Main group metal catalysts

Thiols can be added to alkenes under radical, acidic and basic conditions, as well as by use of main group metal catalysts. In particular, Dunach demonstrated high yields of inter- and intramolecular In(III)-catalyzed hydrothiolation [249]. Both aliphatic and aromatic thiols react efficiently, as do sterically hindered olefins. Functional group compatibility remains to be demonstrated. In addition, these approaches lack selectivity, functional group compatibility and generality. 

In contrast to the transition-metal-based catalysts, main-group metal catalysts for activating H2 are still rare. In 2002, the main-group metal catalyst KO Bu was found by Miiller et al. to catalyze the hydrogenation reaction of ketones through the heterolysis of H2 [66, 67]. 

In 2008, several main-group metal catalysts, such as the following organo-calcium catalyst, were found to readily cleave the H—H bond diuing the hydrogenation reactions of conjugated alkenes [68]. 

The concept of FLPs has also been proven to be very useful for understanding the reactions of H2 with main-group metal catalysts. In exploring the hydrogenation mechanism of a conjugated alkene, 1,1-diphenylethylene, mediated by the early main-group metal catalysts, CaH(dipp-nacnac)(thf) 2 (dippnacnac=CH (CMe)... 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Finally, chain polymerisation can occur via coordination, as is the case for polymerisation involving Ziegler-Natta catalysts. These catalysts are complexes formed between main-group metal alkyls and transition metal salts. Typical components are shown in Table 2.1. 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

A) Ziegler method Catalyst systems can be prepared by reacting a nickel(II) salt with an organometallic compound of the main group metals but aluminum is preferred. 

Of course, main-group metal hydrides such as these cannot be incorporated into a useful catalytic scheme since they do not form readily from H2. The following, apparent criteria for a suitable catalyst are based on the above discussion. 

A dimeric proline-derived diamidobinaphthyl dilithium salt has been introduced as the first example of a chiral main group metal-based catalyst for asymmetric hydroami-nation-cyclization reactions of aminoalkenes.256... 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

Addition of organometallic compounds of main group metals R—M —X (M = B, Al, Zn, Mg, Sn) to alkenes and alkynes is called carbometallation. Some reactions proceed without a catalyst, but they are promoted or accelerated by transition metal... 

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

Addendum Recent Achievements in Polymerisation with Main Group Metal-based Catalysts... 

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