Nickel-Catalyzed Cycloisomerization of 1,6-Dienes

Based on the formal analogy between the intermolecular hydrovinylation and the intramolecular cycloisomerization process, we have chosen catalysts with proven potential for the first reaction type [48, 51] as the starting point of our study. The results are summarized in Table 2.1.5.7 [64]. Despite its excellent performance in the hydrovinylation of styrene [51], the [ Ni(allyl) Br 2]/(Ra, Sc, Sc)-26/NaBARF system led to disappointingly low conversions and selectivities in the cycloisomerization of 27a (entry 1). Similarly, the [ Ni(allyl)Cl 2]/(Ra,Rc)-4cel/Na-BARF system is not effective for the cycloisomerization of 27a (entry 2) even though it is able to promote the hydrovinylation. The other diastereomer, (R ,Sc)-4cel, however, which forms an active nickel catalyst for styrene oligomerization 

Entry Precursor/ Activator Ligand Substrate S S/[Ni] t[h] Conv. l%] TOFfh 7 Select. ee[%] 

The most active catalyst for the cycloisomerization of 27b was generated using the BARF-containing precursor 31a. After 60 min, 64% of the substrate was consumed, corresponding to an average turnover frequency of 128 h (entry 14). 

The regio- and enantioselectivity of the cationic nickel catalyst seems not to be affected by the nature of the counter ion in the case of the cyclization of 27a (entries 7, 8). In contrast, variation of the counter ion for substrate 27b influenced not only the activity but also the catalyst selectivity. The enantioselectivity increased from 33% to 54% in the opposite order to that observed for the activity (entries 12-14). Intriguingly, the order of activity imparted from the different anions for the cyclization of 27a differs signiflcantly from that observed for 27b. 

The studies summarized in this review demonstrate the broad utility of the QUINAPHOS hgand family for a variety of reactions in combination with different metals. Both monodentate and bidentate ligands derived from 1,2-dihydro-quinoline ensure high levels of enantioselectivity and/or activity, demonstrating the suitability of this chiral backbone. 

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