PCy3, catalysts from

Attempted reaction of 1,3-pentadiene with the optically active diboron derived from dialkyl tartrate in the presence of a phosphine-free platinum catalyst gave poor diastereoselectivity (20% de).63 Better selectivity has been attained with a modified platinum catalyst bearing a PCy3 ligand (Scheme 6).64 The reaction of allylborane thus obtained with an aldehyde followed by oxidation with basic hydrogen peroxide affords the corresponding diol derivative with moderate ee. 

In the case of cyclopentenyl carbamate in which a directive group is present at the homoallyl position, the cationic rhodium [Rh(diphos-4)]+ or iridium [Ir(PCy3)(py)(nbd)]+ catalyst cannot interact with the carbamate carbonyl, and thus approaches the double bond from the less-hindered side. This affords a cis-product preferentially, whereas with the chiral rhodium-duphos catalyst, directivity of the carbamate unit is observed (Table 21.7, entry 7). The presence of a hydroxyl group at the allyl position induced hydroxy-directive hydrogenation, and higher diastereoselectivity was obtained (entry 8) [44]. 

An exceptionally simple procedure was developed for the catalytic carbonylation of chloroarenes using [PdCl2(PCy3)] as catalyst. According to this method the neat chloroanene, e.g. m-chlorotoluene and the catalyst are stirred with 20 % (w/w) aqueous KOH at reflux temperature with bubbling CO. The benzoic acids are extracted from the aqueous phase after... 

Cahonic iridium complexes bearing imidazol-2-ylidene hgands were applied as TH catalysts by Hillier et al. [48]. Here, [lr(cod)(py)(L)]Pp6 (L= IMes, l,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (53a), IPr, l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (53b) and ICy, l,3-bis(cyclohexyl)imidazol-2-ylidene (53c) were employed as catalysts for TH from 2-propanol to various unsaturated substrates, and compared to [lr(cod)(py)(PCy3)]PF6 and complexes formed in situ from [Ir(cod)(py)2]PF,5 and diazabutadienes (RN=CHCH=NR, DAB-R R = cyclohexyl, DAB-Cy 2,4,6-trimethylphenyl, DAB-Mes adamantyl, DAB-Ad 2,4,6-trimethoxyphenyl, DAB-trimethoxyphenyl). AH complexes were achve catalysts for the TH of acyclic ketones, with complex 53c being the most achve, showing 100% conversion of BuC(0)Me and PhC(0)Ph in 25 and lOmin, respechvely, using 0.025 mol% of catalyst and KOH in PrOH at reflux (Scheme 4.20). 

Figure 8.8 Comparison of in situ infrared profiles of copolymer production from C02 and cyclohexene oxide utilizing the catalyst in Figure 8.6b and the three classes of cocatalysts. T = PPN+CI" O = PCy3 = N-Melm.

We shall consider reactions catalysed by two different types of pro-catalyst the first (type A) employs Pd-allyl cations ([Pd(a]lyl)(PCy3)]+/Et3SiH or [Pd(allyl)(MeCN)2] + ), and the second (type B) employs Pd-alkyl or chloro complexes ([(phen)Pd(Me)(MeCN)]+, where phen = phenanthroline, and [(RCN)2PdCl2]). These two types of catalysts give very different products in the cyclo-isomerisation of typical 1,6-dienes such as the diallyl-malonates (10), Scheme 12.6. Since there is known to be a clear order of thermodynamic stability 11 < 12 <13, with a difference of ca. 3-4 kcal mol 1 between successive pairs, any isomerisation of products under the reaction conditions will tend towards production of 12 and 13 from 11 and 13 from 12. Clearly, when 11 is the major product (as with pro-catalysts of type A), it must be the kinetic product (see Chapter 2 for a discussion of kinetic and thermodynamic control of product distributions). However, when 12 is generated selectively, as it is with pro-catalysts of type B, there is the possibility that this is either generated by rapid (and selective) isomerisation of 11 or generated directly from 10. 

---