallated and then aminated with 2a in good yields, but with a slower reaction rate (Scheme 19). 

According to NMR data, reaction of lithium tetramethylpiperidine (TMP) and t-Bu2Zn, in a 1 1 molar ratio (Scheme 6) affords the heteroleptic zincate [f-Bu2Zn(TMP)]Li (36) . An interesting feature of this compound is that it is capable of metalate arenes that contain a directing metalating group (DMG) in ortho-posilion and thus allow further derivatization by reaction of the zincate intermediate with electrophiles (E+). 

Regioselective substitution reactions of a series of 2- and 3-hydroxybiaryls including BINOL have been performed via a new directed orf/io-metallation procedure.75 O-Aryl AMsopropylcarbamates, conveniently prepared from phenols and isopropyl isocyanate, have been temporarily and in situ N-protected by means of silyl inflates to form stable intermediates for low-temperature lithiation reactions using n-BuLi-TMEDA in diethyl ether. The IV,IV-dialkyl aryl O-sulfamate has been reported as a new directed metallation group.76... 

DMG, Directed Metalation Group EWG, electron-withdrawing group. 

Certain derivatives of benzene and naphthalene can be lithiated with sec-butyllithium (sec-BuLi). This reaction is regioselective. It takes place exclusively in the ortho-position (Directed ortho Metalation, DoM) to a so-called Directed-Metalation Group (DMG), whose presence, accordingly, is a prerequisite for such a metalation. Figure 5.37 gives examples of DMGs that are bound through a C, an O, or an N atom to the aromatic compound. 

A tetrahedral platinum atom was not possible in these molecules, since the isomers were not optically active. The direct linking to the metal of the non-metallic groups added in the formation of these supposed molecular compounds was known as coordination and the resulting molecules were coordination compounds. Molecules that we consider coordination compounds today include hemoglobin and chlorophyll, which are vital to animal and plant life. 

The concurrent and independent discovery of DoM by Wittig and Gilman over sixty years ago set the stage for the systematic work of Hauser, which was pursued in a number of laboratories in the late 1970s and has led to the current status of Directed Metalation Groups (DMGs), which allow the regioselective ortho introduction of a host of electrophiles (Scheme 2) [9a-c],... 

The C-Si(CH3)3-substituted stannacarboranes have been found to be useful starting materials for the synthesis of the other metallacarboranes. The apical tin is loosely ligated and can be replaced by other metal groups. For example, the reaction of Os3(CO)12 with c/-Sn[(CH3)3Si]2-C2B4H4 in the absence of solvent yields l-Os(CO)3-2,3-[(CH3)3Si]2C2B4H4 (50) in essentially quantitative yield [see Eq. (7)], while direct reaction of... 

The group in the Periodic Table with the least complicated chemistry is almost certainly the alkali-metal group lithium (Z — 3), sodium (Z 11), potassium (Z = 19), rubidium (Z 37), cesium (Z = 55), and the recently discovered francium (Z = 87). These elements follow directly after the inert gases and thus have one single valence electron beyond a well-shielded nucleus. 

---