Phospholanes

Phospholanes usually adopt a folded envelope configuration, with which there are possible alternative arrangements (6.816). Phospholane, (CH2)4PH, mp = -88°C, bp = 100-103 C, can be prepared via the dimethylamine borane adduct or the chloro derivative. 

The hydrolysis of phospholanium salts based on the five-membered ring generally proceeds considerably faster than that of the corresponding phosphoranium salt based on the six-membered ring. 

In the case of (6.819), it is about 1500 times faster and it is likely that a pseudorotation process is involved (Chapter 14.3). 

Dibromomagnesium butane will react with thiophosphoryl chloride to give the five-membered ring analogues of those in scheme (6.813). In addition, crystalline 1,1 biphospholane 1,1 disulphide, mp= 185°C, is readily reduced by iron powder to give the liquid biphospholane. If heated with ethylene and a trace of iodine at 275-300°C for 48 h in an autoclave, it gives sym ethylene 1,1 phos-pholane 1,1 disulphide, mp= 174.5°C. 

Phospholanic acid, (1-hydroxyphospholane 1 oxide), mp=53-54 C, can be obtained by an alternative route via the ester, which is obtained by heating di n-butyl 4 chlorobutylphosphinite. 

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Boranes 1 and 2 in equation 89 react with 2-fluoro-1,3-dioxa-2-phospholanes to give nng expansion products as the corresponding antimony analogues yield the products of msertion of nitrogen into the antimony-fluorme bond [775] (equation 89) (Table 31)... 

C. By Reduction.—The cyclic secondary phosphines phospholan and phosphorinan have been prepared by reduction of the corresponding chlorophosphines with lithium aluminium hydride. ... 

Scheme 5-15 Examples of organolanthanide-cat-alyzed hydrophosphina-tion/q clization. Eq. (6) Phosphinoalkenes can undergo organolanthanide-catalyzed hydrophosphi-nation/cyclization to give phospholanes or an uncatalyzed reaction to give phosphorinanes...

Among electron-rich chiral phosphines, chiral phospholanes have emerged to be one of the most efficient classes of ligands in metal catalyzed enantioselective reactions. We have developed a novel family of bisphospholane ligand namely, catASium M, from laboratory to commercial scale. Trimethylsilylphospholane 1 was employed as a key intermediate to provide access to a large variety of ligands. 

The enantiopure l-chloro-2,5-dimethylphospholane 2 is now available from the corresponding 1-trimethylsilylphospholane 1. The new phospholane 2 was used as an electrophilic building block in a wide range of coupling reactions giving rise to new phospholanes. These proved to be valuable as chiral ligands in transition metals catalysis with Rh, Ir or Ru complexes. 

The pioneering work of Denney et ai19 on the synthetic utility of oxyphosphoranes has been thoroughly exploited by Evans et al. in demonstrating that diethoxytriphenylphosphorane promotes mild and efficient cyclodehydration of diols (e.g. 11) to cyclic ethers (e.g. 13) via the cyclic phosphorane (12)20>21. Simple 1,2-, 1,4-, and 1,5- diols afford good yields of the cyclic ethers but 1,3-propanediol and 1,6-hexandiol give mainly 3-ethoxy-l-pro-panol and 6-ethoxy-l-hexanol respectively whereas tri- and tetra-substituted 1,2-diols afford the relatively stable 1,3,2- diox-phospholanes. In some instances (e.g. 14), ketones (e.g. 16) are formed by a synchronous 1,2-hydride shift within (15). The synthetic utility has been extended to diethoxyphosphoranes supported on a polystyrene backbone22. 

Alkoxy-l,3-dithia-2-phospholane ( O-Alkyl ethylene dithiophosphate )... 

Simple derivatives of the parent compound in this group, exo,endo- and o ,o -2,6-dimethyl-l-phosphabicyclo[2.2.1]-heptane 211 and 212, have been synthesized by the cyclization of 2-methyl-4-(2-propenyl)phospholane in the presence of base <2002ZFA580>. The structures were determined via spectroscopic means as well as X-ray crystallography and further confirmed by reactions with selenium, sulfur, (CH3)2SeO, CH3I, and HS03F. 

