Synthesis of Sterically Hindered Secondary and Tertiary Alkyl(Aryl)Phosphines

Abstract

Cross coupling of silylphosphines with 2-halobenzenecarboxylates or 2-halophenyl ethers, catalyzed with zero-valent palladium complexes can not be used as a procedure for preparing sterically hindered tertiary phosphines. The latter compounds can be successfully prepared by means of phosphorylation of corresponding o-lithiated derivatives.In [1], a procedure for preparing secondary alkyl-(aryl)phosphines of the general formula RArPH containing electron-donor and electron-acceptor substituents in the aromatic ring have been proposed. This procedure is based on cross coupling of alkyl-(trimethylsilyl)phosphines with corresponding aryl halides in the presence of catalytic amounts of zerovalent palladium.In this work we have studied the cross coupling of trimethylsilyl derivatives of secondary phosphines R 1 R 2 PSiMe 3 Ia and Ib [R 1 = i-Pr, R 3 = tert-Bu (a); R 1 = tert-Bu, R 2 = Ph (b)] and also of bis-silylated primary phosphines RP(SiMe 3 ) 2 IIa and IIb [R = i-Pr ÄÄÄÄÄÄÄÄÄÄÄÄ (a); R = tert-Bu (b)] with various aryl halides YC 6 H 4 Hlg III [reactions (1) and (2)] with the purpose to establish the utility of the above procedure for preparing tertiary alkyl(aryl)phosphines R 1 R 2 PC 6 H 4 Y IV and RP(C 6 H 4 Y) 2 V with secondary and tertiary alkyl groups and a series of o-and m-substituents in the aromatic ring. Bulky radicals in phosphines IV and V, including o-substituted in the aromatic ring, were chosen to impart configurational stability to racemic tertiary phosphines which we proposed to resolve into optical antipodes [2]. Substituents containing alkoxycarbonyl groups were introduced to render the synthesized phosphine ligands water-soluble after hydrolysis. R 1 R 2 PSiMe 3 + 4 c = Y Hlg 7777776 3Me 3 SiHlg 235 mol% Pd(0), to 4 c = Y PR 1 R 2 I III IV R(PSiMe 3 ) 2 + 2III 7777776 3Me 3 SiHlg 235 mol% Pd(0), to 4 c = Y PR 2 V II (1) (2) ÄÄÄÄÄÄÄÄÄÄÄÄIt was previously reported that the aromatic functionally substituted tertiary phosphines are successfully prepared by cross coupling of bromo(iodo)arenes III with diphenyl(trimethylsilyl)phosphine [3] and even with diphenylphosphine [4,5], whereas tertiary phosphines with bulky alkyl groups and functional groups in the aromatic ring were not obtained.Preliminary experiments showed [1] that the synthesis of tertiary phosphines by reaction (2) requires much more rigid conditions (1303140oC) than the synthesis of secondary phosphines (503100oC), and the slowest reactions are observed with o-substituted haloarenes III. In view of these results, we primarily studied the cross coupling of bis-silylphosphines IIa and IIb with bromide IIIa (Y = 3-CF 3 ) which was expected to be the most resistant to bromo-and especially iodotrimethylsilane evolved upon prolonged heating of the components of reaction (2). Reactions (1) and (2) were carried out in sealed ampules with

Transition-Metal-Catalyzed C-P Cross-Coupling Reactions

“…Being independent of the electronic nature of the aryl substituents, this cross-coupling displayed useful scope, tolerating a number of functional groups, including methyl ethers (which were reported to be cleaved under similar reaction conditions), 28 nitriles, ketones, esters (albeit transesterification was observed in some cases) 29 or amides. However, hydroxy, amino, nitro and aldehyde groups were not compatible, as they can cleave the Si-P bond and afford instead the trimethylsilylated products.…”

Synthesis of o-Diorganylphosphino-substituted Benzoic Acids and Their Derivatives

“…In [19], we found that the conditions of the synthesis by the Stille reaction of tertiary phosphines I with Y = COOR, even with one secondary or tertiary alkyl radical on phosphorus, become, in view of the steric hindrances, so rigid that the major cross-coupling products in this case are not phosphines I but teriary phosphonium salts formed by the side reactions presented in scheme (1). R 1 R 2 PSiMe 3 + 6 d : X COOR 3 777776 335 mol% Pd(0) 3Me 3 SiCl 6 d : PR 1 R 2 COOR 3 I 7776 Me 3 SiX, ot 6 d : PR 1 R 2 COOSiMe 3 6 d : P + R 1 R 2 R 3 X 3 COOSiMe 3 + R 3 X 76 to (1) R 1 = i-Pr, t-Bu; R 2 = Ph; R 3 = Me, Et; X = Br, I.…”

“…In [19], we found that the conditions of the synthesis by the Stille reaction of tertiary phosphines I with Y = COOR, even with one secondary or tertiary alkyl radical on phosphorus, become, in view of the steric hindrances, so rigid that the major cross-coupling products in this case are not phosphines I but teriary phosphonium salts formed by the side reactions presented in scheme (1 …”

“…Later on the same cross-coupling reaction was performed with phenyl-and diphenylphosphines (or their complexes with BH 3 ) instead of silylphosphines [10314], which allowed iodine-substituted arylcarboxylic and arylsulfonic acids and phenols to be included in the range of electrophilic reagents [15317]. Just recently the first short communication concerning the synthesis of tertiary diphenylarylphosphines with the acetyl and methoxycarbonyl groups in the aryl substituent by the cross coupling of diphenylphosphine with iodoarenes, catalyzed by copper iodide, appeared [18].In [19], we found that the conditions of the synthesis by the Stille reaction of tertiary phosphines I with Y = COOR, even with one secondary or tertiary alkyl radical on phosphorus, become, in view of the steric hindrances, so rigid that the major cross-coupling products in this case are not phosphines I but teriary phosphonium salts formed by the side reactions presented in scheme (1). R 1 R 2 PSiMe 3 + 6 d : X COOR 3 777776 335 mol% Pd(0) 3Me 3 SiCl 6 d : PR 1 R 2 COOR 3 I 7776 Me 3 SiX, ot 6 d : PR 1 R 2 COOSiMe 3 6 d : P + R 1 R 2 R 3 X 3 COOSiMe 3 + R 3 X 76 to (1) R 1 = i-Pr, t-Bu; R 2 = Ph; R 3 = Me, Et; X = Br, I.…”