Biphenylic Bisphosphines through ortho-Metalation of Aryldiphenylphosphine Oxides

Diphenyl-(2-trifluoromethyl)phosphine oxide was metalated next to the diphenylphosphinoyl group with LITMP. Upon iodination and Ullmann coupling, the corresponding 3,3'-disubstituted bis(phosphine oxide) was obtained. When phenyllithium was used as metalating agent, aryl/phenyl group exchange occurred. The torsional barrier for the planarization of the biaryl moiety was equal to 22 ± 1 kcal/mol

The activation barrier to axial torsion in 2,2'-bis(diphenylphosphino)biphenyl

After selective deuteration of 2,2'-bis(diphenylphosphino)biphenyl, it was possible to det. the NMR coalescence temp. of the diastereotopic Ph groups. On this basis, a torsional barrier of 22 kcal/mol was calcd. Thus, the bisphosphine, even if it could be resolved into pure enantiomers, would rapidly re-racemize at temps. around or >25 Deg. [on SciFinder (R)

2,2'-Bis(diphenylphosphino)biphenyl revisited

The reaction between 2,2'-dilithiobiphenyl and two equiv. of chlorodiphenylphosphine is confirmed to afford equal amts. of 9-phenyl-9-phosphafluorene (2) and PPh3. 2,2'-Bis(diphenylphosphino)biphenyl (1,1'-biphenyl-2,2'-diylbis(diphenylphosphine) (1)) can be conveniently prepd. by Ullmann coupling of 2-iodophenyldiphenylphosphine oxide and subsequent redn. with trichlorosilane. [on SciFinder (R)

The preparation of pure allyl- and benzyl-type organoalkali intermediates via organotin compounds

Superbase metalation of alkenes or alkylbenzenes and subsequent condensation with trialkylstannyl chloride affords allyl- or benzyl-type organotin compds. that can be isolated in pure form. Treatment with sol. reagents such as methyllithium, trimethylsilylmethylpotassium and trimethylsilylmethylcesium generates the corresponding organoalkali derivs., almost quant. and in high purity, suitable for kinetic or spectroscopic studies. E.g., treating Me3SiC(:CH2)Me with KOCMe3 and BuLi in THF and then with Me3SnCl gave 45% CH2:C(SiMe3)CH2SnMe3, which when treated with Me3SiCH2Cs gave 95% of the Cs deriv. [on SciFinder (R)

How advantageous is the intramolecular aggregation of 1,4-organodilithio compounds?

1,11-Dilithio-5,5,7,7-tetramethyl-5,7-dihydrodibenz[c,e]oxepin (3; shown as I) is a conformationally confined analog of o,o'-dilithiobiphenyl (1). A temp. variable NMR study of this model compd., monitoring the coalescence of its diastereotopic Me groups, has revealed a barrier of about 12 kcal/mol to planarization, just 2 kcal/mol less than that found with the metal-free heterocycle. Thus, the energetic benefit of intramol. aggregation of o,o'-dilithiobisaryls is significantly smaller than that predicted by ab initio calcns. [on SciFinder (R)

1,(w-1)-Dienes: solvent controlled unilateral or bilateral metalation

In hexane, i.e. under heterogeneous conditions, 1,(w - 1)-dienes readily undergo double deprotonation to give bis(allylmetal) intermediates. In THF at -75 or -50 Deg, however, only monometalation occurs with dienes having <C12 chains. Thus, treatment of 1,7-octadiene with BuLi-KOCMe3 in hexane, followed by MeI, gave (Z)-3-methyl-1,6-nonadiene and (3Z,7Z)-3,7-decadiene; whereas, in THF, with hexane stripped from the BuLi, 3-methyl-1,7-octadiene and (Z)-1,6-nonadiene were obtained. The practical potential of the selective monosubstitution reaction is demonstrated by two novel pheromone syntheses. [on SciFinder (R)

A one-pot preparation of allylsilanes and (Z)-alk-2-enyl silanes

Metalation of 1-alkenes with a mixt. of BuLi and KOCMe3 in tetrahydropyran, followed by stereochem. equilibration and treatment with Me3SiCl, afforded (2-alkenyl)trimethylsilanes in good yield and with Z:E ratios of 95:5 to 98:2. E.g., reaction of RCH2CH:CH2 (R = Pr, Bu, pentyl, hexyl, heptyl, octyl, nonyl) as above gave 52-75% Z-RCH:CHCH2SiMe3. Deprotonation of propene was rapidly and readily accomplished with a stoichiometric amt. of the superbase suspended in pentane on a 1 mol scale. [on SciFinder (R)

Reagent-modulated optional site selectivities: the metalation of o-, m- and p-halobenzotrifluorides

Chloro(trifluoromethyl)benzenes and bromo(trifluoromethyl)benzenes underwent deprotonation at a postion adjacent to the single halogen substituent when treated with alkyllithiums (at -75 DegC) and, resp., lithium 2,2,6,6-tetramethylpiperidide (at -100 DegC) in THF. Positional ambiguities, if existing, were be exploited to establish optional site selectivities. Thus, the reaction of butyllithium with 1-chloro-3-(trifluoromethyl)benzene led to hydrogen/metal interconversion at the 2-position whereas the reaction of sec-butyllithium led exclusively to a reaction at the 6-position. The latter mode of regioselectivity was also exhibited by 1-bromo-3-(trifluoromethyl)benzene in the presence of lithium 2,2,6,6-tetrmethylpiperidide, and only 2-bromo-4-(trifluoromethyl)phenyllithium was produced. 2-Bromo-6-(trifluoromethyl)phenyllithium was directly inaccessible, but was formed when 2-bromo-3-(trifluoromethyl)phenyllithium, (generated at -100 DegC) was allowed to isomerize at -75 DegC. [on SciFinder (R)

Polar allyl type organometallics as key intermediates in regio- and stereocontrolled reactions: conformational mobilities and preferences

A review with 64 refs., including discussion of the structural flexibility of organometallic species, the stereochem. of 2-alkenylmetal compds., and the origin of the endo effect. [on SciFinder (R)