Bis-boryl platinum

Benzyl alcohol, dehydrogenation of, 279 Bifunctional dicarboxylic acids, 85 Biphasic olefin dimerization, 274-276 Bis(boryl) platinum compounds, 228 Bis(catecholato)diborane(4) compounds, 195, 200-201... 

A recent paper by Bomer and Kleeberg discussed the synthesis and reactivity of unsymmetrical diborane(4) compounds (pin)B—B(dmab) (dmab = 1,2-di(methylamino)benzene) and (pin)B—B(dbab) (dbab=l,2-di(benzylamino)benzene) also derived from diaminoboryl ligands. They found that reactions of [Pt(T] -CH2=CH2)(PPh3)2] with these new diborane(4) species gave selective and unprecedented formation of unsymmetrical bis(boryl)platinum(II) complexes (Scheme 42). 

In 1993, Ishiyama et al.60 reported the synthesis of isomerically pure cis-1,2-bis(boryl)alkenes 53a-57a from their corresponding alkynes 53-57 (Scheme 12)60,61 via platinum complexes. The solvents did not play an important role in the reaction, but a comparison of the reaction rates at 50 °C revealed that the addition was apparently accelerated in polar... 

Bis(pinacolato)diborane(4) selectively adds to terminal aikenes and cyclic aikenes having internal strain to provide bis(boryl)alkanes in 76-86% yields 85-89 in the presence of a catalytic amount of Pt(dba)2 at 50 °C67 (Scheme 16). It is interesting to mention that Pt(dba)2 directed 1,2-addition to certain conjugated dienes, whereas 1,4-addition through a 7i-allyl-platinum(II) intermediate is an energetically more favorable process. The 1,4-addition to penta-1,3-diene at 80 °C with Pt(PPh3)4 gives 90, but the same reaction with Pt(dba)2 selectively produced the 1,2-addition product 91 at room temperature (Scheme 16). 

The Pd(0)-catalyzed addition of the B-S bond to alkynes (thioboration) regio- and stereoselectively produces (Z)-2-(organothio)-l-alkenylboron reagents (Scheme 2-25) [63]. Tlie addition of (tetraalkoxy)diboron to alkynes to give cw-bis(boryl)alkenes (diboration) is catalyzed by a platinum(O) catalyst [34]. 

Fig. 36 Crystallographically characterized square planar platinum(II) bis(boryl) complexes 10.16-10.30...

Let us take acrolein as an example to illustrate the findings from the DFT calculations. For a platinum boryl complex (37), the boryl ligand can act as a nucleophile if the Pt-boryl o-bonding electron pair plays the role to initiate the reaction and as an electrophile if the empty p orbital to initiate the reaction. Scheme 9 shows three possible insertion pathways for the reaction of the bis-boryl diimine platinum(II) complex with acrolein whose a -electron density distribution is also illustrated in 37. The free energy barriers for the pathways of the boryl attacks at C4 and C2 were calculated to be 35.7 and 53.3 kcal/mol, respectively [106]. Interestingly, the free energy barrier for the pathway of the boryl attack at O was calculated to be only 18.6 kcal/mol [106]. These results indicate that the boryl ligand, in this case, behaves as an electrophile. 

The last cascade reaction we report here for the synthesis of a quaternary spirocenter employs a platinum catalyst [21 ]. A regio and enantioselective diboration of 1,3-diene 44 could lead to a chiral bis(boryl) intermediates 45 (Scheme 9.12), which could then react in two subsequent asymmetric carbonyl allylations of succinic dialdehyde, leading to the creation of three stereogenic centers in good diastereomeric (9 1) and promising enantiomeric ratios (88 12) 46. 

The platinum precursor species [Pt(T] -CH2=CH2)(PPh3)2], postulated by Smith and Iverson, hold out an inducement since the olefin ligand did not always serve as an innocent mask for the low-valent metal center. In fact, tris(bicyclo[2.2.1]heptene)platinum(0) instantaneously reacts with B2Cat2 at room temperature to give a bis(boryl)bicyclo-[2.2.1]-heptane as the chief organic product (Scheme 8)." It indicated that metal-olefin complexes... 

Ishiyama T, Matsuda N, Murata M, Ozawa P, Suzuki A, Miyaura N. Platinum(O)-catalyzed diboration of alkynes with tetrakis(alkoxo)diborons an efficient and convenient approach to a s-bis(boryl)alkenes. Organometallks. 1996 15 713-720. 

Diboration of a,/ -unsaturated ketones is promoted by platinum(O) complexes. Reaction of 4-phenyl-3-buten-2-one with bis(pinacolato)diboron in the presence of a platinum catalyst affords a boryl-substituted (Z)-boron enolate, that is, a 1,4-diboration product, in high yield with high stereoselectivity (Scheme 8). The isolated boron enolate is easily hydrolyzed by exposure to water, giving / -boryl ketones in high yields.66 Similar diboration of a,/ -unsaturated ketones has also been achieved with Pt(bian)(dmfu) (bian = bis(phenylimino)acenaphthene, dmfu = dimethyl fumarate).67 Although the... 

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