One of the possible synthetic ways to obtain heterocyclic phosphines is the insertion of carbonyl compounds into the P—E (E = Si, Ge) bond of sila- and germa-phospholanes. Thus, the enlargement of the ring takes place and the P—C—O—E fragment is formed (9) [Eq. (7)] (74MI1 75JOMC35 77JOM35). The heterocyclic phosphepanes are obtained as a mixture of stereoisomers. 

The spatial structure of 16 stereoisomers of 1,3-aza-, 1,3-thia-, and 1,3-oxaphospholanes was determined (75OMR470). H NMR spectra of several stereoisomers of phospholanes, containing N and O atoms in the 3-position to phosphorus, have been presented [78PS(4)59 83PS51], However, the conformational equilibrium was not studied (Fig. 2). 

Gusarova, N.K., Shaikhudinova, S.I., Dmitriev, V.I., Malysheva, S.F., Arbuzova, S.N., and Trofimov, B.A., Reaction of red phosphorus with electrophiles in superbasic systems. VII. Phospholanes and phosphorinanes from red phosphorus and a,co-dihaloalkanes in a single preparative step, Zhur. Obshch. Khim., 65, 1096, 1995. 

Bodalski, R. and Pietrusiewicz, K., A new route to the phospholane ring-system, Tetrahedron Lett., 4209, 1972. 

One of the branches in ligand design was provided by Kumada and his introduction of the ferrocene backbone for BPPFA [99-101] (20a) and BPPOH [102] (20b). This development leads us to the next class of ligands - ferrocene-based. Other variations for development include changes in the backbone and incorporation of the phosphorus into a phospholane (see Section 23.6). 

The development of the next major class of ligands occurred during the 1990s, with Burks DuPhos (42) family of phospholane ligands [222, 223]. (An individual member of the family is named after the substituent R in Me-DuPhos, R=Me.) This structure could be considered an improvement on the DIOP-derived ligands, where the stereogenic centers are now closer to phosphorus. In addition to the aromatic spacer of DuPhos, there is also the related BPE (43) family, where the spacer between the two phosphorus atoms is less rigid. In both series the phosphorus is... 

As the chirality with the DuPhos ligands is within the phospholane rings, a wide variety of backbones can be used ranging from ferrocene to heterocycles [61, 62, 77, 222, 251-262],... 

Variations have also been made on the DuPhos theme by changing the nature of the phospholane ring (Fig. 23.2). These ligands retain the high selectivity of... 

Neither of the phosphorus atoms needs not be in an asymmetric phospholane ring, as illustrated by both Saito and Pringle with UCAPs (44) [273, 274]. 

In this chapter, we review the growing family of phospholane-based chiral ligands, and specifically examine their applications in the field of enantioselective hydrogenation. In general, this ligand class has found its broadest applicability in the reduction of prochiral olefins and, to a significantly lesser extent, ketones and imines this is reflected in the composition of the chapter. Several analogous phosphacycle systems have also been included, where appropriate. 

The first reported application of phospholane-based ligands for enantiomeric hydrogenation was described by Brunner and Sievi in 1987 [6], Unfortunately, these trans-3,4-disubstituted phospholanes (1-3) were derived from tartaric acid, and proved to be relatively unselective for the rhodium-catalyzed hydrogenation of (Z)-a-(N-acetamido)cinnamic acid (6.6-16.8% ee). This was, presumably, due to the remoteness of the chiral centers from the metal coordination sphere failing to impart a significant influence. This was also found to be the case with several other bi- and tridentate analogues [7]. 

The fundamental discovery by Burk et al. that the analogous trans-2,5-disub-stituted phospholanes formed a more rigid steric environment led to the introduction of the DuPhos and BPE ligand classes (Fig. 24.1) [8-13]. Subsequently, these ligands have been successfully employed in numerous enantiomeric catalytic systems [4 a, 5], the most fruitful and prolific being Rh-catalyzed hydrogenations. The reduction of N-substituted a- and /1-debydroarnino acid derivatives,... 

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