Hydrophosphorylation, of alkynes

R = n-CeHia, Ph, p-Tol, NC(CH2)a, HC=C(CH2)5, 3-HC=C-C6H4, 1-cyclohexenyl, SiMea Scheme 5-17 Palladium-catalyzed hydrophosphorylation of alkynes... 

Scheme 5-20 Proposed mechanism for palladium-catalyzed bis-hydrophosphorylation of alkynes...

Regioselective (Markovnikov) hydrophosphorylation of alkynes with dialkyl phosphites catalyzed by a series of Pd(0) complexes has been accomplished in excellent yield in THF solution.78 Although a variety of Pd(0) species served as catalysts, common Pd(II) species were ineffective. Several Pt(0) species were also able to catalyze the hydrophosphorylation process, but with lower efficiency. 

Nickel complexes have proved to be active for hydrophosphorylation of alkynes [17a, 25]. Not only the five-membered phosphonate but also commercially available dialkyl phosphonates readily react in the presence of... 

Hydrophosphorylation of alkynes via external attack of //-phosphonate to an (alkyne)metal complex is a possible pathway, although the possibility has been concluded to be less likely as far as the nickel-catalyzed reaction is concerned [12]. However, such a process appears to proceed in early transition metal carbonyl-catalyzed reactiOTis [19]. For instance, refluxing a mixture of phenylacetylene, HP(0)(0Et)2, and Mo(CO)fi (10 mol%) affords the franr-addition product... 

Han et al. have studied hydrophosphorylation of alkynes catalyzed by Ni complexes (Scheme 8.10) [73]. The influence of diphenylphosphinic acid Ph2P(0)OHon the reaction selectivity was found using different Ni systems with phosphorus... 

Scheme 8.13 Hydrophosphorylation of alkynes leading to a-isomer and possible by-products...

Scheme 8.14 Regioselective hydrophosphorylation of alkynes using [PdJ/CFsCOOH system...

The hydrophosphorylation of internal alkynes is somewhat slow. For instance, the reaction of 4-octyne with diethyl phosphonate resulted in 82% yield only after heating for 65 h. Only c/s-isomer was observed in NMR spectroscopy. 

The catalyst (PPh3)3RuCl in the presence of DPPB is an efficient reagent for the hydrophosphorylation of olefins. The reaction is highly sensitive to olefin substitution and monosubstituted olefins can be reliably converted to their aliphatic phosphonates in the presence of other olefins. Additionally, a trimethyl-silyl group is an effective acetylene protecting functionality that reverses the normal preference for alkyne hydrophosphorylation over a terminal olefin (Scheme 54). 

Insertion of alkynes into Pd-P bonds give alkenylpalladium intermediates 2 and 3. Photonolysis of these intermediates by diaUcyl H-phosphonate results in aUcenylphospho-nates 4 and 5 and regenerates the active catalyst 1. Indirect evidence for the insertion of alkynes into Pd-P bonds comes from the isolation of trace amounts of alkynyl phospho-nate 6, which can only result from reductive elimination of the alkenylpalladium intermediate 3. The second hydrophosphorylation reaction proceeds via the insertion of geminal alkenylphosphonate 4 into the Pd-H bonds, followed by reductive elimination of the product 7. This reaction pathway can explain the failure of 5 to react with dialkyl H-phosphonate because of the steric hindrance of the potential insertion intermediate. These bisphosphonates are evalnating as ligands for radionuclides such as and for... 

DFT calculations shed light on the factors governing the selectivity of Pd-catalysed hydrophosphorylation, hydrophosphinylation, and hydrophosphination of alkynes with... 

Hydrophosphorylation of alkenes and alkynes with phosphinic acid derivatives such as alkyl phosphinates [H2P(0)(0R)] and anilinium phosphinate [PhNH3 0P (0)H2] is successfully catalyzed by palladium complexes, in particular those ligated by xantphos and dppf [25]. Polymer-botmd palladium catalyst 11 has proved to catalyze the same reactions with phosphinic acid to give good yields, although the... 

