2- phosphole

Phosphole polymerizes rapidly. 1-Substituted phospholes are thermally more stable. X-Ray studies of 1-benzylphosphole show that the molecule is not planar and that phosphorus retains its pyramidal structure [63]. 

Consistent with this finding, the NMR spectra show that phosphole has a lower aromaticity than furan. 

Phospholes are weak bases and react with strong acids to give phospholium salts. The cleavage of the exocyclic P-C bond by lithium in boiling THF is an interesting reaction of 1-phenyl- and 1-benzylphosphole. 

The phosphole anion is planar, aromatic and iso- r-electronic with fiiran and thiophene. 

Phospholes are accessible by [4+1] cycloaddition of buta-1,3-dienes with alkyl- or aryldibromophos-phanes followed by dehydrobromination. 

The enantioselective synthesis of the insect pheromone (S)-4-methylheptan-3-one (21) by alkylation of pentan-3-one (18) may serve as an example for the use of these chiral auxiliaries [239]  

The ketone 18 forms a hydrazone 19 with SAMP. Due to asymmetric induction by the chiral auxiliary, the subsequent alkylation (a-metalation with lithium diisopropylamide in diethyl ether, followed by 1-iodopropane at —110°C) occurs stereoselectively with formation of the diastereomer 20. In the final step, the auxiliary SAMP is removed from 20 by hydrolysis and the a-alkylated ketone 21 is obtained with ee = 99.5%. The use of RAMP as auxihary produces the (R)-enantiomer of 21. 

4-HydroxyproHne (22) is a proteinogenic amino acid occurring mainly in collagen. It can be separated from the hydrolysis products of gelatin. 

Several alkaloids are derived from pyrroHdine, for example, hygrin (23), a minor alkaloid of the coca plant, as well as nicotine (see p. 378). The vasodilator buflomedil (24) and the antihypertensive captopril (25) are dmgs containing a pyrroHdine ring. 

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McCORMACK - KUCHTIN RAMIREZ PhospholeSynthesis Fomtation of phospholes Irom butadienes (McCormack) or of dwxaphospholes from 1,2-diketones (Kuchtin-Ramirez)... 

The half-sandwich and sandwich complexes of phospholides and phosphole tetramer are known even for the nontransition metals. The half-sandwich arrangement was studied for lithium tetramethylphospholide [89AG(E)1367] and sodium derivative 127 generated from the phospholide tetramer and sodium in the presence of 1,2-dimethoxyethane [94JA3306 96AG(E)1125]. Potassium and 1,2-dimethoxyethane in THE in these conditions give a full dianionic sandwich 128. 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

If the reaction temperature is raised to 430 K and the carbon monoxide pressure to 3 atm, coordination of the metal atom in the rearranged product occurs via the phosphorus site, as in 159 (M = Cr, Mo, W) [84JOM(263)55]. Along with this product (M = W) at 420 K, formation of the dimer of 5-phenyl-3,4-dimethyl-2//-phosphole, 160 (the a complex), is possible as a consequence of [4 - - 2] cycloaddition reactions. Chromium hexacarbonyl in turn forms phospholido-bridged TiyP)-coordinatedcomplex 161. At 420 K in excess 2,3-dimethylbutadiene, a transformation 162 163 takes place (82JA4484). 

The facile route for introduction of the phosphole ring into the coordination sphere of the chromium vinylcarbene complex is via [4 - - 2] intramolecular... 

Fe—Fe bond can be assigned structures 201 or 202 based on spectral data. The other product of this reaction is 193 (R = r-Bu), however, it is produced in minor amounts. Complexes 199 (R = R = r-Bu, R = Ph, R = r-Bu) were obtained. Reaction of 146 (M = Mo, R = Ph, R = R = Ft, R = r" = Me) with (benzyli-deneacetone)iron carbonyl gives rise to the bimetallic complex 200 (M = Mo), which reacts further with the free phosphole to form the bimetallic heteronuclear sandwich 203. The preferable coordination of the molybdenum atom to the dienic system of the second phosphole nucleus is rather unusual. The molybdenum atom is believed to have a greater tendency to coordinate via the trivalent phosphorus atom than via the dienic system. 

The complex cation [Cp(Ti -2,5-dimethylthiophene)Fe] " with 3,4-dimethyl-phosphole gives the a complex 234 on photolysis [84JOM(272)417]. 

The phosphine-phosphole ligand with [Rh(cod)Cl]2 affords complex 254 (99OM4205). 

The classical tiaP) complexes were basically described for the phosphole chemistry of palladium(II) andplatinum(II) (80IC709 82IC2145 84IC449 85JA6939 89IC217). 

Phospholes and analogs offer a wide variety of coordination modes and reactivity patterns, from the ti E) (E = P, As, Sb, Bi) through ri -dienic to ri -donor function, including numerous and different mixed coordination modes. Electrophilic substitution at the carbon atoms and nucleophilic properties of the phosphorus atom are well documented. In the ri -coordinated species, group V heteroles nearly acquire planarity and features of the ir-delocalized moieties (heterocymantrenes and -ferrocenes). 

The fourth chapter of this volume comprises the second part of an ongoing series by Professor A. P. Sadimenko (Fort Hare University, South Africa) dealing with organometallic compounds of pyrrole, indole, carbazole, phospholes, siloles, and boroles. This follows the review in Volume 78 of Advances covering organometallic compounds of thiophene and furan. The enormous recent advances in this area are summarized and classified according to the nature of the heterocycle and of the metals. 

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