Shashin Shah

‘Phospha-variations’ on the themes of Staudinger and Wittig: phosphorus analogs of Wittig reagents

Abstract The Wittig and Aza-Wittig reactions have undergone tremendous development over the past 50 years in light of their potential in organic synthesis to construct carbon–carbon and carbon–nitrogen double bonds. In contrast, the development of the analogous phospha-Wittig reaction has only seen progress over the last 12 years. A phospha-Wittig process is one that uses phosphaylides to convert carbonyl compounds to new materials possessing phosphorus–carbon double bonds (phosphaalkenes). The phospha-Wittig reaction has evolved from initial work involving metal-assisted phospha-Wittig reactions to phospha-Wittig reactions, of terminal phosphinidene complexes to, more recently, free phosphanylidene-σ4-phosphoranes as phospha-Wittig reagents. The major developments in the phospha-Wittig reaction are highlighted and divided into these three methodologies. The complementary nature and the differences between the three approaches is also discussed. Finally, wherein applicable, comparisons between the Wittig reaction and the phospha-Wittig reaction are presented.

Crystal structure of the phosphanylidene-σ4-phosphorane DmpPPMe3 (Dmp=2,6-Mes2C6H3) and reactions with electrophiles

Abstract The stable phosphanylidene-σ4-phosphorane DmpPPMe3 (1, Dmp=2,6-Mes2C6H3) has been examined by single-crystal X-ray diffraction methods. The structure of 1 features a relatively short PP bond length of 2.084(2) A. Reactions of 1 with various electrophiles demonstrate the nucleophilic behavior of the phosphanylidene atom of 1 and also provide access to new organophosphorus compounds. For example, addition of excess BH3 (in the form of either BH3·THF or BH3·SMe2) to 1 leads to formation of a mono-borane adduct DmpP(BH3)PMe3. Reactions of carbon and silicon based electrophiles EX (E=R3C or R3Si; X=halide or OTf−) produce either diphosphanium salts [DmpP(E)PMe3]X or phosphines DmpP(E)X. In some cases equilibrium mixtures of both product types are observed, and the equilibria can be shifted by addition of either X− or PMe3. Compound 1 is also readily protonated by HOTf, HCl and PhOH. As found for the carbon and silicon based electrophiles, the nature of the resulting product depends on the counterion.

A role for free phosphinidenes in the reaction of magnesium and sterically encumbered ArPCl2 in solution at room temperature

Abstract The reduction of ArPCl 2 (Ar=2,6-Trip 2 C 6 H 3 , 2,6-Mes 2 C 6 H 3 , 2,6-(2,6-Me 2 C 6 H 3 ) 2 C 6 H 3 , or 2,4,6- t Bu 3 C 6 H 3 ) by various forms of activated magnesium have been examined and compared with previously reported reductions using unactivated magnesium. The current reactions are more rapid and give rise to products that are reminiscent of products obtained by irradiation of phosphanylidene-σ 4 -phosphoranes ArPPMe 3 bearing the same Ar groups. The correlation in product distributions suggests the involvement of phosphinidenes in the reduction of aryldichlorophosphines by activated magnesium.

‘Phospha-Wittig’ reactions using isolable phosphoranylidenephosphines ArPPR3 (Ar = 2,6-Mes2C6H3 or 2,4,6-But3C6H2)

Phosphoranylidenephosphines DmpPPMe3 (1a, Dmp = 2,6-Mes2C6H3) and Mes*PPMe3 (1b, Mes* = 2,4,6-But3C6H2) act as ‘Phospha-Wittig’ reagents with aldehydes providing phosphaalkenes [ArPC(H)R] in high yields.

Synthesis and solid state structures of increasingly sterically crowded 1,4-diiodo-2,3,5,6-tetraarylbenzenes: a new series of bulky benzenes and aryls

A series of very bulky 1,4-diiodo-2,3,5,6-tetraarylbenzenes I2Ar4C6 (Ar = 4-tBuC6H4, 1; Ar = 3,5-tBu2C6H3, 2; Ar = 2,4,6-Me3C6H2, 3) have been prepared by a rapid, convenient procedure involving sequential reaction of hexabromobenzene with the appropriate Grignard reagent in excess, followed by addition of excess elemental iodine. The new materials 2 and 3 were isolated in 18 and 28% yields, respectively. The solid state structures of 1–3 have been investigated by single crystal X-ray techniques. Interesting distortions in the structures of these hexasubstituted benzenes are observed upon the increasing steric pressures in 1–3. For example, while the structures of 1 and 2 contain central planar benzene rings, steric interactions in 3 produce a non-planar central benzene ring. Regardless of these factors, the carbon–iodine bond lengths in 1–3 are essentially constant (2.108–2.118 A).

Diphosphene and Phosphoranylidenephosphine Formation from a Terminal Phosphinidene Complex

The zirconium phosphinidene complexes [Cp2Zr=PDmp(PR3)] (Dmp = 2,6-Mes2C6H3; R = Me: 1a; R = Bu: 1b) form the diphosphene DmpP=PDmp, [Cp2ZrCl2], and the phosphoranylidenephosphines DmpP=PR3 (3a, 3b) upon reaction with DmpPCl2.

Phosphoranylidenephines (R3P=Pr) as Phospha-Wittig Reagents

Abstract We have recently discovered direct and high yielding routes to phosphoranylidenephosphines ArP=PMe3 (la, Ar = Dmp; lb, Ar = Mes*, eq. l).1