Both the bisphosphine and bisphosphinite pincer complexes ( tBu4 PCP)IrH 2 and ( tBu4 POCOP)IrH 2 can cocatalyze alkane metathesis in tandem with olefin metathesis catalysts, but the two complexes have different resting states during catalysis, suggesting that different steps are turnoverlimiting in each case. This led to the hypothesis that a complex with intermediate properties would be catalytically more active than either of these two species. Accordingly, "hybrid" phosphine−phosphinite pincer ligands (PCOP) and the corresponding iridium complexes were synthesized (3c−e). In tandem with olefin-metathesis catalyst MoF12, ( tBu4 PCOP)-IrH 2 displays significantly higher activity for the metathesis of n-hexane than does ( tBu4 PCP)IrH 2 or ( tBu4 POCOP)IrH 2 . ( tBu2 PCOP iPr2 )IrH 4 (3d) is even more active (>30-fold more active than ( tBu4 POCOP)IrH 2 ) and affords nearly 4.6 M alkane products after 8 h at 125 °C.

Threefold intramolecular ring-closing metatheses of trans-[MCl2(P{(CH2)(m)CH=CH2}3)2] are effected with Grubbs' catalyst. Following hydrogenation catalyzed by [RhCl(PPh3)3], the title complexes trans-[MCl2(P((CH2)n)3P)] (n=2m+2; M/n=Pt/14, 4 c; Pt/16, 4 d; Pt/18, 4 e; Pd/14, 5 c; Pd/18, 5 e) and sometimes isomers partly derived from intraligand metathesis, trans-[MCl2{P(CH2)n(CH2)n}P(CH2)n)] (4'c-e, 5'e), are isolated. These react with LiBr, NaI, and KCN to give the corresponding MBr2, MI2, and M(CN)2 species (58-99%). (13)C NMR data show that the MX2 moieties rapidly rotate within the diphosphine cage on the NMR timescale, even at -120 °C. The reaction of 4 c and KSCN gives separable Pt(NCS)2 and Pt(NCS)(SCN) adducts (13 c, 28%; 14 c, 20%), and those of 4 c,e and Ph2Zn give PtPh2 species (15 c, 61%; 15 e, 90%). (13)C NMR spectra of 13 c-15 c show two sets of CH2 signals (ca. 2:1 intensity ratios), indicating that MX2 rotation is no longer rapid. Reactions of 4 c or 4'c and excess NaC≡CH afford the free diphosphines P{(CH2)14}3P (91%) and (CH2)14P(CH2)14P(CH2)14 (90%). The latter has been crystallographically characterized as a bis(BH3) adduct. The crystal structures of eight complexes with P(CH2)14P linkages (PtCl2, PtBr2, PtI2, Pt(NCS)2, PtPh2, PdCl2, PdBr2, PdI2) and 15 e have been determined, and intramolecular distances analyzed with respect to MX2 rotation. The conformations of the (CH2)14 moieties and features of the crystal lattices are also discussed.

Reactions of Br(CH2)mCH=CH2 with Mg powder and then PCl3 (0.33 equiv.) afford P((CH2)m- CH=CH2)3 (1; m = a, 4; b, 5; c, 6; d, 7; e, 8; f, 9; 52 - 87%). Reactions of 1a - c, e with PdX2(COD) (X = Cl, Br) give trans-PdX2(P((CH2)mCH=CH2)3)2 (35 - 92%). Reactions of 1b - e with PtCl2 in benzene give mainly trans-PtCl2(P((CH2)mCH=CH2)3)2 (trans-5b-e; 52 - 75%), whereas those with K2PtCl4 in water give mainly cis-5b-e (33 - 70%). The reaction of equimolar quantities of 1c and H3B・S(CH3)2 gives the 1 : 1 adduct H3B・P((CH2)6CH=CH2)3 (85%). In none of these transformations are by-products derived from the C=C linkages observed

The direct addition of amine N-H bonds across an unsaturated carbon-carbon linkage gives fast and highly atom-economical access to amines. This review provides an overview of the most effective stereoselective methods using various catalyst systems based on alkali, alkaline earth, and transition metals, as well as enzymatic and organocatalytic approaches. Except for a few sporadic examples in the last century, this field has evolved primarily over the last decade. Catalyst systems have reached enantioselectivities exceeding 90% ee for many substrate classes, but significant challenges remain, in particular in the stereoselective intermolecular hydroamination of unactivated alkenes.

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