Systems functional group compatibility with

The palladium-catalyzed reductive coupling reactions were used in the synthesis of several natural products, including laurene [75], ceratopicanol [80], and dihydrostreptazolin 141 [81]. The cyclization leading to dihydrostrepta-zolin shown in Eq. 26 highlights the diastereoselectivity and functional group compatibility seen with this catalytic system. 

Subsequently, direct incorporation of GO by titanocene(ii) catalyst, Gp2Ti(GO)2, under a GO atmosphere was reported.This catalytic system showed substantially higher TON and broader functional group compatibility. However, this catalyst fails to react with sterically hindered olefins and alkynes. In a recent contribution from the same group, a series of aryloxide titanium complexes 22 (figure 4) are prepared and shown to promote PKR with some sterically hindered enynes." ... 

In terms of functional group compatibility, ethers, alcohols, tertiary amines, acetals, esters, amides and heterocycles are compatible with the Pauson-Khand reaction. In the intramolecular version, relatively few carbon skeletons undergo the cyclization. Most intramolecular PKRs use systems derived from hept-l-en-6-yne (6) or propargyl allyl ethers (7) or amines (8). Other interesting and more recent substrates are enynes connected through aromatic rings like 9-11, which have allowed us and other groups to obtain aromatic polycycles (Fig. 1) [28-31]. 

Related molybdenum catalysts appear to show even more functional group tolerance. To date, the major test of functional group compatibility has been in the synthesis of polymers however, it is anticipated that this activity will persist into acyclic metathesis. Later transition metals are active in the metathesis polymerization of highly functiondized cyclic alkenes. These catalyst systems, which appear to tolerate almost all functional groups, show very low activity for acyclic alkene metathesis. If these systems can be activated, the problems associated with the use of alkene metathesis in the synthesis of multifunctional organics will be solved. 

Similar to the reduction of aryl phosphates, phenols can be reduced to arenes via the corresponding aryl triflates. The reducing systems employed are compatible with a wide array of functional groups. 

Building from Krause s study with allenes,it was discovered that Au(l) catalysts can effect intermolecular hydrothiolation of unactivated olefins (32) [12]. 2-Mer-captoethanol reacts exclusively with sulfur, demonstrating chemoselectivity and functional group compatibility. As with the other systems, both aliphatic and aromatic thiols work well. 

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