Platinum(II)‐Catalyzed Allylation of CO and CN Bonds under Hydrogenation Conditions

Abstract

Abstract:The nucleophilic addition of allylplatinum to a range of electrophiles including carbonyls, oximes, and hydrazones was examined. Platinum(II) catalysts and allene substrates were subject to hydrogenation conditions to generate nucleophilic allylplatinum complexes without using a stoichiometric amount of toxic and sensitive organometallic reagents. The allylplatinum intermediates reacted with C=O and C=N bonds to produce the corresponding homoallylic alcohol and amine derivatives in good yield. The Pt-catalyzed allylation of C=N bonds is the first example performed under hydrogenation conditions. A reaction mechanism initiated with the hydrometalation of allenes by Pt À H species is proposed based on deuterium labeling studies.Keywords: allylation; cyclization; hydrazones; hydrogenation; platinumThe transition metal-catalyzed allylation of aldehydes and imines has attracted considerable attention due to the synthetic utility of corresponding homoallylic alcohol and amine products as well as the interesting reactivity of the allyl-transition metal complex during the reaction. [1,2,3,4] To accomplish such allylations using a catalytic amount of transition metal complexes and allylic acetates or allenes as starting materials, a range of protocols utilizing stoichiometric amounts of organometallic reagents (tin, [2a,b,d,e,3c] zinc, [2c,h,i,j,m,p] indium, [2f,3a,b] borane, [2o,s,t,u] and silane [2k,q,r] ) which reverse the reactivity of the allyl-metal complexes from electrophilic to nucleophilic have been investigated. Recently, the reactions of nucleophilic allyl-iridium complexes were reported under transfer hydrogenation conditions where the use of metallic reductants were avoided.[5]Our continuing effort to develop nucleophilic Pt complex-catalyzed reactions under environmentally benign conditions without the use of large amounts of organometallic reagents prompts us to examine the reaction conditions to form a nucleophilic allyl-platinum complex from allenes, Pt catalysts, and hydrogen. Similar to allyl-palladium complexes, allyl-platinum complexes have been applied to electrophilic catalytic reactions such as allylic alkylations but, to the best of our knowledge, there is no report of the reaction of nucleophilic allyl-platinum complexes.[6] Based on Ptcatalyzed hydrogenative coupling reactions using a range of p systems investigated by our group, [7] Pt À H species formed from the Pt complex and hydrogen react with the allene to form a nucleophilic allyl-platinum species. This paper reports the Pt-catalyzed hydrogen-mediated reductive cyclization of allene-carbonyls, allene-oximes and allene-hydrazones where nucleophilic allyl-platinum complexes were generated without large amounts of organometallic reagents. A remarkable aspect of this protocol is that Pt-catalyzed allylations of the ketone and carbon-nitrogen double bonds (e.g., allyl-oximes and allyl-hydrazones) were realized under hydrogenation conditions for the first time. [8,9] Table 1 summarizes the reaction optimization process. Compound...

Platinum-Catalyzed Substitution of Allylic Fluorides

“…Their work was conducted with rac -1,3-diphenylallyl acetate, and the optimal conditions are shown in Scheme . More recently, platinum-catalyzed allylic amination (86% ee) and allylation of aldehydes (up to 83% ee) have been reported. , Using XantphosPtCl 2 as the precatalyst, direct amination of allylic alcohols has been achieved, in line with the high efficiency of platinum complexes as catalysts for allylic amination . These scattered results highlight the unrealized potential for further examples of Pt catalysis via allylic intermediates.…”

Diastereoselective Reductive Cyclization of Allene‐Tethered Ketoamines via Copper‐Catalyzed Cascade Carboboronation and Protodeborylation

“…Starting materials, 1i and 1j were prepared according to the known procedure 9b . To a suspension of N ‐(buta‐2,3‐dien‐1‐yl)‐4‐methylbenzenesulfonamide (669 mg, 3.00 mmol), n ‐Bu 4 NCl (83.37 mg, 0.30 mmol), and Na 2 CO 3 (635.4 mg, 6.00 mmol) in toluene (24 mL) at 0 °C and was added corresponding 3 chloropropione (1.1 equiv) portion wise and the mixture was stirred at room temperature for 24 h. The reaction was quenched with saturated aqueous NH 4 Cl solution (30 mL) and extracted with EtOAc (3 × 20 mL).…”

“…A Pt‐catalyzed method with SnCl 2 and H 2 has been reported for the reductive cyclization of ketoallene intermediates to obtain 3‐hydroxypyrrolidines (Scheme 1(b)). 9 In this study, we applied the Cu‐catalyzed cascade process for the effective borylative intramolecular cyclization of an allyl copper intermediate to a ketone, followed by in situ protodeborylation to obtain 3‐hydroxypyrrolidines in a highly diastereoselective fashion. This strategy is very attractive, particularly because it requires nontoxic and abundant Cu catalysts for the selective synthesis of highly valuable functional molecules.…”

Platinum-Catalyzed Diastereoselective Intramolecular Coupling of Allyl Halides and Hydrazones

“…We recently reported the platinum-catalyzed reductive cyclization of allene hydrazones. In this reaction, a nucleophilic allyl platinum intermediate was generated from the hydrometalation of an allene by a Pt II /SnCl 2 complex and H 2 . In addition to the hydrometalation route shown in the cyclization of allene hydrazones, other synthetic routes to form allyl platinum complexes exhibiting higher reactivity and selectivity have been explored.…”

“…When electron-rich phosphanes were used instead of P(C 6 H 4 - p -CF 3 ) 3 and P(2-furyl) 3 , 1b was obtained in higher yields (entries 3–6) with good cis diastereoselectivity. The stereochemical assignment of 1b was accomplished by comparing its 1 H NMR spectrum with that of a previously reported compound after chemical modification. − Although 1b was derived from 1c under hydrogenation conditions, reduction of the latter could not be controlled in the presence of a platinum catalyst and H 2 . The reaction with PCy 3 only showed 1c in 9% yield with 1b .…”