Boron transition structure

The reactions of 1-trimethylsilyl- or l-trimethylstannyl-2-butenyl-9-borabicyclo[3.3.1]-nonancs in the presence of pyridine are adequately explained by the usual transition structure 20 since the aldehyde should be capable of displacing pyridine as a ligand on boron. 

The stereoselectivity of the P-carboethoxyallylic boronate derived from the endo-phenyl auxiliary A (p. 803) toward R- and. S -glyccraldchydc acetonide has been investigated. One enantiomer gives the anti product in 98 2 ratio, whereas the other favors the syn product by a 65 35 ratio. Based on the proposed transition structure for this boronate, determine which combination leads to the higher stereoselectivity and which to the lower. Propose the favored transition structure in each case. 

It is well accepted that the high diastereospecificify of aldehyde allylboration reactions is a consequence of the compact cyclic transition structure. Theoretical calculations have shown that the chairlike transition structure shown in Scheme 1 and Fig. 1 is the lowest in energy relative to other possibilities such as the twist-boat conformation. With boronate reagents, it has also been suggested that a weak hydrogen bond between the axial boronate oxygen and the hydrogen of the polarized formyl unit contribntes to the preference for the transition structme with the aldehyde substituent in the psendo-eqnatorial position. ... 

The presence of a stereogenic center on the aldehyde can strongly inlinence the diastereoselectivity in allylboration reactions, especially if this center is in the a-position. Predictive rules for nucleophilic addition on snch a-snbstitnted carbonyl substrates such as the Felkin model are not always snitable for closed transition structures.For a-substituted aldehydes devoid of a polar substituent, Roush has established that the minimization of ganche-ganche ( syn-pentane ) interactions can overrule the influence of stereoelectronic effects. This model is valid for any 3-monosubstituted allylic boron reagent. For example, althongh crotylboronate (E)-7 adds to aldehyde 39 to afford as the major prodnct the diastereomer predicted by the Felkin model (Scheme 2), " it is proposed that the dominant factor is rather the minimization of syn-pentane interactions between the Y-snbstitnents of the allyl unit and the a-carbon of the aldehyde. With this... 

The enantioselectivity of these reagents is explained by comparison of transition structures 72 and 73 shown in Scheme 7. The disfavored transition structure 73 leading to the minor enantiomer displays a steric interaction between the methylene of the allylic unit and the methyl group of one of the pinane units. Unlike the tartrate boronates described above, the directing effect of the bis(isopinocampheyl) allylic boranes is extremely powerful, giving rise to high reagent control in double diastereoselective additions (see section on Double Diastereoselection ). 

Recently, the first examples of catalytic enantioselective preparations of chiral a-substituted allylic boronates have appeared. Cyclic dihydropyranylboronate 76 (Fig. 6) is prepared in very high enantiomeric purity by an inverse electron-demand hetero-Diels-Alder reaction between 3-boronoacrolein pinacolate (87) and ethyl vinyl ether catalyzed by chiral Cr(lll) complex 88 (Eq. 64). The resulting boronate 76 adds stereoselectively to aldehydes to give 2-hydroxyalkyl dihydropyran products 90 in a one-pot process.The diastereoselectiv-ity of the addition is explained by invoking transition structure 89. Key to this process is the fact that the possible self-allylboration between 76 and 87 does not take place at room temperature. Several applications of this three-component reaction to the synthesis of complex natural products have been described (see section on Applications to the Synthesis of Natural Products ). 

The catalytic asymmetric diboration of allenes provides a-substituted 2-boronyl allylic boronates of type 25 (see Eq. 32). One of them, 91, adds to ben-zaldehyde, albeit with a slight erosion of stereoselectivity (Eq. 65). The major P-hydroxy ketone stereoisomer, isolated after an oxidative work-up, originates from the putative chairlike transition structure 92. 

Intramolecular additions generally follow the same trends of stereoselectivity as observed in the bimolecular reactions. Eor example, allylic boronates ( )- and (Z)-118 provide the respective trans- and cis-fused products of intramolecular aUylation. As shown with allylboronate ( )-118, a Yb(OTf)3-catalyzed hydrolysis of the acetal triggers the intramolecular aUylboration and leads to isolation of the trans-fused product 119 in agreement with the usual cyclic transition structure (Eq. 96). 

A masked allylic boron unit can be revealed through a transition-metal-catalyzed borylation reaction. For example, a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates has been developed. The intramolecular allylboration step is very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (see Eq. 33). The use of a non-coordinating solvent like toluene is more suitable for the overall process provided that an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation reaction. With cyclic ketones such as 136, the intramolecular allylation provides cis-fused bicyclic products in agreement with the involvement of the usual chairlike transition structure, 137 (Eq. 102). 

Muetterties and Schunn (1966) suggest that certain boron hydride structures, e.g. 26 and 22 or 23 (C, symmetry, when X=Y), might be used as models for electrophilic substitution. In general, if 21 or 22 were the preferred C3 transition states or intermediates, the reaction would go with retention. Note that these are the analogs of the two-electron three-center species A3 discussed earlier by HMO theory. If, however, X, Y, and R were permuted, there would be several stereochemical possibilities, two of which are indicated in Table 7. To decide what kind of geometry is assumed in SE2 transition states, which are pentacoordinated and electron- and orbital-deficient, calculations on model species are needed to establish preferred geometries. 

To explain the stereochemical outcome of the reaction of allylic boron reagents with carbonyl compounds, Houk and Li carried out calculations on the transition structures of the model reaction of formaldehyde and allylboronic acid6 (Scheme 3.V). The bimolecular complex formed initially between allylboronic acid and formaldehyde would rearrange via a six-membered transition state to form an intermediate. Calculations show that chair transition state A is 8.2kcal/ mol more stable than twist-boat transition structure B, clearly confirming that the six-membered chairlike transition-state model is a legitimate scheme to predict the stereochemical outcome of the boron allylation reaction. 

There are a number of Se2 reactions which are not open-chain reactions. The electrophile is typically an aldehyde coordinated at the time of reaction to an electropositive atom like boron, tin or zinc on the stereogenic centre. These reactions usually use cyclic, chair-like transition structures, are called metallo-ene reactions, and are inherently syn overall. 

The boron-aldol reaction of the p-methoxyben-zyl(PMB)-protected methylketone 16 proceeds with excellent 1,5-anti-selectivity (Scheme 4). In cases where the asymmetric induction is lower it may be improved by a double stereodifferential aldol reaction with chiral boron ligands [7]. The reason for this high stereoselectivity is currently unknown. Ab initio calculations suggest the involvement of twisted boat structures rather than chair transition structures [6]. 

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