Detailed study of the outcome of alkynes hydrophosphorylation reaction have revealed rather complicated picture with several by-products depending on the structure of alkyne, H-phosphonate, catalytic system and reaction conditions (Scheme 8.13) [76]. 

Gulykina NS, Dolgina TM, Bondarenko GN, Beletskaya IP (2003) Hydrophosphorylation of terminal alkynes catalyzed by palladium. Russ J Org Chem 39 797-807... 

Zhao Ch-Q, Han L-B, Goto M, Tanaka M (2001) Rhodium-catalyzed hydrophosphorylation of terminal alkynes leading to highly selective formation of (i )-afkenylphosphonates complete reversal of regioselectivity to the palladium-catalyzed counterpart. Angew Chem Int Ed 40 1929-1932... 

Because organophosphorus compounds are important in the chemical industry and in biology, many methods have been developed for their synthesis [1]. This chapter reviews the formation of phosphorus-carbon (P-C) bonds by the metal-catalyzed addition of phosphorus-hydrogen (P-H) bonds to unsaturated substrates, such as alkenes, alkynes, aldehydes, and imines. Section 5.2 covers reactions of P(lll) substrates (hydrophosphination), and Section 5.3 describes P(V) chemistry (hydrophosphorylation, hydrophosphinylation, hydrophosphonylation). Scheme 5-1 shows some examples of these catalytic reactions. 

Han and Tanaka reported that Pd(0) complexes catalyze the addition of dialkyl phosphites to terminal alkynes to give alkenylphosphonates (Scheme 5-17, hydrophosphorylation) [15]. 

Addition of the P—H bond of hydrogen phosphonates (R0)2P(0)H across alkynes (hydrophosphorylation) may be catalyzed using both Pd° and Pd11 complexes.196 Reaction of oct-l-yne with either (MeO)2P(0)H or (EtO)2P(0)H affords the Markovnikoff adduct (Equation (18)) as the... 

Although various substrates that have two alkene linkages or both alkene and alkyne linkages conform to the catalysis, the chemoselectivity is rather low and mixtures of products are formed. However, when bulky Me3Si is bound to the alkynyl bond in an en-yne substrate, the alkene moiety is selectively hydrophosphorylated (Scheme 34). 

Theoretical calculations have been also carried out to study acetylene insertion into the Pd-P(0)(0Me)2 bond related to hydrophosphorylation process [39]. The activation barriers for alkyne insertion were calculated to be AG = 21.4 and 23.7 kcal/mol depending on the orientation of the phosphorus group. However, much smaller difference in the relative stability of the products was found. The presence of three oxygen atoms attached to the phosphorus center facilitated chelate coordination to the metal in any orientation of the phosphorus group. Thus, the influence of orientation of heteroatom group significantly depends on the number of bonded oxygen atoms (see also Scheme 8). 

Due to the successful development of the catalyzed hydrophosphorylation, its extension to functionalized alkynes is becoming of interest. Nickel-catalyzed addition of //-phosphonate (and also //-phosphinate and sec-phosphine oxide) to propargyhc alcohols is an interesting example, as discussed later (Sect. 4). 

Tanaka et al. were the pioneers of the synthetic application of catalytic hydrophosphorylations and hydrophosphinylations of multiple carbon-carbon bonds. Addition to alkynes was catalyzed by Pd complexes and led to vinyl-phosphonates in high yields under relatively mild conditions (Scheme 8.2) [66]. In some cases an excellent a-regioselectivity was reported. As an exception, trimethylsilylacetylene gave trans-jS-trimethylsilyl-vinylphosphonate in the addition reaction presumably due to steric hindrance. Various a-substituted vinylphospho-nates were synthesized from terminal alkynes in 80-95% yield and 92-96% regioselectivity using c -PdMe2(PPh2Me)2 as the catalyst precursor. 

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