Dedicated to Professor Giambattista Consiglio on the occasion of his 65th birthdayThe chiral diphosphonite ligand (11bR,11'bR)-4,4'-(9,9-dimethyl-9H-xanthene-4,5-diyl)bis[dinaphtho[2,1-d:1',2'-f][1,3,2]dioxaphosphepin] ((R,R)-XantBino; (R)-1), based on a rigid xanthene backbone, was applied in the Pt/Sn-catalyzed hydroformylation of styrene (4a), 4-methylstyrene (4b), vinyl acetate (4c), and allyl acetate (4d), by using a Pt/Sn ratio of 1 : 1. High ee of up to 80% were observed, along with good regioselectivities towards the desired branched aldehydes. For styrene, an interesting inversion in the stereoselection process was observed at elevated temperatures, and a mechanism is proposed considering the temperature dependence of the regioselectivity. Introduction. -The asymmetric hydroformylation of alkenes is a reaction of great potential for the production of optically active aldehydes, which are versatile building blocks in synthetic organic chemistry [1] [2]. Initial work on asymmetric hydroformylation in the presence of cobalt [3], platinum [4], and rhodium catalysts [5] resulted in very low enantioselectivities, but the Pt/Sn-catalyst, modified with chiral diphosphine ligands and pioneered by Consiglio and co-workers led to much better results than any previous system [6]. However, these catalysts showed poor regioselectivities, yielding the branched, chiral aldehydes in only 50 -70% and in some cases of up to 80% [7]. Additionally, the formation of hydrogenated products and double-bond isomerization as unwanted side reactions were observed.Typically, a large excess of up to 6 equiv. of SnCl 2 was necessary to form stable catalysts and to avoid decomposition to Pt-black. However, this excess causes significant product racemization, as observed problem for the early systems, leading to lower overall enantioselectivities [6]. To overcome these drawbacks, trapping agents, e.g., triethyl orthoformate (HC(OEt) 3 ) were added, to achieve in situ protection of the aldehyde and to reach high enantioselectivities of up to 96% [8] [9]. Alternatively, preformation of the Pt/Sn bimetallic catalyst species resulted in higher enantioselectivities as well as chemo-and regioselectivities [10] [11].
