Advantageous use of tBu2P-N=P(iBuNCH2CH2)3N in the Hiyama coupling of aryl bromides and chlorides.

An efficient catalytic route to biaryls by employing (generally) only 0.25 mol % of Pd(OAc)(2) and 0.5 mol % of 1 in the Hiyama coupling reaction is reported. High yields for electron-rich, -neutral, and -deficient aryl chlorides are obtained. A variety of phenylsiloxanes undergo coupling with aryl bromides and chlorides with low Pd(OAc)(2)/1 loadings.

(t-Bu)2PN=P(i-BuNCH2CH2)3N: new efficient ligand for palladium-catalyzed C-N couplings of aryl and heteroaryl bromides and chlorides and for vinyl bromides at room temperature.

By employing Pd(OAc)2, Cs2CO3, or NaOH, and the new ligand (t-Bu)2PN=P(i-BuNCH2CH2)3N (3a), an electronically diverse array of aryl bromides and chlorides possessing base-sensitive substituents (nitro, ester, and keto) provide coupling products with bulky aryl amines in good to excellent yields. Aryl halides possessing other functional groups including cyano, amino, trifluoromethyl, and phenol, coupled with equal ease, producing highly functionalized amines in good to excellent yields. Moreover, an aryl chloro group can be preserved in the presence of a bromo substituent under our reaction conditions. BOC-protected amines also participated efficiently. Heterocyclic bromides and chlorides underwent clean couplings with amines in excellent yields. An important strength of our protocol is the use of lower palladium loadings than those reported earlier, without compromising yields. The air-stable palladium complex (eta3-cinnamyl)PdCl.(3a) (5) was also employed successfully in C-N coupling reactions while the crotyl analogue was less efficacious. The 3a/Pd(OAc)2 catalyst system promotes, for the first time, efficient coupling of vinyl bromides with a variety of amines to produce imines and enamines at room temperature.

Synthesis and characterization of R2PN=P(iBuNCH2CH2)3N: a new bulky electron-rich phosphine for efficient Pd-assisted Suzuki-Miyaura cross-coupling reactions.

Pro-azaphosphatrane 1a [P(iBuNCH2CH2)3N] reacts with iodine under mild conditions to give [IP(iBuNCH2CH2)3N]I in excellent yield, which on subsequent reaction with ammonia followed by deprotonation with KOtBu provided HN=P(iBuNCH2CH2)3N (3a) in quantitative yield. Reaction of 3a with R'2PCl afforded sterically bulky electron-rich phosphines of the type R'2PN=P(iBuNCH2CH2)3N (4) [R'=Ph (4a), iPr (4b), tBu (4c)]. The Pd(OAc)2/4c catalyst system was particularly efficient for the coupling of arylboronic acids with aryl bromides as well as aryl chlorides to give biaryls in excellent yields.

3/2-order chain kinetics involving a postulated dicationic intermediate in the isomerization of a p-isocyano to a p-cyano azaphosphatrane monocation.

Kinetic evidence suggests the possibility of a dicationic intermediate in the title reaction. Thus the linkage isomerization reaction, PNC+ = PCN+, is described by the rate law, nu = 3/2k[PNC+]3/2, which can be interpreted by a chain mechanism with the propagation reaction PNC+ + P2+ --> P2+ + PCN+. Such propagation is unusual in that the intermediate regenerates itself in this single step rather than forming a different intermediate for a second propagation step. Cyanide ions inhibit the rate because they participate in the termination step, P2+ + CN- --> PCN+. The rate constant in CD3CN at 100 degrees C is 3/2k = 7.2 +/- 0.6 x 10-5 L1/2 mol-1/2 s-1; 3/2k represents the composite (kinit/kterm)1/2 kprop. When the reaction is carried out in the presence of PBr+, however, the reaction becomes much faster and is described by the rate law, nu = kBr[PBr+][PNC+]; because [PBr+] remains at constant concentration, the time-course experiments follow first-order kinetics.

Mechanistic insight into fragmentation reactions of titanapinacolate complexes.

Reactions between terminal alkynes or aromatic ketones and titanapinacolate complexes (DMSC)Ti(OCAr(2)CAr(2)O) (2, Ar = Ph, and 3, Ar = p-MeC(6)H(4); DMSC = 1,2-alternate dimethylsilyl-bridged p-tert-butylcalix[4]arene dianion) occur via rupture of the C-C bond of the titanacycle. Thus, reactions of 2 and 3 with terminal alkynes produce 2-oxatitanacyclopent-4-ene or 2-oxatitanacycloheptadiene complexes along with free Ar(2)CO. These compounds have been characterized spectroscopically and by X-ray crystallography. Because metallapinacolate intermediates have been implicated in important C-C bond-forming reactions, such as pinacol coupling and McMurry chemistry, the mechanism of the fragmentation reactions was studied. Analysis of the kinetics of the reaction of (DMSC)Ti[OC(p-MeC(6)H(4))(2)C(p-MeC(6)H(4))(2)O] (3) with Bu(t)Ctbd1;CH revealed that the fragmentation reactions proceed via a preequilibrium mechanism, involving reversible dissociation of titanapinacolate complexes into (DMSC)Ti(eta(2)-OCAr(2)) species with release of a ketone molecule, followed by rate-limiting reaction of (DMSC)Ti(eta(2)-OCAr(2)) species with an alkyne or ketone molecule.